On the advantages of using a hypoeutectic Sn–Zn as lead-free solder material

Hypoeutectic Sn–Zn may be a better choice than the eutectic Sn–9Zn as a lead-free solder. We checked the non-equilibrium melting behaviors of a series of Sn–Zn alloys (2.5–9 wt.% Zn) by differential thermal analysis, and found that at a heating rate of 5 °C/min, Sn–6.5Zn behaves the same way as the eutectic Sn–9Zn in melting. Dipping and spreading tests were carried out to characterize the wettability of Sn–Zn alloys on Cu. Both tests indicated that Sn–6.5Zn has significantly better wettability to Cu than Sn–9Zn does. The reaction layers formed during the spreading tests were examined. For all samples with 2.5–9 wt.% Zn, two reaction layers are formed at the interface, a thick and flat Cu5Zn8 layer adjacent to Cu and a thin and irregular Cu–Zn–Sn layer adjacent to the alloy. The total thickness of the reaction layers between the alloy and Cu was found to remarkably decrease with decrease of the Zn concentration.