Creep test method for determination of Anand parameters for lead free solders and their variation with aging

Experimental testing and microstructural characterization have revealed that lead-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 our prior work, the Anand viscoplastic model has been used to establish theoretical equations for the uniaxial stress-strain response (constant strain rate) and creep response of lead-free solders. Procedures were also developed for extracting the Anand model constants from experimental stress-strain or creep data. A revised Anand constitutive model for lead free solder material that includes aging effects was also investigated. In that work, the evolutions of the Anand parameters with aging were found using stress-strain testing. In this study, we have extended our prior work to investigate aging effects on the creep curves of lead free solder materials, and to use the observed changes to determine the evolution of the Anand model parameters with aging using the creep test procedure. In particular, Anand parameters for SAC305 (Sn-3.0Ag-0.5Cu) lead free solder were found from aging dependent creep test data for samples subjected to prior aging at 100 C for various time durations (0-6 months). For each set of aging conditions, creep data were measured at several stress levels (σ = 6, 8, 10, 12 and 15 MPa) and several testing temperatures (T = 25, 50, 75, 100, and 125 C). Mathematical expressions were also developed to model the evolution of the Anand model parameter with aging time. Our findings show that 2 of the 9 Anand parameters remain essentially constant during aging, while the other 7 show large changes (30-70%) with up to 6 months of aging at 100 C. The results also show that the changes are rapid during first 20 days of aging. After that, they change slowly and linearly for longer aging times. The determined variation of Anand parameters from the creep test method was also compared with our prior results obtained from using stressstrain test data, and good agreement was found between the results of the two methods. Finally, the creep curves for different aging times have been predicted using the different sets of Anand parameters that are appropriate for those aging times. Good correlation was observed between the experimental curves and the model predictions.