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This study investigates aging effects on interfacial reactions between Sn-9wt%Zn-x wt% (SZ-xCu) alloys and Au substrate. The Au3Zn7/AuZn2/AuZn and Au3Zn7/AuZn phases respectively formed in the SZ/Au and SZ-1Cu/Au couples aged at 160Â°C for 24 hours. Only the AuSn phase was found at the SZ-4 Cu/Au interface. Extending the aging time to 800 hours, Sn became a dominant diffusion element. Binary Au-Sn phases and the metastable Au-Zn-Sn ternary phase, Au33-36 Zn35-36Sn29-31, was formed at the interface. The aging effect causing the changes of dominant diffusion element and concentration of Zn, Cu in the solder is the main reasons to change the sequence of the IMC formation in the SZ-xCu/Au systems.