Investigation of phase equilibria of the Au–Ni–Sn ternary system and interfacial reactions in the Au/Sn/Ni/Cu multilayer couple

Abstract

Au and Ni are widely used in the under-bump metallization, whereas Sn and Cu serve as common bonding materials and substrates, respectively. Solder joints consisting of an Au/Sn/Ni/Cu multilayer structure are widely found in the electronic packaging. Both phase equilibria of the Au–Ni–Sn ternary alloys and interfacial reactions of the Au/Sn/Ni/Cu multilayer structure were applied to the solid/solid-state reaction couple technique. The results showed that the Ni3Sn2 and AuSn4 phases respectively had a great solubility of the third element and the ternary AuNi2Sn4 phase existing in the Au–Ni–Sn ternary alloys was formed at 180 and 240 °C. The Sn/Ni/Cu multilayer structure was prepared by electroplating Sn/Ni layers on the Cu substrate. Afterward, the Sn/Ni/Cu multilayer and Au foil were screwed together to obtain the Au/Sn/Ni/Cu multilayer structure, followed by reactions at 150, 180 and 200 °C for different durations. The thickness of the intermetallic compounds (IMCs) increased with the increase in the reaction time and temperature. The IMC evolution in the couple remained unchanged at different temperatures. An increase in the reaction time converted the AuSn4 and AuSn2 phases to the AuSn phase. The voids formed in the couple were attributed to the volume shrinkage effect, which decreased with an increase in the reaction temperature. The (Ni, Au)3Sn2 phase was formed at the Ni side; the Ni layer functioned as an effective diffusion barrier to prevent the formation of the Cu–Sn IMCs.