Effect of high temperature bake on evolution of interfacial structure in Cu wire bonds and its impact on Cu/Al interfacial corrosion

Although Cu wire bonding has been successful and in high volume production in the industry, sporadic failure of ball bonds on Al pad due to interfacial corrosion has been reported. Different corrosion reactions have been proposed in the literature, but it is still not clear how these reactions could have occurred in the environment of green mold compounds whose contents of free chloride ions were at low ppm levels. In this paper, an experiment was performed to create cracks by bake at 300°C in the interface between Cu ball and Al pad to study the mechanism of interfacial corrosion. It was found that after autoclave test interfacial corrosion occurred only in the units where the Cu/Al interface was cracked. In those units where the Cu/Al interface was not cracked, autoclave test did not result in failure. Evolution of microstructure of inter-metallic compounds and formation of micro voids and cracks in the interface during bake is discussed. The theory of crevice corrosion is applied to explain how the IMC layer and Al pad get corroded in a relatively clean environment. Possible correlation of interfacial corrosion to sustained event of high current density or high voltage in packages is proposed.