a b s t r a c tIn this study, the interfacial reactions on Sn, Sn-3.0Ag-0.5Cu, Sn-0.7Cu, Sn-58Bi and Sn-9Zn lead-free solders with the Au/Pd/Ni/Cu multilayer substrate at 240-270 • C for 20 min to 20 h were investigated. The experimental results showed that the (Ni, Cu) 3 Sn 4 phase is converted to the (Cu, Ni) 6 Sn 5 and Cu 3 Sn phases in the Sn/Au/Pd/Ni/Cu system when the reaction time is longer than 4 h. In Sn-3.0Ag-0.5Cu/Au/Pd/Ni/Cu and Sn-0.7Cu/Au/Pd/Ni/Cu systems, the (Cu, Ni) 6 Sn 5 and Cu 3 Sn phase were observed, and only the Ni 3 Sn 4 phase was formed at the Sn-58Bi/Au/Pd/Ni/Cu interface. Furthermore, the Pd 2 Zn 9 and NiZn phases were formed in the Sn-9Zn/Au/Pd/Ni/Cu system. When the reaction time was longer than 4 h, the Pd 2 Zn 9 , NiZn, and Ni 5 Zn 21 phases were formed at the interface. The reaction mechanism for all the reaction systems was diffusion-controlled. The Sn-58Bi/Au/Pd/Ni/Cu system was found to have the lowest activation energy when compared with other systems, and its value was 17.43 kJ/mol.

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