Broadband insertion loss measurement of flexible copper clad laminate with direct metallization

The technological trend of replacing the existing ceramic antenna and coaxial cable with the flexible PCB (FPCB) and feed line integrated module has recently attracted much attention. Several studies have been done to improve the insertion loss of the FPCB, e.g., an attempt to develop a lower insertion loss than the conventional FPCB with an adhesive layer by manufacturing the FPCB with a direct metallization method. This paper presents a novel manufactured low-loss flexible copper clad laminate (FCCL) by the direct metallization method fabricated on a polyimide substrate and the measurement of its insertion loss over a broad frequency range from 0.1 to 40 GHz. A comparison of insertion loss results between FCCL direct metallization and FCCL with an adhesive layer has been provided to show its efficiency. The measurement is done using a ground-signal-ground (GSG) probe and a multiline TRL (ML-TRL) calibration method to obtain more precise measurement findings. The FCCL direct metallization achieves insertion losses of 0.03 dB/mm, 0.04 dB/mm, and 0.06 dB/mm at 20 GHz, 28 GHz, and 40 GHz, respectively, better than the adhesive type and are comparable to the other high-frequency substrates. Furthermore, an additional analysis is performed to assess the losses caused by the material's surface roughness in the millimeter wave (mmWave) frequency range.