What is the thickness of the inner copper layers?
For a 4 layer stack-up, the inner copper thickness is 0.017mm (0.67mils) per 0.5oz copper.
1oz copper
FR4
0.5oz copper
1.2mm FR4
0.5oz copper
FR4
1oz copper
| Inner Layer Copper Thickness (example for 4 layers) | 0.017mm(0.5oz) | 0.67mils |  | |