Auger Electron Spectroscopic (AES) Measurements on High Aspect Ratio Tin Whiskers

With the implementation of RoHS directives regarding Pb-free electronics, pure tin (Sn) films and board finishes offer potentially serious reliability issues due to Sn whisker formation. A key aspect of Sn whiskers is their material composition, which has been assumed pure crystalline Sn since 1951. Due to the submicron width (_~ 0.25 mum) of high aspect ratio whiskers, it has been difficult for even state- of-the-art materials techniques to provide clear, unambiguous data on Sn whiskers and, in particular, the surfaces ofSn whiskers. In this study, high resolution Auger electron spectroscopy (AES) has been used to determine both the surface and bulk composition of high aspect ratio Sn whiskers. The whiskers were grown from intrinsically stressed thin films (_~ 6000 A) of Sn on brass, deposited using cylindrical magnetron sputtering techniques. Results show that the whiskers are 100% Sn at the whisker base, shaft, tip, and up to a substantial depth into the whisker bulk. No evidence of pull-up from the brass substrate or surface contaminants is observed in the whiskers. A remarkable aspect of the growth is that high aspect ratio whiskers _~ 10-100m in length containing no brass are grown from a _~ 0.6 mum thin film of Sn on brass.