Ceramic circuit board

Abstract

To provide a ceramic circuit board that is intentionally warped to enable: eliminating a defect; facilitating handling thereof; and improving a manufacturing yield.SOLUTION: A ceramic circuit board 10 comprises: a ceramic substrate 1 which includes a first surface 1a and a second surface 1b, in which the first surface 1a includes a first region and a second region; first metal plates 2 joined to the first region; and a second metal plate 3 joined to a second surface 1b. The first surface 1a extends in a first direction and in a second direction crossing the first direction. A first waviness curvature in the second region measured along a first side extending in the first direction of the first surface 1a is linear. A second waviness curvature in the second region measured along a second side extending in the second direction of the first surface 1a is circular.SELECTED DRAWING: Figure 2

Description

[Technical field]

[0001] This embodiment relates to a ceramic circuit board. [Background technology]

[0002] In recent years, with the progress of miniaturization, power generation, and performance improvement of motors, power conversion devices, etc., circuit boards for power semiconductor elements mounted thereon are required to have high thermal conductivity capable of handling higher power, and high reliability with a longer temperature cycle life. In order to meet this demand, various proposals have been made for ceramic circuit boards for power semiconductor elements having high thermal conductivity and high reliability.

[0003] For example, in a ceramic circuit board in which a metal circuit plate is brazed to the front surface of a ceramic substrate and a metal heat sink plate is brazed to the rear surface, even if the metal circuit plate is made thick, a ceramic circuit board having a metal circuit and a manufacturing method thereof are known (see, for example, Patent Document 1). In this ceramic circuit board, the brazed joint between the metal circuit plate and the ceramic substrate is cooled at -110°C to relieve residual stress, and the amount of warping is suppressed by adjusting various thicknesses.

[0004] Also, as a ceramic circuit board that suppresses bending during mounting while taking into consideration bonding with a lead frame and also has good thermal cycle test (TCT) characteristics, there is known a ceramic circuit board in which a metal plate is bonded to a ceramic substrate of a specified thickness, and the ceramic substrate is made to have a specified undulation curve (see, for example, Patent Document 2). [Prior art documents] [Patent documents]

[0005] [Patent Document 1] JP 2003-309210 A [Patent Document 2] Patent No. 7048493 Summary of the Invention [Problem to be solved by the invention]

[0006] As described in Patent Document 1 above, warping and undulation in ceramic circuit boards are generally caused by the difference in thermal expansion coefficient between the metal plates provided on the front and back surfaces of the board and the ceramic substrate, and various efforts have been made to eliminate such warping and undulation and to achieve flatness.

[0007] However, even when the ceramic circuit board is flattened, warping or undulation may occur due to manufacturing variations, and since it is difficult to control which side of the ceramic circuit board the warping or undulation will occur on (for example, the long side or the short side in the case of a rectangular ceramic circuit board), there are cases where the expected operation cannot be performed during transportation by suction or mounting of semiconductor elements due to the state of the warping or undulation. In other words, there are cases where suction failure occurs due to the warping direction, or the mounting position of semiconductor elements, etc. is shifted, making subsequent handling difficult or causing problems such as product defects.

[0008] SUMMARY OF THE PRESENT EMBODIMENTS The present invention aims to provide a ceramic circuit board which is intentionally warped to eliminate the above-mentioned problems, to facilitate handling, and to improve the production yield. [Means for solving the problem]

[0009] The ceramic circuit board of this embodiment comprises a ceramic substrate having a first surface and a second surface, the first surface including a first region and a second region, a first metal plate bonded to the first region, and a second metal plate bonded to the second surface, the first surface extending in a first direction and a second direction intersecting the first direction, a first undulation curve of the second region measured along a first side of the first surface extending in the first direction is linear, and a second undulation curve of the second region measured along a second side of the first surface extending in the second direction is arc-shaped. Effect of the Invention

[0010] By intentionally warping the ceramic circuit board, it becomes easier to control the direction of the warp even if there are manufacturing variations.

[0011] In addition, since the warping direction is fixed, the warping direction can be taken into consideration when transporting the ceramic circuit board or when mounting semiconductor elements on the ceramic circuit board, making it easier to handle the ceramic circuit board. [Brief description of the drawings]

[0012] [Figure 1] 1 is a plan view showing a schematic configuration of a ceramic circuit board according to one embodiment. [Diagram 2] 2 is a side cross-sectional view showing a schematic configuration of the ceramic circuit board shown in FIG. [Diagram 3] FIG. 13 is a diagram for explaining a wavy curve. [Figure 4] FIG. 13 is a diagram for explaining a wavy curve. [Figure 5A] FIG. 13 is a diagram showing an example of an arrangement structure of metal plates. [Figure 5B] FIG. 13 is a diagram showing an example of an arrangement structure of metal plates. [Figure 6] FIG. 4 is a diagram showing a waviness curve of Example 1. [Figure 7] FIG. 13 is a diagram showing a waviness curve of Example 2. [Figure 8]FIG. 13 is a diagram showing a waviness curve of Example 3. [Figure 9] FIG. 13 is a diagram showing a waviness curve of Example 4. [Figure 10] FIG. 13 is a diagram showing a waviness curve of Example 5. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] A ceramic circuit board and a manufacturing method thereof according to the present embodiment will be described below.

[0014] [Ceramic circuit board] The ceramic circuit board according to this embodiment has the above-mentioned configuration, and can be exemplified by the ceramic circuit board shown in Figures 1 and 2. Figure 1 is a plan view showing a schematic configuration of a ceramic circuit board 10, and Figure 2 is a side view thereof.

[0015] This ceramic circuit board 10 is illustrated with a rectangular ceramic substrate 1, metal plates (front metal plate 2, rear metal plate 3), length L11 of the long side of the ceramic substrate 1, length L12 of the short side of the ceramic substrate 1, and measurement point W of the waviness curve. FIGS. 1 and 2 show an example in which the ceramic circuit board 10 has two front metal plates 2 and one rear metal plate 3. The structure of the ceramic circuit board according to the embodiment is not limited to the above structure, and for example, the ceramic circuit board 10 may have one front metal plate 2 or three or more front metal plates 2. Similarly, the ceramic circuit board 10 may have two or more rear metal plates 3.

[0016] The ceramic substrate 1 has a surface 1a and a surface 1b facing each other. The surfaces 1a and 1b extend in a first direction and a second direction, respectively. The second direction intersects with the first direction. When the surfaces 1a and 1b are, for example, rectangular surfaces, the first direction is, for example, the long side direction of the rectangle, and the second direction is, for example, the short side direction of the rectangle.

[0017] The surface 1a includes a bonding region for bonding to the front metal plate 2 and a non-bonding region around the bonding region. The non-bonding region extends, for example, from the contour of the bonding region to the end of the surface 1a. When a plurality of front metal plates 2 are bonded to the ceramic substrate 1, the non-bonding region includes the region between the plurality of front metal plates 2. The surface 1b may have a bonding region for receiving the back metal plate 3 and a non-bonding region around the bonding region. The non-bonding region extends, for example, from the contour of the bonding region to the end of the surface 1b. When a plurality of back metal plates 3 are bonded to the ceramic substrate 1, the non-bonding region includes the region between the plurality of back metal plates 3.

[0018] The thickness of the ceramic substrate 1 is preferably 1.0 mm or less. If the thickness of the ceramic substrate 1 exceeds 1.0 mm, it becomes difficult to realize a predetermined undulation curve. By making the thickness of the ceramic substrate 1 thinner, it becomes easier to realize a predetermined undulation curve.

[0019] The three-point bending strength of the ceramic substrate 1 is preferably 500 MPa or more. By using a ceramic substrate having a three-point bending strength of 500 MPa or more, the substrate thickness can be reduced to 0.4 mm or less.

[0020] Examples of ceramic substrates having a three-point bending strength of 500 MPa or more include silicon nitride substrates. Examples of the ceramic substrates include strengthened aluminum nitride substrates, strengthened alumina substrates, and strengthened zirconia-containing alumina substrates.

[0021] As the ceramic substrate, a silicon nitride substrate is particularly preferable. The three-point bending strength of ordinary aluminum nitride substrates and alumina substrates is about 300 to 450 MPa. In the case of substrates with a strength of less than 500 MPa, the TCT characteristics decrease when the thickness is reduced to 0.4 mm or less. As the silicon nitride substrate, there are high-strength silicon nitride substrates with a three-point bending strength of 500 MPa or more, and even 600 MPa or more.

[0022] Silicon nitride substrates can have a thermal conductivity of 50 W / m·K or more, or even 80 W / m·K or more. In recent years, silicon nitride substrates that combine high strength and high thermal conductivity have become available. Silicon nitride substrates with a three-point bending strength of 500 MPa or more and a thermal conductivity of 80 W / m·K or more can be made as thin as 0.33 mm or less. Three-point bending strength is measured in accordance with JIS-R-1601, and thermal conductivity is measured in accordance with JIS-R-1611.

[0023] The front metal plate 2 is joined to region 11. The back metal plate 3 is joined to region 13. That is, region 11 indicates the region joined to the front metal plate 2 on face 1a. Region 13 indicates the region joined to the back metal plate 3 on face 1b.

[0024] The front metal plate 2 and the back metal plate 3 preferably contain copper, aluminum, or an alloy mainly composed of these. These metal plates have low electrical resistance and are therefore easy to use in circuits. In addition, copper and aluminum both have high thermal conductivity, 398 W / m K and 237 W / m K, respectively. This allows for improved heat dissipation.

[0025] Taking advantage of these characteristics, it is preferable to set the thickness of the metal plates (front metal plate 2 and back metal plate 3) to 0.6 mm or more, and even 0.8 mm or more. By making the metal plates thicker, it is possible to ensure both current carrying capacity and improved heat dissipation. There is no particular upper limit to the thickness of the metal plates, but it is preferably 5 mm or less, and more preferably 3 mm or less. If the thickness exceeds 5 mm, the semiconductor module will become larger when lead terminals are attached.

[0026] Examples of methods for joining the ceramic substrate and the metal plate include a method of joining them via a brazing material and a method of directly joining them without using a brazing material. Note that a structure using a brazing material is preferable, and Fig. 2 shows a case where the metal plates 2 and 3 are joined to the ceramic substrate 1 via a brazing material layer 4. In this case, a known brazing material can be used for the brazing material layer 4.

[0027] The brazing material used here may be a known brazing material layer used in this type of ceramic circuit board, and is generally composed of a paste containing brazing material powder and an organic binder. The brazing material powder used here may be, for example, a brazing material powder containing silver, copper, etc. in a predetermined composition, and various organic resins may be used as the organic binder.

[0028] Moreover, the brazing material preferably contains an active metal. The active metal preferably contains at least one element selected from Ti (titanium), Zr (zirconium), Hf (hafnium), Nb (niobium), and Al (aluminum). For example, an Ag-Cu-based active brazing material, which is mainly composed of Ag and Cu as a eutectic composition and has active metals such as Ti, Zr, and Hf added thereto, is preferable in that it can provide high strength and high sealing properties. Furthermore, from the viewpoint of the bonding strength between the ceramic substrate and the metal plate, a ternary Ag-Cu-In-based active brazing material, in which In is added to the Ag-Cu-based active brazing material, is more preferable. It is preferable to use this brazing material in a paste form for bonding.

[0029] Here, the undulation curve of the region measured along a first side of surface 1a extending in the first direction (first undulation curve) is linear and has no extreme values. That is, the number of extreme values ​​of the undulation curve is 0. In contrast, the undulation curve of the region measured along a second side of surface 1a extending in the second direction (second undulation curve) has one extreme value. That is, the number of extreme values ​​of the undulation curve is 1. Each extreme value is a maximum value or a minimum value.

[0030] When the surface 1a is a rectangular surface, it is preferable that one of the undulation curves along the long side and the short side of the surface 1a is linear and the other is approximately arc-shaped. The undulation curves along the long side and the short side of the surface 1a are linear and arc-shaped, respectively. The undulation curves along the long side and the short side of the surface 1a are linear and arc-shaped, respectively. The undulation curves along the short side and the long ... long side and the short side of the surface 1a are linear and arc-shaped, respectively. The undulation

[0031] The waviness profile is measured according to JIS-B0601. The waviness profile is a profile curve obtained by sequentially applying profile filters with cutoff values ​​λf and λc to the cross-sectional curve, and is different from the roughness curve. Examples of the measurement method include a laser method and a contact method. The waviness profile along the long side of the surface 1a is measured. It is preferable to measure the long side at once. If it is not possible to measure at once due to the size of the substrate or the size of the device, the measurement is made in several steps to obtain the measurement value of one waviness curve. The same is true for the short side. Furthermore, the same is true when measuring the waviness curve for the surface 1b. The measurement point of the waviness curve is within a range of 0.5 to 1 mm from the end of the ceramic substrate 1.

[0032] One of the undulation curves along the long side of surface 1a and the undulation curves along the short side is linear, and the other is arc-shaped. Figures 3 and 4 are diagrams for explaining the undulation curves WP. Figure 3 is a diagram for explaining the linear undulation curve WP, and Figure 4 is a diagram for explaining the linear or approximately arc-shaped undulation curve WP.

[0033] A straight line refers to a shape that has no substantial curves and no extreme points between the start and end of the undulation curve. In other words, the number of extreme points is 0. However, while a typical straight line as shown in FIG. 3 is of course included, in reality, there are almost no cases where the shape is completely straight, and there may be some undulation as shown in FIG. 4.

[0034] The term "approximately arc-shaped" refers to a shape in which the swell curve WP rises in a curved manner, and then curves down after passing the maximum point (highest point), as shown in FIG. 4. In this case, there is only one extreme point. The extreme point may be on either the front side or the back side. When the extreme point faces the back side, the shape is one in which the curve falls in a curved manner, and then curves up after passing the minimum point (lowest point). The beginning and end parts of the swell curve WP may be in a wide shape, like the foot of Mount Fuji. In other words, the term "approximately arc-shaped" refers to a shape in which the swell curve WP has one extreme point.

[0035] When the surface 1a is a rectangular surface, the surface 1a has two long sides that face each other. Of the two first wavy curves measured along the two long sides, at least one of the first wavy curves preferably extends linearly, and more preferably the two first wavy curves extend linearly. The two first wavy curves measured along the two opposing long sides only need to extend linearly, and the shapes of the two first wavy curves do not have to completely match each other.

[0036] Of the two second undulation curves, at least one of the second undulation curves preferably extends in a substantially arc-like shape, and more preferably the two second undulation curves extend in a substantially arc-like shape. The two second undulation curves only need to extend in a substantially arc-like shape, and the shapes do not have to completely match each other.

[0037] Fig. 3 shows a schematic diagram of an example in which the first swell curve WP is a straight line. Fig. 3 shows a perfect straight line, but in reality, as described above, there may be some swell, and the swell may be so small that there is no point that can be evaluated as a maximum or minimum value. For example, when the difference Z (Z1) between the maximum and minimum values ​​in Fig. 4 is 20 μm or less, it is included in the straight line as there is no point that can be evaluated as a maximum or minimum value. In this case, this difference Z (Z1) is preferably 10 μm or less, more preferably 5 μm or less, even more preferably 3 μm or less, and particularly preferably 1 μm or less.

[0038] FIG. 4 is a schematic diagram of an example in which the second swell curve WP is arc-shaped, and illustrates the difference Z between the maximum and minimum values ​​of the swell curve WP (a difference Z of 20 μm or less is not included as an arc-shaped curve). The difference Z (Z2) between the maximum and minimum values ​​of the swell curve WP in FIG. 4 is preferably 30 μm or more. The maximum value is the largest value among the uppermost points of the swell curve WP. The minimum value is the smallest value among the lowermost points of the swell curve WP. The difference Z (Z2) between the maximum and minimum values ​​of the swell curve WP is preferably 30 μm or more, and more preferably 50 μm or more.

[0039] By increasing the difference Z between the maximum and minimum values ​​of the undulation curve WP, the shape direction of the ceramic circuit board 10 can be unified, and handling in the subsequent manufacturing process becomes easier. The maximum value of the difference Z (Z2) between the maximum and minimum values ​​of the undulation curve WP is not particularly limited, but is preferably 400 μm or less. If it exceeds 400 μm, the undulation of the ceramic substrate 1 is so large that the undulation remains even after bonding the lead terminals, which may reduce mountability.

[0040] In addition, the ratio (Z2 / Z1) between the difference (Z1) between the maximum and minimum values ​​of the first undulation curve and the difference (Z2) between the maximum and minimum values ​​of the second undulation curve is preferably 2.0 or more. By making this ratio 2.0 or more, the characteristics of the shape direction become clear, and the ease of handling can be improved.

[0041] The ratio of the thickness of the ceramic substrate 1 to the thickness of the metal plate [thickness of the ceramic substrate 1 / thickness of the metal plate] is preferably 1.5 or less. When multiple metal plates including the front metal plate 2 and the back metal plate 3 are joined to the ceramic substrate 1, the above ratio is calculated using the thickness of the thickest metal plate among them. In other words, the thickness of the thickest metal plate is preferably 1.5 times or less the thickness of the ceramic substrate 1.

[0042] The ratio of the thickness of the ceramic substrate 1 to the thickness of the metal plate is preferably 1.5 or less, and more preferably 0.5 or less. A ratio of the thickness of the ceramic substrate 1 to the thickness of the metal plate of 0.5 or less indicates that the ceramic substrate 1 is made thin or the metal plate is made thick. This makes it easier to control the waviness curve.

[0043] The ceramic substrate 1 is rectangular, and the ratio of the long side length L11 to the short side length L12 of the ceramic substrate 1 preferably satisfies (L11 / L12) ≧ 1.25. That is, L11 is preferably 1.25 times or more as large as L12. The longer the long side of the substrate, the easier it is to impart a desired waviness.

[0044] In addition, it is also effective to adjust the arrangement of the metal plates in order to control the first undulation curve on the long side of the ceramic substrate 1 and the second undulation curve on the short side of the ceramic substrate 1. Figures 5A and 5B are diagrams showing an example of the arrangement of the metal plates, and show the ceramic substrate 1 having the ceramic substrate 1, the front metal plate 2 consisting of the metal plate 2a and the metal plate 2b, and the rear metal plate 3. Figure 5A is a plan view of the ceramic substrate 1 as viewed from the front metal plate side, and Figure 5B is a bottom view of the ceramic substrate 1 as viewed from the rear metal plate side.

[0045] In this example, the metal plates 2a and 2b are arranged as metal plates of the same shape. The metal plates 2a and 2b are rectangular metal plates with lengths L21a and L21b. The metal plates 2a and 2b of such shapes are arranged side by side so that the distance between each other is a uniform distance L21c.

[0046] In this case, when the total length of the long sides of the metal plates is ΣMn, it is preferable that the ratio of ΣMn to L11 satisfies 0.50≦ΣMn / L11≦0.95. ΣMn is the total length of the metal plates closest to the long side surface of the ceramic substrate 1, and in FIG. 5A, it is L21a+L21a. n is determined by the number of target metal plates. In FIG. 5A, the metal plates 2a and 2b are closest to the long side surface of the ceramic substrate 1. Therefore, ΣMn is the sum of L21a+L21a. When the length of one metal plate is represented by M, it is preferable that M / L11≦0.5. Although the metal plates 2a and 2b are illustrated in FIG. 5, the number of metal plates may be three or more.

[0047] When the total length of the short side of the metal plate is ΣMm, it is preferable that the ratio of ΣMm to L21b satisfies 0.50≦ΣMm / L21b≦0.95. ΣMm is the total value of the lengths of the metal plates closest to the short side surface of the ceramic substrate 2. n is determined by the number of target metal plates. In FIG. 5, the metal plate 2a (or 2b) is closest to the short side surface of the ceramic substrate 1. Since both the metal plate 2a and the metal plate 2b have the same length L21b, ΣMm is L21b. In addition, FIG. 5 illustrates the case where there is one metal plate close to the short side surface, but when there are two or more metal plates, it is preferable that M / L12≧0.5, where M is the length of one metal plate.

[0048] The shape of the undulation curve can be controlled by controlling the ΣMn value, ΣMm value, and M value for the lengths L11 and L12 of the ceramic substrate 1. This makes it possible to make the shape of the undulation curve into a desired shape.

[0049] Taking the case shown in Figures 5A and 5B as an example, the ceramic substrate 1 has two metal plates (2a, 2b) provided on each of the short side surfaces, and these metal plates are provided independently. Therefore, on the long side surface, a gap is formed between the metal plates 2a and 2b. By providing this gap, it becomes easier to form the extreme value of the undulation curve. In other words, there is no gap on the short side, making it easier to form a linear undulation curve, and on the long side, there is a gap, making it easier to form an arc-shaped undulation curve.

[0050] In addition, it is preferable that the rear metal plate 3 of the ceramic substrate 1 is a single continuous metal plate on both the long side and the short side. In this case, it is preferable that the length L31 of the long side of the rear metal plate 3 and the length L11 of the long side of the ceramic substrate 1 satisfy 0.50≦L31 / L11≦0.95, and the length L32 of the short side of the rear metal plate 3 and the length L12 of the short side of the ceramic substrate 1 satisfy 0.50≦L32 / L12≦0.95.

[0051] The ceramic circuit board 10 is suitable for use as a ceramic circuit board with lead terminals. In the ceramic circuit board 10, lead terminals and a semiconductor element are provided so as to be connected to the front metal plate 2, and a heat dissipation member is provided so as to be connected to the back metal plate 3, thereby forming a semiconductor module.

[0052] The ceramic circuit board 10 can be made into a ceramic circuit board with lead terminals by bonding lead terminals to the front metal plate 2 of the ceramic circuit board 10. Here, the lead terminals function as external electrodes for providing electrical continuity to the outside. Examples of the shape of the lead terminals include a lead frame type and a lead pin type. The number of bonded lead terminals is not particularly limited. If necessary, the lead terminals may be used in combination with wire bonding.

[0053] The ceramic circuit board with lead terminals is suitable for a structure in which lead terminals having a thickness of 0.2 mm or more are joined to the front metal plate 2. The thickness of the lead terminal is preferably 0.2 mm or more, and more preferably 0.4 mm or more. The upper limit of the thickness of the lead terminal is not particularly limited, but is preferably 3 mm or less. The thickness of the lead terminal is defined by the thickness of the lead frame plate in the case of a lead frame type, and by the width of the lead pin in the case of a lead pin type. The lead terminal preferably contains a material with good conductivity, such as copper, aluminum, or an alloy thereof. Examples of the joining method between the front metal plate 2 and the lead terminal include solder joining, brazing material joining, ultrasonic joining, and pressure joining. By making the lead terminal thicker, the current carrying capacity can be increased and the inductance can be reduced. This makes it possible to take advantage of the advantages of semiconductor elements with improved performance.

[0054] A semiconductor module can be formed by mounting a semiconductor element on this ceramic circuit board with lead terminals and providing a heat dissipation member. The semiconductor element is bonded to the front metal plate 2. The number of semiconductor elements mounted and the mounting locations on the front metal plate 2 are arbitrary.

[0055] The heat dissipation member is a member for mounting the rear metal plate 3. The heat dissipation member is provided on the rear metal plate 3. Examples of the heat dissipation member include a heat sink, a base plate, and a cooling fin. The heat dissipation member may have various shapes such as a plate shape, a comb shape, a groove shape, and a pin shape. The heat dissipation member is made of a material with good heat dissipation properties such as Cu, Al, or AlSiC. Between the heat dissipation member and the back metal plate 3, an intervening layer such as solder or grease may be provided.

[0056] Resin sealing may be performed as necessary. The resin sealing process may be performed using a method such as a transfer molding method. Since the semiconductor module according to the embodiment has thick lead terminals bonded thereto, the lead terminals will not break during the resin sealing process. On the other hand, since the wires used in wire bonding are thin, breakage is likely to occur when using a method with high mold pressure such as a transfer molding method. In other words, by using thick lead terminals, a module structure that is easy to seal with resin can be achieved.

[0057] [Manufacturing method of ceramic circuit boards] Next, a method for manufacturing the ceramic circuit board will be described. As long as the ceramic circuit board has the above-mentioned configuration, the manufacturing method is not particularly limited, but the following method can be mentioned as a method for obtaining the ceramic circuit board with a good yield.

[0058] The method for manufacturing the ceramic circuit board 10 of this embodiment involves bringing metal plates into contact with the front and back surfaces of the ceramic substrate 1 via a brazing material containing an active metal such as titanium, raising the temperature to the melting point of the brazing material, and then bonding the two by lowering the temperature to room temperature, for example, at a rate of 5°C / min or less to prevent cracks from occurring.

[0059] The coating thickness of the active metal brazing paste is preferably 10 μm or more and 40 μm or less. The joining step is preferably carried out at a temperature of 600° C. or more and 950° C. or less. When the active metal contains at least one element selected from Ti, Zr, Hf, and Nb, the joining step is preferably carried out at a temperature of 800° C. or more and 950° C. or less. When the active metal contains Al, the joining step is preferably carried out at a temperature of 600° C. or more and 800° C. or less. The heating atmosphere is preferably a non-oxidizing atmosphere.

[0060] At this time, for example, metal plates are bonded to the front and back surfaces of the ceramic substrate 1 so as to cover most of the surface, and the bonded metal plates are etched to form a pattern on the front surface. For example, as shown in FIG. 1 and FIG. 2, the center of one metal plate is etched to form a state in which two front metal plates 2 are bonded, and one metal plate is bonded as the back metal plate 3. By this, the arrangement of the metal plates on the front and back surfaces is made different, so that the warp can be provided in a desired direction. In FIG. 1, the front metal plate 2 is provided with two independent metal plates, and there is a gap between them due to etching. Note that the front metal plate 2 may be two independent metal plates bonded to the front surface of the ceramic substrate 1. On the other hand, the back metal plate 3 is provided with one metal plate on the back surface.

[0061] Therefore, the area of ​​the metal plate is larger on the back side, and the degree of shrinkage on the back side is greater during thermal shrinkage, but here, for example, a gap is provided between the front metal plates 2 near the center in the long side direction, so the ceramic substrate 1 is pulled to the back side (back metal plate 3 side) and the front side (front metal plate 2 side) becomes convex. Also, in the short side direction, the front metal plate 2 and back metal plate 3 are both formed continuously with about the same length, so the degree of shrinkage does not change much and flatness is easily maintained along the short side.

[0062] By adopting such a relationship, it is possible to obtain a ceramic circuit board 10 that is warped in a desired direction. Furthermore, the front metal plate 2 can be etched to form a circuit pattern.

[0063] As another manufacturing method, for example, the thickness of the brazing material layer 4 that joins the metal plates on the front and back surfaces can be changed to warp in a desired direction, making the first undulation curve linear and the second undulation curve arc-shaped. For example, by changing the thickness of the brazing material used, it is possible to warp in a desired direction by utilizing the difference in the thermal shrinkage rate between the front and back surfaces.

[0064] Specifically, by making the thickness of the brazing material on the front surface thicker and the thickness of the brazing material on the back surface thinner, the degree of shrinkage on the thick side of the brazing material is higher, and thus the warping can be achieved. At this time, the desired shape can be achieved by balancing the length of the long and short sides of the ceramic substrate 1 with the thickness of the brazing material. In this case, for example, the brazing material layer 4 for joining the front metal plate 2 to the ceramic substrate 1 can be 10 to 30 μm, and the brazing material layer 4 for joining the back metal plate 3 can be 10 to 30 μm, with the difference in thickness being 5 to 20 μm.

[0065] Although the above describes a method for joining a ceramic substrate and a metal plate using a brazing material layer, a direct joining method without using a brazing material layer may also be used. In this case, when the metal plate is a copper plate, a direct joining method using a eutectic of copper and oxygen can be used. When the ceramic substrate is a nitride, an oxide film is formed on the surface. When the metal plate is an Al plate, an AlSi alloy plate is used for direct joining.

[0066] As described above, by carrying out the above steps, the undulation curve of the ceramic substrate can be controlled; for example, the undulation curve of the long side or short side of the ceramic substrate can be made linear and the other side can be made approximately arc-shaped.

[0067] The ceramic circuit board 10 thus obtained can be used to bond lead terminals by a known method to form a ceramic circuit board with lead terminals, and a semiconductor element can be mounted on the ceramic circuit board to form a semiconductor module. At this time, the ceramic circuit board with lead terminals can also be mounted on a heat dissipation member. EXAMPLES

[0068] Example 1 A silicon nitride substrate with a thermal conductivity of 90 W / m K and a three-point bending strength of 700 MPa was prepared. The silicon nitride substrate had a long side length of 47.3 mm, a short side length of 23.6 mm, and a thickness of 0.32 mm. Copper plates were prepared as the front and back metal plates. The front metal plate was oxygen-free copper and had a total area of ​​405 mm2. 2 The back metal plate is made of oxygen-free copper and has a total area of ​​880 mm. 2 , and a thickness of 1.2 mm. The silicon nitride substrate and the copper plate were joined using an active metal brazing material. The active metal brazing material was a paste in which 0.3 parts by mass of TiH2 was added to an alloy powder (total of 100 parts by mass) consisting of 70% by mass of Ag, 3% by mass of In, and 27% by mass of Cu, and further an organic solvent and a binder component were added. After the active metal brazing material paste was dried, copper plates were placed on both sides of the silicon nitride substrate, and the substrate was heated at 800°C while being pressurized in a vacuum, and the copper plates were joined to the silicon nitride substrate. At this time, five ceramic circuit substrates (samples 1 to 5) were manufactured under the same conditions.

[0069] Next, the swell curves of the short sides (first swell curves) and the long sides (second swell curves) of the ceramic circuit boards (samples 1 to 5) according to the examples were measured. The swell curves were measured by laser scanning a portion 1 mm inward from the edge of the ceramic board. The measurement results were converted into swell curves in accordance with JIS-B0601. The difference Z (Z1 and Z2) between the maximum and minimum values ​​of the swell curves was also measured for the first swell curve and the second swell curve obtained at this time. The results are shown in Table 1 and Figures 6 to 10.

[0070] [Table 1]

[0071] From the above results, it was found that the present embodiment provided a ceramic circuit board having a desired waviness curve. By providing a ceramic circuit board having a desired warpage tendency in this way, it was found that the handling of the substrate during the manufacture of semiconductor devices can be facilitated and the manufacturing yield can be improved.

[0072] The invention made by the inventor has been specifically described above based on the embodiment thereof. However, it goes without saying that the present invention is not limited to the above embodiment, and various modifications can be made without departing from the spirit of the invention. [Explanation of symbols]

[0073] 1. Ceramic substrate 1a,1b side 2 Front metal plate 3 Back metal plate 4 Brazing layer 10 Ceramic circuit board WP waviness curve

Claims

1. A ceramic substrate having a first surface and a second surface, the first surface including a first region and a second region; a first metal plate joined to the first region; a second metal plate joined to the second surface, the first surface extends in a first direction and a second direction intersecting the first direction, and a first undulation curve of the second region measured along a first side of the first surface extending in the first direction is linear; a second undulation curve of the second region measured along a second side of the first surface extending in the second direction is arcuate; Ceramic circuit board.

2. The ceramic substrate is rectangular, and both of the first side and the second side are 100 mm or more.

2. The ceramic circuit board according to claim 1.

3. The second side of the ceramic substrate is longer than the first side.

2. The ceramic circuit board according to claim 1.

4. The ceramic substrate has a first side longer than a second side.

2. The ceramic circuit board according to claim 1.

5. The difference (Z1) between the maximum value and the minimum value of the first wavy curve is 20 μm or less, The difference (Z2) between the maximum value and the minimum value of the second wavy curve is 30 μm or more.

4. The ceramic circuit board according to claim 3.

6. The Z2 / Z1 ratio is 2.0 or more.

6. The ceramic circuit board according to claim 5.

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