Highly conductive nano-silver ink for PCB trace repair

Summary form only given. Silver is a the best conductor, but silver powders are very difficult to disperse well because of high density (10.49 g/cm³) and strong van der Waals force (hamaker constant~40*10^-20J). In order to disperse silver powders in organic solvent, insulating polymeric dispersants should be used to create enough steric hindrance between silver powders. However, in order to maintain the conductivity of silver dispersion after sintering, the dosage of polymeric dispersants should be used as less as possible. It's a conflict between stability and conductivity for silver dispersion system. In the study, we have successfully fabricated a highly stable conductive silver nanoparticle-based ink (D_ave~60 nm) for inkjet printing through formulating a novel lab-synthesized dispersant, Ag nano-powders, an organic ECO solvent. Based on the characterization of thermogravimetry analysis, viscometer, surface tension analyzer, and dynamic light scattering, the silver nanoparticle-based ink shows a high silver content of 75wt%, a high uniformity of D9₀/D_ave~2.5, a viscosity of >20000 cps at 25°C, and tunable surface tension from 25-35 dyne/cm. Moreover, the ink can be directly loaded in screen printer to pattern fine conductive line on FR-4 or flexible PI. After baking at 170°C for 30min, the conductive patterns perform a line width of 4 mil, a low resistivity of 5 μΩ-m (only 3 times than pure silver) and a good peel adhesion of class 5; Most important, the printed line can't breakdown when a high voltage of 30V is applied (breakdown voltage for 4mil copper trace on PCB is 60V, and breakdown voltage for all of commercial PCB-repaired silver pastes are below to 15 V), indicating the silver paste and screen print process can be applied for PCB trace pattern and repair.