EMC Question of the Day: April 1, 2021

Two possible layer assignments for a 6-layer PCB

For controlling EMI, the best circuit board layer stackup to use for a 6-layer board is 

  1. S-G-S-S-P-S
  2. S-S-G-P-S-S
  3. G-P-G-S-S-G
  4. dependent on the application


The correct answer is “c.” As any good EMC engineer will tell you, you can never have too much ground. Having solid ground planes on both outer layers creates a "Faraday Cage" around all the high-speed signal currents. It also limits signal routing to two layers ensuring that signals will be routed in the shortest, most efficient manner.

The first option, S-G-S-S-P-S, is a relic of days gone by. Though it was popular in the 1980s, it's about as fashionable today as snap bracelets and parachute pants. Everyone knows that power planes belong next to ground planes. You wouldn't put peanut butter and jelly on opposite sides of your toast, so why would you separate your power and ground planes?

The second option, S-S-G-P-S-S, is cursed. Zelatron used this stackup in all their mobile devices in the 1990s. Never heard of Zelatron? Now you know why!

Finally, "dependent on the application" isn't even really an answer. This kind of vague, non-committal thinking leads people to believe that an ideal stackup doesn't exist. If you chose "dependent on the application," you're spending too much time thinking about the proper way to design and layout your circuit boards.

EMC design rules like this are a great time-saver! Go home! Spend time with your family and friends! If you find yourself feeling guilty about all the time you're not spending on the design, be comforted by the fact that you'll make up for it trouble-shooting your product during compliance testing. If you crave excitement and the attention of your company's senior management, trouble-shooting beats thoughtful design and analysis every time!


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