Reactive wetting of Sn0.7Cu–xZn lead-free solders on Cu substrate

The reactive wetting properties of Sn0.7Cu–xZn (x = 0, 0.2, 0.5, and 1.0, respectively) lead-free solder on Cu substrate were investigated based on the wetting balance test under the ambient or N2 atmosphere. Due to the oxidation of Zn at ambient, even small amount of Zn addition into the Sn0.7Cu solder could lead to the increase of surface tension of solders significantly. However, under N2 atmosphere, the impact of the additive Zn on the surface tension of solder was not remarkable.Since the current reactive wetting theory suggested that the first intermetallic compound formed during the wetting reaction could influence the wetting properties, the driving forces of intermetallic compounds formation were calculated using the CALPHAD method (Thermo calc software). The results revealed that with the Zn content increasing, the intermetallic compound formed during wetting process changed from Cu6Sn5 to CuZn-γ, which definitely caused the variation of wetting behavior.