Evolution of high strain rate and high temperature mechanical properties of SAC305 with long term storage up to 1-year

The effect of aging on mechanical properties of SAC 305 at low strain rate has been investigated. For high strain rate constitutive mechanical behavior, a number of researchers relied on Split Hopkinson Pressure bar and the strain rate range is from 500/s to 3000/s. However, for typical drop and shock, the strain rate range is from 1/s to 100/s. There is a general scarcity of data for solder materials in this strain rate range. Therefore knowing the mechanical properties of lead free solder at this high strain rate range is very important for design and optimization of package reliability. It is possible that failure may happen at initial shock incident or may result from cumulative damage from sequential shock and vibration events. In addition, isothermal aging and thermal cycling may cause significant changes of mechanical properties of solder alloys due to evolving of microstructure. These changes are large especially in harsh environment such as high temperature and long-term aging. Consequently, a complete understanding of high strain rate and high temperature behaviors of solder alloy after long period of aging is necessary to perform a better design and optimization in electronics. A viscoplastic model was proposed by Anand [1982, 1989] to describe materials that depend on both operating temperature and strain rate. Recently, it has been broadly used to characterize viscoplastic deformation of lead-free solder materials. However, the Anand constants of SAC305 for high strain rate and high temperature condition at long-term aging are not available. In order to compute the constants for this model, uniaxial tensile tests have been done at a wide range of high strain rate and high temperature conditions within different aging period. In this study, different weighted impact hammers were introduced which enable attaining different high strain rates around 1 to 100 /s. A load cell is on the top of the specimen-grip, which is used to calculate tensile load dynamically. Additionally, a small thermal chamber is used to control the operating temperatures. High-speed data acquisition system was built to capture the stress-strain curves of specimen. Tensile stress-strain curves have been plotted over a wide range of strain rates (8 =10, 35, 50, 75 /s) and temperatures (T = 25, 50, 75, 100, 125, 150, 175, 200°C) at different aging periods (Pristine, 60, 120, 180, 240, 300, 360 days). Totally, seven groups of Anand constants have been computed based non-linear least square curve fitting procedures. In addition, the correctness of the predicted model has been verified by comparing with experimental data.