Improvement in bond wire reliability by adjustments in bond wire microstructure via heat treatment

Many research related to reliability of power electronic devices has been published to improve the lifetime of power modules. Especially the top side connection between the bond wires and the semiconductor chip of such devices is very prone to fail, often denoted as wire lift-off. This paper focuses how particular microstructures of the bond wires adjusted by different post wire-drawing heat treatments affect the lifetime of the bond connection and the related damage mechanisms. In order to evaluate and compare the grain structures, EBSD measurements of a wire-drawn and bonded microstructure were conducted and the influence of a subsequent heat treatment was analyzed. To evaluate the influence of heat treatments on the lifetime of bond wire connections, power cycling tests of wires either subjected to a heat treatment or without any additional heat-treatment were conducted. In all cases, the bonded wires revealed a gradient in grain size and texture within the wire. By an appropriate heat treatment at 150 °C for 120 h the lifetime can be improved significantly, while shorter or longer heat-treatment durations result in reduced lifetimes. For the latter case, it is argued that due to the recovery and recrystallization of the heat-treated wire the crack growth rate is reduced and therefore improving the lifetime.