The dielectric constant (DK) value of FR4 material is between 4.2 and 4.8?

The dielectric constant (DK) value of FR4 material is between 4.2 and 4.8?

The dielectric constant (Dk or εᵣ) of FR4 typically ranges from 4.2 to 4.8 (measured at 1 GHz), which is a very core and important parameter.

To better apply this information in circuit design, here is a more detailed explanation and important supplement for you:

1. Why is it a range rather than a fixed value?

FR4 is a composite material made of glass fiber cloth and epoxy resin. Its Dk value is not a fixed constant and is mainly influenced by the following factors:

- Ratio of resin to glass cloth: Glass fiber has a higher Dk (about 6.0), while epoxy resin has a lower Dk (about 3.2). Different mixing ratios of the two in the finished product can lead to differences in the final Dk.

- Frequency: The Dk value of FR4 decreases slightly as the frequency increases, reflecting the dispersion characteristic of dielectric materials. For instance, a material with a Dk value of 4.5 at 1GHz may drop to around 4.3 at 10GHz.

- Suppliers and formulations: Different resin formulations, curing processes, and glass cloth types (such as 1080, 2116, 7628) from various board manufacturers result in variations in the Dk values of FR4 available on the market.

- Testing method: Different testing standards (such as IPC TM-650 2.5.5.9) may yield slightly different results.

2. Reference values under different scenarios

- Conventional design and preliminary calculation: To balance performance and cost, engineers often adopt Dk ≈ 4.4 as an intermediate value for theoretical calculations and simulations. This is a widely applicable and safe estimation.

- Low-speed digital circuits/general PCBs: In such applications where impedance control is not critical, a typical value of 4.5 - 4.7 can be directly used.

- High-speed digital circuits (such as DDR3/4, PCIe Gen3): More precise impedance control is required at this time. Suggestion:

    1. Request the official data sheet for the specific FR4 model you are using (such as Isola 370HR, Shengyi S1000-2) from your PCB board supplier, and obtain its Dk/Df test data at different frequencies.

    2. If it cannot be obtained, for conservatism, simulation and design can be carried out according to 4.2 - 4.3, because Dk will decrease at high frequencies, and designing with a lower value is safer.

- RF/Microwave Circuits (> 1GHz): It is generally not recommended to use standard FR4 in critical applications above 3-5GHz due to its high dissipation factor (Df) and poor Dk stability. If it is necessary to use, the actual measured Dk-frequency curve provided by the supplier must be used.

3. The crucial "companion" parameter: loss tangent (Df)

When discussing Dk, another crucial parameter must be mentioned - the tangent of the loss angle (Df, also known as the loss factor). It represents the dielectric loss of a material.

- Df value of FR4: Typically between 0.015 and 0.025 (at 1 GHz).

- Meaning: The larger the Df value, the greater the energy loss (converted into heat) of the signal during transmission, resulting in poorer signal integrity (such as slower edge transitions and amplitude attenuation).

- Impact on design: For high-speed long-line transmission (such as backplanes and long traces), even if the Dk value is accurate, an excessively high Df can severely limit the available bandwidth. In this case, it may be necessary to choose low-loss FR4 (such as FR4.2, Megtron 4 equivalent materials), with a Dk value ranging from 3.8 to 4.2 and a Df as low as 0.005.

4. Practical advice for engineers

1. Clarify application requirements: distinguish between general circuits and high-speed/RF circuits.

2. Communicate with PCB manufacturers: Before submitting boards, it is essential to confirm with the manufacturers the Dk/Df values (preferably with frequency curves) of the specific FR4 board material they use. The manufacturers may possess actual measurement data.

3. Design with margin: When calculating and simulating impedance line width, considering the fluctuation of Dk (such as ±0.2), a certain design margin can be set aside, or the board manufacturer is required to control impedance and fine-tune the line width.

4. Prototype verification: For critical designs, after creating prototypes, impedance is measured using tools such as TDR (Time Domain Reflectometer) to verify the accuracy of the design.

Summary:

It is a common knowledge that the Dk of FR4 is between 4.2 and 4.8. However, in rigorous engineering design, it should be treated as a variable. The best practice is to use the precise Dk and Df values of the corresponding model of FR4 obtained from the board supplier for your specific design frequency. For general applications, take 4.4-4.5 for estimation; for high-speed applications, precise data must be sought and conservative design should be carried out based on the lower value (such as 4.2).

Standard FR4: Located in the upper right region of the coordinate system (Dk ≈ 4.5, Df ≈ 0.02), representing average performance.

Low-loss FR4/FR4.2: Located in the lower left corner of FR4 (Dk ≈ 4.0, Df ≈ 0.01).

High-performance RF board material (such as Rogers RO4350B): located further down on the left (Dk ≈ 3.48, Df ≈ 0.0037), it exhibits superior performance.

Rogers plates (such as Rogers RT/duroid): closer to (Dk 2.2-10.2), with extremely low Df.