Electromigration effects upon interfacial reactions in flip-chip solder joints

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 Cu6Sn5 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/soldei-/Cu/(Ni,V)/die structure. A Sn/Cu/Sn/Ni/Sn/Cu/Sn couple was prepared to simulate the flip-chipjoint. At 200degreesC, the Cu6Sn5 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 Cu6Sn5 phases are different. The Cu6Sn5 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 Cu6Sn5 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.