I am designing an 8 layer board with the following stackup
1 signal
2 ground
3 signal
4 power
5 power
6 signal
7 ground
8 signal
I need to have the signal layer as 50 ohmn impedance. How do I find out what the pcb manufacturer will use for the thickness of the layers so that I can calculate my track widths and spacing to give me 50 ohmn?
Inform the PCB manufacturer on which layers you are planning to rout those 50 ohms traces and you can request the fabricator to give the stackup with the required trace widths. This will work best compare to the available impedance calculators.
They are the best to give you the most close value.