Whisker growth from Sn solder alloys

Studies of tin (Sn) whisker growth from Sn solder alloys shows that SAC deposits can be whisker prone. In fact, even SnPb solders may produce whisker growth. The Sn alloy specimens consisted of 2400 A SAC305 (~3.0%Ag, 0.5%Cu and rest Sn), along with 750 and 1200 Å Sn-37Pb films, each sputtered under compressive stress conditions on electrochemically polished brass and incubated in ambient room temperature/humidity (RT/RH). After a month of incubation whisker growth is already observed on the SAC and 750 Å SnPb films. The thicker, 1200 Å, SnPb film did not whisker till after over a year of incubation with only 524 whiskers/cm². At this time (~400 days) the 750 Å Sn-37Pb film has modest whisker numbers, while SAC has produced > 147,000 whiskers/cm² after 590 days of incubation at RT/RH, making it apparent that whisker growth can occur on high Sn content alloy deposits. Comparing the 750 Å SnPb specimen to 750 A of pure Sn on brass after similar incubation periods, the Sn film produced ~ 1.75X the whisker density of the Sn-37Pb film, with > 5 X the average whisker length, highlighting the ability of incorporated Pb to suppress whiskers. This is in agreement with electroplated films studies, where the addition of Pb in Sn is observed to reduce film stress, mitigating whisker growth [e.g., E. Chason et. al., App. Phys. Letters, 92 (2008) 171901].