FR4 Properties & Datasheet — Mechanical, Thermal, Electrical
FR4 properties reflect its construction: woven E-glass fabric bonded with a brominated epoxy resin, cured under heat and pressure into a rigid thermoset laminate. Mechanical stiffness, a dielectric strength around 500 V/mil, low moisture absorption, and UL94 V-0 flame retardancy make FR4 the default choice for PCB substrates and structural electrical insulation. This datasheet covers all major property categories for standard FR4 and high-Tg FR4, with comparison data across grades.
At a glance:
- Dielectric strength: ~500 V/mil perpendicular (ASTM D149), the single most cited FR4 electrical spec
- Glass transition temperature (Tg): 130–140°C (standard FR4); 170°C+ (high-Tg FR4)
- Tensile strength (lengthwise): ~45,000 psi; anisotropic — crosswise values are 10–15% lower
- UL94 flame rating: V-0 across all FR4 grades
- Comparative Tracking Index (CTI): 175–249 V (Group IIIa per IEC 60112)
- Moisture absorption (24 hr): 0.10–0.20% — low enough for stable copper-trace registration
- Decomposition temperature (Td): 300–340°C
Mechanical Properties
FR4 is stiffer and stronger than most engineering thermoplastics in compression and bending, owing to the glass-fiber reinforcement. Values are anisotropic — lengthwise (warp) direction outperforms crosswise (fill) by 5–15% in tensile and flexural tests, and the z-axis (through-thickness) is the weakest direction for peel and interlaminar shear.
Tensile and Flexural Data
Hardness, Impact, and Physical
FR4's anisotropy is significant in z-axis applications. Through-thickness compressive strength is adequate for most mounting applications, but interlaminar peel (z-axis tensile) is the weakest link. Never spec FR4 as a structural element loaded primarily in through-thickness tension.
Thermal Properties
Thermal behavior is one of the most critical FR4 property categories, especially in PCB applications where lead-free reflow soldering exposes the laminate to 260°C peak temperatures.
Glass Transition Temperature (Tg)
The Tg is the single most important thermal property for PCB laminates. Below Tg, the epoxy matrix is in a glassy, rigid state and the z-axis CTE is modest. Above Tg, the matrix softens and z-axis CTE spikes dramatically, driving delamination risk during thermal cycling.
| FR4 Grade | Tg (DSC method) | Tg (TMA method) | Application |
|---|---|---|---|
| Standard FR4 | 130–140°C | 130–135°C | General-purpose PCBs, low-power |
| Mid-Tg FR4 | 150–160°C | 145–155°C | Lead-free compatible, some industrial |
| High-Tg FR4 | 170–180°C | 165–175°C | Lead-free, automotive, power electronics |
| Very High-Tg | >180°C | >175°C | High-reliability, server, aerospace PCBs |
Thermal Expansion Data
Z-axis CTE mismatch between FR4 (high z-axis CTE above Tg) and copper (low CTE ~17 ppm/°C) is the primary driver of barrel cracking in drilled vias during thermal cycling.
Electrical Properties
Electrical properties are the reason FR4 exists. Its combination of high dielectric strength, moderate dielectric constant, good surface and volume resistivity, and reasonable arc resistance satisfies the requirements of the vast majority of digital, RF, and power electronics applications up to several gigahertz.
Dielectric Strength
FR4 dielectric strength is typically cited as approximately 500 V/mil (about 20 kV/mm) measured perpendicular to the laminate plane (through-thickness), per ASTM D149 short-time test method. This is the most referenced FR4 electrical specification. Values vary with specimen thickness, conditioning, and test method — thicker specimens test at slightly lower V/mil due to the area-effect in dielectric breakdown statistics.
For a dedicated technical treatment of FR4 dielectric strength testing, standards, and influencing factors, see the FR4 dielectric strength page.
Comparative Tracking Index (CTI)
CTI measures resistance to electrical tracking (conductive carbon path formation on the surface under contaminated conditions). FR4 falls in IEC 60664-1 Group IIIa (CTI 175–249 V), which affects the required creepage distances in low-voltage equipment design per IEC 60664. For applications requiring Group II (CTI 400–599 V), FR4 does not qualify — glass-reinforced polyester (GPO-3) or polycarbonate may be more appropriate.
Electrical Properties vs Grade Comparison
Halogen-free FR4 achieves V-0 rating through phosphorus-nitrogen synergist systems rather than brominated epoxy. The dielectric constant is typically 0.1–0.3 higher than standard FR4, which matters for controlled-impedance traces above 1 GHz. Verify with your laminate supplier's specific product datasheet.
Chemical Resistance
FR4's epoxy resin matrix is resistant to most weak acids, bases, and organic solvents at room temperature. Aggressive chemicals and elevated temperatures can cause surface degradation, resin swelling, or delamination.
| Reagent | Resistance | Notes |
|---|---|---|
| Weak acids (HCl, H₂SO₄ <10%) | Good | Surface attack minimal at RT |
| Strong acids (concentrated H₂SO₄) | Poor | Epoxy attack and delamination |
| Weak alkalis (NaOH <10%) | Good | Acceptable for short exposure |
| Strong alkalis | Moderate | Saponification of epoxy at elevated temp |
| Aliphatic hydrocarbons | Excellent | Mineral spirits, hexane |
| Ketones (acetone, MEK) | Moderate | Surface softening; avoid prolonged contact |
| PCB solvents (IPA, flux removers) | Good | Standard cleaning chemistry |
| Water (ambient) | Good | Low absorption; avoid immersion above Tg |
| Steam / high-temp water | Poor | Accelerates delamination above 100°C |
FR4 Properties vs Competing Laminates
The most critical differentiator between FR4 and G10 is the UL94 flame rating. For structural insulation applications that do not require flame retardancy, G10 is typically less expensive. For the detailed side-by-side analysis, see the G10 vs FR4 comparison.
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