Material Characterization of Doped Lead-Free Solders at High Strain Rates and Extreme temperatures and Prediction of Mechanical Properties

Abstract

The reliability of solder joints remains a critical concern in advanced electronic packaging, particularly under harsh service conditions where high strain rates, extreme temperatures, and prolonged thermal aging accelerate material degradation. This dissertation presents a comprehensive investigation of the mechanical behavior, thermal stability, and constitutive modeling of SAC-R, QSAC10, and QSAC20 solder alloys. High strain rate tensile testing (10–75 s⁻¹) was conducted across a broad temperature range (–65 °C to +200 °C) using an impact hammer-based setup, while long-term thermal aging studies were performed at 50 °C for up to 360 days. Experimental results demonstrate that strain rate increases ultimate tensile strength (UTS) and elastic modulus (E) through strain-hardening, whereas elevated temperatures reduce these properties due to thermal softening. Thermal aging leads to progressive mechanical degradation, though doped alloys—particularly QSAC10 and QSAC20—exhibit significantly better property retention compared to undoped SAC-R, with QSAC20 consistently achieving the highest strength values.

To model these behaviors, the Anand visco-plasticity framework was calibrated using experimental stress–strain data, capturing both temperature and strain-rate sensitivity. The study revealed that thermal aging alters key Anand parameters (A, ŝ, h₀) while leaving others (Q/R, ξ) relatively unchanged, with doped alloys showing slower parameter evolution and greater thermal stability. The strong agreement between experimental data and Anand model predictions validates its applicability for finite element simulations of solder joint reliability. By correlating Bi content with mechanical properties and aging trends, predictive frameworks were also established, enabling the extrapolation of performance for untested solder compositions without requiring extensive experimental campaigns.

Overall, this research demonstrates the pivotal role of Bi doping in enhancing solder alloy performance and establishes an integrated experimental–computational framework for predicting solder joint behavior under dynamic and aging conditions. The findings contribute directly to the design of reliable, high-performance electronic systems for aerospace, automotive, defense, and industrial applications, where long-term durability under extreme environments is paramount.