Research Of Contour Comparision-Based Defect Inspection Algorithm For Bare PCB

Printed Circuit Board,a kind of information carrier which integrates varieties of electronic devices,has popular applications in electronic fields nowadays.With the rapid development of manufacture techniques,electronic product tends to lighter,thinner and smaller,and PCB turns to have more layers and higher density,which makes quality detection of PCB become more difficult.Traditional detection methods cannot satisfy the large production for inaccurate,slow and long time detection,then how to realize automated defect detection of PCB becomes a hot topic in semiconductor industry.AOI is a kind of machine vision technology,which combines electronics,photoelectric detection,image processing and computer technology into oneself,is a potential new technology in industrial detection field.Image processing software is the most important component of AOI,and the core of which is inspection algorithm.So far the existing inspection algorithms can be classified into three categories:reference comparison,nonreferential approaches,and hybrid approaches, among which the reference comparison algorithms are the one widely in use now.Most reference comparison algorithms are based on the comparison of content,which have three limitations:firstly,the inspection precision only achieves pixel-level accuracy,secondly,the resampling after image registration is high time-consuming,it will cost very long time to inspect those images with high resolution,thirdly,the way of defect analysis is not direct enough to analyze some special defects.Combining the features of high resolution and high contrast owned by PCB bare board images which are acquired by high performance linear CCD camera,a new inspection algorithm based on contour comparison is proposed to satisfy the real-time and high accuracy defect inspection of PCB bare board.The new inspection algorithm is belong to reference comparison algorithms and consists of image segmentation and region labeling, boundary extraction,sub-pixel boundary computation,contour registration and defect inspection and analysis.The six steps are discussed in sequence and the corresponding solutions have been given,and the innovations lie in the following:(1) A new region labeling algorithm based on run length connectivity analysis is proposed to realize the fast labeling of connected region and the fast extraction of object description just by scanning image data once.(2) A new run based boundary extraction algorithm is proposed to realize the fast and exact extraction of outer and inner boundaries and the deep relationships between them.(3) A new contour normal angle feature based image registration algorithm is proposed to realize exact registration of the contours of the reference image and defect image.(4) The contour comparison theory is proposed to realize the common defects inspection of PCB bare board.Comparing with those content based inspection algorithms,the new contour comparison based inspection algorithm has the following advantages:firstly,less data storage is needed because of the fewer contour data,secondly,higher precision can be achieved by subpixel technology,thirdly,faster inspection speed can be achieved because the contour comparison way does not need the resampling and owns the fewer data size,finally, the contour comparison way make the defect analysis more direct and easy.