Automated 3D X-ray Inspection Of Fine Pitch PCB's

The visuell inspection of printed circuit boards (PCB's) by human controllers is still an often applied method. However, increasing complexity of electronic fine pitch devices and application of spatial modules makes this way of testing more and more problematic and some defect characteristics can't be judged only by visual inspection systems. A typical example is the testing of SMD solder joints, which can be checked properly only by x-ray methods. In this paper the recently developed x-ray system 3D-DELTA (3-dim. Defect Localization and depht Analysis) for automatic PCB testing is presented. The system was developed for faster spatial data extraction by use of only two x-ray images of the PCB's. The result is the 3-dimensional classification of defects within solder joints and of the solder joints itself without the use of complex computer tomographic algorithms. First the x-ray imaging system for data acquisition is shown. In order to achieve the required data for evaluation, two micro focus x-ray sources are used to produce a stereo image pair during one scan. The use of a line scanning x-ray detector allows the image acquisition line by line with contrast enhancement during object movement. Time loss by stop and go of the assembly line is avoided with this method. Second the data preparation by matching and subsequent division of an adapted reference image from the x-ray data is described. Finally the algorithm for testing the solder joints for completeness and faint flaws is outlined. A combination between absolute thickness measurement by evaluation of polychromatic intensity distribution of the solder pads and 3D-localization by stereoscopic technics is presented.