Abstract
Power devices have possessed low switching and conduction loss characteristics with the improvement in semiconductor device manufacturing. Insulated gate bipolar transistors have been widely used in power module because of such characteristics. When the power module is subjected to the cyclic temperature load, the thermal stress resulting from the mismatch between the coefficients of thermal expansion (CTE) of materials causes the interface of the materials to fatigue. According to the different operating requirements, the module is under different temperature profiles. Different dwell times and ramp rates produce different stress relaxation, which eventually affect the reliability of the power module. A two-dimensional finite-element model was established based on real sample. The models were subjected to thermal cycling between -40 and 125 °C to discuss the effect of the dwell time and the ramp rate on the creep behavior of the solder. The results indicated that the dwell time brings accumulative creep strain due to stress relaxation; furthermore, dwell time at the high temperature segment leads to more evident stress relaxation. Compared dwell phase with ramp section, ramp section brings more significant accumulative creep strain than dwell phase in one thermal cycle. About the discussion of ramp rate, decreasing the ramp rate causes more creep strain, and deteriorates the reliability. A creep strain-based life prediction model, i.e. Darveaux-like equation, was proposed in this study. The creep strain was calculated by a three-dimensional finite element model, together with the experimental results, to establish the life prediction model. Finally, the critical crack length was determined to obtain the total life of Sn96.5Ag3.5 solder. The result indicated the longer the period of thermal cycling is, the more damage the power module accumulates.