플렉스 PCB는 얇은 절연 필름과 동박으로 만든 휘어지는 회로 기판입니다. 접착제 번짐은 열과 압력 때문에 커버레이 또는 보강판 수지가 패드, 슬롯, 굽힘 영역으로 흐르는 현상입니다.
2026년 산업용 센서 프로젝트 검토에서 3개 FPC 도면이 0.50 mm ZIF 접점 주변에 0.20 mm만 남긴 것을 확인했습니다. 0.35 mm 후퇴와 10x 초도 검사 후 300개 파일럿 로트는 패드 세정 없이 통과했습니다.
요약
- 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 |
0.50 mm 피치 ZIF 패드는 0.30-0.35 mm 접착제 후퇴를 시작값으로 두고, 초도품을 10배 현미경으로 확인합니다. 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.
