Microstructural and micromechanical characteristics of tin-based solders under self-propagating exothermic reaction heating

In this study, a rapid solder melting process has been examined through a self-propagating exothermic reaction using Al/Ni nanofoil as a localized heat source. Four kinds of tin-based solder preforms were partly melted under this process, and the thicknesses of the fusion zone in solder preforms around the Al/Ni nanofoil were experimentally analyzed. The microstructure and morphology of the fusion zone in different kinds of solder preforms were studied by metallographic analysis. In Sn₄₂Bi₅₈ solder, the mean grain size of Bi-rich phases in the fusion zone was finer than the original structure, and decreased from ~ 2.7 μm to ~ 0.8 μm away from the Al/Ni nanofoil. Similar trends of mean size of grains in the fusion zone were also found in the Sn, Sn-3 wt.%Ag-0.5 wt.%Cu(SAC), and SnPb solder preforms. The widths of the fusion zone in the Sn, SAC, SnPb, and SnBi solder preforms were 150 μm, 173 μm, 188 μm, and 233 μm, respectively. In addition, the nano-hardness and distribution in the fusion zones were evaluated. The results show that the value of nanohardness increased along with the decrease of distance from Al/Ni nanofoil. The average hardness and the lower and upper bounds of Sn, SnPb, SAC, and SnBi solder in the fusion zone are 0.260 (−0.045, +0.039) GPa, 0.246 (−0.059, +0.114) GPa, 0.260 (−0.014, +0.070) GPa, and 0.404 (−0.072, +0.134) GPa, respectively. The statistical significance of solder hardness is related to its alloy component, element content, and microstructure.