EFFECT OF BISMUTH ON THE ISOTHERMAL FATIGUE PROPERTIES OF SN-3.5MASS-PERCENT-AG SOLDER ALLOY

Sn-3.5mass%Ag eutectic solder is selected as a candidate base alloy fo r replacing the eutectic Sn-Pb, and the effect of bismuth (2, 5, 10mas s%) on the fatigue Life of bulk Sn-3.5mass%Ag eutectic at room tempera ture has been studied over the total strain range from 0.3 to 3 percen t in tension-tension mode. Fatigue life is defined as the number of cy cles at which the load decreases to a half of the initial value. The f atigue 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 significantl y increases with an increase in bismuth contents due to solid solution hardening (<5%Bi) or dispersion strengthening of fine bismuth particl es, while ductility of this system dramatically decreases with increas ing bismuth contents. Fatigue life of these alloys depends on ductilit y obtained by tensile test. The fatigue life of Bi containing Sn-3.5%A g alloys can be described by (Delta epsilon(p)/2D).N-f( 0.59) = 0.66 w here N-f is fatigue Life defined by number of cycles to one-half load reduction, Delta epsilon(p) is the plastic strain range for initial cy cles, D is the ductility as measured by reduction in area.