Automated inspection of solder paste by directional LED lighting

Screen printing of solder paste is often the very first step in an SMT line in the manufacturing of PCBs. Conventional inspection method would depend on an image acquired from a camera mounted from the top. This 2D inspection of solder paste based on images is fast but is limited to defect such as bridge or no solder. Defects related to the volume of the printed solder paste or unevenness of the paste cannot be treated from a top image. The proposed method of this paper would involve the use of special directional side lighting. A sequence of four images is acquired and image processing is carried out for the edge information of the printed solder paste. A fuzzy system is developed to give a score on the quality of the solder paste. The proposed method can handle other types of defects that cannot be treated by conventional top light images.