Eine Flex-PCB ist eine biegbare Leiterplatte aus Kupfer und dünnem Dielektrikum, meist Polyimid. Kleberausfluss entsteht, wenn Coverlay- oder Versteiferharz unter Temperatur und Druck in Pads, Schlitze oder Biegezonen fließt.
In einem Sensorprojekt 2026 fanden wir drei Zeichnungen mit nur 0,20 mm Coverlay-Abstand an 0,50-mm-ZIF-Kontakten. Nach 0,35 mm Kleberrückzug und 10x-Erstmusterprüfung lief die 300-teilige Pilotserie ohne Pad-Reinigung.
Kurz gefasst
- Adhesive squeeze-out means resin moves beyond the intended coverlay or stiffener edge.
- Use 0.25-0.35 mm pullback around fine-pitch FPC pads as a starting point.
- Cite IPC-2223 and IPC-6013, then add your own measurable acceptance limits.
- Inspect before SMT with 10x magnification and, for critical bends, a cross-section.
Definitions and standards
A coverlay is a polyimide protection film with adhesive. A stiffener is a local reinforcement under a connector or component. Polyimide is the common flexible dielectric; see polyimide. Flexible printed board design and qualification language is often aligned with IPC standards, but standards do not replace supplier-specific DFM numbers.
"For fine-pitch FPC connectors, a CAD clearance of 0.20 mm can become only 0.05 mm after lamination if resin flow is ignored."
— Hommer Zhao, Engineering Director at FlexiPCB
Practical DFM table
| Feature | Starting control | Reason | Check |
|---|---|---|---|
| 0.50 mm ZIF pads | 0.30-0.35 mm pullback | Keeps contact edge clean | 10x microscope |
| 1.00 mm solder pads | 0.20-0.25 mm pullback | Protects wetting area | Solderability test |
| Dynamic bend tangent | 0.50 mm resin-free zone | Protects copper fatigue | Cross-section |
| Via near coverlay edge | 0.25 mm edge distance | Avoids flux trap | AOI |
| Stiffener perimeter | 0.15-0.30 mm fillet | Balances bond and overflow | Peel test |
Bei 0,50-mm-ZIF-Pads sind 0,30-0,35 mm Kleberrückzug ein sinnvoller Startwert; die Freigabe erfolgt mit 10-facher Mikroskopprüfung. For related design context, compare coverlay opening registration, gold finger flex PCB design, and the flex PCB DFM checklist.
Material and process choices
Acrylic adhesive is flexible but flows more when thickness rises from 12.5 microns to 50 microns. Epoxy systems can improve temperature resistance but may be less friendly to repeated bending. Adhesiveless laminate removes base adhesive, yet coverlay adhesive and stiffener adhesive still remain. Review adhesiveless flex PCB before locking the stack-up.
"Adhesiveless laminate removes one source of resin, not every source. Coverlay, bond ply, and stiffener adhesive still need pullback and first-article evidence."
— Hommer Zhao, Engineering Director at FlexiPCB
Drawing notes that work
A useful drawing note should say where resin is forbidden, how much pullback remains after lamination, and how inspection is done. Example: minimum post-lamination adhesive pullback from ZIF contact edge 0.20 mm; hardened bead forbidden within 0.50 mm of the dynamic bend tangent; first article inspected at 10x and worst opening reported.
"A squeeze-out requirement needs a number, a location, and an inspection method. Without all three, quality will negotiate after the lot is built."
— Hommer Zhao, Engineering Director at FlexiPCB
FAQ
How much pullback is needed for FPC connector pads?
Use 0.30-0.35 mm for 0.50 mm ZIF pads and 0.20-0.25 mm for 1.00 mm solder pads, then confirm on first articles.
Is every resin fillet a defect?
No. A 0.05-0.10 mm fillet outside the functional area can be acceptable, but resin on a pad, gold finger, or bend tangent is a defect.
Does adhesiveless flex stop squeeze-out?
No. It removes base adhesive only. Coverlay, bond ply, and stiffener adhesive can still flow during lamination.
Which standards should be named?
Name IPC-2223 for design and IPC-6013 for qualification, then add project-specific values such as 0.20 mm or 0.35 mm.
When should inspection happen?
Before SMT. Use 10x visual inspection and add one cross-section for high-reliability or dynamic-bend designs.
Request a review
Send Gerbers, stack-up, connector datasheets, and stiffener drawings. Request a flex PCB DFM review before tooling so adhesive pullback and lamination risk are checked together.
