The definition of multilayer PCB is a PCB that is made with three or more conductive copper foil layers. These appear as several layers of double-sided circuit boards, laminated and glued together with layers of heat-protective insulation between them. The entire construction is arranged so that two layers are placed on the surface sides of the PCB to connect to the environment. All electrical connections between the layers are achieved with vias such as plating through holes, blind and buried vias. Application of this method then leads to the generation of highly complex PCBs of varying sizes.

The multilayer PCB came into being due to the evolving changes in the electronics industry. The functions of electronics have become progressively more sophisticated over time, requiring more complex PCBs. Unfortunately, PCBs were limited by problems like noise, stray capacitance and crosstalk, and therefore needed to follow certain design constraints. These design considerations made it difficult to get a satisfactory level of performance from a single or even double-sided PCB - thus the multilayer PCB was born.

Packing the power of a double-layer PCB into a format that's a fraction of the size, multilayer PCBs are becoming increasingly popular in electronics. They come in a wide range of sizes and thicknesses to accommodate the needs of their expanding applications, with variants ranging anywhere from four to twelve layers. Layers most often come in even numbers, since odd numbers of layers can cause issues in the circuit like warping, and are no more cost-effective to produce. Most applications require between four and eight layers, though applications like mobile devices and smartphones tend to use around twelve layers, and some professional PCB manufacturers boast the ability to produce multilayer PCBs with nearly 100 layers. Multilayer PCBs with that many layers are rare to see, however, as they are extremely cost-inefficient.