Multilayer PCB Board

A multilayer PCB board is used in situations where the density of connections needed is too high to be handled by two layers or where there are other reasons such as accurate control of line impedances or for earth screening.

A multilayer PCB contains two reference planes and a signal via. The signal via allows a signal to flow through all the planes. A stitching via is connected to one of the planes next to the signal via and serves to reduce the area through which the signal passes through. This is very important as it may assist in reducing noise and cross talk.

The electrical circuit is completed by interconnecting the different layers with plated through-holes, placed transverse to the board at appropriate places. Multi-layer boards have three or more circuit layers, while some boards have even as many as 50 layers.

Advantages of multilayer PCBs include high reliability and uniform wiring. However, the initial costs are higher than that of one-layered PCBs. Also, repairing a multilayer PCB is quite difficult.

• Wherever weight and volume savings in interconnections are the overriding considerations
• When the complexity of interconnection in sub-systems requires complicated and expensive wiring or harnessing
• When frequency requirements call for careful control and uniformity of conductor wave impedances with minimum distortions and signal propagation, and where the uniformity of these characteristics from board-to-board is important
• When coupling or shielding of a large number of connections is necessary
• With multilayers, all interconnections can be placed on internal layers, and a heat sink of thick solid copper can be placed on the outer surfaces.