Can multiple return paths cause EMI problems?

Question

I am working with a 4-layer board (Signal - GND - PWR - Signal).

The power layer uses routed power so that each chip's power rail can be isolated. Layers 1, 3, and 4 have ground pours and stitching vias. The ground layer is solid, so the routed power has a good return path on the adjacent ground. The stackup has the outer layers 0.065 mm separated from the adjacent layer, and power and ground are separated by 1.2 mm

The possible issue that I see is that the ground pour is over 18 times closer to the power trace than the dedicated ground plane. The coupling to the layer 4 pour is much greater than to the ground plane. Unfortunately the pour is interrupted by traces so that return current on the pour cannot fully follow the power trace.

I would think that when the return current on the pour is interrupted noise will be radiated inside the board. This board fails conducted voltage emissions.

To summarize, there are 2 return paths. One is closely coupled but interrupted. The other has less coupling but is properly decoupled on both ends.

Answer 1

Multiple return paths are the usual case. But there is only one decisive one: the one with the smallest loop area and hence inductance. This is the dominant return path taken by (EMI relevant) high frequency current.

If your intended GND plane return path is too far away, that means the current will return through top layer copper. And this is the reason why your stackup often causes EMI issues, and frankly being a terrible stackup decision when the gap between the two inner layers is big.

Mitigation

1. if possible with a stackup like yours, make both inner layers GND and route power between signals on the outer layers if space permits..That way both power and signals with excellent return loops and negligible coupling to eachother.

2. use the power distribution only for low frequencies such as below 1 kHz, and take high frequency power only from local decoupling that has bottom layer GND close to L3 power. That way you at least solve the power return part. You will still need to care for all the signal on L4 running near the power layer. Make sure their return current are closed with capacitors. In the end, you may realize why (1) is actually often easier.