Interfacial metallurgical reaction between small flip-chip Sn/Au bumps and thin Au/TiW metallization under multiple reflow

The interfacial reaction between Sn/Au bumps and a thin Au/TiW metallization was investigated. It was found that for 60 μm bumps, the pad metallization was not reliable and large voids were generated after multiple reflow cycles due to the dewetting between eutectic AuSn and TiW. For 40 and 20 μm bumps, the Au/TiW was quite stable and the joints could be entirely composed of TiW/Au5Sn/Au because of the wetting on the side walls of the Au socket.