I have a circuit that I am currently designing using a 4 layer PCB (signal, ground, power, signal). The PCB has rather a lot of vias to route the signals around the board (there are size constraints, meaning I can't space the ICs as much as I would like which would make it easier to route the signals without vias). The circuit is primarily an audio intercom system, with the vast majority of signals being of audio frequency. There are only a couple of ICs that have high (10MHz) signals and these I have been able to place close together, i.e. reduce signal trace length.
My question is when should you consider adding an additional signal plane? Is there a point at which it becomes better to add an additional signal plane (e.g. going to 6 layer board instead of 4) rather than use a lot of vias? Is there a frequency of operation above which it is better to do this?
Edited to add PCB images:
Best Answer
Before you expand the board to 6 planes, at least stop wasting what you have. What exactly do you imagine a dedicated power plane does for you? Think about it instead of blindly following someone else's religion. Yes, I know a lot of designs are like that, but without a solid technical reason it's just mass superstition.
Use wide enough traces for the power feeds to support the required current, then locally bypass at each point of use to make sure the impedance at high frequencies is low too. Preferentially route high current traces on the bottom layer, then on the top layer, then in middle layers only as a last resort. The reason for that is that the outer layers are usually thicker, or can be made thicker for a smaller incremental cost.
Here is a good layer stackup:
In some cases this layer is a local ground patch for something that makes nasty high frequency currents thru its power feed, like a lot of digital chips and their immediate surroundings. This local ground is then connected to the master ground in exactly one place.