Rigid‑flex PCB design guidelines: stackup, bend radius, and DFM checklist

A manufacturable rigid‑flex PCB follows IPC‑2223 rules on stackup, bend radius, copper routing in bend areas, and clear DFM keep‑outs.

Stackup choices that survive assembly

A common 4‑layer rigid with 2‑layer flex stackup is: Rigid FR‑4 (e.g., 0.8–1.6 mm total) with 2116/7628 prepregs, and Flex using adhesiveless polyimide (12.5–25 μm) with 12–18 μm rolled‑annealed (RA) copper. Use polyimide coverlay (12.5–25 μm) with acrylic/epoxy adhesive (12–38 μm). Keep all plated through‑holes in rigid regions whenever possible; keep flex layers continuous through the rigid‑flex transition to avoid stubs.

What minimum bend radius should I use?

For static bends, typical minimum radius R is based on total flex thickness t: single‑layer R ≥ 6t, double‑layer R ≥ 12t, multilayer R ≥ 20–30t. For dynamic bends, single‑layer R ≥ 100t, double‑layer R ≥ 150t, multilayer R ≥ 200t. Use RA copper and the thinnest practical polyimide for dynamic applications to reduce strain. If you must bend across multiple copper layers, offset conductors between layers and place the neutral axis near the copper by asymmetrical build‑ups.

How to route copper in bend areas?

Run traces perpendicular to the bend axis (along the direction of flexing), use smooth arcs (minimum radius ≈ 3× trace width), and avoid 90° corners. Stagger traces on adjacent layers to prevent the I‑beam effect. Replace solid planes in bend areas with 45° hatched copper (20–40% open area, 0.5–1.0 mm pitch) to improve flexibility and reduce stress. Keep components, vias, and pads out of bend zones; as a rule of thumb, keep components ≥ 2.0 mm and vias ≥ 1.0 mm away from the nearest bend tangent.

Vias, pads, coverlay, and stiffeners

Avoid vias in flex; if unavoidable, use anchored pads/teardrops and enlarged annular rings (≥ 0.25 mm) with button‑plated coverlay openings. Keep coverlay webbing ≥ 0.25 mm between adjacent pads. Use FR‑4 stiffeners under connectors and fine‑pitch parts; bond length should exceed the connector length by ≥ 1.5 mm on each side, with 1.0 mm corner radii to avoid stress risers. At rigid‑to‑flex transitions, taper copper and use gradual 45° chamfers instead of abrupt edges.

Impedance control on flex and rigid

Target 50 Ω single‑ended and 90 Ω differential as common values. Polyimide Dk is typically ~3.2–3.4; adhesive layers alter effective dielectric height and loss, so include coverlay and adhesive in the field solver. Adhesiveless polyimide tightens impedance tolerance and reduces thickness. Expect ±10% impedance tolerance unless tighter control is negotiated; widen differential gaps in bend areas to limit coupling changes during flexing.

DFM drawing notes and tolerances

• Specify IPC‑2223 for design and IPC‑6013 for flex performance, with acceptance to IPC Class 2 or Class 3

• Minimum trace/space in flex: 100/100 μm (4/4 mil) for robust yield; finer requires review

• Mechanical via drill ≥ 0.20–0.25 mm in rigid; avoid in flex; laser microvias ≥ 75–100 μm where used

• Flex pad annular ring ≥ 0.25 mm; rigid annular ring ≥ 0.15–0.20 mm

• Coverlay registration tolerance ±75–100 μm; board outline tolerance ±0.10 mm

• Keep‑out from bend: components ≥ 2.0 mm; vias ≥ 1.0 mm; rigid copper thieving not required over flex

• Note stiffener thickness, adhesive type, and PSA if any; call out bend radii and fold sequence on the drawing

Reliability and material selection

For dynamic flex, prefer 12–18 μm RA copper on 12.5–25 μm adhesiveless polyimide with hatched reference copper; for static flex, 18–35 μm copper and ED copper can be acceptable. Use fillets at pad entries and strain‑relief tabs at connectors. Avoid large unrelieved copper areas in flex; balance copper to minimize curl.

Standards, acceptance, and compliance

Call out acceptance to IPC Class 2 or Class 3 on the fabrication notes; boards can be built and inspected to these criteria. For quality management, HyperPCB operates an ISO 9001:2015 certified system (SGS certificate CN‑2023‑Q1156, valid until June 2027). RoHS and REACH compliance is supported on request. If you need early risk reduction, request DFM feedback before final release; quotes are returned within 24 hours and DFM feedback within 1 business day.