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 by far 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 SMT Terminal Block, which lead to high concentration of interconnecting lines.
The printed circuit layout leads to 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 form of requirements could not met satisfactory in single-sided and even double-sided printed circuit boards due to limited cross-over which may be realized. Thus, to accomplish satisfactorily performance from the circuit in the existence of a very large number of interconnections and cross-over, the PCB should be extended beyond two-plane approach.
This gives rise to the concept of multi-layer circuit boards. Hence, the key intent of fabricating a multi-layer printed circuit board would be to provide one more degree of freedom in the selection of suitable routing paths for complex Plug In Terminal Block. Multi-layer boards have a minimum of three layers of conductors, in which two layers are on the outside surface while the remaining one is incorporated into the insulating boards. The electrical connector is often completed through plated through-holes, which can be transverse towards the boards. Unless otherwise specified, multi-layer PCBs are assumed to get plated through hole similar to double-sided boards.
Multi-layer boards are fabricated by stacking 2 or more circuits on top of each and every other and establishing a dependable set of pre-determined interconnections between them. The procedure begins with a departure from conventional processing in this all the layers are drilled and plated before these are laminated together. Both innermost layers will comprise conventional two-sided PCB whilst the various outer layers will likely be fabricated as separate single-sided PCBs.
Just before lamination, the inner layer boards will be drilled, plated through, imaged, developed and etched. The drilled outer layers, which can be signal layers, are plated through in such a way that uniform donuts of copper are formed on the underside rims in the through-holes. This is followed by lamination in the various layers in to 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). In the case of Module Box, the prepared material (press stack) is positioned within the cold or pre-heated press (170 to 180 °C for material using a high glass transition point). The glass transition temperature will be the temperature at which the amorphous polymers (resins) or perhaps the amorphous areas of a partially crystalline polymer change coming from a 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 are definitely 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 beneficial in high speed circuitry because more than two planes are offered to the PCB designer for running conductors and providing for big ground and supply areas.