Fatigue properties of lead-free solder joints in electronic packaging assembly investigated by isothermal cyclic shear fatigue

This paper reports the fatigue properties of Sn-Ag-Cu (SAC) and Pb-Sn solder alloys determined from isothermal shear fatigue testing and analysis of resulting data using modified Coffin-Manson fatigue model. In our study, a series of cyclic shear fatigue testing was conducted on the solder joints in PBGA assembly with variation in testing temperature, strain range, and strain rate (or cycle frequency). The number of cycles to the first joint failure in the assembly was determined from the resistance of each solder joint and taken as the fatigue life of the assembly. Analysis of the resulting data reveals that isothermal fatigue behavior of both SAC (Sn-Ag-Cu) and Pb-Sn alloys follow the classic Coffin-Manson fatigue model and provide constants indicative of fatigue properties of the alloy, namely fatigue ductility coefficient and ductility exponent. These fatigue constants are consistent with what is generally expected from a metallic fatigue system except for their dependence on temperature. This deviation, attributable to the involvement of the thermal strain that is added to the applied mechanical shear strain, suggests that consideration of the thermal strain effect needs to be included in the fatigue analysis of solder joints even if the major fatigue mode appears to be pure mechanical shear. The fatigue constants gained from our study provide insights helpful in understanding the mechanism behind variation in fatigue reliability with solder alloy compositions and solder microstructure.