Screen-Printed Flexible Coplanar Waveguide Transmission Lines: Multi-physics Modeling and Measurement

Flexible hybrid electronics (FHE) is a promising technology enabling many applications in biomedical, communication, energy harvesting and internet of things (IoT) areas. To realize FHE applications, the components and devices used in the mentioned technologies need to be electrically characterized under various flexible conditions such as stretching, bending, twisting, and folding. Also, the strain analysis from the mechanical point of view needs to be conducted to justify the reliable applications of FHE under different flexible scenarios. In this paper, the design and electrical characterization of coplanar waveguides (CPWs) in flexible substrates such as Kapton polyimide and polyethylene terephthalate (PET) under uniaxial bending are studied and discussed. The fabricated lines were measured using a vector network analyzer (VNA) up to 8 GHz under both flat and bending conditions. Finite-element models (FEM) of CPW lines were created in ANSYS HFSS to capture the effect of bending on the CPW frequency response. In addition, the variations in the trace width and separations along the CPW lines were modeled accurately to capture the variations in the fabrication process and their effect on the CPW S-parameters in the flat condition. The finite element analysis of strain variation during bending was also performed and the relationship between strain variation and CPW performance was investigated. The bending of the CPW lines was carried out using two parallel plates that had a gap distance varying from 40 mm to 140 mm. The S-parameters were monitored in-situ while the substrate was under bending. The experimental results were compared against simulated results under both flat and bent configurations. Based on the conducted studies, correlation was achieved for the flat and bending scenarios between measurement and simulation results. Also, it was observed that the CPW line has better matching and lower losses compared with the flat case and tensile bending cases.