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Chris M. , 05-31-2024, 07:37 AM

Hey Fedevel Com. Hoping someone can enlighten me on a board stack-up question.
Finished the Mixed-Signals Course with KiCAD and loved it.

**Only one question:** In a situation where you have a 4 layer board for example and you have two ground planes, are those two ground planes directly electrically connected, or are they ground planes for only their respective signal/power layers? I understand that signal layers need their respective ground planes to optimise impedance levels in the signal traces.

I believe the answer is "No, they're not directly electrically connected.", but want to confirm.

To further clarify what I'm asking, see the illustration below:

Layer 1 -------------Signal1/Power1-------------
Layer 2 ----------------Ground1----------------
Core ====================================
Layer 3 ----------------Ground2----------------
Layer 4 -------------Signal2/Power2------------

Lets assume:
* Power 1 = 5V plane
* Power 2 = 12V plane
* Signal 1 is a 300kHz signal layer
* Signal 2 is a 3MHz signal layer

Are Ground1 and Ground2 actually electrically connected though the Core by vias for example in this arrangement, or are they separate and only coupled with their respective Signal/Power layers withing the circuit?

Thanks, I hope my question is clear.

Mini , 05-31-2024, 02:04 PM

Of course they are connected through vias. How would you use 5V and 12V if they are not referenced to same GND? They would be both totally independent. You would get tons of problems just by having them together so close or if you happen to connect them externally. Think about how your currents would flow and you will get the idea. Never separate ground planes without really needing to do so. I think Robert has some good videos about ground planes.

Robert Feranec , 05-31-2024, 03:13 PM

Each VIA has a net assigned. Once you place a VIA, any net/track/polygon on any layer connected to the same net will be connected to this VIA. So if you have GND plane on layer 2 and GND plane on layer 3 and if you place a GND VIA, this VIA will make connection between these two planes.

QDrives , 05-31-2024, 07:01 PM

Signals on layer 1 mainly reference Layer 2 for the return. Signals on layer 4 mainly reference layer 3 for the return.
However, the grounds on layer 2 and 3 should be connected using multiple via's.
If you use a via from layer 1 to layer 4, place a Gnd via close by so that the return currents can use that via.

If you do not have components on the bottom, you could swap layers 3 and 4. In that case, when you switch from layer 1 to layer 3, you do not need a Gnd via as the return currents **can** stay in layer 2. However, this too has a caveat, as layer 4 will be more closely coupled to layer 3 than layer 2 will. Again, stiching vias to the rescue.

Also note that switching frequency is something else than the signal frequency. That is defined by the rise time of the signal.

Mini , 06-01-2024, 10:01 AM

100% agree with previous answers. I would add one link to one of the best sources on PCB layers. It's a video by Altium made with Rick Hartley. Should be ABC textbook for beginners. https://www.youtube.com/watch?v=ySuUZEjARPY

Mini , 06-01-2024, 10:03 AM

Btw Robert has quite a few videos as well with him, maybe theres even some playlist.

QDrives , 06-01-2024, 03:11 PM

I have lost track on the amount of Rick Hartley videos. Problem is that I also did the paid training from him so I do not know if some things are in public videos or just in the training.

Mini , 06-01-2024, 03:13 PM

It's totally fine that's where community is great.