Ceramic substrates, composite substrates, circuit boards, and power modules

Abstract

To provide a ceramic substrate capable of sufficiently reducing foreign matter remaining in laser holes. [Solution] The ceramic substrate has multiple compartments on its main surface, which are demarcated by scribe lines composed of multiple laser holes. When this ceramic substrate is viewed in cross-section perpendicular to the main surface and passing through the bottom points of the multiple laser holes, the bottoms of the multiple laser holes are curved, and the average value of the radius of curvature r of the bottoms is 10 to 30 μm.

Description

Technical Field

[0001] The present disclosure relates to a ceramic substrate, an integrated substrate, a circuit board, and a power module.

Background Art

[0002] An insulating ceramic plate may be used for a circuit board mounted on an electronic device. As a manufacturing method of such a circuit board, the following technique described in Patent Document 1 is known. That is, after a scribeline is provided by irradiating the surface of a ceramic substrate with laser light using a carbon dioxide laser, a YAG laser, or the like, a metal layer is bonded to the surface to form a composite substrate. Then, the metal layer on the surface of the composite substrate is processed into a circuit pattern by etching. Thereafter, the composite substrate is divided along the scribeline to manufacture a plurality of circuit boards. In Patent Document 2, a technique for reducing charring due to laser light irradiation by irradiating laser light in multiple portions when forming a scribeline has been proposed.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0004] The present disclosure provides, in at least one aspect, a ceramic substrate capable of reducing foreign matter, and an integrated substrate including such a ceramic substrate. The present disclosure provides, in another aspect, a circuit board including a ceramic plate with reduced foreign matter, and a power module including such a circuit board.

Means for Solving the Problems

[0005] This disclosure provides [1] to [7] in several respects. [1] A ceramic substrate having a scribe line composed of multiple laser holes on its main surface, When a cross-section perpendicular to the main surface and passing through the bottom points of the plurality of laser holes is viewed, the bottoms of the plurality of laser holes are curved in an arc shape. A ceramic substrate having an average value of 10 to 50 μm for the radius of curvature r at its base. [2] The ceramic substrate according to [1], wherein in the cross-section, the average value of the angle θ between the imaginary lines connecting the vertices of a pair of protrusions forming each of the plurality of laser holes and the base point between them is 40 to 80°. [3] A composite substrate comprising the ceramic substrate described in [1] or [2] above, and at least one conductive portion bonded to the main surface of the ceramic substrate. [4] A circuit board obtained by fracturing the ceramic substrate in the aggregate substrate described in [3] above along the scribe line. [5] A circuit board comprising a ceramic plate and a conductive portion bonded to the main surface of the ceramic plate, The side surface of the ceramic plate has a laser-processed portion and a fractured portion adjacent to each other in a direction perpendicular to the main surface, and the laser-processed portion and the fractured portion are separated by a curved wave pattern. A circuit board in which, when the side surface is viewed from above, the first protrusions of the waveform pattern that protrude toward the fracture portion are curved in an arc shape, and the average value of the radius of curvature r of the first protrusions is 10 to 50 μm. [6] The circuit board according to [5], wherein the average value of the angle θ between virtual lines connecting the vertices of a pair of adjacent second protrusions that protrude toward the opposite side from the fractured portion of the waveform pattern and the vertex of the first protrusion between them is 40 to 80°. [7] A power module comprising one of the circuit boards described in [4] to [6] above, and a semiconductor element electrically connected to the conductor portion.

[0006] In the ceramic substrate described in [1] above, the bottom of the laser hole is curved in an arc shape, and the average value of the radius of curvature r of the bottom is within a predetermined range, so foreign matter that has entered the bottom of the laser hole can be smoothly removed. Therefore, this ceramic substrate can reduce foreign matter. In the ceramic substrate described in [2] above, the average value of the angle θ between the imaginary lines connecting the vertices of a pair of protrusions and the bottom points of the bottom is within a predetermined range, so foreign matter can be removed even more smoothly. In addition, fracture along the scribe line can be performed smoothly, so the unevenness of the fractured part that occurs as a result of fracture can be reduced.

[0007] The aggregate substrate described in [3] and the circuit board described in [4] above are equipped with the ceramic substrate described in [1] or [2] above, and therefore foreign matter can be reduced.

[0008] The circuit board described in [5] above has a first projection on its side that is curved in an arc shape, and the average value of the radius of curvature r of the first projection is 10 to 50 μm. A ceramic plate having such a side can reduce foreign matter in the laser-processed area. The circuit board described in [6] above can further reduce foreign matter because the angle θ is within a predetermined range, and can also reduce unevenness in the fractured area because it is smoothly fractured along the scribe line.

[0009] The power module described in [7] above features a circuit board with reduced foreign matter. Such a power module offers, for example, superior insulation reliability. [Effects of the Invention]

[0010] According to this disclosure, it is possible to provide a ceramic substrate capable of reducing foreign matter, and an aggregate substrate equipped with such a ceramic substrate. Furthermore, it is possible to provide a circuit board equipped with a ceramic plate with reduced foreign matter, and a power module equipped with such a circuit board. [Brief explanation of the drawing]

[0011] [Figure 1] This is a perspective view of a ceramic substrate. [Figure 2] It is a cross-sectional view of a ceramic substrate. [Figure 3] It is a plan view showing a partially enlarged main surface of the ceramic substrate. [Figure 4] It is a cross-sectional view showing a partially enlarged cross-section of the ceramic substrate. [Figure 5] It is a cross-sectional view showing a further enlarged part of the cross-section of the ceramic substrate. [Figure 6] It is a perspective view of a bonding body used in the manufacture of a circuit board. [Figure 7] It is a perspective view of an assembly substrate. [Figure 8] It is a perspective view of a circuit board. [Figure 9] It is a view showing a partially enlarged side surface of the circuit board. [Figure 10] It is a cross-sectional view of a power module. [Figure 11] (A) is a photograph of an image when observing the cross-section of the silicon nitride substrate of Example 1 with a microscope. (B) is a photograph of an image when observing the cross-section of the silicon nitride substrate of Comparative Example 1 with a microscope. [Figure 12] (A) is a SEM photograph showing an enlarged side surface of the silicon nitride plate in the circuit board of Example 1. (B) is a SEM photograph showing an enlarged side surface of the silicon nitride plate in the circuit board of Comparative Example 1.

Embodiments for Carrying Out the Invention

[0012] Hereinafter, some embodiments of the present disclosure will be described with reference to the drawings as appropriate. However, the following embodiments are examples for explaining the present disclosure and are not intended to limit the present disclosure to the following contents. In the description, the same reference numerals are used for the same elements or elements having the same function, and redundant descriptions may be omitted as appropriate. Also, the positional relationships such as up, down, left, and right are based on the positional relationships shown in the drawings unless otherwise specified. Furthermore, the dimensional ratios of each element are not limited to the ratios shown in the drawings.

[0013] In the following description, the symbol "~" used with numerical values ​​indicates a numerical range that includes upper and lower limits. For example, "X~Y" indicates a numerical range of "X or less and Y or less". Numerical ranges obtained by replacing the upper and / or lower limits of a numerical range with the numerical values ​​described in the examples are also included in this disclosure. Furthermore, numerical ranges obtained by replacing the upper and / or lower limits of one numerical range with the upper and / or lower limits of another numerical range are also included in this disclosure. In addition, content that applies the content described in one embodiment to other embodiments is also included in this disclosure. When multiple materials or components are exemplified, one of them may be used alone, or multiple may be used in combination.

[0014] A ceramic substrate according to one embodiment has a scribe line on its main surface, which is composed of multiple laser holes. Laser holes are holes formed by irradiating with laser light. A scribe line is composed of multiple laser holes arranged in a row. The main surface is a surface that has a larger area than the other planes among several planes. The main surface is usually a pair of surfaces that are in a front-back relationship, but is not limited to this.

[0015] When a cross-section perpendicular to the main surface of a ceramic substrate is viewed, passing through the bottom points of multiple laser holes, the bottoms of the multiple laser holes are curved in an arc shape. The bottom point refers to the deepest point of the laser hole, and the bottom is the portion containing the bottom point of the laser hole. The ceramic substrate is not particularly limited and includes nitride ceramic substrates, oxide ceramic substrates, carbide ceramic substrates, boride ceramic substrates, and composite ceramic substrates containing two or more types of ceramics. Examples of nitride ceramic substrates include silicon nitride substrates and aluminum nitride substrates. The arc shape curvature can be confirmed by observing a cross-section passing through the bottoms of multiple laser holes under magnification using a commercially available microscope. The average value of the radius of curvature r at the bottom of the laser holes is 10 to 50 μm. This average value is obtained by measuring the radius of curvature r at the bottoms of five or more adjacent laser holes and taking the arithmetic mean of the obtained measurements.

[0016] The ceramic substrate 10 shown in Figure 1, which is an example of this embodiment, has a flat plate shape. The ceramic substrate 10 has two main surfaces 10A and 10B that are in a front-back relationship. Of these, the main surface 10A is divided into multiple sections by scribe lines L1 and L2. In this example, no scribe lines are provided on the main surface 10B, but in modified examples, scribe lines may also be provided on the main surface 10B. The main surface 10A is provided with a plurality of scribe lines L1 that extend along a first direction and are arranged at equal intervals, and a plurality of scribe lines L2 that extend along a second direction perpendicular to the first direction and are arranged at equal intervals.

[0017] Figure 2 is a cross-sectional view taken along line II-II in Figure 1. As shown in Figures 1 and 2, the partitioned portion 50 is composed of a three-dimensional region enclosed by a region of one main surface 10A surrounded by scribe lines L1 and L2, a region of the other main surface 10B corresponding to that region, and virtual lines VL1 and VL2 drawn parallel to the thickness direction of the ceramic substrate 10 from the scribe lines L1 and L2. That is, the ceramic substrate 10 has a plurality of partitioned portions 50 (9 in Figure 1) defined by scribe lines L1 and L2. The ceramic substrate 10 is configured to be divisible into a plurality of ceramic plates along the virtual lines VL1 and VL2.

[0018] Figure 3 is a magnified plan view showing the scribe line L1 (L2) provided on the main surface 10A of the ceramic substrate 10. The scribe line L1 (L2) is composed of a plurality of laser holes 20 that are connected along its longitudinal direction. Adjacent laser holes 20 may be connected to each other or separated. The outer edge 20E of the laser holes 20 on the main surface 10A is formed by an arc. In plan view, the outer edge 20E may have a circular or elliptical shape.

[0019] The average diameter R of multiple laser holes 20 when viewed from above may be 60 μm or more, 70 μm or more, or 80 μm or more, from the viewpoint of sufficiently curving the bottom of the laser holes 20 and ensuring that foreign matter that enters the laser holes 20 can be removed smoothly. The average diameter R may be 200 μm or less or 150 μm or less, from the viewpoint of ensuring sufficient bonding area with the metal plate when used as a circuit board. For example, the average diameter R may be 60 to 200 μm or 70 to 150 μm. If the outer edge 20E is not a perfect circle, the diameter R can be determined as the diameter obtained by converting the area of ​​the laser hole 20 when viewed from above into a circle. The average value of the circle-converted diameter obtained in this way should be calculated. The average value of the diameter R (or average value of the circle-converted diameter) is obtained by measuring the diameter R (or circle-converted diameter) of five or more adjacent laser holes 20 and taking the arithmetic mean of the obtained measurements.

[0020] Figure 4 is a cross-sectional view taken along line IV-IV of Figure 3. That is, Figure 4 is a cross-sectional view taken when the ceramic substrate 10 is cut by a plane perpendicular to the main surface 10A, passing through the bottoms 20B of the multiple laser holes 20 that constitute the scribe line L1 (L2). Each of the multiple laser holes 20 is formed between a pair of protrusions 22. The laser holes 20 have a mortar shape that tapers from the outer edge 20E towards the interior of the ceramic substrate 10. The laser holes 20 have a depth D1 from the apex 22a of the protrusion 22 to the bottom point BP of the bottom 20B. From the viewpoint of facilitating the division of the ceramic substrate 10 and the removal of foreign matter from the laser holes 20, the average value of the depth D1 may be 40-200 μm, 50-160 μm, or 60-120 μm. The average value of the depth D1 is obtained by measuring the depth D1 of five or more adjacent laser holes 20 and taking the arithmetic mean of the obtained measurements. The thickness of the ceramic substrate 10 may be, for example, 0.1 to 0.6 mm.

[0021] Figure 5 shows a further magnified view of one of the laser holes 20 shown in Figure 4. The contour of the bottom 20B of the laser hole 20 coincides with a part of the arc of the virtual circle VC. Thus, the bottom 20B of the laser hole 20 is curved in an arc shape. The average value of the radius of curvature r (radius of the virtual circle VC) of the bottom 20B including the bottom point BP is 10 to 50 μm. If the average value of the radius of curvature r is within this range, foreign matter that has entered the laser hole 20 constituting the scribe line L1 (L2) can be smoothly removed. Examples of foreign matter that may enter the laser hole 20 include brazing material used when joining a metal plate to a ceramic substrate 10.

[0022] The average value of the radius of curvature r may be 15 μm or more, or 20 μm or more, from the viewpoint of making the removal of foreign matter smoother. The average value of the radius of curvature r may be 40 μm or less, or 30 μm or less, from the viewpoint of making the bonding area with the metal plate sufficiently large when it is made into a circuit board. The average value of the radius of curvature r may be, for example, 10 to 40 μm, 10 to 30 μm, or 15 to 30 μm. The standard deviation of the radius of curvature r may be 5 μm or less, 4 μm or less, or 3 μm or less, from the viewpoint of sufficiently reducing foreign matter on the ceramic substrate 10. The average value and standard deviation of the radius of curvature r can be determined using the measured values ​​of the radius of curvature r of the bottom 20B of five or more adjacent laser holes 20. The average value of the radius of curvature r is the arithmetic mean.

[0023] The average angle θ between the virtual lines VL2 connecting the vertices 22a of a pair of protrusions 22 forming the laser holes 20 and their base points BP may be 40 to 80°. This allows for sufficiently smooth division along the scribe lines L1 and L2 and removal of foreign matter that has entered the laser holes 20. The lower limit of the average angle θ may be 50° or 55° from the viewpoint of further reducing foreign matter. The upper limit of the average angle θ may be 70° or 65° from the viewpoint of further smoothing the division of the ceramic substrate 10. The standard deviation of the angle θ may be 5° or less, 4° or less, or 3.2° or less from the viewpoint of sufficiently reducing foreign matter in the ceramic substrate 10. The average angle θ and standard deviation can be determined using the measured values ​​of the angle θ between the virtual lines VL2 drawn in the manner described above for five or more adjacent laser holes 20. The average angle θ is also the arithmetic mean.

[0024] The laser hole 20 can be formed using a CO2 laser processing apparatus. The radius of curvature r and angle θ of the bottom 20B of the laser hole 20 can be adjusted by setting the laser beam irradiation conditions. An fΘ lens may be used to form the laser hole 20. By using an fΘ lens, the uniformity of the focal length of the laser beam when forming the scribe lines L1 and L2 is increased, improving processing accuracy. This makes it possible to form a laser hole 20 having a smoothly curved bottom 20B. In addition, variations in the shape of multiple laser holes 20 can be sufficiently reduced. This makes it easier to adjust the average value of the radius of curvature r and the average value of the angle θ to the above range. The irradiation conditions of the CO2 laser processing apparatus may be, for example, within the following range.

[0025] ·Focal length: 0.00~0.30mm ·Processing speed: 240~480mm / sec Output: 60-100W • Frequency: 2.4~4.8kHz

[0026] In the example shown in Figure 5, the pair of protrusions 22 that form the laser hole 20 have a shape that is perpendicular to the main surface 10A and symmetrical with respect to a straight line passing through the base point BP, but are not limited to this. For example, the base point BP may be located closer to one of the pair of protrusions 22 on the left and right.

[0027] The ceramic substrate 10 may be used as a composite substrate by joining metal plates to, for example, the main surface 10A and the main surface 10B. When forming the composite substrate, a bonding body 60 as shown in Figure 6 is used. The bonding body 60 applies brazing material to all or part of the main surface 10A and the main surface 10B of the ceramic substrate 10. The brazing material contains at least one of Ag and Cu as its main component and an active metal as a secondary component. Examples of active metals include Ti, Zr, Hf, Nb, Cr, etc. These active metals may be included as hydrides such as TiH2. In addition to these inorganic components, the brazing material may contain organic solvents and binders. The viscosity of the brazing material may be, for example, 5 to 20 Pa·s. The content of organic solvents in the brazing material may be, for example, 5 to 25% by mass, and the content of binders may be, for example, 2 to 15% by mass. Such a brazing material is applied as a paste to the entire main surface 10A and main surface 10B of the ceramic substrate 10 by a method such as a roll coater, screen printing, or transfer method. Such a brazing material may be filled into the laser holes 20.

[0028] A pair of metal plates 11 and 12 are stacked so as to cover the brazing material coating layers on the main surfaces 10A and 10B of the ceramic substrate 10, and heated to 700-900°C in a heating furnace. The heating may be carried out while pressing the stack of ceramic substrate 10 and metal plates 11 and 12 in the stacking direction. The metal plates 11 and 12 may be copper plates. The atmosphere inside the furnace may be an inert gas such as nitrogen, and the heating may be carried out under reduced pressure below atmospheric pressure or under vacuum. In this way, a joined body 60 is obtained in which the ceramic substrate 10 and metal plates 11 and 12 are joined via the brazing material layer.

[0029] Next, a portion of the metal plates 11 and 12 in the bonded body 60 is removed by photolithography. Specifically, a photosensitive resist is printed on the surface of the metal plates 11 and 12. Then, an exposure device is used to form a resist pattern having a predetermined shape. The resist may be negative or positive. Uncured resist is removed, for example, by washing.

[0030] After forming the resist pattern, the portions of the metal plates 11 and 12 not covered by the resist pattern are removed by etching. This removes a portion of the metal plates 11 and 12 and the underlying brazing layer, exposing the main surfaces 10A and 10B of the ceramic substrate 10, as well as the scribe lines L1 and L2 formed on those surfaces. At this time, the bottom 20B of the laser holes 20 constituting the scribe lines L1 and L2 are curved in an arc shape, and the average value of the radius of curvature r is within a predetermined range, so the brazing material components filled in the laser holes 20 can be sufficiently removed. After etching, for example, the exposed portions may be cleaned by washing and honing. Subsequently, the resist pattern is removed to form independent conductive portions for each section 50 on the main surfaces 10A and 10B. In the assembled substrate obtained in this way, the brazing material components remaining in the laser holes 20 constituting the scribe lines L1 and L2 are sufficiently reduced.

[0031] A composite substrate according to one embodiment comprises a ceramic substrate according to the above embodiment and a plurality of conductive parts joined to the main surface of the ceramic substrate. A composite substrate is one which is divided into a plurality of circuit boards by cutting the ceramic substrate along the scribe lines. The conductive parts may be provided independently for each section of the ceramic substrate. The conductive parts may be metal terminals or heat sinks. The plurality of conductive parts may be provided on only one main surface of the ceramic substrate, or on both main surfaces that are in a front-back relationship. In a composite substrate, at least a portion of the scribe lines formed on the main surface of the ceramic substrate may be exposed.

[0032] In the example shown in Figure 7, the composite substrate 30 comprises a ceramic substrate 10, a plurality of conductor portions 13 joined to the main surface 10A independently of the partitioned portion 50, and a plurality of conductor portions 14 joined to the main surface 10B independently of the partitioned portion 50. The shapes of the conductor portions 13 and conductor portions 14 may differ from each other. The plurality of conductor portions 13 may be formed by integrating a portion of the metal plate 11 remaining after etching with a brazing layer. The plurality of conductor portions 14 may be formed by integrating a portion of the metal plate 12 remaining after etching with a brazing layer. No brazing components may remain at all in the laser holes 20 that constitute the scribe lines L1 and L2 of the composite substrate 30. In this way, the composite substrate 30 has a sufficiently reduced amount of foreign matter in the laser holes 20. Multiple circuit boards can be obtained by fracturing and dividing the composite substrate 30 along the scribe lines L1 and L2.

[0033] A circuit board according to one embodiment can be obtained by fracturing a composite board along a scribe line. The circuit board (fragmented board) may be used as a component in, for example, a power module. The circuit board comprises a ceramic plate and a conductive portion bonded to its main surface. The conductive portion may be bonded to only one main surface of the ceramic plate, or to each of two main surfaces that are in a front-back relationship. For example, electronic components may be mounted on the conductive portion, but is not limited to this, and the conductive portion may also function as a heat dissipation portion.

[0034] The side surface of the ceramic plate in the circuit board has a laser-processed area and a fractured area adjacent to each other in a direction perpendicular to the main surface. The laser-processed area is a portion formed by irradiation with laser light, and the fractured area is a portion composed of the fracture surface. On the side surface, the laser-processed area and the fractured area are separated by a curved wave pattern.

[0035] The side surface of the ceramic plate may have a first laser-processed area near one main surface of the ceramic plate, a second laser-processed area near the other main surface, and a fractured area between the first and second laser-processed areas. In this case, the first laser-processed area and the fractured area are separated by a first wave pattern, and the second laser-processed area and the fractured area are separated by a second wave pattern. Such a side surface can be obtained, for example, by fracturing a ceramic plate on which scribe lines are formed on both main surfaces.

[0036] If the ceramic substrate can be fractured along all the bottom points of multiple linearly arranged laser holes, the waveform pattern showing the contour of the laser-processed area when the side surface of the ceramic plate is viewed from above will match the waveform pattern in the cross-section of the ceramic substrate as shown in Figure 4. In this case, when the side surface of the ceramic plate is viewed from above, the first protrusions of the waveform patterns (first waveform pattern and second waveform pattern) that protrude toward the fracture area are curved in an arc shape, and the average value of the radius of curvature r at their tips is 10 to 50 μm. This average value can be obtained by measuring the radius of curvature r at the tips of five or more adjacent first protrusions and taking the arithmetic mean of the obtained measurements. The numerical range of this average value of the radius of curvature r may be any of the numerical ranges of the radius of curvature r listed in the embodiment of the ceramic substrate.

[0037] In the example shown in Figure 8, the circuit board 35 comprises a ceramic plate 50a and conductive portions 13 and 14 joined to its main surface 50A and main surface 50B, respectively. When the circuit board 35 is mounted on a semiconductor device, the conductive portions 13 and 14 may constitute part of an electrical circuit or function as a heat sink.

[0038] As shown in the side view in Figure 9, the side surface 50S of the ceramic plate 50a has a laser-processed portion 52 and a fractured portion 54 adjacent to each other in a direction perpendicular to the main surface 50A and the main surface 50B. The laser-processed portion 52 and the fractured portion 54 are demarcated by a waveform pattern WP. The first protrusion 52b of the waveform pattern WP that protrudes toward the fractured portion 54 is curved in an arc shape. The circuit board 35 may be obtained by fracturing the ceramic substrate 10 along the scribe lines L1 and L2. That is, the ceramic plate 50a may be divided into sections 50 of the ceramic substrate 10. The waveform pattern WP on the side surface 50S of the ceramic plate 50a may have a shape derived from the shape of the laser hole 20.

[0039] The average value of the radius of curvature r of the first protrusion 52b is 10 to 50 μm. The numerical range of this average value of the radius of curvature r may be the same as the numerical range of the radius of curvature r of the bottom 20B of the laser hole 20. The average value of the angle θ between the virtual lines VL3 connecting the vertices VE2 of a pair of adjacent second protrusions 52a that protrude toward the opposite side of the fracture portion 54 in the waveform pattern WP and the vertex VE1 of the first protrusion 52b between them may be 40 to 80°. This average value is obtained by measuring the angle θ at the tips TP2 of five or more adjacent first protrusions 52b and taking the arithmetic mean of the obtained measurements. The numerical range of this average value of the angle θ may be the same as the numerical range of the average value of the angle θ between the virtual lines VL2 connecting the vertices 22a of a pair of convex portions 22 that form the laser hole 20 and the bottom point BP between them.

[0040] The numerical range of the average value of the distance D2 between vertices VE1 and VE2 along the direction perpendicular to the main surfaces 50A and 50B may be the same as the numerical range of the average value of the depth D1 of the laser hole 20. The numerical range of the average value of the distance R2 between vertices VE2 of adjacent second protrusions 52a may be the same as the numerical range of the average value of the diameter R of the laser hole 20.

[0041] The circuit board 35 can suppress the retention of foreign matter in the laser-processed portion 52 on the side surface 50S. Therefore, the ceramic plate 50a has excellent insulation reliability. Furthermore, since the circuit board 35 is obtained by smoothly dividing the assembled board 30, the unevenness of the fracture portion 54 on the side surface 50S can be reduced. Such a circuit board 35 has excellent dimensional accuracy. With reduced foreign matter and excellent dimensional accuracy, the circuit board 35 is suitable, for example, as a component for a power module.

[0042] A power module according to one embodiment comprises a circuit board according to the above embodiment and a semiconductor element electrically connected to the conductor portion of the circuit board. A power module is an electronic component for power conversion and control, and its performance and reliability are influenced by the circuit board used. The power module of this embodiment includes a circuit board with reduced foreign matter. Such a power module has, for example, excellent insulation reliability. Note that the application of the circuit board is not limited to power modules.

[0043] In the example shown in Figure 10, the power module 100 comprises a base plate 70 and a circuit board 35 joined to one side of the base plate 70 via solder 82. A conductor portion 14 on one side of the circuit board 35 is joined to the base plate 70 via solder 82. A semiconductor element 90 is attached to at least one of the conductor portions 13 on the other side of the circuit board 35 via solder 81. The semiconductor element 90 is connected to a predetermined location on the conductor portion 14 by a metal wire 84 such as an aluminum wire. In this way, the semiconductor element 90 and the conductor portion 13 are electrically connected. To electrically connect the outside of the housing 86 to the conductor portion 13, one of the conductor portions 13, conductor portion 13a, is connected to an electrode 83 that penetrates the housing 86 via solder 85.

[0044] A housing 86 is arranged on one main surface of the base plate 70, and is integrated with the main surface to house the circuit board 35 and the semiconductor element 90. The housing space formed by the one main surface of the base plate 70 and the housing 86 is filled with resin 80. The resin 80 seals the circuit board 35 and the semiconductor element 90. The resin may be, for example, a thermosetting resin or a photocuring resin.

[0045] Cooling fins 72, which form a heat dissipation section, are bonded to the other main surface of the base plate 70 via grease 74. Screws 73 for fixing the cooling fins 72 to the base plate 70 are attached to the end of the base plate 70. The base plate 70 and cooling fins 72 may be made of aluminum. The base plate 70 and cooling fins 72 function well as a heat dissipation section due to their high thermal conductivity. Since the power module 100 is equipped with a circuit board 35, it operates stably even when used in high-temperature environments and has high reliability.

[0046] Although several embodiments of the present disclosure have been described above, the present disclosure is not limited in any way to the above embodiments. For example, the shape and structure of the ceramic substrate, assembly substrate, circuit board, and power module of the present disclosure are not limited to the illustrated structure and shape. For example, the shape of the conductor portion provided in each section of the ceramic substrate does not have to be the same, and each section 50 may have a different shape. The ceramic substrate, ceramic plate, and metal plate may have a shape other than a rectangular prism. Any surface treatment may be applied to the conductor portion in the assembly substrate and circuit board. For example, a part of the surface of the conductor portion may be covered with a protective layer such as solder resist, and the other part of the surface of the conductor portion may be plated. [Examples]

[0047] The contents of this disclosure will be explained in more detail with reference to specific examples, but this disclosure is not limited to these examples.

[0048] [Fabrication and evaluation of silicon nitride substrates with scribe lines] (Example 1) Silicon nitride powder and sintering aids, magnesium oxide powder, yttrium oxide powder, and silicon dioxide powder were prepared. The silicon nitride powder, magnesium oxide powder, yttrium oxide powder, and silicon dioxide powder were blended in a mass ratio of Si3N4:MgO:Y2O3:SiO2 = 91.35:1.58:6.00:1.07 to obtain the raw material powder. A binder, dispersant, and dispersion medium were added to this raw material powder to prepare a mixed raw material (raw material slurry). Next, the raw material slurry was applied to a release film using the doctor blade method, and the application thickness was adjusted to 0.440 mm to produce a green sheet.

[0049] The prepared green sheet was placed on a boron nitride setter and placed in an electric furnace equipped with a carbon heater. It was then degreased by heating in air at 500°C for 18 hours. The resulting degreased material was fired in a nitrogen gas atmosphere at 1800°C for 6 hours to obtain a flat silicon nitride plate (length × width × thickness = 200 mm × 150 mm × 0.32 mm).

[0050] Multiple laser holes were formed on the main surface of the silicon nitride substrate using a CO2 laser processing device, creating scribe lines. Multiple scribe lines were created along directions that intersect each other, dividing the main surface of the silicon nitride substrate into 24 regions. An fΘ lens was used as the laser lens for the CO2 laser processing device. The laser irradiation conditions were as follows:

[0051] <Laser light irradiation conditions> ·Focal length: 0.20mm ·Processing speed: 360mm / sec Output: 80W • Frequency: 3.60kHz

[0052] A cross-section was obtained by dividing the silicon nitride substrate along one of several scribe lines. The cut was made perpendicular to the main surface of the silicon nitride substrate and passed through the bottom points of several laser holes. The obtained cross-section was observed using a microscope (OPTO SCIENCE, product name: Dino-Lite).

[0053] Figure 11(A) is a photograph of a cross-section of the silicon nitride substrate of Example 1, magnified 200 times with a microscope. In the image shown in Figure 11(A), the radius of curvature r at the bottom of the laser holes was measured. The measurement was performed at five arbitrarily selected adjacent laser holes. In addition, the angle θ made by the imaginary lines connecting the vertices of the pair of protrusions forming each of these five laser holes and the bottom point of the laser hole between them was measured. The radius of curvature r and angle θ were measured using the measurement function installed in the microscope. The measured values, mean, and standard deviation of the radius of curvature r and angle θ are shown in Table 1.

[0054] (Example 2) A silicon nitride substrate was fabricated using the same procedure as in Example 1. Multiple laser holes were formed on the main surface of the silicon nitride substrate using a CO2 laser processing apparatus, and scribe lines were created using the same procedure as in Example 1. The silicon nitride substrate was divided along the scribe lines to obtain a cross-section. The cross-section of the silicon nitride substrate was observed using a microscope in the same manner as in Example 1. The measured values, mean, and standard deviation of the radius of curvature r and angle θ of five arbitrarily selected adjacent laser holes are shown in Table 1.

[0055] (Comparative Example 1) A silicon nitride substrate was fabricated using the same procedure as in Example 1. As the laser lens for the CO2 laser processing apparatus, a lens with a sharper angle of laser beam irradiated onto the silicon nitride substrate than in Example 1 was used, and a scribe line was formed in the same manner as in Example 1. The silicon nitride substrate was divided along the scribe line to obtain a cross-section. The cross-section of the silicon nitride substrate was observed using a microscope in the same manner as in Example 1.

[0056] Figure 11(B) is a photograph of a cross-section of the silicon nitride substrate of Comparative Example 1, magnified 200 times with a microscope. Following the same procedure as in Example 1, the radius of curvature r and angle θ of the bottom of five arbitrarily selected adjacent laser holes were measured. The measured values, mean, and standard deviation of the radius of curvature r and angle θ are shown in Table 1. The radius of curvature r of Comparative Example 1 was below the lower limit of measurement (10 μm) and therefore could not be measured.

[0057] [Table 1]

[0058] [Circuit board fabrication and evaluation] Using the silicon nitride substrates of Examples 1 and 2 and Comparative Example 1, composite substrates and circuit boards were fabricated according to the following procedure. A brazing material containing Ag, Cu, Sn, and TiH2 was prepared. The mass ratio of Ag to Cu in the brazing material was 9:1. This brazing material contained 3 parts by mass of Sn and 3.5 parts by mass of TiH2 for every 100 mass of Ag and Cu combined.

[0059] A coating layer was formed by screen printing a brazing material to cover the entire main surface of one of the silicon nitride substrates. After forming the coating layer, the silicon nitride substrate and the copper plate were stacked so that the main surface of the silicon nitride substrate and the main surface of the copper plate faced each other through the coating layer, and the copper plate on the main surface of the silicon nitride substrate was pressed toward the silicon nitride substrate at 0.015 MPa to obtain a laminate.

[0060] The laminate is placed in a vacuum (1.0 × 10 -3 The substrate was heated at 790°C for 1 hour. In this way, a bonded body was obtained in which a copper plate was bonded to the main surface of a silicon nitride substrate via a brazing layer. A resist was applied to a predetermined area on the main surface of the copper plate of the bonded body. Then, etching was performed by spraying an etching solution onto the main surface of the copper plate of the bonded body, followed by cleaning and honing to clean the exposed areas. In this way, the scribe lines in the silicon nitride substrate were exposed, and a composite substrate having 24 electrically independent conductive parts on the main surface of the silicon nitride substrate was obtained.

[0061] Multiple circuit boards were obtained by splitting the silicon nitride substrate along the scribe lines. Figure 12(A) is an SEM image showing a magnified view of the side surface of the silicon nitride plate in the circuit board of Example 1. Figure 12(B) is an SEM image showing a magnified view of the side surface of the silicon nitride plate in the circuit board of Comparative Example 1. As shown in Figures 12(A) and 12(B), the side surface of the silicon nitride plate in the circuit board consisted of laser-processed areas, which are traces of laser holes, and fractured areas. As shown in Figure 12(B), it was confirmed that brazing material components (black parts) remained in the laser-processed areas on the side surface of the silicon nitride plate in the circuit board of Comparative Example 1. On the other hand, as shown in Figure 12(A), it was confirmed that no brazing material components remained in the laser-processed areas on the side surface of the silicon nitride plate in the circuit board of Example 1. No brazing material components remained in the laser-processed areas on the side surface of the silicon nitride plate in the circuit board of Example 2 either. Note that the white areas in Figures 12(A) and 12(B) are oxides derived from the sintering aid and are not foreign matter. [Industrial applicability]

[0062] This disclosure provides a ceramic substrate capable of reducing foreign matter, and a composite substrate comprising such a ceramic substrate. It also provides a circuit board comprising a ceramic plate with reduced foreign matter, and a power module comprising such a circuit board. [Explanation of symbols]

[0063] 10...Ceramic substrate, 10A, 10B...Main surface, 11, 12...Metal plate, 13, 13a, 14...Conductor part, 20...Laser hole, 20B...Bottom, 20E...Outer edge, 22...Protrusion, 22a...Vertex, 30...Assembled substrate, 35...Circuit board, 50...Partitioned part, 50A, 50B...Main surface, 50S...Side, 50a...Ceramic plate, 52...Laser processed part, 52a...Second protrusion, 52b...First protrusion, 54...Fracture section, 60...Joint, 70...Base plate, 72...Cooling fin, 73...Screw, 74...Grease, 80...Resin, 81,82,85...Solder, 83...Electrode, 84...Metal wire, 86...Housing, 90...Semiconductor element, 100...Power module, BP...Bottom point, L1,L2...Scribing lines, TP2...Tip, VC...Virtual circle, VE1,VE2...Vertices, WP...Wave pattern.

Claims

1. A ceramic substrate having a scribe line composed of multiple laser holes on its main surface, When a cross-section perpendicular to the main surface and passing through the bottom points of the plurality of laser holes is viewed, the bottoms of the plurality of laser holes are curved in an arc shape. A ceramic substrate having an average value of 10 to 50 μm for the radius of curvature r at its base.

2. The ceramic substrate according to claim 1, wherein in the cross-section, the average value of the angle θ between the imaginary lines connecting the vertices of a pair of protrusions forming each of the plurality of laser holes and the base point between them is 40 to 80°.

3. A composite substrate comprising a ceramic substrate according to claim 1 or 2, and a plurality of conductive portions bonded to the main surface of the ceramic substrate.

4. A circuit board obtained by fracturing the ceramic substrate in the aggregate substrate according to claim 3 along the scribe line.

5. A circuit board comprising a ceramic plate and a conductive portion bonded to the main surface of the ceramic plate, The side surface of the ceramic plate has a laser-processed portion and a fractured portion adjacent to each other in a direction perpendicular to the main surface, and the laser-processed portion and the fractured portion are separated by a curved wave pattern. A circuit board in which, when the side surface is viewed from above, the first protrusions of the waveform pattern that protrude toward the fracture portion are curved in an arc shape, and the average value of the radius of curvature r of the first protrusions is 10 to 50 μm.

6. The circuit board according to claim 5, wherein the average value of the angle θ between virtual lines connecting the vertices of a pair of adjacent second protrusions that protrude toward the opposite side from the fractured portion of the waveform pattern and the vertices of the first protrusions between them is 40 to 80°.

7. A power module comprising a circuit board according to claim 5 or 6, and a semiconductor element electrically connected to the conductor portion.

Images