Effect of stand-off height on microstructure and tensile strength of Cu/Sn-8Zn-3Bi/Cu and Cu/Sn-9Zn/Cu solder joints

In this paper, the effect of the stand-off height (SOH) on the microstructure and tensile strength of Cu/Sn-8Zn-3Bi/Cu and Cu/Sn-9Zn/Cu solder joints with SOH of 100 mum, 50 mum, 20 mum and 10 mum were studied. The results show that as the SOH is reduced, the needle-like Zn-rich phase steadily decrease until completely disappeared in both of the two solder joints. For Cu/Sn-9Zn/Cu, Cu₅Zn₈ IMC is formed, while, for Cu/Sn-8Zn-3Bi/Cu, two IMC layers are formed, the first layer adjacent to the Cu substrate is a flat Cu5Zn8 phase, and the second layer adjacent to the solder bulk is a mixture of Cu-Sn + Cu-Zn. In Cu/Sn-9Zn/Cu solder joints, the dissociative Cu₅Zn₈ intermetallic compound (IMC) increases with the reduced SOH, while there are less dissociative IMCs found in the Cu/Sn-8Zn-3Bi/Cu solder joints. Bi dissolves into the solder bulk and does not participate in the interfacial reaction. Due to the addition of Bi, the tensile strength of the Cu/Sn-8Zn-3Bi/Cu solder joint is higher than that of the Cu/Sn9Zn/Cu solder joint. From the results of the tensile test, it is found that the ultimate tensile strength (UTS) of Cu/Sn-9Zn/Cu solder joints is decreased with the reduced SOH, while for Cu/Sn-8Zn-3Bi/Cu solder joints the UTS is increased.