A Radiofrequency Printed Circuit Board is a highly complex yet fast-growing part of the PCB manufacturing industry. In the PCB industry, a circuit board that operates above 100MHz is classified as an RF PCB. The bar, however, stops at 2GHz. Adding more, any board that works on frequencies above 2GHz is named a microwave board. RF PCB boards have components that use radio frequencies to operate. An RF PCB is a particular class of the PCB manufacturing industry and is estimated to be the fastest-growing sector.
Let’s delve deeper to find out the foremost techniques and steps of designing RF PCB.
What is a microwave PCB board?
Any RF circuit board operating at a frequency greater than 2GHz is categorized as a microwave PCB. The radiofrequency at which they function is the critical distinction between RF PCB and Microwave PCB.
Any application that calls for receiving and transmitting radio signals employs RF PCB and uses microwave PCB for communication signals. Radar stations and cell phones are two examples of RF PCB typical applications.
Different types of materials used in making an RF PCB BOARD
- Ceramic-filled PTFE composites
They are exceptionally mechanically and electrically stable. Regardless of the dielectric constant (Dk) chosen, Rogers RO3000 series circuit materials offer consistent mechanical properties, enabling multi-layer board designs that use various dielectric constant materials without experiencing warpage or reliability issues. Unlike its hydrocarbon-based competitors, the Taconic RF family has a low dissipation factor and high thermal conductivity potential, preventing oxidation, yellowing, upward dielectric constant, and dissipation factor drift.
- Woven Glass Reinforced PTFE laminates
Compared to chopped fiber-reinforced PTFE composites, they are more dimensionally stable and made of very lightweight woven fiberglass. Low dissipation factors in materials like those found in the Taconic TL products make them ideal for millimeter-wave antennas and 77 GHz radar applications.
- Hydrocarbon ceramic laminates
Due to their reduced loss and more streamlined features than conventional PTFE materials, manufacturers use them in microwave and millimeter-wave frequency designs. The Rogers RO4000 products have above-average thermal conductivity and a wide range of DK values (2.55–6.15). (.6-.8).
- Thermoset microwave laminates
These offer good mechanical dependability, a copper-matched coefficient of thermal expansion, and a low thermal coefficient of dielectric constant (Dk). Great-frequency laminates made by Rogers TMM are perfect for strip-line and micro-strip applications requiring high reliability.
Some other types of materials
- Ultra-low Loss, Highly Heat-Resistant, Halogen Free Megtron 6
The material of these circuit boards with their hydrocarbon resin-based MEGTRON 6 is suited for High-Density Interconnect (HDI) and high-speed (over 3 GHz) constructions because of their high glass transition temperature (Tg) and low expansion ratio.
- Filled PTFE (random glass or ceramic)
Composite laminates, such as the Rogers RT/duroid® high-frequency circuit materials, are chosen in space applications due to their low electrical loss, low moisture absorption, and low outgassing characteristics.
Factors affecting the designing of an RF PCB
- Matching the impedance
Maximum power transmission without distortion from source to load happens in a controlled impedance RF circuit when the impedance is constant along the trace. This impedance is the trace’s characteristic impedance (Z0). Moreover, the trace geometry, such as trace width, the dielectric constant of the PCB material, trace thickness, and height from the reference ground plane, all affect the characteristic impedance. Additionally, matching circuits are created to match these impedances.
- The material of the board
RF PCB production involves specific materials that meet the needs of the high-frequency operation. These materials should absorb extensive heat, have low signal losses, and be stable during high-frequency operations. Consistency over a broad frequency range is also an essential requirement for the dielectric constant (DK), loss tangent (tan), and coefficient of thermal expansion (CTE) measurements. For these boards, the typical dielectric constant ranges from 3 to 3.5. For the frequency range of 10-30GHz, loss tangent values are in the field of 0.0022 to 0.0095. Besides these specific requirements, considering the materials cost and the manufacturing’s simplicity is also significant.
Commonly utilized materials include those constructed of PTFE (polytetrafluoroethylene), ceramics, and hydrocarbons combined with glass. RF circuit boards commonly use Rogers material. There are various versions of Rogers’ writings accessible. Following is a list of a few of them:
- Rogers TMM
- RF PCB Stack
Details like isolation between traces and components, power supply decoupling, the number of layers and their arrangement, the placement of elements, etc., must be considered when stacking radio-frequency boards. The top layer is where the traces and radio-frequency components are placed. The ground plane and the power plane come right after this layer, followed by traces and non-RF parts. This configuration minimizes the interference between RF and non-RF components. The ground return current has a minimum route provided by the nearby ground plane. In conclusion, this stack-up is appropriate for a compact radio-frequency board.
- RF Trace design
RF traces propagate High-frequency signals, which makes them vulnerable to transmission losses and interference problems. The designers’ primary focus is on the traces’ characteristic impedance. The traces on radio-frequency boards are thought of as transmission lines. Coplanar waveguide (CPWC), microstrip, and strip-line transmission lines are the three most often designed transmission line types. The elements of radio frequency trace design that influence proper operation and minimal losses are listed below:
• The trace’s length ought to be as minimal as possible, resulting in reduced Attenuation.
• In the arrangement, RF and normal traces should never be parallel. Interference between the two will happen if positioned that way.
• Ground aircraft are necessary to give sicals return routes.
It is forbidden to place the test points on the traces. The trace’s impedance matching values will be interrupted. The performance of a trace is improved by gradually curving bends instead of continuing acute right turns.
- Designing ground plate
• Any trace or component used in radio-frequency technology needs a return path for the current passing through it.
• An earth plane handles this. The ground planes, however, require some additional design factors. We’ll look at these now.
• Each RF layer should have its separate ground plane. This ground plane is positioned directly beneath the layer for the current flow path to be as short as possible.
• There should be no breaks in the ground plane. Breaks are not permitted.
• A minimum of two grounding vias are required for every shunt component used in an RF transmission line, as these breaks may allow for shorter paths for the current to return.
- Via design
As much as possible, RF traces should be free of vias. Consider opting for specific widths and lengths as per the demand. A through causes a circuit board’s parasitic capacitance. This capacitance affects the high-frequency operation of radio-frequency boards. Therefore, it’s crucial to build vias with the following principles in mind to minimize interference at these frequencies:
• Increase the number of parallel vias to lower parasitic capacitance.
• A dedicated via must be present for each component’s pins or pads.
• Use stitching to implement a ground plane where appropriate. As a result, the current’s ground return path shortens.
• Minimize the use of vias for RF trace routing between layers.
• Use as many vias as the design permits to connect the inner layer planes with the top layer ground plane. Plus, position these vias no farther than 1/20th of the signal wavelength away.
- Decoupling the power supply
Using this method, noise that enters the circuit from the power supply is filtered. Decoupling capacitors are the type of capacitors used for this. These capacitors are connected across the power supply. Every RF circuit board must have impedance matching. As a result, the impedance of the entire circuit shouldn’t change after the decoupling capacitors are connected. To prevent the change in impedance, adopt these design guidelines:
- For decoupling, always connect capacitors with the lowest possible impedance.
- To produce the most negligible impedance, run the capacitors at their self-resonant frequency (SRF). A capacitor’s SRF value is inversely correlated with its capacitance value.
- Select capacitors with SRFs that are around the noise frequency.
- Placement of decoupling capacitors
A successful RF design also depends on the location of decoupling capacitors. A short circuit that shows two decoupling capacitors connected in parallel with an IC component is provided below.
The higher capacitor’s purpose is to store energy while filtering the system’s low-frequency noise, while the lower capacitor filters out the high-frequency noise.
Additional placement-related rules include:
- The components and the decoupling capacitors should be on the same layer.
- Line up the capacitors parallel to the direction of the signal.
- Maintain separate ground vias for every capacitor.
- The capacitors should be arranged according to the supply’s capacitance in increasing sequence. So, the component with the lowest capacitance is closest to the power source.
It is safe to say RF PCBs can essentially contribute to the future of communication. The manufacturers vitally use these in communication devices such as mobile phones. Learning to craft an RF PCB can be quite beneficial and is one of the leading fields in the PCB industry. So follow these techniques, and you will thank us.