High-Temperature Degradation of Wire Bonds in Plastic Encapsulated Microcircuits

Application of low-cost commercial plastic encapsulated microcircuits (PEMs) for military and aerospace applications requires rigorous analysis of their quality and reliability in harsh environments. It is known that degradation of Au/Al wire bonds limits reliability of PEMs at high temperatures; however, there is only limited information on acceleration factors of wire-bond failures. In this work a technique, which is based on in-situ evaluation of contact resistances (Rc), has been used to monitor degradation processes in wire bonds in two types of microcircuits encapsulated in 80-pin and 44-pin QFP plastic packages. The parts were stored at temperatures varying from 175 to 250 C for up to 2,500 hours in some cases. Kinetics of Rc variations and temperature dependence of parameters of Weibull distributions were used to calculate activation energy of the degradation process and predict failure rates at temperatures below 175 C. The mechanism of wire-bond failures and factors affecting the degradation process are discussed.