SMT assembly (PCBA)

Solder paste is printed onto the bare board through a stencil, pick-and-place machines position the components, and a reflow oven melts the paste to form the joints. Automated optical inspection (AOI) checks placement and solder, and X-ray inspection checks hidden joints under BGAs and similar packages. Through-hole parts (large connectors, some power components) are added by wave, selective or hand soldering.

IPC-A-610 is the widely used acceptability standard for assembled boards, with classes for general, dedicated-service and high-reliability products.

Where it bites: tiny packages, many through-hole parts, components on both sides and moisture-sensitive parts all add cost or defect risk. Naming the IPC class you need avoids paying for, or failing to get, the wrong level of workmanship.