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This paper presents a simulation method to evaluate the thermal fatigue life of a power module. A coupled electrical-thermal analysis was performed to obtain the nonuniform temperature distribution of electric current. Then, a thermomechanical analysis was carried out based on the temperature distribution from the electrical-thermal analysis. Since crack propagation can change the route of heat transfer, a crack path simulation technique was used to investigate the fracture behavior of the power module. The crack initiates in the solder joint below the Al bonding wire of the insulated gate bipolar transistor module and propagates by increasing the diameter. The effect of the bonding type on power cycling fatigue life is also discussed. The fracture process was found to depend on the type of bonding. Lead frame bonding was found to be more effective than wire bonding.

In this study we propose a new mounting structure for SiC power devices that operate at high temperatures and evaluate its reliability. In this new structure the stress relaxation function rests with the circuit metal on the substrate rather than the joint layer, so high purity aluminum, which has similar characteristics to conventional solder, was chosen as the circuit metal. By conducting a Finite-Element-Method analysis using the measured nonlinear material properties of aluminum, it was possible to make a stress-strain evaluation of the structure. In order to investigate the practical fatigue properties of aluminum we devised a mechanical test method which makes local strain concentration of the chip joint appear, and this method enabled prediction of the thermal fatigue life cycle of the structure. Moreover, a harsh Thermal Cycle Test of the chip mounting samples was conducted between Ϫ50 and 300 degrees Celsius, and a positive correlation was obtained between the predictions and the test results.

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