Microwave Tech - Choosing the right circuit board material to reduce the size of the RF circuit

Choosing the right circuit board material to reduce the size of the RF circuit

With the increasing demand for mobility and portability of electronic devices, the miniaturization of circuits is becoming more and more important. Before starting to design electronic products, choosing an appropriate circuit board material will help design smaller RF and microwave circuit board. For a given frequency range, the use of a circuit board material with a higher dielectric constant (Dk) usually makes the design size and structure of the circuit smaller. However, the use of plates with a higher Dk value will increase the insertion loss of the circuit and may also reduce the performance of other aspects of the circuit. At the same time, the Dk value of the circuit board material will also affect the index parameters of the circuit, such as radiation loss, dispersion, coupling, and so on.

For a given frequency, the wavelength in the medium will decrease with the increase of the circuit board material Dk, resulting in a circuit size designed on a circuit board material with a higher Dk value than a circuit with a lower Dk value The circuit size designed on the board material is smaller. In addition, circuit board materials with higher Dk values will also reduce the phase velocity of electromagnetic waves (EM) passing through these materials. The Dk of the circuit board material is usually a value measured in the z-axis direction (ie, the thickness direction) of the material at 10 GHz. The z-axis Dk value of commercial circuit board materials can be as high as 10 (or higher) or as low as 2 (compared to air with Dk equal to 1). But objectively speaking, but usually with a Dk value of 6 or higher, it can be considered as a high dielectric constant sheet.

The transmission line made of circuit board material with lower Dk value has faster phase velocity. For the miniaturization of phase-sensitive circuits (such as phased array antennas), the influence of Dk must be considered. In addition, a circuit board material with a higher Dk value exhibits greater dispersion than a circuit board material with a lower Dk value. Circuit board materials with higher Dk values are usually used in directional couplers and other circuits that require higher coupling coefficients.

As far as Dk is concerned, circuit board materials are usually anisotropic. Although the Dk values of the materials on the three axes are different, people are usually used to compare them based on the Dk value of the material in the z-axis direction. For materials with higher Dk values, the difference in Dk between the z-axis and the x-y plane of the circuit is often greater than that of materials with lower Dk values. The Dk values in all three dimensions of the circuit board material will jointly determine the performance of the transmission line (such as the microstrip line) made on the material. For many high-frequency circuit board, it is usually not necessary to consider the anisotropy of the circuit board material Dk, but anisotropy does bring some potential unknown problems, especially when the Dk value of the xy plane and the Dk on the z axis When the values differ greatly. This difference may cause unexpected problems in the edge-parallel coupling circuit, because the coupling is highly dependent on the Dk value on the x-y plane.

When trying to miniaturize the circuit, the easiest way to think of is to minimize the thickness of the circuit board material, but the thickness of the circuit board material will affect the performance of multiple indicators of the high-frequency circuit. Although the radiation loss of high-frequency circuits increases with frequency, thicker circuit board materials will also exhibit higher radiation losses than thinner circuit board materials with the same Dk value. For a given circuit layout and design, the choice of Dk will also affect the size of the radiation loss, because the radiation loss of the circuit board material with a higher Dk value is lower than that of the circuit board material with a lower Dk value.

For circuits that may cause resonance or stray interference (for example, between circuits in a multilayer PCB), it is beneficial to use a thinner circuit board material. The degree of resonance spurs usually depends on the type of transmission line in the circuit. For example, microstrip transmission lines are often more susceptible to resonance and propagation problems than other types of RF/microwave transmission lines (such as strip lines, coplanar waveguide CPW transmission lines). Thinner circuit board materials can help reduce the size of the PCB while limiting radiation loss and transmission line propagation problems, such as resonance and intermodulation. Common engineering experience is to use a circuit board material thinner than a quarter-wavelength of the highest operating frequency of the circuit. But a safer method is to choose a circuit board material that is thinner than one-eighth wavelength of the highest operating frequency in terms of thickness.

The line width of a transmission line (such as a microstrip line) will depend on the thickness of the circuit board material (such as a circuit laminate or prepreg material). Circuits with thicker substrates will widen the conductor width, which can reduce the conductor loss and insertion loss of the circuit. However, in this case, some electromagnetic wave propagation problems may occur. In order to select the thickness of the circuit board material suitable for high-frequency board design, usually the conductor width should also be less than one-eighth wavelength of the highest operating frequency. The Dk of the circuit board material plays an important role in determining the width of the transmission line conductor, because the same size conductor designed on the high-Dk circuit board material has a lower impedance than the same circuit on the low-Dk material. Therefore, in order to keep the circuit with a characteristic impedance of 50Ω, the circuit designed on the circuit board material with a higher Dk value will be narrower.

Wise choice

When designing a circuit using circuit board materials with different Dk values, many trade-offs need to be considered. The use of high-Dk circuit board materials can not only reduce the circuit size, but also realize high-performance miniaturized circuits by combining high-Dk and low-Dk circuit board materials. For example, a band-pass filter composed of a resonant unit, its size depends on the Dk of the circuit board material. Because of the spacing between each filter unit, the coupling strength in the circuit affected by the circuit board material Dk is determined. The circuit board material with high Dk provides stronger coupling and allows more space between the filter resonant units.

In order to verify the advantages of using circuit board materials with different Dk values (combining materials with different Dk values into a composite component), a band-pass filter was designed on the composite material of high Dk and low Dk circuit boards. The high-Dk material used in this filter is RT/duroid® 6010.2LM circuit laminate with a Dk value of 10.7; and the low-Dk material used is 2929 prepreg with a Dk value of 2.9. Both materials are from Rogers Corporation . Since circuit board materials with different Dk values will bring differences in circuit performance, computer simulations are needed to determine the required ratio of two different material thicknesses through modeling. This modeling method can help us design a perfect composite filter. The experimental results show that the size of the filter designed by composite materials not only maintains the size on a single high-Dk material, but also has improved electrical performance. . For example, the high-order harmonic resonance is significantly reduced, and the stop-band characteristics of the filter have also been significantly improved. Studies have shown that by using more than one circuit board material in the circuit, miniaturization of the circuit is often possible without sacrificing performance.