Highly Conductive Copper-Silver Bimodal Paste for Low-Cost Printed Electronics

Printed electronics are circuits that are additively manufactured using conductive pastes composed of micro-/nanoconductive metal particles. Silver-based compounds are the most widely used metals for such pastes due to their superior conductivity and oxidation stability. However, the high cost of silver (Ag) has demanded its replacement with more cost-effective and abundant metals such as copper (Cu). Despite its cost-effectiveness and abundance, Cu suffers from high oxidation tendency and sintering temperature that have limited its widespread utilization in printed electronics. In this work, we have developed a low-cost hybrid bimodal paste composed of Cu microparticles (1-5 mu m) and Ag nanoparticles (20-30 nm) (CuMPs/AgNPs) via nondestructive photonic sintering. The concurrent melting of AgNPs and catalytic reduction of CuMPs allow the paste to be sintered at considerably low temperatures using an intense-pulsed light (IPL) source. The required light energy density for effective sintering of different mixing ratios of AgNPs and CuMPs was systematically measured using electrical, optical, and mechanical characterization techniques. These analyses revealed that a minimum of 16 wt % AgNPs in the bimodal CuMP/AgNP paste with an IPL irradiation energy of 10.6 J/cm(2) and pulse duration of 5 ms achieved a minimum sheet resistance of 0.072 Omega/square that results from localized melting of AgNPs between adjacent CuMPs. Furthermore, the CuMP/AgNP films with a minimum of 6 wt % AgNPs showed significantly improved oxidation stability characteristics even after 7 days of incubation in accelerated oxidation conditions [70 degrees C and 100% relative humidity (RH)]. As a proof of concept, we demonstrated an application of the developed paste (CuMPs-6 wt % AgNPs) by directly printing a wireless resonant moisture sensor onto the interior region of a cardboard package box, which is capable of performing in situ monitoring of the moisture ranging from 30 to 85% RH with an average linear sensitivity of -3.08 % RH/MHz.