Effect of Heat Generation on Fatigue-Crack Propagation of Solder in Power Devices

Power devices are used in inverters in a variety of electrical equipment, for instance, hybrid-power cars, electric vehicles, and generators. These types of equipment are used to decrease the negative impact on the environment, and thus, the power devices need to function effectively as electric power converters for the long-term stability of the equipment. In short, the long-term reliability, i.e., the life, of the power device is important, and a high level of reliability is required. In the development process of power devices, it is necessary to conduct thermal fatigue tests to evaluate the reliability. However, such tests are extended over a long period of time, which makes it difficult to shorten the development period. Therefore, a simulation technique needs to be developed to forecast the life of a thermal fatigue test in order to reduce the development period. During the thermal fatigue test, thermal stress is caused by differences in the line expansion coefficient between solder joint materials. Thermal stress causes crack generation and propagation in solder. The thermal resistance of a device increases steadily as the cracks grow. This raises the temperature of the device and increases thermal stress. As a result, crack propagation is accelerated. However, conventional crack propagation analysis does not take this phenomenon into account. We developed a method of crack propagation analysis that takes into account the changes in thermal and electrical boundary conditions resulting from the crack propagation. The method is a combination of electrical conduction analysis, heat transfer analysis, and crack propagation analysis. The boundary condition of the heat transfer analysis is determined from the results of the electrical conduction analysis. The boundary condition of the crack propagation analysis is determined from the results of the heat transfer analysis. The crack propagation behavior in solder is calculated by repeating these analyses. This method reproduces the drastic increase in thermal resistance in the latter part of the thermal fatigue test, and the results agree well with the experimental results. We confirmed that the temperature distribution of the device changes as the crack propagates and that thermal and electrical coupled analysis has a major effect on the prediction of fatigue life of power device products. We also revealed that the thermal fatigue life is affected by the position of the heat source and cracks.