Solid State Diffusion in the Bi-Pd System

Growth of intermetallic compounds in the Bi-Pd system during solid state diffusion was studied since this system gains relevance for the electronic industry where Pd is used as a protective coating on substrates in combination with lead-free solders (containing Bi.This system is still not completely established, especially on the Pd-rich side of the system, and no diffusion studies have been performed yet.In the present work diffusion couples of Bi and Pd were investigated after annealing for various times (96h-1000h) in the temperature range 200-250°C, to establish the growth kinetics of the intermetallic phases.It was found that the total layer thickness grows parabolically with time, which implies a diffusion-controlled process.The intermediate phase gamma, which forms below 400°C, was found to exist as a stable phase at a temperature down to 200°C. In order to confirm the existence of this phase at low temperature an equilibrated Bi/Pd alloy was examined. The composition of the gamma-phase was found to be 62.4 at %of Pd.This phase was also observed in an incremental diffusion couple of BiPd/Pd after annealing. Another intermetallic layer is that couple, although thin, was identified as the BiPd3 intermetallic. The latter phase was not reported to be stable at such low temperatures. Integrated diffusion coefficients for gamma-phase, BiPd and Bi were determined at different temperatures.