Processing of nano-micro copper materials for the production of conductive circuits

Abstract

Copper inks potentially provide a cost-effective alternative to silver for printed electronic circuits. In glass-based applications such as PV or smart glass, they can provide a means of conductivity enhancement or additional functionality. Three inks consisting of a mixture of nano and micro copper particles were systematically studied to examine the relationship between sintering temperature, sintering time and gaseous environment on the electrical qualities of the sintered printed films deposited on FTO coated glass. There is a definite interaction between the particulate nature of the ink, the sintering conditions, and the conductive properties of the film. Films containing only nano-particles provide the most conductive films with optimum sintering conditions of temperature of 225 °C for 60 minutes. The inclusion of micro particles increased the ideal sintering temperature but lowered the sintering time. An ink containing an equal mixture of nano and micro particles exhibited the lowest performance. This could be attributed to partial oxidation of the nano-particles along the conductive path, which occurs as a result of the presence of the micro particles. Other samples were photonically sintered using a PulseForge 1200 laboratory photonic sintering unit where the number of pulses, pulse power, pulse frequency and the intra pulse gap could be varied. An initial optimization study identified an operational range of photonic energy profile. The best possible line conductivity obtained using these optimum conditions was around a 1/3 of that obtained by conventional thermal sintering. This relative conductivity of photonically sintered features further deviated from conventionally sintered features as the film thickness increased and as the line width reduced. Laser / NIR techniques were found ineffective to sinter the copper ink used in this study. The possibility to manually blend copper and silver paste ink was investigated and an optimum blend of 25% silver and 75% copper could be used which had maintained conductivity, cost, and adhesion benefits.