Thermal fatigue resistance evaluation of solder joints in IGBT power modules for traction applications

Several ageing test campaigns have been carried out to evaluate thermal fatigue resistance of solder alloys used in IGBT power modules. The solder joint between the ceramic substrate and the copper baseplate of this stack packaging has been the focus of these experiments. Indeed, this interconnection layer is highly subject to shear strains and stresses due to the mismatched expansion on either side of the solder joint during operation. The impact of the substrate metallization and of the cooling rate has been evaluated by measuring crack growth rate on cross sections of cycled solder joints. Two solder alloy compositions have been selected: a lead-free one and a lead-bearing one. Scanning electronic microscopy analyses have allowed a study of cracking at the microstructural level. Moreover, additional analyses based on scanning acoustic microscopy have led to a ranking of the batches under test. Best results in terms of thermal fatigue resistance have been obtained with the lead-bearing solder alloy. Furthermore, it has been shown that the use of bare copper substrates, and above, all fast cooling rate after the solder reflow, delay solder joint failure.