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

Reliability enhancement of thick Al wire bonds during thermal fatigue test has been investigated from a metallurgical viewpoint. Al wire bonds degrade with the increase of crack length during thermal fatigue tests with high /spl Delta/T/sub j/ due to the tensile stress generated by the thermal expansion coefficient mismatch between Al wires and Si. It is also found that cracks propagate along the small grain boundaries of Al wires at the bonding interface. It is predicted that the Al wire bonds may not degrade due to thermal fatigue if /spl Delta/T/sub j/ is controlled below 40 K, i.e., keeping it within the actual temperature fluctuation range in IGBT modules for traction motor drives. The reliability of Al wire bonds can be enhanced by increasing the grain size of the Al wire at the bonding interface. The high temperature bonding is considered to be a good candidate for enhancing the reliability of Al wire bonds.