Sequential computer algorithms for printed circuit board inspection

Surface mounted technology (SMT) in automated assembly facilities requires the use of automatic surface-mountdevice (SMD) placement machines. One of the problems involved in the electronic printed circuit board (PCB) assembly process is the verification of the SMD placement operation within tight tolerances. The high throughput of modem manufacturing lines along with the required accuracy, demand the use of automatic inspection systems to verify SMD placement. Image complexity of the board makes the use of machine vision for the inspection process a difficult task. This is complicated by the fact that missclassification errors should be kept to a minimum. Additionally, it is desirable that the inspection results provide enough information to be used for statistical process control (SPC). The strategy adopted to solve this problem was to simplify the complexity of the image by means of special illumination devices. The simplified image was then suitable for analysis by simple processing, segmentation and detection algorithms that, sequentially applied to the image, met the required repeatability and accuracy specifications for the inspection system. The scope of this paper is to describe the techniques explored by the authors to solve the SMD inspection problem in order to develop a working industrial SMD inspection system.