barcode reading using c#.net Copyright 2008 by The McGraw-Hill Companies. Click here for terms of use. in Software

Print Quick Response Code in Software Copyright 2008 by The McGraw-Hill Companies. Click here for terms of use.

Copyright 2008 by The McGraw-Hill Companies. Click here for terms of use.
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This is true for trace pitches, but it is also true for linewidths and spaces. Three-dimensional measurements are required to guarantee the quality of microbump arrays. Usually, it takes time to measure exact dimensions for each circuit, and this is one of the major costs of the circuits when the specification is very tight compared to the process capability. There have been several basic technical issues regarding quality assurance for high-density flex circuits, as listed in Table 67.1. Because of the fine circuits and the severe acceptable level of the defects, low-magnification scopes and low-resolution AOI do not work effectively. Magnification of 20 to 50 is required for visual inspections of fine circuits, and higher resolutions are required for AOI systems. Low contrast between copper and polyimide base creates another difficulty for traditional AOI equipment. It is not easy for common AOI systems to distinguish the copper traces from the circuits on the other side of a double-sided circuit built on adhesiveless substrate, because the base layer is very transparent and the sensor detects both sides of the circuits as one.
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TABLE 67.1 Key Inspection Issues for High-Density Flex Circuit Item Finer traces Micro blind via holes Unstable materials Photoimageable coverlay Fragile traces Fragile flying leads Microbump arrays Cosmetic defects Issues Resolution of AOI Low contrast for AOI Resolution of AOI Dimensional accuracy Low contrast for AOI Damage during processes Damage from test probe pins Damage after AOI 3-D constructions Serious failures caused by small defects
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67.3 AUTOMATIC OPTICAL INSPECTION SYSTEMS
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AOI is the most valuable inspection equipment for detecting dimensional defects in the traces and spaces of the printed circuits, especially for high-density traces. It s also very useful for the detection of cosmetic defects on the surfaces. Further sensitivities are required for AOI systems to detect small defects on coverlay, flying leads, microbumps, narrow space perimeters, and so on. These small defects have low contrast, different kinds of brightness, and different defect shapes, among other things. Traditional AOI systems are usually not appropriate for inspection in these circumstances. Traditional AOI systems were designed for rigid circuit boards, and they were originally only good for the detection of copper trace qualities. They could work for 50-mm-line/space fine traces, but their inspection speed could be very slow. Therefore, remarkable progress has been made in the resolution and productivity of the newer AOI systems; and as a result, systems can now detect 1.5-mm defects on 15-mm traces. Also, the systems have other capabilities for detecting various defects on polyimide films and coverlay films.A careful review of AOI capabilities, compared to the specified design requirements, should be done prior to committing to a particular equipment.
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67.4 DIMENSIONAL MEASUREMENTS
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It seems to be an impossibility to require accurate dimensions on an unstable, thin, flexible material. However, high-density interconnects in particular demand very high accuracy in the termination area of the circuits, even though the dimensional allowance is smaller than the dimensional stability of flexible substrate materials.Therefore, high-accuracy 2-D or 3-D dimension measurement technologies are required. There has been continuous progress in dimension-measuring equipment. The newest 3-D measurement equipment has a resolution higher than 0.2-mm and accuracy higher than 1 mm, with statistical data in a 200-mm square 100-mm high. It has automatic measuring capabilities to detect each measuring point, producing statistical data. It can provide standard deviation data for each point with Cpk value. Usually, there are several critical points that require exact dimensional accuracy in a flexible circuit. They are guide marks and termination areas, and customers ask exact tolerance for each dimension. The stable dimensions, such as punched perimeters processed by blanking dies, can be represented with the first article. However, unstable dimensions such as line width and pad pitch should be checked for each circuit. A statistical method is available. The measurement can be conducted by sampling when Cpk is larger than 1.3, but 100 percent measurement should be done when Cpk is smaller than 1.0. See MIL-STD-414 for help in reducing the number of samples.
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