Significance of Nucleation Kinetics in Sn Whisker Formation

Abstract

Sn whiskers are believed to form in response to stress in layers used as protective coatings. However, what makes them form at specific sites on the surface is not known. We have used thermal expansion mismatch to induce stress and observe the resulting whisker formation. Cross-sectional measurements of the region around whiskers show that there are oblique grain boundaries under the whiskers that are not seen in the as-deposited columnar structure. The kinetics also suggest that the whiskering sites may be formed by a nucleation process. Based on these results, we propose a nucleation mechanism in which the boundaries of the surrounding grains migrate due to strain energy differences and create oblique boundaries at which whiskers can form. A simple model is developed to predict the stress-dependence of the nucleation rate.

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

“…Therefore, it is thought that the whisker grain forms from an initially columnar grain structure, and the orientation of the grain does not change during this transformation. 18,104 The recrystallization mechanism cannot rationalize the whisker growth from a pre-existing grain which also retains its orientation during the transformation. In addition, hollow whisker shells formed by diffusion of Sn atoms from the whisker interior back to the Sn coatings have also been observed, 105 which indicates that the driving force for whisker growth is reversible.…”

“…16,19,106 As shown schematically in Fig. 30, Chason et al 104 proposed a mechanism wherein strain-assisted grain boundary migration leads to the formation of a surface grain. Figure 30a shows initial columnar grains whose boundaries are pinned at the top and bottom due to the presence of the native oxide layer and the interfacial IMC layer, respectively.…”

Measuring the Stress Dependence of Nucleation and Growth Processes in Sn Whisker Formation

“…17,18 The results are shown in Fig. 2 for thermal treatments with maximum temperatures of 45°C (Fig.…”

“…It is also similar to the morphology beneath whiskers found in IMC studies. 11 The stress dependence of the nucleation barrier from this mechanism can be calculated from the balance between the strain energy reduced by the grain growth and the increase in the area of the grain boundaries 17 (details of this model can be found in Ref. 37).…”

“…The thermally induced stress [16][17][18] has enabled us to measure the coevolution of the stress, whisker density, and whisker volume. The results have been used to determine quantitatively how the stress influences the rates of nucleation rate, growth, and plastic strain relaxation.…”