Observation of chip solder degradation by electrical measurements during power cycling

IGBT power modules used in traction applications undergo large load variations. Thermo-mechanically induced stress then leads to material fatigue which limits the module´s lifetime. In this paper a method is presented for non-destructively quantifying the chip solder degradation during a power cycling experiment. The method is based on measuring the cooling rate after turn-off. A 20% increase in the chip solder thermal resistance has been measured after 1´800´000 cycles from 73°C to 127°C junction temperature. The solder degradation is also analysed using X-ray inspection and scanning acoustic microscopy. Based on these images and the simulated stress profile in the solder, two failure mechanisms have been identified.