Damage Evolution in SAC Solder Alloys

Solder joints in electronic packages often experience fatigue failures due to cyclic mechanical stresses and strains in fluctuating temperature environments. This cyclic loading of the solder is induced by mismatches in coefficients of thermal expansion and elastic modulus, and leads to damage accumulation that contributes to crack initiation, crack propagation, and eventually failure.In this work, we are investigating the accumulation of damage in SAC alloys during isothermal mechanical cycling at room temperature. In the experimental testing, small uniaxial cylindrical samples of SAC305 solder were prepared and reflowed in a reflow oven. These specimens were then mechanically cycled under strain control and several different sets of conditions to induce various levels of damage in the samples. It was found that the total energy dissipation that had occurred in the sample (sum of ΔW for all cycles) could be used as a governing failure variable independent of the damage level applied during each cycle. This has motivated using a scalar damage parameter in the constitutive model for the investigated SAC305 alloy following the basic concepts of continuum damage mechanics. As damage accumulates, the elastic, plastic and creep properties degrade accordingly. The damage law is incorporated into ANSYS UPFs to simulate the stress-strain hysteresis loop in the test. Simulation results show that it can capture the loop area decrease, maximum stress and modulus reduction. The damage model is also used to simulate the progressive failure of a solder sphere under cyclic iso-thermal mechanical load.