Circuit board and method for manufacturing semiconductor module

Abstract

To provide a silicon nitride insulating heat dissipation circuit board with excellent solder joint reliability with a heat dissipation plate.SOLUTION: A circuit board for semiconductor chip mounting comprises a silicon nitride ceramic substrate consisting of rolled copper foil bonded to both main surfaces, and an electrolytic copper foil superimposed on the rolled copper foil at least on the bonding side with the heat dissipation plate.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to the construction of silicon nitride insulating heat dissipating circuit boards. [Background technology]

[0002] Widely known ceramic insulating heat dissipation circuit boards on which electronic components such as semiconductor chips are mounted include silicon nitride insulating heat dissipation circuit boards and alumina insulating heat dissipation circuit boards. Ceramic insulating heat dissipation circuit boards have the role of dissipating heat generated by the mounted electronic components to the outside, and also serve as an electrical connection between the electronic components and the outside.

[0003] A silicon nitride insulating heat dissipation circuit board is made by bonding copper foil (sometimes called copper plate, copper circuit plate, copper heat sink, etc.) whose main component is metallic copper to both sides of a silicon nitride ceramic substrate using a brazing material containing active metal, etc. Usually, a semiconductor chip is bonded (mounted) to one copper foil surface by silver sintering bonding, and a metal heat sink, for example, is soldered to the other copper foil surface.

[0004] Silicon nitride insulating heat dissipation circuit boards are often used for in-vehicle applications because they have superior heat dissipation properties and reliability compared to alumina-based insulating heat dissipation circuit boards using alumina-based ceramic substrates. In such cases, silver plating is often applied to the surface of the copper foil constituting the silicon nitride insulating heat dissipation circuit board in order to improve the bonding reliability of the silver sintered bonding between the semiconductor chip and the silicon nitride insulating heat dissipation circuit board. For example, a form in which silver plating is applied to the surface of a copper circuit board provided on one side of a silicon nitride insulating heat dissipation circuit board is already known (see, for example, Patent Document 1). [Prior art documents] [Patent documents]

[0005] [Patent Document 1] International Publication No. 2020 / 218193 Summary of the Invention [Problem to be solved by the invention]

[0006] Electroless silver plating is mainly used for silver plating on silicon nitride insulating heat dissipation circuit boards. Among them, displacement silver plating, which requires a short plating time, is often used. Reduction silver plating is not usually used because of its slow reaction rate.

[0007] In addition, silicon nitride insulating heat dissipation circuit boards often use copper foil with a thickness of 300 μm or more, and rolled copper foil is usually used for such copper foil.

[0008] However, there is a problem in that the plating speed (silver deposition speed) of displacement silver plating on the surface of such thick rolled copper foil is slower than the plating speed of displacement silver plating on the surface of electrolytic copper foil used in circuits such as printed circuit boards.

[0009] Moreover, when displacement silver plating is performed on the surface of rolled copper foil, due to the slow plating speed, while the silver plating film grows, the corrosion of the rolled copper foil progresses underneath, and voids are generated between the silver plating film and the rolled copper foil. If a heat sink is soldered to the silver-plated rolled copper foil surface in the presence of such voids, the silver plating film diffuses into the solder, but voids still remain between the solder layer and the rolled copper foil.

[0010] This means that in a silicon nitride insulating heat dissipation circuit board with silver-plated rolled copper foil, it is difficult to ensure sufficient bonding reliability when soldering a heat sink to the surface of the rolled copper foil, although the location, size, shape, and number of voids will differ before and after soldering.

[0011] It is possible to apply silver plating only to the rolled copper foil on which the semiconductor chip is mounted, out of the rolled copper foil provided on both sides of the silicon nitride insulating heat dissipation circuit board, but this is not realistic because the process becomes complicated.

[0012] The present invention has been made in view of the above problems, and has an object to provide a silicon nitride insulating heat dissipation circuit board having excellent reliability of solder joints with a heat sink. [Means for solving the problem]

[0013] In order to solve the above problems, a first aspect of the present invention is a circuit board for mounting a semiconductor chip, characterized in comprising a silicon nitride ceramic substrate having rolled copper foil bonded to both main surfaces thereof, and an electrolytic copper foil superimposed on the rolled copper foil at least on the bonding surface side with a heat sink.

[0014] A second aspect of the present invention is the circuit board according to the first aspect, characterized in that the copper particles constituting the rolled copper foil have an average particle size of 100 μm to 1000 μm, and the copper particles constituting the electrolytic copper foil have an average particle size of less than 100 μm.

[0015] A third aspect of the present invention is the circuit board according to the first or second aspect, characterized in that the rolled copper foil has a thickness of 300 μm to 2500 μm, and the electrolytic copper foil has a thickness of 5 μm to 50 μm.

[0016] A fourth aspect of the present invention is the circuit board according to any one of the first to third aspects, characterized in that the electrolytic copper foil is superimposed on both of the rolled copper foils.

[0017] A fifth aspect of the present invention is a circuit board for mounting a semiconductor chip, comprising a silicon nitride ceramic substrate having copper foil bonded to both main surfaces, the copper foil at least on the bonding surface side with a heat sink having a two-layer structure including a first portion including a bonding portion with the silicon nitride ceramic substrate and a second portion including a surface portion of the copper foil, and the copper particles in the second portion have an average particle size smaller than that of the copper particles in the first portion.

[0018] A sixth aspect of the present invention is a circuit board according to the fifth aspect, characterized in that the copper particles in the first portion have an average particle size of 100 μm to 1000 μm, and the copper particles in the second portion have an average particle size of less than 100 μm.

[0019] A seventh aspect of the present invention is a circuit board according to the fifth or sixth aspect, characterized in that the first portion has a thickness of 300 μm to 2500 μm, and the second portion has a thickness of 5 μm to 50 μm.

[0020] An eighth aspect of the present invention is a circuit board according to any one of the fifth to seventh aspects, characterized in that the copper foils bonded to both main surfaces each have the first portion and the second portion.

[0021] A ninth aspect of the present invention is a method for manufacturing a semiconductor module, comprising: a plating step of forming a silver plating film by displacement silver plating on a circuit board according to any one of the first to fourth aspects; a chip mounting step of mounting a semiconductor chip by silver sintering bonding on a main surface opposite to the bonding surface of the circuit board that has undergone the plating step; and a heat sink bonding step of bonding the heat sink by soldering to the bonding surface of the circuit board that has undergone the plating step, wherein in the heat sink bonding step, the heat sink is bonded by solder to the electrolytic copper foil on which the silver plating film has been formed.

[0022] A tenth aspect of the present invention is a method for manufacturing a semiconductor module, comprising: a plating step of forming a silver plating film by displacement silver plating on a circuit board relating to any one of the fifth to eighth aspects; a chip mounting step of mounting a semiconductor chip by silver sintering bonding on a main surface opposite to the bonding surface of the circuit board that has undergone the plating step; and a heat sink bonding step of soldering the heat sink to the bonding surface of the circuit board that has undergone the plating step, characterized in that in the heat sink bonding step, the heat sink is soldered to the second portion of the copper foil on which the silver plating film is formed. Effect of the Invention

[0023] According to the first to tenth aspects of the present invention, in a semiconductor module obtained by forming a silver plating film by displacement silver plating, mounting a semiconductor chip on one main surface of a circuit board, and soldering a heat sink to the other main surface, the bonding reliability of the silver sintered bond with the circuit board can be ensured to be equal to that of the conventional method, while the bonding reliability between the circuit board and the heat sink can be improved compared to the conventional method. [Brief description of the drawings]

[0024] [Figure 1] 1 is a schematic cross-sectional view showing a configuration of a circuit board 10 according to a first embodiment. [Diagram 2] 2A to 2C are schematic cross-sectional views for explaining a manufacturing process of the circuit board 10. [Diagram 3] 1 is a schematic cross-sectional view showing a configuration of a circuit board 20 according to a second embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0025] <First embodiment> <Structure of silicon nitride insulating heat dissipation circuit board> 1 is a schematic cross-sectional view showing the configuration of a silicon nitride insulating heat dissipation circuit board (hereinafter also simply referred to as a circuit board) 10 according to a first embodiment of the present invention. The circuit board 10 is a board on which semiconductor chips are mounted, and serves to electrically connect the semiconductor chips mounted on one main surface side to the outside. For example, when used in a power semiconductor module for in-vehicle use, a power semiconductor element is mounted on the circuit board. In addition, the circuit board 10 also serves to release heat generated by the semiconductor chips to the outside through a heat sink joined to the other main surface side.

[0026] The circuit board 10 is made of silicon nitride (Si 3 N 4A rolled copper foil 2 (2a, 2b) and an electrolytic copper foil 3 (3a, 3b) are laminated in this order and integrated by bonding on both main surfaces of a ceramic substrate (hereinafter also referred to as a silicon nitride substrate) 1. Hereinafter, the rolled copper foil 2a on one main surface side and the electrolytic copper foil 3a laminated thereon will be referred to as the circuit surface side copper foil 4a, and the rolled copper foil 2b on the other main surface side and the electrolytic copper foil 3b laminated thereon will be referred to as the solder bonding surface side copper foil 4b.

[0027] The silicon nitride substrate 1 is a substrate made of silicon nitride ceramics having high thermal conductivity and high insulation properties. There are no particular limitations on the planar shape or size of the silicon nitride substrate 1, but from the perspective of miniaturizing the power semiconductor module, a silicon nitride substrate 1 having a rectangular shape in plan view with a side length L of about 100 mm to 250 mm and a thickness W of 0.20 mm to 0.40 mm is exemplified.

[0028] The rolled copper foil 2 is a copper foil produced by rolling a copper ingot. The average particle size of the copper particles constituting the rolled copper foil 2 when the rolled copper foil 2 is integrated into the circuit board 10 is approximately 100 μm to 1000 μm. In addition, it is preferable that the thickness RC1 of the rolled copper foil 2a and the thickness RC2 of the rolled copper foil 2b in the circuit board 10 are both approximately 300 μm to 2500 μm. However, it is not necessary that the two have the same value.

[0029] As long as the thickness falls within the above range, a copper "sheet" may be used instead of a copper "foil." In this embodiment, however, the two will be referred to as the rolled copper foil 2 without distinction.

[0030] On the other hand, the electrolytic copper foil 3 is a copper foil produced by passing electricity through a copper sulfate bath and depositing copper on a rotating cathode drum. The average particle size of the copper particles constituting the electrolytic copper foil 3 in a state in which the electrolytic copper foil 3 is integrated into the circuit board 10 is generally less than 100 μm, preferably less than 10 μm, and more preferably less than 5 μm. The thickness EC1 of the electrolytic copper foil 3a and the thickness EC2 of the electrolytic copper foil 3b are both preferably about 5 μm to 50 μm. However, it is not necessary that the two have the same value. The ten-point average height Rz of the surface of the electrolytic copper foil 3 in the circuit board 10 is 1 μm to 5 μm, and the maximum height Rmax is 1 μm to 10 μm.

[0031] The silicon nitride substrate 1 and the rolled copper foil 2 are bonded together by a bonding layer (not shown). Such bonding is achieved by an active metal method. Meanwhile, the rolled copper foil 2 and the electrolytic copper foil 3 are bonded together directly. Therefore, depending on the method of bonding, elements contained in the active metal paste used for bonding may diffuse into at least the rolled copper foil 2, and in some cases even into the electrolytic copper foil 3. The manufacturing method of the circuit board 10 will be described later.

[0032] Both the rolled copper foil 2 and the electrolytic copper foil 3 are made of copper particles, but since the particle size is significantly different as described above, the two can be clearly distinguished from each other in a cross-sectional SEM (scanning electron microscope) image of the circuit board 10, for example.

[0033] Focusing on this difference in grain size, the circuit board 10 can also be seen as having copper foil on both main surfaces of the silicon nitride substrate 1, the copper foil having a two-layer structure consisting of a first portion (rolled copper foil 2) having a relatively large grain size adjacent to the silicon nitride substrate 1, and a second portion (electrolytic copper foil 3) having a relatively small grain size that forms the surface layer of the copper foil main surface and is adjacent to the first portion.

[0034] 1, there are gaps in the circuit surface side copper foil 4a, and these gaps become grooves G where the silicon nitride substrate 1 is exposed. This is because the circuit surface side copper foil 4a is patterned into a circuit pattern corresponding to the semiconductor chip to be mounted thereon. In other words, the gaps in the circuit pattern become the grooves G.

[0035] <Circuit board manufacturing process> Next, a description will be given of a manufacturing process for the circuit board 10. Figure 2 is a schematic cross-sectional view for explaining the manufacturing process.

[0036] In manufacturing the circuit board 10, bonding by an active metal method is utilized. First, an active metal paste is applied to substantially the entire surfaces of both main surfaces of the silicon nitride substrate 1 by a known application method such as screen printing to form paste films 7 (7a, 7b). The paste films 7 are preferably formed to a thickness of about 0.5 μm to 3.0 μm.

[0037] The active metal paste is a paste that contains at least a powder of an active metal, which is at least one metal selected from the group consisting of titanium and zirconium, and a silver powder as metal powder, and also contains a binder and a solvent as organic components. In addition, it may optionally contain a powder of at least one metal selected from the group consisting of copper, indium, and tin. It may also contain a dispersant, an antifoaming agent, etc. One suitable example is to use an active metal paste that contains titanium as an active metal.

[0038] Next, the rolled copper foil 2a and the electrolytic copper foil 3a are laminated on the paste film 7a, and the rolled copper foil 2b and the electrolytic copper foil 3b are laminated on the paste film 7b. This results in a laminate in which the silicon nitride substrate 1, on both main surfaces of which the paste films 7a and 7b are applied, is sandwiched between the rolled copper foils 2a and 2b, and further sandwiched on the outside by the electrolytic copper foils 3a and 3b. At this point, the rolled copper foil 2a and the electrolytic copper foil 3a have not been patterned, and therefore the grooves G have not been formed.

[0039] Next, the obtained laminate is subjected to a heat and pressure treatment (hot pressing). For example, the laminate is heated in a vacuum or in an inert gas according to a predetermined temperature profile with a maximum temperature of 800° C. to 900° C., and pressurized according to a surface pressure profile with a maximum surface pressure of 5 MPa to 30 MPa, as shown by the arrow AR in FIG. 2.

[0040] By this hot pressing, the active metal (e.g., titanium) present in the paste film 7 reacts with the nitrogen of the silicon nitride substrate 1, while the silver also present in the paste film 7 diffuses into the rolled copper foil 2. At that time, other metal components contained in the active metal paste may diffuse into the rolled copper foil 2, and silicon contained in the silicon nitride substrate 1 may diffuse into the active metal paste. In addition, the electrolytic copper foil 3 is pressure-bonded to the rolled copper foil 2.

[0041] In addition, the hot pressing also causes grain growth of the copper particles constituting the rolled copper foil 2 and the electrolytic copper foil 3. Therefore, the above-mentioned range of the average particle size of the copper particles constituting the rolled copper foil 2 (2a, 2b) of 100 μm to 1000 μm and the range of the average particle size of the copper particles constituting the electrolytic copper foil 3 (3a, 3b) of less than 100 μm only need to be satisfied in the circuit board 10 finally obtained (as a finished product) through the hot pressing. In other words, at least until the laminate is obtained, the average particle size of the copper particles constituting the rolled copper foil 2 (2a, 2b) and the electrolytic copper foil 3 (3a, 3b) does not necessarily need to satisfy these ranges, and at most, it is sufficient that the relationship that the average particle size of the copper particles constituting the electrolytic copper foil 3 (3a, 3b) is smaller than the average particle size of the copper particles constituting the rolled copper foil 2 (2a, 2b) is satisfied.

[0042] For example, when the average particle size of the copper particles in the rolled copper foil 2 (before bonding) used to prepare the laminate is about 10 μm to 100 μm, the average particle size of the copper particles constituting the rolled copper foil 2 in the circuit board 10 finally obtained after hot pressing will be about 300 μm to 1000 μm. When the average particle size of the copper particles in the electrolytic copper foil 3 (before bonding) used to prepare the laminate is less than 1 μm, the average particle size of the copper particles constituting the electrolytic copper foil 3 in the circuit board 10 finally obtained after hot pressing will be about 2 μm to 15 μm.

[0043] Similarly, the above-mentioned ranges of the thickness of the rolled copper foil 2 (2a, 2b) of about 300 μm to 2500 μm and the thickness of the electrolytic copper foil 3 (3a, 3b) of about 5 μm to 50 μm need only be satisfied for the circuit board 10 to be finally obtained, and the thickness of each copper foil may still be greater than the ranges at the time when the laminate is constructed.

[0044] Finally, the silicon nitride substrate 1 and the rolled copper foil 2 are bonded to each other by a bonding layer (not shown) mainly composed of an active metal nitride, and the electrolytic copper foil 3 is bonded to the rolled copper foil 2. That is, the circuit board 10 is formed in an unpatterned state.

[0045] Thereafter, the circuit surface side copper foil 4a is patterned by a known patterning method such as etching to obtain a circuit board 10 having grooves G as shown in Fig. 1. For example, when the side surface of the circuit surface side copper foil 4a is observed at the location where the grooves G are provided, it can be seen that the circuit surface side copper foil 4a has a two-layer structure of a piezoelectric copper foil 2a and an electrolytic copper foil 3a.

[0046] <Use of silicon nitride insulating heat dissipation circuit board> Next, a description will be given of a mode of use of the circuit board 10. In the circuit board 10, a semiconductor chip is mounted on the surface 5a of the circuit surface side copper foil 4a (more specifically, the electrolytic copper foil 3a), and a metal heat sink is joined to the surface 5b of the solder bonding surface side copper foil 4b (more specifically, the electrolytic copper foil 3b).

[0047] The semiconductor chip is mounted on the surface 5a by a known silver sintering bonding method. In brief, a silver paste is applied to the surface of the circuit side copper foil 4a, and then the semiconductor chip is placed on the silver paste. The silver paste is then sintered under a predetermined temperature and pressure, thereby fixing the semiconductor chip to the circuit board 10.

[0048] On the other hand, following mounting of the semiconductor chip on the surface 5a, a heat sink is soldered to the surface 5b. Therefore, the solder-bonding side copper foil 4b is provided on almost the entire surface of the silicon nitride substrate 1. As the solder, a conventionally known product (for example, a six-component Sn-Ag-Cu-Ni-Sb-Bi solder) can be used.

[0049] However, in general, prior to silver sintering of the semiconductor chip and soldering of the heat sink, the surfaces of the copper foil 4a on the circuit side and the copper foil 4b on the soldering side, i.e., the surfaces 5a and 5b of the electrolytic copper foils 3a and 3b, are plated with silver by displacement silver plating. An example of the plating bath is the Stirling System manufactured by MacDermid.

[0050] The displacement silver plating of the electrolytic copper foil 3 proceeds faster than the conventional displacement silver plating of the rolled copper foil 2. Therefore, unlike the conventional method, the corrosion of the copper foil does not progress due to the slow plating speed, and voids do not occur between the silver plating and the copper foil.

[0051] Rather, since the absence of voids ensures sufficient adhesion of the silver plating to the surfaces 5a, 5b of the electrolytic copper foils 3a, 3b, in this embodiment, the reliability of the solder bonding of the heat sink to the solder bonding surface side copper foil 4b, which was not necessarily sufficient in the past, is suitably ensured.

[0052] The reliability of the solder joint can be evaluated, for example, by the number of cycles at which cracks occur when a temperature cycle test is performed in which exposure to a low temperature atmosphere and exposure to a high temperature atmosphere are alternately repeated. If there is a gap under the silver plating film (between the copper foil), the crack will progress through the gap, so the reliability of the solder joint can be evaluated by the ease with which cracks occur in the temperature cycle test.

[0053] On the other hand, in the case of silver sinter bonding between the circuit side copper foil 4a and the semiconductor chip, silver paste is applied onto the electrolytic copper foil 3a having a silver plating film formed on the surface 5a. Since silver sinter bonding is performed in a high-temperature pressurized atmosphere, even if there is a gap between the silver plating and the circuit side copper foil 4a at the start of bonding, no gap will remain in the end, and therefore the reliability of the bond is ensured.

[0054] Therefore, when no voids exist due to the formation of a silver plating film on the surface 5a of the electrolytic copper foil 3a as in this embodiment, the bonding reliability between the circuit surface side copper foil 4a and the semiconductor chip is more suitably ensured.

[0055] As described above, according to this embodiment, in a silicon nitride insulating heat dissipation circuit board formed by bonding copper foil to both main surfaces of a silicon nitride ceramic substrate, in which a semiconductor chip is mounted on one main surface and a heat sink is bonded to the other main surface, the copper foil provided on both main surfaces has a two-layer structure of a first portion (rolled copper foil) having a relatively large grain size and a second portion (electrolytic copper foil) adjacent to the first portion and having a relatively small grain size. This suppresses the occurrence of voids between the silver plating film and the copper foil when silver plating is applied to the surface of each copper foil by displacement silver plating in order to mount a semiconductor chip by silver sintering bonding on one main surface and solder bond a heat sink on the other main surface. As a result, the bonding reliability between the semiconductor chip and the heat sink and the circuit board is suitably ensured. In particular, the bonding reliability between the heat sink and the circuit board is improved compared to the conventional case.

[0056] <Second embodiment> 3 is a schematic cross-sectional view showing the configuration of a circuit board 20 according to a second embodiment of the present invention. Like the circuit board 10 according to the first embodiment, the circuit board 20 is also a board having a semiconductor chip mounted on one main surface side. That is, the heat sink bonded to the other main surface side has a role of releasing heat generated by the semiconductor chip to the outside, and also serves as an electrical connection between the semiconductor chip and the outside.

[0057] The circuit board 20 has the same configuration as the circuit board 10, except that it does not include the electrolytic copper foil 3a. In other words, only the rolled copper foil 2a is used as the copper foil on the circuit surface side.

[0058] The circuit board 20 can be produced by the same manufacturing process as the circuit board 10 shown in the first embodiment, except that the electrolytic copper foils 3a are not overlapped when creating the laminate prior to hot pressing.

[0059] Like the circuit board 10, the circuit board 20 is usually silver-plated by displacement silver plating before use. The silver plating is applied to the surface 6a of the rolled copper foil 2a and the surface 5b of the electrolytic copper foil 3b. Then, like the circuit board 10, the heat sink is soldered to the silver-plated surface 5b of the electrolytic copper foil 3b.

[0060] Therefore, the joining mode of the heat sink to the circuit board 20 is the same as the joining mode of the heat sink to the circuit board 10 in the first embodiment. Therefore, in the case where the circuit board 20 is used, as in the case where the circuit board 10 is used, the reliability of the solder joining of the heat sink to the solder joining surface side copper foil 4b is suitably ensured.

[0061] On the other hand, unlike the first embodiment, the mounting of the semiconductor chip is realized by silver sinter bonding to the surface 6a of the silver-plated rolled copper foil 2a. This bonding mode is the same as that of the conventional technology. Therefore, during silver plating, gaps may occur between the plating film and the rolled copper foil 2a, but as described above, since silver sinter bonding is performed in a high-temperature pressurized atmosphere, even if there are gaps between the silver plating and the rolled copper foil 2a at the start of bonding, no gaps will remain in the end, and therefore the reliability of the bonding is ensured.

[0062] In other words, according to this embodiment, the bonding reliability of the silver sintered bond between the semiconductor chip and the circuit board can be ensured to be the same as in the conventional case, while the bonding reliability of the solder bond between the heat sink and the circuit board can be improved compared to the conventional case. EXAMPLES

[0063] Two types of circuit boards 10 (Example 1 and Example 2) in which the average particle size of the copper particles in the electrolytic copper foils 3a, 3b constituting the circuit side copper foil 4a and the solder bonding side copper foil 4b, respectively, are different, and two types of circuit boards 20 (Example 3 and Example 4) in which the average particle size of the electrolytic copper foil 3b constituting the solder bonding side copper foil 4b is different, were produced by the above-mentioned diffusion bonding method.

[0064] The average particle diameter of the copper foil in the circuit boards 10 and 20 was determined as follows. First, the circuit boards 10 and 20 were cut, and the cut surfaces were polished. Then, the polished surfaces were observed by SEM. A straight line was drawn across a plurality of copper particles in the obtained observation image, and the length of each of the straight line segments that crossed each individual copper particle was measured. The average length of the line segments for 50 copper particles was determined as the average particle diameter of the copper particles.

[0065] Displacement silver plating was performed on each of the obtained circuit boards, and the cross section of each circuit board after the plating film was formed was observed by SEM to evaluate the number of voids generated between the silver plating and the copper foil.

[0066] In addition, a heat sink was soldered to the surface 5b of the electrolytic copper foil 3b of each circuit board after the plating film was formed. Then, a temperature cycle life test was performed to evaluate the reliability of the soldered joint.

[0067] On the other hand, as comparative examples, two types of circuit boards were produced in which only rolled copper foil 2 (2a, 2b) was bonded to silicon nitride substrate 1 and neither electrolytic copper foil 3a nor 3b was included, with the average particle size of the copper particles in rolled copper foil 2 (2a, 2b) being different (Comparative Example 1 and Comparative Example 2).

[0068] Specifically, the planar sizes and thicknesses of the silicon nitride substrate 1, rolled copper foil 2, and electrolytic copper foil 3 used in each circuit board are as follows:

[0069] Silicon nitride substrate: 140mm x 190mm, 0.320mm; Rolled copper foil: 130 mm × 185 mm, 0.500 mm (0.700 mm only in Comparative Example 2); Electrolytic copper foil: 130mm x 185mm, 0.035mm.

[0070] Moreover, the average particle size of the copper particles in the electrolytic copper foils in Examples 1 to 4 was varied to two levels, 3 μm and 10 μm.

[0071] The thickness of the active metal paste during diffusion bonding was set to 1 μm on average, and the thickness of the plating film formed by displacement silver plating was set to 0.2 μm on average.

[0072] In evaluating the voids between the silver plating and the copper foil, the number of voids of 1 μm or more per mm of interface length between the silver plating and the electrolytic copper foil 3b of the solder joint side copper foil 4b in the cross section of the board was counted.

[0073] The temperature cycle life test was performed by repeating 500 cycles of heating and cooling between -40°C and 150°C at a rate of two cycles per hour, and checking every 50 cycles for the presence or absence of cracks that progress through the gaps under the silver plating film (between the copper foil) at the joint.

[0074] Table 1 shows the surface copper foil materials on the circuit surfaces and solder joint surfaces of the circuit boards of Examples 1 to 4 and Comparative Examples 1 and 2, the average particle size of the copper particles in the materials, the number of voids that occurred under the silver plating film (between the copper foil) after the silver plating film was formed, and the temperature cycle life as an index of solder joint reliability.

[0075] The "circuit surface" refers to the main surface of the circuit boards 10 and 20 on which a semiconductor chip is mounted, and is specifically surface 5a in Examples 1 and 2, and surface 6a in Examples 3 and 4 and Comparative Examples 1 and 3. On the other hand, the "solder joint surface" refers to the main surface of the circuit boards 10 and 20 on which a heat sink is solder jointed, and is specifically surface 5b in Examples 1 to 4, and the surface of the rolled copper foil 2b in Comparative Examples 1 and 2.

[0076] The "surface copper foil material" refers to the type of copper foil that is present at the outermost position of the copper foil and is the target of silver plating. Specifically, in a circuit board having electrolytic copper foil, the electrolytic copper foil is the surface copper foil material, and in a copper foil board not having electrolytic copper foil, rolled copper foil is the surface electrode material.

[0077] [Table 1]

[0078] As can be seen from Table 1, in Examples 1 to 4 in which the surface copper foil material on the solder joint surface was an electrolytic copper foil, no voids were observed under the silver plating film. On the other hand, in Comparative Example 1 and Comparative Example 2, voids of 80 pcs / mm and 85 pcs / mm were confirmed, respectively.

[0079] In addition, in the temperature cycle life test, no cracks were observed until 500 cycles were completed in Examples 1 to 4. On the other hand, in both Comparative Examples 1 and 2, cracks were observed at the end of 50 cycles.

[0080] The above results indicate that using an electrolytic copper foil having a small copper particle size as the surface layer of the copper foil of a circuit board has the effect of improving the bonding reliability when a heat sink is bonded to the surface layer by soldering after silver plating. [Explanation of symbols]

[0081] 1. Silicon nitride (ceramics) substrate 2(2a, 2b) Rolled copper foil 3(3a, 3b) Electrolytic copper foil 4a Circuit side copper foil 4b Copper foil on solder joint side 7(7a, 7b) Paste film 10, 20 (Silicon nitride insulating heat dissipation) circuit board

Claims

1. A circuit board for mounting a semiconductor chip, a silicon nitride ceramic substrate having rolled copper foils bonded to both main surfaces; an electrolytic copper foil overlapping the rolled copper foil at least on the bonding surface side with a heat sink; A circuit board comprising:

2. 2. The circuit board according to claim 1, The average particle size of the copper particles constituting the rolled copper foil is 100 μm to 1000 μm, The average particle size of the copper particles constituting the electrolytic copper foil is less than 100 μm. A circuit board comprising:

3. 3. The circuit board according to claim 1, The thickness of the rolled copper foil is 300 μm to 2500 μm, The thickness of the electrolytic copper foil is 5 μm to 50 μm. A circuit board comprising:

4. 4. The circuit board according to claim 1, The electrolytic copper foil is superimposed on both of the rolled copper foils. A circuit board comprising:

5. A circuit board for mounting a semiconductor chip, A silicon nitride ceramic substrate having copper foil bonded to both main surfaces thereof, At least the copper foil on the side of the joint surface with the heat sink is a first portion including a bonding portion with the silicon nitride ceramic substrate; a second portion including a surface portion of the copper foil; It has a two-layer structure, The copper particles in the second portion have a smaller average particle size than the copper particles in the first portion. A circuit board comprising:

6. 6. The circuit board according to claim 5, The average particle size of the copper particles in the first portion is 100 μm to 1000 μm, The average particle size of the copper particles in the second portion is less than 100 μm; A circuit board comprising:

7. 7. The circuit board according to claim 5 or 6, The thickness of the first portion is 300 μm to 2500 μm, The thickness of the second portion is 5 μm to 50 μm. A circuit board comprising:

8. 8. The circuit board according to claim 5, The copper foils bonded to both main surfaces each include the first portion and the second portion. A circuit board comprising:

9. A method for manufacturing a semiconductor module, comprising the steps of: a plating step of forming a silver plating film on the circuit board according to any one of claims 1 to 4 by displacement silver plating; a chip mounting process of mounting a semiconductor chip by silver sintering bonding on a main surface of the circuit board opposite to the bonding surface that has been subjected to the plating process; a heat sink joining step of joining the heat sink to the joining surface of the circuit board that has been subjected to the plating step by using solder; Equipped with In the heat sink joining step, the heat sink is joined to the electrolytic copper foil on which the silver plating film is formed by soldering. A method for manufacturing a semiconductor module.

10. A method for manufacturing a semiconductor module, comprising the steps of: a plating step of forming a silver plating film on the circuit board according to any one of claims 5 to 8 by displacement silver plating; a chip mounting process of mounting a semiconductor chip by silver sintering bonding on a main surface of the circuit board opposite to the bonding surface that has been subjected to the plating process; a heat sink joining step of joining the heat sink to the joining surface of the circuit board that has been subjected to the plating step by using solder; Equipped with In the heat sink bonding step, the heat sink is bonded to the second portion of the copper foil on which the silver plating film is formed by soldering. A method for manufacturing a semiconductor module.

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