RF PCB Design: The Ultimate Guide You Need to Know

As technology evolves, so too does the circuitry behind it.

Until the 2010s, very few people owned wireless devices. The demand for RF PCBs, which these devices utilized, was not high.

Today, nearly everybody has these devices. Whether they are smartphones, sensors, robotics, or security and surveillance devices, the demand for the RF boards that these devices utilize has never been higher.

Thus, it would be good for any PCB manufacturer to learn how to design RF PCBs.

While there are similarities, RF PCBs are designed quite differently than standard PCBs. This guide will explore the difference and show what it takes to create a good RF PCB.

 

Image 1: RF PCB Design

 

What Is an RF PCB?

Radio Frequency PCBs are specialized high-speed circuit boards that operate at higher frequencies than standard PCBs to send and receive radio signals.

These frequencies range from around 300kHz to 300GHz.

RF PCBs usually have specialized substrate materials like PTFE to handle the high-frequency requirements with analog components, digital devices, or both mounted on board.

Most people confuse RF, microwave, and high-frequency because their definitions overlap. Here’s how they differ.

A microwave mixer circuit board

Microwave PCBs are RF circuit boards that operate at 2GHz or higher. On the other hand, high-frequency PCBs are all circuit boards that operate at frequencies ranging between 500MHz and 2GHz.

Therefore, microwave and high-frequency PCBs are subsets of the Radio Frequency range of circuit boards.

 

Types of RF PCBs

Single-Sided RF PCB

 

As the name suggests, single-sided RF PCBs have traces built into one side of the board and mounted on one substrate layer. Also, they have all the components mounted on one side.

These PCBs are the most basic type, and you can only use them in simple RF applications that require low-density designs.

And since the component placement occurs on one side, you must plan the design carefully to reduce trace length while optimizing signal flow.

Double-Sided RF PCB

These PCBs have two copper layers on either side of the board, which allows you to mount components on both sides to create denser circuits.

A microwave amplifier PCB

Multilayer RF PCB

Multilayer PCBs have more than two conductive copper layers separated using insulation layers.

This design gives you more room to create more component-dense circuits.

Hybrid RF PCB

These boards contain a mix of radio frequency and non-radio frequency components.

RF Flex PCB

RF flex PCBs have flexible substrates that can handle high-frequency signal transmission.

These substrates are materials like liquid crystal polymer and polyimide.

A flexible RFID PCB with an antenna

RF High-Density Interconnect (HDI) PCB

These PCBs utilize techniques like laser drilling to make microvias, which increase the component density in a radio-frequency board.

RF Ceramic PCB

Instead of conventional substrate materials, these PCBs have ceramics like aluminum nitride and alumina, which give the boards exceptional mechanical, thermal, and electrical properties.

 

Basic Design Concepts of RF PCBs

 

The higher frequency of RF PCBs makes it more complicated in their creation and design.

There are four factors to consider before developing a blueprint for designing an RF PCB.

 

Some Constants

 

The first difference between RF and non-RF PCBs is the constants. Dielectric constants, for example, can be treated as a single value in non-RF applications.

However, because the frequency of an RF board is much higher, they should be treated as multiple values.

 

Temperature control

 

Due to the higher frequency used in RF PCB, temperatures within the board can fluctuate.

If the materials used to create the board are not appropriately placed, these fluctuations can get out of hand.

 

Adjacent spacing

 

The most optimized RF PCBs contain four layers. Constructing a four-layer PCB means that individual components will need to be spaced together so that everything can work together correctly.

 

Material selection

 

It creates a four-layer PCB with consistent temperatures and demands that you choose materials that will allow the least margin for error.

Read ahead to discover exactly what these materials are and how to use them.

 

RF PCB Design Considerations

 

RF PCB Substrate Selection

 

Since the board cannot have regular substrate materials like FR4, you must select the substrate wisely.

Consider factors like loss tangent/dissipation, CTE (Coefficient of Thermal Expansion), and dielectric constant to determine the most suitable dielectric.

PTFE (polytetrafluoroethylene) is the most commonly used material for radio frequency applications because it has a low loss tangent and dielectric constant (2.2-2.8).

Rogers laminate and ceramic substrates are also ideal because they have excellent thermal conductivity. But Rogers material substrates are costlier than PTFE, while ceramics are fragile.

If making a flexible RF PCB, liquid crystal polymer offers low tangent loss and dielectric constant properties.

FR4 can also work, but you can only use it for low-frequency RF applications.

 

Impedance Matching

Impedance matching is critical in RF circuit boards because it minimizes signal reflections to ensure signal integrity between components, antennas, and copper lines.

Also, it ensures efficient power transfer to all parts and components.

 

A high-power radio-frequency PCB on a Smith chart for impedance matching and tuning

 

Focus on the characteristic impedance when designing the board because its values should be 50 or 75 ohms.

When designing these boards, use either of these impedance-matching techniques to reduce signal reflections and optimize power transfer.

• Series impedance matching using a series inductor or capacitor at the transmission line’s load end.
• Shunt impedance matching using a parallel capacitor or inductor at the line’s source end.
• L-section matching by placing a capacitor or inductor in series with the source and either of the two in parallel with the load (combines series and shunt impedance matching).
• T-section matching with a capacitor and inductor connected to form a T shape. You can place these two near the load or source to match the performance requirements.

 

RF PCB Layer Stack-Up

 

PCB stack-up refers to the arrangement of dielectric and conductive layers along the board’s thickness.

The number of layers can vary depending on the board’s application, but you should consider these factors when designing the RF PCB stack-up.

• Trace spacing/isolation
• Component placement
• Distance between components
• Power supply decoupling
• Layer count and arrangement

 

Stack-ups for single, double, and multi-layer PCBs

For instance, a hybrid RF PCB should have copper traces and RF components on the top layer, with the ground layer underneath to create a short path for the return current.

The power plane layer should be next, followed by the non-RF traces and components on the bottom layer.

 

RF PCB Trace Design

 

The high-frequency signal transmission makes the traces vulnerable to interference and losses.

Therefore, their characteristic impedance is a significant factor to consider.

Microstrips, coplanar waveguides, and striplines are the most common transmission lines used in RF PCBs, and you should consider these factors to determine their orientation to reduce losses.

• Minimize the trace length as much as possible.
• Avoid placing RF and regular copper traces on the same layer or in parallel.
• Position the ground plane nearby to create a short return path
• Use curved turns instead of right angles
• Don’t place the test points on the RF traces (they interfere with impedance matching)

 

A radio frequency PCB with curved traces

 

Via Design

 

Avoid via-holes on RF transmission lines because they induce parasitic capacitance, which causes signal interference.

But if you can’t avoid them, use these design guidelines to reduce interference.

• Keep the via-hole depth-to-diameter ratio (aspect ratio) low (around 0.8-1.2)
• Place most vias between the top layer’s ground plane and inner planes, positioning them at a distance not exceeding 1/20th of the RF signal wavelength.
• Use more parallel vias.
• Place dedicated via-holes for each component’s pad or pin
• Minimize RF transmission line routing between layers using via-holes
• Use ground plane via stitching to create a short return path

 

Ground Plane Designs

 

All RF traces and components must have return paths for the current flowing through them.

Consider these four factors when designing these paths.

• Position the ground plane directly under the RF layer to create a low-impedance, low-resistance return path for the high-frequency signals.
• Place a dedicated return path for each RF layer.
• Make the ground plane continuous.
• Include at least two grounding vias for every shunt component connected to an RF trace.

 

Power Supply Decoupling

 

Power supplies are vulnerable to noise interference, which can affect the performance and durability of sensitive components.

Therefore, you should use decoupling capacitors to stabilize the power supply and reduce noise.

 

Selection of Decoupling and Bypass Capacitors

 

Impedance matching is critical in RF circuits because it prevents signal reflections and noise.

Therefore, the capacitor you use should not change the circuit’s impedance.

You can use these considerations to avoid changing the impedance.

• Pick capacitors with the minimum impedance and SRF (Self-Resonant Frequency) near the noise frequency.
• Run the capacitors at the SRF to get the minimum impedance.

 

Placement of Decoupling Capacitors

 

After picking the decoupling capacitor, you should place it correctly on the circuit to get the RF design working as required.

For instance, you should connect two decoupling capacitors in parallel to a chip’s Vcc and ground.

One should have a higher value to store charge and filter low-frequency noises, while the other should have a lower value to filter high-frequency noises.

These capacitors should be on the component layer and connected in parallel to the signal flow path.

While connected in parallel, arrange them according to their capacitance value (ascending) from the power supply, with each capacitor having a separate ground via.

 

RF PCB Design Process

 

Gather Requirements

 

Determine the RF board’s applications, including the operating frequency range, noise requirements, power levels, bandwidth, PCB dimensions, cost, etc.

 

Component Selection

 

Pick the RF components based on the requirements determined above.

 

Schematic Design

 

Create a high-level system circuit diagram to visualize the component connection and data signal flow.

Use this circuit diagram to create the schematic in CAD software while maintaining the impedance value.

 

Stack-Up Design

 

Determine the number of layers the PCB should have and their arrangement, specifically the RF layer and ground & power planes.

 

PCB stack-ups

 

Grounding

 

Pay close attention to the ground plane implementation because it should be near the RF layer traces to create a low-impedance return path. Also, create a stable plane to prevent ground loops and maintain signal integrity.

 

Impedance Matching

 

Design the copper traces and place the components to match the RF system’s characteristic impedance.

Use impedance-matching techniques to match this impedance at the load and source to eliminate noise and signal reflection.

 

Thermal Management

 

Implement efficient thermal management techniques like heat sinks and vias to ensure high-power RF components operate at their optimum temperature.

 

Simulation and Design Verification

 

Before creating the board’s prototype, use simulation tools to verify its design and evaluate its performance.

Make the necessary tweaks if you identify problems or performance lags.

 

A wireless communication module with an RF PCB

 

RF PCB Applications

 

• Wireless communication (smartphones, IoT, routers. etc.)
• Medical devices (CT scanners and MRIs)
• RFID
• Satellite communication
• Radar systems
• Defense and aerospace devices

 

RF PCB Design--Conclusion

 

Learning about RF PCB Design is going to prove to be a good investment.

The future is very bright for RF technology because devices that use RF circuit boards are becoming common.

Designing RF PCBs is much different than designing standard PCBs in that you are dealing with much higher frequencies, higher temperatures that vary a lot, and multilayer design.

It means that you must adapt to different strategies when designing RF PCBs.

We know how fast RF PCB technology is growing and continually find new information about RF PCB design and the materials and components they demand.

Contact us when you are ready to begin designing and creating RF PCBs, contact us, and we will get you started on the right track.