RF PCB: Your Ultimate Guide to Radio Frequency PCB

RF PCBs or radio frequency printed circuit boards are among the fastest-growing areas in PCB manufacturing. They function with high-frequency signals, especially those in the RF (radio frequency) range.

It is important to handle the layout and routing carefully to prevent issues related to signal integrity. This is because RF PCB layouts could contain digital components and, with careful layout, could prevent possible interference between the digital & RF signals.

This article explains what RF PCB is, its design principles, material considerations, and its layout design. We will also discuss the common challenges faced during RF PCB design and give possible tips and solutions. Lastly, we will provide tips and factors to consider before selecting the best RF PCB manufacturer. Please stay on this page as we explain the topic further.

What is RF PCB?

RF describes any frequency where the emissions of radio waves occur. The RF frequency range usually falls within 300 KHz and 300GHz. The major difference between a microwave PCB and an RF PCB is the radio frequency they operate on.

An RF PCB is a high-frequency PCB that functions above 100 MHz, while PCBs operating at frequencies higher than 2GHz are categorized as microwave PCBs. Both microwave PCBs and RF PCBs are essential for communication signals in applications requiring the transmission and receiving of radio signals i.e. radar installations.

RF PCB uses materials with high thermal conductivity and low dielectric loss. To maintain their signal integrity, they work with structures of transmission lines and controlled impedance. Furthermore, RF PCBs have shielding features that help to reduce EMI (electromagnetic interference). This feature also ensures better signal transmission.

The applications of RF circuit boards are diverse. These include radar systems, microwave equipment, and wire communication systems. Also, these PCBs help to reduce possible issues such as signal interference, signal loss, and noise. The problems can reduce the functionality of these high-frequency circuits.

The primary function of a conventional PCB is to offer electrical connections and mechanical support by mounting electrical components like capacitors, resistors, and integrated circuits on its surface and interconnecting them utilizing copper traces.

In contrast, RF circuit boards can handle radio-frequency and high-frequency signals. Compared to conventional PCBs, RF PCBs feature unique design requirements and characteristics that allow effective transmission of signals within a high-frequency range.

RF PCB Comparison Chart: Applications, Materials, Bonding Materials and Attributes

Core Design Principles for RF PCBs

It is essential to reduce noise, signal loss, and electromagnetic interference (EMI) during RF PCB design. It is also essential to maximize signal stability, strength, and bandwidth. Here are the main design principles for RF PCBs.

Design Requirements

Before you begin with the RF PCB design, it is crucial to determine the PCB’s impedance matching, power level, and frequency range requirements. Also, know the electronic components used during the PCBA (PCB Assembly).

PCB Layout

Placing the RF PCB components should help reduce the length of the signal wire. It should also make use of a few vias.

In addition, attach smaller components to critical parts. Remember that the RF lines should be short. This prevents routing them close to high-speed power planes, digital traces, and other noise sources.

Grounding

There should be a solid ground plane beneath the RF traces, to reduce return paths, radiation, and crosstalk.

Impedance Matching

It is vital to use microstrip transmission lines. This ensures proper matching of the RF traces’ impedance to those of the connectors and components. Also, ensure the trace space and width meet the required tolerances and impedance value.

Shielding

Utilize RF shields or shielded connectors to avoid possible interference from other sources and RF signal leakage.

Test

After completing the layout, it is essential to test the circuit to evaluate its performance. Use an oscilloscope or network analyzer to measure the bandwidth, return loss, and insertion loss.

Optimization

Make adjustments to the design to optimize its performance. This is possible by adjusting the trace space, width, and length. Also, analyze the circuits’ RF behavior using electromagnetic solution tools.

Generally, RF PCB design requires proper and meticulous planning. It also requires a vast knowledge of the principles of RF design. It is possible to create high-performance RF circuits by following these design guidelines and working with the appropriate techniques and tools.

Material Considerations

Below are some material considerations when manufacturing RF circuit boards.

Dielectric Constant

The dielectric constant describes the medium’s electrical signal propagation speed. This speed is inversely related to the square root of the dielectric constant. Lower values of the dielectric constant mean faster signal transmissions.

Furthermore, measuring this property isn’t an easy task. It isn’t just related to the medium’s characteristics but is closely related to the material’s state during and before the test, the test frequency, and the test method.

Also, a change in temperature affects a change in the dielectric constant. This is why you must consider the temperature while developing some materials. Another factor that affects the dielectric constant is humidity. This is because water’s dielectric constant value is 70. Just a small quantity of water will lead to significant changes.

The best material for high-frequency and high-speed applications is an air medium with copper foil wrapping. Also, aside from affecting the signal’s transmission speed directly, the dielectric constant helps to some extent in determining the impedance. In several parts, this impedance plays a significant role in microwave communication.

Since the dielectric constant usually changes with temperature change, the microwave materials are traditionally outdoors, even in space environments.

Loss Tangent

Loss tangent is another factor that affects an rf pcb material’s electrical properties. Other names for loss tangent are loss factor and dielectric loss. This indicates a signal or energy loss in a medium. This is because whenever high-frequency signals pass through a medium, molecules in that medium adjust or adapt themselves to conform to the electromagnetic signals.

However, the molecules are cross-linked, which makes it almost impossible to adjust itself to, but with frequency changes, the scorers will continue to move, generate more heat, and lead to energy loss.

Furthermore, some materials like PTFE are usually nonpolar. This means an electromagnetic field does not affect them and, therefore, experiences minimal loss. Also, the loss tangent relates to the test method and frequency.

Here, there is a general rule – higher frequencies result in more significant losses. A good example here is electrical energy consumption. When there is a slight loss in the circuit design, there can be a massive increase in the battery life. When an antenna receives the signal, it becomes more sensitive to material loss, and there will be a clearer signal.

Coefficient of Thermal Expansion

Denoted as CTE, it indicates how a material contracts and expands when subjected to different temperatures. In addition, it is an essential criterion when choosing materials during fabrication.

Electrical Strength

Electrical strength has a close relationship with the dielectric strength of a material to be utilized in a circuit. When working with low-power boards, this factor may not be significant. However, for high-power applications such as RF PCBs, electrical strength is very essential.

Thermal coefficient

It is crucial to consider the material’s thermal coefficient before you choose. In some cases, little changes in the value of the thermal coefficient will cause a shift in frequency responses. Because RF PCBs are usually exposed to fluctuating temperatures, it is advisable to use materials with a thermal coefficient value below 50 ppm/°C.

Glass Transition Temperature

Denoted as Tg, it is the temperature at which a PCB substrate can change from its glass state to a softened one, which could easily be deformed. After the material cools down, it returns to its former state.

Moisture Absorption

A material’s moisture absorption is its ability to restrict water absorption, most especially when inserted in water. Usually, moisture absorption affects a material’s electrical and thermal properties. Make sure you understand whatever environment the board will work on.

Decomposition Temperature

The decomposition temperature, denoted as Td, signifies the temperature whereby an RF PCB material undergoes mechanical decomposition. Make sure that the materials utilized will sustain whatever temperature the board will have to operate under. This is necessary because immediately after a material reaches its decomposition temperature, it is irreversible.

RF PCB Layout and Component Placement

RF circuit boards are usually more sensitive to interference. This is why RF PCB designs require tighter tolerances. Specifically, reflections, ringing, and noise have to be accounted for. This requires handling the impedance matching with care during routing.

Furthermore, signal return paths are essential in an RF design's high-frequency circuits. These signal returns currents follow the route of the lowest impedances. During the RF PCB layout process, circuit simulation becomes important to discover problem areas and find solutions.

There are deviations to the traditional PCB layout rules which should deliver the most appropriate signal performance. These include smaller pad sizes and tight placement clearances. However, it would help if these modifications were made when necessary to maintain the required DFM rules to ensure error-free assembly and fabrication.

Let's consider some of the requirements to achieve the best performance.

RF Component Placement

During RF circuit block component placement, the circuitry usually requires a tighter & firmer part placement. Modifying this component placement is one compromise the designer has to manage carefully to be sure you can achieve the board creation with standard and automated equipment for assembly. This helps to prevent the escalation of manufacturing costs.

Furthermore, an RF board usually has extra metal on its external layers for use by its power networks, while components are typically placed in those areas. Although this connection might work great for the RF circuitry’s performance, PCB assembly issues may arise.

Also, during layout, you may need to alter the components. These could include changing the PCB footprint pads' shape and size and incorporating metal shapes and features in the footprint. These tasks are handled individually to the board's components rather than altering the base library section. Now, let's consider some trace routing requirements that may come into play during RF PCB design.

Trace Routing

Like all high-speed routings, direct, short routing with different widths will be necessary for higher current power traces and controlled impedance routing. Below are some points on RF routing to consider.

● The majority of trace routing has to be added manually for some RF topologies

● Setting up and utilizing the design rules is vital for success

● Also, routing features like copy and paste could be important when replicating similar circuitry areas

● Vias provide fencing around the ground plane areas for shielding

● Mitered corners help to reduce any impedance fluctuation

For several RF designs, some routing areas are treated as components. Routing of this type requires tools that accommodate parametric component routing to send the trace segments into the RF analysis systems.

Furthermore, ground and power planes require higher power in RF designs and ensure thermal reliefs handle the current during automated soldering. Also, it is essential to create metal areas that are oddly shaped for RF elements, utilizing your CAD tools’ drafting capabilities and then converting these areas to intelligent net objects.

Common Challenges and Solutions During RF PCB Design

Common challenges in RF PCBs include minimizing weight, outgassing, bleeding prevention, and impedance matching.Below are some other challenges:

Difficult design process

For multiple-layer satellite PCBs, you must keep different components separately.  This decreases possible issues during the assembly process. Furthermore, RF PCBs require many layers, with the top layer featuring RF signal lines and a power stage. Also, the board requires ground layers underneath components that have RF signals.

Noise

RF signals are susceptible to noise and even more sensitive to different noise types than standard PCBs.The4e are different noise types, including band limited, pink, or white and shot, thermal, flicker, phase, and avalanche noise. Reducing the signal noise, reflection, and ringing to a feasible extent is important.

Wide Range of Temperature

A change in temperature level is another common challenge to consider. Changing a satellite’s temperature could alter the material features of the PCB.  Checking the temperature coefficient is necessary for comparing board materials and their suitability for various satellite applications.

Furthermore, RF PCBs must have the ability to dissipate heat properly, most especially in small and constricted areas. Look into the thermal conductivity of the PCB to understand the manner how the RF PCB material dissipates heat.

Reliability

Reducing weight concerns the increasing use of multi-layer circuit boards and increased RF circuitry density. Furthermore, the plated through holes reliability for interconnections between the layers has become a concern when operating in extreme temperatures in space.

Choosing a Reliable RF PCB Manufacturer

Before you choose an RF PCB manufacturer, there are some features to consider

Experience

You wouldn’t want to work with a company without experience in RF PCB manufacturing. Perfection usually comes when there is experience. Experienced manufacturers use recent technologies and machinery during high-frequency PCB manufacturing.

With experience, the chances of the RF PCB failing is low. To achieve the best results, hire a manufacturer that has vast experience in the field.

Certification

Quality should be a crucial factor before choosing an RF PCB manufacturer. The first thing to consider is the manufacturer’s quality management system (QMS). The minimum requirement is the ISO 9001 certification. It indicates the presence of a basic QMS.

Included are work instructions, procedures, processes, quality manuals, quality policies, preventive actions, etc. Some certifications help to prove a company’s competence. These include ISO 13485, A-610, A-600, and IPC J-STD.

Technical Capabilities

It is vital to work with an RF PCB manufacturer with outstanding technical capabilities like PCBasic. These capabilities include appropriate designs and material selection, processing technology, strict quality control, and condition of equipment used.

Fair Pricing

Before you work with an RF PCB manufacturer, know the cost. You can achieve this by checking out the websites of different manufacturers to understand the different market prices.

Conclusion

The design and manufacturing of RF PCBs require the appropriate material selection, design rules, and manufacturing processes. This is why it is important to work with a reliable RF PCB manufacturer like PCBasic.