First-principles Investigation of Bi Segregation at the Solder Interface of Cu/Cu₃Sn(010)

Density functional theory was employed to investigate the bismuth segregation at Cu/Cu₃Sn(010) interface. Five initial constructions were introduced by adopting the adhesion energy criterion. Among them, the so-called " between-Cu" construction in which the interface Cu atoms of Cu slab locate along Cu-Cu bond direction in Cu₃Sn slab was found to be the most energy-favored at an adhesion energy of 1.96 J/m². Based on this construction, five possible segregation sites were examined, and the most likely segregation site was determined with adhesion energy as low as 1.06 J/m², which was almost half of the initial one. Comparing with other sites' adhesion energies, it was concluded that size effect took a large part in embrittlement. The analyses of atomic structure and electronic density revealed that the slabs shifted away from interfaces due to bismuth segregation, and the atoms around Bi atom were pressed away. This calculated work agreed qualitatively with reported experimental results.