A Unified Creep-Plasticity Theory for Solder Alloys

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Abstract
Sn-Pb solders exhibit creep-plasticity interaction and significant strain-rate dependence at operating temperatures of electronic circuits. A major problem is fatigue and creep-fatigue interaction due to thermomechanical cycling. Reversed plasticity, creep, and creep-plasticity interactions dominate the behavior for these types of histories. Furthermore, thermal softening plays a strong role in the time dependence of the mechanical behavior. However, classical forms of decoupled plasticity and creep theories have been shown to be quite inferior for modeling cyclic plasticity, creep, and interaction effects. The goals of this study were to: (a) develop a thermomechanical unified creep-plasticity model which can characterize the response of solders, and (b) characterize a selected solder alloy through mechanical testing and correlation with the model across a range of strain rates, temperatures, aging times, and loading-unloading conditions. Solder alloys in general are mechanically soft and are employed routinely at very high homologous temperatures. The microstructure is often unstable at normal circuit board operating temperatures and the extent of alteration to the microstructure is a function of the applied stress or strain, temperature, etc. Many alloys recrystallize below 77°F (25°C) and therefore can change with time at room temperature. A program of characterization involving tension tests at various temperatures and strain rates, creep tests, cyclic loading tests, and tension tests after prior high-temperature exposure was pursued to develop a broad-based understanding of the behavior of eutectic Sn-Pb solder, the most commonly used industrial solder alloy. A model for thermomechanical temperature and rate-dependent behavior of metallic alloys has been developed. Many of the typical, complex behaviors of solders can be addressed with this model, including temperature dependence and strain-rate dependence, creep-plasticity interaction, viscous cyclic strain ratchetting, softening effects due to static thermal recovery, and potential effects of aging (microstructural changes due to exposure at high temperature).