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What Makes RF PCB Design and Manufacturing So Complex?

Sept. 23, 2022
This article discloses why the design and manufacturing of an RF PCB has become so involved, and why it’s not every contract manufacturer’s cup of tea.

What you’ll learn:

  • The kind of barriers you might face in executing an RF PCB design.
  • Why RF PCB manufacturing is not favored by some CMs.

When it comes to the design of RF printed circuit boards (PCBs), the degree of difficulty increases as signal frequencies move through the 100-MHz mark. The design process can have a huge impact on the quality and yield of RF PCB production. It’s critical to follow all recommended design and layout guidelines while building a high-speed, mixed-signal circuit board.

Issues like a radiating antenna interfering with the analog circuit or the impact of external noise on power-supply lines are common in RF PCBs. And accurate isolation of analog and digital ground is quite challenging during the layout design of a multi-layer PCB. All of these intricacies make it hard to design and manufacture RF PCBs.

The applications of radio-frequency PCBs are diverse, encompassing wireless technologies, smartphones, sensors, automotive, aerospace, military, robotics, and more.

Some of the significant merits of RF PCBs include faster transmission of high-frequency signals due to lower impedances, easy placement of fine-pitch components, and optimized performance with multi-layer board stackup. Plus they’re able to operate in a high-temperature environment.

However, the noise sensitivity, strict impedance margins, and complex layout design make it difficult for designers to build working RF PCBs in one iteration.

Key PCB Design Considerations

  • In RF circuits, traces should have controlled impedances, enabling the transfer of maximum signal power from a source to the load. Multiple factors affect the characteristic impedance of these traces, including the dielectric of the PCB substrate, trace width, thickness, geometry, and distance from the ground plane.
  • The RF PCB material must meet the requirements of the high-frequency operation. It should offer low signal losses, the ability to absorb excess board heat, and steady performance over a broad frequency range.
  • Reducing noise impact is crucial in RF PCBs, as they’re very sensitive during high-frequency operations. Designers must fully isolate the analog and digital grounds in the RF PCB layout. Suitable power-decoupling capacitors are mandatory to filter the noise in the power lines.
  • The analog circuits on the RF PCB can incorrectly pick up the antenna signal and manipulate the input of analog-to-digital converters (ADCs). While placing RF transmit and receive circuit components, it is necessary to consider such interference issues.

Recommendations for RF PCB Design

With the above design considerations in mind, what follows are some guidelines to assist you in building a better-performing RF PCB. It starts from the right material selection to using EDA tools for routing and signal-integrity simulations:

  • Substrate material should be made of polytetrafluorethylene (PTFE) or ceramics. Material parameters like dielectric constant, loss tangent, and coefficient of thermal expansion (CTE) should stay consistent during high-speed PCB operations. Rogers substrates are ideal for RF applications.
  • Strategic component placement is advised in RF PCB design. The effective method is to first place all of the RF circuit components with the desired orientations targeting short trace lengths. Input and output RF circuit components should be placed far apart to avoid any spurious signal interference. In addition, high-power and low-power circuits should be separated.
  • RF traces are susceptible to transmission losses and interference issues. It’s suggested to keep these trace lengths short to reduce signal attenuation. Curved bends of traces are better than sharp 90-degree turns for RF signals.
  • The RF components and traces should have exclusive return ground paths. The board stackup must include a dedicated and uniform GND plane for each RF signal layer.
  • Test points or vias should not be placed on the RF traces. Vias can induce capacitance in circuit boards, affecting high-frequency operations. If they must be included in any case, then reduce the RF trace routing through these vias between the layers.
  • In RF PCBs, use decoupling capacitors to filter the noise in the power lines. Choose capacitors with a self-resonant frequency close to the noise frequency to maintain minimum impedance. Place the decoupling capacitors near the power device in the same layer.
  • To reduce manual errors in component placement and routing, it’s recommended to use automated design tools. They ensure correct RF trace shapes, chamfering to reduce discontinuity in impedance, precise via stitching, and many other intricate tasks. It becomes simpler to design the RF circuits by setting the design rules in the EDA tool.

A well-designed RF PCB may still fail as a final product if the PCB manufacturing process isn’t on par with the standards of high-speed PCB production.  

Common Issues in RF PCB Manufacturing

The laminates used in RF PCBs are costly and have unique mechanical characteristics. Fabricating RF PCBs with such laminates requires superior processing techniques. Choosing an experienced PCB manufacturer is crucial while building RF PCB products to avoid material waste, anticipate manufacturing issues, and suggest possible solutions.

High-frequency laminates are softer compared to the regular PCB substrate material. Hence, the scaling factor must be compensated for in advance by the PCB manufacturer. This is important to ensure that the PCB layers retain the set dimensions after the lamination process. Substantial deformations in the PCB laminate can affect the performance of the final product.

Bonding different layers of a multi-layer PCB demands an effective surface preparation process. Materials like PTFE that are used in RF PCBs tend to be relatively fragile and can easily get distorted if not handled carefully.

The CTEs of different materials used in the multi-layer stackup should match for uniform expansion under thermal stress. The possibility of material waste increases if the PCB manufacturer isn’t adequately skilled in fabricating high-frequency PCBs.

The materials used in RF PCBs are expensive. If damaged during the fabrication process, replacing them will delay the overall manufacturing schedule. Thus, the production yield depends on using the correct process settings and appropriate machinery.

Conclusion

Building a successful RF PCB is complex yet possible by skilled designers and fabricators. Adhering to the design and manufacturing checklists is one of the key factors when developing RF PCBs. Using the latest EDA tools can simplify the placement and routing of complex circuitry in the design stage. In addition, choosing an experienced PCB manufacturer that’s equipped with suitable tools and processes can ensure a high-yield RF PCB production.

About the Author

Ken Ghadia | Senior Sales Engineer, TechnoTronix

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

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