Automatic optical inspection (AOI) systems are commonly used to test the inner layer before the formation of layers. After stratification, X-ray systems monitor alignment accuracy and minor defects; A scanning laser system provides a way to detect the layer before reflow. These systems, along with line visual inspection technology and component integrity testing for automatically placed components, help ensure the reliability of the final assembly and welded plates.

　　However, even if these efforts minimize defects, the final inspection of the assembled printed circuit board is still required, which is perhaps the most important, as it is the final unit for product and overall process evaluation.

　　The final inspection of the assembled PCB board may be done by an active method or by an automated system, and often both methods are used together. Manual "means that an operator uses optical instruments to visually inspect the board and make a correct judgment about the gap. Automated systems use computer-aided graphic analysis to identify defects, and many people also believe that automated systems include all detection methods except manual light detection.

　　The X-ray technique provides a method for assessing solder thickness, distribution, internal voids, cracks, de-soldering, and the presence of solder balls (Markstein, 1993). Ultrasonics will detect voids, cracks, and unconnected interfaces. Automatic optical inspection evaluates external features such as bridging, tin flux and shape. Laser detection can provide three-dimensional images of external features. Infrared detection detects internal weld faults by comparing the heat signature of the weld with that of a known good weld.

　　Notably, all defects that these automated inspection techniques could not detect with limited inspection of assembled printed circuit boards have been found. Therefore, manual visual inspection methods must be used in conjunction with automatic inspection methods, especially for those small number of applications. The combination of X-ray inspection and manual optical inspection is the best method to detect the defects of assembly plates.