FR4 Dielectric Constant Chart

Frequency characteristics of standard FR4 PCB materials

FR4 Dielectric Constant Chart

Use this reference chart to estimate how standard FR4 dielectric constant and dissipation factor shift with frequency before final impedance and material decisions.

When designing high-speed or RF PCBs with FR4, it's vital to understand that the Dielectric Constant (Dk, εr) and Dissipation Factor (Df, loss tangent) are not static values. They change with frequency, typically with Dk decreasing and Df increasing as frequency rises. Use this chart as a general reference for standard FR4 materials (Tg 130-140°C).

Frequency	Dielectric Constant (Dk)	Dissipation Factor (Df)
1 MHz	4.7	0.015
10 MHz	4.6	0.018
100 MHz	4.5	0.020
1 GHz	4.35	0.022
2.5 GHz	4.30	0.023
5 GHz	4.25	0.025
10 GHz	4.20	0.028
20 GHz	4.15	0.030

These values are directional engineering reference data for standard FR4 only. Actual dielectric behavior depends on laminate family, resin system, glass style, copper profile, processing, and test method. Final controlled-impedance work should use exact material data from your PCB manufacturer.

When to use this tool

Impedance planning

Check whether your assumed dielectric constant is directionally reasonable before estimating controlled impedance and trace geometry.

High-speed and RF material screening

Use it to understand where standard FR4 starts to become a weak fit for loss-sensitive or higher-frequency routing.

Manufacturer discussion

Bring this into stackup discussions when you need to compare standard FR4 assumptions with real laminate data and fabrication capability.

Practical notes

• Standard FR4 can work for many digital and general-purpose boards, but it is not automatically the right material for every impedance-critical design.

• Dk shifts affect propagation speed and impedance; Df shifts affect insertion loss and signal attenuation.

• At higher frequencies, simplified assumptions become less reliable and material selection becomes more important.

Why Dk Matters?

The dielectric constant (Dk) determines the signal propagation speed and the characteristic impedance of the traces. At higher frequencies, a lower Dk results in faster signal speeds. If you assume a constant Dk of 4.2 but it drops to 4.0 at your operating frequency, your impedance calculations will be off, potentially causing signal reflections.

Why Df Matters?

The dissipation factor (Df) represents the signal loss as heat within the dielectric material. Standard FR4 has a relatively high Df (~0.02), making it unsuitable for ultra-high frequency applications (e.g., >5GHz long-range signals) where 'low-loss' materials (like Rogers or Megtron) with Df < 0.005 are preferred.

Related resources & next steps

Online Impedance Calculator

Use dielectric assumptions from this reference to estimate single-ended impedance targets.

PCB Trace Width Calculator

Cross-check geometry decisions when signal and current requirements need to coexist on the same board.

PCB Manufacturing Service

Move from generic FR4 assumptions to stackup, laminate, and fabrication-ready review.

Send your target impedance, frequency range, and material questions to engineering.

Frequently Asked Questions

Is FR4 suitable for 5G antenna design?

Generally no. For 5G mmWave frequencies, the signal loss (Df) of standard FR4 is too high and Dk stability is poor. You should consider specialized high-frequency laminates. However, for sub-6GHz bands, high-performance FR4 may still be acceptable depending on trace length.

Does temperature affect Dk?

Yes. Dk typically increases with temperature. If your device operates in extreme thermal environments, consistent impedance requires materials with a stable 'Thermal Coefficient of Dielectric Constant' (TcDk).

Need help choosing laminate assumptions for impedance-critical PCB work?

We can help review FR4 suitability, dielectric targets, frequency risk, and manufacturing stackup decisions before you lock routing or controlled impedance requirements.