Viscoplastic characteristics during the stress relaxation of chilled eutectic Sn-Pb, of which microstructure is typical of Sn-Pb solder joint, was analyzed with the respect to kinetics and micromechanisms. Constitutive relations of stress exponent and relaxation activation energy were calculated from collected data, and possible deformation mechanisms were phenomenologically inferred. By the spectral processing of load relaxation data, abrupt change of stress exponent was revealed at certain unreleased stress level, which implied series operation of different predominant viscous flow mechanisms. Stress exponent was '~3' and '5~6' at low and high stress regime, respectively, while the activation energy was '45~65KJ/mole' regardless of unrelaxed stress magnitude. It was considered that stress relaxation was proceeded by sequential mechanisms of dislocation climb and diffusion creep at high and low stress range, and accommodation between grain boundaries was accomplished through dislocation pipe diffusion. Viscoplastic characteristics at different testing mode of load relaxation, uniaxial creep and displacement-controlled tensile were comparably analyzed. Details on the mathematical data processing and summarized results will be discussed in this presentation.

Key words: Viscoplastic characteristic, eutectic Sn-Pb, stress relaxation, constitutive relation