Multilayer PCB is made up of three or more conductive layers(copper foil layer), these layers are pressed together and form multilayer PCB. Copper foil layer is bonded together by PP(prepreg), Multilayer PCB is one of the most complex types in printed circuit board. Because of the complexity in PCB manufacturing process, low yield rate and difficulty of rework, making the price is relatively high.
Due to the packaging density increase in integrated circuit, which bring about interconnect highly concentrated, thus using multilayer PCB become necessary. In the layout of printed circuit boards, sometimes it will have some unforeseen design problems, such as noise, stray capacitance, crosstalk and so on, therefore, when design printed circuit board, it must be committed to make the length of signal wire as short as possible and avoid parallel circuits, etc. Obviously, it is hard to get satisfied solution from single-sided PCB or even double-sided PCB, due to the cross circuit number is limited. In the demand of mass interconnection and interdigitation, if printed circuit board wants to achieve a satisfactory performance, it must be extend to more than two layers, thus the multilayer PCB appeared.
Multilayer PCB at least has three conductive layers, two layers on surface side, and the rest of the layers are synthesized inside the insulation part. The electrical connection between each layer is usually realized by plated through hole. Unless special instructions, both multilayer PCB and double-sided PCB are plated through hole printed circuit boards.