What Is FR4? The Material Inside Nearly Every Circuit Board
Open any consumer device — a phone, a laptop, a router, a TV — and somewhere inside it sits a green board studded with copper traces. That board is almost certainly made from FR4. The acronym comes up constantly in electronics manufacturing, but the material itself is oddly opaque: most engineers use it every day without fully understanding what it is, where it came from, or why it dominates rigid PCB production so thoroughly.
FR4 is a NEMA grade designation for a glass-reinforced epoxy laminate. “FR” stands for flame retardant, and the number “4” refers to the grade of the specification. The designation was created by the National Electrical Manufacturers Association (NEMA) in 1968, originally as a flame-retardant evolution of the older G-10 grade. Since then, FR4 has become the default substrate for rigid printed circuit boards worldwide.
Source: Wikipedia
Key Takeaways
- FR4 is a NEMA grade (LI 1-1998) glass-reinforced epoxy laminate used as the standard substrate for rigid PCBs since the 1970s
- It consists of woven fiberglass cloth bonded with a brominated epoxy resin, meeting the UL94V-0 flame retardancy standard
- Standard FR4 has a dielectric constant of 3.9–4.7, a dissipation factor of 0.017–0.020 at 1 MHz, and a glass transition temperature of 120–140°C
- FR4 is cost-effective and versatile, but its dielectric properties degrade above 1 GHz — Rogers laminates are the standard upgrade for RF and high-speed digital designs
- High-Tg FR4 variants (180–210°C) serve demanding thermal environments in automotive and industrial applications
What Is FR4 Made Of?
At its core, FR4 is a composite of two materials: woven fiberglass and epoxy resin.
The fiberglass provides structural stability — the woven cloth acts like the rebar in concrete, distributing mechanical stress across the board. The epoxy resin binds the fiberglass together and fills the spaces between the weave, creating a non-conductive matrix that isolates the copper traces on the surface. The combination is rigid, dimensionally stable, and electrically inert at low frequencies.
The epoxy used in standard FR4 is brominated — bromine atoms are embedded in the resin chemistry to inhibit combustion. This is what earns FR4 its UL94V-0 classification: the material will ignite and self-extinguish within 10 seconds when tested vertically, and will not drip flaming particles. This is not optional for most consumer and industrial electronics; it is a safety requirement.
Source: Sierra Circuits
The fiberglass weave itself is not uniform. Different manufacturers use different glass styles (E-glass is most common), and the weave pattern — plain weave versus twill weave — affects the board’s surface texture, resin fill, and even its dielectric properties slightly. In high-precision designs, this variation in the glass weave is one source of inconsistency in the dielectric constant.
Source: MCL PCB
FR4 Properties: A Practical Reference Table
Engineering specs for FR4 vary by manufacturer and specific resin system. The values below are representative of standard Isola-family materials — always check the manufacturer’s datasheet for the exact laminate you are using.
| Property | Value | Notes |
|---|---|---|
| Density | 1.850 g/cm³ | |
| Water absorption | < 0.10% | After 24 hours immersion |
| Dielectric constant (Dk) | 3.9–4.7 @ 1 GHz | Varies with resin content and glass weave |
| Dissipation factor (Df) | 0.017–0.020 @ 1 MHz | Increases with frequency |
| Flexural strength (lengthwise) | > 415 MPa | |
| Dielectric breakdown | > 50 kV | |
| Thermal conductivity | 0.29 W/(m·K) | Through-plane |
| Glass transition temperature (Tg) | 120–140°C (standard); 170–210°C (high-Tg) | Exceeding Tg causes the resin to soften irreversibly |
| Thermal decomposition (Td) | 340–360°C | For high-Tg grades |
| Flame rating | UL94V-0 | Self-extinguishing within 10 seconds |
| Operating temperature | 50–115°C (standard) |
Source: Wikipedia / NEMA LI 1-1998; Sierra Circuits
Two properties in this table deserve special attention: the dissipation factor (Df) and the glass transition temperature (Tg). Both are frequently cited in material selection decisions.
Dissipation Factor and Signal Loss
The dissipation factor — also called loss tangent (tan δ) — measures how much electrical energy the material dissipates as heat rather than transmitting as a signal. Standard FR4 has a Df of approximately 0.020 at 1 MHz. By comparison, high-frequency laminates like Rogers RO4350B sit around 0.004 — five times better.
This matters in practice. As signal frequencies push past 1 GHz — which is routine in 5G, Wi-Fi 6/7, and millimeter-wave applications — the Df of FR4 causes measurable signal attenuation, increased crosstalk, and timing errors. FR4 is not categorically forbidden in these applications, but engineers designing to tight impedance budgets at high frequencies generally select a low-loss laminate.
Source: MCL PCB
Glass Transition Temperature (Tg)
The glass transition temperature is the point at which the epoxy resin transitions from a rigid glassy state to a rubbery, softened state. For standard FR4, Tg is between 120°C and 140°C. Exceeding this temperature — which can happen during multiple reflow passes, high-current operations, or hot-swap environments — causes the board to soften. If the board cools back to room temperature, the resin does not fully recover its original mechanical properties.
High-Tg FR4 variants push Tg to 170°C or higher. Isola’s 370HR sits at 180°C; their FR408HR reaches 200°C. These materials are increasingly common in automotive electronics under the hood, where ambient temperatures can exceed 125°C. The cost premium over standard FR4 is typically 20–40%, which is still a fraction of the cost of a full Rogers laminate.
Source: Sierra Circuits
FR4 Thickness: What Actually Matters in Practice
Thickness selection is one of the first decisions in any PCB layout, yet it is frequently treated as an afterthought. In reality, thickness affects impedance, component compatibility, mechanical strength, and manufacturing yield.
The most common thicknesses for rigid PCBs: 0.8 mm (approximately 0.031″), 1.0 mm (0.040″), 1.2 mm (0.047″), and 1.6 mm (0.063″). The standard for most consumer boards is 1.6 mm. Source: MCL PCB
Here is what actually drives the choice:
Space constraints. Thinner boards reduce the device’s overall height. USB connectors, Bluetooth modules, and wearable electronics almost always use thinner stacks. Thinner boards also introduce more flex, which is either a feature (flexible interconnect sections) or a liability (solder joint cracks under thermal cycling).
Impedance control. Every layer in a multilayer board acts as a capacitor with adjacent layers. The dielectric thickness directly determines this capacitance, which feeds into transmission line impedance. Getting the impedance right — especially on high-speed differential pairs — requires precise control of the prepreg and core thickness, not just the total board thickness.
Component compatibility. Many connectors and switches have specific thickness tolerances. Edge connectors, for instance, have a mating interface that must match the board thickness precisely; a board that is even 0.1 mm off can cause intermittent connections or damage.
Manufacturing yield. Thinner, larger boards are more prone to warping during the reflow process. If your design includes heavy components — large MOSFETs, inductors, connectors — a thicker board provides the mechanical backing needed to survive assembly and thermal cycling.
FR4 Grades: FR1 Through FR5
The FR designation covers a family of PCB substrate grades. Most engineers encounter only FR4, but understanding the others helps contextualize where FR4 sits in the spectrum.
| Grade | Base | Resin | Flame Retardant | Notes |
|---|---|---|---|---|
| FR1 | Paper | Phenolic | No | Early consumer electronics; largely obsolete |
| FR2 | Paper + cotton | Phenolic | No | Used into the 1960s; replaced by FR4 |
| FR3 | Woven fiberglass | Epoxy | No | Better electrical stability; used in industrial equipment |
| **FR4** | **Woven fiberglass** | **Brominated epoxy** | **Yes (UL94V-0)** | **Dominant grade since 1970s; replaced G-10** |
| FR5 | Woven fiberglass | High-Tg epoxy | Yes | Aerospace and military; highest thermal performance |
| G-10 | Woven fiberglass | Epoxy | No | FR4's predecessor; used where flame retardancy is not required |
G-10 and FR4 are mechanically similar — same fiberglass and epoxy base — but G-10 lacks flame retardancy. Where FR4 self-extinguishes under UL94V-0 testing, G-10 continues burning. For most applications, FR4 replaced G-10 entirely.
Source: MCL PCB; Custom Materials
When Standard FR4 Is Not Enough
Standard FR4 handles the majority of PCB applications without issue. But there are situations where a different material makes more sense.
High-frequency RF and microwave designs. As discussed above, the Df and Dk instability of standard FR4 above 1 GHz makes it a poor choice for 5G transceivers, radar modules, and microwave backhaul. Rogers RO4350B (Df: 0.004, Dk: 6.15) is the standard cost-effective replacement. The dielectric constant of Rogers is also more stable across frequency and temperature — FR4’s Dk tolerance can be as high as 10% from supplier to supplier, whereas high-speed laminates are held to within 2%.
Extreme thermal environments. Automotive under-hood applications, industrial motor drives, and LED lighting can push ambient temperatures past the Tg of standard FR4. High-Tg FR4 (Tg ≥ 180°C) or polyimide-based laminates are the standard response. Polyimide can handle temperatures above 250°C, though at significantly higher cost and with more challenging manufacturing requirements.
High-voltage applications. FR4’s dielectric strength of approximately 20 MV/m (500 V/mil) handles most consumer and industrial voltages. At very high voltages — above 10 kV — the risk of conductive anodic filament (CAF) formation increases. CAF is a failure mode where ions migrate along the glass fiber bundles under voltage stress, creating a conductive path between layers. High-voltage designs may require specialized high-voltage laminates or conformal coating.
High-layer-count multilayer boards. As board complexity increases, the Z-axis expansion of standard FR4 during thermal cycling can cause barrel cracking in blind and buried vias. High-Tg FR4 and multifunctional epoxy systems reduce Z-axis CTE and improve long-term reliability.
FR4 and the Environment: A Realistic Assessment
FR4 is a thermoset plastic. Unlike thermoplastics such as ABS or HDPE, it cannot be melted and reprocessed. Once the epoxy cross-links during the initial lamination, the chemical reaction is irreversible — the material is set for life.
This has practical consequences. FR4 in the form of manufacturing scrap and end-of-life boards typically goes to landfill. Efforts to recycle FR4 involve grinding it for filler material or using it as aggregate in construction, but true closed-loop recycling is not currently economical at scale. The brominated flame retardants in standard FR4 also raise questions under RoHS and REACH regulations, particularly in jurisdictions with strict chemical registration requirements.
What this means in practice: if your product needs to meet environmental regulations, you should verify that your specific FR4 laminate is RoHS-compliant and check whether the brominated chemistry creates any supply chain flags. Most major laminate manufacturers (Isola, Nelco, Ventec) offer RoHS-compliant FR4 grades.
Source: Custom Materials
Common FR4 Defects and How IPC Standards Classify Them
If you have a board fabricated to an IPC standard, the defects your board can have — and whether they pass or fail — are governed by IPC-A-600 and IPC-6012. Here are the most common base material conditions:
Weave exposure occurs when the fiberglass cloth is visible through gaps in the resin on the surface. It is permissible on all IPC classes as long as the spacing between remaining conductors meets the minimum design rule. It is most common on boards with very thin prepreg.
Measling appears as discrete white spots in the base material — small resin-starved areas where the glass bundles are visible. Measling develops when resin application during lamination is suboptimal. It is permissible on most boards but can reduce electrical and mechanical performance if widespread.
Crazing is a connected network of white spots, indicating separation between glass bundle junctions. On Class 2 and 3 boards, the spacing between crazing patterns must not exceed 50% of the spacing between adjacent conductors.
Delamination — any planar separation between plies inside the base material, or between the base material and copper foil — is the most serious of these conditions. A delamination that spans more than 1% of the board’s total area or that is closer than 25% of the conductor spacing to an adjacent trace typically fails Class 2 and 3 criteria.
Source: Sierra Circuits
Conclusion
FR4 is not glamorous. It is a fiberglass-and-epoxy composite that has been the default PCB substrate for over 50 years — not because anyone mandated it, but because it works. The balance of electrical performance, mechanical strength, flame retardancy, and cost is difficult to beat, and the manufacturing ecosystem around FR4 is mature, deep, and globally distributed.
Understanding FR4 — its dielectric properties, its Tg limits, where its performance falls short, and what the IPC standards actually say about its defects — does not require a materials science degree. It requires reading the manufacturer’s datasheet and asking one simple question: is my signal frequency, my operating temperature, or my voltage stress pushing against the edges of what this material can handle?
If the answer is no, FR4 is almost certainly the right choice. If the answer is yes, you know exactly which direction to look next — and you have the vocabulary to ask the right questions of your laminate supplier.
Frequently Asked Questions
What does FR4 stand for?
FR4 stands for Flame Retardant grade 4 — a NEMA (National Electrical Manufacturers Association) designation for glass-reinforced epoxy laminate. “FR” indicates the material meets UL94V-0 flame retardancy standards, and “4” refers to the specific grade of the specification established in NEMA LI 1-1998.
What is FR4 made of?
FR4 is a composite of woven fiberglass cloth bonded with a brominated epoxy resin. The fiberglass provides structural rigidity and tensile strength; the epoxy fills the spaces between the weave and acts as the electrical insulator. The bromine in the resin chemistry is what gives FR4 its self-extinguishing flame retardant properties.
What is the dielectric constant of FR4?
The dielectric constant (Dk) of standard FR4 ranges from 3.9 to 4.7 at 1 GHz, with 4.4 as a commonly cited typical value (per Isola 370HR datasheet). The exact value varies based on the glass weave style, resin content, and board thickness. This Dk variation — tolerances up to 10% between suppliers — is one reason FR4 is not ideal for controlled-impedance designs at high frequencies.
What is the Tg of FR4?
Standard FR4 has a glass transition temperature (Tg) between 120°C and 140°C. High-Tg variants range from 170°C to 210°C. Once a board’s internal temperature exceeds its Tg during assembly or operation, the epoxy resin softens and does not fully recover its original mechanical properties upon cooling.
What is the difference between FR4 and G-10?
G-10 is FR4’s predecessor — the same woven fiberglass and epoxy construction, but without the brominated flame retardant chemistry. G-10 is not self-extinguishing and will continue burning if the ignition source is removed. FR4 replaced G-10 in virtually all consumer and industrial applications where flame retardancy is a safety requirement.
When should you not use FR4?
Avoid standard FR4 for designs operating above 1 GHz where signal integrity is critical (use Rogers or similar low-loss laminates), for applications where the operating temperature exceeds the material’s Tg (use high-Tg FR4 or polyimide), and for high-voltage designs where CAF (conductive anodic filament) formation is a concern (use specialized high-voltage laminates).
Is FR4 recyclable?
FR4 is a thermoset plastic — the epoxy cross-links irreversibly during initial lamination and cannot be melted and reprocessed. End-of-life FR4 boards typically go to landfill or are used as construction aggregate. The brominated flame retardants in standard FR4 also require RoHS compliance verification for products sold in regulated markets.
