Why Do High-Frequency PCB Materials Have Low DK/Df Values?

We all know high-frequency PCBs should have low DK and Df. Otherwise, the signal performance will be affected. But do you know why DK and Df are two important parameters for high-frequency products to focus on? And how low should DK and Df for the high-frequency PCB materials be? This article makes an analysis and makes everything clear about DK and Df.

Part 1: What are DK and Df

DK is the abbreviation of dielectric constant, and it can be represented by ε and εr. DK's definition can be the ratio of the capacitance of a capacitor containing a dielectric to the capacitance of a vacuum capacitor of the same volume. Its calculation formula is below:

DK represents a dielectric's capacity to store electrical energy and hinder signal transmission. High-frequency/high-speed PCBs function as antennas to generate and receive high-frequency/high-speed signals and signal delay on the circuit board is not expected. So a high-frequency PCB should use material of low DK.

Besides, for signal stability, high-frequency PCB materials should also have low TCDK, which represents the dielectric's ability to maintain stable DK at changing temperatures.

Df is the abbreviation of dielectric loss factor, and it can be represented by tanδ. Df's definition can be: in an alternating electric field, due to the hysteresis effect of the dielectric conductivity and polarization, a phase difference occurs between the current phasor and voltage phasor flowing in the dielectric, and a phase angle is formed. And Df is the tangent value of this phase angle. From Df's definition, you can see that when a dielectric has high Dk, its hysteresis effect of conductivity and polarization is conspicuous, and as a result, signal loss is much. We use high-frequency materials to transmit high-frequency radiation with the smallest possible attenuation, so we definitely do not wish for much signal loss to occur. Besides, signal loss leads to energy loss and generates heat in the circuit board. That's why high-frequency/high-speed PCB materials should have low Df.

Part 2: How Do DK and Df Change at Different Frequencies, Temperatures, and Moisture Levels

High-frequency/high-speed PCBs' DK and Df are not fixed because they change at different frequencies, temperatures, and moisture levels. For signal stability, stable DK and Df are expected. In other words, we do not wish DK and Df to change much at different frequencies, temperatures, and moisture levels. So how do DK and Df change at different frequencies, temperatures, and moisture levels?

At different frequencies:

Generally, a dielectric's DK decreases in a small range as the signal frequency increases. For example, at normal temperatures, the normal FR4's DK value is 4.7 at 1MHz, 4.19 at 1GHz, and 4.15 at 10GHz. While a dielectrics' Df increases as the signal frequency increases.

At different temperatures:

Generally, a dielectric's DK and Df both increase when temperature increases, and Df increases more inconspicuously.

At different moisture levels:

The PCB's DK and Df both increase when the base materials absorb moisture. You should know that water's DK is 70, and moisture makes the circuit board's DK increase. Elevated moisture increases polarization, and Df also increases. That's why high-frequency PCB materials must have a low water absorption rate.

From the above, we can conclude that high-frequency PCB materials increase DK and Df when temperature and moisture levels rise, and when frequency increases, DK decreases and Df increases.

Part 3: What DK and Df Values Are Expected for High-Frequency/High-Speed PCBs

DK affects impedance line width, and Df affects attenuation of the signal in different frequency domains. High-frequency/high-speed PCBs should use base materials of stable and low DK and Df.

High-frequency/high-speed PCB base materials are generally made of epoxy and glass fiber (FR4 and PTFE), and some can be ceramic or ceramic hybrid. Epoxy has a smaller DK (3.6 at 1MHz) than glass fiber (at 1MHz, E-glass 6.6, NE-glass 4.6, D-glass 4.7, Q-glass 3.9), but its Df (0.025 at 1MHZ) is larger than glass fiber. The ratio of epoxy and glass fiber affects the DK and Df of high-frequency PCB. When you choose materials for high-frequency PCBs, remember this and make a good balance between DK and Df.

How low are DK and Df of high-frequency/high-speed PCBs? Different applications have different requirements for DK and Df of PCBs.

If the PCB transmits high-speed signals, the priority is low Df. But this is not absolute. High-speed signals may have high-frequency components, and in this case, the material should also focus on low DK.

If the PCB transmits high-frequency signals, the priority is low DK and TCDK. High-frequency products require low DK because the signal transmission speed at high frequencies is inversely proportional to the square root of DK.

For microwave applications, the high-frequency PCBs should have low Df and DK of 2 to 4 at 1MHz. For example, RO3003 has a DK of 3 and Df of 0.0013, and RO4350B has a DK of 3.66 and a Df of 0.0031.

In the past, most electrical applications have signal frequencies below 1GHz, and the traditional FR4 with high DK and Df meets the product requirements. As technology develops, applications' frequencies get higher, and products with frequencies above 2GHz are the mainstream. High-frequency/high-speed materials with low DK and Df will play a key role in PCB projects.

From the above, we can conclude that high-frequency/high-speed PCB materials should have both low DK and Df. When the material can't have both low DK and Df, for applications that transmit high-speed signals, the priority is low Df. For applications that transmit high-frequency signals, the priority is low DK and TCDK. But for high-frequency/high-speed projects, nothing is absolute. You should communicate comprehensively with the high-frequency/high-speed PCB manufacturer before having your PCBs manufactured.

Part 4: Finding a Reliable High-Frequency/High-Speed PCB Manufacturer

A reliable PCB manufacturer helps you solve issues in high-frequency/high-speed PCBs and make your products successful. PCBONLINE, a high-frequency/high-speed PCB manufacturer, provides one-stop solutions for high-frequency/high-speed projects. 

Here are the reasons to choose PCBONLINE as your high-frequency PCB solution specialist:

PCBONLOINE usually manufactures high-frequency PCBs of 4 to 8 layers. But if you want, PCBONLINE can manufacture up to 24-layer high-frequency PCBs for you. Online quote for high-frequency/high-speed PCB manufacturing now. Or you can send an email to PCBONNLINE or chat online for suggestions for choosing materials of low DK and Df.

Conclusion

This article tells what DK and Df are, the factors that cause their changes, and how to make a balance when the material can't have both low DK and Df. Finding a reliable high-frequency/high-speed PCB manufacturer is important. By choosing PCBONLINE as your high-frequency PCB manufacturer, you can make sure your PCBs have low DK and Df best suitable for the product application.