Mechanical Behavior and Microstructure Evolution in SAC+Bi Lead Free Solders Subjected to Mechanical Cycling

Solder joints in electronic packages often experience fatigue failures due to cyclic mechanical stresses and strains induced by temperature fluctuations. These fluctuating stresses and strains can be attributed to the mismatches in coefficients of thermal expansion, which lead to damage accumulation that contributes to crack initiation, crack propagation, and eventually failure. In this paper, the authors have quantified the evolution of the properties of SAC+3% Bi lead free solder subjected to mechanical cycling. Cylindrical specimens were prepared and reflowed with a standard reflow profile similar to that utilized in industry. The reflowed samples were then mechanically cycled at five different total strain ranges (0.003, 0.005, 0.007, 0.009, and 0.012), and the corresponding initial cyclic stress-strain curves were recorded and used to calculate the initial plastic work accumulation per cycle. A relationship relating the applied strain range to plastic work accumulation per cycle (damage level) was developed.The samples were then mechanically cycled at four different damage levels (∆W = 0.25, 0.5, 0.75, and 1.0 MJ/m
³
) at room temperature for various durations, and uniaxial tensile tests were subsequently conducted on these prior cycled (damaged) samples. Using the data from these tests, we have been able to characterize and quantify the cycling induced damage through the observed degradations of several mechanical properties (elastic modulus, yield strength, and ultimate strength) with the amount of prior cycling and initial damage level. In addition, rectangular cross-sectioned and polished samples were also cycled at the highest damage level (∆W = 1.0 MJ/m3) to correlate the change in microstructure with degradation of the properties during mechanical cycling.