Printed Circuit Boards
Printed Circuit Boards (PCB), or Printed Wiring Boards (PWB) as they are sometimes called, have come along way since their invention in 1942 by Dr. Paul Eisler. Originally intended as a replacement for costly cable harnesses they are now found in virtually all electronic equipment.
As the manufacturing technology improved the number of internal layers were increased and the width of both the tracks and the spacing between these tracks decreased. Further improvements in the tracking density were realised by reducing the size of the holes drilled in the boards and by moving to blind and buried vias which only penetrate those layers necessary to achieve the required connections. By the mid seventies PCBs incorporating 16 layers were being produced, albeit at a high price. In the early eighties the emergence of surface mount techniques drove component densities upward and prompted further size reductions. Line widths and spacing for tracks as small as 0.006" (152 micron) is quite common
Toward the end of the 20th century some advanced manufacturers were beginning to embed low tolerance passive components such as resistors and capacitors within the material of the PCB itself. This technique was developed to allow increased packing densities and reduce costs. Another technique some manufacturers are using is to produce a 3-dimensional circuit on the inside surface of a moulded plastic enclosure. Although this is of limited application, as it is restricted to single sided surface mount usage, it does allow for significant cost savings on simple consumer products where a separate circuit board can be eliminated completely.
It is interesting to reflect on how far PCB technology has come in the short time since its invention. It is also worth noting that the original invention was never patented. If it had been the take up of the technology may have been much slower and early products would have been more costly as a result of royalty costs relating to the patent. Dr. Eisler probably never imagined the universal application of his original idea, and consequently never got rich on his invention.
There are a number of materials used in the construction of PCBs.
Some are for very specialist applications such as the boards that
incorporate an invar metal layer to aid heat dissipation in high-density
power assemblies. Others are flexible circuit boards, and hybrid flex-rigid
boards, that can be folded around the mechanical assembly of their
enclosures. The vast majority are of conventional rigid construction.
The most common substrates are summarized below.