Sn whisker growth in Sn–9Zn–0.5Ga–0.7Pr lead-free solder

The spontaneous growth of Sn whisker in Sn–Zn series solder is newly reported in this work. It is found that during the exposure of Sn–9Zn–0.5Ga–0.7Pr bulk solder to ambient conditions for a few hours, many different lengths of needle-like Sn whiskers originate spontaneously from the Sn–Pr intermetallic compounds of the solder and grow rapidly at a rate of about 3.5 Å/s. It is proposed that the driving force for whisker formation is the compressive stress resulting from the oxidation of Sn–Pr compounds, and that the free Sn atoms released from oxidation reaction feed the whisker growth during exposure.