SMT, surface-mount technology (SMD stands for surface mount device; SMT refers to the assembly technology), uses electronic components without long through-hole leads: they are soldered directly onto copper pads on the surface of the printed circuit board. Resistors, capacitors, integrated circuits and connectors exist in SMD versions, in a wide variety of packages (0402, SOIC, QFN, BGA…). A 0402 component measures about 1 mm by 0.5 mm.
Assembly follows an automated line. Solder paste is deposited by screen printing through a stencil; a placement machine positions the components precisely; the board passes through a reflow oven which, following a controlled temperature profile, melts the paste and forms the solder joints. Automated optical inspection (AOI) checks the presence and position of components, X-rays examine solder joints hidden beneath BGA packages, and an electrical or functional test validates the board. Solder is most often lead-free, in application of the RoHS directive.
Compared with through-hole components (THT), whose leads pass through holes and are soldered by wave, selectively or by hand, SMT allows more compact boards, components on both sides and high throughput. Many boards are mixed: SMT for most parts, through-hole for connectors or components subject to mechanical stress. For a small batch as for a prototype, assembly can be entrusted to an electronics workshop that combines SMT equipment and manual finishing. The acceptability of solder joints is often defined by IPC-A-610.
In a quote request
Attach the bill of materials (BOM) with manufacturer part numbers and quantities, the Gerber files, the component placement file and the assembly drawing, and specify who supplies the components: yourself, or the assembler through its purchasing. State the IPC class, the inspections (AOI, X-ray, test), the traceability of component batches and any protective coating. See the guide electronic prototype and PCB and the electronics workshops.