Printed Circuit Design & Fab - June 2008 - (Page 36)

DFA taBlE 1. AOI solder bump analysis procedure operations Threshold Bar Count Limits Area Limits Size Limits Invert Image Check Boundaries Source: YesTech Functions Binarizes the image similar to the Histogram setup User can preset the minimum and maximum number of bumps to be found within the inspection region. Similar to the Histogram setup of the minimum and maximum area of bumps within the inspection region. Measures width of the largest extreme bump as a percentage of the length of the inspection region. Inverts the image before bump analysis operation. Will fail inspection for selected direction boundary (N, E, S, W) if a bump touches the region of interest box regardless of the bump’s total area. Specifically, at board assembly, AOI checks for such problems as area defects, component offsets, component polarity, component presence or absence, component skew from surface mount pads, excessive solder joints, flipped components, insufficient solder joints and paste around leads, tombstoning, solder bridges, and solder paste registration. In FiGurE 2, AOI debugs and fine-tunes component verification by comparing it to a similar component in the database parts library. Proper staging and lighting setup reduce variability in component images. They act to properly present the image for the vision computer. Without proper presentation, the vision inspection process can be slowed, preventing AOI from properly inspecting the desired components within given time constraints. New AOI Requirements Today’s AOI systems are being challenged by rapidly advancing technologies that create more complex PCBs than ever before. It may become evident to a manufacturer that the AOI system being used is outdated when it can no longer detect newer and smaller packages or the minute component markings on them. In these instances, extremely high-powered camera magnification and specific angles may be necessary to accurately inspect these small packages and their markings. Accurate color detection should also be incorporated in newer AOI models. Some through-hole components are color coded like resistors and capacitors. AOI detects the colors on the bands of these devices, but often, close colors in inspection images appear similar to an AOI system, making inspection difficult. This is a problem that requires attention in future AOI development. Recognizing odd-shaped PCBs and non-symmetrical components is another improvement that can make AOI systems operate more efficiently. Due to these unusual attributes, some AOI models have difficulty inspecting these types of PCBs and components. As for its limitations, an AOI system can be more useful by giving it the capability to import data from commonly available software applications and platforms, and be able to manipulate that data to obtain desired results. Some AOI systems have size and weight limits for the PCB. For many tools, an 18-inch by 20-inch, three to five pound board is the largest that can be inspected pre/post assembly. The ability to produce statistical process control (SPC) data is one particular feature that would make AOI more useful. SPC comes in handy when there are recurring failures of specific components, and this feature would allow tracking by lot and date code. On the plus side, newer AOI systems have the capability to assign a barcode to all boards passing their inspection. Plus, newer systems incorporate lights that can automatically adjust intensity for the board under inspecJUNE 2008 Other Key Features Foremost among AOI features is the offline programming option, which allows a user to program the AOI system offline and avoids downtime, helping to keep the system continuously inspecting boards. This feature is both cost effective and saves time. Another important AOI feature is solder bump (blob) analysis shown in taBlE 1. This analysis verifies the presence or absence of solder on component leads. Optical character recognition (OCR) is another useful AOI feature. More advanced AOI systems use optical character verification (OCV). Both perform character matching, status checks, and data basing, where OCR and OCV correlate text in the captured image with an image algorithm saved in the database. Based on this comparison, OCR and OCV will flag a particular image if there are flaws in the component. AOI is especially valuable for visually defining component manufacturers’ markings. Many manufacturers can make the same component, but their markings will always be different. Therefore, it’s vital for the AOI software to be sufficiently advanced to correctly associate specific markings with the correct component manufacturer. Staging and Lighting Most AOI systems have built-in “intelligent” staging and lighting, which saves time in inspection and component verification. Using these options sometimes requires the installation of macro programs to utilize these AOI system features. Intelligent AOI can utilize multiple forms of lighting. One source reflects light and causes the highlighting of a particular feature, while other light sources can be used to obscure undesirable component features, which are considered noise. Hence, intelligent AOI determines the required lighting parameters, either ambient or created light, and can determine angles and the correct lighting intensity applied to a given component. 36 printEd circuit dESign & fAB

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Printed Circuit Design & Fab - June 2008

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