Eine Flex-PCB reisst selten in der Mitte eines geraden Bereichs. Risse starten an scharfen Innenkanten, am Ende eines Schlitzes oder direkt neben einer Versteiferkante. In einer Maerz-2026-Pruefung von 1800 doppelseitigen Polyimid-FPCs fanden wir nach Steckereinsetzen und 5000 Biegezyklen 22 Coverlay-Risse an einem 0,45-mm-Entlastungsschlitz. Mit 0,30 mm Schlitzradius, breiterem Hals und 4 mm Abstand zur Versteifung verschwand der Fehler.
Kurz gefasst
- Round every internal corner that can see bending or pulling load.
- Keep copper 0.50-1.00 mm away from slot edges when production space allows.
- Place stiffener edges 3-5 mm away from the first active bend.
- Specify slot-end radius and edge inspection on the fabrication drawing.
- Test insertion, folding, and bend cycling before freezing tooling.
Design rules and definitions
A flex PCB tear relief is a geometry feature that spreads strain so the polyimide and coverlay do not split from a corner or slot. A bend relief slot is a planned cutout that lets the circuit bend in a controlled zone. A tear stop is a round hole, radius, or shape change that slows crack growth before it reaches copper. These decisions belong with IPC-2223 and IPC-6013 style flex design review, not only mechanical CAD cleanup. Public background for IPC, polyimide, and ISO 9001 helps frame the quality language.
| Feature | Conservative target | Prototype minimum | Reason |
|---|---|---|---|
| Inside corner radius | 0.50 mm+ | 0.25 mm | Reduces crack start |
| Slot end radius | width / 2, min 0.20 mm | 0.15 mm | Avoids knife-edge slots |
| Copper keepout | 0.50-1.00 mm | 0.30 mm | Keeps traces away from strain |
| Stiffener to bend | 3-5 mm | 2 mm | Reduces hard-hinge effect |
| Tear-stop hole | 0.60-1.00 mm | 0.40 mm | Slows crack growth |
| Coverlay web | 150-200 um | 100 um | Prevents lifting near cutouts |
"For flex PCB reliability, the outline is part of the circuit. I review slot ends, neck-downs, stiffener edges, and copper keepouts before I trust a bend-cycle number."
— Hommer Zhao, Engineering Director at FlexiPCB
How to apply the rule
Start with the bend map. Mark static folds, dynamic bends, connector tails, component islands, and rigid-flex transitions. Then place relief slots only where they control deformation. A slot with a sharp end is not relief; it is a crack starter. Use radius ends, avoid copper beside the slot, and keep coverlay from forming thin unsupported slivers.
Stiffeners need the same review. FR-4, polyimide, stainless steel, and aluminum stiffeners support connectors and components, but the stiffener edge becomes a stiffness boundary. Keep the fold away from that boundary, taper or radius stiffener corners, and avoid vias or solder joints in the transition band. For related design context, use flex PCB bend radius, flex PCB stiffener design, and laser cutting tolerance.
Validation checklist
| Check | Good result | Red flag | Fix |
|---|---|---|---|
| Slot end | Smooth radius | Sharp notch | Increase radius or tune laser |
| Copper clearance | Meets keepout | Trace near slot end | Reroute or widen neck |
| Stiffener edge | Bend starts 3-5 mm away | Fold at adhesive edge | Move stiffener or fold line |
| Coverlay | No lift after handling | Sliver peels | Enlarge coverlay web |
| Bend test | No tear after cycles | Split before open circuit | Reduce strain and add radius |
| Connector insertion | Tail survives force | Tear at shoulder | Add support or relief radius |
"A tear stop is not decorative. It needs enough diameter, enough copper clearance, and the right location. If it is too small or too close to a trace, it becomes another stress riser."
— Hommer Zhao, Engineering Director at FlexiPCB
FAQ
What radius should be used on a flex PCB inside corner?
Use 0.50 mm or larger when possible. Tight products may use 0.25-0.30 mm after supplier review, but sharp 90-degree internal corners are risky in dynamic polyimide tails.
How far should copper stay from a relief slot?
Use 0.50-1.00 mm for production when space allows. A 0.30 mm keepout may work in prototypes, but dynamic bend zones need more margin.
Should every flex circuit include bend relief slots?
No. Slots help when they guide deformation around a tail, boss, or transition. They hurt reliability when they narrow the circuit too much or end sharply.
Can a tear-stop hole be plated?
Usually no. A tear-stop hole is a mechanical crack-arrest feature. Plating adds stiffness and can create another fatigue point.
Which outline process is best for small relief features?
UV laser cutting is often best for thin polyimide slots and details below about 0.20 mm. Routing or hybrid processing may be better for rigid-flex sections.
How should tear relief be tested?
Run insertion, folding, service handling, and bend cycling on 10-30 samples at minimum. Dynamic products need cycle count, radius, temperature, and humidity targets that match the real product.
Get the outline reviewed
Send Gerbers, DXF outline, bend locations, stiffener drawing, copper weight, and expected handling cycle. FlexiPCB can review relief slots, radius corners, copper keepouts, and stiffener clearances before tooling. Request a flex PCB DFM review or send an RFQ.
