Low-cycle fatigue of Ag-based solders dependent on alloying composition and thermal cycle conditions

Many solder creep and fatigue models have been developed to predict the fatigue life of solder joints under lowcycle fatigue conditions. However, because of the diversity of leadfree solders, additional work has been performed to find out effects of alloying content, surface finish, thermal cycle conditions and microstructure. Based on creep properties, the solder joint creep strain and creep dissipation responses for ceramic chip components were analyzed by FEA. Different thermal cycling conditions were considered. It was observed that with lowered Ag content the maximum creep strain amplitudes increase, but a drop in maximum cyclic stress occurs. On the contrary, a special SAC alloy was tested with five alloying elements, called Innolot (SnAg3.8Cu0.7Bi3.0Sb1.4Ni0.2). It shows a high stress amplitude during cycling, i.e. a high creep resistance, and the creep strain becomes very low. Fatigue behavior of the different alloys is discussed.