Cyclic Stress-Strain Behavior of SAC305 Lead Free Solder: Effects of Aging, Temperature, Strain Rate, and Plastic Strain Range

Solder joints in electronic assemblies are often subjected to cyclic (positive/negative) mechanical strains and stresses. Such exposures can occur in variable temperature application environments or during accelerated life thermal cycling tests used for qualification. Cyclic loading leads to damage accumulation, crack initiation, crack propagation, and eventually to fatigue failure. In this investigation, we have examined the effects of prior aging conditions, strain range, strain rate, and testing temperature on the cyclic stress-strain behavior of SAC305 lead free solder. Newly designed micro-cylinder shaped uniaxial lead free solder test specimens have been prepared in glass tubes using a vacuum suction process. Prior to testing, the samples were aged for various durations (0 to 360 days) at 125 °C. After aging, the fabricated samples were then subjected to cyclic stress-strain loading under several different conditions. From the recorded cyclic stress-strain curves, we have been able to characterize and empirically model the evolution of the solder hysteresis loops with aging duration. It has been observed that aging leads to the microstructural coarsening and degrades the mechanical fatigue properties, and those degradations are much more significant at the first few days of aging. The effects of aging on the cyclic stress-strain behavior have also been quantified for the first time for different testing temperatures, strain rates, and plastic strain ranges. At elevated test temperatures or lower test strain rates, an increase in the plastic strain range and a drop of the peak stress and loop area were found. Additionally, the plastic strain range, loop area, and peak load increased as expected with greater applied total strain range.