Surface-mount components on a keydrive's circuit board

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). Electronic devices so made are called surface-mount devices or SMDs. In industry it has largely replaced the previous construction method of fitting components with wire leads into holes in the circuit board (also called through-hole technology).

An SMT component is smaller than its leaded counterpart (because it has no leads) and has either short pins or flat contacts. The site on the PCB where the component is to be fitted has flat, usually tinned, copper pads without holes. One technique for assembly coats the pads with a thin layer of solder paste, which also acts as a temporary adhesive to hold the component in place during soldering. Soldering, in this technique, consists of heating the circuit board and components in an oven; this drives off the flux from the solder paste and melts the remaining solder. The surface tension in the liquid solder prevents the component from sliding off while the solder is liquid. The circuit board is then cooled to solidify the solder.

The main advantages of SMT over the older through-hole technique are:

• smaller components
• no need to drill holes 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 fitted to both sides of the circuit board
• lower lead resistance and inductance (leading to better performance for high frequency parts)

Package Sizes

Surface-mount components are usually much smaller than their leaded counterparts, and are designed to be handled by machines rather than by humans. The electronics industry has defined a collection of standard package shapes and sizes. These include:

• Rectangular passive components (mostly resistors and capacitors):
  • 0201 - 0.02" × 0.01" (0.6 mm × 0.3 mm), two terminals
  • 0402 - 0.04" × 0.02" (1.0 mm × 0.5 mm), two terminals
  • 0603 - 0.06" × 0.03" (1.5 mm × 0.8 mm), two terminals
  • 0805 - 0.08" × 0.05" (2.0 mm × 1.3 mm), two terminals
  • 1206 - 0.12" × 0.06" (3.0 mm × 1.5 mm), two terminals
• Tantalum capacitors:
  • Size A (EIA 3216-18): 3.2 mm × 1.6 mm × 1.6 mm
  • Size B (EIA 3528-21): 3.5 mm × 2.8 mm × 1.9 mm
  • Size C (EIA 6032-28): 6.0 mm × 3.2 mm × 2.2 mm
  • Size D (EIA 7343-31): 7.3 mm × 4.3 mm × 2.4 mm
  • Size E (EIA 7343-43): 7.3 mm × 4.3 mm × 4.1 mm
• SOIC - small-outline integrated circuit, dual-in-line, 8 or more pins, gull-wing lead form, pin spacing 1.27 mm
• PLCC - plastic leaded chip carrier, square, J-lead, pin spacing 1.27 mm
• TSOP - thin small-outline package, thinner than SOIC with smaller pin spacing of 0.5 mm
• SSOP - shrink small-outline package, pin spacing of 0.635 mm
• QSOP - quarter-size small-outline package, with pin spacing of 0.635 mm
• VSOP - even smaller than QSOP; 0.4, 0.5 mm or 0.65 mm pin spacing
• SOT - small-outline transistor, with three terminals
  • SOT-23 - 3 mm × 1.75 mm × 1.3 mm body - three terminals for a transistor, or up to eight terminals for an integrated circuit
  • SOT-223 - 6.7 mm × 3.7 mm × 1.8 mm body - four terminals, one of which is a large heat-transfer pad
• PQFP - plastic quad flat-pack, a square with pins on all four sides, 44 or more pins
• CQFP - ceramic quad flat-pack, similar to PQFP
• TQFP - thin quad flat pack, a thinner version of PQFP
• BGA - ball grid array, with a square or rectangular array of solder balls on one surface, ball spacing typically 1.27 mm
• CGA - column grid array, circuit package in which the input and output points are high temperature solder cylinders or columns arranged in a grid pattern.
• CCGA - ceramic column grid array, circuit package in which the input and output points are high temperature solder cylinders or columns arranged in a grid pattern. The body of the component is ceramic.
• μBGA - micro-BGA, with ball spacing less than 1 mm
• chip-on-board - a silicon chip containing an integrated circuit is supplied without the usual plastic encapsulation, and soldered directly to the board