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

Thickening behavior of interfacial η (Cu6Sn5) phase and ɛ (Cu3Sn) 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 250°C to 325°C. A scanning electron microscope (SEM) was used to quantify the interfacial microstructure at each processing condition. The η developed with a scalloped morphology, while the ɛ always grew as a somewhat undulated planar layer in phase with the η. The thickness of each phase was quantitatively evaluated from SEM micrographs using imaging software. Thickening kinetics of the ɛ and η 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 η is controlled by a grain-boundary diffusion mechanism, and growth of the ɛ occurs by solid-state diffusion, probably grain-boundary diffusion.