Separating Nucleation from Growth Kinetics of Sn Whiskers using Thermal Pretreatment Followed by Mechanical Loading

The rate of whisker formation in Sn-based coatings is controlled by both the rate of nucleation and the rate of growth. Since the two processes occur in tandem, they are often difficult to separate. In this work, whisker nucleation is separated from growth by briefly annealing a Sn film to induce the creation of small features on the surface due to stress generated by thermal expansion mismatch. This is followed by applying a mechanical load to drive whisker growth. We observed that whiskers mainly grew from the thermally nucleated sites as opposed to forming new sites under the applied load. Comparison with a film that is not annealed but subjected to identical mechanical loading highlights the role of the nucleation process in the development of whiskers.