Effect of the anisotropic characteristics of β-Sn on current-induced solder evolution

Abstract

The miniaturization trend of three-dimensional integrated circuits poses great challenges to solder reliability under high current stressing since anisotropic issues tend to be enhanced in microscale solder bumps. To address these challenges, the solder evolution at the microscale and the corresponding mechanism should be addressed. In this study, by considering the anisotropy of β-Sn in the electrical, thermal, mechanical and diffusional characteristics, the current-induced solder evolution of a typical solder bump was comprehensively explored over time. The results confirmed that the anisotropy of β-Sn aggravated the nonuniform distribution of current-induced stresses, further causing multiple migration modes coupled in the solder bump. The spatial inconsistency between the distribution of β-Sn unit cells and the bump geometry led to accumulation of torsion forces, which became the direct reason for solder rigid rotation. The electron-dislocation interaction promoted dislocation slip, leading to lattice rotation and homogenization of intragranular deformation. In addition, current-induced grain merging via detwinning was observed, and its velocity was faster than imagined. Our study is conducive to thoroughly understanding anisotropic evolution and establishing accurate damage mechanisms for micro solder bump design under current stressing.