Identification of whisker grain in Sn coatings by analyzing crystallographic micro-texture using electron back-scatter diffraction

Here, we attempt to understand the age-old question of “where do whiskers in Sn coatings grow?” by performing grain orientation mapping in conjunction with a simple analysis of the stress field in the vicinity of a whisker grain. Electron back-scatter diffraction (EBSD) was used for orientation mapping of Sn grains in a 4 μm thick Sn coating deposited on brass. It was observed that whiskers consistently grew from low-index grains with (100) or near-(100) orientations that were surrounded by grains with similar orientations, which were then partially surrounded by grains with high-index planes, such as (211), (321) and (420). Strong elastic anisotropy and overall a high fraction of high-angle grain boundaries were also consistently observed in the vicinity of whiskers. In addition, a full-field three-dimensional crystal elasticity simulations were performed using the EBSD orientation maps to analyze local stress variations in the vicinity of the whisker grain. These simulations indicate the presence of a high compressive hydrostatic stress around the whisker grain, which could then possibly create conducive conditions for whisker growth observed experimentally.

The whisker grain in Sn coatings can be identified by observing the orientation of whisker grain and its neighborhood. In this work, the whisker grain was identified by using electron back-scatter diffraction technique. In the above, (a) shows the EBSD pattern of the as-deposited Sn coating, whereas (b) shows the same location as in (a) after whisker growth. Whisker grows from (100) or near-(100) grain orientations surrounded by grains with similar orientation, these first neighbors are then surrounded by high index grain orientation in “double ring fashion”. The whisker grain in (a) is shown by an arrow, whereas the nearest and the second nearest neighbors in both figures are shown by solid and broken circles, respectively.564