Ag nanoparticle-based inkjet printed planar transmission lines for RF and microwave applications: considerations on ink composition, nanoparticle size distribution and sintering time

Sintering of Ag Nanoparticle (NP) – based inkjet printed tracks is a crucial process for the next-generation digitally printed electronics. In particular, while the digital printing, as additive technology, is now well settled for what concerns either DC or signal applications both on rigid and on flexible substrates, this technology has not been demonstrated yet in the RF or microwave field, and a few works appear considering vacuum-evaporated films, screen-printed pastes or inkjet printed inks. We studied the effects of both ink composition and thermal profile on the resulting electrical properties, performing real-time resistance acquisition (DC) and post-annealing microwave measurements. We tested ink compositions featuring both different NP size distributions and different phase compositions, including a pure solvent/salt/metal one and a solvent/salt/metal/polymer one, resulting in a peculiar mass distribution and heat diffusion. The composition strongly affects the onset of electrical percolation and the final resistivity; on the contrary, the heating rate can either have an effect on electrical properties or not depending on the composition. The microwave characterization of microstrip lines printed on alumina substrates, performed up to 26.5 GHz, yield attenuations that are comparable with the best results obtained so far with the same technology.