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Surface-mount technology

Surface mount technology (SMT) is a system for assembling electronic components on printed circuit boards. In industry it has mostly replaced the previous technique of fitting leaded components (i.e. components with wire leads - this name has nothing to do with the metal called lead) into holes in the circuit board.

An SMT component is smaller than its leaded counterpart and has short pins or flat contacts. The site on the PCB where the component is to be fitted has flat copper pads rather than holes. The pads are coated with a thin layer of solder paste, which also acts as an adhesive to hold the component in place during soldering. Soldering consists of heating the circuit board and components in an infrared oven, which drives off the flux from the solder paste and melts the remaining solder. The surface tension in the liquid solder prevents the component from sliding off while the solder is molten. The circuit board is then cooled to solidify the solder.

The main advantages of SMT over traditional leaded components are:

Package Sizes

Surface-mount components are usually much smaller than their leaded counterparts, and are designed to be handled by machines rather than by humans. The electronics industry has defined a collection of standard package shapes and sizes. These include: