Low-temperature sintering of Cu@Ag microparticles in air for recyclable printed electronics

Silver-coated copper microparticles combine the oxidation resistance of silver with the low cost of copper. They are interesting components for printed conductive structures. We studied whether printed films of such particles can be printed and sintered at low temperatures in air to create highly conductive films and whether it is possible to recover the particles from them for recycling. Pastes containing 1.5 mu m to 5 mu m spheres and 3 mu m flakes with l-ascorbic acid were prepared, screen-printed, and treated at temperatures of 110 degrees C to 300 degrees C in air. The bulk resistance of films treated below 160 degrees C were two orders of magnitude higher than that of bulk copper, rho Cu, and limited by particle-particle contact resistances. They were reduced by treating the prints at 160 degrees C to 250 degrees C, leading to bulk film resistances down to 41 rho Cu. We demonstrate that the high mobility of silver enables the formation of necks that bridge the copper cores and reduce resistivity in this temperature window. The sintered prints retained their conductivity for at least 6 months. Treatments at higher temperatures in air were detrimental: resistances increased above 250 degrees C. These temperatures led to dewetting of the silver coating and fast copper oxidation, resulting in a continuously increasing resistance. In a final study, we demonstrated that films treated below 200 degrees C can be recycled by recovering the metal powder from the printed conductors and that the powder can be printed again. An optimal temperature for printed conductors based on silver-coated copper microparticles is 160-250 degrees C, leading to silver sinter necks that enhance the conductivity. Treatments are done in air and the particles can be recycled from the prints.