Aging and creep behavior of Sn3.9Ag0.6Cu solder alloy

Aging effects and creep behavior along with microstructure changes in eutectic PbSn and the lead-free solder alloy Sn3.9Ag0.6Cu were studied. The room temperature aged Sn3.9Ag0.6Cu alloy continually age-softened due to the growth of relatively large tin-rich crystals. The 180°C aged Sn3.9Ag0.6Cu alloy initially age-softened, and the minimum flow strength was reached after one day at 180°C. This softening correlated with the growth of relatively large tin-rich crystals and with the coarsening of Ag₃Sn particles. When aged at 180°C beyond one day the Sn3.9Ag0.6Cu alloy age-hardened corresponding to the dispersion of Ag₃Sn particles into tin-rich crystals which previously had not contained intermetallic precipitates. The Sn3.9Ag0.6Cu alloy showed much lower absolute creep rates than the PbSn eutectic. This extreme increase in creep resistance may result from finely dispersed IMC precipitates in the tin matrix. The β-Sn dendrites after creep appear to have some orientational features (aligned at approximately 45° to the flow direction). The size and distributions of IMC is somewhat coarsened with increasing creep temperature. A number of coarsened precipitates of Cu₆Sn₅ segregate around the β-Sn grain boundary. The simple power-law model was found inapplicable to the whole stress regime for both types of alloy. In addition, two hyperbolic-sine models were developed to describe the creep behavior of both alloys.