Multi-layer PCBs are circuit boards made up of a lot 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 expense is relatively high, owing to the complexity of the manufacturing process, lower production yields and difficulty of re-working on them. The requirement for multi-layer boards has been necessitated by the increasing packaging density of SMT Terminal Block, which lead 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 size 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 because of limited cross-over that could be realized. Thus, to accomplish satisfactorily performance through the circuit in the presence of a really large number of interconnections and cross-over, the PCB should be extended beyond two-plane approach.
This offers rise to the idea of multi-layer circuit boards. Hence, the key intent of fabricating a multi-layer printed circuit board would be to provide an additional degree of freedom in selecting suitable routing paths for complex Feed Through Terminal Block. Multi-layer boards have at the very least three layers of conductors, where two layers are on the outside surface as the remaining one is integrated into the insulating boards. The electrical connector is commonly completed through plated through-holes, that are transverse for the boards. Unless otherwise specified, multi-layer PCBs are assumed to get plated through hole just like double-sided boards.
Multi-layer boards are fabricated by stacking 2 or more circuits on top of each and every other and establishing a trusted set of pre-determined interconnections between the two. The process begins with a departure from conventional processing in that all of the layers are drilled and plated before they may be laminated together. The two innermost layers will comprise conventional two-sided PCB as the various outer layers will likely be fabricated as separate single-sided PCBs.
Before lamination, the inner layer boards will likely be drilled, plated through, imaged, developed and etched. The drilled outer layers, which can be signal layers, are plated through in a manner that uniform donuts of copper are formed on the underside rims from the through-holes. This can be followed by lamination of the various layers right into a composite multi-layer with wave-solderable interconnections. The lamination may be done in a hydraulic press or even in an over-pressure chamber (autoclave). Inside the case of Transformer Terminal Block, the prepared material (press stack) is placed inside the cold or pre-heated press (170 to 180 °C for material with a high glass transition point). The glass transition temperature is the temperature in which the amorphous polymers (resins) or perhaps the amorphous parts of a partially crystalline polymer change from the hard and relatively brittle state to some viscous, rubbery state. Multi-layer boards find applications in professional eqrfdn (computers, military equipment), particularly whenever weight and volume would be the over-riding considerations.
However, there should be a trade-off which is just the cost for space and weight versus the board’s costs. Also, they are very useful in high speed circuitry because a lot more than two planes are offered to the PCB designer for running conductors and providing for big ground and supply areas.