Surface mount technology (SMT) is a method for constructing electronic circuits in which the components (SMC, or Surface Mounted Components) are mounted directly onto the surface of printed circuit boards (PCBs). Electronic devices so made are called surface mount devices or SMDs. In the industry it 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. It may have short pins or leads of various styles, flat contacts, a matrix of solder balls (BGAs), or terminations on the body of the component.
History
Surface mount technology was developed in the 1960s and became widely used in the late 1980s. Much of the pioneering work in this technology was by IBM. The design approach first demonstrated by IBM in 1960 in a small-scale computer was later applied in the Launch Vehicle Digital Computer used in the Instrument Unit that guided all Saturn IV and Saturn V vehicles. (See Saturn Launch Vehicle Digital Computer article for a description of this type of electronic packaging as of 1964. See [1] for high-resolution photos of components/PCBs.) Components were mechanically redesigned to have small metal tabs or end caps that could be directly soldered to the surface of the PCB. Components became much smaller and component placement on both sides of a board became far more common with surface mounting than through-hole mounting, allowing much higher circuit densities. Often only the solder joints hold the parts to the board, although parts on the bottom or "second" side of the board are temporarily secured with a dot of adhesive as well. Surface mounted devices (SMDs) are usually made physically small and lightweight for this reason. Surface mounting lends itself well to a high degree of automation, reducing labor cost and greatly 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.
SMD Terms
| SMD DIALECT | Expanded Form |
|---|---|
| SMD | Surface Mounted Devices (active, passive and electromechanical components) |
| SMT | Surface Mounted Technology (assembling and montage technology) |
| SMA | Surface Mounted Assembly (module assembled with SMT) |
| SMD / C | Surface Mounted Devices / Components (components for SMT) |
| SMP | Surface Mounted Packages (SMD case forms) |
| SME | Surface Mounted Equipment (SMT assembling machines) |
| SO | Small Outline (4 to 28 pins) |
| VSO | Very Small Outline (40 pins) |
| SOP | Small Outline Package (case) |
| SOD | Small Outline Diode |
| SOT | Small Outline Transistor |
| SOIC | Small Outline Integrated Circuit |
| CC | Chip Carrier |
| LCC | Leadless Chip Carrier |
| PLCC | Plastic Leadless Chip Carrier |
| LCCC | Leadless Ceramic Chip Carrier |
| MELF | Metal Electrode Face Bonding |
| MINI MELF | Mini Metal Electrode Face Bonding |
| MICRO MELF | Micro Metal Electrode Face Bonding |
Main advantages
The main advantages of SMT over the older through-hole technique are:
Smaller components. Smallest is currently 0.4 x 0.2 mm. (.01" x .005" - 01005)
Much higher number of components and many more connections per component.
Fewer holes need to be drilled through abrasive boards.
Simpler automated assembly.
Small errors in component placement are corrected automatically (the surface tension of the molten solder pulls the component into alignment with the solder pads).
Components can be placed on both sides of the circuit board.
Lower resistance and inductance at the connection (leading to better performance for high frequency parts).
Better mechanical performance under shake and vibration conditions.
SMT parts generally cost less than through-hole parts.
Fewer unwanted RF signal effects in SMT parts when compared to leaded parts, yielding better predictability of component characteristics.
Faster assembly. Some placement machines are capable of placing more than 136,000 components per hour.
Main disadvantages
The manufacturing processes for SMT are much more sophisticated than through-hole boards, raising the initial cost and time of setting up for production.
Manual prototype assembly or component-level repair is more difficult (more so without a steady hand and the right tools) given the very small sizes and lead spacings of many SMDs.
SMDs can't be used with breadboards (a quick snap-and-play prototyping tool), requiring a custom PCB for every prototype. The PCB costs dozens to hundreds of dollars to fabricate and must be designed with specialized software. For prototyping around a specific SMD component, a less-expensive breakout board may be used.
SMDs' solder connections may be damaged by potting compounds going through thermal cycling.
Package sizes
Surface mount components are usually smaller than their counterparts with leads, and are designed to be handled by machines rather than by humans. The electronics industry has standardized package shapes and sizes (the leading 
standardisation body is JEDEC).
Rectangular passive components (mostly resistors and capacitors):
01005 (0402 metric) : 0.016" × 0.008" (0.4 mm × 0.2 mm) Typical power rating for resistors 1/32 watt
0201 (0603 metric) : 0.024" × 0.012" (0.6 mm × 0.3 mm) Typical power rating for resistors 1/20 watt
0402 (1005 metric) : 0.04" × 0.02" (1.0 mm × 0.5 mm) Typical power rating for resistors 1/16 watt
0603 (1608 metric) : 0.063" × 0.031" (1.6 mm × 0.8 mm) Typical power rating for resistors 1/16 watt
0805 (2012 metric) : 0.08" × 0.05" (2.0 mm × 1.25 mm) Typical power rating for resistors 1/10 or 1/8 watt
1206 (3216 metric) : 0.126" × 0.063" (3.2 mm × 1.6 mm) Typical power rating for resistors 1/4 watt
1210 (3225 metric) : 0.126" × 0.1" (3.2 mm × 2.5 mm) Typical power rating for resistors 1/2 watt
1806 (4516 metric) : 0.177" × 0.063" (4.5 mm × 1.6 mm)
1812 (4532 metric) : 0.18" × 0.12" (4.5 mm × 3.2 mm) Typical power rating for resistors 1/2 watt
2010 (5025 metric) : 0.2" × 0.1" (5.0 mm × 2.5 mm) Typical power rating for resistors 1/2 watt
2512 (6332 metric) : 0.25" × 0.12" (6.35 mm × 3.0 mm) Typical power rating for resistors 1 watt