Low Operating-Temperature High Strain Rate Constitutive Behavior of SnAgCu Solder Alloys after Prolonged Storage at High Temperature

Abstract

Extreme environmental applications, including a downhole drilling, aerospace and automotive, may subject the electronics to high-g acceleration in addition to operation at high and low temperatures for extended periods of time. An electronic assembly may experience strain-rates in the range of 1-100 per sec of strain and operating temperatures in the range of -65 to +200°C. Degradation in traditional SAC solder materials properties for lead free solder alloys can be caused by change in microstructure due to variation in temperatures. Doped solder alloy, SAC-Q, is formulated through addition of Bi (SAC+Bi). Adding Bismuth (Bi) to SAC alloy can play an important role to make the solder alloy resistant to aging-induced degradations. Use of doped solders in extreme environments requires non-linear material properties. However, data on thermally aged SAC solder alloys at high strain rate levels at low operating temperatures are not available. In this paper, materials characterization of thermally aged SAC (SAC105 and SAC-Q) solder at low operating temperatures (-65°C-0 °C) and at high strain rates (10-75 per sec) has been studied. Stress-Strain curves have been measured at low operating temperatures using impact hammer based tensile tests with cooling chamber. The fabricated SAC leadfree solder specimen was isothermally aged up to 6 months at 50°C before testing. Anand Viscoplastic model has been used to compute the nine Anand parameters to describe the material constitutive behavior. Anand Model parameters evolution due to thermal aging has been studied for SAC solders. The computed Anand parameters from experimental data then were used to simulate the tensile test to predict the stress-strain curve and compared to experimental stress-strain curves to verify the accuracy of the model. A good correlation was found between experimental data and Anand predicted data.