Strong Zn concentration effect on the soldering reactions between Sn-based solders and Cu

The acute Zn concentration sensitivity of the reaction between Sn-based solders and Cu substrate is reported and explained in this article. Three Sn-xZn solders (x = 0.5, 0.7, and 2 wt%) were reacted with Cu substrates at 250 °C for 2–10 min. 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. When the Zn concentration was slightly increased to 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.