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

Abstract

A Cu 6 Sn 5 -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 Cu 6 Sn 5 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 Cu 6 Sn 5 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 Cu 6 Sn 5 -based structure and form more thermodynamically stable (Cu,Ni) 6 Sn 5 and (Cu,Co) 6 Sn 5 phases. In comparison, Ni is more effective than Co at stabilizing the Cu 6 Sn 5 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 Cu 4 Ni 2 Sn 5 , where Ni occupies the 8f2 Cu sublattice, Cu 4 Co 2 Sn 5 is less stable due to the lower amplitude of the Co-d PDOS peak and its position mismatch with the Sn-p PDOS peak.

Effects of Co nanoparticle addition to Sn–3.8Ag–0.7Cu solder on interfacial structure after reflow and ageing

“…It has been observed [36] that during IMC formation, Co atoms enter the Cu sublattice of Cu 6 Sn 5 and substitutes some of the Cu atoms. Theoretical calculation revealed [37] that the presence of Co in the IMC causes a greater reduction in the heat of formation and hence makes Cu 6 Sn 5 more stable. It may be noted that similar results were obtained for nickel addition [38,39].…”

Influence of Ni nanoparticle on the morphology and growth of interfacial intermetallic compounds between Sn–3.8Ag–0.7Cu lead-free solder and copper substrate

“…These then participate in the form of (Cu,Ni) 6 Sn5 in the bulk solder as well as at the interface. Ni atoms entered the Cu sublattice of the Cu 6 Sn 5 phase and substituted the Cu atoms to form (Cu,Ni) 6 Sn5 [46,47]. The effects of nickel, as an alloying element, on the interfacial CueSn IMC layers have been explained elsewhere [7,35,48].…”

Reduction of electromigration damage in SAC305 solder joints by adding Ni nanoparticles through flux doping

“…Crystal structure of Ni atoms is cubic as similar to crystal structure of Cu atoms. Therefore, Ni atoms substitute Cu atoms in Cu 6 Sn 5 and create (Cu, Ni) 6 Sn 5 at the interfaces and in the solder bulk [31]. Gao et al have shown, based on thermodynamic calculation, that (Cu, Ni) 6 Sn 5 is more stable than Cu 6 Sn 5 [32] .…”