Magnetic substrate and manufacturing method thereof, coil component and circuit board

Abstract

To provide a magnetic substrate that is hard to break when a circuit board is deformed and has high magnetic permeability, a method for manufacturing the same, a coil component, and a circuit board.SOLUTION: A magnetic substrate 10a includes sintered particles made of ferrite containing Ni and Zn, further includes Bi as a constituent element, and is formed of a sintered body in which the ratio of the average particle diameter of the sintered particles in the central portion to the average particle diameter of the sintered particles in the surface part is 1.3 or more when a portion whose distance from the surface is within 10% of the maximum dimension is defined as a surface portion 11a, and a portion located inside the surface portion is defined as the center portion 12a.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to a magnetic substrate, a method for manufacturing the same, a coil component, and a circuit board. [Background technology]

[0002] In recent years, as the applications of electronic components and the circuit boards on which they are mounted have expanded, there has been a demand for electronic components that can perform well in a variety of environments. For example, electronic components for automotive applications are required to be moisture-resistant, have mechanical strength to withstand vibration and impact, and operate stably over a wide temperature range (e.g., from -40°C to 150°C). Because electronic components are mounted on circuit boards for use, when the circuit board deforms due to vibration, impact, or expansion and contraction caused by temperature changes, stress is generated due to the deformation. Therefore, electronic components are also required to withstand such stresses.

[0003] Among electronic components, coil components having a magnetic substrate made of sintered ferrite exhibit high strength against compressive stress but relatively low strength against tensile stress. Therefore, there is a concern that the tensile stress generated in the magnetic substrate due to deformation of the circuit board may cause cracks and break the component. Therefore, attempts have been made to alleviate the tensile stress generated in the magnetic substrate.

[0004] For example, Patent Documents 1 and 2 disclose ferrite materials each containing 0.03 wt%, 0.06 wt%, 0.1 wt%, 0.3 wt%, 0.6 wt%, 1.0 wt%, 2.0 wt%, or 3.0 wt% of Bi2O3 in a ferrite material made of Fe2O3, ZnO, CuO, and NiO.

[0005] Furthermore, Patent Document 3 discloses ferrites containing Bi2O3 in addition to the main components consisting of Fe2O3, ZnO, CuO, and NiO, and containing 0.25 mass %, 0.3 mass %, or 0.5 mass % of Bi2O3 relative to the main components. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Publication No. 4-323806 [Patent Document 2] Japanese Patent Application Publication No. 4-325458 [Patent Document 3] Japanese Patent Application Laid-Open No. 2011-18913 Summary of the Invention [Problem to be solved by the invention]

[0007] Patent Document 1 states that the magnetic substrate has excellent thermal shock resistance if the crystal grain size of the ferrite of the above composition constituting the magnetic substrate is 20 to 60 μm. However, because the magnetic substrate is formed from ferrite particles with a relatively large crystal grain size, the effect of suppressing damage caused by deformation of the circuit board is limited.

[0008] Patent Document 2 states that a ferrite of the above composition constituting a magnetic substrate has excellent thermal shock resistance if it has grain boundaries formed by amorphous layers with a thickness of 2 to 50 nm. The amorphous layers present at the grain boundaries function as stress relief layers and contribute to suppressing damage caused by deformation of the circuit board. However, since the magnetic permeability of the magnetic substrate decreases as the thickness of the amorphous layers present at the grain boundaries increases, it has been difficult to achieve both high permeability and suppression of damage caused by deformation of the circuit board.

[0009] Patent Document 3 reports that the average crystal grain size of ferrite containing 0.25% and 0.3% Bi2O3 by mass relative to the main components was 0.9 to 1.4 μm. The same document also reports that the crystalline structure of ferrite containing 0.5% Bi2O3 by mass relative to the main components was a mixture of crystal grains with grain sizes of approximately 30 μm and crystal grains with grain sizes of approximately 1 μm. The problem with these ferrites was that the presence of small crystal grains reduced the size of magnetic domains, resulting in insufficient magnetic permeability.

[0010] The present invention has been made in consideration of the above-mentioned problems, and aims to provide a magnetic base that is resistant to breakage when a circuit board is deformed and has high magnetic permeability, and a coil component including the same. [Means for solving the problem]

[0011] The present inventors have conducted various studies to achieve the above-mentioned object, and have found that the above-mentioned problems can be solved by forming the magnetic substrate from a Ni-Zn ferrite sintered body and adjusting the crystal grain size of the sintered body so that it is small near the surface and large inside, and have thus completed the present invention.

[0012] That is, one aspect of the present invention for solving the above-mentioned problems is a magnetic substrate formed of a sintered body that includes sintered particles formed of ferrite containing Ni and Zn, and further contains Bi as a constituent element, and when a portion that is within 10% of the maximum dimension from the surface is defined as a surface portion and a portion located inside the surface portion is defined as a central portion, the ratio of the average particle size of the sintered particles in the surface portion to the average particle size of the sintered particles in the central portion is 1.3 or more.

[0013] Another aspect of the present invention is a method for producing a magnetic base, comprising: preparing a raw material powder containing ferrite containing Ni and Zn and bismuth oxide; molding the raw material powder to obtain a molded body in which a ratio of density of the surface portion to density of the central portion is 0.98 or less, where a portion located within 10% of a maximum dimension from a surface is defined as a surface portion and a portion located inside the surface portion is defined as a central portion; and sintering the molded body to obtain a sintered body.

[0014] Furthermore, the present invention includes, as one aspect, a coil component including the magnetic base, a conductor wound around the magnetic base, and an external electrode provided on the surface of the magnetic base and electrically connected to the conductor, and a circuit board including the coil component. [Effects of the Invention]

[0015] According to the present invention, it is possible to provide a magnetic base that is resistant to breakage when a circuit board is deformed and has high magnetic permeability, and a coil component including the magnetic base. [Brief explanation of the drawings]

[0016] [Figure 1] 1 is a schematic diagram showing a cross-sectional microstructure of a magnetic substrate according to a first aspect of the present invention; [Figure 2] FIG. 1 is an explanatory diagram showing the particle size distribution of sintered particles in a magnetic substrate according to a first aspect of the present invention. [Figure 3] FIG. 1 is an explanatory diagram showing the maximum dimension smax in a magnetic substrate according to a first aspect of the present invention. [Figure 4] 1 is a schematic diagram showing the structure of a coil component according to a third aspect of the present invention; [Figure 5] 1 is a cross-sectional view showing the structure of a circuit board according to a fourth aspect of the present invention; DETAILED DESCRIPTION OF THE INVENTION

[0017] The configuration and effects of the present invention will be described below, along with the technical concept, with reference to the drawings. However, the mechanism of action includes assumptions, and the correctness of such assumptions does not limit the present invention.

[0018] [Magnetic substrate] As shown schematically in FIG. 1 , a magnetic substrate according to one aspect of the present invention (hereinafter sometimes simply referred to as the “magnetic substrate according to the first aspect”) is formed of a sintered body 10a that includes sintered particles 1 formed of ferrite containing Ni and Zn and further contains Bi as a constituent element. This sintered body 10a may have an interface layer 2 at the interface between the sintered particles 1. When the portion of this sintered body 10a that is within 10% of the maximum dimension from the surface is defined as a surface portion 11a and the portion located inside the surface portion is defined as a central portion 12a, the ratio of the average particle size of the sintered particles 1 in the surface portion 11a to the average particle size of the sintered particles 1 in the central portion 12a is 1.3 or more.

[0019] In the magnetic substrate according to the first aspect, the ferrite constituting the sintered particles 1 contains Ni and Zn in addition to Fe as constituent elements and has a spinel-type crystal structure, which is called Ni-Zn ferrite. Ferrites containing Cu in addition to the above elements are sometimes called Ni-Zn-Cu ferrites. A typical composition of Ni-Zn ferrite, calculated as Fe2O3, NiO, and ZnO, is 47.3 mol% to 49.8 mol% Fe2O3, 15.0 mol% to 36.9 mol% NiO, and 15.0 mol% to 36.9 mol% ZnO, when the total of these oxides is taken as 100 mol%. A typical composition of Ni-Zn ferrite expressed in mass % is 64.4 to 67.4 mass % Fe2O3, 9.4 to 23.8 mass % NiO, and 10.4 to 25.6 mass % ZnO, calculated as Fe2O3, ZnO, and NiO, when the total of these oxides is taken as 100 mass %. A typical composition of Ni-Zn-Cu ferrite expressed in mass % is 41.6 to 49.5 mol % Fe2O3, 13.3 to 36.5 mol % NiO, 13.3 to 36.5 mol % ZnO, and 1.0 to 12.1 mol % CuO, calculated as Fe2O3, NiO, ZnO, and CuO, when the total of these oxides is taken as 100 mol %. Furthermore, a typical composition of Ni-Zn-Cu ferrite expressed in mass % is, converted into Fe2O3, ZnO, NiO, and CuO, and when the total of these oxides is taken as 100 mass %, 58.9 mass % to 66.9 mass % Fe2O3, 8.6 mass % to 23.6 mass % NiO, 9.5 mass % to 25.4 mass % ZnO, and 0.6 mass % to 8.6 mass % CuO.

[0020] In the Ni-Zn ferrite and Ni-Zn-Cu ferrite described above, increasing the Fe2O3 content improves the relative permeability and saturation magnetic flux density of the magnetic substrate. Furthermore, in these ferrites, the magnitude of the relative permeability of the magnetic substrate varies depending on the NiO to ZnO ratio (NiO / ZnO). Furthermore, in Ni-Zn-Cu ferrite, increasing the CuO content improves the relative permeability of the magnetic substrate. Thus, the ferrite of the first aspect allows the properties of the magnetic substrate to be adjusted by the composition.

[0021] The composition of the ferrite constituting the sintered particle 1 is determined by the following procedure. First, the sintered body 10a is cut along a plane that divides its volume equally, or is surface-ground until its volume is reduced to approximately half, exposing a smooth surface. Next, carbon is vapor-deposited on the exposed surface to impart conductivity and create a measurement surface. Next, the measurement surface is observed using a scanning electron microscope (SEM) equipped with either an energy-dispersive X-ray spectrometer (EDS) or a wavelength-dispersive X-ray spectrometer (WDS), and the sintered particle 1 is identified as a relatively bright area surrounded by a dark area. Next, the type and amount of contained elements are measured and calculated using EDS or WDS at an arbitrary location near the center of the sintered particle 1. The content percentages of Fe2O3, ZnO, NiO, and CuO are calculated in mol% or mass% from the content of Fe, Ni, Zn, and Cu. This measurement and calculation is performed at five locations, and the average content percentage of each component is calculated. In a sintered body obtained by a general manufacturing method, the content ratio of each element near the center of the sintered particle 1 is constant regardless of position, and there is usually no significant difference in the content ratio of each element between different sintered particles 1, so in this case the measurement points may be multiple points within one sintered particle 1, or one point may be selected from each different sintered particle 1. Finally, each calculated average value is divided by the sum of the average values ​​and then multiplied by 100 to calculate the content ratio of each oxide when the sum of the oxides is 100%, and this is the composition of the ferrite.

[0022] In the magnetic substrate according to the first aspect, the sintered body 10a further contains Bi as a constituent element. This improves the mechanical strength of the sintered body 10a, making it possible to form a coil component that is less susceptible to breakage due to deformation of a circuit board when mounted on the circuit board. The Bi content is not particularly limited, but in order to significantly increase the mechanical strength of the sintered body 10a, it is preferably more than 0.04 mass% and not more than 0.3 mass%, and more preferably 0.05 mass% to 0.2 mass%, calculated as Bi2O3, relative to the ferrite.

[0023] The Bi content of the sintered body 10a is determined by the following method. First, a measurement surface is formed using the same procedure as the above-mentioned method for determining the ferrite composition. Next, an area of ​​50 μm square or larger located near the center of gravity of the measurement surface is determined as the analysis area. Next, the analysis area is spot analyzed using laser ablation inductively coupled plasma (LA-ICP-MS), and the contents of Fe2O3, ZnO, NiO, CuO, and Bi2O3 are calculated in mass%. This measurement and calculation is performed for five different analysis areas, and the average content of each component is calculated. Finally, the percentage of the Bi2O3 content relative to the total content of Fe2O3, ZnO, NiO, and CuO is calculated, and this is the Bi content in Bi2O3 terms for the ferrite.

[0024] In the magnetic substrate according to the first aspect, various subcomponents may be added in addition to the essential components described above, within the scope that allows the object of the present invention to be achieved, in order to further improve the electrical insulation, mechanical strength, or magnetic properties.

[0025] In the magnetic substrate according to the first aspect, the sintered body is allowed to contain unavoidable impurities up to several hundred ppm, such as typical elements B, C, S, Cl, Se, Br, Te, I, Li, Na, Mg, Al, K, Ga, Ge, Sr, In, Sn, Sb, Ba, and Pb, as well as transition elements Sc, Ti, V, Cr, Mn, Y, Nb, Mo, Pd, Ag, Cd, Hf, and Ta.

[0026] In the magnetic substrate according to the first aspect, the sintered body 10a has a surface portion 11a that is within 10% of the maximum dimension from the surface, and a central portion 12a that is located inside the surface portion. The ratio of the average particle size of the sintered particles 1 in the surface portion 11a to the average particle size of the sintered particles 1 in the central portion 12a is 1.3 or greater. That is, as shown in FIG. 2, the sintered body 10a has a small particle size of the sintered particles 1 in the surface portion 11a and a large particle size of the sintered particles 1 in the central portion 12a. This allows for the formation of coil components that are less susceptible to damage due to deformation of a circuit board when mounted on the circuit board, and also provides a magnetic substrate with excellent magnetic properties such as magnetic permeability. This is believed to be due to the following mechanism. When a coil component is formed and mounted on a circuit board, the surface portion 11a of the sintered body 10a is susceptible to large tensile stress due to deformation of the circuit board. Therefore, if the particle size of the sintered particles 1 contained in this portion is small and the area of ​​the particle interface is large, the occurrence of cracks is suppressed by the stress relaxation effect of the particle interface. On the other hand, the tensile stress generated by the deformation of the circuit board in the central portion 12a of the sintered body 10a is not so large, so even if the particle size of the sintered particles 1 contained in this portion is large, the risk of occurrence of cracks is limited. Furthermore, the central portion 12a, which accounts for the majority of the sintered body 10a, is formed by sintered particles 1 with large particle size and has large magnetic domains, resulting in a magnetic base with excellent magnetic properties.

[0027] To make the magnetic base less susceptible to damage when mounted on a circuit board and to achieve superior magnetic properties, the ratio of the average particle size of the sintered particles 1 in the central portion 12a to the average particle size of the sintered particles 1 in the surface portion 11a is preferably 3.0 or more, and more preferably 5.0 or more. While there are no particular limitations on the upper limit of this average particle size ratio, to effectively prevent damage to the magnetic base due to stress generated at the interface between the surface portion 11a and the central portion 12a, it is preferably 9.5 or less, and more preferably 8.5 or less. For these reasons, the preferred range for this average particle size ratio is 3.0 or more and 9.5 or less, and more preferably 5.0 or more and 8.5 or less.

[0028] The average particle size of the sintered particles 1 contained in the surface portion 11a of the sintered body 10a is not particularly limited, but is preferably 1 μm or more and 20 μm or less in order to significantly suppress cracking. It is more preferable that the sintered particles 1 have the above average particle size but do not contain sintered particles 1 of 20 μm or more. The average particle size of the sintered particles 1 contained in the central portion 12a of the sintered body 10a is also not particularly limited, but is preferably 30 μm or more and 100 μm or less in order to obtain better magnetic properties. It is more preferable that the sintered particles 1 have the above average particle size but do not contain sintered particles 1 of 100 μm or more.

[0029] The average particle size of the sintered particles 1 in the surface portion 11a and the central portion 12a of the sintered body 10a is determined by the following procedure. First, as shown in FIG. 3, of any two points located on the surface of the sintered body 10a constituting the magnetic base, the point with the greatest distance is determined, and the distance is defined as the maximum dimension s max Next, a measurement surface is formed using the same procedure as in the above-mentioned method for determining the composition of ferrite. Then, the measurement surface is observed using a scanning electron microscope (SEM), and the distance from the outer edge of the measurement surface is (1 / 10)s. max SEM images are taken of the surface portion 11a located within the periphery of the sintered particle 1 and the central portion 12a located inside the surface portion 11a. Next, a straight line is drawn in the obtained SEM image, and the length of the line segment cut at the periphery of each sintered particle 1 (the distance between the two points where the line intersects with the periphery of the sintered particle 1) is measured. This value is divided by the magnification of the SEM image to obtain the particle size of each sintered particle 1. This particle size is calculated for 400 or more sintered particles 1 in each of the surface portion 11a and the central portion 12a. Note that if the number of sintered particles 1 observed in one SEM image is small, the particle size of the sintered particles 1 may be calculated from multiple SEM images taken at different observation positions. Alternatively, multiple lines may be drawn in one SEM image to increase the number of sintered particles for which particle size calculation is performed. However, in this case, the multiple lines must be parallel to each other. Finally, the sum of the particle sizes of the obtained sintered particles 1 is divided by the number of sintered particles 1 for which particle size calculation was performed, and the obtained value is the average particle size in the surface portion 11a and the central portion 12a.

[0030] In the magnetic substrate according to the first aspect, the sintered body 10a is preferably drum-shaped, having a flange and a shaft. In the sintered body 10a having this shape, the proportion of the surface portion 11a in the flange is significantly higher than that in the shaft. Therefore, the effect of suppressing cracks in the flange by reducing the average particle size of the sintered particles 1 in the surface portion 11a and the effect of improving the magnetic properties by increasing the average particle size of the sintered particles 1 in the central portion 12a are significant. These effects are particularly significant when the thickness of the flange is ⅓ or less of the minimum outer dimension of the sintered body 10a excluding the thickness.

[0031] [Method for manufacturing magnetic substrate] A method for producing a magnetic substrate according to another aspect of the present invention (hereinafter, sometimes simply referred to as "a method for producing a magnetic substrate according to a second aspect") includes preparing a raw material powder containing ferrite containing Ni and Zn and bismuth oxide; molding the raw material powder to obtain a molded body in which, when a portion located within 10% of the maximum dimension from the surface is defined as a surface portion and a portion located inside the surface portion is defined as a central portion, the ratio of the density of the surface portion to the density of the central portion is 0.98 or less; and firing the molded body to obtain a sintered body.

[0032] The ferrite used as the raw material powder in the method for producing a magnetic substrate according to the second aspect contains Ni, Zn, and Fe, and may also contain other components such as Cu. The ferrite may be a ready-made product or may be prepared by personal production. A method for preparing ferrite will be described below.

[0033] The raw materials used to produce ferrite are not particularly limited as long as they contain Fe, Ni, and Zn, and optionally Cu, and various compounds, including metal elements, alloys, and oxides, can be used. Compounds may also be those containing multiple types of the elements mentioned above, such as composite oxides. Among these, the oxides Fe2O3, NiO, ZnO, and CuO are preferred because they have little variation in particle shape and particle size, and powders consisting of small particles are easily available.

[0034] The particle size of the raw material used in this case is not particularly limited, and may be, for example, an average particle size (median diameter (D 50 )) can be set to 0.1 μm or more and 5 μm or less. The average particle size is preferably 0.5 μm or more and 3.5 μm or less, more preferably 0.5 μm or more and 2.5 μm or less, and even more preferably 0.5 μm or more and 1.5 μm or less. When the average particle size is equal to or more than the lower limit, handling becomes easy. On the other hand, when the average particle size is equal to or less than the upper limit, the synthesized ferrite tends to be uniform. This average particle size can be measured, for example, using a particle size distribution measuring device using a laser diffraction / scattering method.

[0035] These raw materials are blended and mixed in a ratio that produces ferrite of the desired composition. The blending and mixing method for the raw material powders is not particularly limited as long as it can uniformly mix the powders while preventing the inclusion of impurities, and either dry mixing or wet mixing can be used. When wet mixing using a ball mill is used, the mixing time can be, for example, 8 hours or more and 24 hours or less.

[0036] The mixed powder is heat-treated to obtain ferrite. The heat treatment conditions are not limited as long as they allow the raw materials to react to produce Ni-Zn ferrite or Ni-Zn-Cu ferrite having the desired composition. For example, the heat treatment may be performed in air at 500°C to 1000°C for 1 hour to 2 hours. When determining the heat treatment conditions, it is advisable to consider the following: low temperatures or short treatment times may result in unreacted raw materials or intermediate products remaining; high temperatures or long treatment times may result in the evaporation of components, making it impossible to obtain a compound with the desired composition, or the product may solidify and become difficult to disintegrate, resulting in reduced productivity.

[0037] If the calcined powder obtained by the heat treatment described above is agglomerated, it is preferable to pulverize it before mixing it with bismuth oxide to obtain a raw material powder. The pulverization is carried out to break down the agglomerations of the calcined powder and obtain a powder with appropriate sinterability. The pulverization may be carried out in a dry manner using a vibration mill, hammer mill, roller mill, or the like, but when the calcined powder forms large lumps, it is preferable to carry out the pulverization in a wet manner using a ball mill, attritor, or the like after coarse pulverization. The pulverization is carried out until the average particle size of the calcined powder is 0.5 μm or more and 2 μm or less, or until the BET specific surface area of ​​the calcined powder is 2.0 m 2 / g or more 3.0m 2 It is preferable to perform the process until the sintering temperature reaches 1 / g or less in terms of moldability, shape retention, and sinterability.

[0038] In the method for producing a magnetic substrate according to the second aspect, the bismuth oxide used as a raw material powder together with ferrite may be Bi2O3.

[0039] Ferrite and bismuth oxide are mixed in a predetermined ratio to form a raw material powder. Other components may be mixed in. The mixing method can be the same as that used for producing ferrite described above.

[0040] In a method for producing a magnetic substrate according to a second aspect, a raw material powder is molded to obtain a compact. The molding is performed so that the density ratio of the portion of the compact within 10% of the maximum dimension of the surface to the portion inside the surface, i.e., the density ratio of the surface to the center, is 0.98 or less. Obtaining a compact with such a density ratio results in a magnetic substrate produced through sintering in which the sintered particles present in the surface portion have a small particle size and the sintered particles present in the center have a large particle size. To obtain a magnetic substrate with a large difference in particle size between the surface and center portions, the density ratio is preferably 0.95 or less, more preferably 0.90 or less. On the other hand, to ensure sufficient shape retention of the compact, the density ratio is preferably 0.80 or more, more preferably 0.85 or more. For these reasons, the density ratio is preferably in the range of 0.80 to 0.95, more preferably 0.85 to 0.90. An example of a method for obtaining a compact with the aforementioned density ratio is press molding using a mold.

[0041] The shape of the molded body is not particularly limited, and may be appropriately selected from known shapes such as rod, plate, toroidal, and drum shapes depending on the application. A drum-shaped molded body having a flange and a shaft is preferred because it is easy to obtain a molded body with a low density ratio of the surface portion to the center portion. Of these, a drum shape in which the thickness of the flange is 1 / 3 or less of the minimum outer dimension of the molded body excluding the thickness is more preferred. In such a drum-shaped molded body, a ratio of the density of the flange to the density of the shaft portion of 0.95 or less is particularly preferred because it allows for a magnetic substrate that is less likely to crack or chip at the flange.

[0042] The density of each part of a molded body is determined by the following procedure. First, a portion of the molded body whose density is to be determined is cut out as a test piece, and its volume is calculated from the dimensions of each part. Next, the mass of the test piece is measured. Finally, the density of the test piece obtained by dividing the obtained mass by the volume of the test piece is used as the density of the part from which the test piece was cut out. Note that when determining the density of the surface part or flange part, if the volume of these parts is small and it is difficult to obtain a test piece, the density is determined by the following procedure instead of the above-mentioned procedure. First, the volume of the entire molded body is calculated from the dimensions of the molded body. Next, the mass of the entire molded body is measured. Next, the surface part or flange part is scraped off from the molded body by polishing or grinding. Next, the volume of the remaining molded body is calculated from its dimensions. Next, the mass of the remaining molded body is measured. Next, the difference between the volume of the entire molded body and the volume of the remaining molded body is calculated, and this is used as the volume of the scraped-off surface part or flange part. Next, the difference between the mass of the entire molded body and the mass of the remaining molded body is calculated, and this is used as the mass of the scraped-off surface part or flange part. Finally, the mass of the surface or collar obtained is divided by its volume to obtain the density of the surface or collar.

[0043] In the method for producing a magnetic substrate according to the second aspect, the raw material powder may be granulated prior to molding to obtain a granulated product (granules). Granulation is carried out to convert the raw material powder into agglomerated particles of an appropriate size and convert them into a form suitable for molding. Examples of such granulation methods include pressurized granulation and spray drying.

[0044] Furthermore, in the method for manufacturing a magnetic substrate according to the second aspect, a binder may be mixed with the raw material powder prior to molding the raw material powder in order to improve shape retention after molding. The binder used is preferably one that can bond particles of the calcined powder together to enable molding and shape retention, and that decomposes and volatilizes at temperatures of 500°C or less. Examples of such binders include acrylic resin, butyral resin, and vinyl resin. The amount of binder added may be determined appropriately taking into consideration moldability, shape retention, and the like, and may be, for example, 0.1 to 5 parts by mass per 100 parts by mass of the calcined powder. An example of a method for mixing the calcined powder and binder is mixing using a ball mill.

[0045] In a method for producing a magnetic base according to a second aspect, the compact obtained by molding is fired to obtain a sintered body. The firing conditions are not limited as long as a dense sintered body is obtained, and can be, for example, in air at a temperature of 900°C to 1200°C for 1 hour to 5 hours. If the compact contains a binder, a degreasing treatment to remove the binder may be performed prior to firing. The conditions for the degreasing treatment are not particularly limited as long as they can oxidize and remove most of the binder without sintering the calcined powder particles in the compact. One example is holding the compact in air at a temperature of 300°C to 450°C for 2 hours to 4 hours.

[0046] The above-mentioned degreasing treatment and heat treatment may be carried out continuously using a single heat treatment device capable of changing the atmosphere and temperature settings, or may be carried out intermittently using two or more different heat treatment devices.

[0047] [Coil parts] A coil component 100 according to yet another aspect of the present invention (hereinafter, sometimes simply referred to as a "coil component according to the third aspect") comprises, as shown in Figure 4, the magnetic substrate 10 according to the first aspect described above, a conductor 20 wound around the magnetic substrate 10, and an external electrode 30 provided on the surface of the magnetic substrate 10 and electrically connected to the conductor 20.

[0048] The material, cross-sectional shape, and dimensions of the conductor 20 are not particularly limited and may be determined appropriately depending on the required characteristics. Examples of the material include silver, copper, or alloys thereof. Examples of the cross-sectional shape include circular, elliptical, and rectangular, and the conductor 20 may have a twisted wire shape in which multiple linear conductors are twisted together.

[0049] The external electrode 30 functions as an electrical contact between the conductor 20 and a circuit board on which the coil component is mounted. The external electrode 30 is usually physically attached to the circuit board by soldering or the like. For this reason, in a coil component having the external electrode 30 on the surface of the magnetic base 10, when the circuit board on which the coil component is mounted is deformed, large tensile stress is likely to occur in the magnetic base 10 located near the external electrode 30. However, in the coil component according to the third aspect, the surface portion of the magnetic base 10, i.e., the vicinity of the external electrode 30, is made up of sintered particles with small particle diameters, so that tensile stress is alleviated and cracks are suppressed.

[0050] In this way, when the coil component according to the third aspect is mounted on a circuit board, it is less likely to break even if the circuit board is deformed. Moreover, as described above, the magnetic base 10 has a high proportion of large magnetic domains and is excellent in magnetic properties such as magnetic permeability, so it also has excellent electrical properties such as inductance.

[0051] [Circuit board] A circuit board according to yet another aspect of the present invention (hereinafter sometimes simply referred to as "circuit board according to the fourth aspect") is a circuit board on which the coil component according to the third aspect described above is mounted.

[0052] The structure of the circuit board and the mounting form of the coil component are not limited, and may be selected according to the purpose. As an example, as shown in Fig. 5, an external electrode 30 of a coil component 100 is joined to a land portion 51 formed on a substrate 50 using solder 52.

[0053] The circuit board according to the fourth aspect uses the coil component according to the third aspect, and thus has a low failure rate during use and high performance. [Example]

[0054] The present invention will be explained in more detail below with reference to examples, but the present invention is not limited to these examples.

[0055] [Example 1] <Magnetic substrate and coil component manufacturing> First, powders of Fe2O3, NiO, ZnO, and CuO were prepared as raw materials for preparing ferrite. Next, these raw materials were weighed so that Fe2O3 was 49 mol%, ZnO was 25 mol%, NiO was 21 mol%, and CuO was 5 mol%, and mixed in a wet ball mill. Next, the dispersion medium was removed by evaporation, and the resulting mixed powder was heat-treated in air at 900°C for 2 hours to obtain ferrite powder. Next, 0.05 mass% of Bi2O3 was added to the obtained ferrite powder, and a ferrite powder having a BET specific surface area of ​​2.0 m was obtained. 2 / g or more 3.0m 2 The powder was crushed to a density of 1 / g or less. Next, distilled water as a dispersion medium and PVA (polyvinyl alcohol) as a binder were added to the crushed mixed powder, and the mixture was spray-dried using a spray dryer to obtain a granulated powder. The resulting granulated powder was then fed into a mold and uniaxially compressed to produce a drum-shaped compact. The shape and dimensions of the compact were such that a 2.5 mm x 2.5 mm x 3.0 mm shaft portion was sandwiched axially between a pair of flange portions, one flange portion being plate-shaped and measuring 6.0 mm x 6.0 mm x 0.6 mm, and the other flange portion being plate-shaped and measuring 6.0 mm x 6.0 mm x 0.8 mm, with a recess for forming an external electrode on the side opposite the shaft portion. The density of this compact was 3.2 g / cm at the flange portion. 3 , 3.4g / cm at the shaft 3 The obtained compact was then sintered in air at 1100°C for 3 hours to obtain a magnetic base (sintered body). Finally, a conductive wire was wound 10.5 turns around the shaft of the obtained magnetic base, and both ends of the conductive wire were soldered to recesses formed in the flange to obtain the coil component according to Example 1.

[0056] [Example 2] The amount of Bi2O3 added to the ferrite powder was set to 0.1 mass%, and the density of the compact was set to 3.1 g / cm 3 , shaft is 3.5g / cm 3 A magnetic base and a coil component according to Example 2 were produced in the same manner as in Example 1, except that the magnetic base and the coil component were formed so as to have the following properties.

[0057] [Example 3] The amount of Bi2O3 added to the ferrite powder was set to 0.2 mass%, and the density of the compact was set to 3.1 g / cm 3 , shaft is 3.6g / cm 3 A magnetic base and a coil component according to Example 3 were produced in the same manner as in Example 1, except that the magnetic base and the coil component were formed so as to have the following properties.

[0058] [Comparative Example 1] A magnetic base and a coil component according to Comparative Example 1 were produced in the same manner as in Example 1, except that Bi2O3 was not added to the ferrite powder.

[0059] Comparative Example 2 The amount of Bi2O3 added to the ferrite powder was set to 0.04 mass%, and the density of the compact was set to 3.3 g / cm3 for both the flange and the shaft. 3 A magnetic base and a coil component according to Comparative Example 2 were produced in the same manner as in Example 1, except that the magnetic base and the coil component were formed so as to have the following properties.

[0060] Comparative Example 3 A magnetic base and a coil component according to Comparative Example 3 were produced in the same manner as in Comparative Example 2, except that the amount of Bi2O3 added to the ferrite powder was 0.2 mass %.

[0061] [evaluation] <Average particle size of magnetic substrate (sintered body)> For each of the magnetic substrates obtained in Examples 1 to 3 and Comparative Examples 1 to 3, the average particle size of the sintered particles located in the surface portion and the center portion was determined by the method described above.

[0062] <Inductance characteristics> For each of the coil components obtained in Examples 1 to 3 and Comparative Examples 1 to 3, inductance was measured using an impedance analyzer (E4990A, manufactured by Keysight Technologies, Inc.) at room temperature, an OSC level of 500 mV, and a frequency of 1 MHz. This measurement was performed on 10 different coil components, and the average value of the obtained results was taken as the inductance of the coil component according to Example 1.

[0063] <Bending test> Ten test specimens were prepared by mounting each of the coil components obtained in Examples 1 to 3 and Comparative Examples 1 to 3 at the center of a 30 mm × 110 mm × 1.6 mm circuit board. These test specimens were deflected once toward the surface facing the mounting surface so as to maximize the displacement of the coil component mounting position. The coil components of each test specimen were then visually observed. If no cracks were found on the surface of the magnetic base for any of the ten specimens, it was determined that the coil component would not break at the deflection displacement. The deflection amounts were determined to be 1 cm, 2 cm, and 3 cm, respectively, and the maximum deflection amount at which the coil component did not break was recorded.

[0064] <Heat cycle test> Ten test specimens were prepared by mounting the coil components obtained in Examples 1 to 3 and Comparative Examples 1 to 3 at the center of a 30 mm × 110 mm × 1.6 mm circuit board. These test specimens were cooled to a temperature below 0°C and then heated to a temperature exceeding 100°C. The coil components of each test specimen were then visually observed. If no cracks were observed on the surface of the magnetic substrate for all ten specimens, it was determined that the coil components would not break due to the temperature change. The temperature changes were: (1) cooling from room temperature to -10°C and then heating to 105°C; (2) cooling from room temperature to -25°C and then heating to 125°C; and (3) cooling from room temperature to -40°C and then heating to 150°C. The lowest temperature of the temperature change with the largest change that did not break the coil components was recorded.

[0065] The evaluation results for the examples and comparative examples described above are summarized in Table 1. The inductance values ​​are shown as ratios when the value for Comparative Example 1 is set to 1.0. The average particle size values ​​are shown in units of μm, with values ​​less than 10 μm rounded to the nearest tenth, and values ​​10 μm or greater rounded to the nearest tenth.

[0066] [Table 1]

[0067] A comparison of the Examples and Comparative Examples in Table 1 reveals that a magnetic base formed from a sintered body containing sintered particles formed from ferrite containing Ni and Zn, and containing Bi as a constituent element, in which the ratio of the average particle size of the sintered particles in the center to the average particle size of the sintered particles in the surface region is 1.3 or more, has excellent magnetic properties, and when mounted on a circuit board as a coil component, fracture due to deformation of the circuit board is suppressed. Furthermore, a comparison of Example 1 with Example 2 and Example 3 reveals that fracture due to deformation of the circuit board is significantly suppressed when the ratio of the average particle sizes in the sintered body forming the magnetic base is 9.5 or less. [Industrial Applicability]

[0068] According to the present invention, it is possible to provide a magnetic base that is resistant to breakage when a circuit board is deformed and has high magnetic permeability, and a coil component including the magnetic base, which is therefore useful in that it is possible to obtain a high-performance coil component that is resistant to breakdowns during use. [Explanation of symbols]

[0069] 100 Coil parts 10 Magnetic substrate 10a Sintered body 11a Surface part 12a central part 1. Sintered particles 2 Interface layer 20 Conductors 30 External electrode 50 boards 51 Land Department 52 Solder

Claims

1. The sintered particles are formed of ferrite containing Ni and Zn, Further containing Bi as a constituent element, When a portion that is within 10% of the maximum dimension from the surface is defined as a surface portion, and a portion located inside the surface portion is defined as a central portion, the ratio of the average particle size of the sintered particles in the central portion to the average particle size of the sintered particles in the surface portion is 7.0 or more and 10.0 or less. A magnetic substrate formed from a sintered body.

2. 2. The magnetic substrate according to claim 1, wherein the ferrite further contains Cu.

3. 3. The magnetic substrate according to claim 1, wherein the sintered body has a drum shape having a flange and a shaft.

4. 4. The magnetic substrate according to claim 3, wherein the thickness of said flange is not more than one-third of the minimum outer dimension of the sintered body excluding said thickness.

5. Preparing a raw material powder containing ferrite containing Ni and Zn and bismuth oxide; The raw material powder is molded to obtain a molded body in which, when a portion located within 10% of the maximum dimension from the surface is defined as a flange portion and a portion located inside the flange portion is defined as a shaft portion, the ratio of the density of the flange portion to the density of the shaft portion is 0.86 or more and 0.95 or less; Firing the molded body to obtain a sintered body. A method for manufacturing a magnetic substrate, comprising:

6. The method for manufacturing a magnetic substrate according to claim 5 , wherein the molded body is formed into a drum shape having the flange portion and the shaft portion.

7. 7. The method for producing a magnetic substrate according to claim 6, wherein the thickness of the flange is set to 1 / 3 or less of the minimum outer dimension of the molded body excluding the flange thickness.

8. The magnetic substrate according to any one of claims 1 to 4. a conductor wound around the magnetic substrate; and an external electrode provided on the surface of the magnetic substrate and electrically connected to the conductor; A coil component comprising:

9. A circuit board on which the coil component according to claim 8 is mounted.

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