FR4 FAQ — Common Questions About FR4 Material Answered

Answers to the most frequently asked questions about FR4 laminate, covering material identity, properties, grades, machining, regulatory status, and procurement.

What is FR4 material?

FR4 is a NEMA-grade glass-epoxy thermoset laminate with a UL94 V-0 flame-retardant rating. It is constructed from layers of woven E-glass fabric impregnated with a brominated epoxy resin, cured under heat and pressure. The "FR" prefix stands for Flame Retardant. FR4 is the dominant substrate material for printed circuit boards (PCBs) worldwide and is also widely used as a structural electrical insulator in switchgear, transformers, and electronic enclosures.

For a complete overview, see the FR4 material hub.

Is FR4 the same as G10?

No. FR4 and G10 are both glass-epoxy laminates but are distinct materials. The critical difference is flame retardancy: FR4 contains a brominated flame-retardant additive (typically TBBPA) and achieves UL94 V-0 rating. G10 uses a standard epoxy resin without brominated FR additives and carries no UL94 V-0 flame rating.

Mechanically and electrically, the two materials are nearly identical — same tensile strength, dielectric strength (~500 V/mil), and Tg range. The V-0 rating is what separates them for applications governed by UL, IEC, or NEC standards requiring flame-retardant insulation. For a full side-by-side breakdown, see G10 vs FR4.

What is the dielectric strength of FR4?

FR4 dielectric strength is approximately 500 V/mil (about 20 kV/mm), measured perpendicular to the laminate (through-thickness) using the short-time test method per ASTM D149. This is the most commonly cited FR4 electrical specification.

The actual value varies with laminate thickness, conditioning, and test method: thicker specimens test slightly lower on a V/mil basis (statistical area effect in dielectric breakdown), and step-by-step test method results are typically 400–480 V/mil for standard FR4.

For a detailed technical discussion of testing methods, influential variables, and data for different grades, see FR4 dielectric strength.

What is the Tg of FR4?

The glass transition temperature (Tg) of standard FR4 is 130–140°C, measured by DSC (differential scanning calorimetry). High-Tg FR4 grades achieve Tg of 150–185°C through modified epoxy resin chemistry.

The Tg is critical for PCB applications because the z-axis CTE of FR4 spikes dramatically above Tg, driving via barrel cracking and delamination during thermal cycling. For lead-free solder assembly (peak reflow 260°C), high-Tg FR4 (Tg ≥ 150°C) is recommended for multilayer boards.

See FR4 grades for the complete Tg breakdown by grade and IPC-4101 sub-class.

What is the UL94 rating for FR4?

FR4 achieves UL94 V-0 — the highest standard UL94 rating for vertical flame testing. V-0 requires:

Flame extinguishes within 10 seconds after each of two 10-second ignition applications
No flaming drips
Total flaming time across five specimens: ≤ 50 seconds

This V-0 rating applies to all standard, high-Tg, and halogen-free FR4 grades. It is achieved through the brominated flame-retardant package in standard FR4 and through phosphorus-nitrogen systems in halogen-free FR4.

What is IPC-4101 and how does it relate to FR4?

IPC-4101 is the primary procurement standard for PCB base materials. It defines:

Sub-classes for different resin systems, Tg ranges, and copper-clad configurations
Minimum property requirements (Tg, Td, peel strength, dielectric properties)
Accepted test methods via IPC-TM-650

For FR4 specifically:

Sub-class /21: Unclad, standard Tg, structural use
Sub-class /24: Copper-clad, standard Tg, PCB use
Sub-class /41–/42: High-Tg (Tg ≥ 150°C)
Sub-class /99: High-Tg, halogen-free

See FR4 specifications for the full sub-class table.

Is FR4 food-safe or FDA approved?

No. FR4 is not FDA approved for food contact and cannot be made food-grade. The brominated flame-retardant system (TBBPA) in standard FR4 is not a permitted food-contact substance under FDA 21 CFR. Halogen-free FR4 eliminates TBBPA but retains an epoxy resin matrix, which is also not cleared for food contact in bulk laminate form under FDA 21 CFR 177.

If you need a structural electrical insulator that is also FDA-compliant, PEEK (21 CFR 177.2415) or PTFE (21 CFR 177.1550) are the only practical options that combine V-0 flame rating with FDA food-contact clearance.

See FR4 FDA and food-grade compliance for the complete regulatory analysis.

What is halogen-free FR4 and when do I need it?

Halogen-free FR4 achieves UL94 V-0 using a phosphorus-nitrogen flame-retardant system rather than TBBPA, resulting in bromine and chlorine content below 900 ppm each (per IEC 61249-2-21). Properties are similar to standard FR4 with modest differences: dielectric constant slightly higher (4.6–4.8 vs 4.5), moisture absorption slightly higher.

Specify halogen-free FR4 when:

OEM procurement specifications mandate halogen-free laminates (common in Japanese and EU consumer electronics)
REACH compliance documentation for SVHC substances (TBBPA) creates supply chain burden
Your product's end-of-life sustainability profile is a specification requirement

For most North American commercial and industrial applications, standard FR4 remains the specification — unless explicitly required otherwise.

Can FR4 be machined, drilled, and routed?

Yes. FR4 machines on standard CNC mills, routers, and drill presses using carbide tooling. The primary machining consideration is abrasiveness: the glass fiber content dulls carbide tooling significantly faster than machining thermoplastics or soft metals. Key parameters:

Drilling: 200–400 SFM, parabolic-flute carbide drills, air blast chip clearing
Routing: 18,000–24,000 RPM, up-cut spiral carbide bits
V-scoring: standard 30° groove for PCB panel singulation

Brominated glass-fiber dust requires mandatory dust collection and respiratory protection. For the complete parameter guide, see FR4 machining.

What is the difference between standard FR4 and high-Tg FR4 for lead-free assembly?

The critical metric for lead-free compatibility is how the laminate performs during and after solder reflow at 260°C peak — significantly above the 130–140°C Tg of standard FR4.

Standard FR4 can survive single-pass lead-free reflow for simple 2-layer boards, but the brief excursion above Tg causes z-axis expansion that stresses drilled vias. In multilayer boards (4+ layers), this risk accumulates with repeated thermal cycles, increasing the probability of via barrel cracking over the product's service life.

High-Tg FR4 (Tg ≥ 150°C, IPC-4101 /41 or /42) reduces — but does not eliminate — the delta between peak reflow temperature and Tg, providing a wider margin for thermal cycling reliability. For high-reliability multilayer boards requiring 10+ year service life, high-Tg FR4 is the standard specification.

What sizes does FR4 sheet and plate come in?

Standard structural FR4 panel sizes are 36" × 48" and 48" × 96" from most distributors. Cut-to-size pieces are widely available. Thickness range for sheet: 0.031" to 0.500"; plate: 0.500" to 2.0"+.

For PCB-grade copper-clad laminate, standard laminate sizes used by PCB fabricators are typically 18" × 24" and 24" × 36" (panel-cut from larger master sheets). Most end-users purchase finished PCBs rather than raw CCL.

See the FR4 sheet and FR4 plate pages for complete size and stocking information.

How does FR4 compare to cotton phenolic laminates?

FR4 outperforms cotton phenolic in every electrical property category: dielectric strength (~500 V/mil vs 250–350 V/mil), moisture resistance (0.10–0.20% vs 0.80–1.50% water absorption), and flame rating (V-0 vs HB at best). Cotton phenolic is less expensive and machines more easily with less tool wear.

Cotton phenolic remains the choice for applications with moderate electrical requirements and cost sensitivity — particularly at low voltage where its lower dielectric strength is acceptable, and where the material's excellent machinability matters more than flame retardancy. See FR4 comparisons for the full laminate comparison table.

Is FR4 RoHS compliant?

Standard FR4 containing TBBPA has been subject to RoHS review. TBBPA falls under a specific RoHS Annex IV exemption covering brominated flame retardants in electronic equipment — check the current exemption status and expiry date for your specific equipment category, as exemptions are reviewed periodically.

Halogen-free FR4 is RoHS compliant without requiring exemptions. If RoHS documentation without exemption reliance is required, specify halogen-free FR4. See FR4 grades for details on halogen-free options.

// JSON-LD FAQ Schema — for site engineer implementation
{
  "@context": "https://schema.org",
  "@type": "FAQPage",
  "mainEntity": [
    {
      "@type": "Question",
      "name": "What is FR4 material?",
      "acceptedAnswer": {
        "@type": "Answer",
        "text": "FR4 is a NEMA-grade glass-epoxy thermoset laminate with UL94 V-0 flame-retardant rating, constructed from woven E-glass fabric and brominated epoxy resin. It is the dominant PCB substrate material and is used as structural electrical insulation in switchgear and electronic enclosures."
      }
    },
    {
      "@type": "Question",
      "name": "Is FR4 the same as G10?",
      "acceptedAnswer": {
        "@type": "Answer",
        "text": "No. Both are glass-epoxy laminates, but FR4 contains a brominated flame-retardant additive and achieves UL94 V-0 rating. G10 uses standard epoxy without brominated FR additives and carries no UL94 V-0 flame rating."
      }
    },
    {
      "@type": "Question",
      "name": "What is the dielectric strength of FR4?",
      "acceptedAnswer": {
        "@type": "Answer",
        "text": "FR4 dielectric strength is approximately 500 V/mil (20 kV/mm) measured perpendicular to the laminate per ASTM D149 short-time test method."
      }
    },
    {
      "@type": "Question",
      "name": "Is FR4 food safe or FDA approved?",
      "acceptedAnswer": {
        "@type": "Answer",
        "text": "No. FR4 is not FDA approved for food contact. The brominated flame-retardant system (TBBPA) and epoxy resin matrix are not cleared under FDA 21 CFR for direct food contact. No standard FR4 grade is food-grade."
      }
    },
    {
      "@type": "Question",
      "name": "What is the Tg of FR4?",
      "acceptedAnswer": {
        "@type": "Answer",
        "text": "Standard FR4 has a glass transition temperature (Tg) of 130–140°C. High-Tg FR4 grades achieve 150–185°C through modified epoxy resin chemistry."
      }
    }
  ]
}

Get a quote on FR4 sheet, plate, rod, or tube

Request a Quote →

More related guides

Product forms

Compare to related materials

Applications — FR4 Applications — PCB, Switchgear & Enclosures