The effect of electric current and surface oxidization on the growth of Sn whiskers

Electric current was applied on pure Sn-plated leadframes to evaluate the effects of current-induced stress on the growth of Sn whiskers. The samples were stored at room temperate and 55 °C/85% relative humidity (RH) conditions with an induced current range of 0.1 A to 0.5 A. The samples stored at the room temperature did not grow the whiskers at any of the current conditions until 3000 hrs. As the current flow increased, irregular intermetallic compounds (IMCs) grew at the interface between the Sn finish and Cu substrate. However, various lengths of columnar and bent whiskers were observed under all current conditions, after exposure to 55 °C/85% RH conditions for 1000 hours. At the same temperature, the higher current levels showed longer whiskers than lower current levels. The Sn oxide had the α-SnO2 structure of the rutile phase which was non-uniformly formed on the surface of the Sn finish. The grain size of the SnO2 was estimated to be several nanometers. The SnO2 film was up to a thickness of ∼23 nm on the Sn whisker surface stored at 55 °C/85% RH conditions for 3000 hours.