In situ tensile testing of tin (Sn) whiskers in a focused ion beam (FIB)/scanning electron microscope (SEM)

Abstract

Tin and tin-alloyed electroplated films are known to be susceptible to whisker growth under a range of conditions, many of which result in the generation of compressive stresses in the film. Compressive stress is considered to be one of the primary causes for whisker nucleation and growth. While extensive investigations have been performed on whisker growth, there have been few studies on the mechanical properties of tin whiskers themselves. We report on the tensile behavior of tin whiskers that were obtained by indentation and furnace aging of electroplated tin films on copper disks. Tensile tests of the whiskers were conducted in situ in a dual beam focused ion beam (FIB)-scanning electron microscope (SEM) system using a micro electro-mechanical systems (MEMS) based tensile testing stage. The strength of the whiskers was found to decrease with an increase in gage length and aged whiskers were found to be weaker than their indented counterparts. The observed gage length effect can be attributed to the probability of finding more defects as the whisker length increases. The effect of processing on the observed strength variation was investigated by analyzing the oxygen content in the whiskers via energy dispersive spectroscopy and the microstructure through transmission electron microscopy (TEM). The deformation mechanisms of whiskers were also inferred using post-mortem TEM. It was observed that the whiskers grown by indentation were dislocation free both before and after deformation. In contrast, whiskers grown by aging showed notable dislocation content (arranged in low energy configurations) even before deformation.