フレキ基板は、薄い絶縁フィルムと銅箔で作る曲げられるプリント回路です。接着剤はみ出しは、熱圧着中にカバーレイや補強板の樹脂がパッド、スロット、曲げ開始点へ流れる現象です。
2026年の産業センサー案件では、3枚のFPC図面が0.50 mmピッチZIF接点の周囲に0.20 mmしか余裕を持っていませんでした。退避を0.35 mmにし、10倍で初品確認した結果、300枚の試作ロットはパッド清掃なしで通過しました。
要約
- 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 |
FPCコネクタ周辺では0.30-0.35 mmの接着剤退避を初期値にし、10倍顕微鏡で初品を確認します。 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.
