SMT also called Surface-mount technology
Look inside any piece of commercially made electronic equipment these days and it is filled with minute devices. Rather than using traditional components with wire leads like those that may be used for home construction and kits, these components are mounted onto the surface of the boards and many are minute in size.
This technology is known as Surface Mount Technology, SMT and SMT components. Virtually all today’s equipment, manufactured commercially uses surface mount technology, SMT, because it offers significant advantages during manufacture, and in view of the size the use of SMT components enables far more electronics to be packed into a much smaller space.
In addition to the size, surface mount technology allows automated production and soldering to be used, and this brings significant improvements in reliability.
Typical PCB using surface mount technology
The advantages and disadvantage of SMT
The Advantages of SMT
The main advantages of SMT over the older through-hole technique are:
Smaller components. As of 2012 smallest was 0.4 × 0.2 mm (0.016 in × 0.008 in: 01005).
Much higher component density (components per unit area) and many more connections per component.
Lower initial cost and time of setting up for production.
Fewer holes need to be drilled.
Simpler and faster automated assembly. Some placement machines are capable of placing more than 136,000 components per hour.
Small errors in component placement are corrected automatically as the surface tension of molten solder pulls components into alignment with solder pads.
Components can be placed on both sides of the circuit board.
Lower resistance and inductance at the connection; consequently, fewer unwanted RF signal effects and better and more predictable high-frequency performance.
Better mechanical performance under shake and vibration conditions.
Many SMT parts cost less than equivalent through-hole parts.
Better EMC compatibility (lower radiated emissions) due to the smaller radiation loop area (because of the smaller package) and the smaller lead inductance?
The Disadvantages of SMT
Manual prototype assembly or component-level repair is more difficult and requires skilled operators and more expensive tools, due to the small sizes and lead spacings of many SMDs.
SMDs cannot be used directly with plug-in breadboards (a quick snap-and-play prototyping tool), requiring either a custom PCB for every prototype or the mounting of the SMD upon a pin-leaded carrier. For prototyping around a specific SMD component, a less-expensive breakout board may be used. Additionally, strip board style protoboards can be used, some of which include pads for standard sized
SMD comportments. For prototyping, “dead bug” breadboarding can be used.
SMDs’ solder connections may be damaged by potting compounds going through thermal cycling.
Solder joint dimensions in SMT quickly become much smaller as advances are made toward ultra-fine pitch technology. The reliability of solder joints become more of a concern, as less and less solder is allowed for each joint. Voiding is a fault commonly associated with solder joints, especially when reflowing a solder paste in the SMT application. The presence of voids can deteriorate the joint strength and eventually lead to joint failure.
SMT is unsuitable for large, high-power, or high-voltage parts, for example in power circuitry. It is common to combine SMT and through-hole construction, with transformers, heat-sinked power semiconductors, physically large capacitors, fuses, connectors, and so on mounted on one side of the PCB through holes.
SMT is unsuitable as the sole attachment method for components that are subject to frequent mechanical stress, such as connectors that are used to interface with external devices that are frequently attached and detached.
SMT in Use
Wired components were always difficult to place automatically because the wires needed to be pre-formed to fit the relevant hole spacing, and even then they were prone to problems with placement.
Although many connectors and some other components still require assisted placement, printed circuit boards are normally developed to reduce this to an absolute minimum, even to the extent of altering the design to use components that can be placed automatically. In addition to this, component manufacturers have developed some specialised surface mount versions of components that enable virtually complete automated assembly for most boards.
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