Analysis of thermomechanically related failures of traction IGBT power modules at short circuit switching

Reliability issues of IGBT power inverters for multi-kilowatt traction motor drivers are investigated. Due to high power loss densities and different material properties any operation of the switching module is connected to internal stress. These forces are known as main causes of thermo mechanical fatigue. For lifetime estimations power and thermal cycling test methods have been established. However, in switching operations extremely high power loss peaks occur particularly under short circuit conditions. These switch losses can be up to 200 times higher compared to normal operation condition. Although, the switching modules withstand even repeated short circuit switching in the regular case, testing and investigating of the devices is usually not done under these extreme conditions. In order to identify weak points we measured the losses at distinct real operation conditions (normal operation, short circuit with medium inductive load, and short circuit with low inductive load) and investigated temperature distributions and mechanical stress/strain distributions in a typical package using the finite element method. Von Mises stress and strain distributions due to thermal expansion were calculated under quasi-static and pulse load conditions. According to our findings the highest strain levels are caused by pulse load operation at the interface between the bonding wire of the gate and the die, and in the solder interface layer between the die and the upper copper metallization. Their meaning for aging processes and reliability relevant material degradation is also discussed.