Impression Creep Behavior of Zn-Sn High-Temperature Lead-Free Solders

This study examines the microstructure and impression creep behavior of the high-temperature Zn-20 wt.%Sn, Zn-30 wt.%Sn, and Zn-40 wt.%Sn solders under constant punch stress in the range of 25 MPa to 300 MPa and at temperatures in the range of 298 K to 425 K. Analysis of the data showed that, for all loads and temperatures, the Zn-20Sn alloy had the lowest creep rates, and thus the highest creep resistance, among all materials tested. This is attributed to the lower volume fraction of the soft Sn-rich phase with a continuous morphology which acts as the matrix encompassing the harder Zn phase. The stress exponents and activation energies were in the range of 4.0 to 6.1 and 40.0 kJ mol⁻¹ to 45.3 kJ mol⁻¹, respectively. Based on the obtained stress exponents and activation energy data, it is proposed that dislocation climb is the controlling creep mechanism. However, the observed decreasing trend of creep activation energy with stress suggests that two parallel mechanisms of lattice-diffusion-controlled and pipe-diffusion-controlled dislocation climb are competing. Dislocation climb controlled by dislocation pipe diffusion is the controlling mechanism at high stresses, whereas climb of edge dislocations is the controlling mechanism at low stresses.