Inspection technique of foreign objects in BGA solder joint using oblique X-ray computed tomography

Foreign objects in a solder joint could deteriorate the joint strength when mounting BGA (Ball Grid Array) packages or CSP (Chip Scale Package) on printed circuit boards. However, foreign objects within solder bumps are not detected by optical inspection which has been used widely. Recently, X-ray computed tomography (CT) for the inspection of printed circuit boards has been developed. X-ray CT visualizes the foreign objects inside a solder joint. If they are automatically detected, higher soldering quality is expected. In this paper, we propose an automated detection method of foreign objects in solder joints using CT slice images. In the method, some characteristic values are identified which show the solder joint deformation. In the experiments, actual mounting substrates involving solder bumps contaminated by foreign objects were checked using the proposed method. The detection accuracy rate reached 98.9%, which clearly indicates that the proposed method is useful in practice.