Damage accumulation and fracture in aged lead-free Sn-3.5Ag solder joints

Owing to concerns regarding the environment and health, lead-containing solders have now been eliminated and substituted by their lead-free counterparts. Hence, the present article is devoted to the clarification of their mechanical strength and reliability. Lead-free Sn-3.5Ag solder joints of various thicknesses were exposed to different thermal treatments in order to study the effect of material changes due to ageing. Thereafter, tensile tests were performed showing a pronounced decrease of strength after excessive heat treatment. The theoretical analysis is facilitated by simulations according to the finite element method. Thereby, the influence of material changes in the solder could be separated from the effect of thermal recovery in the copper base material. Crack initiation in the solder is described by an approach of damage mechanics derived from a thermodynamic framework. Excessive heat treatment leads to Kirkendall voids reducing the ultimate tensile strength of solder joints. Therefrom, one can estimate the reduction of tensile strength as a function of time and temperature.