Lateral propagation of intermetallic phases in coarse- and nano-grained Cu/Sn diffusion couples

The kinetics of lateral Cu ₆Sn ₅ and Cu ₃Sn phase propagation in nano-grained thin-film and coarsegrained massive diffusion couples Cu-Sn is studied in a temperature range 160-180°C by optical microscopy, AFM, SEM, and energy-dispersive X-ray spectroscopy (EDS). To prevent surface diffusion and thus separate GB-diffusion contribution into the kinetics of phase propagation in nanograined structures, the surfaces of thin-film couples are covered by a thin carbon layer. It is found that the rates of lateral Cu ₆Sn ₅ and Cu ₃Sn phase spreading in thin-film couples exceed several times the spreading rates of the same phases over the surface of coarse-grained samples. The kinetics of lateral phase spreading both in thin-film and in massive diffusion couples obeys parabolic law. The kinetics of the phase propagation in thin-film couples turns out to be independent of the film thickness (in the range 40-200 nm), whereas it is rather sensitive to the grain size and GB structure. A and B regimes of GB reactive diffusion are found in the spreading Cu ₃Sn phase. Theoretical analysis of the phase propagation kinetics accelerated by GB diffusion is done and the phase propagation rates are calculated.