Experimental and Modeling Study of RF Performance of a Screen-Printed Coplanar Waveguide (CPW) up to 20 GHz under Monotonic Stretching

To meet the increasing need of wireless communication, radio frequency (RF) modules are commonly included in the design of flexible hybrid electronics (FHE). The reliability of printed flexible electronics under deformation has been widely studied under direct current (DC) conditions, while limited studies have been conducted under RF conditions. This work provides an insight into how monotonic stretching changes the RF performance at a frequency range of 2-20 GHz of a coplanar waveguide (CPW) transmission line. The CPW is a silver ink screen-printed on a polyethylene terephthalate (PET) substrate, with a thin encapsulation on top of the silver conductor. The sample is subjected to monotonic stretching at a rate of 10 mm/min up to 70% strain while in-situ scattering parameters (S-parameters) are recorded by a vector network analyzer (VNA). Greater changes are observed in the insertion loss than in the return loss as the CPW undergoes stretching. Normalized insertion loss decreases as the strain increases, and at higher frequencies the amount of increase is greater. However, when the CPW is stretched to 65% and above, the overall shape of S-parameters changes indicating the open circuit of the CPW. Numerical models at different strain levels are also developed in CST
^TM
Studio Suite to compare predictions with experimental data. It is seen in addition to geometry and conductivity changes in CPW, the dielectric constant of the substrate and the encapsulant and the surface roughness of the conductive silver ink need to be changed with stretching in the numerical model to be able to mimic the experimental data. With such changes, the numerical CPW simulation results match both the overall trend and the magnitude of insertion loss in measurement data.