Enhanced microstructural, thermal and tensile characteristics of heat treated Sn-5.0Sb-0.3Cu (SSC-503) Pb-free solder alloy under high pressure

Abstract

The Sn-5.0Sb-0.3Cu (SSC-503) solder alloy was pressurized by 1.0 GPa during heat treatment at 130 °C for 2 h (SSC-PHT) in order to refine its β-Sn grains and enhance the thermal and tensile properties. Microstructure of SSC-503 solder has irregular wavy coarsened grains. Refined grains with random irregular texture were observed after pressuring. This can be attributed to work hardening and shortened diffusion paths which promoted the crystal defects. XRD calculations revealed that the crystallite size of β-Sn phase of SSC-PHT sample was decreased by ~23% with respect to SSC-503 sample while the average lattice strain (η) was higher than heat treated sample (SSC-HT) by ~ 51%. Hence, pressuring during heat treatment causes an increase in dislocation density which strains the lattice and stores more strain energy. Differential scanning calorimeter (DSC) measurements indicated that the applied pressure inhibited the degree of undercooling from 3.04° to 1.47°C. This could endorse the solidification and restrain the growth rate of β-Sn dendrites due to the creation of new nucleation paths through preferential heterogeneous starting sites. The solidification activation energy of SSC-HT and SSC-PHT samples was evaluated as 60.52 and 52.31 kJ/mol respectively by utilizing modified Kissinger equation. After Appling pressure, a detectable enhancement was recorded in ultimate tensile strength (σUTS) by 63% and in yield stress (σYS) by 61% without deterioration of the ductility. These improvements related to the change in microstructure of SSC-PHT solder since high-pressure generated plenty of mixed dislocations as well as more defects. Moreover, the solid solution and dispersion hardening strengthening restricted the motion of the dislocations.