Mechanical fatigue characteristics of Sn-3.5Ag-X (X=Bi, Cu, Zn and In) solder alloys

In our previous study, the fatigue life of Sn-3.5Ag-Bi alloy was found to be dominated by the fracture ductility of the alloy and to obey a modified Coffin-Manson's law: (Δε_p/2D)· N^α_f = C, where Δε_p is plastic strain range, N_f is fatigue life, and α and = C are nondimensional constants. In this study, copper, zinc, and indium are selected as the third element, and the effect of these elements on the isothermal fatigue properties of Sn-3.5%Ag alloy has been investigated. The relationship between fatigue life and crack propagation rate estimated from load drop curve during fatigue test is also discussed. The addition of copper, indium, and zinc up to 2% slightly decreases the fatigue life of Sn-3.5Ag alloy due to the loss of ductility, while the life still remains higher than that of tin-lead eutectic alloy. The modified Coffin-Manson's equation can also be applied to ternary Sn-3.5Ag-X. It is found that both ductility and fatigue life are significantly responsible for the load drop rate of the alloy, which reflects the extent of crack propagation. The fatigue life of Sn-3.5Ag-X alloy is therefore dominated not by the kinds and amount of third element but by the ductility of each alloy.