Joint Scale Dependence of Aging Kinetics in Sn-Ag-Cu Solders

Sn-Ag-Cu (SAC) solders are susceptible to appreciable microstructural coarsening during storage or service, resulting in evolution of joint properties, and hence reliability, over time. Although the problem of microstructural coarsening in joints has been studied extensively, the effect of joint scale on aging kinetics has never been investigated. Understanding the scale dependence of aging is particularly relevant in light of the continual decrease in joint sizes and the increased reliability demands placed on solder joints. This paper compares the coarsening behavior of a bulk Sn-Ag-Cu (SAC) solder with that of small joints (650 ¿m diameter) attached to bond-pads with a Ni-based surface finish and shows that the aging behavior displays significant joint-scale dependence. Not only do Ag₃Sn precipitates in small joints coarsen faster, the coarsening is more rapid nearer the bond-pads than farther away. Through energy dispersive x-ray (EDX) analysis of the solder-pad interfaces, it is determined that these effects are attributable to the depletion of Cu from the small solder joints due to solution / reaction with the Ni present in the bond-pads. Longer aging times enhance this depletion. Based on an analytical model, it is shown that the coarsening kinetics of the Ag₃Sn precipitates depends directly on the solubility limit of Ag in Sn As a result, smaller joints, which would show greater Cu depletion and hence higher Ag solubility, would display faster Ag₃Sn coarsening kinetics. Furthermore, there is greater Cu depletion from the solder nearer the solder-pad interfaces, and hence the coarsening kinetics are faster near the interfaces than in the middle of the joint, giving rise to spatially inhomogeneous coarsening..