Recently, it was reported that adding Zn to solder was an effective way for reducing the formation of both Cu3Sn and Cu6Sn5 and inhibiting Kirkendall voids formation. The objective of this study is to investigate this Zn effect in detail. Three Sn-xZn solders (x = 0.5, 0.7, and 2 wt. %) were reacted with Cu substrates at 250°J for 2-10 mins. A slight variation in the Zn concentration changed the reaction product formed at the interface. When the Zn concentration was low (x = 0.5 wt. %), the reaction product was Cu6Sn5. At high Zn concentration (x = 2 wt. %), the reaction product became Cu5Zn8. When Zn concentration was in-between (x = 0.7 wt. %), Cu6Sn5 and CuZn co-existed. The above findings are explained using the Cu–Sn–Zn phase diagram. The implication is that the type of compound forms at the interface can be controlled by adjusting the Zn concentration of the Sn-based solders.