Buyers often arrive at our RFQ inbox holding a drawing and one question: "Should this part be carbon fiber board, FR4, or electrical board (phenolic)?" Choosing wrong costs either performance (part flexes, overheats, or conducts where it must not) or budget (paying for carbon fiber stiffness you don't need).
Here is a factory-floor comparison of the three most requested CNC-machined sheets, with a decision table you can apply directly to your next part.
The Three Materials in One Paragraph Each
Carbon fiber board — layers of carbon fiber fabric (typically 3K twill or plain weave) in an epoxy matrix, pressed into panels. Its defining property is stiffness-to-weight: comparable bending stiffness to steel at a fraction of the mass. It is dimensionally stable, fatigue-resistant and premium-looking. The critical caveat: it is electrically conductive, so it never belongs in an isolation path.
FR4 fiberglass sheet — woven glass cloth + epoxy. The workhorse of electrical insulation: flame retardant (UL94 V-0 equivalent), excellent dielectric properties, low water absorption, and strong enough for structural use. The sensible default for busbar plates, spacers, fixtures and any part that must insulate.
Phenolic electrical board — paper or fabric reinforcement with phenolic resin. Lower cost, decent insulation, easier punching. Heat and moisture resistance are below FR4, which is why it earns its place in dry, cost-driven assemblies rather than harsh environments.
Head-to-Head Comparison
| Property | Carbon fiber board | FR4 fiberglass | Phenolic (electrical board) |
|---|---|---|---|
| Density | ≈ 1.5 – 1.6 g/cm³ | ≈ 1.9 – 2.1 g/cm³ | ≈ 1.3 – 1.4 g/cm³ |
| Stiffness (relative) | ★★★★★ | ★★★☆ | ★★ |
| Electrical insulation | ❌ Conductive | ✅ Excellent | ✅ Good |
| Continuous heat (resin-dependent) | 120–150 °C (epoxy) | 130–170 °C (Tg) | ~110–120 °C |
| Water absorption | Very low | ≤ 0.2% | Moderate |
| Flame retardance | Resin-dependent | V-0 equivalent | Grade-dependent |
| CNC machinability | Good (sharp tooling, dust control) | Good (abrasive on tools) | Excellent (can be punched) |
| Cosmetic finish | Premium (visible weave) | Functional | Utilitarian |
| Relative cost | ★★★★★ (highest) | ★★ | ★ (lowest) |
Decision Table: Pick Your Material in 10 Seconds
| Your requirement | Choose |
|---|---|
| Must insulate 1 kV+ between conductors | FR4 |
| Lightweight robot arm link, camera gimbal, drone plate | Carbon fiber board |
| Thousands of cheap spacer washers, dry indoor use | Phenolic |
| Part gets wet, humid or outdoor | FR4 or carbon fiber |
| Safety cert requires flame rating | FR4 (V-0) |
| Visible cosmetic surface for high-end product | Carbon fiber (3K twill) |
| Part sits near RF and must not shield/short | FR4 |
| Stiffness-critical plate under 3 mm thick | Carbon fiber board |
The most common mistake we see: carbon fiber specified where FR4 was needed — the part conducts, the assembly shorts, and the redesign costs more than the material saving ever was. The second most common: FR4 specified where phenolic was plenty, overspending by 30–40% on hidden parts.
CNC Machining Notes Per Material
Carbon fiber board - Use diamond-coated or compressed-tool-geometry cutters; delamination and fuzzing are the failure modes to control - Dust is conductive — extraction and machine protection matter (our shop runs dedicated CF extraction) - Thin panels (< 1 mm) route cleanly with backing plates to avoid breakout
FR4 - Glass cloth is abrasive: carbide tooling with controlled wear - Tight holes: ream after drill; edges can be sanded for cosmetic parts - Stress-relieve before final pass when flatness is specified
Phenolic - The most forgiving: drilling, milling and even punching - Watch for moisture-related dimensional drift in storage — keep parts wrapped until assembly
What We Need From You for a Fast, Accurate Quote
- Part drawing (DXF/PDF) or a sample to reverse-engineer
- Material + thickness — or just the application and we propose it
- Quantity and batch schedule
- Tolerance class (see our FR4 machining tolerance guide)
- Certifications required (RoHS/REACH, UL, material certs)
Source All Three From One Factory
ChaoRong Electronics (ChaoRong Electronics, Dongguan, China) CNC-machines carbon fiber boards, FR4 fiberglass sheets and phenolic electrical board into finished parts for B2B customers — one supplier, consistent quality system, and honest advice when the drawing calls for a different material than you assumed.
→ Send your drawing for a same-week quotation · Carbon fiber board product page
Related reading: FR4 Fiberglass Sheet Guide · Common Mode Choke Selection Guide · Polycarbonate (PC) Sheet Guide · Acrylic Sheet (PMMA) Machining Guide