This study investigates interfacial reactions of (Sn–9Zn) + xCu/Ni systems. Ni5Zn21, Cu5Zn8, (Ni,Zn,Cu)3Sn4, (Cu,Ni,Zn)6Sn5, and Cu6Sn5 phases were formed on the Sn–9Zn/Ni interface at 240–270 °C, when 0–10 wt% Cu was added to the Sn–9Zn solder. Experimental results indicate that changing the concentration of Cu in the Sn–9Zn solder dramatically changes the formation of intermetallic compounds (IMCs) in the (Sn–9Zn) + xCu/Ni system. Different diffusion and segregation rates of elements are the main reasons for a change in the IMC evolution.