Growth of Sn Whiskers under Net Compressive and Tensile Stress States

Studies of tin (Sn) whiskers have emphasized the role of compressive stress as the driving force for whisker production, thought to be a necessary condition for Sn whisker growth. The goal of this work is to characterize Sn whisker growth in film systems having measurable net compressive, zero, and tensile intrinsic stress states. The stress states were generated for the case of ~ 0.2 µm sputtered Sn films on Si (which has no interfacial intermetallic) by controlling the background Ar pressure in the magnetron sputtering system, which produces differing bond energies (and stresses) in the resultant films. Verification of the stress states was accomplished by curvature techniques using stylus profilometry and Stoney's equation. The results show that, after three months of incubation at room temperature, high whisker densities are observed under both tensile (12,000 whiskers/cm2) and compressive (16,000 whiskers/cm2) stress conditions, with a minimum whisker density generated in the zero stress (4,000 whiskers/cm2) condition. It is likely that the zero stress condition produced whiskers due to the difficulty in achieving the narrow range (7-9 mTorr) of background pressures necessary to produce the zero stress state. Relative average whisker lengths were 56µm (tensile stress); 4µm (compressive stress); and 1.5µm (zero stress). Subsequent measurements of whisker density (currently at 96 days of incubation) show no reversals/variations in the stress-density relationship.