PCB Technology - How to select different values dielectric constant for circuit board data

How to select different values dielectric constant (DK) for circuit board data. With the rapid development of high-frequency circuits and the increasing demand for portability and mobility, the miniaturisation of circuit boards has received more and more attention. The selection of materials for circuit PCBs usually starts with the dielectric constant (Dk) of the circuit board material, which is at the top of the list of many parameters to be considered for the circuit board. Circuits with slow wave propagation structures are one of the circuit miniaturisation techniques, but choosing a PCB material with a higher Dk value is a more direct way to have a smaller circuit size. Of course, in some specific cases, it is not always possible to choose a PCB material with a high dielectric constant. In practice, the design should be able to meet the miniaturised dimensions of the electronic circuits, while keeping the Dk value of the PCB material from being too high.

The physical size of a high-frequency printed circuit board (PCB) is usually related to the wavelength of the signals travelling on the circuit, which decreases as the frequency of the signals increases. A quick comparison of the difference between high and low Dk values in the circuit board materials below shows the difference between two PCB materials with relatively low and high Dk values. Depending on the characteristics of the high-frequency data, this comparison is also important in that it can help engineers, to a certain extent, to quickly filter out the PCB materials that will probably satisfy their design requirements.

Difference between high and low Dk values in PCB materials

Taking the above two different types of PCB materials as an example, for two RF/microwave circuits operating at the same frequency, the circuits processed on the PCB material with the lower Dk value will have a longer wavelength. Since most RF/microwave circuit designs are based on the dimensional characteristics of the wavelength, PCB materials with higher Dk values produce smaller wavelengths at a given frequency, resulting in PCBs with smaller circuit sizes.

The difference in phase velocity between circuit board materials with different dielectric constant (DK) can also be a factor in the selection of circuit board data, especially if the realised circuit function is sensitive to parameters such as phase velocity, group delay or propagation delay. A board material with a lower Dk value has a faster phase speed than a board material with a higher Dk value. The magnitude of the Dk value of the selected circuit board material is especially important for circuits that are sensitive to delay accuracy (e.g., delay lines and high-speed digital circuits), which may be affected by transmission lines with different phase speeds or propagation delays.   

Typically, circuit board data has different Dk values in the three dimensions (x, y, z), and each of these values affects the performance of the processed circuit. However, circuit board data is usually compared with each other based on the Dk value in the z-axis (thickness) direction as a typical value. It is normal for circuit board data to vary in the x-axis, y-axis, and z-axis, and this characteristic is called anisotropy of the circuit board data. The anisotropy of board data with higher Dk values is usually greater than the anisotropy of board data with lower Dk values, and this difference is caused by the manufacturing pipeline of high frequency board data. Although the circuit board data have different Dk values in the x-, y-plane and z-axis, the Dk values in the x- and y-axis are relatively closer to each other.

The difference between the Dk values in the x-y plane and the z-axis Dk values is usually not that important for many types of high frequency circuits (e.g., single transmission lines or short cutoffs), but it does affect the performance of some circuits with coupling characteristics. In order to address the effects of high anisotropy on coupled circuits, some high Dk board materials have been designed to minimise the anisotropy of the data, taking this effect into account (for a comparison of the anisotropy of the high-frequency board materials below, the Dk values of the two board materials, TMM 10i and TMM 13i, are very close to each other in both the x-y direction and the z-axis, compared to the other materials). (the Dk values in the x-y direction and z-axis of the TMM 10i and TMM 13i materials in the table are very close to each other).

Comparison of High Frequency Circuit Board Material Anisotropy

The Dk value of the circuit board material also affects other high-frequency parameters, including radiation loss, dispersion, coupling and insertion loss. Radiation loss is caused by a number of factors, especially the design of the circuit itself and its operating frequency, and high-frequency circuits have greater radiation loss than low-frequency circuits. In addition, the thickness of the substrate material is also one of the factors affecting the radiation loss. For the same circuit, a circuit processed on a thicker substrate will show greater radiation loss compared to a thinner substrate. Secondly, the Dk value of the circuit board data also affects the radiation loss. Circuits designed with higher Dk values tend to have lower radiation loss than those with lower Dk values.

It is well known that microstrip transmission lines are dispersive, whereas high-frequency circuit transmission lines based on strip lines are non-dispersive. However, the circuit board data itself is dispersive, and the degree of dispersion depends on how much the Dk of the circuit board data varies with frequency. Typically, circuit board data with a higher Dk value will show a greater change in dispersion than circuit board material with a lower Dk value. However, there are exceptions and the formulation of the circuit board material can be changed if necessary to reduce the dispersion of the material.

Differences in coupling characteristics of DK circuits with different dielectric constants are often easily visible in coupled microstrip line bandpass filters. The coupling coefficients between different resonant units in a bandpass filter can be added by using data with high dielectric constants. The coupling between the resonant units of a microstrip line filter is through the x-y plane of the circuit board material, and the anisotropic value of the circuit board material is very important for this type of filter. Circuit board data with higher Dk values can be used for directional couplers and other circuits that require higher coupling coefficients. For example, high Dk circuit board data allows for stronger coupling in microstrip circuits because less energy is radiated from the circuit and circuit data due to tighter coupling between the circuit signal layer and the ground layer.

When two similar circuits are processed on two circuit boards with different Dk values, it can be seen that the circuits processed on the higher Dk boards generally have higher insertion losses than the lower Dk circuit boards. Although the material itself does not directly cause the difference in insertion loss, board data with a higher Dk value results in a smaller waveguide signal wavelength and reduced circuit characteristics. Higher Dk values narrow the signal path, and a narrower signal path results in greater conductor loss than a wider signal conductor with a lower Dk. This means that when circuits are processed on board data with higher Dk values, the insertion loss of the circuit is usually higher than on boards with lower Dk values.

When attempting to miniaturise circuits using high Dk board data, engineers should take into account the different effects of Dk variations on the RF performance of the circuit.