Long term isothermal aging effects on the cyclic stress-strain behavior of Sn-Ag-Cu solders

When exposed to a temperature changing environment, solder joints in electronic assemblies are subjected to cyclic mechanical loading due to the mismatches in coefficients of thermal expansion (CTE) of different assembly materials. Eventually, the cyclic loading results in fatigue failure of solder joints, which is one of the common failure modes in electronic packaging. Aging leads to solder microstructure changes such as grain and phase coarsening, and these effects are closely correlated to the damage that occurs during cyclic loading. In this investigation, we have studied long term isothermal aging effects on the cyclic stress-strain behavior and microstructure of Sn-Ag-Cu (SAC) lead free solders. Cylindrical uniaxial specimens were produced using a vacuum suction process and then aged for various aging times up to one year before testing. All the specimens were tested at room temperature (25 °C) using strain controlled cycling method. We have found that aging causes the degradation of solder mechanical properties. The evolution of cyclic stress-strain curve (hysteresis loop) induced by aging has been characterized. Also, the effects of aging on hysteresis loop area, plastic strain range and peak stress have been quantified and modeled for different aging times. Lastly, aging induced microstructural changes in a small fixed region of a single solder sample have been examined, and the coarsening of solder microstructure has been observed.