Additive Occupancy in the Cu₆Sn₅-Based Intermetallic Compound Between Sn-3.5Ag Solder and Cu Studied Using a First-Principles Approach

A Cu₆Sn₅-based intermetallic compound containing a certain amount of Co or Ni is commonly formed at the interface between a Cu substrate and Sn-based solder. The Co or Ni additive is often found to occupy the Cu atom sublattice in the Cu₆Sn₅ crystal structure. In this paper, a first-principles approach based on density-functional theory is employed to explore the most favorable occupancy sites of Ni and Co dopants in the Cu₆Sn₅ crystal structure. It is found that, for up to 27.3 at.% concentration, both Ni and Co atoms tend to substitute for Cu in the Cu₆Sn₅-based structure and form more thermodynamically stable (Cu,Ni)₆Sn₅ and (Cu,Co)₆Sn₅ phases. In comparison, Ni is more effective than Co at stabilizing the Cu₆Sn₅ phase. At a lower concentration level (9.1 at.%), the Ni or Co atoms prefer to occupy the 4e Cu sublattice. At a higher concentration (27.3 at.%), the Ni atoms will likely be located on the 4e + 8f2 Cu sublattice. Analysis of density of states (DOS) and partial density of states (PDOS) indicates that hybridization between Ni-d (or Co-d) and Sn-p states plays a dominant role in structural stability. Compared with Cu₄Ni₂Sn₅, where Ni occupies the 8f2 Cu sublattice, Cu₄Co₂Sn₅ is less stable due to the lower amplitude of the Co-d PDOS peak and its position mismatch with the Sn-p PDOS peak.