Controlling βSn grain orientations in electronic interconnects with single-crystal Cobalt substrates

The reliability of electronic solder joints is strongly influenced by βSn grain orientations. Single-crystal Cobalt substrates were used to control βSn nucleation and grain orientations in solder joints. On (0001)Co, (112¯0)Co, (101¯0)Co, and (11¯02)Co, interfacial αCoSn₃ crystals that are potent to catalyze βSn nucleation exhibited strong textures with 2–5 symmetric orientations featuring reproducible orientation relationships (ORs) with the Co. Preferred ORs and orientation selections of αCoSn₃ were examined by exploring i) thermodynamic stabilities of interfaces represented by interfacial atomic matches and work of adhesion and interfacial energies calculated based on density functional theory (DFT) and ii) crystal growth kinetics indicated by the angle between (100)CoSn₃ and the substrate plane. In sharp contrast to the innumerable grain orientations in joints on commonly used substrates, dramatically reduced orientations are attributed to the strongly textured αCoSn₃, on which βSn grains nucleated by fixed ORs. On (112¯0)Co, the total number of orientations including twinned grains were reduced to 20. On (101¯0)Co, none of the βSn orientations exhibit the c-axis perpendicular to the substrate (i.e. parallel to the electron flow direction), which should reduce fast diffusion paths across the joint and improve the reliability of solder joints.