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October 2, 2008

Printed Circuit Boards Part-I

In this series of Basics of Printed Circuit Boards i will try to provide information about PCB's. Things will be provided are what is a PCB,how to make a PCB and what are the advantages and disadvantages of it and etc. This will be a series on printed circuit boards. Keep watching for all the Parts to come.

PCBs are the key components in almost all electronic assemblies. The idea of the PCB originated from the need of placing components and devices on a non-conductive carrier and adding functional and electrical connections with conductive paths. With the invention of PCBs, the former three-dimensional wiring of valves, coins and resistances has been replaced with a two-dimensional pattern on an insulating board.

The history of the PCB begins with the end of the first quarter of the 20th century. The fundamentals of the PCB technology had originated from a patent taken by Cesar Parolini in 1925,but the idea of today’s modern PCB was first created by Dr. Paul Eisler in 1930 and patented in 1943. The first applications of this concept delayed until the end of World War II when PCBs have been used in the USA for building electronically complex military devices like radars and missile controllers. The development of transistors by the late 1950s, which caused to a considerably smaller size and lower dissipation compared to thermionic tubes, and the usage of multi-layer boards made it possible to mount a variety of components and reduce the
overall size of the equipments.

A PCB is a substrate of a paper or glass fabric impregnated with a resin, commonly epoxy,phenolic or silicone. It consists of one or more layers of metal conductors and insulating material which allow electronic components for being electronically interconnected and mechanically supported.

The simplest form of PCB is the single-layer, single-sided board, which
contains metalized conductors only on one side of the board. Greater levels of complexity and component density can be achieved by producing double-sided and multi-layered boards. In a double-sided assembly, the PCB is assembled with components on both sides of the substrate. PCBs can be produced either in an additive or a subtractive process.

In an additive process, the copper is applied directly to the surface of the substrate. In a subtractive process, the copper foil is added first to the whole surface of the substrate. Next, the track pattern is defined and covered with an etch resist. Finally, an etchant is applied on the board removing the excess conductive material and leaving the required track.

Applying the etch resist is referred to as printing and this is the reason
why PCBs are actually called printed circuit boards. Sometimes, both processes
may be combined to produce PCBs with more than one layer of conductive track.

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