Thickening kinetics of interfacial Cu6Sn5 and Cu3Sn layers during reaction of liquid tin with solid copper

Thickening behavior of interfacial eta (Cu(6)Sn(5)) phase and epsilon (Cu(3)Sn) phase intermetallic layers was investigated in liquid tin/solid copper reaction couples over reaction times from 30 sec to over 4,000 min and temperatures from 250degreesC to 325degreesC. A scanning electron microscope (SEM) was used to quantify the interfacial microstructure at each processing condition. The eta developed with a scalloped morphology, while the epsilon always grew as a somewhat undulated planar layer in phase with the eta. The thickness of each phase was quantitatively evaluated from SEM micrographs using imaging software. Thickening kinetics of the epsilon and eta compounds were modeled using time- and temperature-dependent empirical power-law equations. From the model, values for the kinetic exponent, rate constant, and activation energy were established for each intermetallic layer. Measured values for the kinetic exponents and activation energies suggest that thickening of the eta is controlled by a grain-boundary diffusion mechanism, and growth of the epsilon occurs by solid-state diffusion, probably grain-boundary diffusion.