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Placing Power Planes next to each other...Will that cause EMI problems?

Tom Yunghans , 04-06-2021, 08:40 AM
Hi Robert,

Just watched your video chat with Rick Hartley (a couple of times). Really good insight in that. Thank you.

Rick made a lot of negative comments about placing different power planes next to each other in your stackup. In your IMX6 Rex design that you used in your Advanced PCB Design course, you placed four power planes right in the middle of that stackup (layers 5,6,7 and 8). They have very minimal distance between them (4-5 mils). You do have solid ground layers on layers 4 and 9 so the fields between these layers will not intermix with the signal layers, but I believe Rick is saying that the interaction between the power fields on those power layers might result in high frequency noise on the power layers and will likely cause your design to emit unacceptable EMI radiation levels.. He never did mention the EMI levels he was referring to. For example, the difference between FCC EMI (used on home appliances) and DO-160 EMI levels (used in aircraft) can be quite dramatic.

Was the iMX6 Rex design subjected to any EMI testing? What other approaches might you consider for that board after this discussion with Rick?

Thank you!
Lakshmi , 04-06-2021, 09:06 AM
Here you can find the setup, scripts and results of EMC testing. Content Hardware configuration Test description Test results - Radiated emission Test
Tom Yunghans , 04-06-2021, 11:01 AM
Thank you for that post Lakshmi. I am wondering what specification was used to derive that requirements line on those plots; one of the FCC standards? So it looks like the design passed this particular test. However, I am still interested if Robert would have used a different approach for the power distribution on this board after having that discussion with Rick Hartley.
ibocakir06 , 04-07-2021, 12:01 AM
Hi Tom,

I also have watched the video and thought the same as you did. However when I look to my designs which I designed 3 years ago, 2 years ago, 1 year ago, I see some mistakes. Some of them are not critical but some of are really annoying me.

A good designer always have to improve the designs. Mr. Feranec designed Openrex in 2016, 5 years ago.

Besides in the video you mentioned above, Rick also complains about the recommended stackup given by chip manufacturer. I think he is talking about NXP IMX6 recommended stackup. In NXP's documents the power planes are adjacent.
robertferanec , 04-09-2021, 02:40 AM
Yes, we are all learning ... I keep learning all the time.

About the stackup. We have never had EMC problems, even if I have used 4 power planes in the middle of stackup - and the board was always working perfectly fine. From all my talks, I learned, that it is good to know a lot of new things, but it doesn't mean that if you are not following them, your board is going to automatically fail. In reality, it is not possible to follow all the rules and recommendations, my philosophy is try to follow as much of them as I can.

BTW: There are more EMC measurements on the iMX6 Rex website: https://www.imx6rex.com/?s=emc&submit=Search
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