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I'm trying to understand how signals flows in the GND layer.

Let's assume that I have 4 layers PCB:

  1. Signal
  2. GND
  3. POWER
  4. Signal

4 layers PCB -- CPU connect the VCC on top and with VIA to GND

The CPU connect to the GND layer with VIA and I want to know, how the CPU return current will flow to the "Negative Terminal"?

I read that the return current in the GND layer flow below it signal, so the current flow directly from the via to the negative terminal in the shortest way?

Or does it flow below the VCC trace (in RED) and from there to the negative terminal?

About the VCC layer: I once told that the VCC layer can act like a GND layer with every thing related to return current.

I don't understand how this is possible for current to return via VCC layer?

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  • \$\begingroup\$ "I read": where? Context! We can very much reduce the answer to: "in this simplicity, the thing you've read is wrong", but usually, you could get a better answer by giving more context. \$\endgroup\$ – Marcus Müller Mar 26 '17 at 8:36
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The basic rules are very simple. The consequences of those rules can be quite complicated.

A DC current will flow in the path of least resistance.

An AC current will flow in the path of least impedance.

For DC, the path of least resistance means that current will spread out and cross most of the ground plane, between the IC connection and the power supply connection. This may or may not be what you want, if the current flows through parts of the ground plane that have uV sensing across them. This is where careful placement on the plane, and even cutting slots in the plane can be required to control where current flows.

For very high frequency, loop area is so important that where there is a signal trace above ground, the signal current will flow on the underneath of the trace, and the return current will flow on top of the ground plane below the trace.

At lower frequencies, resistance plays a bigger part in impedance, and the situation is somewhere between the DC case and the AC case.

Ground planes are a mixed blessing. If you're lucky, they avoid the need to think about return current paths. If you're unlucky, they can get you into all sorts of trouble, because you haven't thought about return current paths.

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