Tin whisker growth from tin thin film

In this paper, the tin whisker growth from vacuum evaporated Sn thin film is studied. The aim was to characterize the whiskering behavior of submicron thick tin layers which have large grain size. For this purpose, 99.99% pure tin was vacuum evaporated onto 1.5 mm thick copper and ceramic substrates. The average thickness of the tin films was ~400nm with ~1-1.5μm grain size. The samples were stored at room environment for 150 days. Whisker growth was monitored every 15 days by Scanning Electron Microscope (SEM) and at the end of the investigation by a HIROX KH-7700 3D optical microscope. Cross-sections of the layers were prepared by Focused Ion Beam (FIB) to study the change of the layer structure during the test by FIB - Scanning Ion Microscope (SIM). It was found that the submicron thick tin layers with larger grain size on copper substrate can develop large amount and long tin whiskers, as compared to the samples with ceramic substrate which surface remained whisker free. The results indicated that the whiskering was caused by copper-tin intermetallic layer growth between the copper substrate and Sn thin film which resulted in large compressive stress in the Sn thin film.