Modeling fatigue life of power semiconductor devices with ε-N fields

In this study, fatigue life of power semiconductor devices measured in cycles to failure during an accelerated stress test in a climate chamber is analyzed. The tested devices fail mainly in a short circuit event and their physical inspection reveals cracks in the power metallization. Commonly, the time till fracture of macroscopic metal layers is modeled with S-N or ε-N fields, this means that the lifetime (N) depends on the mechanical stress (S) or the strain (ε), respectively. Metal layers of semiconductor devices are microscopic (≤ 20μm) and, in general, their ageing mechanisms are different than for macroscopic layers, nevertheless the application of the macroscopic based ε-N model to semiconductor lifetime data shows good results. Hence, fatigue life due to micro-mechanisms can be described by parameters representing the mechanical load (strain) in the device.