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 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 requirement for multi-layer boards has been necessitated by the increasing packaging density of PCB Spring Terminal Block, which bring about 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 achieve satisfactorily performance through the circuit in the actual existence of an extremely great number of interconnections and cross-over, the PCB must be extended beyond two-plane approach.
This provides rise to the concept of multi-layer circuit boards. Hence, the main 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, where two layers are on the outside surface while the remaining one is incorporated into the insulating boards. The electrical connector is normally completed through plated through-holes, which are transverse towards the boards. Unless otherwise specified, multi-layer PCBs are assumed to get plated through hole as with double-sided boards.
Multi-layer boards are fabricated by stacking two or more circuits on top of every other and establishing a trusted set of pre-determined interconnections between the two. The technique begins with a departure from conventional processing in that each of the layers are drilled and plated before they may be laminated together. Both innermost layers will comprise conventional two-sided PCB whilst the various outer layers is going to 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, that are signal layers, are plated through in a way that uniform donuts of copper are formed on the underside rims from the through-holes. This can be then lamination from the various layers into a composite multi-layer with wave-solderable interconnections. The lamination may be done in a hydraulic press or perhaps in an over-pressure chamber (autoclave). Within the case of Plug In Terminal Block, the prepared material (press stack) is positioned within 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 at which the amorphous polymers (resins) or perhaps the amorphous parts 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 needs to be a trade-off which is the cost for space and weight versus the board’s costs. They are also 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 provide areas.