Effect of bismuth on the isothermal fatigue properties of Sn-3.5mass%Ag solder alloy

Sn-3.5mass%Ag eutectic solder is selected as a candidate base alloy for replacing the eutectic Sn-Pb, and the effect of bismuth (2, 5, 10mass%) on the fatigue life of bulk Sn-3.5mass%Ag eutectic at room temperature has been studied over the total strain range from 0.3 to 3 percent in tension-tension mode. Fatigue life is defined as the number of cycles at which the load decreases to a half of the initial value. The fatigue life dramatically decreases with increasing contents of bismuth and adding this element over 2% makes fatigue life shorter than that of tin-lead eutectic alloy. Tensile strength of the alloy significantly increases with an increase in bismuth contents due to solid solution hardening (<5%Bi) or dispersion strengthning of fine bismuth particles, while ductility of this system dramatically decreases with increasing bismuth contents. Fatigue life of these alloys depends on ductility obtained by tensile test. The fatigue life of Bi containing Sn-3.5%Ag alloys can be described by, (Δε_p/2D)·N _f ^0.59 =0.66 where N_f is fatigue life defined by number of cycles to one-half load reduction, Δε_p is the plastic strain range for initial cycles, D is the ductility as measured by reduction in area.