I've been back and forth with various PCB manufacturers this week regarding their 'standard' 6-layer configurations. The following diagram represents the configuration I'm after:
I only need 3 signal layers, and want each to have a closely coupled GND. From my discussions with the various manufacturers, it seems that the above stack isn't typically catered for in 'standard' configurations, as I've not had much luck determining a suitable configuration. The below is the closest I've come across (outside of going with a custom layer configuration):
My concern with the above, for which I'm seeking some input, is the spacing of the internal layers. In the top image, the separation between layers 3 and 4 is quite pronounced, to ensure the top GND plane (layer 2) is closely coupled to the second signal layer (layer 3). In the above stack, this separation is 14 mil, while the 'coupling' between layers 3 and 2 is 13 mil, which seems like much less of a pronounced separation than I would expect.
Is there any issue with the above stack? My concern is the power plane (layer 4) being used as the return path for layer 3, as the power plane is fragmented with multiple different voltage. I've used plenty of stitching vias for signal transitions between the various signal layers, as they share a reference GND plane; I don't have the space to use stitching capacitors, hence the need to ensure layer 3 used layer 2 as the return reference.
The top diagram specifies 'largest distance', which the above configuration satisfies; am I just being paranoid/pedantic? I'm not sure on the cost difference, but would it be worth specifying a custom configuration which increases the separation between layers 3 and 4, and reduces the dielectric height between layers 2 and 3, and layers 4 and 5?