Soft magnetic powder and inductors

Abstract

The present invention provides a soft magnetic powder for forming a metallic magnetic body used in inductors. When formed into a metallic magnetic body, the powder ensures mechanical strength and achieves high magnetic permeability. The soft magnetic powder comprises soft magnetic particles composed of a particle core comprising a soft magnetic metal and an insulating film located on the surface of the particle core. The insulating film contains Si and a hydrocarbon group having a linear chain portion with 8 or more carbon atoms, and the weight ratio of Si to C in the insulating film is 7.6 to 42.8.

Description

Technical Field

[0001] The present invention relates to soft magnetic powder and an inductor using the same. Background Art

[0002] Inductors (coil components) using metallic magnetic materials, such as surface-mountable chip inductors, are widely used in various electronic devices such as smartphones. Known metallic magnetic materials used in such inductors include powdered magnetic cores or unit cells, which are obtained by adding resin to soft magnetic powder composed of soft magnetic metal particles and then compression molding.

[0003] Patent Document 1 describes magnetic particles formed by forming an insulating coating on the surface of a tiny magnetic material core (particle core). This insulating coating is composed of a sol-gel reaction product of an organic phosphoric acid having a hydrocarbon group with a carbon chain length of 5 or more atoms and a metal alkoxide. These magnetic particles improve smoothness during compression molding to form a metallic magnetic body, increase the magnetic particle filling rate within the metallic magnetic body, and enhance the magnetic permeability of the metallic magnetic body.

[0004] However, increasing the smoothness of magnetic particles reduces the bonding strength between the magnetic particles and the surrounding resin, potentially leading to a decrease in the strength of the metallic magnetic body. In other words, the aforementioned conventional magnetic particles still have room for improvement in terms of the balance between the high magnetic permeability and mechanical strength of the metallic magnetic body formed from them.

[0005] Prior art literature

[0006] Patent Literature

[0007] Patent Document 1: International Publication No. 2018 / 131536 Summary of the Invention

[0008] An object of the present invention is to provide a soft magnetic powder which, when compression-molded to form a metal magnetic body, can achieve high magnetic permeability while ensuring the mechanical strength of the metal magnetic body.

[0009] One embodiment of the present invention is a soft magnetic powder comprising soft magnetic particles, wherein the soft magnetic particles are composed of a particle core comprising a soft magnetic metal and an insulating film located on the surface of the particle core; the insulating film comprises Si and a hydrocarbon group having a straight chain portion with a carbon number of 8 or more, and the weight ratio of Si to C in the above insulating film is 7.6 to 42.8.

[0010] According to the soft magnetic powder of the present invention, when it is compression-molded to form a metal magnetic body, high magnetic permeability can be achieved while ensuring the mechanical strength of the metal magnetic body. BRIEF DESCRIPTION OF THE DRAWINGS

[0011] Figure 1 The diagram schematically shows the configuration of an inductor according to an embodiment of the present invention, and is a perspective view of the inductor as viewed from the top surface side.

[0012] Figure 2 This figure schematically shows the structure of the same inductor and is a perspective view viewed from the mounting surface side of the inductor.

[0013] Figure 3 This is a perspective view showing the internal structure of the same inductor.

[0014] Figure 4 This is a cross-sectional view of a conductor used for a coil, taken along a section perpendicular to the longitudinal direction.

[0015] Figure 5 A diagram showing an overview of the manufacturing process of an inductor.

[0016] Figure 6 A diagram showing a method of forming a unit body using a flat plate formed of mixed powder.

[0017] Figure 7 Schematic diagram showing the state of the core of the unit cell after molding.

[0018] Figure 8 This is a diagram showing the structure of the first soft magnetic particles constituting the mixed powder.

[0019] Figure 9 This is an electron microscope photograph of the surface of an oxide film formed on the particle core of the Cr-free first soft magnetic particle.

[0020] Figure 10 This is a graph showing changes in the withstand voltage of a molded body with respect to the oxygen content in the first soft magnetic particles.

[0021] Figure 11 This is a graph showing changes in the specific magnetic permeability and saturation magnetic flux density of a molded body with respect to the oxygen content in the first soft magnetic particles.

[0022] Figure 12 This is a graph showing changes in the magnetic properties of a molded body with respect to the oxygen content in the first soft magnetic particles.

[0023] Figure 13 A diagram showing the structure of the second soft magnetic particles constituting the mixed powder.

[0024] Figure 14 This figure shows the flow of the powder mixture around the coil during the unit body molding and curing process.

[0025] Figure 15It is a diagram showing the state of the gap between the surface region and the central region of the first flat plate during the unit body molding and curing steps.

[0026] Figure 16 This diagram illustrates the reference surface of an inductor.

[0027] Figure 17 A diagram showing how the side surfaces of a unit cell are filled with resin.

[0028] Figure 18 It is a graph showing the measurement results of the surface roughness of the unit cell.

[0029] Figure 19 A diagram illustrating the distance between the side surface of the unit body and the coil.

[0030] Figure 20 This is a diagram illustrating the relationship between the amount of resin in the mixed powder and the density of the unit cell.

[0031] Figure 21 (A) is an image showing the winding portion of the lower section of the coil together with the surrounding materials, and (B) is an image showing the winding portion of the upper section of the coil together with the surrounding materials.

[0032] Figure 22 A diagram illustrating the pressure applied when molding a unit body.

[0033] Figure 23 This is a characteristic curve diagram showing the simulation results of the magnetic powder structure between wires.

[0034] Figure 24 This is an image of a case where an air gap serving as a magnetic gap is provided near the winding portion.

[0035] Figure 25 The characteristic curve diagram shows the simulation results corresponding to the presence or absence of the air gap.

[0036] Figure 26 This is a diagram schematically showing an example of a grinding device for grinding a unit body.

[0037] Figure 27 This is an illustration of the side clearance.

[0038] Figure 28 This is a diagram schematically showing an example of a protective film forming apparatus for forming a unit cell protective film.

[0039] Figure 29 The graph shows the experimental results of the relationship between the content of nano-silica and the drying speed.

[0040] Figure 30 The graph shows the experimental results of the average particle size of nano-silica silica particles and the adhesion generation rate.

[0041] Figure 31 This is an image showing cracks generated in the unit cell protective film.

[0042] Figure 32 This graph shows the results of measuring the number of "plating jumps" by changing the thickness of the unit cell protective film.

[0043] Description of Reference Numerals

[0044] 1 Inductor

[0045] 10 units

[0046] 12 Mounting surface

[0047] 14 Top

[0048] 16 Side 1

[0049] 18 Side 2

[0050] 20 External electrodes

[0051] 30 coils

[0052] 32 Winding section

[0053] 32L winding unit

[0054] 34 extension

[0055] 36 copper wire

[0056] 40 cores

[0057] 40K air gap (magnetic gap)

[0058] 50 unit protective film

[0059] 60 Insulation coating material

[0060] 61 Insulation coating

[0061] 62 welding layer

[0062] 81 First soft magnetic particle

[0063] 81A particle nucleus

[0064] 81B oxide film

[0065] 81C Insulation Film

[0066] 82 Second soft magnetic particles

[0067] 82A particle core

[0068] 82B insulation film

[0069] T Thickness

[0070] W Width

[0071] L length

[0072] P pressure

[0073] CW Coil width of the winding section

[0074] LS Length of the second soft magnetic particle

[0075] KL air gap length

[0076] KW Air gap width DETAILED DESCRIPTION

[0077] [Overall structure of inductor]

[0078] Figure 1 and Figure 2 1 is a diagram schematically showing the configuration of the inductor 1 according to this embodiment. Figure 1 is a perspective view of the inductor 1 as viewed from the top surface 14 side. Figure 2 This is a perspective view of the inductor 1 as viewed from the mounting surface 12 side.

[0079] The inductor 1 of this embodiment is configured as a surface-mount electronic component and includes a substantially rectangular parallelepiped unit 10, a pair of external electrodes 20 provided on the surface of the unit 10, and one surface of the unit 10 serving as a mounting surface 12 ( Figure 2 ), and the unit cell 10 is covered with a unit cell protection film 50 except for the external electrode 20 .

[0080] Hereinafter, in the unit body 10, the surface opposite to the mounting surface 12 is referred to as the top surface 14 ( Figure 1 ), of the four side surfaces other than the mounting surface 12 and the top surface 14, the pair of side surfaces located in the extension portion 34 of the coil 30, described later, are referred to as first side surfaces 16, and the remaining pair of side surfaces are referred to as second side surfaces 18. These first side surfaces 16 and second side surfaces 18 are also surfaces of the unit body 10 located in the radial direction of the winding portion 32 of the coil 30, described later. Hereinafter, the opposing mounting surface 12 and top surface 14 are referred to as a pair of principal surfaces.

[0081] In addition, if Figure 1 As shown, the length from the mounting surface 12 to the top surface 14 is defined as the thickness T of the unit body 10 , the length of the short side of the top surface 14 is defined as the width W of the unit body 10 , and the length of the long side is defined as the length L of the unit body 10 .

[0082] Figure 3It is a perspective view showing the internal structure of the inductor 1 according to this embodiment.

[0083] The unit body 10 includes a coil 30 and a core 40 in which the coil 30 is embedded, and is configured as a coil-enclosing magnetic component in which the coil 30 is enclosed in the core 40 .

[0084] The coil 30 is an air-core coil component in which a conducting wire 31 is wound.

[0085] The core 40 is formed by compressing a mixed powder obtained by mixing soft magnetic powder and resin in a state where the coil 30 is enclosed, thereby forming a compact into a substantially rectangular parallelepiped shape.

[0086] The coil 30 includes a winding portion 32 around which a conductive wire 31 is wound, and a pair of extension portions 34 extending from the winding portion 32. The winding portion 32 is formed by spirally winding the conductive wire 31, with both ends of the conductive wire 31 located on the outer periphery and the inner periphery connected to each other. Within the unit cell 10, the coil 30 is embedded in the core 40 with the central axis K of the winding portion 32 aligned with the thickness T of the unit cell 10. The extension portions 34 extend from the winding portion 32 to extend from the pair of first side surfaces 16.

[0087] Figure 4 The figure shows a cross-sectional view of the conductive wire 31 used for the coil 30, taken along a section perpendicular to the longitudinal direction. The conductive wire 31 used to form the coil 30 is composed of a copper wire 36 and an insulating coating material 60 covering the copper wire 36. The insulating coating material 60 includes an insulating coating layer 61 having electrical insulation properties and a welding layer 62 formed on the insulating coating layer 61. During the coil forming process, the conductive wire 31 is heated and wound, melting the welding layer 62, thereby bonding the conductive wire 31 in the winding portion 32 to each other. This can prevent the shape of the winding portion 32 from collapsing after the coil is formed. In addition, the insulating coating layer 61 can reliably insulate the coil 30 from the core 40.

[0088] The pair of external electrodes 20 are L-shaped members extending from the first side surface 16 of the unit body 10 across the mounting surface 12. The external electrodes 20 are connected to the extension portion 34 of the coil 30 on the first side surface 16 and the portion 20A ( Figure 2 ) is electrically connected to the wiring of the circuit board through appropriate mounting means such as welding.

[0089] The inductor 1 with this configuration is, for example, a power inductor and can be used as a choke coil in DC-DC converter circuits and power supply circuits where large currents flow. It can be used in electronic equipment such as personal computers, DVD players, digital cameras, TVs, mobile phones, smartphones, automotive electronics, and medical and industrial machinery. However, the applications of the inductor 1 are not limited to these, and can also be used in tuning circuits, filter circuits, and rectification / smoothing circuits, for example.

[0090] [Inductor Manufacturing Process Overview]

[0091] Figure 5 1 is a diagram schematically showing the manufacturing process of the inductor 1 .

[0092] As shown in the figure, the manufacturing process of the inductor 1 includes a granulation step, a coil forming step, a unit body molding and curing step, a unit body grinding step, a unit body protective film forming step, a unit body protective film removing step, and an external electrode forming step.

[0093] The granulation step is a step of granulating a mixed powder obtained by mixing the soft magnetic powder contained in the core 40 with a resin. The soft magnetic powder is formed of particles whose surfaces are covered with an insulating film.

[0094] The coil forming process forms the coil 30 from the conductive wire 31 coated with the insulating coating material 60. In this process, the coil 30 is formed by winding the conductive wire 31 using a winding method known as "α-winding," resulting in a shape having the aforementioned wound portion 32 and a pair of extended portions 34. α-winding refers to a state in which the conductive wire 31, functioning as a conductor, is wound in a two-stage spiral shape, with the extended portions 34 at the beginning and end of the winding located on the outer periphery. The number of turns of the coil 30 is not particularly limited; for example, 6.5 turns are used.

[0095] The unit body molding and curing step is a step of molding a molded body serving as a base of the unit body 10 .

[0096] As the molding material of the molded body, the mixed powder obtained in the granulation step is used.

[0097] In this process, the mixed powder is preformed into a flat plate (a solid object of a predetermined shape). This plate and coil 30 are then placed in the cavity of a forming mold. Next, the cavity is heated and pressurized with a punch to compress the molded body containing coil 30. The solidified molded body is then removed from the cavity and polished. This polishing process uses a barrel polishing method, which allows for chamfering of the corners of the molded body.

[0098] For preformed flat sheets, e.g. Figure 6 As shown, two types of flat plates can be used: a first flat plate 70 having a groove 71 of an appropriate shape (e.g., E-shaped) for placing the coil 30 therein, and a second flat plate 72 having an appropriate shape (e.g., I-shaped, plate-shaped, etc.) for covering the groove 71 of the first flat plate 70. During compression molding, the first flat plate 70 and the second flat plate 72 are overlapped in the cavity 75 of the molding die 74, and the coil 30 is embedded in the groove 71 of the first flat plate 70. Then, the first flat plate 70 and the second flat plate 72 are heated, and in the overlapping direction, the first flat plate 70 or / and the second flat plate 72 are heated from one side ( Figure 6In this example, the second flat plate 72 side is pressed using a punch 76 to integrate the first flat plate 70, the coil 30 and the second flat plate 72.

[0099] In addition, not only the preformed flat plate but also the mixed powder obtained in the granulation step can be directly put into the cavity and compression-molded.

[0100] The pressure P during compression molding is as follows Figure 7 As shown, it is preferable to apply a pressure lower than the pressure before molding so that each particle 80 of the soft magnetic powder constituting the unit body 10 does not collapse and maintains its shape before molding. This pressure P can suppress damage to the surface insulating film of each particle 80 constituting the soft magnetic powder, thereby suppressing a decrease in insulation performance (i.e., a decrease in withstand voltage performance).

[0101] In addition, if Figure 7 As shown, the particle sizes of the particles 80 constituting the soft magnetic powder are preferably two or more ( Figure 7 In the example, the first soft magnetic particles 81 are large particles with a larger average particle size, and the second soft magnetic particles 82 are small particles with a smaller average particle size). According to the soft magnetic powder, when compression molding is performed, as shown in FIG. Figure 7 As shown, resin 90 and second soft magnetic particles 82, which are small particles, are placed between first soft magnetic particles 81, which are large particles. This allows for a molded body (unit body 10) having a high filling rate of particles 80. The embodiments of the first soft magnetic particles 81 and second soft magnetic particles 82 constituting the core 40 will be described later.

[0102] The unit body grinding step is a step of applying abrasive grains to the second side surface 18 of the molded body obtained in the unit body molding and curing step to thereby shave (ie, grind) the second side surface 18 until the width W becomes a predetermined width.

[0103] Through this process, a unit body 10 can be obtained in which the width W of the molded body is reduced to a predetermined width. This reduction shortens the distance between the coil 30 and the second side surface 18 (also referred to as the side gap) within the unit body 10, thereby increasing the occupancy rate of the coil 30 in the radial direction of the winding portion 32 of the coil 30. In addition, since the molded body obtained by compression molding is ground to a predetermined size to obtain the unit body 10, the dimensional unevenness of the unit body 10 can be reduced compared to the case where the unit body 10 is controlled to a predetermined size by compression molding alone.

[0104] In addition, in the unit body grinding step, polishing (for example, barrel polishing) may be performed to chamfer the corners produced by grinding on the second side surface 18 .

[0105] The unit body protection film forming step is a step of forming the unit body protection film 50 on the entire surface of the unit body 10 ground to a predetermined size in the unit body grinding step.

[0106] The cell protection film 50 may be made of thermosetting resins such as epoxy resin, polyimide resin, and phenol resin, or thermoplastic resins such as polyethylene resin and polyamide resin. These resins may further contain fillers such as silicon oxide and titanium oxide.

[0107] In this step, the material of the unit body protection film 50 is applied to the entire surface of the unit body 10 by appropriate means such as coating and dipping, and then cured to form the unit body protection film 50 .

[0108] The unit body protective film removal process is a process of irradiating the unit body 10 whose entire surface is covered with the unit body protective film 50 with a laser, thereby removing the unit body protective film 50 at the electrode formation location (in this embodiment, the first side surface 16 and the specified location within the mounting surface 12) where the external electrode 20 is formed, and the insulating coating material 60 of the extension portion 34 of the coil 30 exposed at the electrode formation location.

[0109] In the unit cell protective film removal step, after the insulating coating material 60 is removed by laser, etching may be performed to clean the surface of the electrode formation portion.

[0110] The external electrode forming step is a step of forming the external electrodes 20 by plating at the electrode forming portion of the unit cell protective film 50 removed in the unit cell protective film removing step.

[0111] In this process, the external electrodes 20 are formed by plating the soft magnetic powder exposed on the surface of the unit cell 10 and the extension portion 34 of the coil 30. In this plating process, a layer made of copper (Cu) is formed by plating growth, thereby forming the external electrodes 20.

[0112] It should be noted that a nickel (Ni) layer and a tin (Sn) layer are sequentially stacked on the copper (Cu) layer by plating growth. Alternatively, aluminum (Al), silver (Ag), gold (Au), or palladium (Pd) layers may be used instead of the copper (Cu) layer.

[0113] In addition, the external electrodes can be formed using sputtering, conductive resin, copper plate, or the like.

[0114] The external electrode 20 is not limited to the L-shape shown in the figure, and may have a so-called five-surface electrode structure or a bottom-surface electrode.

[0115] The inductor 1 manufactured as described above can maintain the mechanical strength of the core 40 while improving the specific resistance and the ratio of the soft magnetic metal portion of the core 40 , thereby achieving high reliability and excellent withstand voltage, magnetic permeability, saturation magnetic flux density, and DC superposition characteristics.

[0116] Next, an embodiment of the inductor 1 is described below.

[0117] In each embodiment, unless otherwise specified, the dimensions of the inductor 1 are a length L of 2.0±0.2 mm, a width W of 1.2±0.2 mm, a thickness T of 0.7±0.1 mm, and a withstand voltage of approximately 20 V.

[0118] The inductor 1 can use any of the embodiments described below in [A-1-1. First soft magnetic particles], [A-1-2. Second soft magnetic particles], [A-2. Resin], [B. Coil], [C. Magnetic circuit], [D. Unit grinding], and [E. Unit protective film], and can be configured by any combination of these embodiments.

[0119] [A. Mixed powder]

[0120] The mixed powder used to form the core 40 contains soft magnetic powder and resin.

[0121] [A-1. Soft magnetic powder]

[0122] The soft magnetic powder contained in the mixed powder is composed of soft magnetic metal particles. For example, the soft magnetic powder includes first soft magnetic particles 81 (large particles) and second soft magnetic particles 82 (small particles) having an average particle size smaller than that of the first soft magnetic particles 81. It should be noted that in this specification, "average particle size" refers to the volume-based median particle size.

[0123] The average grain size separately of the 1st soft magnetic particle 81 and the 2nd soft magnetic particle 82 can be before these are mixed each other, use particle size distribution meter to measure respectively.In addition, as the situation of measuring under the state of the core 40 of the molded body of mixed powder compression molding, by analyzing the electron microscope image of the section of the soft magnetic particle that core 40 is polished and obtained, thereby measure.For example, from above-mentioned electron microscope photo, obtain the equivalent circle diameter of each soft magnetic particle section, each soft magnetic particle is assumed to have the ball of above-mentioned equivalent circle diameter, obtain the volume of each ball, calculate average grain size from the median of this volume value distribution.

[0124] The average particle size of the first soft magnetic particles 81 is 20 μm to 28 μm, preferably 21.4 μm to 27.4 μm. The average particle size of the second soft magnetic particles 82 is 1 μm to 6 μm, preferably 1.5 μm to 1.8 μm. In this way, by forming a mixed powder from the first soft magnetic particles 81 and the second soft magnetic particles 82 having different average particle sizes, the saturation magnetic flux density of the core 40 is increased by the first soft magnetic particles 81 having a large average particle size, the DC superposition characteristics are improved, and the second soft magnetic particles 82 having a small average particle size enter the gaps between the first soft magnetic particles 81, thereby increasing the filling rate of the soft magnetic particles in the core 40 and improving the specific magnetic permeability.

[0125] The amount of the second soft magnetic particles 82 contained in the mixed powder is 15% to 30% by weight, preferably 20% to 30% by weight, based on the total weight of the soft magnetic particles contained in the mixed powder. If the content of the second soft magnetic particles 82 in the soft magnetic powder is within the above range, the filling rate of the soft magnetic particles in the core 40 of the molded body of the mixed powder can be further increased.

[0126] The composition of the soft magnetic metal constituting the second soft magnetic particle 82 may be the same as the composition of the soft magnetic metal constituting the first soft magnetic particle 81, but preferably has different compositions from each other and has almost the same hardness. The hardness of the first soft magnetic particle 81 and the second soft magnetic particle 82 can be measured using the nanoindentation method. For example, the hardness of the first soft magnetic particle 81 is 600HV (kgf / mm 2 ) to 1200 HV, preferably 800 HV to 1000 HV. In addition, the hardness of the second soft magnetic particles 82 is 900 HV (kgf / mm 2 )~1400HV, expected to be 900HV~1100HV.

[0127] Furthermore, the ratio of the hardness of the second soft magnetic particles 82 to the hardness of the first soft magnetic particles 81 is preferably 0.7 to 1.2. Thus, when the mixed powder containing these soft magnetic particles is compression-molded to form the core 40, the soft magnetic particle with the lower hardness, either the first soft magnetic particle 81 or the second soft magnetic particle 82, can be prevented from deforming, thereby preventing a decrease in the insulation resistance of the core 40.

[0128] [A-1-1. First Soft Magnetic Particle]

[0129] [A-1-1-1. Embodiment of the First Soft Magnetic Particle]

[0130] Figure 8: This figure shows the structure of the first soft magnetic particle 81. The first soft magnetic particle 81 is composed of a particle core 81A containing a soft magnetic metal and an insulating film 81C formed on the surface of the particle core 81A. The particle core 81A has an oxide film 81B formed by oxidizing the soft magnetic metal constituting the particle core 81A on the surface of the particle core 81A.

[0131] In order to stably achieve high withstand voltage in core 40, insulating film 81C must maintain a strong bond with oxide film 81B, preventing it from peeling off from the underlying oxide film 81B. If insulating film 81C peels off from oxide film 81B, the insulation resistance of core 40 decreases, reducing its withstand voltage as an inductor. On the other hand, the formation of oxide film 81B reduces the amount of soft magnetic metal in particle core 81A, reducing the specific magnetic permeability of core 40 formed using particle core 81A. Therefore, from the perspective of specific magnetic permeability, the thickness of oxide film 81B is preferably as thin as possible.

[0132] The present inventors have discovered that when the particle core 81A is composed of a soft magnetic metal containing Cr, the oxide film 81B formed on the surface of the particle core 81A becomes thinner, the surface tends to become smoother, and there is a possibility that the bonding strength of the insulating film 81C in the oxide film 81B cannot be sufficiently achieved. Furthermore, the present inventors have discovered that as a solution to this problem, while limiting the Cr content in the particle core 81A, the film thickness of the oxide film 81B formed on the surface of the particle core 81A is set within a predetermined range. This ensures the bonding strength of the insulating film 81C in the oxide film 81B, and suppresses the reduction in the specific permeability of the core 40 formed using the particle core 81A to a certain range.

[0133] Specifically, the soft magnetic metal constituting the particle core 81A is an iron-based soft magnetic metal having a Cr content of 1.5% by weight or less. By keeping the Cr content within this range, the iron content is increased, thereby increasing the specific magnetic permeability of the particle core 81A. A mottled passivation film is formed on the surface of the particle core 81A, thereby making the oxide film 81B uneven. This increases the contact surface area between the oxide film 81B and the insulating film 81C, thereby improving the bonding strength between the insulating film 81C and the oxide film 81B.

[0134] The particle core 81A may also be made of a Cr-free (Cr-free) iron-based soft magnetic metal. Here, "Cr-free" means that the material contains substantially no Cr. Even if Cr is present, the amount is minimal (e.g., 500 ppm or less) enough to be introduced from the environment during the manufacturing process of the particle core 81A.

[0135] More specifically, particle core 81A is an amorphous or crystalline metallic magnetic material of an Fe-Si-Cr alloy or Fe-Si alloy having a Cr content within the aforementioned numerical range. The Fe-Si-Cr alloy or Fe-Si alloy may contain, for example, 87% by weight or more of Fe and 3% by weight or more of Si, and may also contain boron.

[0136] The particle core 81A of the first soft magnetic particle 81 is not limited to the Fe-Si-Cr alloy or Fe-Si alloy described above, and may be formed of an iron-based soft magnetic metal. Such an iron-based soft magnetic metal may be, for example, an amorphous or crystalline alloy of Fe-Si-Cr-Al or Fe-Si-Al, wherein the Cr content is within the above-mentioned numerical range.

[0137] When a Cr-free alloy is used as the particle core 81A of the first soft magnetic particle 81, the weight ratio of Fe in the particle core 81A can be increased, and the saturation magnetic flux density of the core 40 made using the particle core 81A can be further increased, so that as an inductor, better DC superposition characteristics can be obtained.

[0138] The oxide film 81B can be formed by oxidizing the soft magnetic metal on the surface of the particle core 81A during the production process of the particle core 81A. For example, the oxide film 81B can be formed by providing an active oxidation step during the production process of the particle core 81A, that is, exposing the particle core 81A to a water or oxygen atmosphere and / or exposing the particle core 81A to a high-temperature oxygen atmosphere.

[0139] As oxidation of the soft magnetic metal on the surface of particle core 81A progresses and the film thickness increases, the surface roughness of oxide film 81B increases, thereby improving the bonding strength between insulating film 81C formed on the surface and oxide film 81B. On the other hand, as oxidation of the soft magnetic metal progresses, the film thickness of oxide film 81B increases, reducing the amount of metal contained in particle core 81A, and the specific permeability of core 40 formed from particle core 81A decreases. To ensure the bonding strength of insulating film 81C and suppress the decrease in specific permeability within a certain range, the oxygen content of particle core 81A is preferably 900 ppm to 2800 ppm.

[0140] The insulating film 81C formed on the oxide film 81B is, for example, an inorganic glass film formed by a mechanochemical method. Examples of the inorganic glass film include phosphate glass, such as zinc phosphate or manganese phosphate, or glass. Alternatively, the insulating film 81C may be composed of an organic polymer film, an organic-inorganic hybrid film, or an inorganic insulating film. These insulating films 81C may be formed by mechanochemical methods, sol-gel reactions of metal alkoxides, or the like, depending on their materials.

[0141] The thickness of the insulating film 81C is 10 nm to 50 nm. By making the thickness of the insulating film 81C 10 nm or greater, the resistivity of the first soft magnetic particle 81 can be increased. Furthermore, by making the thickness of the insulating film 81C 50 nm or less, the proportion of metal in the first soft magnetic particle 81 can be increased, thereby achieving good magnetic properties in the core 40 using the first soft magnetic particle 81.

[0142] The first soft magnetic particle 81 having the above-described structure can ensure the bonding strength of the insulating film 81C formed on the oxide film 81B of the particle core 81A, stably achieve high withstand voltage in the core 40 , and maintain the specific magnetic permeability of the core 40 high.

[0143] [A-1-1-2. Method for producing first soft magnetic particles]

[0144] Next, a method for producing the first soft magnetic particle 81 according to one embodiment of the present invention will be described. Note that the method described below is merely an example, and the method for producing the first soft magnetic particle 81 according to one embodiment of the present invention is not limited to the method described below.

[0145] The particle core 81A of the first soft magnetic particle 81 can be obtained, for example, by a gas atomization method. That is, each metal serving as the basis of the particle core 81A is heated and melted in an induction furnace to form a molten metal, and the obtained molten metal is ejected from an ejection hole together with a jet stream of argon gas, which is an inert gas, to obtain metal particles. Thereafter, the obtained particles are cooled in water and dried to form the particle core 81A of the first soft magnetic particle 81. The average particle size of the particle core 81A can be adjusted, for example, by adjusting the velocity of the jet stream of argon gas used in ejecting the molten metal in the gas atomization method and / or the caliber of the ejection hole.

[0146] When the particle cores 81A are formed of amorphous metal and have an average particle diameter of 20 μm or more, for example, the SWAP method (Spinning Water Atomization Process) can be used, in which metal particles formed from the molten metal are rapidly cooled by a high-speed rotating water flow.

[0147] During the cooling in water and subsequent drying process, the particle core 81A is exposed to water and / or an oxygen atmosphere, thereby forming an oxide film 81B on the surface of the particle core 81A. The thickness of the oxide film 81B can be adjusted to a desired thickness by controlling the exposure time in the water or oxygen atmosphere and / or controlling the oxygen concentration in the manufacturing environment of the particle core 81A. In addition, by exposing the dried particle core 81A to a high-temperature oxygen atmosphere, a thicker oxide film 81B can be formed on the surface of the particle core 81A. It should be noted that the average particle size of the particle core 81A is not expected to change substantially before and after the formation of the oxide film 81B and the formation of the insulating film 81C described later.

[0148] Furthermore, the oxide film 81B formed on the surface of the particle core 81A does not necessarily need to have the metal oxides uniformly distributed within the film. For example, if one or more metals constituting the particle core 81A can form multiple oxides, the different oxides may be unevenly distributed within the oxide film 81B. Alternatively, the oxide film 81B may be composed of multiple layers formed from different oxides.

[0149] Next, an insulating film 81C is formed on the oxide film 81B formed on the particle core 81A. The insulating film 81C is, for example, a phosphate glass film formed by a mechanochemical method.

[0150] [A-1-1-3. Example of the first soft magnetic particle]

[0151] Twenty-seven samples (samples A1-01 to A1-27) were prepared, each with different Cr contents in the particle core 81A and thicknesses of the oxide film 81B on the surface of the particle core 81A, and their properties were evaluated. An overview of the particles in samples A1-01 to A1-27, along with their evaluation results, is shown in Table 1 below. Samples A1-03 to A1-08, samples A1-12 to A1-16, and samples A1-20 to A1-24 are examples of the first soft magnetic particle 81 according to one embodiment of the present invention.

[0152] Hereinafter, each sample will be described.

[0153] <Sample A1-01>

[0154] (Preparation of particle cores)

[0155] Amorphous metal microparticles of a Fe-Si alloy with zero Cr content (Cr-free) were prepared using the aforementioned SWAP method to serve as particle cores 81A. The Fe and Si contents of the prepared particle cores 81A were: 93% by weight Fe, 3.5% by weight Si, 3% by weight B, and the remainder C. The average thickness of the oxide film 81B resulting from surface oxidation of the particle cores 81A was 5 nm. Furthermore, the hardness of the prepared particle cores 81A was 953 HV.

[0156] Here, the contents of Fe and Si were measured by ICP-OES optical emission spectrometry (spark discharge optical emission spectrometry), and the hardness of the particle core 81A was measured by nanoindentation.

[0157] (Formation of Insulating Film)

[0158] Next, an insulating film 81C made of zinc phosphate, which is a phosphate glass, was formed on the surface of the particle core 81A (including the oxide film 81B) obtained above by a mechanochemical method. The soft magnetic particles after the insulating film 81C was formed were designated as sample A1-01 of the first soft magnetic particle 81. The formed insulating film 81C had a thickness of 23 nm.

[0159] The average particle size (median size) of the first soft magnetic particles 81 is 25.3 μm.

[0160] The average particle size is measured using a particle size distribution analyzer.

[0161] The average film thickness of the oxide film 81B is measured as follows. The average film thickness (average thickness) of the oxide film 81B refers to the average value of the thickness of the oxide film 81B measured at multiple points in the cross section of the particle core 81A in a broad sense, and refers to the value derived according to the steps described below in a narrow sense. First, one particle core 81A is cut using a focused ion beam (FIB) to produce a thin slice. In this case, the cutting position is arbitrary. Using an electron microscope (TEM) set to a magnification of 100,000 times, one particle core 81A is set at three arbitrary points at equal intervals along the periphery, and the cross section of the particle core 81A is photographed for three fields of view. For each TEM image, the thickness of the oxide film 81A is measured at four arbitrary points at equal intervals. The above measurement is performed on the three particle cores 81A, and the average value is calculated based on the thickness of the oxide film measured at all points (3 fields of view × 4 points × 3 points = 36 points), which is used as the "average thickness". The film thickness of the insulating film 81C can also be measured in the same way.

[0162] (Evaluation of the unevenness of oxide film thickness and oxygen content)

[0163] The difference between the maximum film thickness and the minimum film thickness of the oxide film 81B in the cross section of the particle core 81A (hereinafter referred to as the difference in the thickness of the oxide film) is measured as an indicator value representing the unevenness of the thickness of the oxide film 81B. The difference between the maximum film thickness and the minimum film thickness of the above-mentioned oxide film 81B can be measured as follows. First, one particle of the particle core 81A is cut with a focused ion beam (FIB) to make a thin slice. In this case, the cutting position is arbitrary. Using an electron microscope (TEM) set to a magnification of 100,000 times, the cross section of the particle core 81A is observed along the periphery of the particle core 81A, and 3 points are set at each of the seemingly thinner places and the seemingly thicker places, and photographed on 3 fields of view. Then, for each TEM image, the maximum and minimum values are measured, and the maximum value in the 3 fields of view is taken as the maximum film thickness, and the minimum value is taken as the minimum film thickness. The evaluation results are shown in Table 1.

[0164] Furthermore, the amount of oxide film 81B formed on the surface of particle core 81A was used as a parameter that could be estimated after the formation of insulating film 81C to evaluate the oxygen content in Sample A1-01. The oxygen content was measured and evaluated by weighing 1 gram of the soft magnetic particles produced above using an inert gas fusion method. The evaluation results are shown in Table 1.

[0165] (Evaluation of insulation film bonding strength)

[0166] The bonding strength of the insulating film 81C in the soft magnetic particles produced above was evaluated as follows using a powder resistance tester (Hiresta). First, 10 g of the powder composed of the soft magnetic particles produced above was weighed and placed in a cylinder (a cylinder with an electrical insulator as the side wall and a metal plate connected to the ground potential as the bottom plate) for measurement provided by the powder resistance tester. An upper plate composed of a metal plate with a diameter the same as the above inner diameter was brought into contact with the top of the powder placed in the cylinder, and a voltage was applied between the bottom plate and the upper plate. A load was applied to the upper plate in the direction toward the bottom plate, the load was increased, and the current flowing between the upper plate and the bottom plate was observed. The load value (unit: MPa (megapascals)) when the current exceeded the specified threshold was measured as an evaluation value representing the degree of bonding strength of the insulating film 81C. The evaluation results were expressed as ◎0×, where a measured value of 60 MPa or more was ◎, 20 MPa or more and less than 60 MPa was 0, and less than 20 MPa was ×. The evaluation results are shown in Table 1.

[0167] (Preparation of test pieces)

[0168] In order to evaluate the withstand voltage, specific permeability, and saturation magnetic flux density of a molded body composed of the soft magnetic particles of Sample A1-01 prepared above, a test piece was prepared for Sample A1-01. The test piece was a ring-shaped test piece obtained by compression molding the first soft magnetic particles 81, second soft magnetic particles 82, and epoxy resin of Sample A01-01. The second soft magnetic particles 82 used in the test piece were Sample A2-05, which will be described later.

[0169] The second soft magnetic particles 82 used in the test piece are soft magnetic particles having an average particle size of 3 μm and an insulating film 82B (described later) having a thickness of 2 nm and containing an alkyl group having a long chain portion and having 16 carbon atoms formed on a particle core 82A (described later) composed of crystalline pure iron. The weight ratio of the first soft magnetic particles 81 and the second soft magnetic particles 82 used in the test piece is 75:25. In addition, the weight ratio of the total of the first soft magnetic particles 81 and the second soft magnetic particles 82 to the epoxy resin is 100:3.1. The test piece is in the shape of a ring with an inner diameter of 8 mm, an outer diameter of 13 mm, and a thickness of 5 mm.

[0170] (Evaluation of withstand voltage)

[0171] The withstand voltage was evaluated using the test piece of Sample A1-01 prepared above. The withstand voltage was measured using an AC / DC withstand voltage insulation resistance tester. The evaluation results are shown in Table 1.

[0172] (Evaluation of magnetic permeability)

[0173] The test piece for Sample A1-01 prepared above was used to evaluate the specific magnetic permeability. The specific magnetic permeability was measured using a BH analyzer and an impedance material analyzer using a high-frequency signal of 1 MHz. The evaluation results are shown in Table 1.

[0174] (Evaluation of saturation magnetic flux density)

[0175] The saturation magnetic flux density was evaluated using the test piece prepared for Sample A1-01. The inductance change of the test piece was measured using an LCR meter and a DC power supply. The BH data was reversed and the value at which magnetic flux saturated was used as the saturation magnetic flux density. The evaluation results are shown in Table 1.

[0176] <Samples A1-02 to A1-27>

[0177] Samples A1-02 to A1-27 were prepared using the same procedure as for Sample A1-01, with the Cr content of particle core 81A set to the values shown in Table 1 and the Fe, Si, and oxygen contents varied. Bond strength, withstand voltage, specific magnetic permeability, and saturation magnetic flux density were evaluated. It should be noted that the average film thickness of oxide film 81B formed by surface oxidation of particle core 81A increased with increasing oxygen content, regardless of the Cr content.

[0178]

[0179] Figure 9 The electron microscope images of the surfaces of the particle cores 81A were taken for the particle cores 81A with oxygen contents of 500, 1200, 1500, 2500, and 2600 ppm, i.e., the particle cores 81A of samples A1-01, A1-04, A1-05, A1-06, and A1-07, among the particle cores 81A with zero Cr content (Cr-free). Figure 9 The differences in the surface conditions of particle core 81A between these samples indicate that the thicker the oxide film 81B, the higher the oxygen content of particle core 81A before the formation of insulating film 81C, and the deeper the surface irregularities of oxide film 81B. This increase in the depth of the irregularities with increasing oxide film 81B thickness is believed to be due to, for example, differences in the contact state of particle core 81A, which results in differences in the dryness of the surface of particle core 81A, and the fact that the oxidation resistance of the Fe-Si alloy varies depending on the location of the particle core.

[0180] Furthermore, when the oxygen content is 900 ppm or greater, the average thickness of oxide film 81B increases, the difference in thickness of oxide film 81B increases, and the bonding strength of insulating film 81C meets the reference value. In other words, from the perspective of the bonding strength of insulating film 81C, the oxygen content in first soft magnetic particles 81 is preferably 900 ppm or greater.

[0181] It can be considered that the improvement in the bonding strength of the insulating film 81C as the thickness of the oxide film 81B increases is due to the fact that as the thickness of the oxide film 81B increases, the unevenness formed on the surface of the oxide film 81B becomes deeper, resulting in an increase in the anchoring effect of the unevenness.

[0182] Then, according to the comparison of the bonding strength between samples A1-26 and A1-27, other samples A1-03 to A1-08, samples A1-12 to A1-16 and samples A1-20 to A1-24 in Table 1, it can be seen that the effect of improving the bonding strength due to such an increase in surface roughness can be obtained when the Cr content is less than 1.5 weight%, especially when sample A1-01 is Cr-free (content 0).

[0183] Figure 10 This is a graph showing the dependence of the withstand voltage on the oxygen content in the Cr-free samples A1-01 to A1-09. Figure 11 It is a graph showing the dependence tendency of the specific magnetic permeability and saturation magnetic flux density on the oxygen content in samples A1-01 to A1-09.

[0184] in addition, Figure 12 This is a graph showing the dependence of the magnetic properties coefficient on the oxygen content in samples A1-01 to A1-09.

[0185] Figure 10 The breakdown voltage shown increases with increasing oxygen content. This is presumably because the increase in the thickness of the oxide film 81B and the increase in the bonding strength of the insulating film 81C increase the insulation resistance of the particle core 81A of the first soft magnetic particle 81 to the surroundings, thereby increasing the resistivity of the test piece (molded body).

[0186] Figure 11 The specific magnetic permeability shown decreases as the oxygen content increases. This is presumably because the increase in oxygen content, i.e., the increase in the oxidized portion of the Fe-Si alloy constituting particle core 81A, leads to a decrease in the Fe-Si alloy content in particle core 81A, i.e., the content of the soft magnetic metal portion. Furthermore, it was found that within the oxygen content range of 2800 ppm or less, the decrease in specific magnetic permeability associated with an increase in the thickness of oxide film 81B can be suppressed to approximately 15% relative to the value at an oxygen content of 500 ppm.

[0187] The saturation magnetic flux density increases along with the oxygen content. It is believed that the saturation magnetic flux density increases along with the oxygen content because, as described above, the oxidized portion of the Fe-Si alloy, which is the soft magnetic metal constituting the particle core 81A, increases, and the cross-sectional area of the Fe-Si alloy portion of the particle core 81A decreases. As a result, the effective magnetic flux passing through the Fe-Si alloy portion of the particle core 81A decreases among the magnetic flux passing through the test piece. It should be noted that it is believed that Figure 11 The decrease in saturation magnetic flux density at oxygen levels of 2500ppm, 2600ppm, and 3000ppm is due to the influence of measurement conditions, test piece workmanship, and other factors. Post-evaluation verification confirmed that the abnormal measured values at 2500ppm, 2600ppm, and 3000ppm were caused by errors in volume calculation of the respective test pieces, errors in measurement condition settings, heat generation of the test pieces, and abnormal test piece conditions.

[0188] The above results show that the desired oxygen content of the first soft magnetic particles 81 constituting the core 40 is 900 ppm to 2800 ppm, which ensures the bonding strength of the insulating film 81C, maintains the withstand voltage at a practical level, and suppresses a significant decrease in the specific magnetic permeability.

[0189] As described above, the soft magnetic powder of the mixed powder used to form core 40 includes first soft magnetic particles 81. First soft magnetic particles 81 are composed of a core 81A containing a soft magnetic metal and an insulating film 81C located on the surface of core 81A. Core 81A also has an oxide film 81B composed of an oxide of the soft magnetic metal between insulating film 81C. Core 81A contains no chromium or contains 1.5% or less of chromium by weight, and has an oxygen content of 900 to 2800 ppm by weight.

[0190] According to this configuration, when the mixed powder including the first soft magnetic particles 81 is compression-molded to form the core 40 as a metallic magnetic body, a decrease in magnetic permeability in the core 40 can be suppressed and high withstand voltage can be stably achieved.

[0191] The soft magnetic metal contained in the particle core 81A of the first soft magnetic particle 81 may be an iron-based soft magnetic metal containing Fe and Si. With this configuration, the oxide film 81B can be easily formed on the surface of the particle core 81A.

[0192] Furthermore, the iron-based soft magnetic metal may be crystalline. According to this configuration, the oxide film 81B can be formed more easily on the surface of the particle core 81A.

[0193] Furthermore, the soft magnetic powder constituting the mixed powder may further include, in addition to the first soft magnetic particles 81, second soft magnetic particles 82 containing a soft magnetic metal and having a smaller average particle size than the first soft magnetic particles. By using the second soft magnetic particles 82 (described later) in addition to the first soft magnetic particles 81, the filling rate of the soft magnetic particles in the core 40 can be increased, resulting in a higher magnetic permeability.

[0194] Furthermore, the inductor 1 can be formed by a metal magnetic body composed of soft magnetic powder containing the first soft magnetic particles 81 according to any of the above embodiments and a wound conductive wire 31. This configuration can realize a compact, high-voltage, and highly reliable inductor.

[0195] [A-1-2. Second Soft Magnetic Particles]

[0196] [A-1-2-1. Embodiment of the Second Soft Magnetic Particle]

[0197] Figure 13: is a diagram showing the structure of the second soft magnetic particle 82. The second soft magnetic particle 82 is composed of a particle core 82A containing a soft magnetic metal and an insulating film 82B formed on the surface of the particle core 82A. The soft magnetic metal constituting the particle core 82A is, for example, crystalline or amorphous iron (Fe). Specifically, the granular body of the second soft magnetic particle 82 is, for example, carbonyl iron powder with an onion-skin structure, and the Fe content is 95% to 99.8% by weight, preferably 97% to 99.8% by weight. The carbonyl iron powder may contain carbon C, oxygen O, nitrogen N, and sulfur S as impurities. In addition, the carbonyl iron powder serving as the particle core 82A may have an Fe oxide film on its surface.

[0198] The soft magnetic metal constituting the particle core 82A of the second soft magnetic particle 82 is not limited to Fe, as in the first soft magnetic particle 81 , and may be an iron-based soft magnetic metal containing Fe and other metals.

[0199] The insulating film 82B of the second soft magnetic particle 82 is composed of, for example, a sol-gel reaction product with silicon dioxide as a component, and contains a hydrocarbon group with a straight chain portion of more than 8 carbon atoms. Specifically, the hydrocarbon group with a straight chain portion of more than 8 carbon atoms is, for example, an alkyl group that is a chain saturated hydrocarbon group. It should be noted that the hydrocarbon group with a straight chain portion of more than 8 carbon atoms can be a hydrocarbon group selected from one or more of octyl, nonyl, decyl, undecyl, dodecyl, tridecyl, tetradecyl, pentadecyl, hexadecyl, heptadecyl and octadecyl. In addition, the alkyl group can be any one of a primary alkyl group, a secondary alkyl group or a tertiary alkyl group.

[0200] The hydrocarbon group having a long chain portion can be formed as a product of a sol-gel reaction obtained using a mixture of tetraethoxysilane (TEOS) and a silane coupling agent having the hydrocarbon group, for example.

[0201] By giving the insulating film 82B of the second soft magnetic particle 82 a hydrocarbon group having a straight chain portion with more than 8 carbon atoms, the filling rate of these soft magnetic particles in the core 40 can be increased when the mixed powder containing the first soft magnetic particle 81 and the second soft magnetic particle 82 is compression-molded to form the core 40.

[0202] The mechanism of improving the filling rate of soft magnetic powder is not limited to a specific theory, and can be inferred as follows. As mentioned above, the core 40 is formed by compression molding a mixed powder comprising the first soft magnetic particle 81, the second soft magnetic particle 82 and a thermosetting resin (epoxy resin, etc.). In this case, if the second soft magnetic particle 82 (small particle), which is a soft magnetic particle on one side, has a hydrocarbon group with a straight chain portion of more than 8 carbon atoms on its surface, the hydrogen bond and / or dipole interaction between the polar group (epoxy group and / or hydroacid group, etc.) possessed by the second soft magnetic particle 82 and the above-mentioned epoxy resin can be reduced, thereby improving the fluidity (smoothness) of the second soft magnetic particle 82 during compression molding.

[0203] As a result, the highly smooth second soft magnetic particles 82 can enter the gaps between the first soft magnetic particles 81 (large particles). It is believed that this mechanism can increase the filling rate of the soft magnetic particles in the core 40 compared to a case where the soft magnetic particles do not have long-chain hydrocarbon groups. By increasing the filling rate of the soft magnetic particles, the density of the soft magnetic powder in the core 40 can be increased, resulting in an increase in the specific magnetic permeability of the core 40.

[0204] Here, the smoothness of the second soft magnetic particle 82 can be measured by the following steps using the single-sided shear tester used in JISZ 8835. More specifically, the smoothness can be measured using a single-sided shear tester of the direct-acting type of the lower unit (Nano Seeds Corpration. powder layer shear force measuring device NS-S500) according to the following steps. The inner diameter of the upper unit (Ring) and the inner diameter of the lower unit (Base) are both set to 15 mm, and the gap (micro-interval) between the upper unit and the lower unit is set to 0.2 mm. In a manner that a laser sensor can be used to measure the thickness of the powder layer, before the powder is placed, the upper pestle (Lid) is configured in the upper lower unit and the zero point is set. 10 g of the powder sample of the second soft magnetic particle 82 is evenly filled in the upper and lower divided units, and after the upper pestle (Lid) is calmly configured, a compression load of 150 N is applied with a vertical servo motor. When the vertical servo motor applies an extrusion load of 150N, the position of the load cell of the vertical servo motor is fixed. The extrusion speed is set to 0.2mm / second. 100 seconds after the position of the load cell of the vertical servo motor is fixed, the side planing starts. That is, the start delay of the side planing is set to 100 seconds. After the side planing starts by the operation of the horizontal servo motor, the pressure is measured every 0.1 seconds. The side planing speed is set to 5μm / second. For each measurement sample, N=50 points or more are continuously measured while the horizontal servo motor is operating, and the measurement is stopped when the coefficient of variation (CV value) of the measured value is less than 0.4%. The thickness of the final compacted powder layer (final powder layer thickness) is measured using a laser sensor.

[0205] Then, based on the load (maximum extrusion load) applied to the load sensor of the vertical servo motor when the position of the load sensor is fixed obtained from the above measurement, the load applied to the load sensor at the start of operation of the horizontal servo motor (extrusion load at the start of side sliding) and the value of the final powder layer thickness, the smoothness can be calculated using the following formula (for details, refer to Japanese Patent Application No. 2019-224678, for example).

[0206] [Number 1]

[0207]

[0208]

[0209] As described above, by imparting a hydrocarbon group having a linear chain portion with 8 or more carbon atoms to the surface of the second soft magnetic particle 82, the smoothness of the second soft magnetic particle 82 is improved, and the filling rate of the soft magnetic particles in the core 40 when forming the core 40 is increased, thereby improving the magnetic permeability. However, as a result of improving the smoothness by including a long-chain hydrocarbon group on the surface of the second soft magnetic particle 82, the adhesion or bonding between the second soft magnetic particle 82 and the surrounding resin or other soft magnetic particles (the first soft magnetic particle 81 and / or other second soft magnetic particles 82) is reduced, and the mechanical strength of the core 40 as a molded body may be reduced.

[0210] The present inventors have discovered that by reducing the number of long-chain hydrocarbon groups having 8 or more carbon atoms formed on the surface of the particle core 82A of the second soft magnetic particle 82 and controlling the smoothness, the mechanical strength of the core 40 as a molded body can be improved.

[0211] The number of long-chain hydrocarbon groups on the surface of the second soft magnetic particle 82 can be controlled by, for example, the mixing ratio of tetraethoxysilane and the silane coupling agent in the sol-gel reaction mixture used when forming the insulating film 82B on the surface of the particle core 82A.

[0212] The number of long-chain hydrocarbon groups on the surface of the second soft magnetic particle 82 can be evaluated by the content ratio of silicon Si to carbon C in the insulating film 82B. Furthermore, when the particle core 82A of the second soft magnetic particle 82 contains no Si and C, the number of long-chain hydrocarbon groups can be evaluated by the weight ratio of Si to C contained in the entire second soft magnetic particle 82 (Si / C weight ratio). From the perspective of suppressing a decrease in the mechanical strength of the core 40 while maintaining high magnetic permeability, the Si / C weight ratio in the second soft magnetic particle 82 is preferably 7.6 to 42.8.

[0213] It should be noted that in this embodiment, among the first and second soft magnetic particles 81, 82, which comprise the mixed powder and have different average particle sizes, the surface of the particle core 82A of the second soft magnetic particle 82 having the smaller average particle size has long-chain hydrocarbon groups. However, the soft magnetic particles forming the long-chain hydrocarbon groups are not limited to the second soft magnetic particles 82. For example, instead of the second soft magnetic particles 82, an insulating film 82B containing hydrocarbon groups having a long-chain portion with 8 or more carbon atoms as described above may be formed on the surface of the first soft magnetic particle 81, or on the surfaces of the first and second soft magnetic particles 81, 82. This improves the smoothness of the surface of the first soft magnetic particle 81, or the surfaces of the first and second soft magnetic particles 81, 82, thereby suppressing a decrease in the mechanical strength of the core 40 and achieving a high magnetic permeability within the core 40.

[0214] [A-1-2-2. Second Method for Producing Soft Magnetic Particles]

[0215] Next, a method for producing the second soft magnetic particles 82 according to one embodiment of the present invention will be described. Note that the method described below is merely an example, and the method for producing the second soft magnetic particles 82 according to the present invention is not limited to the method described below.

[0216] (Preparation of Soft Magnetic Metal Core Particles)

[0217] First, metal fine particles are prepared as the particle cores 82A of the second soft magnetic particles 82. The details of the average particle size of the second soft magnetic particles 82 and the composition of the particle cores 82A are as described above. It should be noted that the average particle size of the particle cores 82A is not substantially changed before and after the surface treatment described below.

[0218] (Formation of an Insulating Film on the Surface of a Particle Core)

[0219] Next, an insulating film 82B containing hydrocarbon groups having linear chains of 8 or more carbon atoms is formed on the surface of the particle core 82A. This insulating film 82B can be formed, for example, by a sol-gel reaction using a surface treatment agent containing tetraethoxysilane as an alkoxide and a silane coupling agent. Thus, the insulating film 82B containing hydrocarbon groups having linear chains, which is a sol-gel reaction product, can be formed on the particle core 82A.

[0220] The alkoxide is not limited to tetraethoxysilane and can be any metal alkoxide represented by the chemical formula M-(OR)n. In the formula, the metal species M of the metal alkoxide is preferably at least one selected from the group consisting of Li, Na, Mg, Al, Si, K, Ca, Ti, Cu, Sr, Y, Zr, Ba, Ce, Ta, and Bi. The alkoxy group OR of the metal alkoxide can be any alkoxy group such as methoxy, ethoxy, and / or propoxy.

[0221] Silane coupling agents can be represented by the chemical formula R'-Si(OR)3. In the formula, R' is a hydrocarbon group having a linear portion with 8 or more carbon atoms, and can be one or more hydrocarbon groups selected from octyl, nonyl, decyl, undecyl, dodecyl, tridecyl, tetradecyl, pentadecyl, hexadecyl, heptadecyl, and octadecyl. In the formula, OR is an alkoxy group, preferably a methoxy group or an ethoxy group. For example, when forming a hydrocarbon group having a linear portion with 16 carbon atoms, hexadecyltrimethoxysilane can be used.

[0222] The second soft magnetic particles 82 thus obtained, which are endowed with smoothness, will not excessively constrain the epoxy resin when they are compression-molded together with the above-mentioned first soft magnetic particles 81 and epoxy resin to form the core 40. Instead, they can effectively fill the spaces between the first soft magnetic particles 81, and can serve as the core 40 to realize a magnetic body (magnetic core) with high specific magnetic permeability.

[0223] Here, the formation of insulating film 82B on particle core 82A is preferably performed through a first-stage process of forming a film of tetraethoxysilane on the surface of particle core 82A, followed by a second-stage process of forming a film containing long-chain hydrocarbon groups having linear carbon atoms of 8 or more on the formed tetraethoxysilane film through a sol-gel reaction between tetraethoxysilane and a silane coupling agent. This prevents the long-chain hydrocarbon groups from being buried within the insulating film 82B layer, allowing the long-chain hydrocarbon groups to be efficiently arranged on the surface of insulating film 82B. Consequently, the amount of silane coupling agent used in forming insulating film 82B can be reduced.

[0224] In addition, the surface treatment agent can include a surfactant in the process of the first stage. By adding a surfactant to the surface treatment agent, the hydrophilic group portion of the surfactant that becomes micelles forms a hydrogen bond with the silanol group formed by the hydrolysis reaction of tetraethoxysilane. Thus, in the process of the first stage of forming the film of tetraethoxysilane, the micelles are configured on the surface of the particle core 82A of the soft magnetic metal. There can be a sparse part and a dense part of the tetraethoxysilane molecule on the surface of the particle core 82A. Therefore, in the process of the second stage, the long-chain hydrocarbon groups with more than 8 carbon atoms can be spaced apart and configured. As a result, the long-chain hydrocarbon groups are dispersed on the surface of the insulating film 82B and configured, so that smoothness can be distributed on the entire surface of the second soft magnetic particle.

[0225] [A-1-2-3. Example of the Second Soft Magnetic Particle]

[0226] Twenty-seven soft magnetic particle samples were prepared, each with different carbon number of the long chain portion of the hydrocarbon group contained in the insulating film 82B and different Si to C weight ratio (Si / C weight ratio) in the insulating film 82B. These samples, designated as samples A2-01 to A2-27, were then evaluated for their properties. Table 2 summarizes samples A2-01 to A2-27. Samples A2-01 to A2-09, samples A2-15 to A2-18, and samples A2-24 to A2-27 are examples of the second soft magnetic particle 82. Table 2 also lists the silane coupling agent used in preparing the insulating film 82B for each of samples A2-01 to A2-27.

[0227] [Table 2]

[0228]

[0229] Hereinafter, each sample will be described.

[0230] <Sample A2-01>

[0231] (Preparation of particle cores)

[0232] Carbonyl iron powder containing 97% to 99.8% iron by weight was selected as the core 82A of the second soft magnetic particle 82. This core 82A had a hardness of 952 HV, which is approximately equal to the hardness of the core 81A of the first soft magnetic particle 81 used in samples A1-01 to A1-08.

[0233] (Formation of an Insulating Film Containing a Long-Chain Hydrocarbon Group)

[0234] On the prepared particle core 82A, an insulating film 82B containing an alkyl group having a hydrocarbon group as a linear portion with 16 carbon atoms is formed. A mixed solution containing tetraethoxysilane as an alkoxide, hexadecyltrimethoxysilane as a silane coupling agent, and a phosphate ester type anionic surfactant as a surfactant is used as a surface treatment agent for forming the insulating film 82B.

[0235] The specific steps are as follows. Tetraethoxysilane and hexadecyltrimethoxysilane were KBE04 (manufacturer: Shin-Etsu Chemical Co., Ltd.) and X-88-422 (manufacturer: Shin-Etsu Chemical Co., Ltd.), respectively. PLYSURF AL (manufacturer: Daiichi Kogyo Seiyaku Co., Ltd.) was used as a phosphate ester anionic surfactant.

[0236] First, the first stage of the process involves forming a film of tetraethoxysilane on the surface of particle cores 82A. An aqueous solution of isopropyl alcohol, ammonia, and PLYSURF AL is added and stirred to produce Dispersion 1. A predetermined amount of the prepared particle cores 82A is weighed, isopropyl alcohol is added, and ultrasonic vibration is applied to disperse the powder of particle cores 82A, producing Dispersion 2. Subsequently, Dispersion 1 is added to Dispersion 2, and the mixture is stirred using a stirrer to produce Dispersion 3.

[0237] Next, tetraethoxysilane was added to isopropyl alcohol and mixed to prepare surface treatment liquid 1. This surface treatment liquid 1 was added to dispersion liquid 3 to prepare reaction liquid 1, which was stirred with a stirrer to form a tetraethoxysilane film on the surface of particle core 82A.

[0238] Next, a second step is performed to form an alkyl group having a linear chain portion with 16 carbon atoms on the surface of the tetraethoxysilane film formed in the first step. First, isopropyl alcohol, hexadecyltrimethoxysilane, and tetraethoxysilane are mixed to prepare surface treatment solution 2. This surface treatment solution 2 is added to reaction solution 1 formed in the first step to prepare reaction solution 2, which is then stirred with a stirrer to form an alkyl group having a linear chain portion with 16 carbon atoms on the surface of the tetraethoxysilane film formed above.

[0239] Afterwards, the reaction solution 2 was filtered through a membrane filter to separate the particles forming the insulating film 82B. The separated particles were appropriately washed with acetone and dried in a natural environment at room temperature. The dried particles were filtered through a metal mesh, and the remaining particles were used to prepare sample A2-01 for the second soft magnetic particle 82.

[0240] The average particle size (median diameter) of the second soft magnetic particles 82 produced as described above was 1.7 μm. The average particle size was measured using a particle size distribution analyzer.

[0241] (Method for Confirming the Si / C Weight Ratio in Particle Cores After Insulating Film Formation)

[0242] As a parameter indicating the number of hydrocarbon groups having a long chain portion with 16 carbon atoms on the surface of the formed insulating film 82B, the Si / C weight ratio in the sample A2-01 after forming the insulating film 82B was determined as follows.

[0243] First, using an X-ray photoelectron spectrometer, X-rays are irradiated onto the second soft magnetic particles 82 forming the insulating film 82B. Using a method known as wide-scan spectrometry, information on the peak intensities of the elements contained in the insulating film 82B is obtained. Next, using a method known as narrow-scan spectrometry, attention is focused on the Si and C elements contained in the insulating film 82B. The area intensity of these peak intensities is determined, and the relative sensitivity coefficients of the element orbitals are normalized to 100% to calculate the atm% concentration. This atm% is then multiplied by the atomic weight of each element to obtain the weight ratio of Si to C.

[0244] (Evaluation of Smoothness)

[0245] 10 g of the soft magnetic particles of Sample A2-01 prepared above were weighed and evaluated for smoothness using a single-sided shear tester with a direct-acting lower unit (Nano Seeds Corporation, Inc., NS-S500 Powder Shear Tester). The inner diameters of the upper unit (ring) and the lower unit (base) were both set to 15 mm, and the extrusion load was set to 150 N. The evaluation results are shown in Table 3.

[0246] (Preparation of test pieces)

[0247] To evaluate the molded body formed from the soft magnetic particles of Sample A2-01 prepared above, a test piece was prepared for Sample A2-01. The test piece was a ring-shaped test piece obtained by compression molding the first soft magnetic particles 81, the second soft magnetic particles 82 of Sample A02-01, and epoxy resin.

[0248] The first soft magnetic particles 81 used in the test piece are sample A1-04, which is composed of 5 nm oxide film 81B and 23 nm insulating film 81C formed on particles of an amorphous Fe-Si alloy containing no Cr and having an average particle size of 25.3 μm. The mixing ratio of the first soft magnetic particles 81 to the second soft magnetic particles 82 is 75:25 by weight. The mixing ratio of the first soft magnetic particles 81 and the second soft magnetic particles to the epoxy resin is 100:3.1 by weight. The test piece is annular with an inner diameter of 8 mm, an outer diameter of 13 mm, and a thickness of 4 mm.

[0249] (Evaluation of magnetic permeability)

[0250] The specific magnetic permeability of the test piece of Sample A02-01 prepared above was evaluated. The specific magnetic permeability was measured using a BH analyzer and an impedance material analyzer using a high-frequency signal at a frequency of 1 MHz. The evaluation results of the specific magnetic permeability are represented by 0×, with a value of 0 indicating a value greater than or equal to the reference value of 30, and a value of × indicating a value less than or equal to the reference value of 30. The measured values and evaluation results of the specific magnetic permeability are shown in Table 3.

[0251] (Evaluation of radial compressive strength)

[0252] The radial compressive strength of the test piece of sample A02-01 prepared above was evaluated. The radial compressive strength was measured by measuring the pressure at which the annular test piece was broken by applying radial pressure. The evaluation results of the radial compressive strength are expressed as 0×, and the measured value is a reference value of 85 N / mm. 2 If the value is less than the reference value 85N / mm, it is 0. 2 The measured values and evaluation results of the radial compressive strength are shown in Table 3.

[0253] (Evaluation of withstand voltage)

[0254] The test piece for Sample A2-01, prepared above, was used to evaluate the withstand voltage. The withstand voltage was measured using an AC / DC withstand voltage insulation resistance tester. The evaluation results are indicated by 0×, with a value of 0 indicating a value greater than or equal to the reference value of 50 V / mm and a value of × indicating a value less than or equal to the reference value of 50 V / mm. The evaluation results are shown in Table 3.

[0255] <Samples A2-02 to A2-27>

[0256] For samples A2-02 to A2-27, the silane coupling agents listed in Table 2 were used, respectively, to adjust the number of carbon atoms in the long chain portion of the hydrocarbon group contained in insulating film 82B to the number listed in Table 2, and the Si / C weight ratio in insulating film 82B to the value listed in Table 2. Otherwise, insulating film 82B was formed using the same procedures as sample A2-01, and its smoothness was evaluated. Furthermore, for each of samples A2-02 to A2-27, the soft magnetic particles of each sample were used as the second soft magnetic particles 82, and test pieces similar to those used for sample A2-01 were prepared. Using these test pieces, the specific magnetic permeability, radial compressive strength, and withstand voltage were evaluated using the same procedures as for sample A2-01.

[0257] Here, the Si / C weight ratio in samples A2-01 to A2-27 was adjusted by changing the mixing ratio of tetraethoxysilane and the silane coupling agent used in forming the insulating film 82B.

[0258] The evaluation results of samples A2-01 to A2-09 are shown in Table 3. In addition, the evaluation results of samples A2-10 to A2-18 are shown in Table 4, and the evaluation results of samples A2-19 to A2-27 are shown in Table 5.

[0259]

[0260]

[0261]

[0262] According to the measured values of smoothness and the evaluation results of magnetic permeability in Table 3 or Table 5, it can be seen that: the insulating film 82B formed on the surface of the particle core 82A contains hydrocarbon groups with long chain portions having a carbon number of 8 or more, thereby improving the smoothness of the soft magnetic particles, improving the molding density of the molded body (test piece), and improving the specific magnetic permeability.

[0263] There is a trade-off between specific magnetic permeability, radial compressive strength and Si / C weight ratio.

[0264] Furthermore, in insulating film 82B having 8 or more carbon atoms in its linear portion, if the Si / C weight ratio is in the range of 7.6 to 42.8, the radial compressive strength and specific magnetic permeability, which are trade-offs, can be achieved, with radial compressive strength of 85 or more and specific magnetic permeability of 30 or more. This allows for a core having high specific magnetic permeability while maintaining practical mechanical strength. In particular, from the perspective of manufacturing reproducibility, the Si / C weight ratio is more preferably in the range of 9.7 to 13.4.

[0265] As described above, the soft magnetic powder constituting the mixed powder includes second soft magnetic particles 82. Second soft magnetic particles 82 are composed of a particle core 82A containing a soft magnetic metal and an insulating film 82B located on the surface of particle core 82A. Insulating film 82B contains Si and a hydrocarbon group having a linear chain portion with 8 or more carbon atoms. The weight ratio of Si to C in insulating film 82B is 7.6 to 42.8.

[0266] With this configuration, when the mixed powder containing the second soft magnetic particles 82 is compression-molded to form the core 40 as a metallic magnetic body, both high mechanical strength and high magnetic permeability can be achieved in the core 40. Here, the Si is derived from, for example, a silane coupling agent used to form the insulating film 82B.

[0267] Alternatively, the hydrocarbon group included in the insulating film 82B of the second soft magnetic particle 82 may be an alkyl group. This configuration allows a hydrocarbon group having a linear chain portion having 8 or more carbon atoms to be easily formed on the surface of the particle core 82A.

[0268] Furthermore, the particle core 82A of the second soft magnetic particle 82 may be composed of carbonyl iron. With this configuration, when the core 40 is formed as a metallic magnetic body, a higher magnetic permeability can be achieved in the core 40.

[0269] Furthermore, the soft magnetic powder constituting the mixed powder may further include, in addition to the second soft magnetic particles 82, first soft magnetic particles 81 containing a soft magnetic metal and having particle cores 81A having a larger average particle diameter than the particle cores 82A of the second soft magnetic particles. This configuration further increases the filling rate of the soft magnetic particles in the core 40, resulting in a higher magnetic permeability.

[0270] Furthermore, the inductor 1 can be formed by a metal magnetic body composed of soft magnetic powder containing the second soft magnetic particles 82 according to any of the above-described embodiments and the wound conductive wire 31. This configuration can realize a compact and highly reliable inductor.

[0271] [A-2. Resin]

[0272] The ratio of the resin is 2.0 wt% to 3.5 wt% based on the total weight of the soft magnetic powder and the resin. The resin contains at least a bisphenol A epoxy resin and a rubber-modified epoxy resin, and may further contain a novolac epoxy resin.

[0273] The present inventors, through evaluation experiments described below, discovered a first resin blending ratio, which is a preferred blending ratio of a bisphenol A-type epoxy resin to a rubber-modified epoxy resin when no novolac-type epoxy resin is added to the mixed powder. The first resin blending ratio is 50% to 90% by weight of the bisphenol A-type epoxy resin and 10% to 50% by weight of the rubber-modified epoxy resin, based on the total weight of the resins included in the mixed powder.

[0274] Here, bisphenol A epoxy resin is the main component of the resin contained in the mixed powder. If the resin contained in the mixed powder is solely bisphenol A epoxy resin, the formed unit body 10 tends to be brittle. Therefore, by adding a rubber-modified epoxy resin to the resin contained in the mixed powder, the formed unit body 10 can be made tougher and its brittleness can be improved. The ratio of bisphenol A resin to rubber-modified epoxy resin in the resin contained in the mixed powder can then be adjusted based on the first resin blending ratio. Through the above-described unit body molding and curing steps, the unit body 10 encapsulating the coil 30 can be molded, thereby producing an inductor 1 with improved unit body strength.

[0275] The present inventors have discovered, through evaluation experiments described below, a second resin blending ratio as a preferred blending ratio of a bisphenol A-type epoxy resin, a rubber-modified epoxy resin, and a novolac-type epoxy resin when a novolac-type epoxy resin is blended into a powder mix. The second resin blending ratio, based on the total weight of the resins contained in the powder mix, is 40% to 80% by weight of the bisphenol A-type epoxy resin, 10% to 50% by weight of the rubber-modified epoxy resin, and 1% to 30% by weight of the novolac-type epoxy resin.

[0276] Here, the novolac-type epoxy resin performs the functions of adjusting the viscosity of the powder mix during unit formation and the glass transition temperature of the unit during the unit molding and curing process, thereby improving the strength of the unit at high temperatures. Therefore, by appropriately adding the novolac-type epoxy resin according to the second resin blending ratio, an inductor 1 with enhanced unit strength can be manufactured.

[0277] Furthermore, the present inventors have discovered that by forming a unit body using a mixed powder obtained by blending resins at the first or second resin blending ratio, inductors having the following specific configurations 1 to 3 can be produced.

[0278] Specific configuration 1...For the cross-section of the unit body 10, the ratio of the area of the voids to the total area of the soft magnetic particles (the first soft magnetic particles and the second soft magnetic particles) and the resin in the surface region from the surface of the unit body 10 to 1 μm to 100 μm is smaller than the ratio of the area of the voids to the total area of the soft magnetic particles and the resin in the central region of the unit body 10, and the surface region is denser than the central region.

[0279] Specific structure 2...See Figures 1 to 3 The amount of resin in the ridgeline portion where the main surfaces 12 , 14 are in contact with the second side surface 18 is smaller than the amount of resin in the ridgeline portion where the main surfaces 12 , 14 are in contact with the first side surface 16 .

[0280] Specific structure 3...See Figures 1 to 3 The polished first soft magnetic particles or the polished second soft magnetic particles are exposed from the core, and the first soft magnetic particles or the second soft magnetic particles exposed from the core are covered by the unit body protective film 50. Furthermore, the surface roughness of the second side surface 18 is greater than the surface roughness of the first side surface 16, and the minimum distance between the second side surface 18 and the winding portion 32 of the coil 30 is 1 times larger and 4 times smaller than the diameter of the first soft magnetic particle.

[0281] The mechanism obtained by the above-mentioned specific configurations 1 to 3 will be described. Figure 6 As described above, the coil 30 is placed on the first flat plate 70 and is sandwiched between the first flat plate 70 and the second flat plate 72 to form a single body. The first flat plate 70 and the second flat plate 72 are heated and pressurized in the overlapping direction of the first flat plate 70 and the second flat plate 72 to cause the mixed powder to flow, thereby obtaining a core in which the coil 30 is embedded.

[0282] Figure 14 for Figure 6 The enlarged image of the peripheral portion Ar of the coil 30 shown is shown in which the coil 30 is sandwiched between the first flat plate 70 and the second flat plate 72 and is pressurized in the overlapping direction. Figure 15 is a cross-sectional view of the first flat plate 70. Figure 14 As shown, the lateral area of the coil 30 can be the gaps s1, s2, and s3. Therefore, when the first and second flat plates 70 and 72 are pressed in the overlapping direction, the mixed powder can be first filled from the gaps near the outer periphery of the core.

[0283] That is, near the outer periphery of the coil 30, the amount of mixed powder moving increases, thereby easily filling gaps and thus increasing the packing density. In contrast, in the area inside the coil 30, the amount of mixed powder moving decreases, making it difficult to fill gaps and thus decreasing the packing density. Figure 15As shown, the filling rate of the mixed powder in the outer peripheral portions s10 to s13 of the coil 30 is higher than that in the inner portion s14 of the coil 30. Therefore, the inductor 1 having the above-mentioned specific structure 1 can be obtained.

[0284] If the surface area of the unit cell has gaps, moisture can enter the unit cell through these gaps, adversely affecting the moisture resistance of the inductor. Furthermore, when forming the external electrodes, plating solution can enter the unit cell through these gaps, accelerating the aging and degradation of the unit cell. Therefore, increasing the density of the unit cell surface area through the above-mentioned specific configuration can prevent these adverse effects.

[0285] Here, Figure 16 LT and WT are explanatory diagrams of the reference surfaces of the inductor 1. Figure 17 for Figure 16 The cross-sectional images of the inductor 1 taken along the LT and WT planes are shown. As described above, before forming the unit cell protective film on the unit cell 10, the second side surface 18 is ground in the unit cell grinding process, but the first side surface 16 is not ground. Therefore, in the ridges s22 and s23 where the main surface (here, the mounting surface) 12 and the second side surface 18 meet, the metal magnetic powder is ground flush with the second side surface 18. This increases the exposed area of the metal magnetic powder in these ridges s22 and s23, and the amount of resin in these ridges s22 and s23 is less than the amount of resin in the ridges s20 and s21 where the main surface 12 and the first side surface 16 meet, as shown in the cross-sectional image taken along the LT plane. This results in an inductor 1 having the aforementioned specific structure 2. According to the specific configuration 2, by grinding the vicinity of the ridgeline portion where the main surface 12 and the second side surface 18 meet, it is possible to prevent a decrease in the soft magnetic particles protruding from the unit cell protective film and a decrease in the insulation properties of the inductor.

[0286] in addition, Figure 18 This table compares microscopic photographs of the LT and WT surfaces after the unit molding and curing process and after the unit grinding process, along with their maximum heights. Maximum height Sz serves as an indicator of surface roughness. A larger maximum height Sz indicates greater surface roughness.

[0287] The LT surface is ground by the unit body grinding process. At this time, due to the threshing of the first soft magnetic particle or the second soft magnetic particle, the surface roughness becomes larger. Therefore, the maximum height Sz (50 μm) of the LT surface is larger than the maximum height Sz (43 μm) of the WT surface that is not ground. By making the surface roughness of the LT surface larger, the bonding property of the unit body protective film and the core in the LT surface can be improved. This surface roughness is obtained by using a shape analysis laser microscope (Keyence company VK-X250) to scan the center of the surface of the LT surface and the WT surface along the long side direction and measure the maximum height (Sz).

[0288] In addition, if Figure 19 As shown, in the intervals SG1 and SG2 between the second side surface 18 and the coil 30, the narrower interval is set to a range larger than one first soft magnetic particle and smaller than four, thereby ensuring the moisture resistance of the unit body even if it becomes smaller, and obtaining the inductor 1 having the above-mentioned specific structure 3.

[0289] [A-2-1. Embodiment using the resin having the first resin blending ratio]

[0290] Using the mixed powder containing the resin at the first resin blend ratio, inductor unit samples were produced through the aforementioned steps of granulation, coil formation, unit molding and curing, unit grinding, unit protective film formation, unit protective film removal, and external electrode formation. Each unit sample was evaluated. The manufacturing conditions during the unit molding and curing steps were a temperature of 135°C and a pressure of 10 MPa.

[0291] (Evaluation of unit strength)

[0292] The strength of each sample was evaluated based on the breaking load during three-point bending using a 3-point bending tester (Shimadzu Corporation AGS-5kNX). A breaking load of 30 MPa or greater was rated G (pass), while a load less than 30 MPa was rated NG (fail).

[0293] (Density Evaluation)

[0294] For each sample, the cross-sectional image was processed to extract the void image portion, and the ratio of the total area of the void image portion to the area of the cross section was calculated to measure the porosity.

[0295] The porosity was measured by cutting the unit cell in half lengthwise and photographing four locations (one location on each surface) from the surface of the unit cell to the region of 1 μm to 100 μm on the cut surface at a magnification of 1000x using a scanning electron microscope (SEM). The voids contained in the cut surface were measured and the average value was calculated. Separately, the unit cell was cut in half lengthwise and photographed four locations in the central region of the unit cell on the cut surface at a magnification of 1000x using a scanning electron microscope (SEM). The voids contained in the cut surface were measured and the average value was calculated to determine the porosity.

[0296] As a result, except for samples b4 to b8, b22, b23 and b27, the average porosity of 8 points in the surface area and central area of the unit body is small, the proportion of voids in the surface area of the unit body is smaller than the proportion of voids in the central area of the unit body, and the central area is denser than the surface area.

[0297] (Proportion of resin in mixed powder)

[0298] Figure 20 The relationship between the amount of resin in the mixed powder and the density of the unit body is shown, with the vertical axis being the density of the unit body (g / cm 3 ), and the horizontal axis is set to the resin amount (weight %) in the mixed powder. The molding conditions of the unit body are temperature 180 degrees, pressure 30 MPa, and pressing time 100 seconds. Figure 20 If the resin content is less than 2.0%, the density of the unit cell decreases. This is presumably because the fluidity of the powder mixture decreases, which deteriorates the filling properties of the powder mixture during unit molding.

[0299]

[0300] The evaluation results of the samples in Table 6 indicate that the preferred resin ratios in the powder mixture for forming an inductor with improved unit strength are 50% to 90% by weight of bisphenol A epoxy resin and 10% to 50% by weight of rubber-modified epoxy resin (the first resin ratio).

[0301] [A-2-2. Embodiment using resin having the second resin blending ratio]

[0302] Similar to the embodiment described in A-2-1 above, samples of inductor units were prepared using a powder mix containing the resin at the second resin blend ratio, and each sample unit was evaluated. The proportion of the resin in the powder mix was the same as in the embodiment described in A-2-1 above, ranging from 2.0% to 3.5% by weight of the powder mix.

[0303]

[0304] The evaluation results of the samples in Table 7 indicate that the preferred resin blend ratios in the mixed material for forming an inductor with improved unit strength are 40% to 80% by weight of bisphenol A epoxy resin, 10% to 50% by weight of rubber-modified epoxy resin, and 1% to 30% by weight of novolac epoxy resin (the second resin blend ratio).

[0305] [A-2-3. Embodiments Related to Side Clearance]

[0306] For evaluation Figure 19 To determine the relationship between the intervals SG1 and SG2 between the second side surface 18 and the coil 30 and the moisture resistance of the inductor 1, samples shown in Tables 8 and 9 below were prepared and evaluated for moisture resistance.

[0307] (Evaluation of moisture resistance)

[0308] Each sample was subjected to a humidity test using a humidity chamber set at a temperature of 85°C and a humidity of 85%. A weight increase of 2% or less due to water absorption was rated G (pass), and a weight increase of more than 2% was rated NG (failure).

[0309] (Specifications of mixed powder)

[0310] The resin ratio in the mixed powder was set to 2.0% to 3.5% by weight, and the resin was blended at the first resin blending ratio described above. The average particle size of the large soft magnetic particles (first soft magnetic particles) in the mixed powder was 21 μm (samples in Table 8) or 28 μm (samples in Table 9), and the average particle size of the small soft magnetic particles (second soft magnetic particles) was 2 μm.

[0311] The following describes the evaluation of samples b51 to b60 shown in Table 8. Samples b54 to b60 are examples of the present invention, and samples b51 to b53 are comparative examples.

[0312] [Table 8]

[0313] Large particles: average particle size 21μm, small particles: average particle size 2μm

[0314]

[0315] Samples b51, b52, and b53 marked with * are comparative examples.

[0316] (Example A-2-3-1)

[0317] <Example A-2-3-11 (Sample b54)>

[0318] A unit cell was formed in which the side gap was smaller, 25 μm, and the side gap was larger, 85 μm. Evaluation results: Moisture resistance G.

[0319] <Example A-2-3-12 (Sample b55)>

[0320] A unit cell was formed in which the side gap was smaller, 29 μm, and the side gap was larger, 81 μm. Evaluation results: Moisture resistance G.

[0321] <Example A-2-3-13 (Sample b56)>

[0322] A unit cell was formed in which the side gap was smaller, 33 μm, and the side gap was larger, 77 μm. Evaluation results: Moisture resistance G.

[0323] <Example A-2-3-14 (Sample b57)>

[0324] A unit cell was formed in which the side gap was smaller, 40 μm, and the side gap was larger, 70 μm. Evaluation results: Moisture resistance G.

[0325] <Example A-2-3-15 (Sample b58)>

[0326] A unit cell was formed in which the side gap was smaller, 45 μm, and the side gap was larger, 65 μm. Evaluation results: Moisture resistance G.

[0327] <Example A-2-3-16 (Sample b59)>

[0328] A unit cell was formed in which the side gap was smaller, 50 μm, and the side gap was larger, 60 μm. Evaluation results: Moisture resistance G.

[0329] <Example A-2-3-17 (Sample b60)>

[0330] A unit cell was formed in which the side gap was smaller at 55 μm and the side gap was larger at 55 μm. Evaluation results: Moisture resistance G.

[0331] (Comparative Example A-2-3-1)

[0332] <Comparative Example A-2-3-11 (Sample b51)>

[0333] A unit cell was formed in which the side gap with a smaller side was 0 μm and the side gap with a larger side was 110 μm. Evaluation results: Moisture resistance NG.

[0334] <Comparative Example A-2-3-12 (Sample b52)>

[0335] A unit cell was formed in which the side gap was smaller at 10 μm and the side gap was larger at 100 μm. Evaluation results: Moisture resistance NG.

[0336] <Comparative Example A-2-3-13 (Sample b53)>

[0337] The unit cell had a smaller side gap of 18 μm and a larger side gap of 92 μm. Evaluation results showed that the moisture resistance was NG.

[0338] The evaluation results in Table 8 show that, for first soft magnetic particles having an average particle size of 21 μm, units having a side gap smaller than one first soft magnetic particle and smaller than four first soft magnetic particles can achieve good moisture resistance.

[0339] The following describes the evaluation of samples b61 to b70 shown in Table 9. Samples b65 to b70 are examples of the present invention, and samples b61 to b64 are comparative examples.

[0340] [Table 9]

[0341] Large particles: average particle size 28μm, small particles: average particle size 2μm

[0342]

[0343] Samples b61, b62, b63, and b64 marked with * are comparative examples.

[0344] (Example A-2-3-2)

[0345] <Example A-2-3-21 (Sample b65)

[0346] The unit cell had a smaller side gap of 29 μm and a larger side gap of 81 μm. Evaluation results showed that the unit cell had a moisture resistance of G.

[0347] <Example A-2-3-22 (Sample b66)>

[0348] The unit cell had a smaller side gap of 33 μm and a larger side gap of 77 μm. Evaluation results: Moisture resistance G.

[0349] <Example A-2-3-23 (Sample b67)>

[0350] A unit cell was formed in which the side gap was smaller, 40 μm, and the side gap was larger, 70 μm. Evaluation results: Moisture resistance G.

[0351] <Example A-2-3-24 (Sample b68)>

[0352] A unit cell was formed in which the side gap was smaller, 45 μm, and the side gap was larger, 65 μm. Evaluation results: Moisture resistance G.

[0353] <Example A-2-3-25 (Sample b69)>

[0354] A unit cell was formed in which the side gap was smaller, 50 μm, and the side gap was larger, 60 μm. Evaluation results: Moisture resistance G.

[0355] <Example A-2-3-26 (Sample b70)>

[0356] A unit cell was formed in which the side gap was smaller at 55 μm and the side gap was larger at 55 μm. Evaluation Result: G.

[0357] (Comparative Example A-2-3-2)

[0358] <Comparative Example A-2-3-21 (Sample b61)>

[0359] A unit cell was formed in which the side gap with a smaller side was 0 μm and the side gap with a larger side was 110 μm. Evaluation results: Moisture resistance NG.

[0360] <Comparative Example A-2-3-22 (Sample b62)>

[0361] A unit cell was formed in which the side gap was smaller at 10 μm and the side gap was larger at 100 μm. Evaluation results: Moisture resistance NG.

[0362] <Comparative Example A-2-3-23 (Sample b63)>

[0363] A unit cell was formed in which the side gap was smaller, 18 μm, and the side gap was larger, 92 μm. Evaluation results: Moisture resistance: NG.

[0364] <Comparative Example A-2-3-24 (Sample b64)>

[0365] A unit cell was formed in which the side gap was smaller, 25 μm, and the side gap was larger, 85 μm. Evaluation results: Moisture resistance NG.

[0366] The evaluation results in Table 9 show that, for first soft magnetic particles having an average particle size of 28 μm, units having a side gap smaller than one first soft magnetic particle and smaller than four first soft magnetic particles can achieve good moisture resistance.

[0367] [A-2-4. Other research matters]

[0368] In the above embodiment, the resins contained in the mixed powder include bisphenol A epoxy resin, rubber-modified epoxy resin, and novolac epoxy resin. The general concept of bisphenol A epoxy resin is epoxy resin, and the general concept of rubber-modified epoxy resin is flexible rubber or resin.

[0369] Therefore, examples of resins that have been studied as replacements for bisphenol A epoxy resins include bisphenol A, F, and S phenoxy resins. Furthermore, examples of resins or rubbers that have been studied as replacements for rubber-modified epoxy resins include urethane-modified, NBR (Acrylonitrile Butadiene Rubber) rubber-modified, CTBN (Carboxyl Terminated Butadiene Acrylonitrile) rubber-modified, and CTBN rubber. Furthermore, examples of resins that have been studied as replacements for novolac epoxy resins include, if limited to novolacs, cresol, dicyclopentadiene, phenolarane, biphenyl, naphthol, xylene, triphenylmethane, and tetraphenolethane. Examples of resins that have been studied as replacements for novolacs include, if limited to novolacs, naphthalene, biphenyl, and triazine.

[0370] [B. Coil]

[0371] Next, the coil 30 portion of the inductor 10 including the core 40 formed of the mixed powder of the soft magnetic particles and the resin described in [A. Mixed Powder] will be described.

[0372] (wire)

[0373] In the inductor 1, the conductive wire 31 used in the coil 30 can be either a round wire or a flat wire ( Figure 3 By using the flat wire as the conductive wire 31, no gaps are generated between the conductive wires 31 when the winding portion 32 is formed, making winding easier.

[0374] The number of turns of the winding portion 32 can be appropriately determined according to the characteristics to be achieved by the inductor 1 .

[0375] Furthermore, as the conductive wire 31 , a copper wire 36 made of copper is preferably used.

[0376] For example, in an inductor 1 having a length L of 2.0±0.2 mm, a width W of 1.2±0.2 mm, and a thickness T of 0.7±0.1 mm, the winding portion 32 of the coil 30 has a height of 0.4 mm, an outer diameter of 1.17 mm, and an inner diameter of 0.55 mm in the width W direction.

[0377] Here, when the conductive wire 31 of the coil 30 is a flat wire, the length of the short side of the cross section of the flat wire is, for example, 0.118 mm or less. The length of the short side of the cross section of the flat wire is preferably 0.052 mm or more.

[0378] Furthermore, the length of the long side of the cross section of the flat line may be, for example, 0.203 mm or less. Furthermore, the length of the long side of the cross section of the flat line is preferably 0.141 mm or more.

[0379] Furthermore, the aspect ratio (long side / short side) of the cross section of the plane is, for example, between 1.3 and 3.4.

[0380] In the case of an inductor 1 with the aforementioned dimensions but with only the thickness T changed to 0.55±0.1 mm (so-called miniaturized) , the dimensions of the winding portion 32 of the coil 30 are, for example, an outer diameter of 1.17 mm, an inner diameter of 0.48 mm, and a height of 0.30 mm in the width direction W. The conductive wire 31 of the coil 30 constituting the winding portion 32 of this preferred dimension is, for example, a flat wire having an aspect ratio (long side / short side) of 1.3, with a short side of the cross section of 0.11 mm and a long side of 0.14 mm.

[0381] (Insulation coating material)

[0382] The material forming the insulating coating layer 61 of the insulating coating material 60 is not particularly limited, and examples thereof include polyurethane resin, polyester resin, epoxy resin, and polyimide amide resin, with polyimide amide resin being preferred.

[0383] Furthermore, the thickness of the insulating coating layer 61 is preferably 4 μm.

[0384] Examples of the material for forming the welding layer 62 of the insulating coating material 60 include polyamide resin.

[0385] The thickness of the welding layer 62 is preferably 1 μm to 25 μm, more preferably 2 μm to 25 μm, and even more preferably 2 μm to 4 μm.

[0386] By setting the thickness of the welding layer 62 to the above value, the size of the winding portion 32 of the coil 30 can be suppressed, and sufficient welding force can be obtained to suppress peeling due to springback of the conductive wire 31 at the outermost periphery of the winding portion 32, thereby preventing shape defects of the coil 30.

[0387] As described above, the welding layer 62 is melted by heating the conductive wire 31 during winding in the coil forming step, thereby fixing the conductive wires 31 in the winding portion 32 . Therefore, the material of the welding layer 62 can be selected so that the melting point temperature is, for example, 180°C.

[0388] This melting point temperature is comparable to the heating temperature during the reflow welding process when mounting the finished inductor 1 on a printed wiring board. Therefore, the welding layer 62 can be melted during this reflow welding process. The material of the melting welding layer 62 during the reflow welding process typically partially penetrates into the interior of the unit cell 10 and solidifies. However, due to the viscosity of the molten welding layer 62 material, this penetration area remains near the coil 30, thus posing no problem.

[0389] However, when the gap Sg on one side of the unit body 10 (the distance between the coil 30 in the unit body 10 and the second side face 18 ) is 50 μm or less, the material of the welding layer 62 melted during the reflow welding process may expand outside the second side face 18 .

[0390] Therefore, when the second side surface of the unit cell 10 is formed to be thinner than 50 μm as described above, it is important that the welding layer 62 be made of a material having a melting point temperature equivalent to that described above and having a higher viscosity when melted.

[0391] The material of such a welding layer 62 can be, for example, composed of a material comprising multiple resins of varying molecular weights. Generally speaking, the lower the molecular weight, the lower the viscosity of the molten resin. Therefore, by forming the welding layer 62 from a material comprising multiple resins of varying molecular weights, the weight ratio of the various resins can be adjusted to adjust the molten viscosity of the welding layer 62 to a desired value, thereby preventing the material from bulging out of the unit body 10 during the reflow welding process.

[0392] The above-mentioned material containing multiple resins with different molecular weights can be produced, for example, as follows: in addition to mixing resins with different molecular weights, it can be produced by adding a catalyst to a resin with a small molecular weight to polymerize part of the resin, or adding a catalyst to a resin with a large molecular weight to depolymerize part of the resin.

[0393] In one embodiment, the welding layer 62 is made of, for example, two types of polyamides having different molecular weights.

[0394] [C. Magnetic circuit]

[0395] Next, the structural relationship between conductive wire 31 of coil 30 and magnetic powder of core 40 in inductor 10 including core 40 formed of the mixed powder of soft magnetic particles and resin described in [A. Mixed Powder] above will be described. Conductive wire 31 is a flat wire.

[0396] [C-1. Magnetic powder between lines]

[0397] In the inductor 1 , since soft magnetic powder composed of metal magnetic particles is used as the magnetic material, better DC superposition characteristics can be obtained compared to the case where a magnetic material such as ferrite is used.

[0398] Here, Figure 21 (A) is an image showing the winding portion 32L of the lower section of the coil 30 together with the surrounding materials. Figure 21 (B) is an image showing the winding portion 32L of the upper stage of the coil 30 together with the surrounding materials. Figure 21 In (A) and (B), the vertical direction on the paper corresponds to the thickness direction of the unit cell 10, and the horizontal direction on the paper corresponds to the radial direction of the winding portion 32. In the drawings, symbol CW represents the winding width of the winding portion 32.

[0399] In this composition, Figure 21 As shown in (A) and (B), a portion of the second soft magnetic particle 82 as a small particle enters the winding portion 32L. A portion of the second soft magnetic particle 82 is located near the outer periphery of the winding portion 32, and the symbol 10S represents the area near the outer periphery. The second soft magnetic particle 82 in the area of symbol 10S forms a magnetic circuit near the winding portion 32L along the flow of magnetic flux, which can suppress the local saturation of the magnetic flux density. It should be noted that in this structure, the second soft magnetic particle 82 also enters near the inner periphery of the winding portion 32 (not shown). Among them, a portion of the second soft magnetic particle 82 can enter the winding portion 32L as described later, and the position of entry is not limited to near the outer periphery or the inner periphery of the winding portion 32. The structure in which the second soft magnetic particle 82 enters the winding portion 32L is recorded as an inter-wire magnetic powder structure.

[0400] The structure of the magnetic powder between the wires will be described.

[0401] like Figure 21 As shown in (A) and (B), the soft magnetic powder includes first soft magnetic particles 81 having large particles and second soft magnetic particles 82 having small particles.

[0402] The gaps between the winding portions 32L are small, allowing the small second soft magnetic particles 82 to enter but preventing the large first soft magnetic particles 81 from entering. The second soft magnetic particles 82 have an average diameter that is smaller than the thickness of the welding layer 62 of the winding portion 32L. Therefore, the second soft magnetic particles 82 easily enter the vicinity of the welding layer 62.

[0403] In this configuration, during the unit molding and curing process, in which the coil 30 is arranged within a powder mixture containing the first and second soft magnetic particles 81 and 82 and compression-molded, the pressure P during compression molding is adjusted to a higher value than previously described, such that the second soft magnetic particles 82 actively penetrate the winding portion 32L. Furthermore, heating is applied during this compression molding process, melting the fusion layer 62 of the insulating coating material 60 on the surface of the winding portion 32L, facilitating the penetration of the second soft magnetic particles 82 into the molten fusion layer 62.

[0404] More specifically, if Figure 22 As shown, when pressure P is applied from above, pressure P is applied from above to and around the winding portion 32 of the coil 30. Simultaneously with the pressure P being applied from above, pressure P is also applied from below and to the left and right due to the law of action and reaction. Thus, pressure is applied to the second soft magnetic particles 82 from the outer circumference of the coil 30 toward each winding portion 32L, making it easier to fill the second soft magnetic particles 82 between the winding portions 32L.

[0405] The conditions of the pressure P at this time may include not only the value of the pressure P but also various parameters related to the pressurization, such as the time for applying the pressure P. By appropriately setting this condition, it is easy to fill the second soft magnetic particles 82 into the winding portion 32L. In this case, it is easy to fill the second soft magnetic particles 82 into the winding portion 32L by adjusting the heating conditions or adjusting the distance between the winding portion 32 and the surrounding wall (the inner surface of the molding die 74 and the punch 76).

[0406] like Figure 21 As shown in (A) and (B), by allowing the second soft magnetic particles 82 to enter between the winding portions 32L, local saturation of the magnetic flux density near the winding portion 32 can be suppressed, thereby improving the DC superposition characteristics.

[0407] Next, the length LS of the second soft magnetic particles 82 between the winding portions 32L will be described.

[0408] This length LS corresponds to the length of the second soft magnetic particles 82 that contacts the coil 30 constituting the winding portion 32L.

[0409] The wire width of the winding portion 32L of the coil 30 is 95 μm, the thickness of the winding portion 32L of the coil 30 is 180 μm, the thickness of the welding layer 62 between the winding portions 32L of the coil 30 is 6 μm, the average particle size of the first soft magnetic particles 81 is 10 μm or more, the average particle size of the second soft magnetic particles 82 is 5 μm or less, and the pressure P is 300 kg / cm 2 The study was conducted by varying the length LS of the second soft magnetic particles 8 between the winding portions 32L and calculating the DC superimposed rated current Isat.

[0410] The DC bias current rating, Isat, is the current value at which the inductance decreases by a certain percentage compared to the initial characteristics without bias current. It measures the maximum current that can flow without magnetic saturation. The DC bias current rating, Isat, is defined as the current value at which the inductance decreases by approximately 30% from the initial inductance value. The results of this study are shown in Table 10.

[0411] [Table 10]

[0412] The length LS of the second soft magnetic particles between the winding parts Isat Comparative Example CK-1 (No magnetic powder) 100 Comparative Example CK-2 5% of the length of the wires that meet each other 100.07 Example C1-1 10% of the length of the wires that meet each other 100.14 Example C1-2 50% of the length of the wires that meet each other 100.7 Comparative Example CK-3 55% of the length of the wires that meet each other 100.77

[0413] Comparative example CK-1 is a case where the length LS of the second soft magnetic particles 82 between the winding portions 32L is zero in the cross section (e.g., WT cross section) of the coil 30 constituting the winding portion 32L, that is, the second soft magnetic particles 82 do not exist between the winding portions 32L. Comparative example CK-2 is a case where the above-mentioned length LS is 5% of the length of the winding portions 32L connected to each other in the coil cross section, in other words, it is a case where the length is 5% of the length of the conductors 31 of the coil wire connected to each other in the coil cross section. The ratio of the length LS of the second soft magnetic particles 82 between the winding portions 32L to the length of the winding portions 32L connected to each other is the ratio of the length LS of the second soft magnetic particles 82 between the winding portions 32L to the length of the winding portions 32L connected to each other, which is the center of the L direction of the unit body 10 observed in the WT cross section, and is set to (the length LS of the second soft magnetic particles 82 between all the winding portions 32L) / (the length of all the winding portions 32L connected to each other).

[0414] Example C1-1 shows a case where the length LS is set to 10% of the length of the conductive wires 31 in contact with each other in the coil cross-section. Example C1-2 shows a case where the length LS is set to 50% of the length of the conductive wires 31 in contact with each other in the coil cross-section. Comparative Example C1-3 shows a case where the length LS is set to 55% of the length of the conductive wires 31 in contact with each other in the coil cross-section. It should be noted that the DC superimposed rated current Isat is calculated by assuming the value of the DC superimposed rated current Isat in Comparative Example CK-1 to be 100.

[0415] The inventors have conducted research and have found that if the length LS is at least 10% of the length of the conductive wires 31 in the coil cross-section, the DC superposition rated current Isat increases compared to when the length LS is zero. Therefore, the length LS is preferably at least 10% of the length of the conductive wires 31 in the coil cross-section. However, if the length LS is greater than 55% of the length of the conductive wires 31 in the coil cross-section, cracks are more likely to form in the weld layer 62 that joins the winding portions 32L, increasing the risk of separation of the winding portions 32L.

[0416] Taking these factors into consideration, it can be determined that, from the perspective of suppressing magnetic saturation and improving DC superposition characteristics, it is preferable to set the length LS to at least 10% of the length of the conductive wires 31 in contact with each other in the coil cross section. Furthermore, from the perspective of suppressing separation between the winding portions 32L, it is preferable to set the length LS to 10% to 50% of the length of the conductive wires 31 in contact with each other in the coil cross section. Therefore, it is preferable to set the length LS to a range of 10% to 50% of the length of the conductive wires 31 in contact with each other in the coil cross section.

[0417] Figure 23 This is a characteristic curve diagram showing the simulation results of the magnetic powder structure between wires.

[0418] Figure 23In the figure, the horizontal axis represents the current value, and the vertical axis represents the inductance value (L value). Figure 23 In the figure, comparison example CK-4 shows a situation where no inter-wire magnetic powder structure is provided, that is, the second soft magnetic particles 82 do not exist on any of the surfaces between the upper and lower sections of the winding portion 32 wound in the upper and lower sections, or between the winding portion 32L of the upper section and the winding portion 32L of the lower section.

[0419] Example C1-3 shows a case where the second soft magnetic particles 82 are present on the surface between the upper and lower stages of the winding portion 32 and on the entire circumference of each of the upper winding portions 32L and the lower winding portions 32L.

[0420] Example C1-4 shows a case where the second soft magnetic particles 82 are present only in the outermost periphery between the upper and lower stages of the winding portion 32, the upper half between the upper winding portions 32L, and the lower half between the lower winding portions 32L.

[0421] Example C1-5 shows a case where the second soft magnetic particles 82 are present in the upper half between the upper winding portions 32L and the lower half between the lower winding portions 32L.

[0422] Example C1-6 shows a case where the second soft magnetic particles 82 are present over the entire circumference between the upper winding portions 32L and the lower winding portions 32L.

[0423] Table 11 shows the simulation results for the initial inductance (initial L value) and the DC superposition rated current Isat for Comparative Example CK-4 and Examples C1-3 to C1-6. The conditions for coil 30 wire width and thickness, as well as the thickness of weld layer 62, were the same as those in Table 10.

[0424] [Table 11]

[0425]

[0426] like Figure 23 As shown, Examples C1-3 to C1-6 achieve higher inductance values across a wide current range of 0 A to 10 A compared to Comparative Example CK-4, confirming that magnetic saturation is suppressed and DC bias characteristics are improved. Furthermore, as shown in Table 11, it is clear that Examples C1-3 to C1-6 achieve a higher DC bias rated current Isat than Comparative Example CK-4.

[0427] Furthermore, Examples C1-3 and C1-6 achieved substantially identical characteristics (inductance and DC superposition rated current Isat), and achieved greater inductance values than Examples C1-4 and C1-5. Examples C1-3 and C1-6 share the presence of second soft magnetic particles 82 between the upper and lower winding portions 32L, which is presumably beneficial for suppressing magnetic saturation and improving DC superposition characteristics.

[0428] In this way, the core 40 of the embedded coil 30 includes large-particle first soft magnetic particles 81 and small-particle second soft magnetic particles 82. A portion of the second soft magnetic particles 82 enters the winding portion 32L and forms a magnetic circuit near the winding portion 32L. Therefore, even if magnetic particles that can obtain good DC superposition characteristics are used, local saturation of the magnetic flux density can be suppressed.

[0429] In addition, by making the length LS of the second soft magnetic particles 82 between the winding parts 32L more than 10% of the length of the wires 31 connected to each other in the coil cross section, local saturation of the magnetic flux density, that is, magnetic saturation, can be further suppressed compared to the case where the length is less than 10%.

[0430] Furthermore, by making the length LS of the second soft magnetic particles 82 between the winding parts 32L less than 50% of the length of the wires 31 connected to each other in the coil cross section, it is easy to avoid cracks in the welding layer 62 that connects the winding parts 32L to each other, and the winding parts 32L are easily peeled off from each other.

[0431] In addition, adjacent winding portions 32L are joined to each other by a flux formed by the welding layer 62, and the second soft magnetic particles 82 form a diameter smaller than the thickness of the welding layer 62. A portion of the second soft magnetic particles 82 enters the welding layer 62, forming a magnetic circuit between the winding portions 32L. Therefore, adjacent winding portions 32L are joined to each other and a magnetic circuit is effectively formed between the winding portions 32L, making it easy to effectively suppress magnetic saturation.

[0432] Furthermore, when the material formed by the core 40 material (the first and second soft magnetic particles 81, 82, etc.) and the coil 30 is compression-molded to form the unit body 10, a portion of the second soft magnetic particle 82 enters between the winding parts 32L, at least adjusting the pressure P during compression molding, so that a magnetic circuit can be easily set between the winding parts 32L.

[0433] It should be noted that the method for causing a portion of the second soft magnetic particles 82 to enter the space between the winding portions 32L is not limited to the conditions of pressure P, heating, etc. For example, the second soft magnetic particles 82 can be easily inserted into the space between the winding portions 32L by appropriately combining the adjustment of the diameters of the first and second soft magnetic particles 81, 82, the adjustment of the smoothness of the surface layer between the particles 81, 82, and the selection of the resin.

[0434] Furthermore, when the coil 30 includes a winding portion 32 wound in multiple (including two or more) sections in a state where the conductive wire 31 is connected, it is preferable that the second soft magnetic particles 82 are present between the winding portions 32L of the uppermost and / or lowermost sections. This facilitates the provision of a magnetic circuit that is effective in suppressing magnetic saturation.

[0435] It should be noted that the material of the core 40 can be appropriately increased, decreased, or changed within a range that allows a portion of the second soft magnetic particles 82 to enter the winding portion 32L, and the shape of the coil 30, etc. can also be appropriately changed. In addition, the second soft magnetic particles 82 are not limited to being allowed to enter the winding portion 32L during the unit body molding and curing process. Other methods can also be used to allow the second soft magnetic particles 82 to enter the winding portion 32L.

[0436] The winding method of the coil 30 is not limited to α-winding, and may be, for example, edgewise winding. In edgewise winding, a magnetic path is formed between adjacent winding portions 32L joined by the fusion layer 62 through the second soft magnetic particles 82, making it easier to effectively suppress magnetic saturation.

[0437] [C-2. Magnetic Gap]

[0438] In order to further suppress the saturation of the magnetic flux density in the vicinity of the winding portion 32 of the coil 30 , a magnetic gap may be provided in the vicinity of the winding portion 32 in the inductor 1 .

[0439] Figure 24 This is an image of a case where an air gap 40K serving as a magnetic gap is provided near the winding portion 32 .

[0440] The air gap 40K extends in the arrangement direction of the winding portion 32L and is substantially perpendicular to the magnetic flux. The air gap 40K can be formed during the unit body molding and curing process. Figure 22 As shown in FIG. 1 , by applying pressure P to the coil 30 from top to bottom and left to right, the first and second soft magnetic particles 81, 82, etc., which are the core material, are compressed around the winding portion 32 of the coil 30. Thereafter, by quickly retracting the punch 76 or removing the unit body 10 from the mold before the core material is completely solidified, the first and second soft magnetic particles 81, 82, etc. are rebounded (rebound force) to form a core around the winding portion 32. Figure 24 Air gap 40K shown.

[0441] This springback includes at least one of the springback between the first soft magnetic particles 81, the springback between the second soft magnetic particles 82, and the springback between the first and second soft magnetic particles 81, 82. By appropriately utilizing these springbacks, an air gap 40K that functions as a magnetic gap can be formed. Note that this springback may also include the springback between the coil 30 and the core 40 (the first and second soft magnetic particles 81, 82).

[0442] Specifically, by appropriately adjusting various compression molding conditions such as the compression molding pressure P, the pressing speed, the pressing time, the retraction speed of the punch 76, and the timing of removing the unit body 10, an air gap 40K extending in the arrangement direction of the winding portion 32L is formed around the winding portion 32L. This facilitates the provision of the air gap 40K, which functions as a magnetic gap. This air gap 40K suppresses saturation of the magnetic flux density near the winding portion 32 of the coil 30, thereby improving the DC superposition characteristics.

[0443] Next, the position, length, and width of the air gap 40K will be described.

[0444] The wire width of the winding portion 32L of the coil 30 is 95 μm, the thickness of the winding portion 32L of the coil 30 is 180 μm, the thickness of the welding layer 62 of the winding portion 32L of the coil 30 is 4 μm, the average particle size of the first soft magnetic particles 81 is 10 μm or more, the average particle size of the second soft magnetic particles 82 is 5 μm or less, and the pressure P is 300 kg / cm 2 Under the same conditions, the position, length, and width of the air gap 40K were varied, and the DC superposition rated current Isat was calculated and studied. The DC superposition rated current Isat was defined as the current value at which the inductance drops by approximately 30% from the initial inductance value.

[0445] The results of the examination of the position of the air gap 40K are shown in Table 12. The DC superimposed rated current Isat in Table 12 indicates a value of 100 when the air gap 40K is located at a position 11 μm from the winding portion 32 .

[0446] [Table 12]

[0447] Air gap location Isat 0μm from the winding part 67 1 μm from the winding part 70 2μm from the winding part 73 3μm from the winding part 76 4 μm from the winding part 79 5μm from the winding part 82 6μm from the winding part 85 7μm from the winding part 88 8μm from the winding part 91 9μm from the winding part 94 10 μm from the winding portion (equivalent to twice the average particle size of the first soft magnetic particles) 97 11 μm from the winding part 100 15 μm from the winding part 113 20 μm from the winding portion (equivalent to twice the average particle size of the first soft magnetic particles) 130 30 μm from the winding portion (equivalent to twice the average particle size of the first soft magnetic particles) 130 40 μm from the winding portion (equivalent to four times the average particle size of the first soft magnetic particles) 113 50 μm from the winding portion (equivalent to five times the average particle size of the first soft magnetic particles) 100

[0448] As shown in Table 12, within the range of the air gap 40K from the winding portion 32 to 30 μm, the value of the DC superimposed rated current Isat increases as the distance from the winding portion 32 increases. When the distance is greater than 50 μm, the Isat value falls below 100. The inventors' research has shown that the air gap 40K within the range of the winding portion 32 to 50 μm is an effective range for suppressing magnetic saturation. In other words, the preferred range for suppressing magnetic saturation is within a distance of five times the average particle diameter of the first soft magnetic particles 81. More preferably, the air gap 40K is within the range of 20 to 30 μm from the winding portion 32.

[0449] Table 13 shows the length KL of the air gap 40K (see Figure 24 ) research results. The DC superimposed rated current Isat in Table 13 represents the value obtained by setting the length KL of the air gap 40K to the winding width CW of the winding portion 32 ( Figure 24) is 10% of the DC superimposed rated current Isat as 100.

[0450] [Table 13]

[0451] Air gap location Isat 10% of the roll width of the winding part 100 20% of the roll width of the winding part 102 30% of the roll width of the winding part 103 40% of the roll width of the winding part 105 50% of the roll width of the winding part 107 60% of the roll width of the winding part 109 70% of the roll width of the winding part 111 80% of the roll width of the winding part 113 90% of the roll width of the winding part 114 100% of the roll width of the winding part 116 110% of the roll width of the winding part 118

[0452] As shown in Table 13, the length KL of the air gap 40K extends up to 110% of the winding width CW of the winding portion 32. The longer the air gap 40K, the greater the DC superposition rated current Isat. The inventors' research has shown that an air gap 40K length KL greater than the width of a single winding portion 32L (33% or more of the winding width CW) is effective in suppressing magnetic saturation.

[0453] On the other hand, if the length KL of the air gap 40K is significantly larger than the roll width CW, more specifically, if it is larger than 110% of the roll width CW, there is a risk that the inductance value will decrease.

[0454] Therefore, the inventors and others judged that the length KL of the air gap 40K is preferably greater than the width of a single winding portion 32L (equivalent to more than 33% of the winding width CW of the winding portion 32) to less than 110% of the winding width CW of the winding portion 32, and more preferably greater than 1.5 times the width of a single winding portion 32L (more than 50% of the winding width CW of the winding portion 32) to less than the winding width CW of the winding portion 32 (less than 100%).

[0455] Table 14 shows the width KW of the air gap 40K (see Figure 24 ) research results. This width KW corresponds to the length of the air gap 40K perpendicular to the arrangement direction of the winding portion 32L. The DC superimposed rated current Isat in Table 14 represents the case where the width KW of the air gap 40K is 1 μm less than the diameter of the smallest particle (defined as no air gap) as 100.

[0456] [Table 14]

[0457] Air gap width Isat Smaller than the smallest particle 100 1μm 103 2μm 107 3μm 111 4μm 114 5μm 118 6μm 122 7μm 126 8μm 131 9μm 135 10μm 140 11μm 140

[0458] As shown in Table 14, the DC superposition rated current Isat increases with increasing width KW up to 10 μm. When width KW is 10 μm or 11 μm, the DC superposition rated current Isat has the same value. Furthermore, when width KW of air gap 40K is greater than 11 μm, width KW exceeds the average particle size of first soft magnetic particles 81, weakening the bonding strength of the resin bonded to first soft magnetic particles 81 and making it more likely for cracks to form in unit cell 10 along the extension direction of air gap 40K.

[0459] Therefore, the width KW of the air gap 40K is preferably equal to or larger than the average particle size of the second soft magnetic particles 82 (5 μm) and up to 11 μm, and more preferably close to 10 μm within a range that can suppress cracking of the unit cell 10 .

[0460] Figure 25 4 is a characteristic curve diagram showing simulation results corresponding to the presence or absence of the air gap 40K. Figure 25 In the figure, the horizontal axis represents the current value, the vertical axis represents the inductance value (L value), the characteristic curve K1 represents the case where there is no air gap 40K, and the characteristic curve K2 represents the case where the air gap 40K extends above and below, inside and outside the winding portion 32, and in both the long side direction and short side direction of the winding portion 32.

[0461] Table 15 shows the simulation results for the initial inductance values (initial L values) and the DC superposition rated current Isat for characteristic curves K1 and K2, respectively. The conditions for coil 30 wire width and thickness, as well as the thickness of weld layer 62, are the same as those in Tables 10 and 11.

[0462] [Table 15]

[0463]

[0464] Such as Figure 25 As shown in Table 15, the simulation results confirm that the presence of the air gap of 40K can suppress magnetic saturation compared to the absence of the air gap, and the magnetic saturation suppression effect is particularly achieved in the current value range of 0A to 6A.

[0465] In this way, the unit body 10 has an extended air gap 40K in the direction of arrangement of the winding portion 32L, at the periphery of the winding portion 32 of the coil 30 and within a distance of 5 times the average particle size from the winding portion 32 to the first soft magnetic particle 81. Therefore, even when using magnetic particles obtained with good DC superposition characteristics, magnetic saturation can be suppressed.

[0466] Moreover, the air gap 40K is the length of the width of a single winding portion 32L to the winding width CW of the winding portion 32 in the direction in which the winding portion 32L is arranged, and the width is the average particle size of the second soft magnetic particles 82 to 10 μm in the radial direction of the winding portion 32L, thereby effectively suppressing magnetic saturation.

[0467] Furthermore, the air gap 40K is formed by utilizing the springback when the unit body 10 is formed by compression molding the material including the core 40 and the coil 30 . Therefore, the air gap 40K can be easily provided.

[0468] It should be noted that the material of the core 40 can be appropriately increased, decreased, or changed within a range that allows the air gap 40K to be formed, and the shape of the coil 30, etc. can also be appropriately changed. In addition, the air gap 40K is not limited to being formed during the unit body molding and curing process, and can also be formed by other methods.

[0469] [D. Unit grinding]

[0470] Next, surface grinding of unit cell 10 of inductor 10 including core 40 formed of the mixed powder of soft magnetic particles and resin described in [A. Mixed Powder] will be described.

[0471] As described above, the unit cell 10 of the inductor 1 is a molded body formed by compression-molding the mixed powder with the coil 30 embedded therein, and includes the coil 30 and the core 40 .

[0472] Figure 5 The unit body grinding step shown is as described above, in which abrasive grains act on the second side surface 18 ( Figure 1 ), a process of grinding the width W to a predetermined width. By this grinding, the unit body 10 is reduced to a predetermined size, and the occupancy rate of the coil 30 in the unit body 10 is increased. In addition, by adopting a method of reducing the unit body 10 by grinding and processing it to a predetermined size, the dimensional unevenness of the unit body 10 can be reduced compared to the case where the size of the unit body 10 is controlled to a predetermined size by adjusting the size of the cavity of the forming mold. After this grinding, in order to chamfer the corners generated by grinding the second side surface 18, for example, drum polishing can be performed.

[0473] (Grinding device)

[0474] Figure 26 This is a diagram schematically showing an example of a grinding device 101 for grinding a unit body.

[0475] The grinding device 101 includes a storage tool 102 for storing the unit body 10 (workpiece) as the grinding object, and an upper grinding stone 103 and a lower grinding stone 104 that sandwich the unit body 10 stored in the storage tool 102; the storage tool 102 is stored in such a way that the second side surface 18 of the unit body 10, which serves as the grinding surface, faces up and down.

[0476] When grinding the unit body, the grinding device 101 causes the upper grinding stone 103 and the lower grinding stone 104 to be at a specified load, and presses the upper and lower second side surfaces 18 respectively, so that these upper grinding stones 103 and lower grinding stones 104 move relative to the upper and lower second side surfaces 18, thereby grinding the upper and lower second side surfaces 18 simultaneously through the abrasive grains 105 of the upper grinding stone 103 and the lower grinding stone 104 (so-called double-sided grinding).

[0477] (Size of abrasive grains)

[0478] The size of the abrasive grains 105 is proportional to the grinding rate, and the larger the abrasive grains 105 are, the more the soft magnetic powder particles in the grinding surface are detached, and the greater the surface roughness is. This has been confirmed through experiments by the inventors.

[0479] Specifically, when a soft magnetic powder molded body is ground, the use of abrasive grains 105 causes a considerable amount of soft magnetic powder particles to be detached, resulting in irregularities on the ground surface due to particle defects. In soft magnetic powders containing both large and small particles, large particles are more easily detached than small particles. The larger the abrasive grains 105, the more large particles are detached, resulting in a greater number of large irregularities on the ground surface, which in turn increases the surface roughness of the ground surface.

[0480] Regarding surface roughness, there is no correlation between surface roughness and load, which has been confirmed through experiments by the inventors.

[0481] In this example, the arithmetic mean height is used to evaluate surface roughness. Specifically, multiple (e.g., 3 to 4) measurement areas of a specified size (approximately 200 μm x 290 μm in this example) are set on the surface to be measured. The maximum height in each measurement area is measured using a laser microscope, and the arithmetic mean height is calculated by averaging these maximum heights. The laser microscope used is a Keyence VK-X250.

[0482] (Grinding speed)

[0483] The inventors' experiments confirmed that the higher the grinding speed (the moving speed of the upper grindstone 103 and the lower grindstone 104), the lower the surface roughness of the ground surface produced by the cutting of the soft magnetic powder particles. The grinding speed is proportional to the grinding rate.

[0484] (grinding rate)

[0485] A target value can be appropriately set for the grinding rate, and the size of the abrasive grains 105 and the grinding speed required to achieve that target value can be determined. As described above, the size of the abrasive grains 105 and the grinding speed are related to the surface roughness of the grinding surface. In this embodiment, the size of the abrasive grains 105 and the grinding speed are set so that the surface roughness after grinding is greater than that before grinding, and further, the roughness of the second side surface 18 after grinding is greater than that of the outer surface of the grinding object, namely, the top surface 14 and the mounting surface 12.

[0486] By increasing the surface roughness Sa through grinding, the bonding strength of the unit cell protection film 50 covering the second side surface 18 of the unit cell 10 is improved. Furthermore, the entire surface of the unit cell 10, except for the external electrodes 20, is covered with the unit cell protection film 50. This unit cell protection film 50 improves the moisture resistance, rust resistance, and electrical insulation of the unit cell 10.

[0487] (Grinding time)

[0488] The grinding time is defined as the time from the grinding start time Ts to the grinding end time Te, and can be determined based on the difference between the width W of the unit body 10 before grinding and the target value of the width W, that is, the predetermined width, and the grinding rate.

[0489] Then, when grinding the unit body, the control device (not shown) controls the grinding operation of the grinding device 101 based on the load curve and grinding time, thereby performing grinding until the width W of the compression-molded unit body 10 reaches a predetermined width.

[0490] (Side clearance)

[0491] Side clearance Sg Figure 27 As shown, it is defined as the thickness of the inductor 1 from the coil 30 inside the unit cell 10 to the adjacent second side surface 18. When the unit cell 10 is covered with the unit cell protection film 50, the side gap Sg is the thickness excluding the unit cell protection film 50.

[0492] Then, in this embodiment, in the unit body 10 ground to the predetermined width W, the side gap Sg is larger than the thickness corresponding to the average particle size of one macroparticle of the soft magnetic powder, and smaller than the thickness corresponding to the average particle size of four macroparticles of the soft magnetic powder. In other words, in this embodiment, the side gap Sg of the unit body 10 ground to the predetermined width is adjusted in advance to the predetermined width and the width WLc ( Figure 27 ) of both parties or one party.

[0493] In the unit body 10 after grinding, the thickness of the side gap Sg is made at least larger than the thickness of the average particle size of one large particle equivalent to the soft magnetic powder. Therefore, even if granulation occurs on the second side surface 18 due to grinding, at least one large particle will remain between the second side surface 18 and the coil 30, preventing the coil 30 from being exposed.

[0494] In addition, in the ground unit body 10, by limiting the thickness of the side gap Sg to a range smaller than the thickness equivalent to the average particle size of four large particles of soft magnetic powder, the unit body 10 can be prevented from being enlarged, the occupancy rate of the coil 30 can be maintained at a sufficiently high value, and the reduction of inductance can be prevented.

[0495] Tables 16 and 17 show the maximum and minimum values of the side gap Sg in the inductor 1 , and the measurement results of the inductance and moisture resistance of the inductor 1 .

[0496] Table 16 shows the results of the measurement of Inductor 1 in which the average particle sizes of large and small soft magnetic powder particles were 21 μm and 2 μm, respectively. Table 17 shows the results of the measurement of Inductor 1 in which the average particle sizes of large and small soft magnetic powder particles were 28 μm and 2 μm, respectively.

[0497] This soft magnetic powder is made from chromium-free Fe-Si amorphous alloy powder and crystallized pure iron. The chromium-free Fe-Si amorphous alloy powder represents the large particles, while the pure iron represents the small particles. The surface of the large particles is covered with an oxide film composed of a stack of SiO and Fe₂SiO₄ layers, while the surface of the small particles is covered with an Fe oxide film. The oxide films provide electrical insulation to each particle.

[0498] Then, the mixed powder of the soft magnetic powder and the epoxy resin is compression-molded to form the unit cell 10 of the inductor 1 .

[0499] In addition, sample A1-04 of the above-mentioned first soft magnetic particle is used as a large particle, and any one of the samples A2-02 to 08 of the above-mentioned second soft magnetic particle is used as a small particle, so that resin, Fe or Fe oxide, phosphate glass, and SiO2 and an alkyl group with a chain length of 16 carbon atoms exist on the surface of the unit body.

[0500] The inductance value was measured using an LCR meter. The moisture resistance was tested by exposing the inductor 1 to an environment with a temperature of 85°C and a humidity of 85%. Based on the test results, those that did not meet the prescribed product quality standards for moisture resistance were rated "NG".

[0501] [Table 16]

[0502]

[0503] [Table 17]

[0504]

[0505] As shown in Tables 16 and 17, sufficient moisture resistance can be achieved when the minimum side gap Sg is larger than the average particle size of one large particle. Furthermore, it is found that the larger the maximum side gap Sg, the lower the inductance.

[0506] Furthermore, it was found that within the range where the minimum value of the side gap Sg is larger than the average particle size of one large particle and the maximum value of the side gap Sg is smaller than the average particle size of four large particles, and when the ratio of the minimum value to the maximum value of the side gap Sg is 1:1, an inductor 1 having both excellent moisture resistance and inductance can be obtained.

[0507] As described above, the inductor 1 of this embodiment is an inductor 1 having a pair of external electrodes 20 provided on a unit cell 10 formed from a plate-shaped molded body in which a coil 30 is embedded. The unit cell 10 is molded from a mixture of soft magnetic powder containing large particles and small particles of varying average particle sizes, and a resin. The side gap Sg, the thickness from the second side surface 18 located in the radial direction of the coil 30 to the coil 30, is greater than the thickness corresponding to one large particle and smaller than the thickness corresponding to four large particles.

[0508] By making the thickness of the side gap Sg of the unit body 10 larger than the thickness corresponding to at least the average particle size of one macroparticle of the soft magnetic powder, at least one macroparticle exists between the second side surface 18 and the coil 30 , thereby preventing the coil 30 from being exposed.

[0509] Furthermore, by limiting the thickness of the side gap Sg of the unit cell 10 to a range smaller than the thickness equivalent to the average particle size of four large particles of the soft magnetic powder, it is possible to prevent the unit cell 10 from becoming larger, maintain the occupancy rate of the coil 30 at a sufficiently high value, and prevent a decrease in inductance. This allows for a compact inductor 1 that achieves practical DC resistance and saturation magnetic flux density.

[0510] In the inductor 1 of this embodiment, the second side surface 18 of the unit cell 10 is covered with the unit cell protection film 50 and has a surface roughness greater than that of at least one other surface (the mounting surface 12 and the top surface 14 ).

[0511] This can improve the bonding strength between the second side surface 18 and the unit protection film 50 .

[0512] In the inductor 1 of this embodiment, the surface of the unit cell 10 , except for the external electrode 20 , is covered with the unit cell protection film 50 .

[0513] This can improve the moisture resistance, rust resistance, and electrical insulation properties of the unit cell 10 , and thus provide a high-quality inductor 1 .

[0514] [E. Unit protective film]

[0515] Next, the unit cell protection film 50 formed on the surface of the unit cell 10 in the inductor 10 including the core 40 formed of the mixed powder of soft magnetic particles and resin described in [A. Mixed Powder] above will be described.

[0516] As described above, the unit cell 10 of the inductor 1 is a molded body formed by compression-molding the mixed powder with the coil 30 embedded therein, and includes the coil 30 and the core 40 .

[0517] The unit body protective film 50 is a layer that covers the entire surface of the unit body 10 except for the external electrodes 20 and improves the electrical insulation and rust resistance of the unit body 10. Even if large particles of soft magnetic powder are shed on the ground surface (second side surface 18) during the unit body grinding process, the unit body protective film 50 covering the ground surface can prevent a decrease in electrical insulation, corrosion resistance, and rust resistance.

[0518] (Unit Protective Film Formation Process and Protective Film Formation Apparatus)

[0519] The unit body protection film 50 is as shown in FIG. Figure 5 As described above, in the unit body protective film forming step, a protective film material containing a thermosetting resin is applied to the entire surface of the unit body 10 by an appropriate method such as spraying or dipping.

[0520] Figure 28 1 is a diagram schematically showing an example of a protective film forming apparatus 201 for forming a unit cell protective film.

[0521] The protective film forming apparatus 201 is a device that applies a protective film material by spraying it onto the surfaces of multiple unit bodies 10 (workpieces 208). As shown in the figure, the protective film forming apparatus 201 comprises a drum 203 rotatably mounted on an apparatus body 202, into which the multiple unit bodies 10 (workpieces 208) are loaded, a heater 204 for supplying heat, a duct 205 serving as an exhaust passage for the drum 203, and a spray nozzle 206 disposed within the drum 203.

[0522] When forming the unit protective film, the protective film forming apparatus 201 preheats the drum 203 loaded with the plurality of units 10 by the heater 204 to a temperature (eg, 30 to 70° C.) at which the protective film material does not thermally cure (preheating step).

[0523] Next, in the protective film forming apparatus 201, the drum 203 is rotated (so-called drum rotation) to stir the unit body 10, the protective film material is sprayed onto the unit body 10 from the spray nozzle 206, and hot air 207 is blown onto the unit body 10 from an air nozzle (not shown), thereby coating the unit body protective film 50 on the surface of the unit body 10 (coating process). Next, by stirring the unit body 10 and blowing the hot air 207 onto the unit body 10, the unit body protective film 50 on the unit body 10 is appropriately dried (drying process). Then, after the unit body protective film 50 is dried, the unit body 10 is removed from the drum 203 (workpiece removal process).

[0524] Insufficient drying during the drying process may cause the unit protective film 50 to develop pores and swell, and the adhesion between the unit protective film 50 and the unit body 10 may also deteriorate. Excessive drying may cause the unit protective film 50 to become a so-called discontinuous film, and the adhesion between the unit protective film 50 and the unit body 10 may also deteriorate. Therefore, it is preferable to dry the unit protective film 50 to an appropriate degree so that the unit protective film 50 becomes a so-called continuous film and the adhesion between the unit protective film 50 and the unit body 10 is maintained at a good level.

[0525] (Protective film material)

[0526] As the protective film material, a mixed solution of a resin component serving as a base material of the unit protective film 50 , a solvent component for diluting the resin component, and a filler component serving as an additive can be used.

[0527] (Resin component)

[0528] The resin component is primarily epoxy resin, preferably with one or both of phenoxy resin and novolac resin added. The addition of phenoxy resin can improve the toughness of the unit protective film 50. The addition of novolac resin can also improve the heat resistance of the unit protective film 50.

[0529] The resin of the resin component preferably contains a pigment.

[0530] By using a resin containing a pigment, Figure 5 In the unit cell protection film removal step and the external electrode formation step shown, the surface of the unit cell 10 is irradiated with laser light to remove the unit cell protection film 50, thereby improving processability when forming the external electrode 20. For example, carbon black is preferably used as the pigment.

[0531] (Solvent components)

[0532] As the solvent component, a solvent that can spray the resin component in a mist form in the above-mentioned coating step and obtain appropriate drying properties in the drying step can be used. For example, a solvent containing methyl ethyl ketone (MEK) used as a diluent for paste-like resins is preferably used.

[0533] (Filling ingredients)

[0534] As the filler component, a filler that reduces the gloss of the unit protection film 50 and improves the film quality of the unit protection film 50 can be used, and the filler can be dispersed in a solvent.

[0535] The gloss of the unit cell protection film 50 is reduced, thereby preventing misjudgment due to discoloration during visual inspection of the inductor 1 using a camera. Silica (SiO 2 ) powder is preferably used as the filler.

[0536] In addition, for the filler component, in order to prevent the clogging of the spray nozzle 206 of the spray protective film material and reduce the damage to the surface of the unit body 10 caused by the rotation of the drum 203, the particle size of the filler is preferably as small as possible. When silica powder is used as the filler, nano silica is preferably used.

[0537] (Nanosilica)

[0538] The inventors have found through experiments that when nano-silica is used as a filler, there is a correlation between the drying speed in the drying process performed at the end of the coating process and the content of the nano-silica.

[0539] Figure 29 The graph shows the experimental results of the relationship between the content of nano-silica and the drying speed.

[0540] In this experiment, a protective film material sample was prepared using epoxy resin as the resin component, MEK as the solvent component, and nanosilica as the filler component. This sample was used to form a cell protective film 50 on a cell 10, and external electrodes 20 were further formed to construct an inductor 1. The relationship between the drying time of the cell protective film 50 and its solid content was then examined during the drying process.

[0541] The protective film material samples were prepared with four different nano-silica contents: 0 (none), 50 phr, 100 phr, and 200 phr. In each sample, nano-silica consisting of silica particles with an average particle size of 45 nm was used as a filler.

[0542] The average particle size of the silica particles was determined by connecting the four corners of the top surface 14 of the inductor 1 and crossing the inductor at the intersection of the respective opposite corners. The unit cell 10 was then cut parallel to the second side surface 18. At each point on the upper and lower second side surfaces 18 of the unit cell 10 that equally divided the length L4 of the unit cell 10, a cross-section of the unit cell protective film 50 was photographed at 300,000x magnification using a transmission electron microscope (TEM) to observe the silica particles. A field emission transmission electron microscope (FE-TEM) was used for measurement. A multifunctional electron microscope (model: JEM-F200) manufactured by JEOL Ltd., equipped with an energy dispersive X-ray analyzer (EDX) system (model: NORAN System 7 manufactured by Thermo Fischer Scientific Inc.) was used.

[0543] like Figure 29As shown, the larger the content of nano-silica, the higher the solid content will be even in a short drying time. That is, by increasing the content of nano-silica in the protective film material, the drying speed can be accelerated and the process time of the drying process can be shortened.

[0544] It should be noted that as a result of observing the dried unit body protective film 50 in this experiment, it can be seen that: when the solid content after drying is less than 80%, "adhesion" described later occurs. On the other hand, if the solid content is around 90%, a good film quality unit body protective film 50 can be obtained, but cracks will still appear on the surface of the unit body protective film 50.

[0545] Therefore, in the drying step, it is preferable to dry the mixture so that the solid content is within a range of 80% to 90%.

[0546] (Adhesion)

[0547] "Stick-up" refers to the phenomenon in which the unit protective films 50 of the unit bodies 10 adhere to each other when multiple units are placed in the drum 203 for spray coating during the coating process. This is a major factor in reducing the quality of the unit protective film 50. The inventors have discovered through experiments that when nano-silica is used as a filler, the particle size of the silica particles in the nano-silica can be changed to suppress "stick-up" during the coating process.

[0548] Figure 30 The graph shows the experimental results of the average particle size of nano-silica silica particles and the adhesion generation rate.

[0549] In this experiment, two samples 1 and 2 were prepared as protective film materials.

[0550] Sample 1 is a protective film material using epoxy resin as the resin component, PGM as the solvent component, and nano-silica as the filler component. Sample 2 is a protective film material using epoxy resin as the resin component, MEK as the solvent component, and nano-silica as the filler component. The nano-silica content in both Samples 1 and 2 is 200 phr.

[0551] Then, after the unit body protection film 50 is applied to the unit body 10 using samples 1 and 2 respectively in the coating process, the number of unit bodies 10 in the bonded state is counted when the unit bodies 10 are taken out from the drum 203 in the workpiece removal process to determine the sticking rate.

[0552] like Figure 30 As shown, it is understood that the smaller the average particle size of the silica particles, the lower the adhesion occurrence rate.

[0553] Comparison of Sample 2 using MEK as the solvent component and Sample 1 using PGM as the solvent component reveals that, given the same average particle size of the silica particles, Sample 2 has a lower adhesion occurrence rate.

[0554] In addition, it was found that for Sample 2, when the average particle size of the silica particles was 45 nm or less, the sticking occurrence rate was significantly reduced.

[0555] In both samples 1 and 2, if the average particle size of the silica particles is as small as approximately 12 nm, the adhesion generation rate can be suppressed to approximately zero.

[0556] However, when the average particle size of the silica particles was 12 nm, the surface of the unit cell protective film 50 was observed in both samples 1 and 2. Figure 31 By setting the average particle size of the silica particles to 15 nm, the generation of cracks on the surface of the unit cell protection film 50 can be suppressed. Therefore, the average particle size of the silica particles is preferably greater than 12 nm, and more preferably greater than 15 nm to more reliably suppress the generation of cracks.

[0557] Furthermore, it has been observed that if the average particle size of the silica particles exceeds 75 nm, significant filler precipitation occurs in the protective film material. However, if the average particle size of the silica particles is 75 nm, filler precipitation in the protective film material can be prevented. Even if adhesion is avoided, a uniform unit protective film 50 cannot be obtained if a protective film material that causes filler precipitation is used in the coating. Therefore, the average particle size of the silica particles is preferably 75 nm or less.

[0558] It should be noted that when a protective film material containing only silica particles (silica powder) with an average particle size of 15 nm to 75 nm in a content (150 phr to 250 phr) that can achieve a sufficient drying speed is used to form the unit body protective film 50, the weight ratio of silica particles to resin in the formed unit body protective film 50 is approximately 150% to 250%.

[0559] In other words, when the unit protection film 50 is formed at this weight ratio, it is shown that the unit protection film 50 is formed at a fast drying speed without causing "sticking", and it can be said that a high-quality unit protection film 50 can be obtained.

[0560] (Plating jump)

[0561] Figure 32 This is a graph showing the results of measuring the number of “plating jumps” while changing the thickness of the unit cell protective film 50 .

[0562] "Plating skipping" occurs when plating is formed in unintended locations on the surface of the unit body 10, where the unit body protective film 50 is not applied. For example, when grinding the unit body causes large particles of soft magnetic powder to fall off, resulting in large irregularities on the surface of the unit body 10, the protective film material does not fully penetrate these irregularities, causing "plating skipping."

[0563] In this measurement, the presence or absence of "plating jumps" is checked at predetermined measurement intervals along the entire edge portion where the top surface 14 and mounting surface 12, and the first side surface 16 and second side surface 18 of the unit body 10 intersect, and the number of "plating jumps" is counted.

[0564] As can be seen from the same figure, if the thickness of the unit cell protection film 50 becomes smaller, a large number of "plating jumps" will occur. If the thickness is greater than 5μm, the number of "plating jumps" will be significantly reduced. If the thickness is greater than 10μm, "plating jumps" will almost not occur.

[0565] Therefore, the thickness of the unit cell protection film 50 is preferably 10 μm or more. With such a thickness, the occurrence of “plating skipping” can be suppressed, and the entire surface of the unit cell 10 can be reliably protected by the unit cell protection film 50 .

[0566] When the unit cell 10 of the inductor 1 is of a predetermined size, the thickness of the unit cell protection film 50 increases, which reduces the size of the unit cell 10 other than the unit cell protection film 50 and the size of the coil 30 . This reduces the performance of the inductor 1 .

[0567] Furthermore, in the inductor 1, the external electrodes 20 are formed where the unit cell protection film 50 is removed during the external electrode formation step. Therefore, if the unit cell protection film 50 is thicker than the external electrodes 20, the external electrodes 20 will not protrude beyond the surface of the unit cell protection film 50, and the contact between the external electrodes 20 and the substrate will deteriorate.

[0568] Therefore, the thickness of the unit cell protection film 50 is preferably at least equal to or smaller than the thickness of the external electrode 20 .

[0569] In order to obtain a high-performance inductor 1 , the thickness of the unit cell protection film 50 is more preferably 30 μm or less within a range equal to or less than the thickness of the external electrode 20 .

[0570] The thickness of the external electrode 20 is measured as follows. Specifically, the four corners of the top surface 14 of the inductor 1 are connected and intersected at the intersection of each of the opposite corners. The unit cell 10 is then cut parallel to the second side surface 18. The film thickness of the external electrode 20 formed on the mounting surface 12 of the unit cell 10 is measured at 1000x magnification along the longitudinal direction of the four points. The thickness is then measured using a microscope, and the average of these measurements is determined as the first measured value. This first measured value is then determined for ten inductors 1, and the average of these first measured values is used as the thickness of the external electrode 20. A Keyence VHX-7000 microscope is used as the microscope.

[0571] (Threshing countermeasures)

[0572] When grinding the surface of the molded body, i.e., the unit body 10, a considerable amount of the soft magnetic powder particles are degranulated during grinding, as described above. In this embodiment, a soft magnetic powder containing large particles with a large average particle size and small particles with a small average particle size is used. The degranulation of the large particles produces deep irregularities on the ground surface (the second side surface 18 in this embodiment).

[0573] Table 18 is a graph showing the experimental results of the thickness of the unit body protection film 50, the depth of the unevenness caused by threshing, and the rust resistance.

[0574] As the protective film material of this experiment, Figure 30 The same sample was used in the experiments shown. As the soft magnetic powder used for molding the unit cell 10, a soft magnetic powder having a large particle size of 21 μm to 28 μm was used.

[0575] The thickness of the unit cell protective film 50 is measured as follows. Specifically, in the inductor 1, the four corners of the top surface 14 are connected and intersected at the intersection of each of the opposite corners. The unit cell 10 is then cut parallel to the second side surface 18. At the points on the upper and lower second side surfaces 18 of the unit cell 10 that divide the length L4 of the unit cell 10 into equal parts, the film thickness of the unit cell protective film 50 is measured using a microscope at 1000x magnification. The average of these values is then used as the second measured value. This second measured value is then obtained for ten inductors 2, and the average of these values is used as the thickness measurement value (average thickness). A Keyence VHX-7000 microscope was used as the microscope.

[0576] The rust resistance is based on the prescribed product quality standards. If the rust resistance is not satisfied, it is rated "NG", and if it is satisfied, it is rated "G".

[0577] [Table 18]

[0578] Average thickness [μm] Depth of concave and convex [μm] Average thickness / depth of bumps Rust resistance 4 39 0.10 NG 11 38 0.29 NG 16 40 0.40 G 21 42 0.50 G 27 41 0.66 G 31 43 0.72 G 36 38 0.95 G

[0579] As shown in Table 18, it can be seen that, with respect to the depth of the irregularities caused by threshing, a small thickness of the unit protective film 50 results in poor rust resistance and insufficient quality of the unit protective film 50. However, sufficient rust resistance is achieved when the ratio of the unit protective film 50 thickness to the irregularity depth is 0.4 or greater. Since the irregularity depth is roughly equivalent to the average particle size of the macroparticles, a unit protective film 50 of sufficient quality can be achieved when the thickness of the unit protective film 50 is 0.4 times or greater the average particle size of the macroparticles.

[0580] As described above, the inductor 1 of this embodiment comprises a unit cell 10 formed from soft magnetic powder and resin, a coil 30 embedded in the unit cell 10, an external electrode 20 provided on the unit cell 10, and a unit cell protective film 50 on the surface of the unit cell 10. The unit cell protective film 50 has a thickness of at least 10 μm and comprises silica particles and a resin. The average particle size of the silica particles is 15 to 75 nm, and the weight ratio of the silica particles to the resin is 150 to 250%.

[0581] By setting the thickness of the unit cell protection film 50 to be 10 μm or more, the occurrence of “plating skipping” can be suppressed, and the entire surface of the unit cell 10 can be reliably protected by the unit cell protection film 50 .

[0582] By including silica particles in the cell protection film 50 , glossiness can be reduced, thereby preventing misjudgment during appearance inspection using an optical technique.

[0583] In addition, in the unit body protection film 50, the average particle size of the silica particles is 15 to 75 nm, and the weight ratio of the silica particles to the resin is 150 to 250%, thereby obtaining a high-quality unit body protection film 50 that is not degraded by "sticking" during the formation of the unit body protection film.

[0584] In this embodiment, the thickness of the unit cell protection film 50 is less than the thickness of the external electrode 20 .

[0585] Thus, the external electrode 20 can be in good contact with the circuit of the substrate without reducing the thickness of the unit cell protection film 50 .

[0586] In this embodiment, the unit cell protection film 50 contains carbon black.

[0587] This can improve the workability when removing the unit cell protection film 50 by laser light in order to form the external electrodes 20 .

[0588] In this embodiment, the unit cell protection film 50 contains a phenoxy resin.

[0589] Thereby, the toughness of the unit cell 10 can be improved.

[0590] In this embodiment, the unit body protection film 50 contains novolac resin.

[0591] Thereby, the heat resistance of the unit cell 10 can be improved.

[0592] In this embodiment, the thickness of the unit cell protection film 50 is greater than or equal to 0.4 times the average particle diameter of the macroparticles.

[0593] Thus, even if graining occurs on the surface, a unit protection film 50 of sufficient quality can be obtained.

[0594] In addition, in this embodiment, titanium oxide, zirconium oxide, and aluminum oxide can be used as filler components.

Claims

1. A soft magnetic powder comprising: soft magnetic particles comprising a first core particle containing a soft magnetic metal and an insulating film located on the surface of the first core particle; and soft magnetic particles comprising a second core particle containing a soft magnetic metal and having an average particle size larger than that of the first core particle. The insulating film of the first particle core contains Si and a hydrocarbon group having a linear portion with 8 or more carbon atoms. The weight ratio of Si to C in the insulating film of the first particle core is 7.6 to 42.

8.

2. The soft magnetic powder according to claim 1, wherein The hydrocarbon group is an alkyl group.

3. The soft magnetic powder according to claim 1, wherein The first particle core is composed of carbonyl iron.

4. An inductor having: A metal magnetic body composed of the soft magnetic powder according to any one of claims 1 to 3, and a wound conductive wire.

Images