Deformation Behavior of Sn-3.8Ag-0.7Cu Solder at Intermediate Strain Rates: Effect of Microstructure and Test Conditions

Abstract

When mobile electronic devices drop during service, solder interconnects are loaded under intermediate to high strain rates. Therefore, the strain response of solders at elevated strain rates is critical to reliability prediction. This paper presents the plastic flow behavior of Sn-3.8Ag-0.7Cu solder under compression over strain rates ranging from 0.1 s−1 to 30 s−1 at several different temperatures and under various aging conditions. Both yield strength and work hardening rate were observed to increase substantially with increasing strain rate, with the strain rate sensitivity at higher temperatures being greater. Empirical expressions capturing the strain rate and temperature dependence of the yield and work hardening parameters are presented. Microstructural observations revealed greater strain localization following testing at higher strain rates, with the development of distinct flow patterns and localized kinking of dendrites due to twin formation. Low-temperature aging (35°C) appeared to enhance yield strength slightly relative to the as-reflowed condition, but decreased the work hardening rate. With aging at higher temperatures (100°C and 180°C), and commensurate coarsening of the precipitate structure, both yield strength and work hardening rate decreased dramatically.