ANALYSIS OF LOW-TEMPERATURE INTERMETALLIC GROWTH IN COPPER-TIN DIFFUSION COUPLES

A multiphase diffusion model was constructed and used to analyze the growth of the epsilon- and eta-phase intermetallic layers at a plane Cu-Sn interface in a semi-infinite diffusion couple. Experimental measurements of intermetallic layer growth were used to compute the interdiffusivities in the epsilon and eta phases and the positions of the interfaces as a function of time. The results suggest that interdiffusion in the epsilon phase (D(epsilon) approximately) is well fit by an Arrhenius expression with D0 = 5.48 x 10(-9) m2/s and Q = 61.9 kJ/mole, while that in the eta phase (D(eta) approximately) has D0 = 1.84 x 10(-9) m2/s and Q = 53.9 kJ/mole. These values are in reasonable numerical agreement with previous results. The higher interdiffusivity in the eta phase has the consequence that the eta phase predominates in the intermetallic bilayer. However, the lower activation energy for interdiffusion in the eta phase has the result that the epsilon phase fills an increasing fraction of the intermetallic layer at higher temperature: at 20-degrees-C, the predicted epsilon-phase thickness is almost-equal-to 10 pct of that of eta, while at 200-degrees-C, its thickness is 66 pct of that of eta. In the absence of a strong Kirkendall effect, the original Cu-Sn interface is located within the eta-phase layer after diffusion. It lies near the midpoint of the eta-phase layer at higher temperature (220-degrees-C) and, hence, appears to shift toward the Sn side of the couple. The results are compared to experimental observations on intermetallic growth at solder-Cu interfaces.