Interfacial reactions between Ni substrate and the component Bi in solders

The reactions between Ni and liquid Bi at 300, 360, 420, and 480 degrees 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 NiBi(3). The other intermetallic compound NiBi, which is thermodynamically stable at these temperatures, did not form. Reaction at 300 degrees C produced a thick reaction zone, which is a two-phase mixture of NiBi(3) needles dispersed in Bi matrix. The thickness of the reaction zone increase;i rapidly with reaction time, reaching 400 mu m after 360 min. Reactions at 360 and 420 degrees C produced very thin reaction zones, and the major interaction was the dissolution of Ni into liquid Bi. Reaction at 480 degrees 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 NiBi(3) formed within the reaction zone, NiBi, 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.