Fabrication of Flexible Printed Circuits on Polyimide Substrate by Using Ag Nanoparticle Ink through 3D Direct-writing and Reliability of the Printed Circuits

A $\mathbf{3{D}}$ printable conductive ink was successfully prepared using polyacrylic acid coated Ag nanoparticles (Ag NPs) as conductive fillers. The Ag NP ink was printed on polyimide (PI) substrate to fabricate circuits by a 3D direct-write printer. The average width and thickness of the printed Ag film circuits are 178 μm and 6 ${\mu \mathrm{m}}$, respectively. The resistivity, morphology and microstructure of sintered Ag films subjected sintering at different temperatures for various times were studied systematically. For the Ag film sintered at 300°C for 60 minutes, the resistivity of the Ag film is greatly reduced to ${4.8\times 10^{-8}\Omega\cdot \mathrm{m}}$, which is only 3 times of that of bulk silver. Finally, the cyclic bending reliability of the printed circuits was evaluated by cyclically bending the printed circuit-on-substrate at a radius of 4 mm, and the results show that the relative resistance of the circuits can be maintained at a value smaller than 1.6 after 1000 bending cycles.