Electronic structure mechanism for the wettability of Sn-based solder alloys
The developments of quantum theory, solid-state physics, and computational methods make it feasible to understand properties of materials by means of calculation. In this work, five octahedron clusters were designed to study the wettability of Sn-based solder alloys, which are applied in modern electronic mounting and packaging. Then, relativistic DV-Xα calculation, which is a molecular orbital method based on Hartree-Fock-Dirac approximation, was carried out. Heavy atoms, such as Pb, Bi, Sn, and Sb, were included in our clusters, so relativistic effects were taken into account in the calculation. The electronic parameter, Bo, the orbital interaction between atoms, was obtained through a Mulliken analysis of electronic structure. The electronic structure mechanism for the wettability of Sn-based solder alloys on a Cu substrate was put forward based on the analysis of orbital interactions between atoms. We believe that the wettability of the SnxMy alloy would be improved only if orbital interactions between Sn atoms and Cu atoms are enforced because of the existence of the M element. The spreading and wetting behavior of Sn-based solder alloys were predicted and then explained by this quantum method on the basis of electronic structure theory. Predictions from analysis on calculation results were validated by wettability experiments and energy-dispersive x-ray (EDX) analysis.