The ‘’‘’‘’ represents a relatively wider distance, approximately 0.5mm. The key difference between the two choices is the reference plane of L6, whether it’s closer to PWR plane 5 or closer to GND plane 7. Since I have multiple power rails and need to split the PWR plane, I’m inclined to think that positioning L6 closer to L7 might be more beneficial for maintaining continuous transmission impedance. Would this approach be better than bringing it closer to the PWR plane? Seeking insights on this decision.
Stack-up 1 comes the closest to ideal. Ideal, being “every signal layer requires an adjacent solid return plane and every power plane or routed power requires an adjacent solid return plane.” This will adequately confine the digital signals propagating as electromagnetic waves between the trace and return plane, as well as confine any power rail transients between the power plane (or routed power) and the return plane.
Change layer 4 to a solid return plane and then run power planes or routed power all on layer 5.
I don’t advocate referencing signals to power plane (as in stack-up 2), unless you’re dealing with relatively low clock speeds. Most digital return currents referenced to a power plane must “somehow” return to digital return. Unless you define a solid return path, you’re asking for EMC trouble.