SMTs

Surface-mount technology (SMT) is an electronic assembly method in which components are mounted directly on the surface of printed circuit boards (PCBs). Compared with through-hole assembly, SMT allows higher component density, smaller form factors, and greater automation, and it is now the dominant method for most commercial electronics.

SMT emerged from advances in component miniaturization and automated assembly during the 1980s and 1990s, and has since become the standard for mass production. The process typically begins with applying solder paste to PCB pads using a stencil, followed by placing surface-mount devices (SMDs) with pick-and-place machines. Reflow soldering then melts the paste to form joints as the board passes through a controlled temperature profile. After soldering, boards are inspected using automated optical inspection (AOI), and may be cleaned or subjected to X-ray inspection for solder joints such as BGAs or QFNs. Mixed technology layouts can combine SMT with through-hole components.

SMDs cover a wide range of packages, including resistors, capacitors, and inductors in compact packages (for example 0402, 0603 sizes) and integrated circuits in QFP, QFN, BGA, and other form factors. Footprints and land patterns are critical design considerations to ensure reliable solder joints and manufacturability.

The approach enables high-speed, high-volume production, but it also requires precise process control, specialized equipment, and careful consideration of heat sensitivity, rework difficulty, and inspection challenges.

Environmental and quality standards have driven widespread use of lead-free solders and RoHS-compliant processes. SMT is used across consumer electronics, automotive, telecommunications, and industrial equipment.