Rigid-flex circuits have become increasingly popular in recent years due to it combines the flexibility of flex circuits and rigidity & reliability of FR4 PCB. One of the key design considerations when creating a rigid-flex circuit is the impedance value. For general high-frequency signals and RF circuits, 50ohm is a most common value that designers used and manufacturer recommended, so why choose 50ohm? Does 30ohm or 80ohm is available? Today, we will explore the reasons why a 50ohm impedance is the optimal design choice for rigid-flex circuits.