Evolution of Anand Parameters for Thermally Aged Sn-Ag-Cu Lead-Free Alloys at Low Operating Temperature

Abstract

In extreme environmental applications, such as aerospace and automotive, electronics may endure high or low operating temperatures during service, handling, and storage. An electronic assembly may experience strain rates of 1–100 per sec of strain and ambient temperatures of –65 to +200 °C. Electronic assembly's temperature depends mainly on location, energy dissipation, and thermal architecture. Electronic assemblies in automotive applications may be located underhood, on engine, on-transmission, and or in wheel well. Study of property evolution of solders used for interconnection is important for assurance of reliability. Degradation in material properties for lead-free solder alloys can be caused by change in microstructure due to variation in temperatures. There is need for data on the effect of operating period and operating conditions on the material properties. Addition of dopants in Sn-Ag-Cu (SAC) alloys has been shown to improve mechanical properties and minimize deterioration due to aging at lower strain rates. SAC-Q is formulated with Sn-Ag-Cu with addition of Bi (SAC+Bi). It has been observed that adding Bismuth (Bi) to SAC alloy can play an important role to make the solder alloy resistant to aging-induced degradations. In author's prior research the evolution of Anand parameters and materials properties for SAC solder (SAC105, SAC305, and SAC-Q) at high temperatures and high strain rates has been studied. However, data on thermally aged SAC solder alloys at high strain rate levels at low operating temperatures are not available in published literature. In this paper, materials characterization of thermally aged SAC (SAC105 and SAC-Q) solder at low operating temperatures (−65 °C to 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 lead-free solder specimen was isothermally aged up to 6 months at 50 °C before testing. Anand viscoplastic model has been used to compute 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 nine 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.