Characterization of the growth behavior of copper-tin intermetallics in liquid tin/solid copper reaction couples.

摘要

Nucleation and growth kinetics of the interfacial $h$ and $3$ phase intermetallics were quantitatively investigated in liquid tin/solid copper reaction couples over a range of reaction times and temperatures. Heterogeneous nucleation of interfacial $h$ was modeled within the temperature range from 240°C to 300°C from thermodynamic and empirical data, using classical nucleation theory. The predicted model, when fit to the experimental data, gave values for the nucleation rate parameter, K, the contact angle, &thetas;, and the attachment activation energy, ΔG_a. From K, the number of nucleation sites per unit area, N_a, was evaluated. Correcting for coarsening of the nuclei, and saturation of the copper surface, the values for K underestimated N_a. Thus, inhomogeneous nucleation was concluded to occur at specific sites on the copper surface. The effective nucleation rate of $h$ crystallites was also affected by the surface finish of the copper.; Radial growth kinetics of the $h$ was evaluated at 275°C. At short and long reaction times Region I and Region III, respectively, the values for the kinetic exponent were n_I = 0.30 and n_III = 0.39. However, at intermediate times (Region II), growth of the $h$ was much slower; in Region II, n_II = 0.11. In addition to coarsening of the scallops, thickening of the $h$ layer also occurred. At the shortest reaction times, thickening dominated, but at the longer reaction times, coarsening was the primary growth mechanism; the value of n_III = 0.39 is close to ⅓, which is expected from theory. Slow radial growth (coarsening) in Region II was attributed to development of $h$ phase whiskers.; Thickening kinetics of the $h$ and $3$ were modeled using an empirical power law. The calculated kinetic exponent, rate constant, and activation energy for each layer were $nzh$ = 0.298, $kzh$ = 2.52 μm/s0.298 and $Qzh$ = 13 kJ/mol, respectively, for the $h$ and $nz3$ = 0.567, $kz3$ = 2.26 μm/s0.567 and $Qz3$ = 29 kJ/mol, respectively, for the $3$. Measured values for the kinetic exponents and activation energies suggested that thickening of the $h$ was controlled by a grain boundary diffusion mechanism, and growth of the $3$ occurred by solid state diffusion.