Interfacial reactions between Sn-9Zn+Cu lead-free solders and the Au substrate

The effects of added Cu into Sn-9Zn solder reacting with an Au substrate at 160degC for 24 hours were investigated in this study. The intermetallic compound (IMC) evolution sequences at the (Sn-9Zn)+xCu/Au interface and solders were : (i) (Au₃ Zn₇ + AuZn₂ + AuZn) and Au₃ Zn₇ phases at the Sn-Zn/Au couples; (ii) (Au₃ Zn₇ + AuZn), (Au,Cu)₃Zn₇ phases, as x (the Cu content) was 1 wt%; (iii) AuSn and Cu₅ Zn₈ phases, as x was 4 wt%; (iv) ((Cu,Au)Sn+AuSn) and CuZn phases, as x was 7 wt%, and (v) ((Cu,Au)Sn + AuSn) and CuZn phases at Sn-9 Zn+10 wt%Cu /Au couples. When x is less than 3 wt%, the free Zn atoms in the liquid solder can easily react with the Au substrate to form Au-Zn IMC at the interface. The active Zn atoms in the liquid solder decreases with increased Cu addition into the Sn-Zn solder (3< x <6 wt%). Most of the Zn atoms react with Cu to form a Cu-Zn short-range-ordered structure IMC in the solder. Thus, the Sn atoms become the dominant diffusion specimens and react with the Au substrate to form binary Au-Sn IMC at the solder/Au interface. When 6 to 10 wt% Cu is added into Sn-9Zn solders, the solder/Au interface converts completely into the Sn-Cu/Au system.