JP7509358B2 - Solder materials and sintered bodies - Google Patents

Description

This invention relates to solder materials and sintered bodies.

A power semiconductor module is a power semiconductor device that incorporates one or more power semiconductor chips to form part or all of a conversion connection, and has a structure in which the power semiconductor chips are electrically insulated from the laminated substrate or metal substrate. Power semiconductor modules are used in industrial applications such as motor drive control inverters for elevators and the like. In recent years, they have also become widely used in vehicle motor drive control inverters. Vehicle inverters must be small and lightweight to improve fuel efficiency, and long-term reliability is required under high-temperature operation because they are placed near the drive motor in the engine room.

The structure of a conventional power semiconductor module will be explained using a typical IGBT (Insulated Gate Bipolar Transistor) power semiconductor module structure as an example.

Figure 9 is a cross-sectional view showing the configuration of a power semiconductor module with a conventional structure. As shown in Figure 9, the power semiconductor module includes a power semiconductor chip 1, an insulating substrate 2, an electrode pattern 3, a conductive plate 9 arranged on the back surface of the insulating substrate 2, a solder material 14, a heat sink 5, a cooling body 7, a metal wire 10, an external terminal 11, a terminal case 12, and a sealing material 13.

The power semiconductor chip 1 is a semiconductor element such as an IGBT or a diode chip. An electrode pattern 3 and a conductive plate 9 are provided on both sides of an insulating substrate 2. The power semiconductor chip 1 is bonded to the electrode pattern 3 with a solder material 14, which is a bonding material. A heat sink 5 is bonded to the conductive plate 9 on the back side with the solder material 14. The heat sink 5 is bonded to a cooling body 7 provided with heat sink fins via a heat sink grease 6. A substrate on which an electrode pattern 3 is provided on at least one side of an insulating substrate 2 is called a laminated substrate. In addition, a metal wire 10 connects the electrode pattern 3 to the upper surface of the power semiconductor chip 1 as wiring for electrical connection. A metal external terminal 11 for external connection is provided on the upper surface of the electrode pattern 3. In addition, in order to insulate and protect the power semiconductor chip 1, a sealing material 13 such as a low-elasticity silicone gel is filled in the terminal case 12, and the power semiconductor chip 1 is packaged with a lid (not shown).

Compared to industrial power semiconductor modules, automotive power semiconductor modules are required to be smaller and lighter due to space restrictions. In addition, as the output power density for driving the motor increases, the temperature of the semiconductor chip during operation increases, and the demand for long-term reliability during high-temperature operation is also increasing. For this reason, there is a demand for a power semiconductor module structure that can operate at high temperatures and has long-term reliability.

The power semiconductor module of the above configuration is fitted with a cooling body 7 provided with heat dissipation fins, and the heat generated by the power semiconductor chip 1 when current is passed through it is transferred to the heat dissipation fins and dissipated outside the system. If the surface of the cooling body 7 and the surface of the heat sink 5 are not in close contact with each other, the contact thermal resistance between them increases, reducing heat dissipation.

In conventional semiconductor devices, in order to ensure high heat dissipation performance, the surface flatness and surface roughness of the heat sink 5 and cooling body 7 are finished to be as small as possible, and further, a thermal compound such as thermal grease 6 is applied to the surface of the cooling body 7 to keep the contact thermal resistance between the heat sink 5 and cooling body 7 low.

The power semiconductor chip 1 and the electrode pattern 3, and the conductive plate 9 and the heat sink 5 are joined using solder material 14. For example, the joints below the power semiconductor chip 1 are joined using Pb (lead)-free solder, flux-containing paste solder, or plate solder.

One Pb-free solder used in semiconductor modules is a composite solder that is used for temperature-hierarchical connections such as die bonding of semiconductor chips, and is made up of a metal mesh made of Cu (copper) sandwiched between two pieces of solder foil and pressed together (see, for example, Patent Document 1).

JP 2004-174522 A

The thermal conductivity of the solder material 14 used for bonding below the power semiconductor chip 1 is 40 to 60 W/m·K. This value is low compared to the thermal conductivity of copper, which is 390 W/m·K. As a result, the heat generated by the power semiconductor chip 1 when current is passed cannot be sufficiently transferred to the electrode pattern 3, and the generated heat cannot reach the heat dissipation fins, resulting in the problem that the power semiconductor chip 1 cannot be sufficiently cooled. In addition, because the thermal conductivity of the solder material 14 is low, the temperature of the solder material 14 itself rises, causing cracks to form in the solder material 14, increasing thermal resistance and further reducing heat dissipation performance.

The purpose of this invention is to provide a solder material and sintered body that can efficiently dissipate heat generated by power semiconductor chips and prevent cracks from occurring in the solder, which would increase thermal resistance, in order to solve the problems associated with the conventional technology described above.

In order to solve the above-mentioned problems and achieve the object of the present invention, the solder material according to the present invention has the following features: A solder material used in a semiconductor device having an assembly structure in which a semiconductor element is mounted on a laminated substrate, the solder material contains metal fibers, and spaces between the metal fibers are filled with solder, and the solder material is a bonding layer that bonds the semiconductor element to an electrode pattern on the laminated substrate. In the bonding layer, the occupancy rate of the metal fibers is 20 to 30% by weight of the total amount of the solder and the metal fibers, and the metal fibers are sintered and have contact points with each other.

The solder material according to the present invention is characterized in that, in the above-mentioned invention, the assembly structure further includes a heat sink on which the laminated substrate is mounted, and the solder material is a bonding layer that bonds the laminated substrate and the heat sink.

The solder material according to the present invention is characterized in that, in the above-mentioned invention, the diameter of the metal fibers is equal to or less than the thickness of the solder material.

The solder material according to the present invention is characterized in that the metal fibers are folded in two or more layers in the above-mentioned invention.

The solder material according to the present invention is characterized in that, in the above-mentioned invention, the metal fibers are copper fibers and have a diameter of 20 μm or less.

In addition, the solder material according to the present invention is characterized in that, in the above-mentioned invention, the length of the metal fibers is 50 μm or more and 10 mm or less .

The solder material according to the present invention is characterized in that, in the above-mentioned invention, the metal fibers are Ni-plated.

The solder material according to the present invention is characterized in that in the above-mentioned invention, the solder is a Sn-Sb solder or a Sn-Ag solder.

The semiconductor device according to the present invention is characterized in that, in the above-mentioned invention, the metal fibers have contacts with each other.

The semiconductor device according to the present invention is also characterized in that, in the above-mentioned invention, the solder does not contain copper.

The semiconductor device according to the present invention is characterized in that in the above-mentioned invention, the solder is a Sn-Sb solder containing Ni or Co, a Sn-Bi solder containing Ni or Co, or a Sn-Ag solder containing Ni or Co.

The semiconductor device according to the present invention is characterized in that, in the above-mentioned invention, the metal fibers are Ni-plated.

The semiconductor device according to the present invention is characterized in that, in the above-mentioned invention, the metal fibers are Co-plated.

The semiconductor device according to the present invention is characterized in that, in the above-mentioned invention, the spaces between the metal fibers are filled with a sintered material of Ag or Cu.

The semiconductor device according to the present invention is characterized in that, in the above-mentioned invention, the solder material has two or more layers of the metal fibers folded over each other.

In order to solve the above-mentioned problems and achieve the object of the present invention, the method for manufacturing a semiconductor device according to the present invention has the following features. First, a process is performed in which an electrode pattern on a laminated substrate and a semiconductor element are joined using a solder material containing metal fibers with the spaces between the metal fibers filled with solder, thereby mounting the semiconductor element on the laminated substrate. Then, a process is performed in which the laminated substrate is assembled into a laminated assembly. Next, a process is performed in which the semiconductor element and the electrode pattern on the laminated substrate are electrically connected. Next, a process is performed in which a resin case is assembled to the laminated assembly.

The manufacturing method of the semiconductor device according to the present invention is characterized in that in the electrical connection step, the semiconductor element and the electrode pattern on the laminated substrate are electrically connected using the solder material.

The manufacturing method of the semiconductor device according to the present invention is characterized in that in the assembling step, the laminated substrate is joined to the heat sink of the laminated assembly using the solder material.

According to the above-mentioned invention, the joint that joins the power semiconductor element and the electrode pattern is a copper fiber-containing solder material that contains copper fibers and fills the spaces between the copper fibers with solder. This improves the thermal conductivity compared to paste solder or plate solder containing flux, so the heat generated by the power semiconductor chip can be dissipated efficiently. In addition, since the solder contains copper fibers, even if cracks occur in the solder, the cracks will progress by bypassing them, so the life of the solder is improved. Furthermore, since the solder contains copper fibers, it is possible to prevent fillets from occurring and the solder is less likely to protrude to the sides. In addition, since the copper fibers are in contact with each other and form a thermal path, it is possible to suppress an increase in thermal resistance even if cracks occur in the solder. In addition, by impregnating the copper fiber member with solder to make plate solder, it is possible to handle it in the same way as before and to control the solder thickness more uniformly than before.

The solder material and sintered body of the present invention have the effect of efficiently dissipating heat generated by power semiconductor chips and suppressing the occurrence of cracks in the solder and an increase in thermal resistance.

FIG. 1 is a cross-sectional view showing a configuration of a power semiconductor module according to an embodiment. FIG. 2 is a cross-sectional view (part 1) showing the details of the solder material that joins the power semiconductor chip and the electrode pattern. FIG. 3 is a cross-sectional view (part 2) showing details of the solder material that joins the power semiconductor chip and the electrode pattern. FIG. 4 is a table showing the results of the power cycle test of the solder according to the embodiment. FIG. 5 is a graph showing the relationship between solder thickness and equivalent thermal conductivity. FIG. 6 is a graph showing the relationship between the copper occupancy rate and the equivalent thermal conductivity. FIG. 7 is a cross-sectional view showing an embodiment of a solder material containing copper fibers. FIG. 8 is a cross-sectional view of a joint of an example of a copper fiber containing solder material. FIG. 9 is a cross-sectional view showing the configuration of a power semiconductor module having a conventional structure.

Below, preferred embodiments of the solder material and sintered body according to the present invention will be described in detail with reference to the attached drawings. Figure 1 is a cross-sectional view showing the configuration of a power semiconductor module according to the embodiment.

(Embodiment)
As shown in Fig. 1, the power semiconductor module includes a power semiconductor chip 1, an insulating substrate 2, an electrode pattern 3, a joint 4, a heat sink 5, a lead frame wiring 8, and a conductive plate 9 disposed on the back surface of the insulating substrate 2. Since the power semiconductor module is similar to a power semiconductor module of a conventional structure, illustration of a cooling body 7, an external terminal 11, a terminal case 12, a sealing material 13, etc. is omitted here. In Fig. 1, the power semiconductor chip 1 and the electrode pattern 3 are connected using the lead frame wiring 8, but they may be connected using a metal wire 10 as in the conventional structure.

The power semiconductor chip 1 is a semiconductor element such as an IGBT or a diode chip. An electrode pattern 3 made of a conductive plate such as copper (Cu) is provided on the front surface (the power semiconductor chip 1 side) and the back surface (the heat sink 5 side) of an insulating substrate 2 such as a ceramic substrate that ensures insulation. A substrate on which the electrode pattern 3 is provided on at least one side of the insulating substrate 2 is called a laminated substrate. The power semiconductor chip 1 is bonded to the electrode pattern 3 at the joint 4. The heat sink 5 is bonded to the conductive plate 9 on the back surface at the joint 4. The heat sink 5 is bonded to a cooling body (not shown) on which a heat sink fin is provided. The conductive plate such as copper on the front surface of the laminated substrate is called the electrode pattern, and the conductive plate such as copper on the back surface is called the conductive plate. In addition, one end of the lead frame wiring 8 is bonded to the upper surface of the power semiconductor chip 1 (the surface opposite to the surface in contact with the joint 4) at the joint 4 as wiring for electrical connection. The other end of the lead frame wiring 8 is bonded to the electrode pattern 3. In addition to the above locations, the joints of the present invention can also be used in locations where solder materials are used in conventional semiconductor modules.

The joint 4 is formed by a metal fiber-containing solder material containing a metal fiber member. The metal fiber-containing solder material contains fibrous metal (hereinafter referred to as metal fiber), and the metal fibers have contacts with each other to form a heat path, and the spaces between the metal fibers are filled with solder. The metal is preferably a metal with high thermal conductivity, such as copper. Hereinafter, fibrous copper is referred to as copper fiber, and the joint 4 is referred to as copper fiber-containing solder material 4. Copper fibers will be described below. Hereinafter, fibrous refers to an elongated shape, that is, a shape whose length is extremely large compared to its diameter. In the embodiment, the diameter of one copper fiber is preferably 20 μm or less. In addition, the length of the copper fiber is preferably 50 μm or more, and more preferably 1 mm or more. This is because the above length allows more contact between the copper fibers and makes it easier to form a three-dimensional structure. In addition, the length is preferably 10 mm or less, which is about the length of the copper fiber member.

The contact is a point where the copper fiber of the copper fiber-containing solder material 4 is in contact with another copper fiber. The copper fiber member is formed of multiple copper fibers. The copper fiber member may be in the form of a cloth in which copper fibers are woven like a fabric, or may be formed in the form of a net or mesh. Multiple sheets of these cloth-like or net-like copper fibers may be laminated. Multiple fibers may be randomly accumulated and laminated to form a sheet. Furthermore, the laminated sheet may be pressed to pressurize the copper fibers together. It is also preferable that these are formed into a sheet. The thickness of the sheet-like copper fiber member is preferably 50 μm to 200 μm. This is because a predetermined bonding strength can be obtained when the thickness is greater than 50 μm. Furthermore, if the thickness is greater than 200 μm, the thermal resistance itself increases, voids are generated, the thermal conductivity decreases, and the thermal resistance increases. The spaces between the copper fibers may be filled with a sintered material of silver (Ag) or Cu instead of filling with solder.

In this way, the copper fiber-containing solder material 4 of the embodiment includes copper fibers that have contacts with each other and form a thermal path, and is different from a solder material containing spherical copper. The thermal path is a path for transferring heat generated by a power semiconductor chip or the like. A solder material containing spherical copper has fewer thermal paths than copper fibers, has a large thermal resistance, and has the same joint strength as the solder itself. Note that simply placing a metal such as copper in a plate or foil shape in the solder does not provide the same joint strength or thermal conductivity as a solder containing a copper fiber member. This is because the thickness of the solder layer itself does not change in order to obtain a predetermined joint strength, even if a copper plate or the like is placed in the solder. In other words, the thermal resistance does not change. By impregnating the solder between the three-dimensional copper fibers, the thermal resistance decreases and the joint strength can also be improved.

The copper fiber-containing solder material 4 may be a solder material formed by weaving copper fibers, sintering the copper fibers to form a copper fiber member in which the fibers are folded so that they have contact points with each other, and impregnating the copper fiber member with solder. By impregnating the copper fiber member with solder in advance, it can be handled as a plate solder. Specifically, a copper fiber-containing solder is formed by impregnating the copper fiber member with solder in advance, and the solder is placed between the joined materials and heated to join them. In addition, when assembling a semiconductor module, the solder and the copper fiber member may be placed between the joined materials and heated to join them. Here, it is preferable that the copper fibers are folded in two or more layers. Folded in two or more layers means that there are two or more copper fibers that have contact points with each other in the thickness direction (from the power semiconductor chip 1 to the heat sink 5). In the example of FIG. 1, the copper fibers are folded in three layers. Here, because the copper is folded in a fibrous form, the copper occupancy is higher than when it is processed into a mesh form. For example, in copper fiber-containing solder material 4, the copper occupancy of the copper fiber member is 22 to 30% by weight relative to the total amount of solder and copper fiber member. The higher the copper occupancy of the copper fiber member, the better the thermal conductivity, but if the amount of solder impregnated is small, the bondability will be poor. Therefore, the copper occupancy of copper fiber members of various forms is preferably 5 to 50% by weight, and more preferably 20 to 30% by weight.

In this way, by disposing the copper fiber member in the solder, the thermal conductivity of the copper fiber-containing solder material 4 is improved, and the generated heat can be efficiently dissipated. In addition, since the solder contains copper fibers, even if a crack occurs in the solder, the crack will progress by detouring, and the joint strength is improved. In addition, since the copper fibers themselves have strength, the joint strength is high. In addition, the life of the solder is improved. In addition, by using the above-mentioned sheet-like copper fiber member, the thickness of the copper fiber-containing solder material 4 can be made uniform. In the case of conventional bonding using only solder material, it was difficult to make the thickness of the solder material 1014 uniform because the power semiconductor chip 1 was misaligned when placed on the solder material, or the solder material flowed during heat bonding. However, by introducing the copper fiber member, the above problems are eliminated, and the thickness of the copper fiber-containing solder material 4 can be made uniform.

In addition, in order to efficiently diffuse the heat generated by the power semiconductor chip 1, the copper fiber-containing solder material 4 is preferably used under the power semiconductor chip 1, that is, as a bonding layer between the power semiconductor chip 1 and the electrode pattern 3. The copper fiber-containing solder material 4 may also be used as a bonding layer between the conductive plate 9 and the heat sink 5, and as a bonding layer between the power semiconductor chip 1 and the lead frame wiring 8.

In the method for manufacturing a power semiconductor module, first, the power semiconductor chip 1 is mounted on the laminated substrate by bonding the power semiconductor chip 1 to the laminated substrate using the copper fiber-containing solder material 4. Here, the copper fiber-containing solder material 4 may be created by impregnating the copper fiber member with solder prior to the manufacture of the power semiconductor module. Also, when bonding the power semiconductor chip 1 to the laminated substrate, the copper fiber member and the solder may be overlapped, for example, by sandwiching the copper fiber member between the solder, to create the copper fiber-containing solder material 4.

Next, the power semiconductor chip 1 and the electrode pattern 3 provided on the insulating substrate 2 are electrically connected by the lead frame wiring 8. Next, these are joined to the heat sink 5 using the copper fiber-containing solder material 4 to assemble a laminated assembly consisting of the power semiconductor chip 1, laminated substrate, and heat sink 5. A resin case is bonded to this laminated assembly with an adhesive such as silicone. Note that the power semiconductor chip 1 and the electrode pattern 3 provided on the insulating substrate 2 may also be electrically connected by a metal wire.

Next, the electrode pattern 3 and the metal external terminal 11 are connected with metal wires, and the resin case is filled with a sealing material such as a hard resin such as epoxy. This completes the power semiconductor module according to the embodiment shown in FIG. 1. If the sealing material is not a sealing material such as epoxy resin, a lid is attached to prevent the sealing material from leaking out.

Next, the copper fiber-containing solder material 4 will be described. The copper fiber-containing solder material 4 is arranged on almost the entire back surface of the power semiconductor chip. Figures 2 and 3 are cross-sectional views showing the details of the solder material that joins the power semiconductor chip and the electrode pattern. As shown in Figure 2, the copper fiber member 20 is arranged in the center inside the copper fiber-containing solder material 4. Also, solder 23 that does not contain copper fibers may be arranged on the power semiconductor chip 1 or electrode pattern 3 side. In this case, the thickness of each solder 23 is preferably 25 μm to 100 μm. This is because by setting it in this range, both the bonding strength and the reduction of voids can be achieved. This is because the heat generated by the power semiconductor chip 1 is dissipated from the center where the copper fiber member 20 is arranged. Here, by separating the copper fiber member 20 by a predetermined distance d from the end of the copper fiber-containing solder material 4, the voids can be reduced and the bonding property can be improved. The copper fiber member 20 has a structure in which the copper fibers are bent in a complex manner and cross each other, and voids exist. When the solder is heated and bonded, the solder is impregnated into the copper fiber member 20, but voids tend to form near the depletion. However, it is presumed that by separating the solder by a certain distance d, the voids are easily discharged, resulting in a reduction in the number of voids. Regardless of the size of the power semiconductor chip 1, the distance d is preferably 0.1 mm or more and 1 mm or less. More preferably, it is 0.2 mm or more. This is because if it is shorter than 0.1 mm, voids will form, which will deteriorate the bond with the electrode pattern 3 and make it difficult to control.

As shown in FIG. 3, the solder 23 may be sandwiched between the copper fiber members 20 inside the copper fiber-containing solder material 4. In this case, a layer of the solder 23 with a predetermined thickness t is present between the layers of the copper fiber members. In this form, the copper fiber member 20 is placed on the side of the power semiconductor chip 1 or the electrode pattern 3, improving thermal conductivity. For example, the form of FIG. 3 has improved thermal conductivity compared to the form of FIG. 2 in which the solder 23 is present on the side of the power semiconductor chip 1 or the electrode pattern 3. In addition, this structure makes it difficult for voids to occur near the copper fiber member 20. The thickness t of the center portion is preferably 5 μm or more and 20 μm or less. This is because if it is 20 μm or more, the thermal resistance increases, and if it is 5 μm or less, voids are likely to occur. In addition, the ratio of the thickness t to the thickness of the copper fiber-containing solder material 4 is preferably 5% to 20%.

In addition, by using a solder of a specific composition in the solder of the copper fiber-containing solder material, it is possible to produce effects in the joint strength and thermal properties. For example, in the solder of the copper fiber-containing solder material 4, since heat is transferred through the copper fibers, a Cu-free solder is preferable, which suppresses the diffusion of copper and copper alloys in the copper fibers. For example, when the copper in the copper fibers is alloyed with tin (Sn) and diffuses, the area that transfers heat decreases and the thermal conductivity decreases, so it is preferable to add nickel (Ni) and cobalt (Co), which can suppress the diffusion of copper and copper alloys, to the solder. Note that Cu-free means that it does not contain Cu other than as an impurity.

As a specific solder composition, it is preferable to use Sn-(5-10)Sb-(0.1-1)Ni,Co solder (Example 1), which is a Sn-Sb (antimony) based solder with Ni and Co added. The unit here is weight percent (wt%). For example, in Example 1, Sb is contained in the solder at 5-10 wt%. Also, since Ni and Co have the same effect, only Ni or only Co may be contained. Furthermore, it is more preferable that Ni and Co are contained in the solder at 0.2-0.5 wt%.

It is also preferable to use Sn-(40-70)Bi-(0.1-1)Ni,Co solder (Example 2), which is a Sn-Bi (bismuth) solder containing Ni and Co. Solder of this composition is brittle and cannot normally be used to join power semiconductor chips 1, but when used together with a copper fiber member as in the embodiment, it has sufficient strength and can be used to join power semiconductor chips 1. In addition, the Ni and Co are similar to Sn-Sb solder.

It is also preferable to use Sn-(1-6)Ag-(0.1-1)Ni,Co solder (Example 3), which is a Sn-Ag (silver) solder containing Ni and Co. The Ni and Co content is the same as for Sn-Sb solder.

In addition, in the above example, the diffusion of copper and copper alloy in the copper fibers was suppressed by adding Ni and Co to the solder, but the same effect can be obtained by plating the copper fibers with Ni or Co.

Figure 4 is a table showing the results of a power cycle test of the solder according to the embodiment. In addition to the solders of Examples 1 to 3 above, a Sn-Sb-Cu-Ni solder was also tested as a comparative example. In the case of Example 1, a test was also conducted in which the solder contained 0.4% by weight of Ni and Co. In the power cycle test, the power was repeatedly turned on and off, and the number of times that the thermal resistance increased by 20% or more from the initial value was measured.

As shown in Figure 4, in the case of Examples 1 to 3, even after the power was turned on and off more than 100,000 times, the thermal resistance did not increase by more than 20% from the initial value. In particular, when the solder contained 0.4% by weight of Ni and Co, the thermal resistance did not increase by more than 20% from the initial value even after more than 200,000 times. In contrast, in the case of the comparative example, the thermal resistance increased by more than 20% from the initial value after 50,000 or fewer cycles. This shows that the use of the solders of Examples 1 to 3 produces effects on joint strength and thermal properties.

Next, the effect of the present invention was confirmed by an actual example. First, the thickness of the solder is determined when the solder is sandwiched between the copper fiber member 20 from above and below. FIG. 5 is a graph showing the relationship between the solder thickness and the equivalent thermal conductivity. The horizontal axis is the sum of the thicknesses of the upper and lower solder in μm, and the vertical axis is the equivalent thermal conductivity in W/m·K. Here, the equivalent thermal conductivity is the thermal conductivity given to a part made of multiple materials as if it were one block. The distance d from the end of the copper fiber-containing solder material 4 to the end of the copper fiber member was set to 0.5 mm.

5 shows the calculation results when the thermal conductivity of the solder is 40 W/m·K, the thermal conductivity of copper is 390 W/m·K, the copper occupancy rate of the copper fiber member is 22%, and the thickness of the copper fiber member is 100 μm. As shown in FIG. 5, the relationship between the equivalent thermal conductivity y and the sum x of the thicknesses of the upper and lower solders is as follows:
y = 113.04x ^-0.151
For example, if the thickness of the upper solder, the thickness of the copper fiber member, and the thickness of the lower solder are 10 μm, 100 μm, and 10 μm, respectively, the equivalent thermal conductivity is about 80 W/m·K, which is about twice the 40 W/m·K of the solder alone. Also, if they are 20 μm, 100 μm, and 20 μm, respectively, the equivalent thermal conductivity is about 72 W/m·K.

Fig. 6 is a graph showing the relationship between the copper occupancy rate and the equivalent thermal conductivity. The horizontal axis is the copper occupancy rate in weight percent, and the vertical axis is the equivalent thermal conductivity in W/m·K. Here, the copper occupancy rate is the weight percent of copper contained in the copper fiber member, and the remainder is the weight of the solder. In Fig. 6, the lines marked with ● represent the relationship between the copper occupancy rate x and the equivalent thermal conductivity y when the thickness of the upper solder, the thickness of the copper fiber member, and the thickness of the lower solder are 10 μm, 100 μm, and 10 μm, respectively. In this case,
y = 3.25x + 6.6667
The straight lines with circles represent the relationship between the copper occupancy rate x and the equivalent thermal conductivity y when the thickness is 50 μm, 100 μm, and 50 μm, respectively. In this case,
y = 1.95x + 20
It can be seen from Fig. 6 that in the case of the straight line marked with a black circle, if the copper occupancy rate is set to 22% by weight, the equivalent thermal conductivity is approximately 80 W/m·K.

Based on the above results, the results are shown for a 100 μm thick copper fiber member 20 sandwiched between 10 μm thick Sn-Sb solder from above and below. Figure 7 is a cross-sectional view showing an example of copper fiber-containing solder material 4. As shown in Figure 7, copper fiber-containing solder material 4 is composed of a copper fiber portion 22 in which the copper fibers are folded over so that they have contact with each other, and a solder-soaked portion 21 in which solder has soaked between the copper fiber members 20.

Figure 8 is a cross-sectional view of the joint of an embodiment of the copper fiber-containing solder material. In Figure 8, the right figure is an enlarged view of the center of the left figure. As shown in Figure 8, the copper fiber members 20 have contacts with each other, and it can be seen that the spaces between the copper fiber members 20 are filled with solder 23. The thermal conductivity of the copper fiber-containing solder material created in this way is calculated to be 72.2 W/m·K. As a result of actual measurement using the laser flash method, it was confirmed that while the thermal conductivity of only Sn-Sb-based solder is about 40 W/m·K, the thermal conductivity of the copper fiber-containing solder material is improved to 75.8 W/m·K. Here, the laser flash method is a method of obtaining thermal diffusivity by uniformly pulse-heating the surface of a flat plate-shaped sample placed in an adiabatic vacuum and observing the one-dimensional thermal diffusion phenomenon from the front surface to the back surface.

As described above, according to the solder material and sintered body of the embodiment, the joint that joins the power semiconductor element and the electrode pattern is a copper fiber-containing solder material that contains copper fibers and fills the spaces between the copper fibers with solder. This improves the thermal conductivity compared to paste solder or plate solder containing flux, so that the heat generated by the power semiconductor chip can be dissipated efficiently. In addition, since the solder contains copper fibers, even if cracks occur in the solder, the cracks will progress by bypassing them, so the life of the solder is improved. Furthermore, since the solder contains copper fibers, it is possible to prevent fillets from occurring and the solder is less likely to protrude to the sides. In addition, since the copper fibers are in contact with each other and form a thermal path, it is possible to suppress an increase in thermal resistance even if cracks occur in the solder. In addition, by impregnating the copper fiber member with solder to make plate solder, it is possible to handle it in the same way as before and to control the solder thickness more uniformly than before.

In addition, the copper fiber-containing solder material has the internal copper fibers arranged in the center, away from the power semiconductor element and the electrode pattern, and no copper fibers are arranged at the ends of the copper fiber-containing solder material. This allows the heat generated by the power semiconductor chip to be dissipated from the center where the copper fibers are arranged, reducing voids and improving bonding.

In addition, copper fiber-containing solder material is joined by sandwiching the solder between the copper fibers, with the copper fibers being placed on the power semiconductor chip or electrode pattern side. This further improves thermal conductivity. When solder is sandwiched between copper fiber members, the thermal conductivity is improved by about 5% compared to the above result where the copper fiber member is sandwiched between solder. The copper fiber member and solder are of the same thickness. Voids are also less likely to occur near the copper fibers.

The composition of the solder contained in the copper fiber-containing solder material is Sn-(5-10)Sb-(0.1-1)Ni, Co, Sn-(40-70)Sb-(0.1-1)Ni, Co, or Sn-(1-6)Sb-(0.1-1)Ni, Co. The Ni and Co can suppress the diffusion of copper and copper alloys in the copper fibers, and suppress the decrease in thermal conductivity. Furthermore, the use of these solders can produce effects on joint strength and thermal properties.

The copper fibers may also be Ni-plated or Co-plated. Ni and Co can suppress the diffusion of copper and copper alloys in the copper fibers, preventing a decrease in thermal conductivity.

As described above, the solder material and sintered body of the present invention are useful for power semiconductor devices used in power conversion devices such as inverters, power supply devices for various industrial machines, and igniters for automobiles.

1 Power semiconductor chip 2 Insulating substrate 3 Electrode pattern 4 Joint (copper fiber-containing solder material)
5 Heat sink plate 6 Heat sink grease 7 Cooling body 8 Lead frame wiring 9 Conductive plate 10 Metal wire 11 External terminal 12 Terminal case 13 Sealing material 14 Solder material 20 Copper fiber member 21 Solder-immersed portion 22 Copper fiber portion 23 Solder

Claims (8)

A solder material used in a semiconductor device having an assembly structure in which a semiconductor element is mounted on a laminated substrate,
The solder material includes metal fibers, and spaces between the metal fibers are filled with solder.
the solder material is a bonding layer that bonds the semiconductor element and an electrode pattern on the laminated substrate,
In the bonding layer, the occupancy rate of the metal fibers is 20 to 30% by weight with respect to the total amount of the solder and the metal fibers;
The metal fibers are sintered and have contacts with each other. The solder material according to claim 1, characterized in that the assembly structure further includes a heat sink on which the laminated substrate is mounted, and the solder material is a bonding layer that bonds the laminated substrate and the heat sink. The solder material according to claim 1 or 2, characterized in that the diameter of the metal fibers is equal to or less than the thickness of the solder material. The solder material according to any one of claims 1 to 3, characterized in that the metal fibers are folded in two or more layers. The solder material according to any one of claims 1 to 4, characterized in that the metal fibers are copper fibers and have a diameter of 20 μm or less. The solder material according to any one of claims 1 to 5, characterized in that the length of the metal fibers is 50 μm or more and 10 mm or less. The solder material according to any one of claims 1 to 6, characterized in that the metal fibers are Ni-plated. 8. The solder material according to claim 1, wherein the solder is a Sn--Sb based solder or a Sn--Ag based solder .

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