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A surface-mount device (SMD) is an electronic device whose components are placed or mounted onto the surface of the printed circuit board (PCB). This method of manufacturing electronic circuit boards is based on the surface-mount technology (SMT), which has largely replaced the through-hole technology (THT) especially in devices that need to be small or flat. Compared to the latter, SMT allows both sides of a PCB to be used when required.
The most common example of an SMD is the smart phone. It has components that need to be packed very tightly in a very slim case, so it is not feasible to use THT components. The latter also take up space at the bottom of or behind the PCB since that is where they are soldered, resulting in pointy tips where the solder meets the leads. SMT components can be smaller than THT components since they can have either smaller leads or no leads at all, which makes it easier to shrink the components down.