Reliability model application for power devices using mechanical strain real time mapping

The reliability characterization is a milestone for the design of semiconductor power devices operating in those applications where a high robustness is a critical point, as in the case of the automotive field. In order to realize a reliability model that can describe the degradation of power devices, due to a cyclic thermo-mechanical stress, the Coffin-Manson equation is widely applied. Generally speaking the Coffin-Manson relationship shows a correlation between the fatigue life and the plastic strain but this equation is often customized in order to correlate the fatigue life with the temperature variations, being these simpler to measure. In this paper a methodology based on a direct measurement of the mechanical strain applied on operating devices will be shown. Furthermore, the application of the results for the reliability evaluation without taking into account the temperature variations will be described. This target has been accomplished with a suitable scanning instrument based on the laser interferometry able to perform very fast acquisitions and to reconstruct nanometric mechanical displacements.