Insulated Circuit Board

Abstract

To prevent cracking of ceramic substrates in insulated circuit boards consisting of a circuit layer made of copper or copper alloy bonded to a ceramic substrate and a heat sink.SOLUTION: An insulated circuit board 10 consists of a rectangular ceramic substrate 20 in plan view with a circuit layer 30 made of copper or copper alloy bonded to one side and a heat sink 40 made of copper or copper alloy bonded to the other side, and the heat sink 40 has non-bonded areas 51 between the side edges at least at the four corners of the ceramic substrate 20 and the heat sink 40.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to an insulating circuit board such as a power module substrate used in a semiconductor device that controls a large current and a high voltage. [Background technology]

[0002] A known insulated circuit board is a power module substrate, which has a circuit layer formed on one side of an insulating substrate made of a ceramic substrate such as aluminum nitride or silicon nitride, and a metal layer formed on the other side. A heat sink for heat dissipation is bonded to the metal layer of this insulated circuit board. In order to reduce the thermal resistance of such insulating circuit boards, attempts have been made to omit the metal layer and bond a heat sink directly to the ceramic substrate.

[0003] For example, Patent Document 1 discloses a fin-integrated substrate in which a metal circuit plate made of aluminum or an aluminum alloy is bonded to one surface of a ceramic substrate, and one surface of a flat metal base plate made of aluminum or an aluminum alloy is bonded to the other surface, and a plurality of heat dissipation fins are integrally formed with the metal base plate so as to protrude from the other surface of the metal base plate and be arranged at predetermined intervals from each other.

[0004] Furthermore, Patent Document 2 discloses a heatsink-integrated insulating circuit board that includes a heatsink having a top plate portion and heat dissipation fins, an insulating resin layer formed on the top plate portion of the heatsink, and a circuit layer made of metal pieces arranged in a circuit pattern on the surface of the insulating resin layer opposite the heatsink. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2011-166122 [Patent Document 2] Japanese Patent Publication No. 2021-145094 Summary of the Invention [Problem to be solved by the invention]

[0006] However, when this insulating circuit board is heated to solder a semiconductor element, thermal stress occurs between the ceramic substrate and the circuit layer, and between the ceramic substrate and the heat sink due to the difference in thermal expansion coefficients between them. In particular, when the circuit layer and the heat sink are made of copper or a copper alloy, the thermal stress is greater than when made of aluminum, and there is a risk of cracking the ceramic substrate.

[0007] The present invention has been made in view of the above circumstances, and has an object to prevent cracks from occurring in a ceramic substrate in an insulated circuit board formed by joining a circuit layer made of copper or a copper alloy and a heat sink to the ceramic substrate. [Means for solving the problem]

[0008] The insulated circuit board of the present invention comprises a ceramic substrate having a rectangular shape in plan view, a circuit layer made of copper or a copper alloy bonded to one surface thereof, and a heat sink made of copper or a copper alloy bonded to the other surface thereof, and the heat sink has non-bonded portions between the side edges of at least four corners of the ceramic substrate and the heat sink.

[0009] When an insulating circuit board is heated, thermal stress is particularly large at the four corners of the ceramic substrate. In the present invention, the side edges at at least the four corners of the ceramic substrate are non-bonded, so that the thermal stress at the four corners of the ceramic substrate can be effectively reduced and cracks can be prevented.

[0010] In the insulating circuit board of the present invention, the non-bonded portion may be formed in a ring shape along the circumferential direction of the ceramic substrate, and the non-bonded portion may be between each side edge of the ceramic substrate and the heat sink.

[0011] By forming non-bonded portions not only at the four corners of the ceramic substrate but also along the entire periphery, thermal stress at the side edge portions of the ceramic substrate can be reduced, making it less likely to crack.

[0012] In the insulating circuit board of the present invention, the non-bonded portion may be configured as a recess formed in the heat sink.

[0013] The non-bonded portion need only be such that the ceramic substrate and the heat sink are not bonded, and it does not matter if they are in contact with each other; however, by forming a recess in the heat sink, the non-bonded portion can be formed reliably, and there is no influence of frictional force, which increases the stress reduction effect.

[0014] In the insulating circuit board of the present invention, the circuit layer is bonded from the side edge of the ceramic substrate to the inside, and in a cross section in the lamination direction, if the distance from the side edge of the ceramic substrate to the side edge of the circuit board is A mm and the width of the non-bonded portion from the side edge of the ceramic substrate is L mm, then 0 <L<2Aであるとよい。

[0015] If there is even a small non-bonded portion directly below the side edge of the ceramic substrate, it is possible to reduce the stress on the ceramic substrate. However, if the width L of the non-bonded portion is 2A or more, the width of the non-bonded portion becomes large and the bonded portion of the heat sink becomes small, which may result in a decrease in heat dissipation characteristics.

[0016] In the insulating circuit board of the present invention, when the thickness of the top plate portion of the heat sink joined to the ceramic substrate is B mm and the depth of the recess is t mm, 0 <t<Bであるとよい。 If t is equal to or greater than B, the heat sink will be penetrated, which may result in a decrease in heat dissipation characteristics. [Effects of the Invention]

[0017] According to the present invention, the portions between the side edges of the ceramic substrate and the heat sink at least at the four corners are non-bonded, thereby reducing thermal stress at the four corners of the ceramic substrate, where thermal stress is most likely to concentrate, and suppressing the occurrence of cracks. [Brief explanation of the drawings]

[0018] [Figure 1] 1 is a cross-sectional view showing an insulating circuit board according to a first embodiment of the present invention. [Figure 2] FIG. 2 is a top view of the insulating circuit board according to the embodiment. [Figure 3] FIG. 2 is a top view of a heat sink in the insulating circuit board according to the embodiment. [Figure 4] FIG. 2 is an enlarged cross-sectional view of a main part of the insulating circuit board according to the embodiment. [Figure 5] FIG. 10 is a top view of a heat sink in an insulating circuit board according to a second embodiment of the present invention. [Figure 6] FIG. 10 is a cross-sectional view of an insulating circuit board according to a third embodiment of the present invention. [Figure 7] FIG. 10 is a cross-sectional view of an insulating circuit board according to a fourth embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0019] Hereinafter, an embodiment of an insulating circuit board according to the present invention will be described. 1 to 4 show an insulating circuit board 10 of the first embodiment. As shown in Figures 1 and 2, the insulating circuit board 10 of the first embodiment includes a ceramic substrate 20, a circuit layer 30 bonded to one surface of the ceramic substrate 20, and a heat sink 40 bonded to the other surface of the ceramic substrate 20.

[0020] The ceramic substrate 20 is an insulating substrate that prevents electrical connection between the circuit layer 30 and the heat sink 40, and is made of nitride ceramics such as AlN (aluminum nitride) or Si3N4 (silicon nitride), or oxide ceramics such as Al2O3 (alumina), with Si3N4 being preferred due to its high yield strength. The ceramic substrate 20 has a thickness of 0.3 mm to 1.0 mm. The ceramic substrate 20 is formed into a rectangular plate shape (square in the illustrated example) in plan view, and is slightly larger than the circuit layer 30.

[0021] The circuit layer 30 is made of copper such as oxygen-free copper or a copper alloy such as a zirconium-added copper alloy. The circuit layer 30 has a thickness of, for example, 0.3 mm to 1.6 mm. The circuit layer 30 is formed in a rectangular plate shape (square in the illustrated example) in a plan view, and is formed slightly smaller than the ceramic substrate 20.

[0022] Like the circuit layer 30, the heat sink 40 is made of copper such as oxygen-free copper or a copper alloy such as a zirconium-added copper alloy. The heat sink 40 is integrally formed with a flat top plate 41 and a plurality of fins 42 protruding from one side of the top plate 41. The top plate 41 is formed in a rectangular shape (square in the illustrated example) in a plan view, and has a thickness of 0.3 mm to 1.6 mm. In the illustrated example, the fins 42 are formed in a plate shape and are provided parallel to each other and at equal intervals along one side of the top plate 41.

[0023] Therefore, the heat sink 40 is formed by extrusion molding of copper or a copper alloy, and flow paths 42a for circulating a coolant such as water or air are formed between the fin portions 42. Note that in this embodiment, the fin portions 42 are formed in a plate shape, but this is not limiting, and the fin portions may be pin-shaped, or the heat sink may have only a top plate portion 41 without fin portions. Furthermore, the heat sink may be formed in a hollow shape, and flow paths for circulating a coolant may be formed inside.

[0024] And the central portion 43 of the top plate portion 41 of this heat sink 40 is joined to the ceramic substrate 20 (this portion is referred to as the joint portion 50), and a groove-shaped recess 44 is formed on the surface of the ceramic substrate 20 on the side where it is joined so as to avoid joining with the peripheral edge portion of the ceramic substrate 20. As shown in FIG. 3, this recess 44 is formed in an annular rectangular frame shape along the peripheral edge of the ceramic substrate 20. Further, its cross section (the cross section in the direction orthogonal to the length direction of the groove, which is the longitudinal section of the heat sink) is formed in a rectangular shape, and the side edge 20a of the ceramic substrate 20 is disposed directly above the recess 44.

[0025] Therefore, the side edge portion of the ceramic substrate 20 disposed on the recess 44 is a non-joined portion 51 that is not joined to the heat sink 40. The non-joined portion 51 is formed with a predetermined width including the side edge position of the ceramic substrate 20. Also, a flange portion 45 is formed outside the recess 44, and the fin portions 42 are evenly arranged including this flange portion 45. In other words, in this embodiment, the top plate portion 41 of the heat sink 40 is composed of a central portion 43 joined to the ceramic substrate 20, a recess 44 for forming a non-joined portion 51 outside the central portion 43, and a flange portion 45 that projects outside the recess 44 and also outside the ceramic substrate 20.

[0026] As shown in FIG. 4, in the cross section in the stacking direction of the recess 44, when the distance from the side edge 20a of the ceramic substrate 20 to the side edge of the circuit layer 30 is A mm and the width of the non-joined portion 51 from the side edge 20a of the ceramic substrate 20 (in other words, the distance from the side edge 20a of the ceramic substrate 20 to the inner peripheral edge of the recess 44) is L mm, it is set to 0 < L < 2A. The position of the outer peripheral edge of the recess 44 only needs to be outside the side edge 20a of the ceramic substrate 20, and the outer peripheral edge of the recess 44 does not need to contact the side edge 20a of the ceramic substrate 20. In this case, the width L of the non-joined portion 51 is the distance from the side edge 20a of the ceramic substrate 20, and in the case of the ceramic substrate 20 having a rectangular shape in plan view, at the four corner portions, it is the distance measured on the diagonal line. In addition, when the thickness of the central portion 43 of the top plate portion 41 of the heat sink 40 joined to the ceramic substrate 20 is B mm and the depth of the recessed portion 44 is t mm, 0 <t<Bであるとよい。 In Figure 1, the thickness of the top plate portion 41 is set to the same dimension for the central portion 43 of the ceramic substrate 20 and the flange portion 45 located outside the recess 44, except for the area where the recess 44 is formed. However, this flange portion 45 may be made thinner than the central portion 43, or may be made thicker than the central portion 43 as long as it does not come into contact with the ceramic substrate 20.

[0027] The insulating circuit board 10 configured in this manner is manufactured by first preparing the ceramic substrate 20, the metal plate for the circuit layer (not shown), and the heat sink 40, then stacking the metal plate for the circuit layer on one side of the ceramic substrate 20 and the top plate portion 43 of the heat sink 40 on the other side via a brazing material, and then sandwiching the stacked body between a pressure device and heating it while applying pressure in the stacking direction, thereby joining the metal plate for the circuit layer and the heat sink 40 to the ceramic substrate 20. For example, an Ag-Cu-Ti based brazing filler metal is used as the brazing filler metal for joining the circuit layer 30 and the heat sink 40 to the ceramic substrate 20. Furthermore, the pressure in the stacking direction during joining is preferably 0.1 MPa to 8 MPa, and the heating temperature is preferably 800°C to 930°C.

[0028] In this insulating circuit board 10, various semiconductor elements (not shown), such as IGBTs (Insulated Gate Bipolar Transistors), MOSFETs (Metal Oxide Semiconductor Field Effect Transistors), and FWDs (Free Wheeling Diodes), are soldered onto the circuit layer 30. During the soldering process, the substrate is heated to, for example, 290°C for solder reflow.

[0029] In this insulating circuit board 10, the heat sink 40 is directly bonded to the ceramic substrate 20, so that the thermal resistance is small and the heat generated by the semiconductor elements on the circuit layer 30 can be quickly transferred to the heat sink 40 and dissipated. Furthermore, when the semiconductor elements are bonded or in the subsequent use environment, they may be exposed to high heat, causing thermal expansion and contraction in the circuit layer 30, the ceramic substrate 20, and the heat sink 40. In this case, since the circuit layer 30 and the heat sink 40 are made of copper or a copper alloy, thermal stress occurs on both sides of the ceramic substrate 20 due to the difference in thermal expansion coefficient between them.

[0030] Here, the heat sink 40 generally has a planar shape larger than the ceramic substrate 20 in order to increase the heat dissipation area. On the other hand, the circuit layer 30 is formed to have a planar shape smaller than the ceramic substrate 20 in order to ensure a creepage distance. For this reason, in a conventional structure in which the heat sink 40 is bonded to the entire surface of the ceramic substrate 20, the heat sink 40 is bonded only to one surface of the outside of the circuit layer 30, i.e., the peripheral portion of the ceramic substrate 20, and nothing is bonded to the other surface, which causes stress concentration during heating and causes the ceramic substrate 20 to crack.

[0031] In this embodiment, by providing a recess 44 in the heat sink 40 and positioning the side edge 20a of the ceramic substrate 20 over the recess 44, the vicinity of the side edge 20a of the ceramic substrate 20 is left free and unconstrained, preventing stress concentration at the periphery of the ceramic substrate 20 at high temperatures and suppressing the occurrence of cracks. This prevents damage to the ceramic substrate 20 and improves the reliability of the insulating circuit board 10.

[0032] In this case, in the cross-section in the stacking direction, regarding the distance Amm from the side edge 20a of the ceramic substrate 20 to the side edge of the circuit layer 30 and the width Lmm from the side edge 20a of the ceramic substrate 20 of the non-bonded portion 51, since 0 < L < 2A, cracking of the ceramic substrate 20 can be more reliably prevented. When L is 2A or more, the width of the non-bonded portion 51 becomes large and the bonded portion (central portion 43) 50 of the heat sink 40 becomes small, so there is a risk that the heat dissipation characteristics will deteriorate. Also, regarding the thickness Bmm of the top plate portion 41 bonded to the ceramic substrate 20 in the heat sink 40 and the depth tmm of the recess 44, 0 < t < B is preferable. When t is B or more, the recess 44 may penetrate the heat sink 40, and there is a risk that a problem of deterioration of the heat dissipation characteristics will occur.

[0033] FIG. 5 shows the heat sink 400 in the insulating circuit board of the second embodiment. The ceramic substrate 20 and the circuit layer (not shown) are the same as in the case of the first embodiment. In this heat sink 400, recesses 440 are formed only at the four corners of the ceramic substrate 20 having a rectangular shape in plan view. When heated when mounting the aforementioned semiconductor element on the circuit layer, stress particularly concentrates on the four corners of the ceramic substrate 20, and higher stress remains at the four corners than in other portions even after cooling. Therefore, in order to relieve the stress generated at the four corners of this ceramic substrate 20, the recesses 440 of the heat sink 400 are formed near the positions where the four corners of the ceramic substrate 20 are arranged. Therefore, the recesses 440 are formed at four locations. As a result, most of the four sides of the ceramic substrate 20 are bonded to the heat sink 400, so the heat dissipation characteristics are also excellent. Note that 0 < L < 2A and 0 < t < B described in the first embodiment are also applicable to the second embodiment. As described above, at the four corners of the ceramic substrate 20, L is the distance measured on the diagonal line. Also, the central portion 430 of the heat sink 400 is set to the same thickness continuously to the flange portion 450 except for the four corners.

[0034] Also, the present invention may be in the form shown in FIG. 6 or FIG. 7. FIG. 6 shows the insulating circuit board 100 of the third embodiment. In the insulating circuit board 100 of this embodiment, the recess 441 formed in the top plate portion 410 of the heat sink 401 is not rectangular in cross section as in the first embodiment, but is formed in a triangular shape. In this case, it is formed in a right-angled triangular shape, one side of which is arranged inside the side edge 20a of the ceramic substrate 20 and perpendicular to the surface of the ceramic substrate 20, the hypotenuse is formed obliquely upward from the lower end thereof, and the upper end of the hypotenuse is arranged outside the ceramic substrate 20. Therefore, the side edge 20a of the ceramic substrate 20 is arranged on the recess 441.

[0035] FIG. 7 shows the insulating circuit board 101 of the fourth embodiment. The recess 4 forty-two formed in the top plate portion 411 of the heat sink 402 is formed in a semi-arc shape in cross section, the inner peripheral edge of which is arranged inside the side edge 20a of the ceramic substrate 20, and the outer peripheral edge is arranged outside the side edge 20a of the ceramic substrate 20.

[0036] Also in these embodiments, although illustration is omitted, in the cross section in the stacking direction, regarding the distance Amm from the side edge 20a of the ceramic substrate 20 to the side edge 30a of the circuit board 30 and the width Lmm from the side edge 20a of the ceramic substrate 20 of the non-bonded portion 51, 0 < L < 2A, and regarding the thickness Bmm of the top plate portions 430, 431, 432 joined to the ceramic substrate 20 in the heat sinks 400, 401, 402 and the depth tmm of the deepest part of the recesses 440, 441, 442, 0 < t < B is set (see FIG. 4).

Example

[0037] Next, the effects of the present invention will be described using examples, but the present invention is not limited to the following examples.

[0038] ​The ceramic substrate, circuit layer, and heat sink were made of the materials and had the thicknesses shown in Table 1, and the stress on the ceramic substrate was calculated through simulation. The heat sink was a flat top plate only. All plane shapes were square, with the length of one side being 40 mm for the ceramic substrate, 37 mm for the circuit layer, and 50 mm for the heat sink.

[0039] In addition, for Examples 1 to 4, groove-shaped recesses were formed in the heat sink around the entire periphery of the ceramic substrate. The outer periphery of the recesses was a square with sides of 43 mm. For Example 5, recesses were formed only in the areas where the four corners of the ceramic substrate were located. The distance between the outer edges of two recesses located on one side of the ceramic substrate was 43 mm. The width L from the side edge of the ceramic substrate at the non-bonded portion and the depth t of the recesses were as shown in Table 1. In the comparative example, no such recess was formed, and one entire surface of the ceramic substrate was bonded to the heat sink.

[0040] Simulations were then performed on the stress applied to the ceramic substrate at room temperature (25°C), the stress applied to the ceramic substrate when heated to 290°C, which simulates solder reflow, and the stress applied to the ceramic substrate when cooled to room temperature (25°C) after heating. Table 2 shows the maximum stress values.

[0041] [Table 1]

[0042] [Table 2]

[0043] As can be seen from these tables, in Examples in which non-bonded portions with respect to the heat sink were formed around the entire periphery or at the four corners of the ceramic substrate, the stress on the ceramic substrate was small. Between Example 1, in which non-bonded portions were formed around the entire periphery of the ceramic substrate, and Example 5, in which non-bonded portions were formed only at the four corners, the stress in Example 5 was slightly higher. Furthermore, in Example 2, in which the circuit layer was thick at 1.5 mm, the stress when heated to 290°C, which simulates solder reflow when mounting a semiconductor element, was higher than in the other Examples. However, it is not expected that cracks would occur in any of the Examples. Since the ceramic substrate has a side of 40 mm and the circuit layer has a side of 37 mm, the distance A from the side edge of the ceramic substrate to the side edge of the circuit board is 1.5 mm, and the width L of the non-bonded portion from the side edge of the ceramic substrate is a maximum of 2.7 mm, satisfying L<2A. Also, the depth t of the recess relative to the thickness B of the heat sink is t <Bとなっている。

[0044] On the other hand, in the comparative example in which no non-bonded portion was formed between the ceramic substrate and the heat sink, the stress on the ceramic substrate was large, particularly at 25°C, which corresponds to the temperature after bonding, and there was concern that cracks might occur. [Explanation of symbols]

[0045] 10. Insulated circuit board 20 Ceramic substrate 20a side edge 30 circuit layers 40 Heatsink 41 Top plate 42 Fin section 42a Channel 43 Central part 44 Recess 45 flange 50 Joint 51 Non-joint part 100,101 Insulated circuit board 400,401,402 Heatsink 410,411 Top plate 430,431,432 central part 440,441,442 recess 450, 451, 452 Flange part

Claims

1. 1. An insulated circuit board comprising a ceramic substrate having a rectangular shape in plan view, a circuit layer made of copper or a copper alloy bonded to one surface thereof, and a heat sink made of copper or a copper alloy bonded to the other surface thereof, wherein the only non-bonded portions of the heat sink are between the side edges of the ceramic substrate at the four corners and the heat sink.

2. 2. The insulating circuit board according to claim 1, wherein the non-bonded portion is formed by a recess formed in the heat sink.

3. 3. The insulating circuit board according to claim 1, wherein the circuit layer is bonded from a side edge of the ceramic substrate to an inner side thereof, and in a cross section in the stacking direction, where A mm is the distance from the side edge of the ceramic substrate to the side edge of the circuit layer and L mm is the width of the non-bonded portion from the side edge of the ceramic substrate, the relationship 0<L<2A is satisfied.

4. 3. The insulating circuit board according to claim 2, wherein the thickness of the top plate portion of the heat sink joined to the ceramic substrate is B mm, and the depth of the recess is t mm, where 0<t<B.

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