The influence of thermal cycling methods on the interconnection reliability evaluation within IGBT modules

High temperature application and long term reliability are the future trends for IGBT (Insulated Gate Bipolar Transistor) power modules. Interconnection degradation is a main mechanism leading to IGBT module failures under high temperature and thermo-mechanical cycling loads. Thermal cycling is a common test for verifying the bonding quality and reliability in electronics packaging. However, it is difficult to compare cycling results with different cycling profiles as the failure mechanism can be different and there are many different lifetime models. In this paper, both thermal cycling experiments and thermo-mechanical simulations were used to understand the influence of the key thermal cycling parameters on the degradation results. In a first step, Air-to-air temperature cycling tests and liquid-to-liquid thermal shock cycling tests with different temperature swings were performed and compared in this study. Crack length analysis based on SAM (Scanning Acoustic Microscopy) images defined the life time of solder joints. SAM and SEM (Scanning Electron Microscopy) characterizations were able to identify the same failure mechanism for all the cycling conditions. In a second step, Thermo-mechanical simulations showed quantitatively mechanical behavior of substrate solder layer and derived stabilized inelastic strain changes per cycle under different cycling profiles. Finally, Life time calculations using strain-based model of solder joints in all these three conditions achieved a very good agreement with experimental values. Therefore, liquid to liquid thermal shock cycling could replace air-to-air cycling for time saving during substrate solder qualification if the same failure mechanism applies. Strain-based model could be used for solder lifetime estimation in these high temperature cycling conditions.