One sheet of copper. 2-layer = top + bottom; 4-layer = top + 2 inner + bottom.glossary, Core concepts
One sheet of copper in the board; CORE3588 has four, signals top and bottom, ground in the middle.glossary, Physical / electrical concepts
How it looks
Real geometry: CORE3588 copper, one panel per layer
The four copper layers of CORE3588, each drawn flat from our board file, all seen from the top. Grey = copper, light dots = pads and vias that pass through.
1Top copper (F.Cu): parts sit here; most tracks.
2Inner 1: almost solid ground copper.
3Inner 2: also ground.
4Bottom copper (B.Cu): more tracks, plus ground fill.
How it works
Where it fits
The order and thickness of the layers is the stackup. KiCad calls the outer ones F.Cu and B.Cu; the insulator between them is the substrate.