FR4 vs G7: Flame-Rated Epoxy Glass vs Silicone Glass for High-Temperature Duty

FR4 and G7 are glass-reinforced thermoset laminates serving overlapping but distinct electrical insulation markets. FR4 dominates electronics, PCBs, and panel-building work where a UL94 V-0 flame rating is the entry requirement. G7 serves the high-temperature insulation market — motors, transformers, and power electronics where Class H (180°C) continuous service is required and flame rating may be secondary to thermal endurance. The key differences: FR4 carries UL94 V-0; G7 does not. G7 survives 180°C continuously; FR4 tops out at 130°C. FR4 has higher room-temperature mechanical strength; G7 maintains structural properties better at elevated temperature. These are complementary materials for different application domains — not direct substitutes.

TL;DR

• Flame rating: FR4 = UL94 V-0 (mandatory for PCBs, UL-listed panels); G7 = no V-0 rating.
• Temperature: G7 rated 180°C (Class H); FR4 rated 130°C (Class B) — G7 wins decisively in high-temperature duty.
• Resin: FR4 = brominated epoxy; G7 = silicone — different resin systems, not interchangeable.
• Strength: FR4 has higher room-temperature tensile and flexural strength; G7 retains properties better at elevated temperature.
• Moisture: Both absorb minimal moisture; FR4 marginally lower (<0.10% vs <0.15%).
• Cost: G7 is significantly more expensive than FR4 — specify only when Class H rating is genuinely required.

Chemistry & Origin

FR4 achieves its UL94 V-0 flame rating through brominated epoxy resin — specifically tetrabromobisphenol A (TBBPA) or equivalent bound into the epoxy matrix during laminate manufacture. The woven E-glass reinforcement is identical to that in G7. The brominated resin self-extinguishes on ignition by releasing HBr, which interrupts the combustion chain reaction.

G7 uses a silicone resin — a polyorganosiloxane system with an Si-O backbone. This backbone has substantially higher bond energy than the C-C and C-O bonds in epoxy, enabling stable operation at 180°C. Silicone resins do not achieve the same V-0 classification as brominated epoxy in standard laminate form, which is the fundamental reason G7 does not carry a UL94 V-0 rating despite being a glass-reinforced thermoset like FR4.

Mechanical Properties

FR4 is the stronger material at room temperature, benefiting from the highly crosslinked epoxy network. Tensile strength advantage is approximately 15–25%. This strength advantage, combined with V-0 rating, explains FR4's dominance in electronics and panel building where structural and compliance requirements coexist.

At elevated temperatures, G7's silicone resin provides better property retention. Epoxy begins to soften above its glass transition temperature (typically 130–150°C for standard FR4); silicone's Si-O backbone remains stable to 180°C and beyond. For motor coil supports, bus-bar insulators in furnace switchgear, or any load-bearing insulator exposed to sustained high temperature, G7's elevated-temperature structural performance is the determining factor.

Electrical Properties

Both materials are high-quality electrical insulators. FR4's dry dielectric strength (>40 kV/mm) is marginally higher than G7's (~35 kV/mm) at ambient temperature. G7 retains its dielectric properties closer to its service temperature ceiling — a critical distinction for insulation that must perform reliably in Class H equipment over decades of service.

G7's silicone resin provides superior resistance to corona discharge and ozone attack — degradation mechanisms that are relevant in high-voltage equipment where corona forms near sharp edges or in dry air. This makes G7 preferred for some high-voltage insulation applications even below its thermal limit.

Thermal Properties

The 50°C service temperature gap defines the application domains:

• FR4 at 130°C covers the vast majority of electronics, instrumentation, and panel-building applications.
• G7 at 180°C covers Class H motors and generators, high-temperature transformers, oven and furnace controls, and aerospace electrical components.

High-Tg FR4 variants (typically 150°C or 170°C Tg) exist and are used in lead-free soldering processes — but even these do not reach G7's 180°C service rating and do not gain a flame-rating advantage over standard FR4.

Chemical Resistance

Both materials resist common dielectric fluids, oils, and dilute aqueous solutions. G7's silicone resin provides somewhat better resistance to ozone and oxidation at elevated temperatures. FR4's brominated resin does not introduce significant chemical resistance differences from standard epoxy for most industrial fluids.

Machinability

Both FR4 and G7 are glass-reinforced thermosets that require carbide tooling. Woven E-glass is among the most abrasive reinforcements encountered in plastics machining — HSS tooling dulls extremely quickly on both materials. Dust management is critical: glass-fiber dust from either material is a respiratory hazard, and machining should be performed with positive air blast or flood coolant and appropriate respiratory protection.

G7's silicone resin is somewhat more flexible than FR4's epoxy, which can make it slightly more prone to delamination in thin sections if feed rates are too aggressive. Both materials drill cleanly with carbide-tipped drills at moderate speed; slow entry and exit rates prevent delamination at hole edges. Thread tapping is possible in both materials with appropriate coated taps and cutting fluid.

Cost & Availability

FR4 is a commodity laminate available in wide size ranges from numerous distributors at competitive prices. G7 is a specialty material — significantly more expensive (often 3–5× FR4 pricing) and available from specialty thermoset distributors with potentially longer lead times. The cost premium is justified only when the application genuinely requires Class H thermal performance.

When to Choose FR4 vs G7

Choose FR4 when:

• UL94 V-0, IPC-4101, or equivalent flame compliance is required.
• Service temperature is below 130°C.
• Maximum mechanical strength in the laminate is required.
• Cost and availability are important factors.

Choose G7 when:

• Class H (180°C) insulation rating is required by design specification.
• The application is in high-temperature motors, generators, or transformers.
• Thermal cycling or sustained high-temperature exposure would degrade FR4.
• Corona and ozone resistance at high voltage is a design concern.

More related guides

Applications

• Electrical Insulation
• Motor and Transformer Components

Industries

• Electronics
• Aerospace

Other comparisons

• G10 vs G7
• FR4 vs G9
• G10 vs FR4

Spec sheets

• FR4 hub
• G10 hub