Evolution of SAC305 Mechanical Behavior Due to Damage Accumulation During Cycling

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 leads to damage accumulation that contributes to crack initiation, crack propagation, and eventually to failure. In our previous papers, we have investigated the accumulation of damage in several lead free solder materials (SAC305, SAC+Bi, and SAC+Bi-Ni-Sb) during mechanical cycling at room temperature (25 C) and elevated temperature (100 C). Circular cross-section solder specimens were first reflowed, and these samples were then mechanically cycled for various durations using a Micro-Mechanical tester. Monotonic stress-strain and creep tests were subsequently conducted on the prior cycled samples to characterize the change in mechanical behavior occurring in the solder due to damage accumulation. 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 (initial elastic modulus, ultimate tensile strength, yield stress, and creep strain rate) with the amount of prior cycling. All of the mechanical cyclic testing in our prior work were performed for a single applied level of cyclic strain = +/- 0.01 (single level of damage per cycle), which corresponded to a hysteresis loop area (energy dissipated per cycle) during room temperature cycling of SAC305 of ΔW = 1.2 MJ/m3.In the current work, we have extended the experimental work in our prior studies on SAC305 to examine several levels of damage during cycling, as well as several cycling temperatures. Material behaviors of the pre-cycled solder were characterized for the various damage levels per cycle and durations of cycling. One goal of this investigation was to identify a damage parameter that can be used to predict the observed material property degradations occurring during cyclic loading of solder irrespective of the way that the damage is accumulated. The total energy dissipation occurring in the solder during cycling was found to correlate well with the evolution of mechanical properties, independent of the damage level applied during each cycle.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 several different sets of conditions to induce various levels of damage in the samples. In particular, four levels of initial damage per cycle were considered (ΔW = 0.25, 0.50, 0.75 and 1.00 MJ/m3), as well as three cycling temperatures (T = 25, 100, and 125 °C). For each of these damage levels per cycle, various durations of cycling were applied (e.g. 0, 50, 100, 200, 300, 600, and 1200 cycles). This test matrix generated a large set of prior damaged samples, where the damage had been accumulated at different rates (different damage amounts per cycle), different cycling temperatures, and for different durations. In this paper, results obtained for isothermal mechanical cycling at 25°C will be presented in detail, as well as limited results for cycling at 100 °C.Mechanical stress-strain testing was then performed on the prior damaged samples. This allowed us to study the degradations of the constitutive behavior of the solder alloy that occurred due to the various conditions that induced the damage. In particular, the degradations of the initial elastic modulus, ultimate tensile strength, and yield stress with duration of mechanical cycling were evaluated and plotted versus the duration of cycling for the various prior applied levels of damage per cycle. Exponential empirical models were found to fit the material property degradations well for any one set of conditions. More importantly, 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. In particular, all of the material property data for a selected property and cycling temperature were modeled well using a single degradation curve independent of that rate the damage was accumulated. By considering the degradation curves for each cycling temperature, values of a temperature dependent damage parameter can be identified that can be used in thermal cycling simulations. Using the results of this study, we are working to develop better fatigue criteria for lead free solders that are subjected to variable temperature applications.