While designing PCB, considering the FR4 dielectric constant is a must to ensure stable signal transfer. It also has a greater influence on impedance control, power integrity, crosstalk, and EMI of the circuit board.
Let’s explore the ins and outs of the FR4 dielectric constant and find out how it influences PCB performance.
FR4 stands for ‘Flame Retardant 4’, a widely used material in PCBs. This is a type of epoxy-reinforced fiberglass laminate with high-insulating properties that can withstand increasing temperatures.
Moreover, FR4 is cheap and available in different thicknesses (from 0.4mm to 6mm), making it suitable for PCB fabrication. Additionally, they are affordable and provide greater stability.
Dielectric constant (Dk) refers to an insulator material’s capacity to store electrical energy in an electric field. It is defined as the proportion of a material’s permittivity to that of empty space. High Dk means more electric storage ability, but it often causes signal delay.
FR4 is ideal for PCB due to its stable Dk that maintains consistent signal integrity. However, the dielectric constant varies for variations in FR4 properties. For instance, high resin content in FR4 results in low Dk. Again, water absorption in the material can raise the Dk value. We will explore the impact of these factors in-depth in the upcoming discussions.
There is no constant Dk value for FR4 as it varies for the compositional and environmental factors of the material. For frequencies up to 1GHz, FR4 Dk typically ranges from 4.2-4.7. However, it differs for various FR4 laminate materials and compositions.
The common dielectric constant for different FR4 laminate materials is as follows:
The relation between frequency and dielectric constant is inversely proportional. That is, with the increase in frequency, the value of Dk decreases. For an increase of frequency from 1 MHz to 10 GHz, the dielectric constant can be reduced from 10% to 20%. This reduction greatly impacts applications where precise signal integrity is crucial.
Frequency ⇑ Dielectric Constant ⇓
As the temperature of the FR4 increases, changes occur in the physical properties of FR4. This eventually shifts the dielectric constant. This is why to maintain Dk consistency, maintaining thermal stability is crucial.
The dielectric constant of FR4 is greatly affected by the ratio of resin to fiberglass. High resin content decreases the dielectric constant of FR4.
Resin Content ⇑ Dielectric Constant ⇓
Filler mixing can shift the value of Dk. The presence of high filler content within the resin increases the dielectric constant. However, the range of Dk shift also depends on the type of filler material. So, you can adjust the Dk by adding suitable filler during manufacturing.
The Dk of glass weave is much higher than that of epoxy resin. So, for tight glass weave, the FR4 Dk increases. In contrast, loose glass weave reduces the dielectric constant, speeding up the signal propagation.
Usually, with the increases in layer thickness, the dielectric constant of FR4 shows more variation. In contrast, thinner layer offers more consistent Dk. For example, the thinner lamina of FR4 1080 commonly offers a more stable Dk value than the thicker layer of FR4 2116. However, this widely depends on the resin-glass fiber ratio, filler material, etc.
With the increase in moisture content, the dielectric constant of FR4 increases. This occurs due to higher Dk of water in contrast to FR4.
The material composition and manufacturing process of the FR4 material affects its dielectric constant. This brings a variation of Dk in different axes of the PCB.
In high Dk FR4, the electric field has a stronger coupling with the material. As a result, the increase of Dk reduces the speed of signal transmission. So, for high-frequency applications like computing, design the PCB keeping Dk low. This will facilitate boosting the signal propagation velocity.
However, the dielectric constant (Dk) itself does not affect whether a low-frequency circuit is “viable,” but it does affect the speed of signal transmission and the design of the impedance characteristics.
When designing any PCB, you must remember that the dielectric constant affects the placement of PCB capacitors. As a result, it directly influences the power-distributing network. Thus, by ensuring a stable FR4 Dk, you can maintain proper power delivery to the PCB components.
FR4 usually has a high Dk, which shows non-uniform values in high-frequency applications. So, while designing PCBs for high-frequency applications, choosing PCB material FR4 might not be a suitable choice in most cases. However, for some applications, you can use low-loss FR4 like Isola 370HR, Isola FR408HR, and Nelco N4000-13.
Variation in Dk causes impedance mismatch that leads to signal issues. So, to avoid this issue, maintaining a constant Dk is essential. This offers controlled impedance traces, ensuring consistent signal integrity.
The dielectric constant plays a greater role in the spacing and width of PCB traces. For instance, a lower Dk requires wider traces, meanwhile a higher Dk needs narrower traces. This way, while designing PCB, considering the Dk of FR4 is very important as it maintains the impedance consistency and stabilizes the signal flow.
The coupling between adjacent PCB traces is greatly affected by the dielectric constant. This eventually influences the crosstalk and electromagnetic interference (EMI).
A higher dielectric constant increases electric field interaction between traces, leading to more crosstalk. In contrast, a lower dielectric constant reduces unwanted signal coupling. Thus, it helps to maintain signal integrity and minimizes interference.
For controlling the FR4 dielectric constant, you must consider the factors affecting it. For instance, tailoring resin content, adjusting pressure & temperature, adding filler particles like ceramic, brominated flame retardants, etc.
The typical Dk for FR4 ranges from around 4.2-4.6. However, for applications requiring higher or lower Dk than this range, you can engineer them to your desired value.
For applications where you need more signal speed, lowering the Dk will help. Here is some method to low-down the dielectric constant of FR4:
Here are ways you can achieve high Dk in FR4:
Some international standards are set to ensure that the Dk of FR4 meets the specification. These are as follows:
This method measures the dielectric properties by applying a wide range of frequencies to the material. This allows you to track how the value of the dielectric constant shifts at varying frequencies. It is a reliable Dk testing method, especially at high frequency.
TDR deals with observing the reflection of PCB’s transmission lines. Using this technique of measuring Dk, you can get precise impedance values.
This method is used when you need a quick measurement of Dk. It determines the dielectric constant by analyzing how the material affects resonant frequency.
Engineers use different techniques to test the impact of FR4 Dk on the circuit board while designing a PCB.
With these tools, engineers can identify the performance of signal integrity, power distribution, and EMI behavior. Thus, they analyze and optimize circuit performance before fabrication.
At high frequencies (above 1 GHz), the dielectric constant in FR4 decreases. This results in signal integrity problems. So, FR4 is not the best choice for applications that require high-speed signal transmission.
Again, in RF circuits, matching precise impedance is essential to avoid signal reflections. But FR4 shows limitations here as its dielectric constant greatly varies with frequency.
Besides, the dielectric constant, there are many other properties that you should consider while using FR4 as your PCB material. These are as follows:
FR4 has exceptional dimensional stability. As a result, it maintains its shape and size at varying temperatures. Besides, it has great impact resistance, making FR4 PCB robust. A high tensile strength of 70,000 PSI also contributes to this robustness. Moreover, it is also flexible enough to withstand cracking.
The thermal conductivity of FR4 is poor, and it doesn’t offer good heat dispersion. As a result, overheating is an issue with PCBs made of FR4 material. To minimize this effect, heat sinks are used with FR4 PCB.
Tg determines the maximum operating temperature of FR4. It ranges from 130° C to 180° C. If the value decreases from this range, it impacts the mechanical property of FR4.
It determines the expansion of FR4 for temperature variation. The coefficient of thermal expansion (CTE) is, however, related to Glass transition temperature (Tg). CTE is low for temperatures below Tg and vice-versa.
To pick the right FR4 Dk value for your PCB, always consider your application. For general application, FR4 Dk of about 4.5 is suitable. However, for RF/microwave circuits, low Dk materials are needed to ensure fast signal propagation and stable signal integrity.
FR4 usually has a Dk range of 4.2-4.7, which makes it less ideal for high-frequency applications. For RF/microwave circuits, materials with a lower Dk (typically in the range of 2 to 4 at 10 GHz) are preferred, as they allow better performance at high frequencies. Therefore, if you’re designing a PCB for RF or microwave, it’s better to use
