The effects of aging on the Anand viscoplastic constitutive model for SAC305 solder

Lead free solder materials are widely used in electronic packaging industry due to environmental concerns. However, experimental testing and microstructural characterization have revealed that Pb-free solders exhibit evolving properties that change significantly with environmental exposures such as isothermal aging and thermal cycling. These changes are especially large in harsh environments, where the effects of aging on solder joint behavior must be accounted for and included in constitutive models when predicting reliability. In this work, we have adapted the Anand viscoplastic stress-strain relations for Pb-free solders to include material parameters that evolve with the thermal history of the solder material. In particular, aging effects have been examined by performing uniaxial tensile tests on SAC305 samples that were aged for various durations (0-12 months) at temperature of 100 C. For each set of aging conditions, several sets of constant strain rate and temperature tests were conducted on the aged solder samples. Testing conditions included strain rates of 0.001, 0.0001, and 0.00001 (1/sec), and temperatures of 25, 50, 75, 100, and 125 C. Using the measured uniaxial test data, the Anand parameters were calculated for each set of aging conditions, and the effects of aging on the nine Anand model parameters were determined. Mechanical tests have been performed using both water quenched (WQ) and reflowed (RF) SAC305 samples (two unique specimen microstructures). In the case of the water quenched samples, there is rapid microstructural transitioning during the brief time that occurs between placing molten solder into the glass tubes and immersing the tubes in water bath. On the other hand, the reflowed samples are first cooled by water quenching, and then sent through a reflow oven to re-melt the solder in the tubes and subject them to a desired temperature profile matching that used in PCB assembly. From the experimental results, the differences between the extracted Anand model parameters of water quenched and reflowed samples were high for samples with no prior aging. As expected, the water quenched samples had much higher mechanical properties (stiffness and strength) than reflowed samples prior to aging. For both the water quenched and reflowed specimens, significant degradation of the mechanical properties was observed with aging. The variations of the Anand model parameters with aging time have been characterized, and empirical relationships were established to model the observed changes. After long aging times, the water quenched and reflowed SAC305 materials were found to exhibit similar mechanical properties, and thus their Anand parameters converged and became nearly identical.