Abstract Whisker and hillock formation are a stress relaxation response in Sn thin films and have become a critical reliability problem in Pb-free electronics. Whiskers grow by diffusion of atoms driven by stress gradients and usually have shallow grains at their roots. Therefore, the local conditions under which surface grains form need to be understood to mitigate whisker growth in Sn thin films. Two possible mechanisms for the nucleation of shallow grains are investigated using finite element simulations: grain boundary migration and pinch-off, and subgrain formation by local lattice rotation. By incorporating the role of localized plasticity, we find that shallow grains do not form by motion and bowing of grain boundaries. The simulations show that deformation-induced lattice rotation creates subgrains close to the thin film surface. These shallow grains form by the local heterogeneity of the strain in Sn polycrystals leading to high dislocation densities that subdivide grains into subgrains that may be potential sites for whisker formation.

中文翻译：

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Sn薄膜中的浅晶粒形成

摘要 晶须和小丘的形成是 Sn 薄膜中的应力松弛响应，已成为无铅电子产品中的关键可靠性问题。晶须通过由应力梯度驱动的原子扩散而生长，并且通常在其根部具有浅晶粒。因此，需要了解表面晶粒形成的局部条件，以减轻 Sn 薄膜中的晶须生长。使用有限元模拟研究了浅晶粒成核的两种可能机制：晶界迁移和夹断，以及通过局部晶格旋转形成的亚晶粒。通过结合局部塑性的作用，我们发现浅晶粒不是由晶界的运动和弯曲形成的。模拟表明，变形引起的晶格旋转会在薄膜表面附近产生亚晶粒。