Creep behavior of the ternary 95.5Sn-3.9Ag-0.6Cu solder—Part I: As-cast condition

The compression creep behavior was studied for the ternary solder alloy 95.5Sn-3.9Ag-0.6Cu in the as-cast condition. Samples were tested under stresses of 2–45 MPa and temperatures of −25–160°C. There was a significant variability in the creep curve shape, strain magnitude, and steady-state strain-rate properties. A multivariable linear-regression analysis of the steady-state strain-rate data, using the sinh-law stress representation, indicated two mechanisms distinguished by low- and high-temperature regimes of −25–75°C and 75–160°C, respectively. The sinh-law stress exponent (n) and apparent-activation energy (ΔH) in the −25–75°C regime were 4.4 ± 0.7 kJ/mol and 25 ± 7 kJ/mol (63% confidence intervals), respectively. Those same parameters in the 75–160°C regime were 5.2±0.8 kJ/mol and 95±14 kJ/mol, respectively, for the high-temperature regime. The values of ΔH suggested a short-circuit diffusion mechanism at low temperatures and a lattice or bulk-diffusion mechanism at high temperatures. The stress dependency of the steady-state strain rate did not indicate a strong power-law breakdown behavior or a threshold stress phenomenon. Cracks and grain-boundary sliding were not observed in any of the samples. As the creep temperature increased, a coarsened particle boundary and particle depletion zone formed in the region of fine Ag₃Sn particles that existed between the Sn-rich phase areas. The coarsened particle boundary, as well as accelerated coarsening of Ag₃Sn particles, were direct consequences of the creep deformation process.