Study on Cu Wire Wedge Crack and Fatigue Life Prediction during Thermal Cycling Test (TCT)

It is widely believed that wedge crack is resulted from thermal stress occurring at wire bonding area due to coefficient of thermal expansion (CTE) mismatches among epoxy molding compound (EMC), Cu wire, and Leadframe, and wedge bond heel area has the highest risk due to higher stress concentration. To fully understand the wedge crack failure mechanism and the way to improve its fatigue life, in this study, the wedge bond performance under TCT was evaluated in a small outline transistor package with different wire bonding settings, different types of EMC with different CTEs and modulus. The results showed that the TCT performance of Cu wire wedge bond depended on the wire bonding setting, CTE of EMC, adhesion between EMC and Leadframe, wire diameter, etc. Through suitable material selection, package design, process optimization, Cu wire wedge crack during TCT can be avoided to achieve the increasingly higher reliability requirement. Furthermore, numerical modeling was developed to predict Cu wire wedge bond fatigue life with those different factors during TCT. For given packages, modelling can provide a comparison on accumulative plastic strain ($\Delta\varepsilon_{\mathrm{p}}$) at Cu wedge bond to identify the effect of factors on Cu wedge fatigue crack. Fatigue parameters can be derived by simulation results and experimental data. Regarding to these fatigue parameters, TCT fatigue life can be predicted for a given package.