Mechanistic Model for Aging Influenced Steady State Flow Behavior of Sn3.8Ag0.7Cu Solder Alloys

Sn3.8Ag0.7 Cu solder alloys age even at ambient temperatures. This significantly affects both microstructure and behavior of these alloys. In this work, aging influences on both are addressed. In particular, we discuss (a) honey comb type microstructure patterns in unaged samples and their subsequent evolution into a coarsened random microstructure at prolonged aging durations (b) aging effects on primary, secondary creep and on a range of applied loads and test temperatures through double lap shear experiments; the results show an increased tendency of aged solders to flow (c) a modified power law creep-plasticity model to predict aging effects on behavior. Possible mechanisms that result in the above behavior are also discussed; they motivate the mechanistic basis for the developed aging-informed behavior model, (d) procedures to compare alloys in terms of aging effects. Steady state creep strains, monotonic plastic strains and unified creep-plasticity theory are also discussed. Aging temperatures of −10 °C, 25°C, 75°C and 125°C, and aging times of 15, 30, 60 and 90 days (at each aging temperature) were selected as different levels of factors in a statistically designed experiment to study aging effects. Test specimens were selected with due pre-test considerations to joint-geometry, associated stress heterogeneity and joint-microstructures.