Effect of Sn Orientation on Electromigration Failure in CuSn Solders

Sn-Ag-Cu (SAC) has emerged as one of the most widely accepted lead-free solders used as interconnecting material in electronic packaging. However, these systems still have major reliability problems. During manufacturing, the interfacial reaction of Cu with molten Sn-based solder results in the formation of brittle intermetallic compounds (IMC) that accelerate the degradation of these systems. The evolution of IMC follows during operation due to electromigration (EM), which in addition is responsible for Cu depletion. Both mechanisms are strongly affected by the anisotropic diffusion of Sn resulting in large variability of solders life time prediction. We developed a model to study the formation and evolution of IMC and the diffusion of copper due to EM that includes the impact of Sn crystal orientation on solder failure during fabrication and operation. Our findings show that IMC growth is polarized during electromigration, accelerating at the anode and slowing at the cathode. Similarly, copper depletion is more pronounced at the cathode. The anisotropy of Sn strongly affects the rates of IMC growth and copper depletion during electromigration, shaping solder failure during operation.