What are the recommended design rules for RF transmission lines in a flex or rigid-flex PCB?

RF transmission lines on flex and rigid-flex PCBs require special design rules to maintain impedance control and minimize loss in the flexible portions: (1) Substrate considerations: flex materials: polyimide (Kapton): Dk = 3.2-3.5, Df = 0.002-0.008. The most common flex substrate. Liquid Crystal Polymer (LCP): Dk = 2.9, Df = 0.002. Lower loss, better for high-frequency RF. Adhesive layers: flex stackups often include adhesive layers (acrylic or epoxy) between the copper and polyimide. Adhesive Dk = 3.0-4.0, Df = 0.01-0.03 (higher loss than polyimide). For RF: specify adhesiveless flex (the copper is directly plated or laminated onto the polyimide without adhesive). This reduces Dk variation and loss. (2) Transmission line types for flex: microstrip: the most common for flex RF. Signal trace on one side, ground plane on the other. Stripline: signal trace sandwiched between two ground planes. Provides better shielding but requires a multilayer flex stackup (more expensive and less flexible). Coplanar waveguide (CPW): signal trace with ground coplanar on the same layer. No ground plane reference needed on the opposite side. More flexible (the ground is on the same layer, not constraining the flex). Good for short flex transitions. (3) Design rules: minimum bend radius: general rule: bend radius ≥ 6× the flex thickness for single-layer flex, ≥ 12× for multilayer flex. For a 4 mil flex: minimum bend radius = 24-48 mil (0.6-1.2 mm). Tighter bends cause copper cracking and impedance change. Trace orientation: route RF traces perpendicular to the bend axis (the traces run straight through the bend, not along it). Traces parallel to the bend axis experience more strain and are more prone to cracking. Avoid vias in flex regions: vias in the flex portion are stress concentrators and can crack during bending. If vias are necessary: use through-hole vias with strain-relief pads (larger annular rings). Copper weight: use thinner copper for flex (0.5 oz or 18 μm). Thicker copper is stiffer and more prone to fatigue cracking during repeated bending. Impedance control: the flex portion has different dielectric thickness and Dk than the rigid portions. The impedance must be calculated separately for each section. Use transition tapers where the rigid and flex portions meet (the trace width may need to change to maintain 50 Ω across the transition).