An RF PCB is a type of electronic device that uses radio frequency (RF) waves to receive or transfer data. It consists of two major components: an RF transceiver and an antenna. In the following paragraphs, you will learn different types of RF PCB, parameters when choosing an appropriate RF PCB materials, how to choose from them, and how to design RF PCB.
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What is RF PCB?
An RF printed circuit board is one that runs at frequencies more than 100 MHz. Their dimensions are carefully estimated and employed in the circuit design based on the RF frequency for which it is intended.
RF system is required for designing RF circuits, which are required to be printed on circuit boards either using only passive transmission lines like RF filters, RF splitters/combiners, RF couplers, and attenuators; or by using active circuit elements that involve the use of RF transistors or RF integrated circuits in combination of passive transmission lines. Both active and passive circuits are necessary for designing an RF system.
Different Types of RF PCB
First of all, because the minimum number of layers for RF PCBs is two. The most prevalent types of them are double-sided and Multilayered RF PCBs.
The Microwave PCBs are the most often used printed circuit boards with radio frequencies over 2 GHz. There are also RF Flexible PCBs, combining the benefits of both RF and flexible PCBs. Then, RF Ceramic PCBs have high thermal conductivity, often used in Bluetooth devices, routers, and microwave satellite communications.
Finally, as the name suggests, Hybrid RF PCBs have both RF and non-RF components on the same board. They can be used in telecommunication, IoT, and aerospace.
Parameters to Consider When Choosing RF PCB Substrate Material
The materials used for the PCB substrate and the thickness of the substrate sheets must be carefully considered when designing printed circuit boards for RF circuits. RF signal alignments, also known as RF transmission lines, are not your typical connectivity alignments. The following parameters are most often considered when selecting RF PCB substrate materials for a design application.
Dielectric Constant
It is one of the most important parameter for PCB material selection. The dielectric constant of a material is a measure of its ability to store electrical energy. It is an expression of the extent to which a material holds or concentrates electric flux.
Its value determines the length of transmission line and also effects the width of transmission line for a certain impedance value. The wavelength of a signal inside the dielectric medium is given by
Where
C = speed of light in vaccum
f = frequency of signal
ɛr = dielectric constant of material
This is the simplest relation of wavelength propagation inside a dielectric medium. As depicted by the above equation, we can see that higher dielectric constant material provides lesser electrical dimensions and hence results in a miniaturized design.
All transmission lines and waveguides follow a similar, but not exact behavior. Each transmission line has its own geometry and the exact relation depends on its geometry. The most commonly used transmission line is microstrip, which has a ground plane on only one side and hence is Quasi TEM transmission line. The effective wavelength of the microstrip transmission line is given by the following relations:
Generally higher dielectric constant material PCB substrate materials are much suitable for lower frequency designs as they tend to lessen the physical length of the transmission line, while maintaining electrical length (in terms of wavelength or fraction).
Similarly physical track width of the transmission line is also effected by the dielectric constant of the PCB material for a certain characteristic impedance. Higher dielectric constant of a PCB material tend to concentrate electric flux to achieve a certain value of characteristic impedance. Hence transmission line width tend to be lesser in higher dielectric constant materials.
Loss Tangent
Dielectric loss tangent also denoted as “tan(δ)” is a measure of signal attenuation as the RF signal propagates down the transmission line. RF PCB material datasheet commonly denote this loss as dissipation factor (Df). Loss tangent is the result of microwave signal absorption by the dielectric material and depends on the material’s structure and glass-resin composition. Usually minimum value of loss tangent is desirable, which ensures the transmitted signal get through to its destination with minimum losses in signal strength.
Attenuation=2.3 x f x tan(δ) x √εr
Where:
- f is the frequency in GHz
- tan(δ) is the dimensionless loss tangent
- εr is the relative dielectric constant of the material
Ideally, selecting the lowest loss material is the best choice. However, lower loss comes at an increased cost tradeoff.
The table given below shows some of the RF PCB materials from ELE PCB catalog that you can use for your design. Their dielectric constant and loss tangent are given in the accordion table below:
Materials’ Dielectric Constant and Loss Tangent Table
| RF PCB Substrate Material | Dielectric Constant | Loss tangent |
| FR-4 | 4.4 | 0.016 |
| Rogers RO4003 | 3.38 | 0.0027 |
| Rogers RO4350B | 3.48 | 0.0037 |
| Rogers RO3003 | 3 | 0.0013 |
| Rogers RO5880 | 2.2 | 0.0009 |
| RT/duroid 6035HTC | 3.5 | 0.0013 |
| RT/duroid 6006 | 6.15 | 0.0027 |
| RT/duroid 6010.2LM | 10.2 | 0.0023 |
| Nelco 4000-6 | 4.12 | 0.012 |
| Nelco 4000-13 EP | 3.7 | 0.009 |
| Nelco 4000-13 EP SI | 3.2 | 0.008 |
Substrate Thickness
Substrate thickness affects not only the physical strength of PCB, but also the RF transmission lines dimensions and losses. Thicker substrates lead to wider transmission lines with lower conductor losses, due to skin effect and larger surface area. These factors are important for high power RF applications.
Via Holes
There is also another side of the picture to it. Via holes are often used in transmission line circuits to provide a ground connection to a particular point in the circuit layout. For high-frequency circuits, it is often observed that via hole might behave non-ideally to give stray inductance value to the circuit, which is highly undesirable. It is quite possible when you are designing for high-frequency circuits using a thicker substrate. Thicker substrates tend to provide lengthy via holes, hence may result in an inductive effect. To mitigate this issue, it is often recommended to use multiple via holes in parallel (in a wide ground plane) to minimize ground inductance.
Track Width and Its Dependency on Frequency
It is general rule of thumb in RF circuit design, that transmission lines behave like normal circuit wire (and not like RF transmission line), when circuit dimensions are less than λ/10. In RF transmission lines, it must be noted that RF track width and PCB height should always be less than λ/10, as RF signal may behave quite differently. So, substrate thickness should be chosen accordingly.
Cladding
Cladding is the copper layer on the substrate of PCBs. It is etched to form RF tracks or lines. Copper is a good conductor. RF PCBs have cladding on both sides. Clad thickness is 17-70 µm. It affects simulation and design. High-power RF needs thicker cladding to reduce losses. More cladding does not help, as RF current flows on the surface due to the skin effect.
Thermal Conductivity
Thermal conductivity is the ability of a given material to conduct or transfer heat energy. Thermal conductivity of RF PCB substrate is an important parameter, if the target design is an RF power application, like Power amplifier or high power switch, in which the dissipated heat is required to be carried away from circuit components. ELE PCB recommends the designers to use special RF PCB substrates like Rogers RT/Duroid 6035HTC or Rogers TC series for applications requiring high thermal conductivity.
Coefficient of Thermal Expansion
PCB material’s coefficient of thermal expansion shows how PCB size changes with temperature. It is the fractional change in size per degree change in temperature at constant pressure. Lower coefficients mean less change in size. For RF circuits, size change (especially in Z direction) can affect PCB thickness and transmission line impedance, causing mismatch and poor performance.
Rogers TMM series PCB substrates are good for high temperature or high power RF designs, as they are stable and reliable. Their coefficient of thermal expansion is similar to copper, which helps with via holes.
Comparison of the 5 Material Used in RF PCB
| Materials | Dielectric Constant | Cost | Suitable for |
| FR-4 | 4.4 | cost-effective | low-frequency RF applications |
| PTFE | 2.2-2.8 | more expensive than FR-4 material | all high-frequency RF applications |
| Rogers Laminate | Typical 2-10 | more expansive than FR-4 | RF and microwave PCB applications |
| Ceramic-Based Materials | Alumina: 3-9, AN: 9-10.4 | high cost | RF application |
| Liquid Crystal Polymer (LCP) | 2.9-3.1 | more expensive than FR-4 or PTFE | high-frequency RF applications |
# PTFE is one of the most often used materials in RF PCB design, and we suggest it highly.
Steps to Design Your RF PCB
Step 1: Gather Design Rules and Requirements – First, be aware of the frequency range, cost, noise specifications, bandwidth, power levels, PCB size, and any other technical requirements.
Step 2: Create RF PCB Layout – PCB layout design is critical to consider early in the stage because impedance relates to how much and how quickly electricity may move down a trace. The stackup affects how the mechanical engineer designs and fits the PCB into the device.
Step 3: Design RF PCB Stackup – The three most significant points in RF PCB design are isolation, decoupling and layer arrangement (at least 4-layer PCB stackup with internal power and ground planes).
Step 4: Place Components – You need to consider the frequency range, gain, noise figure, power levels, and temperature resistance of each component. Indicating that specific parts cannot be put close to other if they cause electrical noise in the circuit.
Separate the analog and digital parts, and provide room for wiring and tuners around the devices. Then, keep RF input and output ports accessible, and group frequently interacting devices together.
Step 5: Match Impedance – The following techniques can be used to realize impedance matching:
1. Transformer impedance matching technique: impedance matching is achieved by selecting a suitable transformer design.
2. Attenuator impedance matching technique: By connecting a certain number of resistors or attenuators in series in the circuit, the impedance size of the circuit is changed.
3. Filter impedance matching technology: by changing the parameters of the filter to change the impedance size of the circuit to achieve the impedance matching of the input and output ports.
Step 6: Ground – For the ground plane, use continuous copper fills, and keep it close to RF signals and traces.
Step 7: Simulate and Verify – Verify impedance, losses, and frequency response. Then, test its functioning in different conditions.
Step 8: Add Labels and Identifiers – Finally, make any labels, IDs, markers, or reference designators for the layout.
Conclusion
Choosing an adequate RF PCB needs consideration of parameters and performance, material selection, design and production issues, and other variables.
If you are looking to create microwave or RF PCBs and have manufacturing questions or need a price, Your best RF PCB manufacturer in China, ELE, can assist you with over 10 years broad engineering experience.
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