Rigid‑flex PCB bend radius: rules, formulas, and examples

The minimum bend radius in a rigid‑flex PCB is governed by the flex section: plan roughly 6–10× flex thickness for single‑sided static bends, 12–20× for double‑sided static, and 100–150× (200–300× for multilayer) for dynamic bending per IPC‑2223 design practice.

What is the minimum bend radius for rigid‑flex?

Rigid sections don’t bend; the flex tail does. Treat each bend as a hinge located in the flex‑only area and never across the rigid‑to‑flex transition or stiffener edges. The required radius depends on total flex thickness (polyimide, copper, coverlay and adhesives), copper type/weight, and whether the bend is static (one‑time form) or dynamic (repeated cycling).

How do I compute bend radius from strain?

Use the bending strain relationship at the copper outer fiber: ε = y / R, where y is the distance from the neutral axis to the copper surface and R is bend radius. If the stack is symmetric so that the neutral axis is at mid‑thickness, y ≈ t/2 and R ≈ t / (2·ε). Copper, not polyimide, usually limits strain. Typical allowable surface strain for rolled‑annealed (RA) copper is about 0.8–1.0% for static forming and ~0.2–0.3% for dynamic cycling. Example: a symmetric flex with t = 0.10 mm and dynamic ε = 0.25% gives R ≈ 0.10 / (2×0.0025) = 20 mm. If the stack is asymmetric (e.g., single‑sided), the neutral axis shifts; y can be > t/2, so use a larger radius or balance the stack to re‑center the neutral axis.

Typical bend radii by flex construction

• Single‑sided static: 6–10× t (adhesiveless toward the low end; adhesive toward the high end)

• Double‑sided static: 12–20× t

• Multilayer static: 20–30× t

• Single‑sided dynamic: ~100× t

• Double‑sided dynamic: ~150× t

• Multilayer dynamic: ~200–300× t

These are commonly used values derived from IPC‑2223 guidance and large‑volume industry experience. Always verify with your exact stack (materials, copper type/weight, coverlay adhesive) and required cycle life.

Layout and stackup choices that reduce required radius

• Choose RA copper for flex layers and keep copper light in the bend area (e.g., 12–18 µm). Avoid heavy copper in the bend.

• Balance the flex stack (symmetrical polyimide/coverlay) to place the neutral axis near the copper; if single‑sided, add balancing dielectric on the opposite side.

• Route conductors perpendicular to the bend line, use smooth arcs through the bend area, and avoid sharp corners or neck‑downs.

• In multilayer flex, stagger traces across layers to prevent an “I‑beam” effect; use hatched planes or thinned/relieved copper in the bend zone.

• Keep plated holes, vias, and pads out of bend areas; place stiffener edges away from the bend and add generous fillets to coverlay openings.

• Prefer adhesiveless polyimide for tighter radii and better dynamic life; acrylic adhesives generally increase required radius.

• Form bends over a controlled mandrel (> Rmin), not by kinking; for dynamic designs, qualify life at the target radius and bend angle.

How do I verify a chosen radius?

Specify the target R, bend line, and bend angle on the fabrication drawing. For static bends, a mandrel‑bend check is practical. For dynamic applications, run cyclic bend tests at the intended R using IPC‑TM‑650 methods (e.g., the 2.4.3 series) and monitor resistance change or continuity to the required cycle count. Include coupons that replicate the thinnest conductors and highest‑strain nets in the bend.

Manufacturing and conformity notes

Boards can be built and inspected to IPC Class 2 or IPC Class 3 acceptance criteria. HyperPCB operates an ISO 9001:2015 certified quality management system (certificate no. CN‑2023‑Q1156, SGS, valid until June 2027). RoHS and REACH compliance is supported for hazardous substance control. Quotes are returned within 24 hours and DFM feedback within 1 business day.