G10 vs FR4 — Which to Choose? The Definitive Guide

G10 and FR4 are not the same material — they share the same glass-epoxy base but differ fundamentally in flame retardancy: FR4 is UL 94 V-0 (self-extinguishing) due to brominated epoxy resin; G10 is UL 94 HB (horizontally burns) with no halogenated additives. Specifying the wrong grade violates electrical safety codes or costs unnecessary material expense.

TL;DR — Key Takeaways

• G10 ≠ FR4. They are different materials. Always distinguish them on drawings and purchase orders.
• FR4 uses brominated epoxy (TBBPA) to achieve UL 94 V-0; G10 uses plain bisphenol-A epoxy, rated only HB
• Electrical properties (dielectric strength, Dk, Df) are nearly identical between G10 and FR4 at equivalent thickness
• Mechanical properties (flexural, tensile, compressive) are essentially the same
• FR4 is required wherever UL 94 V-0 is mandated: PCBs per IPC standards, industrial control panels per UL 508A, most switchgear
• G10 is appropriate where V-0 is not required: HV insulators in enclosed switchgear, MIL-I-24768/14 GEE applications, cost-driven substitution where code allows
• FR4 typically costs 5–15% more than G10 at equivalent thickness and size

See also: G10 vs FR4 comparison page for a focused property data comparison.

The Chemistry Difference — Why It Matters

G10: Glass-Epoxy Without Flame Retardant

G10 (NEMA Grade G10, ASTM D709, MIL-I-24768/14 type GEE) is fabricated from woven E-glass fabric impregnated with bisphenol-A epoxy resin (typically a standard difunctional epoxide). The cured laminate contains no intentional flame-retardant additives.

In a horizontal burn test (UL 94 HB):

• G10 burns slowly (< 76 mm/min for specimens > 3mm thick)
• It will self-extinguish after the flame source is removed from very thin specimens
• It does NOT reliably extinguish in a vertical burn configuration

UL 94 HB does not satisfy the V-1, V-0, or 5VA requirements of most electrical equipment standards.

FR4: Glass-Epoxy With Brominated Flame Retardant

FR4 (NEMA Grade FR4, ASTM D709, MIL-I-24768/14 type GEF) uses the same woven E-glass reinforcement as G10 but with an epoxy resin system that incorporates tetrabromobisphenol A (TBBPA) as a reactive flame retardant — TBBPA is chemically bonded into the polymer backbone, not simply mixed in as a filler.

When FR4 catches fire:

• TBBPA releases bromine radicals (HBr) that interrupt the vapor-phase combustion chain
• The material extinguishes within 10 seconds in a vertical burn position (UL 94 V-0)
• No burning drips are generated

The brominated resin gives FR4 its V-0 rating and the "FR" (flame-retardant) designation.

Full Property Comparison

When to Use G10

G10 is the right choice when:

1. UL 94 V-0 is not required. If the applicable equipment standard does not mandate V-0 (or if the component is not a load-bearing insulation element in a UL-listed product), G10 provides equivalent electrical and mechanical performance at lower cost.

2. MIL-I-24768/14 GEE is specified. Military drawings that call out type GEE (not GEF) require G10, not FR4. Do not substitute FR4 for GEE without drawing revision — the chemistry differs.

3. Halogen-free or RoHS-sensitive programs. If your program restricts brominated materials (EU REACH SVHC lists, IEC 61249-2-21 halogen-free definition), G10 is the halogen-compliant glass-epoxy laminate. FR4 contains ~10% bromine by laminate weight and does not meet halogen-free criteria (< 900 ppm Br+Cl).

4. Cost optimization in enclosed, non-code-regulated environments. For HV insulators inside a sealed enclosure where NEC, UL, or IEC flame ratings do not apply to the specific component, G10 saves 5–15% material cost over FR4.

Typical G10 Applications

• High-voltage standoffs and bushings in enclosed switchgear
• MIL-spec insulation panels and spacers (GEE designation)
• Structural electrical insulators where V-0 is not required
• Cryogenic insulation where low-temperature properties dominate (see cryogenic applications guide)
• Precision machined electrical fixtures and test jigs

When to Use FR4

FR4 is required when:

1. UL 94 V-0 is required by equipment standard. UL 508A (industrial control panels), UL 60950 (IT equipment), IEC 62368-1, NEC Article 110 all require V-0 for exposed live-part insulation. G10's HB rating does not comply.

2. PCB substrate (universally FR4 in standard PCB fabrication). IPC-4101 (the governing PCB laminate specification) requires V-0 for Class 1 boards and above. G10 was historically used as PCB substrate before flame retardancy requirements were codified — it is no longer appropriate for most PCB applications.

3. MIL-I-24768/14 GEF is called out. GEF requires the brominated formulation (FR4); GEE (G10) is a different type.

4. Customer specification explicitly requires FR4. Many OEM specifications, particularly in industrial, medical, and aerospace electronics, explicitly call out FR4 by grade name.

Typical FR4 Applications

• PCB substrates (dominant market application)
• Industrial control panel insulation (UL 508A-listed panels)
• Switchgear arc barriers and busbar supports
• Terminal boards in electrical equipment requiring V-0
• Aerospace circuit board assemblies (in conjunction with IPC qualification)

The PCB Special Case

For PCB applications, FR4 is the overwhelming industry standard. The IPC-4101 specification, which governs PCB laminates, requires:

• Dielectric material: meets Dk/Df requirements (4.0–5.0 Dk, < 0.035 Df at 1 MHz for standard FR4)
• Flammability: UL 94 V-0 (FR4 qualifies; G10 does not)
• Thermal performance: Tg > 130°C (standard FR4); Tg > 170°C (high-Tg FR4)

G10 should not be used as a PCB substrate in current-production electronics. Its HB rating creates a compliance gap, and its Dk/Df values are not controlled to the same tolerances as IPC-grade FR4.

Cost Comparison

FR4 typically costs 5–15% more than G10 at equivalent thickness and panel size, driven by:

• TBBPA adds cost to the resin system
• FR4 has tighter process control requirements (controlled Dk/Df, V-0 certification)
• Higher testing and documentation burden

For large structural insulation plates (1/4″ × 48″ × 96″ and above), the cost difference per panel may be $15–$50 — meaningful for programs ordering hundreds of panels. For small machined components, the per-piece cost difference is typically negligible relative to machining cost.

G10 vs FR4 — Summary Decision Chart

G11 and FR5 — The High-Temperature Siblings

When continuous service temperature exceeds 130°C, both G10 and standard FR4 are insufficient. The upgraded grades are:

• G11 = G10 equivalent with higher-Tg epoxy (Tg ~170°C); MIL type GEB; no flame retardant; HB rated
• FR5 = FR4 equivalent with higher-Tg epoxy; V-0 rated; higher cost

For applications above 130°C requiring V-0, specify FR5. For applications above 130°C where V-0 is not required, specify G11.

More related guides

Specifications

• ASTM D709 Explained
• Dielectric Strength Testing of Thermosets

Applications

• High-Voltage Standoff Materials
• Switchgear Insulation Materials
• Cryogenic Applications for Thermosets

Comparisons

• Glass-Epoxy vs Paper-Phenolic
• Electrical Insulation Laminates Explained