Investigation on Creep-Fatigue Interaction Failure of Die-Attach Solder Layers in IGBTs Under Power Cycling

Creep plays a significant role in the deterioration of solder layers particularly over extended periods of heating, even though numerous existing studies focused on the mechanism of fatigue failure. In this article, the creep-fatigue interaction failure mechanism of the solder layer under power cycling test (PCT) is investigated. The microstructure of solder layer was successively scanned using an ultrahigh resolution computed tomography scanner to observe the degradation of the microstructure. It is found that the microstructure of the solder layer had obvious changes even when the thermal resistance remains unchanged. The initial voids in the solder layer are constantly collapsing and new voids are constantly growing out. This is different from the previous finding that the solder layer degrades by cracks initiating and propagating under short heating in PCT. The reason for this result is that creep becomes the dominant factor in the degradation of solder layer. Scanning electron microscope (SEM) scanning of the solder layer after failure showed that microcracks also existed, but their distribution was more dispersed, making it difficult to determine their crack initiation locations and propagation paths. Therefore, the failure of the solder layer under longer heating period and higher temperatures is considered to be a complex creep-fatigue interaction failure process. Finally, a thermal resistance degradation model is proposed, which can accurately predict the change of the thermal resistance during module degradation.