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Surface Mount Technology (SMT) is a method used in electronic assembly to mount electronic components directly onto the surface of a printed circuit board (PCB). This stands in contrast to traditional through-hole technology, where components have leads that pass through holes in the PCB and are soldered on the other side. SMT offers several advantages, including smaller PCB size, higher component density, and improved manufacturing automation.

In SMT, electronic components are typically small and lightweight, with no leads or with very short leads called surface mount devices (SMDs). These components are placed on the surface of the PCB using automated pick-and-place machines. The PCB itself is coated with solder paste, a sticky mixture of tiny solder particles and flux. The solder paste is applied to the PCB through a stencil, leaving deposits on the pads where components will be placed.

After the components are accurately placed on the solder paste, the entire assembly is subjected to a reflow soldering process. This process involves heating the PCB to a temperature where the solder paste melts and creates a connection between the component leads and the PCB pads. The molten solder then solidifies, creating a reliable and durable solder joint. Modern SMT processes use infrared or convection reflow ovens for precise temperature control during soldering.

Once the soldering is complete, the PCB undergoes inspection to ensure the quality of the solder joints and the correct placement of components. Automated optical inspection (AOI) and X-ray inspection are commonly used to detect any defects or issues.