From designing to operating an RF PCB: The complete guide

by Ken Ghadia, Sales Engineer, TechnoTronix

Radio Frequency (RF) PCBs are very high-speed circuits operating at frequencies in the range of 100MHz and above, typically from 500MHz to 2 GHz. RF PCBs are the backbone of wireless networks, high-tech gadgets like smartphones, smart TVs, etc. Also, RF PCBs are used in automotive, medical equipment, military, and aerospace applications to achieve proficient measurement, detection, and monitoring functions.

There is a significant difference between an RF circuit and any typical analog and digital circuit. The RF signal comprises very high-frequency analog signals that can be at any voltage or current level within the set limits at any given point of time. The design of such RF PCB is quite complex and has to follow stringent guidelines to mitigate the interference issues caused by high-frequency signals.

During an RF PCB design, the most commonly discussed topic is choosing a suitable material for a target application that impacts the final design. The selected substrate material determines the price, thickness, circuit layout, mounting possibilities, etc., covering a large portion of the overall product functionality.

Guidelines to select the right laminate substrate for high-frequency RF PCB applications

Matching the dielectric constant (Dk) of resin and woven material of a substrate is highly recommended. Non-uniform Dks in a substrate may cause a problem during high-frequency operations. The designer must ensure a close match of Dk in all the substrate layers being considered in the PCB design.
The degree of expansion of the substrate in response to the temperature changes during PCB fabrication or operation is known as the Coefficient of Thermal Expansion (CTE). Different layers of a laminate substrate may expand at a different rate, causing functional failures and reliability concerns. Matching the CTEs of all substrate layers is mandatory in an RF PCB design.
Selecting a laminate substrate having woven properties to mesh tightly is suggested as it may directly impact Dk.
The loss tangent of a material is the amount of energy lost in the dielectric region during signal transmission. Selecting a correct loss tangent material is crucial for signal integrity in an RF PCB design. PolyTetraFluoroEthylene Teflon (PTFE) material is one such material suggested for RF PCB design.
The substrate selected should absorb the least moisture from the environment to avoid performance degradations at high frequencies.
At high frequencies, current signals tend to concentrate within a small area near the trace’s surface. This is known as the skin effect. Using smooth copper foil will reduce such resistive losses at high-frequency operations.

General design and layout guidelines to build a high-quality RF PCB

The main concern is the sensitivity of high-frequency signals to any interference of noise like ringing, reflection, or crosstalk. This demands cautious impedance matching while routing the RF signals. Designers widely use a common impedance value of 50Ω, which simplifies the impedance matching of the RF signals.
Inductance also significantly impacts an RF design and has to be kept as low as possible. Ensure adequate ground connection to the RF components with no discontinuity in the return ground path.
RF traces carry the highest frequency signals, and it is suggested to route them first by providing a smooth bend for the traces if required.
The traces connecting the RF components must be short and sufficiently spaced apart to avoid any crosstalk issues.
Isolating the RF traces is necessary to avoid any overheating issues since these tracks operate at a very high frequency. This can be achieved by stitching vias around the RF traces.
A well-balanced stack up with the uniform copper thickness of layers improves the signal integrity of an RF PCB.
To avoid coupling effects, high-speed signal traces should be routed on different layers other than RF signals.
Power supply lines should be routed in dedicated power planes by inserting necessary decoupling and bypass capacitors.
Vias added on RF traces and near RF components reduce the parasitic inductance effects and reduce the coupling between RF signals and other signal traces on the PCB.

Design guidelines for RF PCB assembly

An RF PCB can be manufactured with a high yield by adhering to some basic design guidelines required for an RF PCB assembly. These rules extend the standard DFM rules that will help manufacture a high-speed RF PCB with high output.

RF design requires higher power support. So, the thermal pads should be constructed for the essential power needs and still be manufacturable with the necessary thermal relief.
The placement of SMT components has to be considered in advance during the component placement stage. In an RF design, a lot of metal is used for power networks and shielding purposes. These metal spaces should not be used to place any SMT components by designers.
The complex signal routing requirements of an RF PCB may lead to a strict component placement strategy. But designers also have to consider the necessary rework, debug requirements during mass production, and provide enough spaces for the component access.

Finding a middle ground to achieve both electrical requirements and the manufacturing feasibility is possible if the designer has prudently considered all the design and assembly requirements of an RF PCB.

Guidelines for the safe operation of RF PCB

The boom of wireless technology has ensured the need for high-frequency RF PCBs in consumer products along with the traditional military or aerospace applications. More RF-based products are leading to a crowded RF spectrum with unwanted interferences. So, it becomes necessary to operate these RF PCB applications with caution.

Shielding the RF circuit boards and their sensitive components can highly reduce Electromagnetic Interference (EMI) threats. But this shielding may interfere in the signal transmission also. To manage the EMI issues, proper grounding of the PCB is essential. Lengthy high-speed traces can act as transmitters and receivers of EMI. Hence sticking to the shortest possible trace length is critical to any RF PCB design.

The design and manufacturing of an RF PCB require good domain knowledge and exhaustive PCB Manufacturing experience. This comprehensive guide provides a combination of design, assembly, and safety factors to be considered while designing an RF PCB. Do not forget to involve your PCB Assembly provider right from the design stage to roll out high-performance RF PCBs to the market.

About the author

Ken works as a Sales Engineer at Technotronix. He brings 15 years of experience in PCB sales and technical client services.