Reliability of thick Al wire bonds in IGBT modules for traction motor drives

Reliability of thick Al wire bonds during the thermal fatigue test has been investigated from a metallurgical viewpoint. The bond interface between Al wires and Al-Si films was found to be a coincidence boundary or a boundary with very fine grain size, and hence so strong that cracks advanced along the grain boundary of the wire at the bonding interface in the accelerated thermal fatigue test. It was also found that crack growth rate could be reduced considerably by enlarging the grain size at the interface. The hypothesis that bonds would not degrade in actual use of traction motor drives was presented on the basis of the fatigue theory of Al.