| Your Parameter | Ultra‑thin FR4 (0.1mm–0.3mm) | FPC (flexible) | Your reason to choose |
|---|---|---|---|
| Base structure | Glass fabric + epoxy resin | Polyimide (PI) film + acrylic/epoxy adhesive | ♪ FR4 has a skeleton, FPC is a pure film. Skeleton supports components; film needs stiffener. |
| Material property | Rigid, flexural modulus 17–22 GPa | Flexible, flexural modulus 2–5 GPa | ♪ FR4 is 8‑10× stiffer. Same thickness, ultra‑thin FR4 feels rigid, FPC feels soft. |
| Finished thickness | 0.1 / 0.15 / 0.2 / 0.25 / 0.3 mm (±0.05mm) | Base film: 12.5/25/50µm + coverlay ≈ 0.05–0.2mm | ♪ 0.1–0.3mm is ultra‑thin FR4's home ground! FPC needs 0.1–0.2mm stiffener → total >0.3mm. |
| Copper type | ED copper (1/3oz 12µm, 1/2oz 18µm, 1oz 35µm) | Dynamic: RA copper; static: ED (higher cost) | ♪ Copper cost 30%+ lower. FPC needs RA copper → complex process, higher price. Ultra‑thin FR4 uses standard ED with great adhesion. |
| Peel strength (copper adhesion) | ≥ 1.2 N/mm (hot‑pressed to glass fabric) | ≥ 0.8 N/mm (glued to PI film) | ♪ 50% higher pad pull strength. "FPC pads are glued; Type‑C ports may peel. Ultra‑thin FR4 pads are anchored in the board – they won't come off." |
| Min. line width/space | Limit: 3/3mil (0.075mm) Recommended: ≥4/4mil (0.1mm) |
Limit: 2/2mil (0.05mm) Recommended: ≥3/3mil (0.075mm) |
♪ FPC can do finer, but yield suffers. Ultra‑thin FR4 is dimensionally stable – fine lines on long panels align better. FPC material soft, expansion hard to control, yield drops. |
| Min. mechanical drill | Limit: 0.2mm (8mil) Recommended: ≥0.25mm (10mil) |
Limit: 0.1mm (4mil) Recommended: ≥0.15mm (6mil) |
♪ FPC can do smaller holes, but at a price: 0.1mm needs laser drill – expensive, slow. Standard connectors use 0.25mm, ultra‑thin FR4 handles it economically. |
| Laser drilling | Not applicable (mechanical only) | Micro‑blind/buried vias ≥0.075mm (3mil) | ♪ HDI boards → FPC mandatory. For high‑density interconnects, stacked vias, you need FPC + laser drilling. |
| Surface finish | ENIG preferred; OSP/ImmAg possible; HASL not recommended | ENIG, ImmAg, selective gold; coverlay protects traces | ♪ Ultra‑thin: push ENIG. "0.2mm board in HASL warps; ENIG stays flat, SMT yield higher, overall cost lower." |
| Solder mask type | Photo‑imageable ink (green/white/black etc.) | Coverlay + window openings | ♪ FPC surface is uneven: coverlay creates steps, paste thickness varies. Ultra‑thin FR4 is perfectly flat – consistent stencil printing. |
| Bending resistance | Static bend: 1‑5 times (during assembly) Dynamic: not applicable |
Static: unlimited Dynamic: >100k cycles (design‑dependent) |
♪ Clear boundary: "Ultra‑thin FR4 bends once to fit, but don't flex it daily. Daily, dynamic, 10k cycles → must be FPC." |
| Min. bend radius | Not applicable (bending damages it) | Dynamic: ≥5‑10× thickness Static: ≥3‑5× thickness |
♪ FPC needs radius – too tight breaks copper. Ultra‑thin FR4 simply isn't designed for bending radius. |
| Outline processing | Routing + mouse‑bite panels V‑CUT forbidden |
Die‑cut / laser cut; need tooling ears | ♪ Panels differ totally: ultra‑thin FR4 must use mouse‑bites, never V‑CUT. FPC needs die‑cutting – lower efficiency, higher cost. |
| Warpage control | ≤ 0.7% – 1.0% | ≤ 1.0% – 2.0% (more prone to warp) | ♪ FPC deforms easily: without borders, FPC is cloth‑soft – SMT needs carriers. Ultra‑thin FR4 has inherent stiffness, easier to handle. |
| Operating temperature | -40°C to +130°C (depends on Tg) | -40°C to +120°C (adhesive limit) | ♪ FR4 slightly better in high‑temp environments. FPC adhesive may age and delaminate. |
| Flammability rating | UL94 V‑0 (standard) | UL94 V‑0 (needs material certification) | ♪ Both meet safety norms, but FPC requires material certification. |
