G10 vs FR4: Same Glass-Epoxy, Different Safety Standard

G10 and FR4 are the two most-searched glass-reinforced epoxy laminates — and also the most commonly confused. They share the same structural backbone: woven E-glass cloth saturated with an epoxy resin system, then pressed into laminate sheets or rods. Their mechanical and electrical properties are nearly indistinguishable on a property table. The critical distinction is flame behavior. FR4 incorporates a brominated flame-retardant (FR) additive that earns it a UL94 V-0 classification. G10 contains no such additive and carries no UL94 V-0 rating. In any application where a regulatory authority, customer specification, or safety standard requires UL94 V-0 compliance — PCB substrates, electrical panels, switchgear, aerospace interiors — G10 is not a substitutable alternative for FR4. Choose the wrong grade and you introduce a compliance gap that material certificates cannot close.

TL;DR

• Same base chemistry, different material: Both are woven glass / epoxy laminates, but FR4 adds a brominated flame retardant; G10 does not.
• Flame rating: FR4 = UL94 V-0; G10 = no UL94 V-0 rating — this is the defining commercial and regulatory distinction.
• Mechanical properties: Effectively identical at room temperature — tensile strength ~45,000 psi, flexural strength ~60,000 psi for both.
• Electrical: Both are excellent insulators with dielectric strength >40 kV/mm; FR4 Dk ~4.5 at 1 MHz, G10 Dk ~4.3.
• Temperature: Both rated to 130°C (Class B) continuous service in standard grade; FR4's bromine can slightly depress thermal conductivity.
• Cost: G10 is typically 5–15% less expensive than FR4 in equivalent form and thickness.
• When to use FR4: Any application with a UL, IPC, NEMA, or MIL-spec requiring V-0 flame performance — virtually all electrical / PCB work.
• When to use G10: Structural, mechanical, and wet-environment applications (marine, firearms, knives) where flame rating is not required and cost matters.

Chemistry & Origin

G10 was developed in the mid-twentieth century as a high-performance structural laminate — the "G" denotes glass reinforcement, and "10" is the NEMA laminate grade number for the woven-glass / epoxy system. It replaced earlier paper-phenolic and canvas-phenolic laminates in demanding electrical and mechanical roles because it combined superior mechanical strength with low moisture uptake and excellent dielectric properties.

FR4 emerged as electronics miniaturization accelerated and regulatory pressure to reduce printed circuit board flammability grew. The "FR" prefix means flame-retardant. Manufacturers achieved V-0 performance by incorporating tetrabromobisphenol A (TBBPA) or equivalent brominated compounds into the epoxy resin system during laminate manufacture. Because TBBPA is chemically bound into the resin rather than simply blended, it does not leach out under normal service conditions — but it does affect combustion behavior dramatically. When exposed to an ignition source, FR4 self-extinguishes; G10 continues to burn.

Mechanical Properties

For strictly structural applications, G10 and FR4 are essentially equivalent. Both achieve tensile strengths in the range of 40,000–48,000 psi and flexural strengths of 55,000–65,000 psi depending on laminate direction and thickness. Compressive strength exceeds 50,000 psi in both grades. The woven glass reinforcement is identical between grades — only the resin formulation differs. Machining behavior is also nearly identical: both grades machine with carbide tooling, produce abrasive glass-fiber chips, and require dust collection and respiratory protection during machining operations.

Electrical Properties

Both G10 and FR4 are superb electrical insulators and were both developed with electrical applications in mind. Dielectric strength exceeds 40 kV/mm for both grades. The dielectric constant of G10 is marginally lower (~4.3 vs ~4.5 at 1 MHz) — a meaningful distinction only in microwave or high-frequency RF applications where signal loss and phase velocity matter. For power-frequency and digital-speed insulation work, neither value creates a practical difference.

Dissipation factor for both grades is in the range of 0.018–0.025 at 1 MHz. Volume resistivity exceeds 10¹⁰ MΩ·cm for both. Surface resistivity after 48-hour immersion remains excellent in both grades due to low water absorption.

The one electrical scenario where FR4 is mandatory: any substrate or insulator that must carry a UL listing or conform to IPC-4101 Class B laminate standards. G10 cannot satisfy those specifications regardless of its electrical properties.

Thermal Properties

Standard G10 and FR4 are both rated for continuous service at 130°C (Class B, per NEMA and IEC standards). Higher-temperature variants exist — FR4 high-Tg formulations push the glass transition temperature to 150°C–180°C and are specified in lead-free soldering processes that expose PCBs to higher reflow temperatures. These high-Tg FR4 grades are not equivalent to standard FR4 and command a significant price premium.

The brominated additive in FR4 has a marginally lower thermal conductivity than the neat epoxy it partially replaces, but the practical effect on part-level thermal performance is negligible in structural applications.

Chemical Resistance

G10 and FR4 show virtually identical chemical resistance profiles. Both resist dilute acids, alkalis, and most organic solvents. Both exhibit low water absorption (<0.10% in 24-hour immersion), which contributes to dimensional stability in wet environments. Neither material withstands strong oxidizing acids (concentrated nitric, chromic) or halogenated solvents at elevated temperatures. The brominated content in FR4 does not materially change its response to chemical attack.

Cost & Availability

G10 carries a slight cost advantage — typically 5–15% less expensive than FR4 of equivalent thickness and sheet size. Both materials are commodity laminates stocked in sheet form (0.030″ through 2.0″+) by major plastics distributors. Rod and tube are stocked in G10 more commonly than FR4, since FR4's primary application domain (PCB substrate) uses sheet. In thicker structural sections — particularly rod and tube for mechanical and marine applications — G10 dominates the market.

Custom cut-to-size, precision blanks, and machined parts are available from both materials on short lead times.

When to Choose G10 vs FR4

Choose G10 when:

• The application is structural, mechanical, or marine — knife handles, gun grips, fishing rod blanks, boatbuilding spacers, jigs and fixtures.
• No UL94 flame rating is required by specification, customer, or regulatory authority.
• Cost difference is meaningful for the volume involved.
• Rod or tube form is required (more commonly stocked in G10).

Choose FR4 when:

• The application involves printed circuits, PCB substrates, or any electronics assembly where IPC-4101 compliance or UL94 V-0 is required.
• The end product must carry a UL, CE, or equivalent listing that references flame performance.
• A customer, government, or industry specification explicitly calls for FR4 or a UL94 V-0 laminate.
• High-Tg performance is needed for lead-free soldering processes.

Common Alternatives

• G10 vs Cotton Phenolic — When lower cost or better machinability matters more than moisture resistance.
• G10 vs G7 — G7 silicone glass laminate for higher-temperature insulation applications.
• FR4 vs G9 — When melamine glass chemistry is under consideration.

More related guides

Explore where glass-epoxy laminates show up across applications and industries:

Applications

• Electrical Insulation
• Jigs and Fixtures
• Structural Components

Industries

• Electronics
• Aerospace
• Marine

Other comparisons

• G10 vs G9
• FR4 vs Phenolic Glass
• G10 vs Phenolic Glass

Spec sheets

• G10 Material Hub
• FR4 Material Hub