RF performance evaluation of microstrip lines printed on flexible polyethylene terephthalate (PET) films

This paper reports an exploratory study of implementing microstrip lines in flexible electronics technology by printing conductive tracks on flexible polyethylene terephthalate (PET) films. Based on the material properties of inkjet printable nanoparticles and of PET films published in the literature, electromagnetic-field simulations with COMSOL are used to determine the radio-frequency (RF) performance of the microstrip lines. With a PET film thickness of 250 μm, a conductive track with a linewidth of 565 μm and a conductive layer thickness of 20 μm above a ground plane gives a characteristic impedance of 50 Ω as a microstrip line. Because of the large thickness of the conductive layer relative to the skin depth, the insertion loss at 1 GHz for such microstrip lines can be as small as 0.25 dB/cm even with the conductivity as low as 5×10⁵ S/m. With the 50-Ω microstrip line design, the characteristic impedance is fairly independent of the conductivity of the printed conductor. Printing resolution of about 100 μm is good enough for printing microstrip lines on PET films of normal thickness.