Abstract
Lead-free flip-chip joining processes have attracted the most research interests recently, and inhomogeneous interfacial reactions were observed in the flip chip solder joints. The Cu 6 Sn 5 phase with high nickel solubility, (Cu,Ni) 6 Sn 5 phase, was formed at the substrate side adjacent to the Ni layer in a flip-chip joint with a substrate/Ni/solder/Cu/(Ni,V)/die structure. A Sn/Cu/Sn/Ni/Sn/Cu/Sn couple was prepared to simulate the flip-chip joint. At 200°C, the Cu 6 Sn 5 phase was formed on both ends of the Sn phase at the Cu/Sn and Sn/Ni interfaces for the couple with an electric current stressing or a longer reaction time, but the nickel contents of the two Cu 6 Sn 5 phases are different. The Cu 6 Sn 5 phase at the Ni side has high nickel content and it has almost no nickel at the Cu side. It is concluded that the Cu at the chip-side diffused through the solder phase, reacted with the Ni layer at the substrate side, and the Cu 6 Sn 5 phase with high nickel solubility, i.e. the (Cu,Ni) 6 Sn 5 phase, was thus formed. Electromigration effects significantly enhance the diffusion rate of Cu, but do not alter the phase formation sequence.