Multilayer PCBs further increases the complexity and density of PCB designs by adding additional layers beyond the top and bottom layers seen in double sided boards. The bottom and top layer are indicated as outer-layers, all other layers are indicated as inner-layers. The number of layers is referred to as the number of separate conductor pattern, usually even and including the two outer layers.
With the availability of over thirty layers in multilayer PCBs configurations, multilayer PCBs allow designers to produce very dense and highly complex designs.
Multilayer PCBs are built by laminating the various layers. The inner-layers, normally double-sided circuit boards, are stacked together, with insulating layers in between and between the copper-foil for the outer-layers. Holes drilled through the board (vias) will make connections with the different layers of the board. Besides these through holes, many other via constructions are possible, often are these blind, buried, stacked or staggered vias, chosen to save space.
Often the layers are classified as Signal, Power or Ground planes by the designer of the PCB. Increasing the number of power planes in a PCB design increase the level of thermal dissipation a PCB can provide, which can be important in high power designs.