SMT or Surface Mount Technology is a rapidly evolving field in the manufature of electronic components, involving either active (transistors, integrated circuits, diodes, etc.) or passive (capacitors, resistors, coils, etc.) which do not have leaded connections.
Terminal leads on the device are part of the component body, thus allowing direct mounting on the surface of printed circuit boards (PCBs). Leaded devices are mounted by passing interconnection leads through holes drilled and then plated in the PCBs.
In both cases, the components are held in place, both mechanically and electrically, by solder. Surface mounted components usually are smaller than an equivalent through hole leaded device and, in some cases, less expensive.
Equipment and design engineers can save valuable wiring board area by mounting surface mounted leadless components on the underside of a board, and conventional leaded components on the top side of boards.
