An SMD (surface mount device) is any electronic component made to use with surface mount technology (SMT). SMT was developed to meet the rising demand for PCB (printed circuit board) manufacturers to utilize smaller, faster, more efficient, and cheaper components. SMDs are smaller than through-hole resistors and, rather than have wire leads that go through PCB, feature terminations that are soldered to pads on the surface of the board. This eliminates the need for holes in the board, thereby allowing both sides to be used more thoroughly.
SMDs are made by placing end connection electrode bases onto an aluminum or ceramic substrate. The resistor is then fired to ensure the electrodes remain in place. Next, a film of resistive material is printed or deposited and the resistor is fired a second time. It is then covered with multiple layers of a protective coat that must dry between applications. These layers help protect the SMD by preventing mechanical damage and ingress of moisture and other contaminants. Lastly, if the surface is large enough, a code is placed on the resistor to denote its resistance values.
The terms SMD and SMT are commonly used interchangeably. However, this is not accurate. One refers to a device, while the other refers to a process. An SMD is an electronic resistor printed on a PCB. SMT, or surface mount technology, refers to the method of placing components like SMDs onto a PCB. SMT speeds up the production process but also increases the risk of defects because of SMD miniaturization and the denser packing of boards. An SMT device is smaller than a conventional through-hole PCB because it has either small leads or no leads at all.
As stated, SMDs are marked with small three- or four-digit codes to indicate their resistance values. These are called SMD resistor codes. In a three-digit code, the first two numbers indicate the numbers of resistance, while the third number is either a multiplier or indicates how many zeroes should be added to the first two numbers. Resistances below 10 ohms will not have a multiplier. Four-digit codes are read the same way, just with an added number before the multiplier/zeroes.
There are many advantages to using SMDs. They are smaller in size, offer much higher component density & more connections per component, and are a simpler and faster automated assembly. Furthermore, connections can be placed on both sides of a PCB, fewer holes need to be drilled into the PCB, and they offer better mechanical performance under shock and vibration conditions. Finally, SMT parts are cheaper than through-hole parts.
That said, SMDs are not without their disadvantages. For example, SMD solder components can be damaged by potted compounds going through thermal cycling. Additionally, SMT may not be suitable as the only attachment method for components that will experience regular mechanical stress, and many types of SMT component packages cannot be installed in socket. Manual assembly of SMD, as well as component repair, is also expensive, requires skilled workers, and expensive tools. Their small surface area makes marking difficult, and SMDs cannot be directly used with plug-in breadboards. Nevertheless, SMDs are a helpful device that are only growing in popularity.
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