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Power cycling capability is one of the most important reliability items in the application of power semiconductor modules. This paper describes the failure mechanism of power cycling by analysis of the structure of lead-based solder and joint failure due to solder fatigue. By the application of some additional elements, it has been found that a newly developed tin-silver-based solder shows both excellent mechanical properties and wettability. Further, we have established, both by experiment and computer calculation, that the dependence of the failure mechanism on ΔTj is completely different between the new tin-silver-based solder and conventional lead-based solder. According to these evaluations, it has become clear that the power cycling lifetime of the new tin-silver-based solder depends on the solder joint at higher than around 110 K, while it depends on the aluminum wire bonds at lower than around 50 K. As a result, higher power cycling capability can be successfully achieved by using this newly developed solder instead of conventional lead-based solder.