Systematics of Structural, Phase Stability, and Cohesive Properties of eta-Cu6(Sn,In)5 Compounds Occurring in In-Sn/Cu Solder Joints

S. B. RAMOS; N. V. GONZÁLEZ LEMUS; C. DELUQUE TORO; G. CABEZA; A. FERNÁNDEZ GUILLERMET

JOURNAL OF ELECTRONIC MATERIALS

SPRINGER

Lugar: Berlin; Año: 2017 vol. 46 p. 4485 - 4496

0361-5235

Motivated by the large solubility of In in the (mC44) η´-Cu6Sn5 compound as well as the occurrence of an In-doped η´ intermetallic in the microstructure of Cu/In-Sn/Cu solder joints, a theoretical study has been performed of the various physical effects of incorporating In in the Sn-Wyckoff sites of the binary η´-phase. The work involves systematic ab initio calculations using the projected augmented wave method and the Vienna ab initio simulation package code. In this way the composition dependence of the structural and cohesive properties of the η´-Cu6(Sn,In)5 compounds is determined, and compared with those expected from the binary, end-member compounds Cu6Sn5 and Cu6In5. It is found that the molar volume shows significant deviations from Vegard´s law. The predicted composition dependence of the cohesive properties is discussed using two complementary approaches, viz., a valence-electron density approach as well as bonds-number approach, both accounting for the roughly linear dependence of the cohesive energy upon the In content. A microscopic interpretation for such general trend is given in terms of the key contributions to the chemical bonding in this class of compounds, namely, the Cu-d electron overlap and the hybridization of the Cu-d states with the p-electron states of In and Sn. Moreover, a crystallographic-site approach is developed to accurately establish the phase-stabilizing effect of incorporating In into specific Wyckoff positions of the (mC44) η´-Cu6Sn5 structure.