JP7680252B2 - Method for manufacturing a circuit board and a semiconductor module - Google Patents

Description

The present invention relates to the configuration of a silicon nitride insulating heat dissipation circuit board.

Silicon nitride insulating heat dissipation circuit boards and alumina insulating heat dissipation circuit boards are widely known as ceramic insulating heat dissipation circuit boards on which electronic components such as semiconductor chips are mounted. 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.

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 metals. Typically, a semiconductor chip is bonded (mounted) to one of the copper foil surfaces by silver sintering bonding, and a metal heat sink, for example, is soldered to the other copper foil surface.

Silicon nitride insulating heat dissipation circuit boards have superior heat dissipation properties and reliability compared to alumina-based insulating heat dissipation circuit boards using alumina-based ceramic substrates, and are therefore often used in automotive applications. 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 reliability of the silver sintered bond 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).

International Publication No. 2020/218193

Electroless silver plating is mainly used for silver plating on silicon nitride insulating heat dissipation circuit boards. Of these, 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.

The copper foil used for silicon nitride insulating heat dissipation circuit boards is often as thick as 300 μm or more. Rolled copper foil is usually used for such copper foil.

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 on printed circuit boards, etc.

Furthermore, when displacement silver plating is performed on the surface of rolled copper foil, the slow plating speed causes the silver plating film to grow while corrosion of the rolled copper foil progresses underneath, resulting in the formation of voids between the silver plating film and the rolled copper foil. If a heat sink is soldered to the silver-plated rolled copper foil surface with such voids present, the silver plating film will diffuse into the solder, but voids will still remain between the solder layer and the rolled copper foil.

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

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

The present invention was made in consideration of the above problems, and aims to provide a silicon nitride insulating heat dissipation circuit board with excellent reliability of solder joint with a heat sink.

In order to solve the above problems, a first aspect of the present invention is a method for manufacturing a circuit board for mounting a semiconductor chip, comprising: a coating step of coating each of two main surfaces of a silicon nitride ceramic substrate with an active metal paste to form a paste film; a lamination step of superimposing rolled copper foils on both of the paste films and superimposing an electrolytic copper foil on at least one of the rolled copper foils to obtain a laminate; and a heating and pressurizing step of heating the laminate in a vacuum or in an inert gas while pressurizing the laminate according to a predetermined surface pressure profile, thereby bonding the rolled copper foil to the silicon nitride ceramic substrate and bonding the electrolytic copper foil to the rolled copper foil.

A second aspect of the present invention is the method for producing a circuit board according to the first aspect, characterized in that in the laminating step, the electrolytic copper foil is superimposed on both of the rolled copper foils.

A third aspect of the present invention is a method for manufacturing a circuit board for mounting a semiconductor chip, comprising: a coating step of coating each of two main surfaces of a silicon nitride ceramic substrate with an active metal paste to form a paste film; a lamination step of superposing a first copper foil on both of the paste films and superposing a second copper foil having an average particle size smaller than the copper particles of the first copper foil on at least one of the first copper foils to obtain a laminate; and a heating and pressurizing step of heating the laminate in a vacuum or in an inert gas while pressurizing the laminate according to a predetermined surface pressure profile, thereby bonding the first copper foil to the silicon nitride ceramic substrate and bonding the second copper foil to the first copper foil.

A fourth aspect of the present invention is a method for manufacturing a circuit board according to the third aspect, characterized in that in the lamination step, the second copper foil is superimposed on both of the first copper foils.

A fifth aspect of the present invention is a method for manufacturing a circuit board according to any one of the first to fourth aspects, characterized in that the active metal paste 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 powders, and contains a binder and a solvent as organic components.

A sixth 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 manufactured by the manufacturing method according to the first or second aspect; a chip mounting step of mounting a semiconductor chip by silver sintering bonding on a main surface of the circuit board opposite to a main surface to which a heat sink is bonded, the circuit board having undergone the plating step; and a heat sink bonding step of bonding the heat sink to one main surface of the circuit board having undergone the plating step by soldering, 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 is formed.
A seventh 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 manufactured by the manufacturing method according to the third or fourth aspect; a chip mounting step of mounting a semiconductor chip by silver sintering bonding on a main surface of the circuit board opposite to a main surface to which a heat sink is bonded, the circuit board having undergone the plating step; and a heat sink bonding step of bonding the heat sink to one main surface of the circuit board having undergone the plating step by soldering, wherein in the heat sink bonding step, the heat sink is bonded by solder to the second copper foil on which the silver plating film is formed.

According to the first to seventh 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.

1 is a schematic cross-sectional view showing a configuration of a circuit board 10 according to a first embodiment. 5A to 5C are schematic cross-sectional views for explaining a manufacturing process of the circuit board 10. FIG. 11 is a schematic cross-sectional view showing a configuration of a circuit board 20 according to a second embodiment.

First Embodiment
<Configuration 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, 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 a semiconductor chip is mounted, and serves to electrically connect the semiconductor chip 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. In addition, the circuit board 10 also serves to release heat generated by the semiconductor chip to the outside through a heat sink bonded to the other main surface side.

The circuit board 10 has a configuration in which rolled copper foils 2 (2a, 2b) and electrolytic copper foils 3 (3a, 3b) are laminated in this order and integrated by bonding on both main surfaces of a silicon _nitride ( _Si3N4 ) ceramic substrate (hereinafter also referred to as 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.

The silicon nitride substrate 1 is a substrate made of silicon nitride ceramics, which has 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, an example of a silicon nitride substrate 1 is one that is rectangular in plan view, with a side length L of approximately 100 mm to 250 mm and a thickness W of 0.20 mm to 0.40 mm.

The rolled copper foil 2 is a copper foil produced by rolling a copper ingot. The average particle size of the copper particles that make up 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, the two do not need to be the same value.

As long as the thickness falls within the above range, a copper "plate" may be used instead of a copper "foil," but in this embodiment, the two will be referred to as rolled copper foil 2 without distinction.

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 when 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. It is preferable that the thickness EC1 of the electrolytic copper foil 3a and the thickness EC2 of the electrolytic copper foil 3b are both 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 on the circuit board 10 is 1 μm to 5 μm, and the maximum height Rmax is 1 μm to 10 μm.

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. On the other hand, 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.

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

Focusing on the difference in grain size, the circuit board 10 can be considered to have copper foil on both main surfaces of the silicon nitride substrate 1, which has a two-layer structure consisting of a first portion (rolled copper foil 2) with a relatively large grain size adjacent to the silicon nitride substrate 1, and a second portion (electrolytic copper foil 3) with a relatively small grain size that forms the surface layer of the copper foil main surface and is adjacent to the first portion.

As shown in FIG. 1, there are gaps in the copper foil 4a on the circuit surface side, which serve as grooves G through which the silicon nitride substrate 1 is exposed. This is because the copper foil 4a on the circuit surface side is patterned into a circuit pattern that corresponds to the semiconductor chip to be mounted thereon. In other words, the gaps in the circuit pattern serve as grooves G.

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

When manufacturing the circuit board 10, bonding using the active metal method is used. First, active metal paste is applied to substantially the entire surface of both main surfaces of the silicon nitride substrate 1 using a known application method such as screen printing to form paste films 7 (7a, 7b). It is preferable that the paste film 7 be formed to a thickness of approximately 0.5 μm to 3.0 μm.

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 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 the active metal.

Next, rolled copper foil 2a and electrolytic copper foil 3a are superimposed on paste film 7a, and rolled copper foil 2b and electrolytic copper foil 3b are superimposed on paste film 7b. This results in a laminate in which silicon nitride substrate 1, with paste films 7a and 7b applied to both main surfaces, is sandwiched between rolled copper foils 2a and 2b, and further sandwiched on the outside by electrolytic copper foils 3a and 3b. At this point, the rolled copper foil 2a and electrolytic copper foil 3a have not been patterned, and therefore no grooves G have been formed.

Then, the obtained laminate is subjected to a heating and pressurizing treatment (hot pressing). For example, the laminate is heated in a vacuum or in an inert gas with a predetermined temperature profile in which the maximum temperature is 800°C or more and 900°C or less, while the laminate is pressurized according to a surface pressure profile in which the maximum surface pressure is 5 MPa or more and 30 MPa or less, as shown by the arrow AR in Figure 2.

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 the silicon contained in the silicon nitride substrate 1 may diffuse into the active metal paste. In addition, the electrolytic copper foil 3 is pressed against the rolled copper foil 2.

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 (the 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.

For example, if the average particle size of the copper particles in the rolled copper foil 2 (before bonding) used to produce 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. Also, if the average particle size of the copper particles in the electrolytic copper foil 3 (before bonding) used to produce 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.

Similarly, the thickness range of the rolled copper foil 2 (2a, 2b) of about 300 μm to 2500 μm and the thickness range of the electrolytic copper foil 3 (3a, 3b) of about 5 μm to 50 μm described above need only be satisfied for the circuit board 10 that is ultimately obtained, and the thickness of each copper foil may still be greater than this range when the laminate is constructed.

Finally, the silicon nitride substrate 1 and the rolled copper foil 2 are bonded together by a bonding layer (not shown) whose main component is an active metal nitride, and the electrolytic copper foil 3 is bonded to the rolled copper foil 2. In other words, a circuit board 10 is formed in an unpatterned state.

Then, the circuit surface side copper foil 4a is patterned by a known patterning method such as etching to obtain a circuit board 10 having a groove portion G as shown in FIG. 1. For example, when observing the side of the circuit surface side copper foil 4a at the location where the groove portion G is 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.

<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).

The semiconductor chip is mounted on the surface 5a by the well-known silver sintering bonding method. In general terms, silver paste is applied to the surface of the circuit side copper foil 4a, and the semiconductor chip is placed on the silver paste. The silver paste is then sintered under a specified temperature and pressure, thereby fixing the semiconductor chip to the circuit board 10.

After mounting the semiconductor chip on surface 5a, a heat sink is soldered to surface 5b. Therefore, the copper foil 4b on the solder joint side is provided over 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.

However, typically, 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 solder bonding side, i.e., the surfaces 5a and 5b of the electrolytic copper foils 3a and 3b, are silver plated by displacement silver plating. An example of a plating bath is the Stirling System manufactured by MacDermid.

The displacement silver plating of the electrolytic copper foil 3 proceeds faster than the displacement silver plating of the rolled copper foil 2 that was conventionally performed. Therefore, unlike the conventional method, the slow plating speed does not cause corrosion of the copper foil to progress and gaps to form between the silver plating and the copper foil.

In fact, the lack of voids ensures sufficient adhesion of the silver plating to the surfaces 5a, 5b of the electrolytic copper foils 3a, 3b, so in this embodiment, the reliability of the solder joint of the heat sink to the solder joint side copper foil 4b, which was not necessarily sufficient in the past, is suitably ensured.

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 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.

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 with 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.

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 appropriately ensured.

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 on both main surfaces has a two-layer structure consisting 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.

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.

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

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 foil 3a is not overlapped when producing the laminate prior to hot pressing.

As with circuit board 10, circuit board 20 is usually plated with silver by displacement silver plating before use. Silver plating is applied to surface 6a of rolled copper foil 2a and surface 5b of electrolytic copper foil 3b. Then, as with circuit board 10, the heat sink is soldered to surface 5b of electrolytic copper foil 3b that has been plated with silver.

The joining mode of the heat sink to the circuit board 20 is therefore the same as the joining mode of the heat sink to the circuit board 10 in the first embodiment. Therefore, when the circuit board 20 is used, as when 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.

On the other hand, unlike the first embodiment, the mounting of the semiconductor chip is achieved by silver sintering 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, silver sintering bonding is performed in a high-temperature pressurized atmosphere, so 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 bond is ensured.

In other words, according to this embodiment, the reliability of the silver sintered bond between the semiconductor chip and the circuit board can be maintained at the same level as in the past, while the reliability of the soldered bond between the heat sink and the circuit board can be improved compared to the past.

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

The average particle size of the copper foil in circuit boards 10 and 20 was determined as follows. First, circuit boards 10 and 20 were cut and the cut surfaces were polished. The polished surfaces were then observed with an SEM. Straight lines were drawn across multiple copper particles in the obtained observation image, and the lengths of the straight line segments that crossed each individual copper particle were measured. The average length of the line segments for 50 copper particles was determined as the average particle size of the copper particles.

Each of the resulting circuit boards was subjected to displacement silver plating, and the cross-section of each circuit board after the plating film was formed was observed using an SEM to evaluate the number of voids that occurred between the silver plating and the copper foil.

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. A temperature cycle life test was then performed to evaluate the reliability of the solder joint.

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, Comparative Example 2).

Specifically, the planar size and thickness of the silicon nitride substrate 1, rolled copper foil 2, and electrolytic copper foil 3 used in each circuit board are as follows:

Silicon nitride substrate: 140 mm x 190 mm, 0.320 mm;
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.

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.

The average thickness of the active metal paste used for diffusion bonding was 1 μm. Furthermore, the average thickness of the plating film formed by displacement silver plating was 0.2 μm.

When evaluating the voids between the silver plating and the copper foil, the number of voids of 1 μm or more in size that exist per 1 mm of interface length between the silver plating on the cross section of the board and the electrolytic copper foil 3b of the solder joint side copper foil 4b were counted.

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.

Table 1 shows the surface copper foil material on the circuit surface and solder joint surface of the circuit boards for Examples 1 to 4 and Comparative Examples 1 and 2, the average particle size of the copper particles in the material, 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.

The "circuit surface" refers to the main surface of the circuit boards 10 and 20 on which the semiconductor chip is mounted, 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 the heat sink is soldered, specifically, surface 5b in Examples 1 to 4, and the surface of the rolled copper foil 2b in Comparative Examples 1 and 2.

In addition, the "surface copper foil material" is 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 that has electrolytic copper foil, the electrolytic copper foil is the surface copper foil material, and in a copper foil that does not have electrolytic copper foil, rolled copper foil is the surface electrode material.

As can be seen from Table 1, in all of Examples 1 to 4, in which the surface copper foil material on the solder joint surface was 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 were confirmed at 80 voids/mm and 85 voids/mm, respectively.

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

These results indicate that using electrolytic copper foil with small copper particle size as the surface layer of copper foil on 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.

REFERENCE SIGNS LIST 1 Silicon nitride (ceramic) substrate 2 (2a, 2b) Rolled copper foil 3 (3a, 3b) Electrolytic copper foil 4a Copper foil on circuit side 4b Copper foil on solder joint side 7 (7a, 7b) Paste film 10, 20 (Silicon nitride insulating heat dissipating) circuit board

Claims (7)

A method for manufacturing a circuit board for mounting a semiconductor chip, comprising the steps of:
a coating step of coating an active metal paste on each of two main surfaces of a silicon nitride ceramic substrate to form a paste film;
a lamination step of overlapping rolled copper foils on both of the paste films and overlapping an electrolytic copper foil on at least one of the rolled copper foils to obtain a laminate;
a heating and pressing step in which the laminate is heated in a vacuum or in an inert gas and pressed according to a predetermined surface pressure profile, thereby bonding the rolled copper foil to the silicon nitride ceramic substrate and bonding the electrolytic copper foil to the rolled copper foil;
A method for manufacturing a circuit board, comprising: A method for manufacturing a circuit board according to claim 1 ,
In the lamination step, the electrolytic copper foil is laminated on both of the rolled copper foils.
A method for manufacturing a circuit board. A method for manufacturing a circuit board for mounting a semiconductor chip, comprising the steps of:
a coating step of coating an active metal paste on each of two main surfaces of a silicon nitride ceramic substrate to form a paste film;
a lamination step of superposing a first copper foil on both of the paste films and a second copper foil having an average particle size smaller than that of the copper particles of the first copper foil on at least one of the first copper foils to obtain a laminate;
a heating and pressing step of heating the laminate in a vacuum or in an inert gas and pressing the laminate according to a predetermined surface pressure profile, thereby bonding the first copper foil to the silicon nitride ceramic substrate and bonding the second copper foil to the first copper foil;
A method for manufacturing a circuit board, comprising: A method for manufacturing a circuit board according to claim 3 , comprising the steps of:
In the lamination step, the second copper foil is laminated on both of the first copper foils.
A method for manufacturing a circuit board. A method for manufacturing a circuit board according to any one of claims 1 to 4 , comprising the steps of:
The active metal paste 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 powders, and contains a binder and a solvent as organic components.
A method for manufacturing a circuit board. A method for manufacturing a semiconductor module, comprising the steps of:
a plating step of forming a silver plating film by displacement silver plating on the circuit board manufactured by the manufacturing method according to claim 1 or 2 ;
a chip mounting process in which a semiconductor chip is mounted by silver sintering bonding on a main surface of the circuit board that has been subjected to the plating process, the main surface being opposite to the main surface to which a heat sink is bonded;
a heat sink joining step of joining the heat sink to one main surface of the circuit board that has been subjected to the plating step by soldering;
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. A method for manufacturing a semiconductor module, comprising the steps of:
a plating step of forming a silver plating film by displacement silver plating on the circuit board manufactured by the manufacturing method according to claim 3 or 4 ;
a chip mounting process in which a semiconductor chip is mounted by silver sintering bonding on a main surface of the circuit board that has been subjected to the plating process, the main surface being opposite to the main surface to which a heat sink is bonded;
a heat sink joining step of joining the heat sink to one main surface of the circuit board that has been subjected to the plating step by soldering;
Equipped with
In the heat sink joining step, the heat sink is joined by soldering to the second copper foil on which the silver plating film is formed.
A method for manufacturing a semiconductor module.

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