Surface-mount technology (SMT) is a method for constructing electronic circuits in which the components are mounted directly onto the surface of printed circuit boards (PCBs) with solder paste. Electronic devices made this way are called surface-mount devices (SMDs). Surface-mount technology has largely replaced the through-hole technology construction method of fitting components with wire leads into holes in the circuit board.
An SMT component is usually smaller than its through-hole counterpart because it has either smaller leads or no leads at all.
The three key steps in surface-mount technology are paste, place, and reflow.
In the first step, solder paste must be accurately placed onto a PCB with the aid of a stencil printer, which deposits the paste into the pattern of the circuit.
Next, the electronic components are precisely placed onto the board using a manual or automatic pick and place machine.
Finally, the solder paste must be heated until it melts and forms strong and reliable joints between the components and the surface of the board. This is accomplished through the use of a reflow oven which heats the solder to the proper temperature and then cools it down to a solid again.
In some instances, a mixed technology board with both SMD and through-hole components will be required. To solder the through-hole components on a mixed technology board, a wave solder machine or selective solder machine may be required.
Surface-mount technology began to be widely used in the late 1980s. IBM first demonstrated the technology in 1960 in a small-scale computer and later applied it in the digital computer used to guide Nasa’s Saturn V rockets.
Since then, SMT components became much smaller and component placement on both sides of a board is far more common, allowing much higher circuit densities.
As a result, surface-mounted devices are usually small and lightweight. Surface mounting lends itself well to a high degree of automation, reducing labor costs and increasing production rates. SMDs can be one-quarter to one-tenth the size and weight, and one-half to one-quarter the cost of equivalent through-hole parts.
See an overview of DDM Novastar SMT Equipment