Fundamental influence of segregated Bi on the mechanical properties of interconnect of bismuth-containing solder and copper

We employed density functional theory to investigate bismuth segregation at Cu/Cu₃Sn interface. Firstly, we considered five initial constructions by adopting the adhesion energies criterion. By comparing adhesion energies of them, the so-called "Between-Cu" construction was found to be the most energy-favored (0.98J/m²). Secondly, based on "Between-Cu" construction, we introduced eight possible segregation sites. Among these eight sites, Site-2 was determined to be the most likely segregation site with adhesion energy as low as 0.53J/m², which is almost half of the adhesion energy of the initial construction. Finally, we analyzed atomic structure and electronic density, from which it was found that the interfacial space was enlarged by bismuth segregation and the bond between the first layer and the second layer at copper side was weakened. Our calculated result is qualitatively consistent with reported experimental results and can explain them well.