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.