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The effects of Zn (1 wt.%, 3 wt.%, and 7 wt.%) additions to Sn-3.5Ag solder and various reaction times on the interfacial reactions between Sn-3.5Ag-xZn solders and Cu substrates a during liquid-state aging were investigated in this study. The composition and morphological evolution of interfacial intermetallic compounds (IMCs) changed significantly with the Zn concentration and reaction time. For the Sn-3.5Ag-1Zn/Cu couple, CuZn and Cu 6 Sn 5 phases formed at the interface. With increasing aging time, the Cu 6 Sn 5 IMC layer grew thicker, while the CuZn IMC layer drifted into the solder and decomposed gradually. Cu 5 Zn 8 and Ag 5 Zn 8 phases formed at the interfaces of Sn-3.5Ag-3Zn/Cu and Sn-3.5Ag-7Zn/Cu couples. With increasing reaction time, the Cu 5 Zn 8 layer grew and Cu atoms diffused from the substrate to the solder, which transformed the Ag 5 Zn 8 to (Cu,Ag) 5 Zn 8 . The Cu 6 Sn 5 layer that formed between the Cu 5 Zn 8 layer and Cu was much thinner at the Sn-3.5Ag-7Zn/Cu interface than at the Sn-3.5Ag-3Zn/Cu interface. Additionally, we measured the thickness of interfacial IMC layers and found that 3 wt.% Zn addition to the solder was the most effective for suppressing IMC growth at the interfaces.

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