What items start mattering as PCB layer count (40+) increases?

[mav1234]

I am looking at a design that may require really high layer count, like 40+ layers. The board will have traces like: 10GbE, SAS, PCIe...

My question is: What items start mattering as layer count increases so much?

(Sorry if the question is a bit weird, but trying to learn about things I should look out for)

 

[thylacine1975]

Is it facetious to say "cost"?

I admit I'm guessing since my own experience maxxes out at only 1/4 of that value but with such thick boards, mechanical aspects will start becoming significant. Electrically, the only things that spring to mind are via related: the inductance and DC resistance of a via spanning the entire board thickness will be considerable. This will need to be considered when routing high current/high frequency signals between layers.

Mechanically, a host of issues would (likely) arise:
* z-axis expansion will lead to significant geometry changes with temperature. Beware of via cracking and or bending/warping due to asymmetries.
* The thermal conductivity will strongly depend on the PCB substrate which may be an issue with power devices. The normal approach of thermal conduction through vias may be inadequate.
* The allowable aspect ratio of the drilling process will affect the minimum hole size. Check with the PCB manufacturer - some require the drill diameter to be as large as depth/4.