Surface-mount technology

Surface mount technology (SMT) is a system for assembling electronic components on printed circuit boards. In industry it has mostly replaced the previous technique of fitting leaded components (i.e. components with wire leads - this name has nothing to do with the metal called lead) into holes in the circuit board.

An SMT component is smaller than its leaded counterpart and has short pins or flat contacts. The site on the PCB where the component is to be fitted has flat copper pads rather than holes. The pads are coated with a thin layer of solder paste, which also acts as an adhesive to hold the component in place during soldering. Soldering consists of heating the circuit board and components in an infrared oven, which 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 molten. The circuit board is then cooled to solidify the solder.

The main advantages of SMT over traditional leaded components are:

• smaller components
• more suitable for automated assembly
• small errors in component placement are corrected automatically as the molten solder pulls the component into place by surface tension
• components can be fitted to both sides of the circuit board

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:

• 1206 - 0.12" x 0.06" (3.0 mm x 1.5 mm), two terminals
• 0805 - 0.08" x 0.05" (2.0 mm x 1.3 mm), two terminals
• 0603 - 0.06" x 0.03" (1.5 mm x 0.8 mm), two terminals
• SOIC - small-outline integrated circuit, dual-in-line, 8 or more pins
• TSOP - thin small-outline package, thinner than SOIC with smaller pin spacing
• QSOP - quarter-size small-outline package, with smaller pin spacing than TSOP
• VSOP - even smaller than QSOP
• SOT - small-outline transistor, with three terminals
• PQFP - plastic quad flat-pack, a square with pins on all four sides, 44 or more pins
• 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
• uBGA - 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