Growth kinetics of bismuth nickel intermetallics

Bismuth (Bi)-based systems are of great interest as it is considered to potentially replace high lead (Pb)-content solders used in high temperature electronics. In particular, molten Bi strongly reacts with Ni to form higher melting point intermetallic compounds (IMCs) via transient liquid phase (TLP), which can offer a joining method alternative to the traditional solder approach. A fundamental understanding of nucleation and growth of intermetallic phases is crucial to create a reliable joint. Two intermetallic phases form between Bi and Ni (Bi3Ni, BiNi). In this study, growth kinetics for Bi3Ni and BiNi was investigated, both of which show a parabolic growth behavior. Bi3Ni exhibits rapid growth and apparent activation energy of 65.5kJ/mol at lower temperatures (from 160 to 240 degrees C) and of 132.9kJ/mol at higher temperatures (>240 degrees C), where the transition is likely due to a viscous-flow nature near melting temperature of Bi. On the other hand, BiNi grows at a later stage with a slower rate with the apparent activation energy of 125.6kJ/mol (from 260 to 340 degrees C). In addition, based on the formation sequence and growth direction of these IMCs, interdiffusion coefficients for each of these IMCs were determined. Micro-hardness tests show that Bi3Ni is softer and more brittle than BiNi.