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I made a 4 layer PCB according to a recommendation from this datasheet

enter image description here https://www.ti.com/lit/ds/symlink/dp83848-ep.pdf?ts=1613467684339

  1. Signal
  2. Ground
  3. 3.3v
  4. Singal

But instead, when I notice how the PCB was actully made, it was:

  1. Signal
  2. 3.3v
  3. Ground
  4. Singal

Is this OK, does it matter? I mean, signal is signal, right? The ethernet Phy will not get noise from the 3.3v signal? I'm using both the bottom layer and top layer for signals, but most top layer.

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  • \$\begingroup\$ It is possible that everything will be OK. Do you have any high speed data busses like DDR memory? I am guessing not in four layers. You may want to take a look at the gerbers. View layers 1 and 2 at the same time to see if any signals cross voids or splits in the VCC plane. Layer 4 should be OK, assuming ground is at least as solid as VCC. \$\endgroup\$
    – mkeith
    Feb 16, 2021 at 10:07

2 Answers 2

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Firstly, given the information you have provided, there is no difference between the two scenarios. Signals are signals and there is no information in the question to differentiate signals that might need a better ground-plane from those that aren't so needy.

So, if the 3.3 volt rail is properly decoupled with capacitors fairly regularly then there shouldn't be any repercussions on the swapping of layers. If the 3.3 volt supply has moderate noise there may be some issues arising with sensitive analogue signals that are relying on a clean 0 volts as a ground-plane. Only you can decide this.

In future, you should mark inner layers so that the board manufacturer can build the stack in the right order. Maybe you did this? Maybe the board manufacturer screwed up?

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  • \$\begingroup\$ No. The 3.3v is clean from noise and I have placed decoupling caps near every IC and I have been thinking about EMC all the time. :) \$\endgroup\$
    – DanM
    Feb 16, 2021 at 10:11
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    \$\begingroup\$ @DanielMårtensson then you should be good to go. \$\endgroup\$
    – Andy aka
    Feb 16, 2021 at 10:19
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Wanna show you a nice trick. Carefully pick your board up. Hold it. Now FLIP IT UPSIDE DOWN. You have just fixed your stackup. Congratulations. Use this trick whenever you need :)

I assume 4-layer PCB is not for a gigahertz frequency circuits like DDR or something, other than that, shouldn't be a problem. If you don't have serious cuts in the planes that would mess up ground currents of the signals, you should be OK

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    \$\begingroup\$ Only 50 Mhz, sir. \$\endgroup\$
    – DanM
    Feb 16, 2021 at 10:12
  • \$\begingroup\$ Should be fine. Of course, give it a test, scope stuff if possible, but I would expect things to work. Check out youtube videos of Robert Feranec about ground, ground currents and stuff. He invited some kickass engineers too. They're a little hard to understand because the topic is complex, even I managed to understand some important points even if not all (and I'm by no means expert or anything) \$\endgroup\$
    – Ilya
    Feb 16, 2021 at 10:15

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