Flex PCB Design Guidelines
Design the flexible region around its actual motion, construction and assembly conditions. Define the bend zones, interfaces and qualification requirements before committing the layout.
The bending requirement comes before the bend radius.
A flex section that is folded during installation and one that moves repeatedly are different engineering problems.
Define whether the circuit is flex-to-install or dynamically flexed, then document the motion, intended cycle count, environment and final shape. A bend radius copied from another construction is not a qualification result.
For rigid-flex designs, describe both rigid and flexible regions. The transition geometry, assembly support and finished interfaces belong in the review from the beginning.
Keep the moving region simple and deliberate.
Define the bend zone on a mechanical layer
Mark where bending occurs, its direction and the boundaries of rigid or stiffened regions. Include the installed shape in the design review rather than relying only on the flat artwork.
Keep pads and vias away from bending
Locate these features outside the active bending region and agree the required transition clearances. Do not treat a rigid-board via rule as the complete flex design rule.
Route across the bend appropriately
For repeated bending, route conductors perpendicular to the bend axis where practical. Avoid abrupt width or direction changes in the flexing zone.
Review copper on both sides
For two-sided flex routing, review staggered conductors rather than directly stacking opposing traces through the bending region. Keep the final arrangement consistent with the electrical requirements.
Use smooth transitions
Avoid sharp internal outline notches and abrupt trace transitions where they could concentrate strain. Review contour radii and strain-relief details with the fabricator.
Coordinate reference copper
Do not automatically hatch every reference plane or retain a full solid plane everywhere. Review flexibility, electrical return behavior and processing needs together; controlled lines may require locally different treatment.
Specify the entire flex region, not just “polyimide.”
Swipe horizontally to read the full table.
| Element | What to discuss | What to record |
|---|---|---|
| Copper construction | Copper type and processing in relation to the intended motion. RA copper is commonly used for dynamic applications, but copper direction and subsequent plating also matter. | The proposed copper construction and the validation requirements for this particular use case. |
| Coverlay or flexible mask | The selected coating system and how its openings are formed. These are different options with different process and design constraints. | Coating identification, openings, exposed contacts and any restricted regions. |
| Stiffeners | Where additional support is needed for connectors, components or handling. A stiffener is not a substitute for an electrical rigid-flex stackup. | Material, location, side, finished interface thickness and the boundary relative to the intended bend zone. |
| Connector interface | The exact connector part and its specified contact orientation and mating dimensions. | Finished contact-region thickness, contact side, insertion direction and permitted tolerances. |
| Rigid-flex transition | How the rigid and flexible areas connect and where local features are permitted. | Transition boundaries, keepouts and any construction details that require specific approval. |
| Controlled signals | The different physical construction of the flexible region. | Target, mode, reference arrangement and any required geometry changes across regions. |
Define what a successful prototype must demonstrate.
- 01
Write the motion brief
Identify bend direction, installed shape, motion range, cycle target and environmental conditions. Specify what the product must do during and after the test.
- 02
Review the construction
Ask the design and manufacturing teams to assess the proposed stackup, bend zone and interfaces together. Record which construction is being evaluated.
- 03
Build a representative sample
Use the intended materials, geometry and assembly interfaces. Distinguish exploratory samples from the configuration proposed for production.
- 04
Record results and changes
Define acceptance criteria before testing. Keep the sample revision, test conditions and results together, and repeat relevant verification after design changes.
Include handling and assembly in the design review.
Handling and support
Discuss how the circuit will be supported during printing, placement, inspection and separation. Mark regions that must not be folded or gripped during handling.
Moisture and drying
Polyimide can absorb moisture. Agree the storage and drying process with the fabricator and assembler; drying parameters need qualification for the actual construction and process.
Installation sequence
Specify the order of insertion, fastening and forming. Include the final shape and the intended support points so the flex is not forced into an unintended bend at a connector or stiffener edge.
Inspection and test access
Identify contact areas and test locations that must remain accessible before and after installation. Distinguish bare-circuit checks from the required assembled-product verification.
Flex PCB release checklist.
Use this as a personal release check. A checkmark records your review, not manufacturer approval.
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Frequently asked questions
Check the design, files and manufacturing requirements before sending your next board for production.
Can I use the same radius for static and dynamic bending?
Do not assume so. Installation bending and repeated motion place different demands on the construction. Specify the intended use and request a reviewed radius and validation plan for that design.
Should I always use rolled-annealed copper?
Copper selection depends on the intended construction and application. RA copper is often used for dynamic bending, but orientation and plating can change the relevant copper condition. Review the finished structure, not only the starting foil name.
Can I place vias in the actively bending region?
Treat active bend regions as restricted for vias and pads unless a specialist has qualified a specific alternative. Move such features to appropriately supported regions and agree the transition clearances.
Are coverlay and flexible solder mask interchangeable?
No. They are different coating systems with different process and layout constraints. Select the covering with the manufacturer and specify the required openings and exposed interfaces.
Does this guide provide a qualified bend radius or bake schedule?
No. It identifies the information to review. Bend geometry, cycle performance and drying conditions must be assessed for the actual construction, assembly process and product requirements.
Related engineering resources
Technical references & scope notes
- Multi Circuit Boards — Flex and rigid-flex design aid
- Würth Elektronik — Flex Solutions Design Guide, 06/2025
- Würth Elektronik — RIGID.flex materials, coatings and processing
References support general engineering guidance. Supplier-specific limits, example constructions and qualification results are not GlobalWellPCB capabilities. Use the approved project specification and confirmed manufacturing rules for each order. Content reviewed: September 2026.
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