A multilayer PCB is a circuit board with more pattern layers than the outer (visible) layers. The number of layer are always even, like 4, 6 or 8 and so on. The layers are connected to each other by plated holes. By using different mechanical and optical processes it is possible to position the layers perfectly over each other. The thickness of the PCB, the distances between the layers as well as the copper thicknesses can be stated in the PCB specification. If the “stack-up” is not specified a standard stack-up will be used.