Printed circuit (PC) boards are thin boards made of materials that do not conduct electrical current, but which have electronic components mounted on a network of conductive tracks which join the components together to form a complete circuit. The term multi-layer boards refers to PC boards that consist of a composite wafer made up of several boards bonded together to reduce the size of the finished board while maintaining the circuit size or complexity. These boards may consist of as few as two layers and as many as 50, depending on the complexity of the circuit. The separate layers are insulated from each other to avoid short circuits, and are interconnected by plated or conductive through holes.
Printed circuit boards (PCBs) first saw the light of day in 1936 when an Austrian engineer, Paul Eisler, incorporated one in a radio set. The PCB grew steadily in popularity and sophistication throughout the 1940s and '50s with the first multi-layer board being developed in 1961. The huge benefits offered by multi-layer PC boards was immediately apparent, and their development has continued apace ever since.
Multi-layer boards have many benefits over conventional double-sided, single-layer PCBs. They allow for considerable savings on space, allow for the easy, simultaneous shielding of large numbers of components, and cut down on the number of interconnection wiring harnesses that would be needed if separate circuit boards were used. These interconnections represent a considerable addition to the space a circuit occupies and add substantially to the overall weight of the system.
These savings are of particular value in industries such as aviation, where space and weight are major considerations when designing and constructing aircraft. The internal connection characteristics of multi-layer boards also allows for the outside surfaces of the completed board to be used to mount larger heat sinks which allow cooler operation. Again, industries such as aviation and aerospace benefit considerably from this feature.
The use of multi-layer boards also holds several benefits for applications where high levels of uniformity in conductor wave impedance are required. In addition, multi-layer boards offer superior reductions in distortion and signal propagation in applications where signal integrity and "cross talk" levels are critical. High levels of overall uniformity of these characteristics can also be maintained across all boards in the laminate through the use of multi-layer construction. Although multi-layer boards are comparatively expensive to produce and very expensive to repair, their benefits are extensive and they have both revolutionized the electronics industry and defined the future of printed circuit board technology as a whole.