Systematics of structural, phase-stability and cohesive properties of the Cu6(Sn,In)5 η´-structure compounds occurring in In-Sn/Cu solder joints

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

JOURNAL OF ELECTRONIC MATERIALS

SPRINGER

Lugar: Berlin; Año: 2017

0361-5235

Motivated by the high solubility of In in (mC44) g¢-Cu6Sn5 compound as wellas the occurrence of an In-doped g¢-intermetallic in the microstructure of Cu/In-Sn/Cu solder joints, a theoretical study has been carried out to investigatethe various physical effects of incorporating In at Sn Wyckoff sites of thebinary g¢-phase. Systematic ab initio calculations using the projected augmentedwave method and Vienna Ab initio Simulation Package were used todetermine the composition dependence of the structural and cohesive propertiesof g¢-Cu6(Sn,In)5 compounds, compared with those expected from thebinary end-member compounds Cu6Sn5 and Cu6In5. The molar volume showssignificant deviations from Vegard’s law. The predicted composition dependenceof the cohesive properties is discussed using two complementary approaches,viz. a valence-electron density approach as well as a bond-numberapproach, both accounting for the roughly linear dependence of the cohesiveenergy on the In content. A microscopic interpretation for this general trend isgiven in terms of the key contributions to chemical bonding in this class ofcompounds, namely Cu d-electron overlap and hybridization of Cu d-stateswith In and Sn p-electron states. Moreover, a crystallographic site approach isdeveloped to accurately establish the phase-stabilizing effect of incorporatingIn at specific Wyckoff positions of the (mC44) g¢-Cu6Sn5 structure.