Glass Fiber Epoxy Unclad Laminate — NEMA Grade FR-4
FR-4 (Flame Retardant Grade 4) — woven fiberglass cloth, brominated epoxy resin binder, hot-pressed under high temperature and pressure. Result: rigid, flame-retardant electrical insulation laminate with near-zero moisture absorption.
ACC Insulations manufactures FR-4 glass epoxy unclad laminate sheets in Nashik, Maharashtra. Standard and custom sizes available. CNC machining available to drawings.
What FR-4 Glass Epoxy Unclad Laminate?
NEMA Grade FR-4 — glass-reinforced epoxy laminate. Woven electronic-grade fiberglass cloth impregnated with brominated epoxy resin, pressed under controlled heat and pressure. Brominated resin creates self-extinguishing behavior on ignition — the source of UL94 V-0 compliance.
Result: rigid insulation boards combining high dielectric strength, low moisture absorption, strong flexural strength, and stable dimensions from –50°C to +155°C. Used as PCB base substrate, transformer insulation, switchgear panel insulation, busbar support, and motor slot wedges across electrical OEMs in India.
Also known as: FR4 epoxy sheet, FR-4 laminate sheet, fiberglass epoxy board, glass-reinforced epoxy laminate, epoxy fiberglass cloth laminated board, G10/FR4 sheet.
Key Features — FR-4 Epoxy Laminate
- Flame Retardant UL94 V-0 — self-extinguishes, no drip, critical for electrical enclosures
- High Dielectric Strength (>20 kV/mm) — reliable high-voltage insulation barriers
- Thermal Class F (155°C) — stable in transformers, motors, and power electronics
- Near-zero Water Absorption — insulation performance sustained in humid conditions
- Superior Flexural & Tensile Strength — resists mechanical load in switchgear structures
- Excellent Arc Resistance — ideal for bus bar supports and terminal boards
- CNC Machinable — drilled, milled, routed, and cut with standard tools
- Dimensional Stability — tight tolerances maintained post-machining
- Dielectric Constant 4.2–4.8 — stable impedance across frequencies
Technical Properties — FR-4 Sheet Data
Reference values per NEMA LI 1-1998 and IEC 60893 standards.
| Property | Value / Range | Test Standard |
|---|---|---|
| Dielectric Strength (perpendicular) | > 20 kV/mm | IEC 60243 |
| Dielectric Constant (@ 1 MHz) | 4.2 – 4.8 | IEC 60250 |
| Glass Transition Temperature (Tg) | 130 – 140°C | IEC 60249 |
| Thermal Class (Max. Operating Temp) | 155°C (Class F) | IEC 60085 |
| Water Absorption (24h immersion) | < 0.10% | IEC 60062 |
| Flexural Strength (LW) | > 415 MPa | IEC 60893 |
| Tensile Strength | > 310 MPa | IEC 60893 |
| Flame Rating | UL94 V-0 | UL 94 |
| Thermal Conductivity | 0.3 – 0.5 W/m·K | ASTM E1461 |
| Arc Resistance | > 60 seconds | ASTM D495 |
| NEMA Grade | FR-4 | NEMA LI 1-1998 |
| MIL Spec | MIL-I-24768/27 GEE-F | MIL Standard |
Applications — Where FR-4 Laminate Used
FR-4 glass epoxy unclad sheets serve structural insulation roles across high-voltage and industrial electrical assemblies:
- Transformer insulation — winding supports, barriers, spacers in distribution transformers
- Switchgear panel insulation — arc shields, phase barriers, mounting plates
- Busbar supports — rigid insulating supports for copper busbars in switchboards
- Terminal boards & strip insulation — high-voltage terminal insulation blocks
- Motor slot wedges — insulation wedges in stator slots of induction motors
- PCB base substrate (when copper-clad) — printed circuit board manufacturing
- High-voltage insulation barriers — creepage distance extenders in MV equipment
- Jigs, fixtures & test boards — mechanical and electrical test assemblies
- Railway electrical systems — traction motor insulation and control gear panels
- Industrial drives & automation — VFD enclosure insulation components
Available Sizes & Customization
| Parameter | Standard | Custom |
|---|---|---|
| Thickness Range | 0.2 mm – 100 mm | Per drawing |
| Sheet Size | 1220 mm × 2440 mm | Cut-to-size available |
| Surface Finish | Natural / Smooth | Matte / Machined |
| Machining | — | CNC routing, drilling, milling |
| Color | Natural / Light Green | Yellow, Black |
Manufacturing Process — FR-4 Glass Epoxy Laminates
Controlled 6-stage hot-press process ensures uniform dielectric strength, minimal void content, and consistent mechanical properties batch-to-batch.
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Fiberglass Fabric Selection — electronic-grade woven E-glass cloth selected for consistent weave density and tensile strength
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Resin Impregnation — brominated epoxy resin uniformly saturated into glass cloth via controlled dip-and-squeeze line; resin content monitored for dielectric consistency
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Prepreg Stacking — B-stage prepreg sheets stacked to target thickness; orientation alternated for balanced mechanical properties
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Hot Press Lamination — multi-opening hydraulic press applies controlled heat (170–180°C) and pressure; resin flows and cross-links fully
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Post-Cure — oven post-cure cycle completes cross-linking, eliminating residual stress and achieving final Tg and dimensional stability
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Finishing & QC — sheets precision-cut, surface-dressed, and tested for dielectric strength, thickness tolerance, and flame rating before dispatch
FR-4 vs Other Insulation Laminates — Comparison
Choosing wrong laminate grade increases failure risk. Direct comparison with common alternatives:
| Property | FR-4 | G-10 | Bakelite (CE/LE) | Polyester (GPO-3) | Paper Laminate |
|---|---|---|---|---|---|
| Flame Retardant UL94 V-0 | ✔ | ✘ | Partial | ✔ | ✘ |
| Dielectric Strength | Very High | Very High | Medium | High | Low–Medium |
| Thermal Class | 155°C (F) | 155°C (F) | 105°C (A) | 130°C (B) | 105°C (A) |
| Moisture Resistance | Excellent | Excellent | Moderate | Good | Poor |
| Mechanical Strength | Very High | Very High | Medium | Medium–High | Low |
| CNC Machinability | ✔ | ✔ | ✔ | Limited | ✔ |
| Can Replace FR-4 | — | No | No | No | No |
* FR-4 substitutes G-10 in all applications. G-10 cannot substitute FR-4 where UL94 V-0 compliance required.
Industries Using FR-4 Laminates
- Power transformer manufacturers — distribution and power transformers, dry-type transformers
- Switchgear & control panel OEMs — LV, MV switchgear insulation components
- Electrical equipment manufacturers — motors, generators, VFDs, UPS systems
- Power generation & distribution — substations, bus ducts, cable tray supports
- Railway electrical systems — traction motor insulation, control gear
- Industrial automation & drives — servo drive enclosures, PLC panel insulation
- PCB manufacturers — FR-4 unclad as backup material and drill entry/exit boards
- Defence & aerospace — MIL-I-24768/27 GEE-F rated applications
Why Choose ACC Insulations for FR-4?
- India manufacturer — direct supply, no middlemen, shorter lead times
- Nashik, Maharashtra location — well connected to transformer and switchgear OEM hubs
- Precision CNC machining — components per customer drawings and tolerances
- Wide thickness range — 0.2 mm to 100 mm in standard and custom sizes
- Consistent resin curing — hot-press process with post-cure quality control
- Industrial-grade raw materials — E-glass woven fabric, brominated epoxy resin
- Strict dielectric & flame testing — batch-level verification before dispatch
- Related products stocked — fiber glass epoxy laminates, glass fiber tubes, transformer insulation systems
Need Bulk FR-4 Sheets or CNC Machined Components?
Share drawings or technical requirements. Precision-machined FR-4 parts supplied with consistent dielectric quality and fast delivery from Nashik.
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Frequently Asked Questions — FR-4 Laminate
FR = Flame Retardant. Grade 4 = NEMA classification for glass-reinforced epoxy laminates, defined under NEMA LI 1-1998. FR-4 meets UL94 V-0 flame resistance — achieved via brominated epoxy resin that self-extinguishes upon ignition without dripping. Also satisfies MIL-I-24768/27 Type GEE-F specification.
Both use woven fiberglass cloth with epoxy resin. FR-4 contains brominated flame-retardant additives — achieves UL94 V-0. G-10 contains no flame retardants — not V-0 rated. FR-4 substitutes G-10 in any application. G-10 cannot substitute FR-4 where fire safety standards apply. Both share similar mechanical and dielectric properties otherwise.
FR-4 dielectric constant (Dk) ranges 4.2–4.8 at 1 MHz, depending on resin content and fiberglass weave density. Stable Dk ensures consistent impedance across frequencies — critical for high-voltage insulation barriers and PCB signal integrity. Dielectric strength exceeds 20 kV/mm perpendicular to laminate plane.
Near-zero water absorption (<0.10% per 24h immersion test). Electrical insulation performance stays stable in high-humidity industrial and coastal environments. Significant advantage over paper-based laminates and polyester boards, which show measurable moisture-induced dielectric degradation.
FR-4 supports CNC routing, drilling, milling, punching, and precision cutting. Maintains dimensional stability and structural integrity during fabrication. Note: low shear strength requires larger tool clearances and reduced rake angles versus metals. ACC Insulations supplies CNC-machined FR-4 components per customer drawings and tolerances.
Standard thickness range: 0.2 mm to 100 mm. Standard sheet: 1220 mm × 2440 mm. Custom cut-to-size panels available. CNC machined components per drawings. Contact ACC Insulations with specific requirements for lead time and pricing.
