Long Term Reliability of Power Modules with Low Amplitude Thermomechanical Stresses and Initial Defects

Solder-attached IGBT power modules are widely use in renewable energy and smart grid applications where the thermomechanical stress cycles are relatively low in amplitude but the service duties of the devices are expected to last for decades. Although the initial defect voids and cracks in the solder layer are widely recognized as the triggers of initial aging, it is still necessary to describe and simulate the physical fatigue behavior in their local regions accurately under low amplitude stress cycling. In order to investigate the growth of the damage, in this paper a 2D symmetrical finite element analysis (FEA) model is developed to evaluate the thermomechanical behavior of the solder layer with initial defects, and this is verified by a combination of power cycling test and micro-resolution computed tomography (CT) scanning. The modeling and its experimental validation provides an understanding that the voids distributed in the solder layer may transfer into initial cracks which then grow progressively rapidly; the voids adjacent to the chip-solder interface will particularly reduce the lifetime. This establishes the basis of a modeling theory for further investigation of the damage progress on solder interface.