G7 Phenolic Glass Silicone — Frequently Asked Questions

The questions below represent the most common technical and procurement issues engineers and buyers encounter when specifying or sourcing NEMA G7 glass silicone laminate. If your question isn't answered here, the G7 material hub links to detailed pages covering properties, grades, machining, specifications, and comparisons.

What is G7 laminate made of?

G7 is a woven E-glass fabric laminate bonded with silicone resin (organopolysiloxane). Multiple plies of glass cloth are impregnated with silicone resin, stacked, and consolidated under heat and pressure into rigid sheet, rod, or tube. The silicone resin is the defining characteristic — it is responsible for G7's superior high-temperature performance compared to epoxy-glass (G10 and FR4) or melamine-glass (G9) laminates. The glass cloth provides mechanical strength and stiffness; the silicone resin holds the plies together and provides the electrical insulation matrix. Color is natural cream/tan.

What is G7's continuous use temperature?

G7 is rated for continuous use at 425–485°F (218–252°C). The exact value depends on the loading condition: the lower end (425°F / 218°C) is appropriate for sustained mechanical load; the upper end (485°F / 252°C) for electrical insulation service without significant mechanical stress. MIL-I-24768/17 specifies a temperature index of 200°C (392°F) as a minimum. Short-term excursions above 500°F are survivable without structural collapse, but prolonged exposure beyond the continuous rating accelerates resin oxidation and should be avoided.

No other standard NEMA glass-fabric laminate grade — not G10 and FR4, not G9, not G11 — matches G7's continuous temperature rating. This is the primary justification for G7's higher cost.

Can G7 replace G10 and FR4 in my design?

It depends on why you're considering the swap. G7 is not a drop-in replacement for G10 and FR4 in most applications — the materials differ meaningfully:

• G7 is more expensive — typically 3–6× the cost of G10 and FR4 per pound
• G10 and FR4 has higher room-temperature flexural strength (~60,000 psi vs. ~50,000 psi for G7)
• G10 and FR4 has higher dielectric strength at room temperature (400+ V/mil vs. 350 V/mil for G7)
• G10 and FR4 carries UL 94 V-0; G7 does not

G7 is the correct choice when:

• Continuous temperature exceeds 300°F (G10 and FR4 softens and loses properties)
• Arc resistance above 120 seconds is required
• MIL-I-24768/17 is the drawing call-out (G10 and FR4 carries a different mil-spec: MIL-I-24768/27)

For applications below 300°F without a specific arc resistance requirement, G10 and FR4 is the more economical and practical choice. See the G10 and FR4 vs. G7 comparison for a detailed side-by-side.

What is the difference between NEMA G7 and MIL-I-24768/17?

Both designations cover glass silicone laminate, but they impose different quality and documentation requirements on the supplier:

If your drawing calls out MIL-I-24768/17, you must procure from a DLA-qualified supplier with full documentation. NEMA G7 commercial stock does not satisfy this requirement even if the material properties are identical. See the G7 grades page for more detail.

Is G7 available in flame-retardant grades?

No. There is no flame-retardant sub-grade of G7 that carries a UL 94 V-0 rating. Silicone resin chemistry does not lend itself to standard halogenated or phosphorus-based flame retardant systems that produce the self-extinguishing char required for UL 94 V-0. When G7 is exposed to direct flame, it produces a silica ash rather than a char; the ash is non-conductive but does not reliably self-extinguish under the UL 94 V-0 test protocol.

If your application requires both high-temperature insulation and UL 94 V-0 listing, you need a different material class (polyimide laminates carry V-0 with high Tg, but at significantly higher cost). For applications where UL 94 V-0 is required at lower temperatures, specify FR4.

What is G7's arc resistance, and why does it matter?

G7's arc resistance is greater than 180 seconds per ASTM D-495. This test applies a high-voltage arc to the surface of the laminate and measures how long before the surface carbonizes and becomes electrically conductive. A longer arc resistance time means the material resists surface tracking longer under arc exposure.

This matters in:

• High-voltage switchgear: a conductive arc track across a bus insulator can sustain a fault
• Missile fuzing: a surface track could create unintended continuity in a safe/arm circuit
• Transformer barriers: arc exposure during dielectric failure could track across an inter-winding barrier

G10 and FR4 achieves 60–120 seconds of arc resistance. G9 (melamine-glass) matches G7 at >180 seconds but only up to 350°F. The combination of >180-second arc resistance AND 425°F+ continuous temperature is unique to G7 among standard NEMA laminates.

How does G7 machine compared to G10 and FR4?

G7 and G10 and FR4 machine similarly because both are woven glass-fabric laminates with the same E-glass fiber reinforcement. Both are abrasive on tooling, require carbide or diamond-tipped tools, and produce a glass silica dust that requires respiratory protection.

G7 does not machine significantly differently from G10 and FR4 at the operational level. The key parameters are the same:

• Use carbide or PCD tooling; HSS dulls rapidly
• Drill with backing boards to prevent exit-face delamination
• Moderate feed rates to avoid fiber pullout
• Flood coolant or mist preferred for dust control; local exhaust ventilation required

The main practical difference is that G7 sheet is more expensive, so machining scrap has a higher cost impact. Closer attention to setup, tooling sharpness, and first-article verification pays off more on G7 than on G10 and FR4. See the complete G7 machining guide for speeds, feeds, and tooling specifications.

What forms is G7 available in?

G7 is commercially available in three forms:

• Sheet: 0.031 in. to 4.0 in. thick; standard panels up to 48×96 in.
• Rod: 0.25 in. to 6.0 in. diameter; 4-ft and 8-ft lengths
• Tube: Custom ID/OD programs; wall thickness from 0.062 in.

Rod and tube are less commonly stocked than sheet and may require 4–8 weeks lead time depending on dimensions. Very thick sheet (>2 in.) and non-standard panel sizes are also non-stock items. See the G7 specifications page for standard dimension tables and lead time guidance.

Can G7 be used in wet or humid environments?

Yes, with appropriate design consideration. G7's water absorption is <0.25% after 24-hour immersion, which is low for a glass-fabric laminate. The silicone resin matrix does not swell on water absorption and returns to near-baseline electrical properties on drying.

Key electrical properties under wet conditions (Condition C per NEMA LI-1):

• Surface resistivity: >10¹⁰ Ω (vs. >10¹² Ω dry)
• Volume resistivity: >10⁹ Ω·cm (vs. >10¹¹ Ω·cm dry)
• Dielectric strength reduction: approximately 10–15% compared to dry condition

For applications requiring sustained immersion (marine or washdown environments), the 10¹⁰ Ω surface resistivity under wet conditions is generally adequate for isolation voltages up to a few kV. For sustained high-voltage isolation in wet environments, design for the wet-condition values rather than dry-condition data.

Is G7 RoHS compliant?

Yes. G7 phenolic glass silicone contains no restricted hazardous substances under the EU RoHS Directive 2011/65/EU (as amended by 2015/863). The glass fiber and silicone resin construction does not incorporate lead, mercury, cadmium, hexavalent chromium, PBB, PBDE, or the four phthalates restricted under the 2015 amendment (DEHP, BBP, DBP, DIBP).

Confirm RoHS compliance with a supplier-provided declaration for your specific lot if regulatory documentation is required for your product's EU compliance file. The material class is inherently compliant, but lot-specific declarations may be required under some EU CE marking schemes.

What is G7's water absorption, and how does it affect dielectric performance?

G7 absorbs <0.25% water by weight after 24-hour immersion at room temperature (ASTM D-570). This is an intermediate value among laminate grades — better than some polyester-glass laminates (>0.50%), similar to standard G10 and FR4 (<0.10–0.20%).

The significance for electrical design is that absorbed moisture reduces surface and volume resistivity and can slightly increase dielectric constant. G7's silicone resin recovers fully on drying — the dielectric properties return to dry-condition values without the residual softening or delamination tendency seen in some epoxy laminates after wet conditioning. For power transformer spacer applications, this recovery behavior is important: transformers cycle thermally and may be exposed to condensation during cooldown in poorly sealed enclosures.

How does G7 compare to G9 for transformer applications?

Both G7 and G9 (phenolic-glass-melamine) are used in dry-type power transformer insulation. The choice depends entirely on the transformer's insulation class:

• Class F transformers (155°C continuous): G9 is adequate; G7 is over-specification
• Class H transformers (180°C continuous): G7 is the standard choice; G9 is marginal at 180°C
• Class N (220°C) or specialty transformers: G7 required; G9 is not rated

Both materials share the same dielectric strength (350 V/mil) and arc resistance (>180 sec), so there is no electrical advantage of G7 over G9 for Class F service — the temperature rating is the only differentiator. See the G7 vs. G9 full comparison for a complete property table.

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Applications

• Electrical Insulation
• Aerospace Interior

Industries

• Aerospace
• Electrical Power

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• FR4 Vs. G7 G7 Vs. G9