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Here, we study the role of stress state and stress gradient in whisker growthin Sn coatings electrodeposited on brass. The bulk stress in Sn coatings wasmeasured using a laser-optics based curvature setup, whereas glancing angleX-ray diffraction was employed to quantify the surface stress; this alsoallowed studying role of out-of-plane stress gradient in whisker growth. Bothbulk stress and surface stress in Sn coating evolved with time, wherein bothwere compressive immediately after the deposition, and thereafter while thebulk stress monotonically became more compressive and subsequently saturatedwith aging at room temperature, the stress near the surface of the Sn coatingcontinually became more tensile with aging. These opposing evolutionarybehaviors of bulk and surface stresses readily established a negativeout-of-plane stress gradient, required for spontaneous growth of whiskers. Theimportance of out-of-plane stress gradient was also validated by externallyimposing widely different stress states and stress gradients in Sn coatingsusing a 3-point bending apparatus. It was consistently observed that whiskergrowth was more in the coatings under external tensile stress, however, withhigher negative out-of-plane stress gradient. The results conclusively indicatethe critical role of negative out-of-plane stress gradient on whisker growth,as compared to only the nature (i.e., sign and magnitude) of stress.

Critical Evaluation of Relative Importance of Stress and Stress Gradient in Whisker Growth in Tin Coatings