Multilayer PCBs are circuit boards made up of more than two electrical layers superimposed on each other. The copper layers are joined by layers of resin. Multilayer boards represent the most complicated type of printed circuit boards. Cost is relatively high, due to the complexity of the manufacturing process, less production and difficulty of rework. The need for multilayer boards has been demanded by the increasing packing density of the integrated circuits, which give rise to a high concentration of interconnection lines.
The layout of the printed circuit results in unpredictable design problems such as noise, parasitic capacitance, cross talk, etc. The design of the PCB should therefore aim to minimize the length of signal lines and avoid parallel routing etc. Obviously, such requirements are not satisfactorily met on single-sided and even double-sided printed circuit boards because of the limited crossover that can be accomplished. Thus, to achieve satisfactory circuit performance in the presence of a very large number of interconnections and cross-over, the PCBA manufacturing must be extended beyond the two-plane approach.
This gives rise to the concept of multilayer circuit boards. Therefore, the primary intent of making a multilayer printed circuit board is to provide a further degree of freedom in selecting suitable routing paths for complex and / or noise-sensitive electronic circuits. Multilayer plates have at least three layers of conductors, in which two layers are on the outer surface, while the other is embedded in the insulation plates. The electrical connector is commonly completed through coated through holes, which are transverse to the plates. Unless otherwise specified, multilayer PCBs are assumed to be bathed through the hole as in double-sided plates.