Isothermal Aging Dependent Anand Parameters of SAC305 Lead Free Solder at Extreme High Temperatures

Abstract

With the emergence of the modern electronic packaging technology over the last few decades, lead free solder alloys have been the primary interconnects material used in electronic packaging industry. The reliability of the electronic packages depend largely on the mechanical properties of the lead free solder materials. The microstructures of popular SAC lead free solders are inherently unstable and experimental testing has revealed that SAC solders exhibit evolving mechanical properties that change significantly with isothermal aging. These changes are especially large in harsh environments, such as at extreme high temperatures where the effects of aging on solder joint behavior must be accounted for and included in constitutive models when predicting reliability. The nine parameter Anand viscoplastic constitutive model is a popular commercial finite element code built in to many commercial FEA packages like ANSYS and ABAQUS, which is widely used in the electronic packaging industry to predict reliability. Reliability prediction results are often highly sensitive to the specified Anand parameters, and there are great variations in the available literature values for common solder alloys. Previous investigations on the nine parameters have mainly emphasized mechanical testing and fitting at temperatures up to 125 °C. In real life, electronic devices frequently experience harsh environment applications where solders are exposed to very high temperatures from 125-200 °C. Mechanical properties as well as Anand model parameters of lead free solders at elevated temperatures are not available in previous literatures.In this work, we have studied the changes in mechanical properties and nine parameters of the Anand viscoplastic constitutive model for SAC305 (96.5Sn-3.0Ag-0.5Cu) lead free solder by considering the isothermal aging effect. The pre aging was done at 125 °C. Uniaxial tensile stress-strain tests were carried out on SAC305 specimens using a micro tension/torsion testing machine with three strain rates (0.001, 0.0001 and 0.00001 (1/sec)), four extreme high testing temperatures (T = 125, 150, 175, and 200 °C), and four different pre-aging conditions (0, 1, 5, and 20 days at 125 °C). Our findings show that 2 of the 9 Anand parameters remain essentially constant during aging, while the other 7 show significant changes with aging. Once the Anand constants were determined, we used the constitutive model to predict the stress-strain curve at each particular temperature and strain rate used in the experimental testing to evaluate the goodness of fit of the constitutive model to the test data. Good correlation was observed between the experimental curves and the model predictions.