A multilayer PCB has multiple copper layers which are interconnected through copper plating vias, such as laser vias, NC vias, buried,blind vias, and plated through vias.

Such a circuit board has copper foil, adhesive, pre-peg, and core materials; all layered together under high pressure and temperature to make a multi-layer PCB. The pressure squeezes out the air, whereas the heat melts the adhesive to bind all layers.

A high-layer count PCB is a board with many layers,from 3 to 16, and more layers. In general, you will see an even number of layers in this reference like 4, 6, 8, 10, 12, 14, 16, 18, 20, etc. A multi-layer circuit board helps engineers create complex products.

A PCB with a high-layer count allowsdesigners to implement high-speed and RF design techniques to have ground returns and a high-power distribution.

The BGA(ball grid arrays) of dense devices demand a multi-layer Printed Circuit Board to teach every possible signal.