Enhancement of the Unified Constitutive Model for Viscoplastic Solders in Wide Strain Rate and Temperature Ranges

Eutectic 63Sn-37Pb solder is a typical viscoplastic material widely used in aerospace applications. In order to benchmark the mechanical reliability of electronic packaging structures in harsh environments, this study focused on the constitutive behavior of the solder. A unified creep-plasticity constitutive model is proposed with an enriched flow rule to satisfactorily describe the plastic and hardening stages. The strain rate range of 10^-4 and 10^-2 s^-1 and the temperature range of -55 and 125°C are covered by a series of comprehensive uniaxial tensile experiments. Moreover, the experimental results are analyzed to elucidate the underlying mechanism of the calibrated parameters. It is found that the drag strength increases with temperature, while its slope is reduced with the strain rate. The short range of back stress is closely associated with temperature to influence the hardening slope of the stress–strain relationships. The value of hardening exponent is sensitive to strain rate and thus determined to be 0.52 and 2.72 for the strain rate greater and less than 2·10^-4 s^-1, respectively. The strain rate and temperature ranges in this study are capable of covering the temperature cycling tests for reliability qualification of electronic devices. Thus, the proposed enhancement of the unified creep-plasticity constitutive model facilitates the finite element simulations of packaging structures under temperature cycling conditions. This framework of the proposed constitutive model is readily extended to other types of viscoplastic solders within wide strain rate and temperature ranges.