In the 1960s, the surface-mount technology was developed. It became widely used in the late 1980s. Components were mechanically redesigned to have small metal tabs or pads that could be directly soldered to the surface of the PCB. Often, only the attaching solder holds the part to the board. Surface-mounted components (or devices: SMD) are usually made as physically small and lightweight as possible for this reason.
Often an automated machine removes the parts from reels, and sticks them to the PCB. A silk-screened application of solder paste, a mixture of solder and flux, holds the parts in place.
The board is pre-heated, passed through an oven containing infrared lamps whose heat melts the solder, then the board is slowly cooled. Today infrared ovens are not used because they do not heat the PCB and its components evenly, thus causing defects. So convection ovens are utilized in modern day manufacturing facilities. The pre-heating and controlled cooling prevent the parts from cracking when one edge of the part is cold and another edge is hot from the solder.
The parts and pads of the PCB are designed so that the surface tension of the molten solder centers the parts on their copper pads.
The result is components that are one quarter to one tenth of the size and weight, and half to a quarter of the cost of wire-mounted parts.
Wed, 2010-09-01 07:41
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