New optical measuring system for solder joint inspection

Due to specularity and complex 3D geometry, the visual inspection of solder joints has been regarded as one of the most difficult tasks and thus has not guaranteed accurate inspection results. This paper deals with an optical sensing system designed to inspect solder joints automatically. Taking into account a specular characteristic, a new optical solder joint inspection system is proposed to obtain the external profile of solder joints accurately. A laser scanning unit scans the area of solder joint and observes the angle of the reflected beam. To realize this measuring principle, the system is so composed that a galvanometer steers the direction of laser beam to a point to be measured, a mirror unit (a parabolic and a conic mirror) gathers the specular components of the reflected beam toward its center and a beam-receiving unit positioned along the mirror unit's center line detects the reflected beam. To classify the defects of solder joint, a statistical pattern recognition method is utilized. To verify the validity of the developed system, a series of experiments was performed for SOPs and QFPs in insufficient, normal and excess soldering condition. Based upon observation of the experimental results, the proposed system is found to show good performance for inspection of solder joint defects.