Circuits Assembly - December 2007 - (Page 30)

Cover Story on the test sample (x and y-plane movements) and to change the height of the sample relative to the x-ray tube position (z-direction). Sample movement in the z-direction changes the magnification achieved by the system. For example, moving the sample closer to the x-ray tube increases the magnification (Figure 2). Magnification and Image Quality The magnification level a particular x-ray inspection system can provide is called the geometric magnification. This is a ratio of Distance between the x-ray tube focal spot and the detector = A Distance between the x-ray tube focal spot and the sample a Historically there have been two main tube types used in 2-D x-ray inspection systems. These tubes are called closed or sealed tubes, and open or demountable tubes. Further information on the various types of x-ray tubes used for inspection, and the effects different tubes have on the inspection capabilities such as on magnification and resolution, can be found within numerous literature. In some x-ray machine specifications, the term “system magnification” or “total magnification” is used in place of geometric magnification to indicate the systems capability. This term indicates the ratio of the sample size as presented on the Figure 2. Geometric magnification in x-ray inspection system. promotions through post-event reporting, including customized, real-time updates on webinar attendees, it was ﬁrst-class service all the way.” John Isaac, Dir. Systems Market Development, Mentor “From pre-event The possibilities are unlimited with Webinars by UP Media Group For more information on how we can help you meet and exceed your marketing expectations, contact: Frances Stewart fstewart@upmediagroup.com 678-817-1286 www.pcbshows.com/webinars operator’s viewing screen compared to its actual size. Therefore, if a substantially larger monitor or flat panel display was used on an existing x-ray system, its system magnification would be substantially increased as the x-ray image would appear larger on-screen, but the geometric magnification and image quality of that system would be unchanged (Figure 3). The geometric magnification values used in x-ray system specifications always assume the test sample is extremely thin, permitting its placement as close as possible to the x-ray tube. However, as the sample grows in thickness, and its proximity to the x-ray tube reduced, then the actual geometric magnification for a specific sample may be substantially less. The best approach to confirm acceptable magnification for a particular application is to try real samples within different x-ray systems. Determining which additional capabilities and functions an x-ray system needs to optimize inspection for a particular application requires answers to a number of questions. Whatever the answers generated to define and satisfy an x-ray inspection system suitable for a particular application, it is strongly recommended that the system provide a top-down view and oblique angle views. The system should also provide a substantially high level of grayscale sensitivity in the image capture device. Oblique angle viewing. When limited to inspection of solder bumps from directly above the sample, the main mass of the solder ball will mask the observation of subtle variations that might occur in the interfacial areas between the solder ball and device on one side, and the pad on the other. These interfacial areas will indicate if the joint quality is acceptable. By viewing the joint at an oblique (slanted) angle, and equally important at several different angles, the interference from the solder ball bulk can be reduced or eliminated. This permits the solder ball shape and any interfacial variations, including voids, to be clearly visualized. Imagine a good BGA solder ball joint as a soccer ball sandwiched between two flat pieces of wood. Compared with the oblique angle view, a top down view limits the information that can be obtained. The larger the oblique angle, the better the separation between the bulk of the ball and the interface. In a 2-D x-ray system, all layers within the board are shown at the 30 Circuits Assembly DECEMBER 2007 circuitsassembly.com http://www.pcbshows.com/webinars http://circuitsassembly.com

Table of Contents Feed for the Digital Edition of Circuits Assembly - December 2007

Circuits Assembly - December 2007 Contents Caveat Lector Industry News Market Watch Talking Heads Global Sourcing Screen Printing Better Manufacturing Metalization Options for COB Assembly A Test Comparison of SAC and Non-SAC Pb-Free Solders An A-to-Z Guide to X-Ray Inspection BEST: A ‘Funky Chicken’ with an EMS Niche Tech Tips Wave Soldering Process Doctor Pb-Free Lessons Learned Getting Lean Eastern Approaches Product Spotlight Ad Index Assembly Insider Technical Abstracts

Circuits Assembly - December 2007

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