Incorporation of Damage in Creep Models for SAC305 Lead Free Solder

Abstract

Abstract 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 paper at InterPACK 2022, we investigated the accumulation of damage in several lead-free solder materials during mechanical cycling. Circular cross-sectioned solder specimens were first reflowed, and these samples were then mechanically cycled at room temperature for various durations using a Micro-Mechanical tester. Monotonic stress-strain and creep tests were then 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 (elastic modulus, yield strength, ultimate strength, and creep strain rate) with the amount of prior cycling. In addition, changes in the microstructure and growth of microcracks during cycling have been characterized. In the current work, we have extended the experimental work in our prior studies on SAC305 to examine the evolution of the creep response due to prior damage accumulation at elevated 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 several different sets of conditions to induce various levels of damage in the samples. Four levels of initial damage per cycle were considered (ΔW = 0.25, 0.50, 0.75 and 1.00 MJ/m3 at 100 °C For each of these damage levels per cycle, various durations of cycling were applied (e.g., 0, 50, 100, 300, and 600 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. Creep tests were performed on the prior damage samples at room and elevated temperatures and at stress level of σ = 10 MPa. The changes in the steady state secondary creep rate were then evaluated and plotted versus the duration of cycling for the various 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 (ΣΔW = sum of ΔW for all cycles) could be used as a governing degradation parameter independent of the damage level applied during each cycle. In particular, all of the material property data for a selected property and temperature were modeled well using a single degradation curve independent of that rate the damage was accumulated. We are also using the data in our studies to incorporate damage parameters into popular constitutive models for solder such as the Norton and Garofalo creep models, and the Anand viscoplastic model.