Rigid-Flex Design Rules That Prevent Field Failures
Most rigid-flex failures start in the bend area, not on the rigid sections. Bend radius, copper orientation, via placement and material choice, with the rules we check on every design.
Keep copper out of the bend where you can, run traces perpendicular to the bend axis rather than parallel, stagger vias away from the bend area, and specify both the minimum bend radius and whether the bend is static or dynamic. Those four decisions cover most rigid-flex failures.
A rigid-flex board fails differently from a rigid board. The failure is usually a cracked trace or a delaminated coverlay in the flex region, and it appears after a number of cycles rather than at first power-on, which means it escapes electrical test and shows up in the field.
01Bend radius and cycle type
The minimum bend radius is normally expressed as a multiple of the flex thickness. For a static bend, installed once and never moved again, 10 times the thickness is a workable target. For a dynamic bend that flexes in service, you need 20 times or more, and the number of cycles has to be specified so the material and copper can be chosen for it.
A design that does not state whether the bend is static or dynamic cannot be reviewed properly. The two cases use different copper, different adhesive and different trace geometry.
| Static, one-time install | Minimum 10x flex thickness. Rolled annealed copper preferred for tight radii |
|---|---|
| Dynamic, repeated flexing | Minimum 20x flex thickness, cycle count specified, hatched ground planes, no vias in the bend |
| Hinge, high cycle count | Dedicated flex construction, single or double layer, adhesiveless polyimide, tested to the stated cycle count |
02Copper and trace geometry in the bend
Electrodeposited copper is brittle in the flex direction. Rolled annealed copper costs more but survives repeated bending, and it is the standard choice for any dynamic application.
Trace orientation matters more than most designers expect. Traces running perpendicular to the bend axis experience less strain than traces running parallel to it, because the bend stretches the outer surface along one direction only. Where you cannot reorient, use a hatched or cross-hatched copper plane instead of a solid one: the hatch lets the copper move instead of cracking.
- Use rolled annealed copper for any dynamic bend
- Run traces perpendicular to the bend axis where the layout allows it
- Replace solid planes in the bend area with a hatch pattern
- Keep the copper thickness in the flex region as low as the current allows, 0.5 oz is common
- Avoid trace width changes and sharp corners inside the bend
03Vias, stiffeners and the transition zone
No via should sit inside the bend area. Vias concentrate strain at the barrel and at the annular ring, and they are the most common crack initiation point in a flexing board. Place them on the rigid sections, or in the transition zone only where the design has no alternative.
The transition from rigid to flex is where stress concentrates, because the stiffness changes abruptly over a short distance. Stiffeners extend the rigid region and move the bend point away from the transition. Specify stiffener material, thickness and whether it goes on the component side or the solder side, and remember that a stiffener adds to the total thickness at the connector.
04Material choice
Adhesiveless polyimide, where the copper is bonded directly by casting or sputtering, is thinner and more flexible than adhesive-bonded construction and handles higher temperature. It costs more. Adhesive-bonded flex is fine for static bends and for boards that will be assembled once.
Coverlay is the flexible equivalent of solder mask. It is a polyimide film bonded over the copper, and it is more resistant to flexing than a liquid photoimageable mask. Use coverlay on any board that will bend in service.
Key takeaway
State the bend radius, the cycle count and whether the bend is static or dynamic on the drawing. Keep vias out of the bend, use rolled annealed copper and hatched planes for dynamic flexing, and add a stiffener to move the bend point away from the rigid-to-flex transition.
Products discussed in this article
Continue reading
Ready to start your project?
Send your Gerber files for a quote. Simple boards are quoted within 1 hour, complex builds within 24 hours.