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Printed circuit boards evolved from electrical connection systems that were developed in the 1850s. Metal strips or rods were originally used to connect large electric components mounted on wooden bases. In time the metal strips were replaced by wires connected to screw terminals, and wooden bases were replaced by metal chassis. But smaller and more compact designs were needed due to the increased operating needs of the products that used circuit boards. In 1925, Charles Ducas of the United States submitted a patent application for a method of creating an electrical path directly on an insulated surface by printing through a stencil with electrically conductive inks. This method gave birth to the name "printed wiring" or "printed circuit." In the 1943, Paul Eisler of the United Kingdom patented a method of etching the conductive pattern, or circuits, on a layer of copper foil bonded to a glass-reinforced, non-conductive base. Widespread use of Eisler's technique did not come until the 1950s when the transistor was introduced for commercial use. Up to that point, the size of vacuum tubes and other components were so large that the traditional mounting and wiring methods were all that was needed. With the advent of transistors, however, the components became very small, and manufacturers turned to printed circuit boards to reduce the overall size of the electronic package. Through hole technology and its use in multi-layer PCBs was patented by the U.S. firm Hazeltyne in 1961. The resulting increase in component density and closely spaced electrical paths started a new era in PCB design. Integrated circuit chips were introduced in the 1970s, and these components were quickly incorporated into printed circuit board design and manufacturing techniques. There is no such thing as standard printed circuit boards. Each board has a unique function for a particular product and must be designed to perform that function in the space allotted. Board designers use computer-aided design systems with special software to layout the circuit pattern on the board. The spaces between electrical conducting paths are often 0.04 inches (1.0 mm) or smaller. The location of the holes for component leads or contact points are also laid out, and this information is translated into instructions for a computer numerical controlled drilling machine or for the automatic solder paster used in the manufacturing process. Once the circuit pattern is laid out, a negative image, or mask, is printed out at exact size on a clear plastic sheet. With a negative image, the areas that are not part of the circuit pattern are shown in black and the circuit pattern is shown as clear. MadeHow.com. “Printed Circuit Board.” How Products Are Made Vol. 2. 29 Dec. 2009 [http://www.madehow.com/Volume-2/Printed-Circuit-Board.html].
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