FR4 PCB material guide
FR4 PCB Material Guide: Grades, Tg, Td, Dk & Selection
FR-4 is a class of glass-epoxy laminates, not one product. Here is how standard, mid-Tg, high-Tg, halogen-free and low-loss FR-4 differ, and which one your board needs.
- Tg range
- 130–180 °C
- Dk range
- 4.0–4.8
- Df @ 1 GHz
- ≈0.02
- Flame retardant
- UL 94 V-0
What FR-4 is
What is an FR4 PCB?
An FR4 PCB is built on FR-4, a flame-retardant (UL 94 V-0) composite of woven fiberglass cloth and epoxy resin. “FR” stands for flame retardant and “4” is the NEMA grade. FR-4 is a class of materials, not a single product, so Tg, Td, Dk, CAF resistance and cost vary widely between formulations.
The glass gives mechanical strength and dimensional stability; the epoxy is the dielectric, bonds the stack during lamination and provides flame retardance through brominated or, in halogen-free grades, phosphorus-based additives. FR-4 is used in the large majority of rigid PCBs worldwide because it balances cost, strength, processability and electrical performance.
Glass transition temperature
Glass transition temperature (Tg) and FR-4 CTE.
Tg is where the epoxy changes from rigid and glassy to soft and rubbery. Above Tg the resin expands rapidly in the z-axis, stressing plated through-holes and via barrels. Standard FR-4 with Tg 130–140 °C was fine for tin-lead assembly (peak about 220 °C). Lead-free reflow peaks of 245–260 °C push the board well above standard Tg, so every reflow cycle adds more z-axis stress.
Mid-Tg (150–170 °C) and high-Tg (170–180 °C, such as Isola 370HR) grades give progressively more margin. For any board that will see lead-free assembly, mid-Tg or high-Tg FR-4 is recommended, and high-Tg is the norm for thick or high-layer-count boards.
Decomposition temperature
Thermal decomposition temperature (Td).
Td is where the resin starts to decompose irreversibly, losing weight and releasing volatiles. It is always well above Tg: about 300 °C for standard FR-4 and 340 °C for high-performance grades such as 370HR. Td matters for anything that holds the board above Tg for a while, including wave soldering, selective soldering, multiple reflows and rework. A higher Td gives more margin before delamination.
Dielectric properties
Dielectric properties and high-frequency performance.
FR-4 Dk ranges from about 4.0 to 4.8 depending on resin content, glass style and frequency, a much wider spread than the ±0.05 of an RF laminate. For controlled impedance, use the Dk of the actual laminate and glass style; assuming a generic 4.2 can cause 5–10% impedance error.
Standard FR-4 Df is about 0.02 at 1 GHz and rises with frequency. That is acceptable for digital designs up to roughly 1–2 GHz, depending on trace length and loss budget. For 10 Gbps+ interfaces such as PCIe Gen 4/5, 25GbE or 56G PAM4, use low-loss laminates such as Megtron 6 or Isola FR408HR.
Common FR-4 grades
FR-4 grades compared.
| Grade | Tg | Typical examples | Use |
|---|---|---|---|
| Standard | 130–140 °C | Shengyi S1141, Kingboard KB-6160 | Low cost, tin-lead or simple lead-free |
| Mid-Tg | 150–170 °C | ITEQ IT-158, Shengyi S1150G | Lead-free with standard reliability |
| High-Tg | 170–180 °C | Shengyi S1000-2M, ITEQ IT-180A, Isola 370HR | Automotive, industrial, thick multilayer |
| Low-loss FR-4 class | ≈ 180 °C+ | Isola FR408HR, Megtron 4 | Moderate high-speed digital |
| Halogen-free | Varies | Phosphorus-based systems | Environmental compliance |
FR-4 brands
Shengyi and ITEQ FR-4 laminates.
Shengyi (SYTECH) and ITEQ are our default general-purpose FR-4 suppliers and cover most rigid, multilayer and HDI builds. Shengyi S1141 is a standard-Tg workhorse; S1000-2M and S1000H are high-Tg, CAF-resistant grades for lead-free and automotive work. ITEQ IT-180A is a widely specified high-Tg FR-4, and IT-968 / IT-988G are low-loss grades for high-speed digital. If your drawing names a specific laminate we build on it; if it allows an equivalent, we propose one at quote and only use it with your approval.
| Brand | Grade | Class | Typical use |
|---|---|---|---|
| Shengyi | S1141 | Standard Tg (≈ 140 °C) | Consumer and general electronics |
| Shengyi | S1000-2M / S1000H | High Tg (≈ 180 °C) | Automotive, industrial, thick multilayer |
| ITEQ | IT-180A | High Tg (≈ 175 °C) | Lead-free multilayer and HDI |
| ITEQ | IT-968 / IT-988G | Low loss | High-speed digital, networking |
Choosing
Choosing the right FR-4 grade.
- Prototype or cost-sensitive, tin-lead: standard Tg
- Lead-free with standard reliability: mid-Tg
- Automotive, industrial, 8+ layers or many reflows: high-Tg with CAF resistance
- Above ~5 Gbps: check channel loss; consider low-loss FR-4 or Megtron 6
- RF above ~1 GHz: Rogers or a hybrid stack-up
- Tight impedance: name the exact laminate, not just “FR-4”
Thickness
Common FR-4 board thicknesses.
| Finished thickness | Typical use |
|---|---|
| 0.4–0.8 mm | Thin modules, wearables |
| 1.0–1.2 mm | Compact consumer boards |
| 1.6 mm (0.062 in) | Industry default |
| 2.0–2.4 mm | Industrial, connectors, heavier parts |
| 3.2 mm+ | Backplanes, high layer count |
Halogen-free
Halogen-free FR-4.
Halogen-free FR-4 replaces brominated flame retardants with phosphorus-based systems. It meets the same UL 94 V-0 rating and supports environmental programs that restrict halogens. It is often slightly stiffer and can have different Dk and drilling behaviour, so treat it as a named material in impedance and DFM planning rather than a drop-in substitute.
Specifying
How to specify FR-4 on a drawing.
“FR-4” alone leaves the fabricator free to use any standard-Tg laminate. If your board needs more, write it down: minimum Tg, minimum Td, CAF resistance, halogen-free status, IPC-4101 slash sheet where you use one, and whether equivalent substitutes are acceptable. For controlled impedance, name the laminate family and glass style, or let us propose a stack-up with the actual Dk values.
- Minimum Tg (e.g. ≥ 170 °C)
- Minimum Td (e.g. ≥ 340 °C)
- CAF-resistant requirement
- Halogen-free if needed
- IPC-4101 slash sheet (optional)
- Substitution allowed: yes / no
Limits
Where FR-4 falls short.
FR-4’s wide Dk spread and Df around 0.02 cause high loss at multi-GHz frequencies, and standard grades are limited to about 130 °C operation. For those designs use a high frequency or high speed laminate, polyimide for high temperature or flex, or a hybrid stack-up that keeps FR-4 on the non-critical layers.
FAQ
Material questions.
What does FR-4 stand for?
Flame Retardant, NEMA grade 4: a glass-reinforced epoxy laminate rated UL 94 V-0.
What is the Dk of FR-4?
About 4.0–4.8 depending on resin content, glass style and frequency.
What is the Tg of FR-4?
Standard grades 130–140 °C, mid-Tg 150–170 °C and high-Tg 170–180 °C.
When should I use high-Tg FR-4?
For lead-free assembly, thick or high-layer-count boards, multiple reflows and harsh environments.
What are the limitations of FR-4?
High dielectric loss above a few GHz, Dk variation with frequency and limited operating temperature for standard grades.
Can FR-4 be used for RF?
Below about 1–2 GHz with short traces, or for digital layers in a hybrid stack-up; above that use Rogers or PTFE.
What is the FR-4 CTE?
FR-4 CTE in-plane (X/Y) CTE is typically about 12–16 ppm/°C. Z-axis CTE is about 40–60 ppm/°C below Tg and rises several times above Tg, which is why high-Tg grades protect plated holes better.
Which FR-4 brands do you use?
Shengyi and ITEQ by default, plus Isola, Panasonic and other specified laminates. Equivalent grades are only used with your approval.
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