Multi-layer PCBs are circuit boards comprised 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 by far the most complex form of printed circuit boards. Their cost is relatively high, due 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 SMT Terminal Block, which give rise to high concentration of interconnecting lines.
The printed circuit layout brings about 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 form of requirements could not really met satisfactory in single-sided and even double-sided printed circuit boards because of limited cross-over which could be realized. Thus, to achieve satisfactorily performance through the circuit in the actual existence of a very large number of interconnections and cross-over, the PCB has to be extended beyond two-plane approach.
This gives rise to the concept of multi-layer circuit boards. Hence, the primary intent of fabricating a multi-layer printed circuit board would be to provide yet another degree of freedom in the selection of suitable routing paths for complex Feed Through Terminal Block. Multi-layer boards have a minimum of three layers of conductors, by which two layers are on the outside surface whilst the remaining the first is integrated into the insulating boards. The electrical connector is normally completed through plated through-holes, which can be transverse to 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 other and establishing a reliable set of pre-determined interconnections between the two. The process begins with a departure from conventional processing because all the layers are drilled and plated before they may be laminated together. Both innermost layers will comprise conventional two-sided PCB while the various outer layers is going to be fabricated as separate single-sided PCBs.
Just before lamination, the inner layer boards will likely be drilled, plated through, imaged, developed and etched. The drilled outer layers, which are signal layers, are plated through in such a way that uniform donuts of copper are formed on the underside rims of the through-holes. This can be accompanied by lamination from the various layers into a composite multi-layer with wave-solderable interconnections. The lamination may be performed in a hydraulic press or in an over-pressure chamber (autoclave). Within the case of Module Box, the prepared material (press stack) is positioned inside the cold or pre-heated press (170 to 180 °C for material having a high glass transition point). The glass transition temperature will be the temperature in which the amorphous polymers (resins) or perhaps the amorphous regions of a partially crystalline polymer change from a hard and relatively brittle state to a viscous, rubbery state. Multi-layer boards find applications in professional eqrfdn (computers, military equipment), particularly whenever weight and volume are definitely the over-riding considerations.
However, there has to be a trade-off which is simply the cost for space and weight versus the board’s costs. Also, they are very useful in high-speed circuitry because greater than two planes are offered to the PCB designer for running conductors and providing for large ground and offer areas.