Effect of Surface Nanostructures and Speciation on Undercooling for Low-Temperature Solder Alloys

Role of surface structure/composition in altering solidification behavior is demonstrated through undercooling of core-shell metal particles. Autonomous surface speciation in the passivating oxide plays a critical role in the relaxation of a molten metal due to divergence in the concentration and composition of oxidizing species across the thickness of the oxide layer. In an unreactive environment, surface speciation is dictated by flux, cohesive energy density, and surface energy minimization. Under oxidizing conditions (e.g., ambient), however, reduction potential, curvature, and surface plasticity dictate the spatial order and concentration(s) across thin passivating oxide layers. It is therefore important to redefine solubility beyond the limitations of Hume-Rothery rules by substituting electronegativity for redox potential and cohesive energy density. Increasing number of components in an alloy does not necessarily lead to increased undercooling, but a maximum is observed around two to three components. These results lead to an empirical observation that Delta G(LS) (Gibbs' free energy for liquid-solid transition) around the freezing point can be understood from enthalpy and surface tension balance, with the degree of undercooling being a proportionality term for the enthalpic component. This work extends our understanding of classical nucleation theory by illustrating the importance of asymmetry in surface work in phase change, largely due to changes in structure of nanoscale passivating oxides. This phenomena is critical in development of low-temperature solders.