Circuit board assembly is a process where individual electronic devices are placed on bare circuit boards. During a second phase of assembly, the parts are permanently soldered in place. Only a few years ago, the dominant circuit board assembly technology required that electronics components be placed on the circuit board (either by hand or by machine) by passing the metal leads emanating from the part trough metal-plated holes drilled through the board. Using this technique the part is placed into its assigned set of holes and then soldered in place. This assembly technology is known as 'through-hole' or 'pin-through-hole' (PTH). PTH technology is in decline because it impossible to drill holes close enough to accommodate highly integrated components.
Because of the great number, fragility, and tight configuration of the leads emanating from today's highly integrated devices (1) , many advanced electronics components cannot be placed on circuit boards by hand. The tolerances required in placing components on a board are simply beyond the limits of human dexterity. Each lead must align exactly with the appropriate wire embedded in the circuit board or the product will not work. Thus, in many factories, workers hand-placing color-coded parts on circuit boards have been largely replaced by expensive robotics equipment that uses machine vision systems to accurately align and place parts.
The drive toward miniaturization of products has increased the density in circuit boards. To accommodate all the required circuitry in a smaller area of board space, many new products contain circuit boards that consist of multiple layers. Today, circuit boards are fabricated with as many as 18 layers. As for PTH technology, multi-layer boards inhibit extensive drilling, because it is difficult for layout designers to avoid the dense network of wire vias embedded in the board. The technical solution to these problems has been to place parts on the surface of the circuit board with a process called 'surface mount technology' (SMT).
SMT does not require holes to be drilled in the circuit board, allowing layout designers to specify much tighter lead spacing. In fact, SMT compo- nents are often placed on both sides of the board, further increasing the density of the circuitry in the product. SMT allows product designers to create electronics products that are smaller and have a vastly increased func- tionality.
'Laptop' computers rely on circuit boards assembled using SMT, as do VCRs, portable phones, pagers, modems, etc. Because of the general avail- ability of electronics components packaged in the SMT format, even products
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