Stitching capacitors

Multilayer PCB boards use a stitching capacitor to increase coupling capacitance between ground and power plane

If planes as above then coupling between power and GND is important

If both middle planes are GND then coupling between them is not important

In the 2nd stack up the gap between GND and power increased compared to the 1st stackeup. there for it reduces coupling between those layers. so that increases impedance between them

C = qA/d

d = gap (0.04 inch 2nd picture )

When d increases C reduces, then impedance increases between layers

Less impedance is important for the return path of signals

Z = 1/jwC

Impedance is inversely proportional to capciatance (impedance formula for capacitor)

So that when c reduces impedance increases -> not good for the return path

Capacitive coupling- in this case, middle layers are in different voltage potential (PwR and GND) then they are coupled capacitive

In this case, can use stitching capacitors between PWR and GND to increase the coupling (capacitive coupling) good for return path, less impedance

A stitching capacitor provides a path for the return current to flow back to its source. ... The capacitor would connect between the ground plane and power plane and must be located adjacent to where the signal trace changes reference planes

Decrease in the gap between signal and GND is better --> increases the coupling between signal and ground

As in the below picture Never Cross a Ground Plane Gap in High-Speed PCB Design - use a stitching capacitor to guide return current