The developments of quantum theory, solid physics and computational methods make it feasible for us to carry out material design by means of calculation. In this thesis, Sn{sub}28M(M=Zn, Ag, Cd, In, Sn, Sb, Pb and Bi) cluster and octahedron clusters were set up respectively to understand mechanical properties and wettability of Sn-based Pb-free solders, which are applied in modern electronic mounting and packaging. Then relativistic DV-Xα calculation, which is a molecular orbital method basing on Hartree-Fock-Dirac approximation, was carried out. Firstly, some electronic parameters such as Mk and Bo were obtained through Mulliken analysis of electronic structure. It was found that Mk had ideal linear relationships with both tensile strength and shear strength of Sn-based solders. What is more, linear relationship also existed between Bo and eutectic temperature of Sn-based eutectic solders. It is sure that we could predict mechanical properties of solder alloys by means of electronic structure calculation. Then electronic structure mechanism for the wettability of Sn-based Pb-free solders on Cu substrate was put forward based on the analysis of orbital interactions between atoms. We believe that the wettability of Sn{sub}xM{sub}y alloy would be improved only if orbital interactions between Sn atoms and Cu atoms are enforced because of the existence of M element. Spreading and wetting behavior of Sn-based solders were predicted and then explained by this quantum method on the basis of electronic structure parameter. Predictions from analysis on calculation results were proved by wettability experiments and EDX analysis. Because heavy atoms such as Pb, Bi, Sn and Sb were included in our clusters, relativistic effects were taken into account in the calculation in order to obtain exact results.
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