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 frequencydue to the expansion of 5G mobile products. Therefore, 5G foldable products require componentsthat facilitate both mm-wave RF transmission and ultra-high flexibility for interconnecting throughthe hinge structure of foldable products. To improve flexibility, a flexible PCB must be thin with noground vias in its bending

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... ection; in contrast, the low-loss flexible PCB for mm-wave transmissionmust be thick and have many ground vias, so there is a trade-off relationship between flexibilityand RF characteristics. This paper proposes a new flexible PCB structure that does not experienceproblems 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; astructure with a via-less and meander ground shape is proposed. The simulated loss of the proposedstructure was 0.0254 dB/mm @ 10 GHz, and the isolation between signals ranged from 21.98 dBto 10 GHz. The simulated results of insertion loss and isolation were experimentally verified. Theproposed structure is currently being applied to the RF flexible PCB that interconnects through thehinge of a foldable phone, and is currently being mass-produced.