For high-speed communication services such as 5G technology, the use of millimeter-wave (mmWave) components substantially increases in mobile applications. The interconnect based on a substrate-integrated waveguide (SIW) is an efficient solution for connecting these devices. However, the SIW characteristics in the mmWave frequency range are not sufficiently presented from the practical viewpoint. In this paper, the experimental characterization of mmWave SIWs in flexible printed circuit boards (FPCBs) and their simulation results are presented. A practical method using balanced/single slot transition is proposed for microstrip-to-SIW transition. Using a full-wave simulation and genetic algorithm, the proposed slot technique is optimized. It is experimentally demonstrated that the cutoff frequency affects the operating band of the SIW differently. The per-unit-length losses of the full-mode and half-mode SIWs are obtained as 0.0375 dB/mm and 0.0609 dB/mm, respectively. Using the measurements, the SIW type effect on the transmission loss is quantitatively analyzed, and the loss is increased up to 62.4% at 39 GHz.

Recently, new form factors, such as foldable, have increased demand for mobile products. Moreover, mobile phones should support the RF signal frequency up to the mm-wave frequency due to the expansion of 5G mobile products. Therefore, 5G foldable products require components that facilitate both mm-wave RF transmission and ultra-high flexibility for interconnecting through the hinge structure of foldable products. To improve flexibility, a flexible PCB must be thin with no ground vias in its bending section; in contrast, the low-loss flexible PCB for mm-wave transmission must be thick and have many ground vias, so there is a trade-off relationship between flexibility and RF characteristics. This paper proposes a new flexible PCB structure that does not experience problems regarding signal transmission to the mm-wave band, even when folded 200,000 times. To overcome the physical limits of the trade-off relationship, an interlayer air-gap was formed; a structure with a via-less and meander ground shape is proposed. The simulated loss of the proposed structure was 0.0254 dB/mm @ 10 GHz, and the isolation between signals ranged from 21.98 dB to 10 GHz. The simulated results of insertion loss and isolation were experimentally verified. The proposed structure is currently being applied to the RF flexible PCB that interconnects through the hinge of a foldable phone, and is currently being mass-produced.