11.1 What are Multi-layers?

Multi-layers, or multi-layer PCBs are circuit boards made up of more than two electrical layers (copper layers) superimposed on each other. The copper layers are bonded together by resin layers (prepreg). Multi-layer boards represent the most complex type of printed circuit boards. Their cost is relatively high, owing to the complexity of the manufacturing process, lower production yields and difficulty of re-working on them.

The need for multi-layer boards has been necessitated by the increasing packaging density of integrated circuits, which give rise to high concentration of interconnecting lines. The printed circuit layout results in unpredictable design problems like noise, stray capacitance, cross-talk etc. The PCB design, therefore, must aim at minimizing the length of the signal lines and avoiding parallel routing etc. Obviously, such type of requirements could not be met satisfactory in single-sided and even double-sided printed circuit boards due to limited cross-over which could be realized. Thus, to achieve satisfactorily performance from the circuit in the presence of a very large number of interconnections and cross-over, the PCB must be extended beyond two-plane approach. This gives rise to the concept of multi-layer circuit boards. Hence, the primary intent of fabricating a multilayer printed circuit board is to provide one more degree of freedom in the selection of suitable routing paths for complex and/or noise-sensitive electronic circuits.

Multi-layer boards have at least three layers of conductors, in which two layers are on the outside surface while the remaining one is incorporated into the insulating...