With the ban of Sn-Pb alloys coated on Cu leads in electronic devices, the electronic packaging industry uses Pb-free Sn alloys as the coating materials. This replacement meets a widely concern that Pb-free Sn alloys have a stronger propensity to grow whiskers on the surface of the coating. The experimental samples have been plated a pure Sn coating on Cu leads by an industrial plating process before the accelerated tests. The study executed three different environment conditions to estimate whiskers growth on the Sn coating of Cu leads. They were Room Temperature/Humidity Storage (RS), High Temperature/Humidity Storage (TH) and Temperature Cycling (TC). No whisker had been found in the samples during the RS tests since this environment condition had not destroyed the surface continuity to promote whiskers grow out of the Sn coating. In the TH tests, the moisture degraded the coating surface to produce the electrochemical corrosion on some local areas. Sn whiskers were easy prone to grow from the corroded areas on the coating surface with a high rate. In the TC tests, Sn whiskers grew from the cracks on the coating surface with a high density based on the mechanism of the thermal fatigue. The alternating stress in the coating surface induced by the temperature fluctuation tore the Sn oxide grains to form cracks and provide a growth path for Sn whiskers on the coating surface.