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Defect detection where rework is easiest, before component placement and solder reflow Process characterization during the lead-free conversion with minimal program tuning Automated inspection of solder paste depositions has the following limitations:
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It is sometimes too slow to cover all paste depositions on an assembly. It does not measure or detect defects in component placement or solder reflow.
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Operating Principles Automated inspection systems designed for 2-D component placement and solder paste quantitative measurements typically use optical techniques consisting of multi-angle light sources and charged-couple device (CCD) cameras to generate images. As Fig. 53.10 indicates, these systems extract specific features, such as edges of the components or solder paste deposits, in the images and then use these features to determine quantitative measurements of component and deposit misalignment or deposit area.
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FIGURE 53.10 Schematic of automated inspection systems for component placement defect detection. The camera sensor obtains images of component positions relative to the printed circuit board. The image processing software extracts features from the image and compares them to present position limits to flag a placement as defective.
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Automated optical inspection systems also image only a small portion, or view, of the assembly at a time. These systems normally can use somewhat bigger views than the 3-D solder paste systems because the features being extracted often do not require as much magnification as do measurements of solder paste depositions. However, inspection of components, such as 0402, 0201, and 01005 passive components, or very-fine-pitch deposits, can require the same level of magnification and therefore views as small as the 3-D solder paste inspection systems. In general, pre-reflow AOI systems are typically two to three times faster than 3-D systems and range in inspection speed between 10 and 40cm2/sec. The prices of 2-D component and solder paste placement automated inspection systems are typically somewhat less than those of the 3-D solder paste systems with the fastest inspection speed capability.
ASSEMBLY INSPECTION
Applications The majority of pre-reflow AOI systems are placed directly following the pick-and-place process step in the production line. These systems therefore only measure the amount of misalignment of components and are generally fast enough to cover all components on a printed circuit board and keep up with the production line s cycle time. So manufacturers often use these systems to detect all misalignment defects as well as missing components. But the most important purpose of these systems is process control, just as it is for the 3-D solder paste inspection systems. The intent is to discover process drift early enough to prevent defects by tracking the key quantitative misalignment measurements against control limits. The systems then alarm any condition where control limits are exceeded or process drifts are detected, allowing the manufacturer to assess the production line and monitor or choose to take action until appropriate adjustments have been made. Some system suppliers in conjunction with pick-and-place equipment suppliers can develop closed-loop feedback software links allowing semiautomatic adjustment of the pick-and-place equipment. Pre-reflow AOI systems also have the ability to inspect 2-D solder paste; however, this ability is utilized only to inspect a small percentage of the solder paste deposits combined with the component misalignment measurements. Component misalignment measurements cover the passive components, whereas the solder paste measurements cover deposits for BGA, CSP, or fine-pitch QFP devices. Therefore, these systems are placed within production lines after the pick-and-place systems for passive devices but before the pick-and-place systems for the larger area-array and leaded devices. These systems serve the same purpose as those only meant for component placement measurement, both detecting defects and monitoring measurements within control limits to discover process drift as early as possible.
Advantages and Disadvantages Pre-reflow AOI has the following major advantages:
Real-time detection of systematic process defects where equipment is out of adjustment Detection of defects such as misalignment where rework is easier, before solder reflow Low-cost option for reasonable test coverage Process characterization during the lead-free conversion with minimal program tuning Automated inspection of component placement has the following limitations:
It does not measure or detect defects in solder reflow, and normally does not detect defects in solder printing. It does not measure 3-D for solder paste.
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