Oxidation failure mechanism of copper alloy lead frame for IC package

The most important factor that causes delamination between copper alloy lead frame and epoxy mold compound is copper oxide. To identify the factors that influence the formation of oxide film on copper substrate, oxidation mechanism of several lead-frame copper alloys was investigated. The oxide film structure of copper alloy was found to be CuO/Cu₂O/Cu by AES analysis. Various copper alloys had great difference in CuO/Cu₂O ratio. Oxidation rate of copper alloy was further studied by measuring the oxide thickness as a function of time at 320°C. The results showed that C5191 and C7025 had lower oxidation rate than EFTEC64T and C194, which promoted the growth of CuO. The calculation from AES depth profile of copper atom confirmed that grain boundary diffusion coefficients of C5191 and C7025 were nearly two magnitudes smaller than those of EFTEC64T and C194. At a low temperature (usually below half of melting point), the growth of oxide film is mainly dominated by the grain boundary diffusion. It was found by SPM analysis that copper alloys with smaller grain size had more paths for copper atom to diffuse from substrate to the interface of oxide and air, thus leading to higher oxidation rate. The segregation effect of solute elements of copper alloys at the oxide/substrate interface also played an important role in the growth of oxide.