Whisker Growth under Controlled Humidity Exposure

Studies of Sn whiskers under controlled, calibrated humidity conditions shows that the highest whisker densities occur for ~ 85% RH. The whisker specimens were 1500 Å Sn films sputtered under compressive stress conditions on silicon and electrochemically polished brass. Subsequently, the samples were exposed to a series of saturated aqueous salt solutions (which generated calibrated relative humidity values of 33, 43, 70, 76, 85, 98% RH) for ~140 days at room temperature. The Sn on brass case at 85% RH produced 6X greater whisker densities than Sn on brass exposed to pure O2, which in turn produced 9X greater whisker densities than Sn on brass exposed to ambient room temperature/humidity. The longest average whisker lengths (6.1 µm for Sn on brass and 9.3 µm for Sn on Si) occurred for 70% RH on both substrates. Corrosion features were observed on all samples, but the 98% RH samples experienced excessive corrosion. Generally, we find a dramatic increase in whisker density at > 60% RH and especially around 85% RH, in agreement with batch processed whisker experiments involving humidity [H. Reynolds et. al., IEEE Trans. Electron. Packag. Manuf. 33 (2010) and P. Oberndorff et. al., IEEE Electronic Comp. and Tech. Conference (2005) 429].