It used to be that long-range communication relied on specially trained operators to manipulate complicated radio equipment to send and receive a message. These technicians were referred to as “Sparks,” or “Sparky,” due to the old spark-gap transmitters that used to be used on ships or in military applications. Today however you can do most of your communicating with a cell phone, but that doesn’t mean that good ol’ Sparky is out of a job.
There is still a need for experts in radio frequency, but now instead of operating giant power-sucking radio equipment, these experts are needed for designing RF circuits on printed circuit boards. Designing a PCB that has RF circuitry on it can be a real shock to a layout engineer if they aren’t prepared for it. PCB shapes for RF may be odd-looking, and the design constraints may end up breaking the standard rules that you are used to. Here are some RF PCB design guidelines that can help.
Why DFM is Important in RF PCB Design
Design for manufacturing (DFM) rules are important in every design. By following good DFM rules, you will be able to reduce the number of potential areas in your design that could cause problems for manufacturing, thereby lowering manufacturing costs and improving yields. In the case of RF designs, however, it becomes even more important.
RF boards by their nature will have some areas where the regular DFM rules will be severely bent if not completely broken. These deviations from the rules could include things like component spacing or metal-to-metal spacing on the board to be less than the minimum allowable values. Pad sizes may also be minimized, or additional metal will be connected to the pads. You may also have heavier grounding requirements, blind via usage, and other design requirements that may make it more difficult for the board to be manufactured.
With the need to spend extra attention on the RF areas of the board where DFM rule deviations may be necessary, it is important to keep the rest of the board as closely within the rules as possible. Deviations to the DFM rules force extra work from your contract manufacturer (CM) to create special processes to make sure those areas are assembled correctly. By keeping the rest of the board strictly within the design rules, you will see lower manufacturing costs, and the CM will be able to focus their attention on the critical RF areas.
RF PCB Design Guidelines You Need to Know for Assembly
Most of the RF specific design problems that come up during assembly are from an application perspective. For example, there may be unexpected power waveforms that cause problems for smaller components and how they are laid out on the board. Many of these issues are tied to how much experience the design engineer has had with previous RF designs, and how that experience is used to correctly lay out the board for the circuit flow. There are some basic rules to keep in mind, however, that will help with manufacturing an RF design, and they are basically extensions of the same standard DFM rules that you will find for any PCB technology:
- Power: You may need to support higher power in your RF design, so make sure that your thermal pads are constructed for both the power needs, and yet are still manufacturable with the proper amount of thermal relief.
- Ground planes: An RF board will often have a lot of metal on it for its power networks, clean signal returns paths, and shielding. This can sometimes lead designers to place SMT components directly on the metal for better connectivity. Remember that this is a big DFM no-no for solderability, so consider the placement and connectivity of these parts carefully before you proceed.
- Component spacing: The signal requirements of RF circuits often result in tight component placement. Remember that for mass production runs of the PCB, or even for rework and debug, there needs to be enough space to access the parts.
Obviously, the performance of the circuitry has to come first, because if the board doesn’t function there isn’t any reason to build it. But it always pays to find a way to satisfy both the electrical requirements and the manufacturing requirements of a circuit board whenever possible.
What Your PCB Contract Manufacturer Needs from You
To be fully prepared to build your RF design, you PCB CM will usually want additional information from you, prompting questions such as these:
- Is this a new design, or a modification of an existing design?
- How much experience does the designer have with the application that the board has been designed for, and how RF experience, in general, do they have?
- What are the critical issues that the designer has encountered with earlier layouts, and were they able to identify the root causes of the problems? Or, were they able to find an approach that worked, but they don’t understand why?
- What are the expected power/reflective waveforms in the application that this PCBA will need to support?
- Are leaded components oriented correctly for high RF power areas, and will the lead spacing support the targeted voltage and RF requirements?
- What are the specific requirements of the ground plane, and what is the general grounding methodology being used?
With questions like these, the CM will be able to determine if the design is ready for manufacturing, and how best to develop the assembly plan for it.
At VSE we have over three decades of experience with RF customers like you, and we will work together with you before we build your board making sure we understand your design and your intentions for it. This will give us an accurate picture of how best to develop and execute the manufacturing plan for it.
