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

Abstract

Here, we study the role of stress state and stress gradient in whisker growth in Sn coatings electrodeposited on brass. The bulk stress in Sn coatings was measured using a laser-optics based curvature setup, whereas glancing angle X-ray diffraction was employed to quantify the surface stress; this also allowed studying role of out-of-plane stress gradient in whisker growth. Both bulk stress and surface stress in Sn coating evolved with time, wherein both were compressive immediately after the deposition, and thereafter while the bulk stress monotonically became more compressive and subsequently saturated with aging at room temperature, the stress near the surface of the Sn coating continually became more tensile with aging. These opposing evolutionary behaviors of bulk and surface stresses readily established a negative out-of-plane stress gradient, required for spontaneous growth of whiskers. The importance of out-of-plane stress gradient was also validated by externally imposing widely different stress states and stress gradients in Sn coatings using a 3-point bending apparatus. It was consistently observed that whisker growth was more in the coatings under external tensile stress, however, with higher negative out-of-plane stress gradient. The results conclusively indicate the 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.

Whisker Growth in Sn Coatings: A Review of Current Status and Future Prospects

“…Hence, the net diffusion is expected to occur along the path of the steepest hydrostatic stress gradient. The importance of both in-plane and out-of-plane stress gradients has been proposed and indeed observed experimentally 52,61,[80][81][82] and numerically through simulations. 83,84 Microdiffraction studies have shown that the stress distribution in the Sn coating is inhomogeneous, with grain-to-grain stress variation.…”

“…Another important driving force for whisker growth is mechanically applied stress on the Sn sample. Numerous studies have investigated the application of mechanical stress using different loading devices, such as a clamping fixture, 21,36,58 three-point or four-point bending [59][60][61] of Sn-coated strips, micro-indentation, [62][63][64][65] and in situ nanoindentation. 66,67 Fisher et al, 21 using a ring clamp (shown in Fig.…”