FR4 Dielectric Constant Chart

Frequency-dependent properties of Standard FR4 PCB Material

When designing high-speed or RF PCBs using FR4, it is crucial to understand that the Dielectric Constant (Dk, εr) and Dissipation Factor (Df, Loss Tangent) are not static values. They vary with frequency, typically decreasing in Dk while increasing in Df 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

Why Does Dk Matter?

The Dielectric Constant (Dk) determines the signal propagation speed and the characteristic impedance of traces. <br/><br/> At higher frequencies, a lower Dk results in faster signal speed. 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 reflection.

Why Does Df Matter?

Dissipation Factor (Df) represents the signal loss as heat within the dielectric material. <br/><br/> Standard FR4 has a relatively high Df (~0.02), making it unsuitable for very high-frequency applications (e.g., >5GHz long-haul signals) where "Low Loss" materials (like Rogers or Megtron) with Df < 0.005 are preferred.

Frequently Asked Questions

Is FR4 suitable for 5G antenna designs?

Generally, no. For 5G mmWave frequencies, the signal loss (Df) in 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 might 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).