RF characteristics of flexible circuits patterned with hybrid Ag paste

Printed electronics based on nanomaterials has been a potential alternative to conventional electronic technology and is favorable for flexible polymer substrates. However, a printed circuit has insufficient electrical performance and mechanical reliability under flexible stress. We designed hybrid Ag pastes containing Ag nanoparticles (NPs, 30–50 nm) and Ag flake (F, 2.5–3.5 μm in diameter) particles to overcome the obstacles of printed electronics. Six types of Ag paste (Ag NPs-x wt% F paste, x = 0, 10, 25, 50, 75, and 100) were screen-printed onto a polyimide substrate and sintered at 200 °C for fabricating a flexible circuit. We simulated a coplanar waveguide pattern before measuring the S-parameter using high frequency structural simulation. To investigate the radio frequency characteristics of various Ag circuits under sliding stress, a network analyzer and Cascade’s probe system in the frequency range from 1 to 10 GHz were employed before and after 100,000 sliding cycles. Hybrid Ag circuits showed stable electrical conductance and signal transmission, while pure Ag particle circuits showed a definite drop of electrical conductance (over 30% increase of electrical resistance) under cyclic sliding stress. The microstructural crack behavior demonstrated that the Ag F content within the Ag hybrid circuits effectively suppressed crack propagation, leading to improved flexibility of the sintered Ag circuit.