Printed circuit boards are an integral part of any electrical or electronic system, which house many electronic circuits, sensors, ICs and passive components; to act as a system or sub system. The purpose of printed circuit boards is to house and connect required circuit components together in an integrated manner and provide them with stable connections with minimum physical (stray) wiring connections with miniaturized / optimum routing of interconnecting wires. This not only provides physical strength to the circuit interconnects but also help in optimizing the size of circuit.
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Introduction to RF PCB
Printed circuit boards required for RF circuits need special attention of the material used for PCB substrate as well as thickness of substrate sheet. The wire tracks that carry RF signals, also called as RF transmission lines are not just ordinary wire track used for interconnection. Their dimensions are carefully calculated and used in design which is also a function of RF frequency for which the circuit is being designed.
Why RF PCBs are required?
RF PCBs are required for designing RF circuits, that are required to be printed on circuit boards with either using only passive transmissions lines like RF filters, RF splitters/combiners, RF couplers, attenuators; or by using active circuit elements which 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.
Parameters to look for while choosing RF PCB substrate material
Following parameters are most commonly required to be looked after while choosing a RF PCB substrate material for a design application.
Dielectric Constant of PCB substrate material
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
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 provide lesser electrical dimensions and hence result in 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. Most commonly used transmission line is microstrip, which has ground plane on only one side and hence is Quasi TEM transmission line. The effective wavelength of microstrip transmission line is given by 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). However for much higher RF frequencies, lower dielectric constant materials are preferred as they ensure significant lengths to make circuit dimensions realizable while keeping in view physical tolerances of the PCB milling tool / process.
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.
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
- 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 table below:-
PCB Substrate Material
Nelco 4000-13 EP
Nelco 4000-13 EP SI
PCB Substrate Thickness
One might think that substrate thickness is meant only for physical strength of PCB. It may be true for low frequency analog or digital circuit, but this may not be the case for RF PCBs where substrate thickness also effect the RF transmission lines dimensions for a certain impedance value.
During design when thicker PCB substrate is used, it result in wider transmission lines as compared to thinner substrate; for a particular characteristic impedance. As an example to microstrip transmission line, thicker microstrip transmission lines tend to loosely bound electric flux as compared to thinner one. So, it means substrate thickness also has an effect on the conductor losses as well. As we know, RF signal tend to flow more on skin of the metallic conductor due to skin effect. When the microstrip (or any other transmission line) has a wider track width, it may result in lesser conductor losses (Ohmic losses) as compared to thinner track width transmission line due to larger surface area (to reduce ohmic losses). These losses are quite considerable and need to be properly investigated, especially if your design application is regarding high power handling.
Substrate thickness and via hole
There is also another side of picture to it. Via holes are often used in transmission line circuits for providing ground connection to a particular point in circuit layout. For high frequency circuits, it is often observed that via hole might behave non- ideally to give stray inductance value to circuit, which is highly undesirable. It is quite possible, when you are designing for high frequency circuits using thicker substrate. Thicker substrate tend to provide lengthy via holes, hence may result in inductive effect. In order 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.
In PCBs, cladding is the layer of copper covering the substrate material. One side of cladding is usually etched in order to form RF tracks or transmission line. Copper is used, because it is an excellent electric conductor. In RF PCB, it is usually used on both sides of the PCB substrate sheet. Usually clad thickness is in micron or micro-meters (µm), usually in the range of 17 µm to 70 µm. It should be chosen with care for simulation and design process. For high power application, designer my need a thicker cladding for RF tracks to minimize conductor losses. Conductor losses increase with increasing frequency. However, increasing the cladding beyond a certain thickness does not add any value to circuit design as most of the RF current travels on the outer surface of copper track due to skin effect.
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
The coefficient of thermal expansion of PCB material describes how the dimension of the PCB changes with a change in temperature. It measures the fractional change in dimensions per degree change in temperature at a constant pressure, such that lower coefficients describe lower tendency for alteration in dimension. From an RF circuit point of view, change in dimension (especially Z dimension) may lead to variation in PCB thickness, which can cause change in characteristic impedance of transmission line tracks to produce mismatching in circuit, which is detrimental for RF circuit design.
Rogers TMM series PCB substrates are usually recommended for designs that are subject to high temperatures during PCB fabrication process or for RF power applications where heat is produced due to dissipation. Due to their electrical and mechanical stability, TMM high frequency laminates are ideal for high reliability strip-line and micro-strip applications. Their Coefficient of thermal expansion matches to copper allowing for high reliability of plated through via holes.