Stress Relaxation Mechanisms of Sn and SnPb Coatings Electrodeposited on Cu: Avoidance of Whiskering

The interrelations of microstructural evolution, phase formation, residual stress development, and whiskering behavior were investigated for the systems of Sn coating on Cu and SnPb coating on Cu during aging at room temperature. It was shown that the whisker-preventing effect of Pb addition to pure Sn can be attributed to a Pb-induced change of the stress relaxation mechanism in the coating: Pure Sn coatings, with a columnar grain morphology, relax mechanical stress via localized, unidirectional grain growth from the surface of the coating (i.e., whisker formation occurs), whereas SnPb coatings, with an equiaxed grain morphology, relax mechanical stress via uniform grain coarsening without whisker formation. It can thus be suggested that tuning of the Sn grain morphology (i.e., establishing an equiaxed grain morphology) is a straightforward method of microstructural control to suppress whisker formation at room temperature. Experimental results obtained in this project validate this conclusion.