Interfacial reactions between Ni substrate and the component Bi in solders

Abstract

The reactions between Ni and liquid Bi at 300, 360, 420, and 480°C were studied. Bismuth is an important element in many electronic solders, while Ni is used in many printed circuit board surface finishes. It was found that the only intermetallic compound formed was NiBi3. The other intermetallic compound NiBi, which is therm odynamically stable at these temperatures, did not form. Reaction at 300°C produced a thick reaction zone, which is a two-phase mixture of NiBi3 needles dispersed in Bi matrix. The thickness of the reaction zone increased rapidly with reaction time, reaching 400 µm after 360 min. Reactions at 360 and 420°C produced very thin reaction zones, and the major interaction was the dissolution of Ni into liquid Bi. Reaction at 480°C produced extremely thin reaction zone, and the dissolution of Ni into liquid Bi was very fast and was the major interaction. It is proposed that the formation of the reaction zone is controlled by two factors: the solubility limit and the diffusivity of Ni in liquid Bi. Small diffusivity and small solubility limit, i.e., lower temperature, tend to favor the formation of a thick reaction zone. In addition to the NiBi3 formed within the reaction zone, NiBi3 also formed outside the reaction zone in the form of long needles with hexagonal cross section. The dissolution rate of Ni into Bi is comparable to that of Ni into Sn at the same temperature, and is much slower than the dissolution rates for Au, Ag, Cu, and Pd into Sn.