Return Currents on the Bottom Layer

General
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In my 4-layer PCB design with the layer stack SIG-GND-PWR-SIG, I’ve been considering the behavior of return currents and electromagnetic fields, particularly when routing signals on the bottom layer.

When routing signals exclusively on the top signal layer, the high-frequency signals have a GND reference plane underneath, facilitating return currents and containing the electromagnetic fields. However, when transitioning from the top to the bottom layer, what happens to the return currents and fields?

Given that the return currents tend to follow the trace path, they likely seek the nearest via to the ground plane and continue along the trace path. However, with the power (PWR) plane situated between the signal and ground planes, there’s a disruption in the coupling of fields to the ground plane. How do the return currents behave in this scenario? Do they disperse throughout the ground plane, or is there another mechanism at play when routing signals on the bottom layer of a 4-layer stack?

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If you ensure effective decoupling between the power and the ground planes, the impedance between them remains minimal at high frequencies. Consequently, your high-frequency signal won’t distinguish between the ground and power planes.

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If you have placed ground vias in the proper places, the current will flow nicely from top to bottom. If not, then it will follow the shortest path. The power plane may act as a reference also if they are properly coupled with the ground plane.

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