Phase reaction in Sn–9Zn solder with Ni/Au surface finish bond-pad at 175 °C ageing

γ-Au3Zn7 and γ2-AuZn3 formed as band-shaped phases in solder after reflow process. During 175 °C ageing, γ and γ2 transformed to γ3-AuZn4 and finally (Zn) phase precipitated out next to ɛ-AuZn8. Ni5Zn21 formed in the bond-pad interface. There were (Zn)-free zones near intermetallic compound (IMC) regions after certain ageing period. Ni could diffuse through the (Zn)-free zone and react with Au–Zn IMC and formed a new (Ni, Au)Zn4 ternary phase. The rest part of Au–Zn IMC such as γ3-AuZn4 transferred toward Au-rich IMC as γ2-AuZn3 and then γ-Au3Zn7 phase. According to the results of this study, the existence of reflowed eutectic (Zn) phase is the key factor to maintain the different phase equilibrium in the Sn–Zn solder.