Interfacial reactions and impact reliability of Sn-Zn solder joints on Cu or electroless Au/Ni(P) bond-pads

Sn-9Zn and Sn-8Zn-3Bi solder balls were bonded to Cu or electroless Au/Ni(P) pads, and the effect of aging on joint reliability, including impact reliability, was investigated. For the purpose of quantitatively evaluating the impact toughness of the solder joints, a test similar to the classic Charpy impact test was performed. The interfacial compounds formed in the solder/Cu joint during soldering were Cu-Zn intermetallic compounds (IMCs), not Cu-Sn IMCs. One of the Cu-Zn IMCs, gamma-Cu5Zn8, thickened remarkably with aging, and eventually its morphology changed from layer-type into discontinuous. The rapid growth of the gamma-Cu5Zn8 and void formation at the bond interface led to the significant degradation of the joint reliability due to a ductile-to-brittle transition of the joint. Meanwhile, the compound formed in the solder/Au/Ni(P) joint during soldering was a Au-Zn IMC, above which Zn redeposited during aging. Both the dissolution and diffusion of Ni into the solders were extremely slow, which contributes to negligible void formation at the bond interface. As a result, the solder bumps on the Au/Ni(P) pads were able to maintain the high joint strength and impact toughness even after prolonged aging.