Investigation on fatigue mechanism of solder layers in IGBT modules under high temperature gradients

With the increase in the power density of power semiconductor chips, the lateral temperature gradient over the chip surface becomes more pronounced. Temperature gradients play an important role in reliability evaluation of IGBTs and have attracted great attentions. In this paper, the influence of IGBT chip temperature gradients on the crack initiation site and propagation path of the solder layer is investigated by finite-element analysis (FEA). Results indicated that the temperature gradient could influence the mechanism of solder layer fatigue. With the temperature gradient increasing, the crack initiation site shifts from the edge of solder layer to the center. The crack propagation path also changed. The fatigue mechanism of the solder layer under high temperature gra-dients is investigated by FEA. The inelastic strain energy density per cycle (Delta W) is found to be unequal at crack initiation and crack propagation stages. Accordingly, a novel energy-based lifetime prediction model based on Darveaux's model for solder layer fatigue of IGBTs is proposed, which takes into account the variation of Delta W within these two stages and can better characterize the processes of crack initiation and propagation in the solder layer. Subsequently, two lifetimes of IGBT modules under accelerated aging tests from the existing publications are used to verify the effectiveness of the proposed model. Finally, the accelerated aging tests were carried out to support the theoretical analysis on the direction of crack propagation under high temperature gradients.