Effects of humidity on tin whisker growth — Investigated on Ni and Ag underplated layer construction

The effect of humidity on nickel and silver underlayered tin platings has been observed in relation to whisker formation. Using nickel or silver underlayers between the copper substrate and the tin coating is known to be a useful technique for mitigating the occurrence of whiskers. The underlayer blocks the formation of Cu6Sn5 intermetallics which is one of the root causes of whisker growth. Samples with a bronze (94Cu/6Sn) substrate covered with a 1–2 μm underlayer (silver or nickel) and 5–7 μm of tin on the top were tested. The samples were stored in high humidity and at various temperature conditions; 40 °C/95% RH (Relative Humidity), 105 °C/100% RH and 50 °C/25% RH for over 4200 h to demonstrate the importance of humidity in whisker growth. Results have shown differences in whisker growth on the samples depending on the nature of the environmental conditions. The differences originate from the various stress-causing mechanisms. Growth differences have also been found on the two types of underlayer materials.