High power IGBT modules: thermal fatigue resistance evaluation of the solder joints

Extensive accelerated aging tests have been carried out with representative test structures in order to evaluate the thermal fatigue resistance of five solder alloys intended for high power IGBT modules. With regard to the chip-to-substrate samples, Pb-free preforms have given excellent results since the thermal impedance of these hybrid assemblies has hardly changed during thermal cycling, even after 2000 shocks between +125°C and -55°C (less than 5%). The best results have been clearly obtained with the finest solder microstructures. In addition, the analysis of the results has led to an evaluation of the acceleration factor of the thermal fatigue tests. Finally, SEM observations of cross-sectioned samples have shown crack propagation during thermal cycling which is responsible for the increased thermal impedance. However, with regard to substrate-to-baseplate samples, the best results have been obtained with a Pb-bearing solder alloy strongly subject to coarsening during thermal cycling. Moreover, it has been demonstrated that fast cooling can cut crack growth rate in the solder joints by half. In addition, EDX analyses have shown copper diffusion all over the solder joint when the DBC (direct bonded copper) substrate is not nickel-plated, which seems to slightly improve its fatigue resistance. Finally, these experiments have shown that fine solder microstructures do not necessarily lead to good fatigue performances at high levels of stress or strain