Effect of Thermal and Mechanical Stresses on Novel Microstrip Lines Printed on Flexible Substrates

Direct writing methods created a revolution in the electronic industry due to their lower cost, fast processing, and lower wasted material. Microstrip line is an important electronic component that transfers the signal and the foundation for the communication between multiple components in any circuit board. Therefore, studying its electromechanical behavior against thermal and mechanical stresses is necessary for real-life applications. In this research, novel microstrip lines were printed on “polyethylene terephthalate” (PET) and “polyimide” (PI) flexible substrates using an aerosol jet printer (AJP) and dispensing system (DS). An advanced posttreatment technique was used to enhance the conductivity of the microstrip lines printed on low glass transition flexible PET substrate. Different microstrip line designs with various mesh ground planes were tested under mild and harsh mechanical bending and different environmental conditions and their losses were characterized. The results showed that the photonic curing enhanced the microstrip lines conductivity by 65% compared to the convectional curing. The in situ resistance measurements during harsh bending demonstrated conclusively that the robustness of the printed microstrip lines increased as the filling percentage of the ground plane became lower. The aging at $85~^{\circ }$ C/85% RH had a significantly stronger effect on the microstrip lines conductivity compared to the aging at $85~^{\circ }$ C without humidity due to the changes in the printed ink’s microstructure and the increment in ionic conductivity. Thermal aging led to a reduction in the microstrip line’s ductility and the cracking became easier in the microstructure of the printed films. The resistance of a sample aged at $85~^{\circ }$ C increased by 81.7% after 10000 bending cycles compared to only 20.5% for a sample without thermal aging. Such findings provide important guidelines for those designing flexible hybrid electronics and for manufacturers who seek the assurance of these technologies for both maturing and reliable products.