Sn whiskers can grow spontaneously on the surface in Sn alloy coatings. This paper focused on the mechanism of whisker growth on the pure Sn coating to Cu leads in electronic components. After the rack plating process the pure Sn coating was annealed at 150°C for one hour and measured about 10mm in thickness. Then samples endured the high temperature/humidity storage tests (60°C and 87%RH) from 1 month to 6 months. The experimental results revealed that Sn whiskers were prone to grow from the surface with local corrosion in the Sn coating. This was the high temperature/humidity storage provided much moisture to cause an electrochemistry corrosion between Sn and H2O. The excess formation of SnO2 had a large molar volume comparing to Sn and this generated a high inner stress in the corroded areas to shear the coating surface producing the defects. The compressive stress deriving from the formation of Cu-Sn intermetallic compounds drove Sn whiskers initially grew from the corroded areas.