Additives participation in Cu6Sn5 phase formed between Sn-3.5Ag solder and Cu by first-principle approach

The Cu6Sn5 intermetallic compound is generated at the interface between OSP substrate and Sn-3.5Ag based solder containing small amount of Co or Ni additives. A small fraction of Co or Ni additive is probed in Cu6Sn5-based crystal structure, which occupies the crystal space sites of Cu atoms. The first-principle approach within the density functional theory is employed herein to explore the favorite sites of Ni and Co atoms in Cu6Sn5-based intermetallic compound. In the additive concentration regime of 0.0mnplus18.2 at.%, the Ni atoms occupancy leads to a more thermodynamically stable (Cu, Ni)6Sn5 phase, while the Co atom occupancy results in a less stable phase, regardless of the occupancy sites. The density of states (DOS) analysis indicates that the Co occupancy in (Cu, Co)6Sn5 also leads to a less stable electronic structure. Basically, the Ni occupancy in (Cu, Ni)6Sn5 phase enhances the electronic structure stabilization. At the low Ni concentration level (9.1 at.% in Cu5Ni1Sn5 phase), the Ni atoms prefers to occupy 4a Cu atom site; while the Ni atoms will be located at 8f2 Cu atom site at the high concentration level (18.2 at.% in Cu4Ni2Sn5 phase).