Ductile-to-brittle transition in Sn–Zn solder joints measured by impact test

Sn–9Zn and Sn–8Zn–3Bi solder balls were bonded to Cu or electroless Au/Ni(P) pads, and the effect of aging on impact reliability was investigated. A test similar to the classic Charpy impact test was performed to measure the impact toughness of the solder joints. In the case of the solder/Cu joints, γ-Cu 5Zn 8 at the bond interface thickened remarkably with the aging time, accompanied by void formation, resulting in a ductile-to-brittle transition of the joint. In the case of the solder/Au/Ni(P) joints, however, the slow dissolution and diffusion of Ni into the solders as well as the redeposition of Zn at the interface contributed to maintaining high impact toughness even after a prolonged aging. All of the bumps on the Au/Ni(P) broke in a ductile manner.