Whisker growth and its dependence on substrate type and applied stress

This article deals with influence of long term mechanical and temperature stress on whisker growth. The experiments were conducted on three different surfaces. FR4 substrate was having copper, gold and HAL surface finish. Copper and gold surface finish was provided with thin layer of solder. Altogether, three solders (Sn63%Pb37%, Sn99%Cu1% and Sn97%Cu3%) were used. The samples were exposed to long term stress. The aging lasted 6500 hours and the temperature was either laboratory for the first batch or elevated (50 °C) for the second batch. The whisker growth was not observed on samples where the solder alloy was deposited in a thick layer, not even after exposure to elevated temperature or bolt induced compressive stress. The highest amount of tin whiskers was observed on a sample with copper surface finish and solder with high tin content, specifically Sn99%Cu1% solder. The presence of lead in the solder hindered the whisker growth. The length of the whiskers varied from tens to hundreds of micrometers. The whisker observation and measurement was conducted using Olympus SZX8 microscope.