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Surface mount technology refers to a manufacturing method for electronic circuits. This method uses specially designed components which do not have leads to go through the circuit board. Compared to the traditional method of using leaded components, there are a number of advantages to surface mount assembly.
When designing for surface mount assembly the individual component are smaller, allowing for smaller assemblies, and corresponding miniaturization of products. Additionally, the surface mounted components are generally cheaper than their leaded equivalent. With these smaller components, smaller circuit boards are used, with corresponding reduced cost. With the high cost of circuit board real estate, more complex circuits can be made on the same size circuit board.
On the manufacturing side, surface mounted components allow for automated assembly techniques, with associated reduced labor costs and increased production volume. Automated pick and place machines can dramatically increase the rate of production. Double-sided boards can be manufactured, where components are mounted on both sides of the printed circuit board. As a final benefit, there is a certain amount of auto-alignment of components that happens in the soldering process.
In a surface mount assembly line, a solder paste (solder and flux mixture) is first applied to the contact pads of the circuit board by a silk-screen process. The boards are then loaded into an assembly line, where robotic pick-and-place machines install the individual components. These components typically arrive at the assembly line on a reel, or in the case of large-scale integrated circuits, in trays, and are removed by the pick-and-place machine and placed in exact positions on the circuit board.
The pick-and-place machine normally holds the individual components using pneumatic suction nozzles. Once placed on the board, the suction is released and the surface tension of the solder paste holds the component in place until soldering.
Soldering is accomplished by a variety of means, all of which have a few characteristics in common. In the first heating stage, the board and placed components are slowly and uniformly raised to a temperature near where the solder will melt; then the solder is melted. Heating can be accomplished by either gas-fired convection, infrared or a method called vapor phase reflow, which uses liquids with a high boiling point, although for some manufacturers, this method is falling out of favor. Finally, depending upon the soldering paste that is used, the boards may have to be cleaned to eliminate flux and any solder balls that could cause short circuits.
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