JP4972503B2 - Semiconductor power module - Google Patents

Description

  The present invention relates to a semiconductor power module, and more particularly to a semiconductor power module suitable for a technique applied to connection of an IGBT, a power MOSFET or the like.

The semiconductor power module has a configuration in which an Si chip, an insulating substrate such as AlN, Al ₂ O ₃ , and Si ₃ N ₄ and a Cu, Al base substrate having excellent thermal conductivity are soldered and filled with silicone gel. Mainstream. Up to now, after the Si chip and the electrodes of the insulating substrate such as AlN, Al ₂ O ₃ , Si ₃ N ₄ are connected with Pb-based high temperature solder, the insulating substrate on which the chip is mounted is a large Cu base substrate or the like And soldering with a low-temperature Pb—Sn eutectic solder.

  In recent years, the use of Pb-free solder is indispensable due to environmental problems. In Japan, Sn-3Ag-0.5Cu solder is widely used for consumer, computer, and communication as a substitute for low-temperature Sn-37Pb eutectic solder. Has been.

  However, Sn-3Ag-0.5Cu solder is not in a situation where high reliability is secured for connecting a large substrate such as a base substrate. If Sn-3Ag-0.5Cu solder (melting point: 217-221 ° C.) is used for low temperature, Sn— (5 -10) Sb (melting point: 235-243 ° C) is known. However, in this combination, it is necessary to perform the soldering temperature of Sn-3Ag-0.5Cu at 225 to 230 ° C. or less, so that there is little temperature variation and the connection is not possible under special conditions. That is, the wettability cannot be secured at this temperature.

  Incidentally, as a Sn—Ag—Cu—In solder composition patent, US Pat. No. 5,520,752 is known.

US Pat. No. 5,520,752

  For the purpose of a high-reliability power module using Pb-free two-layer solder, there is a structure especially intended for mounting on industrial and vehicle high-power chips.

  Conventional low temperature Pb-free solder, Sn-3Ag-0.5Cu, has high creep resistance, but it is difficult for its own creep deformation, so it supports large deformation at the end of large substrates such as power modules After all, it was found that the life was shortened due to high stress.

  In addition, for conventional high-temperature Sn- (5-10) Sb (melting point: 235-243 ° C.), when attaching low-temperature side Pb-free solder [Sn-3Ag-0.5Cu (melting point: 217-221 ° C.)] Since the melting points of the two are close to each other, there is a risk of melting the high-temperature solder that has been joined first, and a sufficient temperature margin cannot be obtained.

  The present invention has been made in view of the above-described points, and an object of the present invention is to provide a high-temperature Pb-free solder that can be reliably bonded to a large-sized substrate with a low-temperature Pb-free solder and can be connected to a temperature hierarchy. It is an object to provide a semiconductor power module that can be combined with the semiconductor power module.

  As a method of achieving both high reliability and a soldering process with Pb-free layered solder, a configuration using Sn- (11-20) Sb (for example, the melting point of Sn-15Sb; 246-290 ° C.) is used as the high-temperature side solder. Thought.

  In the case of the power module structure, fortunately, it is composed of heat-resistant parts. Therefore, when using high-temperature solder, it is possible to sufficiently raise the soldering temperature and ensure wettability. Sn- (11-20) Sb is slightly inferior to Sn- (5-10) Sb, but the wettability can be improved by raising the soldering temperature. In addition, compared to Sn- (5-10) Sb, even though the hardness and deformability of the solder, which is regarded as a disadvantage, are inferior, the difference in thermal expansion coefficient between the Si chip and the AlN substrate is small, so that the solder Strain is small and can withstand temperature cycling.

  In order to further utilize the characteristics of Sn-15Sb, if the solder and the periphery of the chip are coated with an epoxy resin whose physical properties are specified, the life can be improved by restraining the deformation of the solder by the resin. Further, regarding the reaction with metallization, the Sn-Ag-Cu system is the same, but the compound growth is active. For this reason, as described later, by adding Cu, Ni or the like to the Sn—Sb solder, it is possible to cope with severe conditions in consideration of the combination with the metallized film, heat treatment conditions, and the like.

  On the other hand, Sn-3Ag-0.5Cu-5In (203-212 ° C.) is found as a solder composition that can withstand relatively large deformation at the end of a large Cu base substrate as a Pb-free low-temperature side solder. It was. Since this solder can lower the soldering temperature by about 10 ° C. from Sn-3Ag-0.5Cu (217-221 ° C.), the working temperature margin for hierarchical connection can be widened. The soldering temperature of Sn-3Ag-0.5Cu-5In is sufficiently possible at 230 to 240 ° C.

  From this, by using a combination of Sn-15Sb (246-290) ° C. for high temperature and Sn-3Ag-0.5Cu-5In (203-212 ° C.) for low temperature, temperature hierarchical connection with Pb-free solder Thus, a power module that can ensure the reliability of the connecting portion is possible.

  By using the semiconductor power module of the present invention, a low temperature Pb-free solder can be bonded with a high-temperature Pb-free solder that can be reliably connected to a large substrate and can be connected to a temperature hierarchy. It becomes.

  An object of the present invention is to provide a semiconductor power module that can be combined with a high-temperature Pb-free solder that can be reliably connected to a large substrate with a low-temperature Pb-free solder and that can be connected to a temperature hierarchy. This was achieved with a simple solder configuration.

Example 1
In the case of a high-temperature solder for connecting the chips of the power module, taking advantage of the fact that the soldering temperature can be close to 400 ° C., we considered intentionally increasing the soldering temperature to ensure wettability. Furthermore, with Pb-free soldering for power modules, there is no problem with the heat resistance of the parts, so the soldering temperature can be raised even a little when connecting the solder under the chip, improving the yield of the connection, such as measures against voids and ensuring wettability. It leads to. Among the solders that may be used, the solder with the highest melting point is the Sn-Sb system, which has been studied in the range of Sb; 5 to 10% so far. Constrained at the line temperature of 235 ° C.

  On the other hand, if the composition selection exceeding 10% is taken into consideration, the solidus temperature is 246 ° C., and Sn-3Ag-0.5Cu-5In (203 to 212 ° C.) is used on the low temperature side. It is possible to connect the low-temperature side solder at 240 ° C. or lower. In addition, Sn-3Ag-0.5Cu-5In has higher reliability in the temperature cycle test between the ceramic substrate and the large Cu base substrate than Sn-3Ag-0.5Cu, and can ensure high reliability. For the soldering, a high-temperature system and a low-temperature system were either a hydrogen furnace or a vacuum soldering furnace in which a vacuum and a hydrogen atmosphere were combined.

(1) High-temperature Pb-free solder for semiconductor chip side connection 1-1) Selection of Sn-Sb composition Sn- (5-10) Sb is a composition that has been used for connecting Cu pipes, etc. It is the same level as Sn-3.5Ag. When Sb exceeds 10%, the elongation gradually decreases, but does not rapidly decrease. In the tensile test at 150 ° C., when Sb is too 10%, the tensile strength increases, but it becomes saturated. Further, the elongation decreases by 10% too much, but the elongation at Sb: 15% is comparable to Sn, so it seems that there is no problem as a characteristic. From these facts, as mechanical characteristics, when Sb: 10% is exceeded, it tends to be gradually hardened, but up to 20% is considered to be a usable range. Therefore, the appropriate range of Sb is set to 11% or more and 20% or less.

The reason why the solder for connecting under the chip can be used even if it is somewhat hard and the appropriate composition range of Sb is considered as follows. For high-power chips, an AlN substrate having excellent thermal conductivity is generally used. Since the difference in thermal expansion coefficient between AlN (4.3 × 10 ⁻⁶ / ° C.) and Si chip (2 × 10 ⁻⁶ / ° C.) is small, the strain generated by temperature change is small even if the solder is somewhat hard, There are some problems with the chip size, power level, etc., but it can be dealt with if the chip size can be reduced or the output can be reduced. The low cycle fatigue life of Sn-15Sb exhibits a life of Sn-3.5Sb or more when the displacement is small.

  However, if the displacement is large, the life is shortened because it becomes hard. That is, it is not preferable to be too hard because a large stress acts on the chip. Therefore, although not an ideal region, Sb: 10% or more, and 12 to 15% (liquidus temperature; about 290 ° C.) was selected as a good range. That is, the connection can be made at a soldering temperature of about 330 to 350 ° C. (when soldering is performed in a reducing atmosphere containing hydrogen, it is easily reduced by reducing the void intentionally, leading to a reduction in voids). In the case of Sb; 20%, the liquidus temperature of the solder is 325 ° C., and in order to ensure the wettability in hydrogen, as a means of increasing the reducing power by hydrogen, the temperature is 380 ° C., which is sufficiently away from the liquid phase temperature. Even if connected, there is no problem of thermal effects on the parts. Therefore, 11 to 20% was selected as the range of Sb.

1-2) Addition amount of trace elements to Sn-Sb solder (Cu, Ni, Ge, etc.)
Since the Sn-based solder is soldered at a high temperature and used for a long time at a high temperature, the attack action on the electrode and the substrate is strong. When a Cu base substrate or Cu electrode is used, 1 to 5% of Cu is added to the solder to prevent the substrate and the electrode from being bitten by Sn and to stabilize the compound layer even when left at high temperature for a long time. In the case of Ni plating, Ni / Au flash or the like on the surface, adding Cu and Ni into the solder similarly has the same effect as in the case of the Cu electrode. In the case of adding Ni, since it is difficult to be dissolved in the solder, it may be about 0.1 to 0.5%. In addition, in order to prevent coarsening of Sn—Sb crystal grains, the addition of Ge: 0.01 to 0.1% can be expected to be effective for reforming by miniaturization and void reduction by preventing oxidation. In addition, addition of 0.1% or more and 1% or less of Zn, Al, Ag, P, and Bi may also modify the solder and the reaction layer.

(2) Pb-free low-temperature solder for connection between base substrate and ceramic insulation substrate 2-1) Selection of In amount in Sn-Ag-Cu-In system Pb-free low-temperature solder for connection between ceramic insulation substrate and large Cu base substrate For composition selection, solder life evaluation was performed by a temperature cycle test (−55 to 125 ° C.) using a model specimen for a promising composition. To some extent, the solder composition was narrowed down in preliminary experiments. Solder containing Zn with a lot of voids (a small amount of Zn added to Sn-Zn and Sn-Ag-Cu) cannot satisfy the void rate target in the current process, and results in bad results even if evaluated. I had to do it. The basic view of the additive element to Sn-based solder is that, when dissolved in Sn, the internal strain increases and it becomes harder. Therefore, the deformability expected as a solder for board connection is lost. I think it is difficult for elements.

  Various rolled solder foils of 150 μm were placed between a 0.3 mm Cu-laminated alumina substrate (t0.3) and a large Cu base substrate (t4), and reflow was performed at a soldering temperature of max 245 ° C. in a reducing atmosphere. The solder foils were different in properties shown in Table 1 as the narrowed composition, and were narrowed down to seven promising compositions and subjected to a temperature cycle test in a module structure.

  The composition is reduced by about 10 ° C. compared to Sn-3Ag-0.5Cu, which is a base for low-temperature post-installation solder, by adding 5% In to the Sn—Ag—Cu system, and sufficient at the bonding interface. This is to ensure strength. Unlike the addition of Bi, the addition of In is known to have little adverse effect on the bonding strength against various metallizations and the mechanical properties of the bulk material due to the addition of a small amount. Further, since it is a solid solution in Sn crystal, the reliability for high temperature is comparable to Sn—Ag—Cu as long as Sn—In eutectic is not generated.

  However, adding a large amount of In causes further dissolution of In in the Sn matrix, increasing internal strain and making it difficult to extend. In the case of In, because it is an element softer than the base Sn, it is known that the increase in strength is small, the decrease in elongation is small, and the addition of In has little influence on the mechanical strength. When 5% or more of In is added, strength reduction at the bonding interface starts.

Moreover, since the melting point decreases as the In content increases, the temperature layer connection is more advantageous, but the alloy layer (Cu ₆ Sn ₅ , Cu ₃ Sn) at the interface in the long-term high-temperature standing test is likely to grow, A decrease in strength occurs. Up to 7%, it was confirmed that there was little decrease in strength, so the upper limit of In was set to 7%. On the other hand, if the amount of In is small, it approaches Sn-3Ag-0.5Cu, and the life of the temperature cycle test becomes a problem. Even if In is 1%, the effect of slightly changing the properties of the solid solution can be expected, so the lower limit of In was set to 1%.

  Table 1 shows the crack growth rate by measuring the crack length with respect to the total length of the joint by cross-sectional observation after lapse of 2500 cycles at -55 to 125 ° C.

The following new findings were obtained from these results, the observation of the progress of cracks in the section, and the mechanism elucidation experiment.
1. By adding 5% In, the crack progress rate of the Sn-based solder is lowered.
2. Even Sn-based solder containing 3% Ag ((i)) is superior to 0% ((ii)) and 1.5% containing ((iii)).
3. Sn-based solder has a significantly increased crack growth rate compared to conventional Sn-37Pb eutectic solder.

In conclusion, it has been found that Sn-based solder has an excellent composition in which In is contained in a system in which a network of acicular Ag ₃ Sn is firmly developed. This is a fact that cannot be predicted by conventional general knowledge. In particular, it seems to be a deterioration phenomenon that is strongly dominated by creep, which is peculiar to temperature cycle tests with long holding temperature and holding temperature.

2-2) The reason why the life is improved by adding In to the Sn-based solder The reason why the life is improved by adding In is insensitive to the cross-sectional observation of the crack progress, In the Sn system with no crack, the crack tends to advance linearly on one plane (intuitively looks sharp progress). In the tensile test, the necking phenomenon concentrated in one place is likely to occur, and in the torsion test, there is a tendency for linear fracture to occur in the twisted direction (perpendicular to the test piece). It is an indirect proof concentrated in a place. This means that cracks tend to travel linearly on one plane, that is, the solder other than the crack path portion has less stress relaxation. By introducing In, the entire Sn matrix is in a solid solution throughout the entire area of the Sn matrix, so the surrounding stress state is transmitted to the adjacent matrix through this matrix. In addition, a microscopic stress dispersion effect is expected even in the stress concentration part. As a result, the strength and stickiness of the original Sn-based solder are lacking, and by spreading and releasing the stress in addition to the crack propagation direction, it is thought that a delay effect will be created by relaxing the linear progress of the crack. (Looks like a progress lacking sharpness).

  Also in the impact test (Charpy) in which the internal damping property of the material can be confirmed, it has been confirmed that the impact absorption energy is reduced by adding In to the Sn-based solder. It was confirmed that by adding about 5% In, the Sn-37Pb eutectic was obtained. It is unclear whether the impact test results are directly related to the crack growth, but the damping ability that can absorb the impact energy of the material is indirect, but appears when In dissolves in Sn crystals. Think.

2-3) Reason why Ag is sufficient in a system with In added and the appropriate range of Ag A system with a lot of Ag forms a needle-like network of Ag ₃ Sn, so it is strong and stable even at high temperatures. Acicular Ag ₃ Sn exists as a reinforcing effect of the composite material. This effect is thought to prevent crack growth. As a result, (v) Sn-0.5Cu-5In and (vi) Sn-1.5Ag-0.5Cu-5In are more cracked than (iv) Sn-3Ag-0.5Cu-5In, which has a larger amount of Ag added. Estimate that progress has been faster. Since the effect of the In amount is remarkably superior to the effect of the Ag amount in preventing crack progress, the lower limit of Ag addition is set to 0.1%. The upper limit of Ag addition is set to 4.5% of the practical range in consideration of cost and the like.

2-4) Appropriate range of Cu in Sn—Ag—In system It is the same as selection of appropriate value of Cu in Sn—Ag system. That is, it is known that when Cu enters 1% or more, the elongation greatly decreases. In the case of the solder for connecting the base substrate and the ceramic insulating substrate, unlike the solder under the chip, it is generally expected that the use at 150 ° C. or higher is small. For this reason, as means for suppressing the reaction with Cu, the upper limit of Cu is set to 2%, and the lower limit is set to 0.1%. The reason why the lower limit may be small is that the Cu plate inevitably melts. In order to cope with large deformations, the alloy layer reaction was prevented so that the mechanical properties of the solder were not deteriorated.

2-5) Sn-Ag-Cu-In system, appropriate range of Bi addition For power modules that require high reliability, it is fundamental that Bi, which is known to become brittle rapidly when added, should not be added. It is. However, when Bi is slightly contained in about 5% In, workability may be improved (fluidity is improved, leading to a reduction in voids and an improvement in reliability). In this case, the proper amount of Bi added is about 0.5 to 1%. When Bi is 1% or more, brittleness that affects reliability appears, and it becomes impossible to cope with large deformation. In particular, the reduction in impact absorption energy at a low temperature on the minus side becomes remarkable.

(3) Reason why Sn-Ag-Cu-based solder has lower temperature cycle performance compared to Sn-37Pb eutectic solder When consumer products and household electrical appliances are targeted, Sn-3Ag-0. It is known that 5Cu is superior to Sn-37Pb eutectic [for example, Shibuya; Non-linear characteristics and thermal fatigue reliability of solder materials, Japan Institute of Electronics Packaging, Vol. 8 No. 2 (2005)]. Therefore, the reason why the life of the Sn-based solder is greatly reduced in the temperature cycle test of the solder between the insulating substrate and the base substrate of the power module was considered. Strain gauges were affixed to the ceramic insulating substrate and Cu base substrate surface sandwiching the power module solder, and the strain rate was measured during the temperature cycle.

As a result, it was found that the maximum strain rate at the change from high temperature to low temperature was 1.5 to 2 × 10 ⁻⁶ / s, which was very slow.

Up to now, consumer parts joints have been regarded as 10 ^-4 / s level at the latest, even in fields where the strain rate is slow. The reason why the strain rate of the insulating substrate and the base substrate of the power module is low is because of the large heat capacity of the Cu base substrate inherent to the power module. Although the chip portion is affected by cooling from the surface in response to a rapid temperature change, the strain rate of the solder in the vicinity of the Cu base substrate is unlikely to exceed 1.5 to 2 × 10 ⁻⁶ / s. Changes at a slower strain rate.

Therefore, it was found that a clear difference between Sn-3Ag-0.5Cu and Sn-37Pb eutectic appears when comparing the mechanical properties and metallographic properties of the solder at this strain rate and below this strain rate. That is, the Sn-37Pb eutectic exhibits a superplastic phenomenon in which the elongation rapidly increases and the stress rapidly decreases with the strain rate of 10 ⁻⁴ / s or less.

  For this reason, at the solder end where the maximum strain occurs, which is the starting point of the solder crack, the module can follow the large deformation of the module, and the solder is difficult to break down. In terms of metal structure, α-Pb crystal and β-Sn crystal Interdiffusion is active, microcracks are unlikely to occur between the particles, and since there is little deterioration even after creeping, it seems that there is a property of not being damaged. In contrast, Sn-3Ag-0.5Cu is a relatively hard and strong aggregate of β-Sn crystals. When creep deformation occurs, the β-Sn crystals grow, the grains are deformed, and the grain boundaries Microcracks are generated by sliding.

  As a result, it is expected that the creep damage cannot be recovered due to the remarkable deterioration. Another factor that causes Sn-3Ag-0.5Cu to accelerate crack growth is that stress tends to concentrate at one location at the solder end. That is, as proof of the nature that stress tends to concentrate in one place, it is easy to cause necking in the tensile test, and in the torsion test, the fracture occurs linearly in parallel with the torsion direction. On the other hand, Sn-37Pb eutectic exhibits an aspect of deformation throughout the solder layer, and even in a tensile test, a behavior of deformation over the entire distance between the scores is observed without being concentrated in one place.

  As a result, Sn-3Ag-0.5Cu-5In was selected as a representative composition that can be expected to have a significantly longer life than Sn-3Ag-0.5Cu in a temperature cycle test as a low-temperature Pb-free solder for power modules. .

(Example 2)
Since Sn-15Sb is a hard solder, there is a problem in reliability after connection. As a solution, if the thermal expansion coefficient of the solder can be reduced, the stress and strain acting on the solder after mounting can be reduced. Therefore, high reliability was achieved by using a composite structure with a carbon fiber having a low thermal expansion coefficient and excellent thermal conductivity.

  A low thermal expansion fiber (for example, diameter: about 8 μm, length: about 2 to 10 times the diameter), which is heat-treated by applying Cu or Ni plating to carbon in Sn-15Sb, does not interfere with gas release (5 (About 20 vol%), the carbon fiber has high strength and a high aspect ratio, so that the solder can be restrained and low thermal expansion of the entire solder is possible.

  Thereby, stress acting on the Si chip can be reduced, so that reliability can be improved. Other metals and non-metallic fibers that can be reduced in thermal expansion include invar, quartz, W, Mo, SiC, SiN, AlN, etc., but carbon fibers are excellent in view of thermal conductivity, weight reduction, workability, and the like. The metallization needs to be a strong metallization that can withstand soldering without leaving voids around the soldered metallization. The method of mixing and using metalized moderate length fibers (for example, 50 to 100 μm) in a solder ball and forming a paste is the simplest composite solder. There is also a method in which solder balls and metallized fibers are mixed and melted, and rolled into an foil in an inert atmosphere.

  In the case of a solder foil, a structure in which long metallized carbon fibers are arranged in a cross shape and embedded in solder or a net structure knitted in a mesh shape is also possible. If the solder thickness is about 100 μm, the reliability improvement effect due to low thermal expansion can be sufficiently expected if a cloth net of about 25 μm is embedded in one solder layer. Since the net woven into cloth has a greater effect on rigidity than short fibers, it has lower thermal expansion and has a greater impact on reliability.

(Example 3)
1) Metal ball dispersed solder foil for gap control (substrate side solder)
When a ceramic insulating substrate is soldered to a large Cu-based substrate using a low-temperature Pb-free solder foil, the weight of the joined body is also affected, and there is a possibility that the connection is inclined and collapsed at high temperatures. Hard copper balls with excellent thermal conductivity (added with trace elements) that are dispersed as uniformly as possible to ensure minimum solder thickness so that they are not tilted and connected. About 20 hard Cu balls (for example, diameter: 50 to 80 μm) subjected to Ni / Au flash plating are dispersed about 10 mm □.

  As a result, not only during soldering but also during deformation of the substrate during temperature cycling and long-term high-temperature tests, the creep deformation of the solder is prevented, the solder thickness is ensured for a long time, and the life is reduced. Since the ball surface is subjected to a surface treatment excellent in wettability, no void is formed in the vicinity of the ball. Moreover, since it is sporadic dispersion, it does not interfere with the discharge of voids formed inside the solder. Furthermore, since the thermal conductivity of Cu-based balls is higher than that of solder, thermal problems are unlikely to occur. When the Cu ball is deformed, a Cu ball or Ni ball hardened by adding a trace element may be used.

Example 4
Next, as a means for further improving the temperature cycle life of the connection portion between the ceramic insulating substrate and the Cu base substrate, a method of partially reinforcing the joint outer peripheral portion with an epoxy resin having appropriate physical properties is proposed. Thereby, the life of the temperature cycle can be greatly improved by changing the destruction mechanism and preventing or delaying the occurrence of solder cracks.

  FIG. 1A shows a cross-sectional structure of a module using Pb-free solder in a normal structure in which the end of the AlN insulating substrate 8 is not partially reinforced with resin. FIG. 1B is an example in which the end portion of the AlN insulating substrate 8 is partially reinforced with the resin 3, and shows a cross-sectional structure of a module using Pb-free solder. 1 (a) and 1 (b) both have a silicone gel 20 sealing structure.

In the configuration of the AlN insulating substrate 8 -Cu base substrate 11, the connection between the Si chip 1 and the lead electrode 5 is an example of the conventional wire bonding method of the Al wire 6, and the soldering of the chip is the first solder of Sn-15Sb. 4 was used. The Cu electrode 7 of the AlN substrate is Cu or Al (in the case of Al, Ni or Ni / Au plating is applied to the surface for wettability with solder). Since it is desirable that the edge reinforcing resin 3 is applied as uniformly as possible to the periphery of the board-side soldered portion 4 as the third solder 10, an epoxy-based potting resin that is easy to apply is used. . The end portion reinforcing resin 3 is preferably applied so that an appropriate fillet is formed around the board-side soldered portion. FIG. 1B shows a case where resin is applied so as to wrap the end portion of the AlN insulating substrate 8 and the end portion of the solder. As the resin application shape, the fillet portion is not limited to the application structure shown in FIG. 1B, and it is effective only on the lower side of the AlN substrate. In order to increase the adhesion between the resin and the Cu base substrate, the diverged dimple hole 14 provided in the Cu base substrate 11 may be provided around the entire periphery. In addition, the dimple holes 14 may be provided only at the four corners of the AlN insulating substrate where stress is severe, or may be provided at the four corners and the center of the side of the AlN insulating substrate. As a condition required for the resin, since it bears a stress burden at the peripheral edge of the AlN insulating substrate, (i) the adhesion of the resin is essential, and (ii) it has a strong Young's modulus and flexibility to deform. (Iii) The linear expansion coefficient is close to that of Cu, or in terms of thermal stress balance, Cu (α = 17.5 × 10 ⁻⁶ / ° C.) and AlN (α = 4.3 × 10 ⁻⁶ / ° C.) And (iv) excellent stability at high temperatures (high Tg).

The resin physical properties are at least Young's modulus: 300 to 3000 kgf / mm ² , linear expansion coefficient: 6 to 25 × 10 ⁻⁶ / ° C., preferably Young's modulus; 1000 to 1500 kgf / mm ² , linear expansion coefficient ; It is the range of 8-17x10 < ^-6 > / degreeC. Young's modulus lower limit; 300 kgf / mm ² means shortages bound by the resin, the Young's modulus limit; 3000 kgf / mm ² means breaking due to stress concentration of the AlN end interface. Here, a design that does not cause AlN destruction is important, and this is a contradictory view from the viewpoint of resin production, but it is necessary to make the thermal expansion coefficient close to AlN and to reduce the Young's modulus as much as possible. In particular, in the AlN corner portion, when the influence of stress concentration is unavoidable (similar to the correspondence in the periphery of the chip), it is possible to prevent AlN destruction by applying polyimide and curing and partially reinforcing with resin. Since the Young's modulus of polyimide is low, deformation of the AlN corner is absorbed by the polyimide layer.

(Example 5)
FIG. 2 shows an AlN insulating substrate 8 -Cu base substrate 11, in which the connection between the Si chip 1 and the lead electrode 5 is a conventional wire bonding method of the Al wire 6, and the soldering of the chip is the first Sn-15Sb soldering. The high-temperature solder, which is the solder 4, is Sn-3Ag-0.5Cu-5In solder for connection between the Cu electrode of the AlN substrate and the Cu base substrate. Here, not only the periphery of the AlN substrate shown in Example 4 but also the deformation of the high-temperature Sn-15Sb solder under the chip is suppressed by the epoxy resin 19 whose physical properties are specified, and the strain acting on the solder is reduced, thereby reducing the lifetime. A highly reliable structure that can be expected to improve. As for the resin physical properties, only the AlN protection is replaced with the Si chip protection, and the function of the resin is the same as that shown in the fourth embodiment. In the case of a Si chip, since it is important to protect the element portion on the surface of the Si chip, the periphery of the Si chip is applied. Further, by adopting individual mounting for each chip, it is also a structure that reduces the stress burden on the resin and the module as compared with the whole module sealing. If the module is small, it is desirable in terms of cost to encapsulate the entire module by potting or molding. However, when the chips are separated or have a large structure, it is necessary to ensure high reliability by individual chip resin reinforced mounting or resin reinforced mounting including components around the chip.

(Example 6)
FIGS. 3A and 3B are examples in the case where a higher output power semiconductor element is used. Instead of Al wire bonds, the back surface (upper side) of the Si chip 1 and the Cu cooling plate 17 are connected by solder 15. FIG. 3A shows a lead structure in which the Cu cooling plate has a bellows-like stress relaxation function, and FIG. 3B shows a lead structure having a stress relaxation function by bending deformation with two leads. The lead structure may be divided due to the terminal relationship. The other end of the lead is soldered with a second solder 16 on another Cu electrode on the AlN substrate. Part of the heat on the chip is transferred to the other Cu electrode 7 of the AlN substrate away from the bottom of the chip via this bypass circuit, so that the overall thermal efficiency is excellent. The solder composition is Sn-15Sb solder under the chip, the substrate side is Sn-3Ag-0.5Cu-5In solder, and the connection between the Cu lead portion and the Cu electrode is made by using a paste of Sn-3Ag-0.5Cu solder. Is also possible. When the Cu lead portion is retrofitted, the Sn-3Ag-0.5Cu-5In solder on the substrate side is remelted, but the solder thickness after remelting is secured by the metal balls in the solder. The Cu lead portion can be connected simultaneously with the low-temperature substrate-side solder. The on-chip solder 15 can be simultaneously connected with Sn-15Sb solder. Further, simultaneously with the low-temperature substrate-side solder, connection with Sn-3Ag-0.5Cu, Sn-3Ag-0.5Cu-5In, Sn-Cu system (melting point: 228 ° C.) or the like is also possible. When the temperature rise of the element is high and severe, it is desirable to use Sn-15Sb solder and connect it simultaneously with the solder under the chip.

  However, the reliability of the upper part of the chip and the peripheral part of the chip cannot be ensured only by connecting with solder. Therefore, in order to improve the reliability of the soldered portion under the chip as well as the chip, the periphery of the chip soldered to the Cu cooling plate 17 is covered with the same epoxy resin 19. The solder and chip are restrained by this resin, and the life of the solder is improved. If the epoxy resin 19 alone has a strong chip restraining force, a stress adverse effect on the element and chip end may be considered. Therefore, a portion where the stress is to be released can be thinly coated with polyimide in advance, and the epoxy resin can be coated thereon after curing. When priority is given to reliability such as stress and life, polyimide coating is effective. Polyimide coating is effective even if it is applied only to places that are particularly protected, such as stress concentration areas. When polyimide is applied to a place where large displacement and stress are generated, and it is coated with resin, the Young's modulus of polyimide is moderately low, so stress and deformation can be released in the polyimide layer (confirmed by analysis) is doing). For this reason, reliability is improved by releasing from defects due to stress concentration.

  In addition, when a Cu-C, Al-C composite material (coefficient of thermal expansion similar to that of alumina) having a low thermal expansion coefficient and excellent thermal conductivity is used as a cooling plate, the stress burden is reduced and the reliability is increased.

(Example 7)
FIG. 4 shows a cross-sectional structure of a structure in which the high-power power semiconductor element is water-cooled from both sides. A specific example for mounting a power module corresponding to a high heat generation density of a power semiconductor element with high reliability using Pb-free hierarchical solder will be described. A Sn-15Sb solder foil is mounted or spread on both surfaces of the first Si chip 1a and the second Si chip 2a, positioned on the Cu electrode 7 of the AlN insulating substrate 8, fixed with a jig, and using a hydrogen furnace. Reflow. At this time, the soldering of the Cu lead electrode 5 and the Cu electrode 7 may be simultaneously connected. If the connection can be made simultaneously with Sn-15Sb solder, it is possible to connect the Cu base substrate 11 and the AlN insulating substrate 8 in a temperature hierarchy with low-temperature solder. If retrofitted, the same low temperature solder may be connected at the same time when the Cu base substrate 11 and the Cu electrode 7 of the AlN insulating substrate 8 are reflowed with a low temperature solder. Since the peak temperature of the low-temperature solder can be 235 to 240 ° C., the composition for soldering the Cu lead electrode 5 and the Cu electrode 7 is Sn-3Ag-0.5Cu, which is used for general purposes. But it is possible. In this case, between the Cu base substrate 11 and the Cu electrode 7 of the AlN insulating substrate 8 is Sn-3Ag-0.5Cu-5In, and the Cu lead electrode 5 and the Cu electrode 7 are Sn-3Ag-0.5Cu. , Will be retrofitted at the same time. In FIG. 4, the space between the two Cu lead electrodes 7 appears to be narrow, but the gap is not a problem because it is actually shifted in the depth direction. The Cu lead electrode 5 and the Cu electrode 7 can be connected with a paste (in a situation where a cleaning-less paste connection method is also possible).

After soldering, polyimide thinned with a solvent is applied to the periphery of the chip, and is applied to the tip end / periphery where stress is concentrated to a thickness of about 20 to 100 μm. After curing, the glass transition temperature (Tg) is high, and there is adhesion. An epoxy resin with a low thermal expansion coefficient (thermal expansion coefficient; about 10 × 10 ⁻⁶ / ° C.) and a Young's modulus of about 15000 MPa (in order to reduce the stress load on the Si chip, the Young's modulus with the lowest possible thermal expansion coefficient) However, there is a limit inherent in the material system, and a part of the applied polyimide film is responsible for this function).

  As a result, the deformation of the solder can be suppressed by strongly restraining the periphery of the chip with an epoxy resin having specific physical properties (which may be preliminarily coated with polyimide). For this reason, high reliability can be ensured even with Sn-15Sb solder.

  Since the chip and the insulating substrate have a low coefficient of thermal expansion, in order to ensure higher reliability with a resin-constrained structure, the base substrate has a relatively low coefficient of thermal expansion and excellent thermal conductivity. The C, Al-C member has a structurally excellent structure.

  In the structure of FIG. 4 as well, a structure in which only the periphery of the chip is partially resin-reinforced and a structure in which the resin is reinforced so as to cover the periphery of the solder connecting the AlN substrate and the base substrate (the direction opposite to the soldering side of the AlN substrate is covered) From the viewpoint of reliability, a structure in which the resin is separated from the structure in which stress can be released. If Cu-C material with a low thermal expansion coefficient can be used for the base substrate, it can be integrated with a low thermal expansion member (the Cu electrode is soft, so the effect of stress is small), so it is costly to integrate with the transfer mold method But it will be advantageous. It should be noted that the double-sided cooling high output power semiconductor element structure is almost symmetrical in the vertical direction, so that the relative displacement and strain are small, and therefore the acting stress is considerably reduced by analysis. Therefore, it is necessary to select a structure in which reliability, cost, and workability are involved.

  Pb-free solder enables the power module to be connected to the temperature hierarchy because (i) there is no problem with the heat resistance of the components, (ii) the soldering temperature of the high-temperature solder can be increased, and (iii) In This is because the melting point of the low-temperature solder is lowered by about 10 ° C., and (iv) the reliability can be improved, and the temperature margin of soldering can be secured. This configuration can be applied not only to power modules, but also to modules that require temperature hierarchy connection and products that have strict heat resistance, such as alternate diodes. In particular, in the case of a configuration in which the element is connected to an insulating substrate having excellent thermal conductivity, since the difference in thermal expansion coefficient is small, there is a possibility that the Sn-15Sb system in which the solder is somewhat hard can be used sufficiently. Furthermore, it is possible to improve reliability more by using the partial reinforcement effect by resin together.

In this embodiment, an example of an AlN substrate is shown, but a similar effect can be expected with a Si ₃ N ₄ substrate or the like. In particular, the Si ₃ N ₄ substrate is slightly inferior in thermal properties to the AlN substrate, but has excellent mechanical properties against bending, impact, etc., less warpage, and increases the reliability of the joint. Is promising.

(A) Cross-sectional view of a power module having an AlN insulating substrate and a Cu base substrate structure (two types), (b) is a cross-sectional view of the power module in which the periphery of the insulating substrate is reinforced with resin in FIG. Sectional drawing of an AlN insulating substrate and Cu base substrate structure (2 types) module. Sectional drawing of the power module structure which mounts Cu cooling plate on a chip | tip. Sectional drawing of a double-sided cooling power module structure.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Si chip 1a 1st Si chip 2a 2nd Si chip 3 End part reinforcement resin 4 1st solder 5 Lead electrode 6 Al wire 7 Cu electrode 8 AlN insulating substrate 9 Resin 10 3rd solder 11 Cu base substrate 12 Refrigerant (water)
13 Heat sink 14 Dimple hole 15 Solder on chip 16 Second solder 17, 18 Cu cooling plate 19 Epoxy resin 20 Silicone gel 21 Resin bond 22 Resin case 23 Alumina substrate

Claims (11)

In a semiconductor power module in which a base substrate, an AlN insulating substrate, and a semiconductor chip are connected by solder,
A high temperature lead-free solder composed of Sb: 11 to 20 mass% and the remaining Sn is used for soldering the semiconductor chip and the AlN insulating substrate, and Ag: 0.1 is used for soldering the AlN insulating substrate and the base substrate. A semiconductor power module characterized by using a low-temperature lead-free solder composed of ~ 4.5 mass%, Cu: 0.1 to 5.0 mass%, In: 3 to 7 mass%, and remaining Sn.   2. The semiconductor power module according to claim 1, wherein 0.1 to 5 mass% of Cu is included in the high-temperature Sn—Sb solder.   3. The high-temperature Sn—Sb solder contains at least one type of Bi, Zn, Ge, Ni, Al, Ag, P, and In, and 0.01 to 1 mass%. The semiconductor power module described in 1.   The low-temperature solder contains at least one kind of Bi, Zn, Ge, Ni, Al, Sb, and P in an amount of 0.01 to 1 mass%, according to any one of claims 1 to 3. Semiconductor power module. The lead-free solder according to any one of claims 1 to 4, wherein the semiconductor power module is filled with silicone gel .
It is characterized in that an epoxy resin is partially coated so as to wrap around the soldered end portion of the AlN insulating substrate and the base substrate, and the periphery of the AlN insulating substrate and a part of the surface of the base substrate around the periphery. Semiconductor power module. The lead-free solder according to any one of claims 1 to 4, wherein the semiconductor power module is filled with silicone gel .
Soldered end periphery of the AlN insulating substrate and the base substrate, and an epoxy resin so as to enclose the part of the periphery and the base substrate surface to which the dimples grooving around is performed of the AlN insulating substrate around the A semiconductor power module characterized by being partially covered.   7. The semiconductor power module according to claim 1, wherein the semiconductor chip and the periphery of the chip are covered with an epoxy resin. In claim 7,
A semiconductor power module, wherein the semiconductor chip and the periphery of the chip are preliminarily coated with an epoxy resin and polyimide is thinly coated. In any of claims 1 to 8,
A semiconductor power module characterized in that a cooling plate lead is soldered on the upper surface of the semiconductor chip and is integrated with the semiconductor chip to reinforce the resin, or the periphery of the chip is pre-coated with polyimide and then resin reinforced. In any one of claims 1 to 9,
A semiconductor power module comprising 5 to 20 vol% of low thermal expansion fiber in the high-temperature Sn-Sb solder. In any one of claims 1 to 10,
Semiconductor power module which is characterized in that Cu particles of 50 to 100 [mu] m, or on the surface Ni, to disperse the Ni / Au Cu particles were subjected to flash plating or Ni particles child, in the low temperature type solder.

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