A 6-layer PCB is composed of a PCB core that has two PCB layers, and two layers on its two sides. It belongs to multilayer PCBs
A PCB core consists of a substrate material layer and two copper layers. It is cut from a double-sided copper-clad laminate. Between the copper layers and the PCB core is the prepreg. Prepreg and PCB substrate are the same materials, but only that prepreg is semi-cured, while the substrate is cured. An external PCB layer consists of a prepreg riveting with a copper foil.
The 6 layers include signal layers, ground (GND) layers, and power layers. The top layer (layer-1) and bottom layer (layer-6) must be signal layers. The four internal PCB layers include 2 signal layers, 1 ground layer, 1 power layer, or 1 signal layer, 2 ground layers, 1 power layer. So there are 4 6-layer PCB stack-ups.
6-layer PCB stackup one: signal1-GND2-signal3-signal4-power5-signal6
6-layer PCB stackup two: signal1-signal2-GND3-power4-signal5-signal6
6-layer PCB stackup three: signal1-GND2-signal3-GND4-power5-signal6
6-layer PCB stackup four: signal1-GND2-signal3-power4-GND5-signal6