# Rigid-Flex PCB Design Guide
A rigid-flex PCB combines the mechanical stability of rigid boards with the bendability of flex circuits on a single board, enabling three-dimensional interconnection in tight spaces. By eliminating connectors and ribbon cables, it improves reliability while reducing volume and weight. Rigid-flex is increasingly common in wearables, medical devices, aerospace, and camera modules. This guide walks through structure, bend-radius rules, stackup, and applications. Sunking PCB, founded in 2009 with 15+ years of manufacturing experience and certified to ISO9001, ISO13485, and IATF16949, supports complex multilayer rigid-flex fabrication.
Basic Structure of Rigid-Flex
A rigid-flex PCB is made of rigid sections and flexible sections:
- Rigid sections typically use hard substrates such as FR-4 to carry components, connectors, and areas needing mechanical support.
- Flexible sections use flexible substrates such as polyimide (PI) to handle bending and dynamic connection.
- The two are joined by continuous flexible copper traces running through the inner layers, delivering integrated electrical and mechanical connection with no extra soldered connectors.
This structure lets the circuit fold or roll to fit inside irregular or cramped enclosures.
Minimum Bend Radius Rules
Bend radius is the single most important reliability parameter in rigid-flex design. Too tight a radius causes copper fatigue and cracking. Common rules of thumb:
- Static bend (formed once, then largely fixed): minimum bend radius about 6x the total flex-section thickness.
- Dynamic bend (repeated flexing, such as hinges or clamshells): minimum bend radius should reach 10x or more the total flex-section thickness, often higher.
- More layers require a larger radius: a single-layer flex bends most easily, double-layer next; multilayer flex is stiffer and needs a larger radius.
| Bend Type | Recommended Min Bend Radius | Typical Use |
|---|---|---|
| Static bend | ≥ 6× flex thickness | One-time fold on assembly |
| Dynamic bend | ≥ 10× flex thickness | Repeated open/close, hinges |
| Multilayer flex | Larger than the above | Needs more signal layers |
Minimize the number of layers in the flex section, and prefer single- or double-layer flex for dynamic bend areas.
Stackup Design Essentials
- Use rolled-annealed (RA) copper in flex layers; its grain structure resists bending far better than electrodeposited (ED) copper.
- Keep the flex stackup symmetrical so the neutral bend axis sits near the copper, reducing stress.
- Use no-flow / low-flow prepreg to control resin bleed at the rigid-flex boundary.
- Avoid large solid copper planes in the flex area; use cross-hatched (hatched) copper to improve flexibility and reduce stress.
Rigid-to-Flex Transition Design
The boundary between rigid and flex sections is a stress-concentration zone and the most failure-prone location, so design it carefully:
- Route copper traces perpendicular to the bend direction through the boundary; avoid diagonal crossings that concentrate stress.
- Add teardrops and rounded corners in the transition zone; avoid right-angle traces.
- The coverlay opening should extend a short distance into the rigid section to form an "anchor" that prevents delamination.
- Keep vias, pads, and components away from the bend line; vias should be placed well clear of the flex area.
Typical Applications
- Wearables: smartwatches, fitness bands, and TWS earbuds that must fold routing into minimal space.
- Medical devices: endoscopes, hearing aids, and implantables demanding high reliability and miniaturization (Sunking holds ISO13485 medical certification).
- Aerospace and defense: vibration-resistant, environmentally robust designs that remove connector failure points and cut weight.
- Camera modules: lens modules and image-sensor interconnects in phones and industrial cameras, using bends to fit compact optical structures.
Design DOs and DON'Ts
DO:
- Minimize flex-section layer count; prefer single or double layer.
- Use rolled-annealed and cross-hatched copper for bend endurance.
- Route copper perpendicular across the bend line and add teardrops and fillets.
- Confirm stackup and process capability with your manufacturer early.
DON'T:
- Do not place vias, components, or pads in the bend area.
- Do not use an overly small bend radius in dynamic-bend zones.
- Do not use large solid copper in the flex area.
- Do not cross copper traces at right or diagonal angles through the transition zone.
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Rigid-flex design and fabrication depend heavily on close collaboration with the manufacturer, so confirm stackup, bend radius, and process limits early in the design phase. If you are planning a wearable, medical, or camera-module project, contact Sunking PCB for a rigid-flex manufacturability review and quote.