Electronic – The distance of the adjacent signal layer in PCB stackup

pcbstack up

I saw in some PCB stack-up design, sometimes there are two adjacent signal layers, such as:

SIG

GND

SIG

SIG

PWR

SIG

I think there must some crosstalk between them, so how much should be enough of the distance between the two layers to minimize the crosstalk?

Best Answer

It depends on what the signals are.

In a 6-layer PCB, you've probably got quite a lot of parallel buses to get from place to place. So common technique is to route each of the two adjacent signal layers with tracks perpendicularly as much as possible; one of the layers predominantly 'north-south', and the other layer predominantly 'east-west', avoiding parallel runs in close proximity as much as possible.

If that isn't significantly possible, and if the signals are significantly likely to cross-talk to others, then physical separation (in the X/Y plane) can be important.

This can be critical for some combinations of signals: high-speed clocks, high-impedance inputs, analog circuitry, high current loops, etc. Sometimes it's just not possible to keep them separated in X/Y enough, and you might have to give priority to some of these signals being on outer layers where they have the ground/power-plane between them and everything else on other layers.