RF PCB Design: Guidelines for Layout

Electronics may seem mystifying to an outsider, but even within the profession, there’s a tendency to denote RF designers as black magic practitioners. The term is not disparaging but rather half-endearment and half-bewilderment: some of the basics of RF design fly in the face of supposed bedrock circuit principles. Designing a PCB with RF circuitry can upend some of an inexperienced layout engineer’s general best practice guidelines. Negligible aspects of the circuit at low speeds, like the conductor shape or thickness, express entirely new properties. RF PCB design has to assimilate many aspects of design best practices from nonspecific PCB manufacturing while accounting for the unique challenges of high-speed design.

Why DFM is Important to RF PCB Design

Design for manufacturing (DFM) rules are essential in every design. Adherence to DFM rules reduces the number of potential manufacturing issues in a design, lowering production costs and improving yields. At high frequencies – like those found in RF circuits – circuit behaviors – and their governing best practice rules – can change drastically.

By their nature, RF boards will have some areas where the regular DFM rules will severely alter to accommodate differences in circuit response at RF speeds. These deviations from the rules could include component spacing or metal-to-metal spacing on the board being less than the typical minimum allowable values: for example, pad sizes will minimize to avoid additional metal connecting to the pads. Board design may further emphasize return paths for signal integrity, via stitching, or other requirements that complicate manufacturing.

High-speed designs will also have to contend with signal reflections that can reduce the power or coherence of a signal. Through-hole vias span the length of the board, but the signals they carry may not travel that entire distance. In these cases, the excess via barrel length – known as a stub – acts as an antenna and can severely degrade performance. In these cases, manufacturers will want to back drill out this unused length of the via barrel after plating.

With the need to spend extra attention on the RF areas of the board where standard DFM rule exceptions may be necessary, the rest of the board must maintain standard manufacturability best practices. Variations of the DFM rules require extra work from a contract manufacturer (CM); the more restricted the areas where design rules differ by the designer, the more the board can seamlessly integrate into the manufacturers’ extant production methods. Partitioning off the board’s RF sections will help lower manufacturing costs and keep the CM’s attention focused on the outlier RF features.

RF PCB Design Guidelines You Need to Know for Assembly

Most of the RF-specific design problems during assembly are from an application perspective, like unexpected power waveforms that cause problems for smaller components and their layout on the board. Many of these issues relate to the designer’s comfort with RF circuits and how they apply their expertise to optimize layout flow for RF considerations. There are some basic rules to keep in mind that will help with manufacturing an RF design, with most amounting to extensions of the same standard DFM rules:

• Power: RF circuits may require greater power for driving transmissions; designers will want to accommodate higher power by ensuring that your thermal pads support both the power needs and are still manufacturable with the proper amount of thermal relief.
• Ground planes: An RF board often has a lot of metal for its power networks, clean signal returns paths, and shielding. The additional metal can sometimes lead designers to place SMT components directly on a conductor for better connectivity, which inhibits solderability and total circuit reliability. Carefully consider these parts’ placement and connectivity before proceeding to signal routing.
• Component spacing: The signal requirements of RF circuits often result in tight component placement. Wherever possible, designers will want to leave enough space to access the parts for soldering rework or debugging.

It’s essential to remember that DFM is the criteria for producibility; design can sometimes override it, but these cases should be a well-measured decision by both designer and manufacturer. The circuit’s function has to come first, and if a design calls for reducing manufacturability (while remaining feasibly producible), then manufacturers will have to work within the designer’s constraints. Suffice it to say, less producible boards mean lower yield and a rise in per-board cost to the designer, so it’s in the designers’ best interest to simultaneously satisfy a circuit board’s functional and manufacturing requirements whenever possible.