No single inspection method can detect every PCB assembly defect. Effective coverage combines process checks with electrical or functional evidence based on component packages, board access, production volume, and application risk.
SPI and AOI
Solder paste inspection measures deposit area, height, and volume before placement. It is especially useful for fine-pitch and high-density assemblies because printing defects can be corrected before reflow. Automated optical inspection checks visible placement, polarity, marking, solder joints, and component presence after assembly.
X-ray inspection
BGA, QFN, LGA, and other bottom-terminated packages hide critical solder joints. X-ray inspection can reveal opens, bridges, alignment issues, and abnormal voiding that cannot be seen by AOI. Sampling or full inspection should be defined around package risk and production stage.
ICT and functional test
In-circuit test checks electrical networks and component behavior through accessible test points. Functional test verifies that the assembled product performs approved operations under controlled inputs and limits. Fixtures, software, golden units, calibration, and result recording must be included in the production plan.
A practical test strategy connects each known failure mode to a method that can actually detect it.