The insulating material between copper layers. Usually FR4 fiberglass.
Real image: CORE3588, rendered by KiCad from our board file
The front edge of CORE3588, rendered by KiCad, very close up.
- 1The substrate: the brown-grey band is the board material itself, 1.6 mm of glass-fibre and resin. Copper is too thin to see at this size.