Effects of thermal storage and Cu addition on adhesive strength and microstructure of Sn-3.0 mass% Ag-1.5 mass% Sb-xCu solder joints

This paper examines the effects of the thermal storage time and Cu addition on the adhesive strength and microstructure of lead-free Sn-3.0 mass% Ag-1.5 mass% Sb-xCu solder joints. The experimental results show that the adhesive strength of the as-soldered specimens increases with increasing Cu addition and increasing strain rate. Meanwhile, for the aged specimens, the adhesive strength increases with increasing strain rate, but decreases with increasing storage time or with increasing Cu addition beyond 1.0mass%. The microstructures and fracture morphologies of the solder specimens are analyzed by optical microscopy (OM) and scanning electron microscopy (SEM). The observations reveal that the Cu₆Sn₅ and Ag₃Sn particles within the solder microstructure coarsen following high temperature storage and thus reduce the adhesive strength of the solder. Finally, it is found that the prolonged aged specimens with a Cu addition of 0.5 mass% or 1.0 mass% fracture in a combined brittle and ductile failure mode, while those with a Cu addition of 1.5 mass% fail as a result of cleavage after following 200 hours of thermal storage and a strain rate of over 1 s^-1, but otherwise these fracture in a combined brittle and ductile failure mode.