Superplastic Creep of Eutectic Tin-Lead Solder Joints

The creep behavior of air-cooled and liquid nitrogen-quenched soldered joints of 60/40 Sn-Pb at 65°C has been studied. The stress exponent, n, in the power law, γ˙ (steady state strain rate) ∝σⁿ (applied stress), changes from a value of about 6 to values of 2 to 3, as γ˙ decreases below 10⁻⁴ in/in per second. This result, combined with the authors’ previous stepped load creep test results, indicates a transition of the creep deformation mechanism from conventional dislocation climb to superplastic grain boundary sliding. The superplastic creep of the soldered joints is ascribed to their non-lamellar microstructure due to their fast cooling rate. During creep deformation, recrystallization of the soldered joints occurs, causing softening.