Whenever I attend a choir concert at the local high school, there is one announcement that is always made before each performance. The choir director will nicely ask everyone in attendance to turn off their cell phones to prevent them from interfering with the wireless microphones used on stage. I’m suspicious that this has more to do with trying to get people to silence their phones to prevent rings, beeps, and clicks from upstaging the performers, but I still comply anyway.
Electromagnetic interference (EMI) has been a problem that electronic design has grappled with for as long as printed circuit boards have been built. Depending on the function and design of the electronics, a PCB may produce or fall victim to radiated electromagnetic energy. The goal is to design PCBs so that they will not interfere with each other and have good electromagnetic compatibility (EMC). This means paying attention to five key areas on your design:
1. Your layer stackup
2. Your trace routing
3. Circuitry separation
4. Decoupling capacitors
5. Ground shielding
Let’s take a closer look at these PCB EMI and EMC design considerations to consider before your board is manufactured.
Understanding Potential EMI Problems
Before EMI considerations can be factored into how a printed circuit board is laid out for optimal manufacturability, we must first recognize where some potential EMI problems can come from. In other words, what conditions on a PCB could result in EMI? Here are some that you should be on the lookout for:
- Switching noise: Many circuits on the circuit board are constantly switching, and some of this switching can cause EMI problems. Clock signals can be the worst of these with their regular switching causing radiated peaks at harmonics of the clock frequency. With the data rates of high speed designs, these digital signals can also cause problems that are similar to EMI radiated from clocks. Even switch-mode power supplies can radiate noise with their switching.
- Current paths: Current flows in loops and it is very important to make sure that the current paths have an unobstructed path. At lower frequencies, the current will take the path(s) of least resistance, while at higher frequencies it will take the path of least inductance.
- Antenna effects: Large areas of metal can contribute to antenna effects on a printed circuit board. Power or ground planes can cause problems as can metal chassis. Heat sinks and tall components may also create problems as frequencies increase, requiring changes to the schematic or layout.
To guard against the potential EMI problems that can be caused by these conditions, here are some layout tips that you should consider in order to make your board more EMI compliant.
The 5 Most Important EMI/EMC Design Considerations for PCB Layout
There are a lot of things that you can do in your printed circuit board layout that will help control EMI coming from your board, and to protect it from being influenced by external EMI. Here are some of the most important areas that you should consider, starting with using a multilayer stackup configuration for the board design:
1. Board layer stackup: One of the best things that you can do to protect your high speed transmission lines is to route them between the power and ground planes of the board. This will produce a stripline layer configuration which will help reduce crosstalk from broadside coupling with routed traces on other signal layers, and keep any radiated emissions to a minimum. Configuring your board layers so that the power and ground layers shield and protect your routing must be carefully planned before you start your routing.
2. Trace routing: Minimize your trace lengths where possible, except for lines that must be lengthened for trace length matching. Never, ever, route signal traces over split planes, especially sensitive high speed signals. This will cause a world of problems for the return path in the plane as it wanders around trying to find its way back, radiating noise as it goes. To help those signal returns, minimize the use of vias whenever possible to keep the return paths in the planes as open as possible. Route differential pairs carefully to keep them close together and avoid using vias in them. Isolate differential pairs and high speed traces as much as possible from other circuitry, even using a ground strip between them if possible.
3. Keep digital and analog circuitry separate: Isolate digital and analog as much as possible from each other. Don’t share the same general digital ground plane with analog or power supply grounds. Avoid routing digital circuitry on inner layers through areas of analog components.
4. Decoupling capacitors: These capacitors store current for the integrated circuits that are switching. Without them, the current would have to flow much further from its source to the IC, causing larger voltage drops which would, in turn, generate more noise. It is important therefore to place a decoupling capacitor close to every power supply pin on all of the ICs on your board.
5. Ground shielding: Use metal area fills and stitching vias to create a pseudo faraday cage around critical areas of circuitry on the board.
By employing these and other tactics, your board will have a far less chance of either radiating or being influenced by external EMI.
What to Expect from Your Contract Manufacturer
Your PCB contract manufacturer will have a lot of experience building all different types and technologies of printed circuit boards. As such, they usually will have plenty of ideas on how to control EMI that you may find useful. Ultimately, the responsibility of creating a PCB that has good EMC belongs to you, but your CM will do what they can to help you along the way. The important thing is that you create a design that will not fail because of EMI or cause problems for other devices.
At VSE we will work together with you to create the best possible PCB for manufacturing. We rely on the design review with you to ask questions and provide feedback in order to ensure that you have considered every option on your design. This way you can be on the lookout for potential EMI problems and take steps during the design to reduce or eliminate them.
