Influence of P content in electroless plated Ni-P alloy film on interfacial structures and strength between Sn-Zn solder and plated Au/Ni-P alloy film

The interfacial structure and strength of solder joints between Sn-9 mass%Zn solder and plated Au/Ni-P alloy film on a Cu substrate have been investigated. Three reaction layers with 0.2 to 0.5 mum thickness were formed along the interface between the plated Ni-P alloy films and Sn-9 mass%Zn solder. The outermost layer contains a Ni-Sn intermetallic compound. The middle layer contains approximately 40 mass%Au, 35 mass%Zn, 20 mass%Ni and 5 mass%Sn. The thickness of the An layer is 0.1 mum, so the An layer does not dissolve. The innermost layer contains about 63 mass%Zn, 25 mass%Ni, 10 mass%Au and 2 mass%Sn. The strength of the Sn-9 mass%Zn solder joints take almost the same values independent of P concentration. The strength of Sn-Zn solder joints with Sn-Zn/Ni-2 mass%P, Sn-Zn/Ni-4 mass%P and Sn-Zn/Ni-8 mass%P joints were found to be almost constantly independent of reflow cycles. Therefore, Sn-9 mass%Zn solder is considered to be an excellent solder material for plated Ni-P alloy films.