Automated optical inspection

In recent years automated optical inspection (AOI)-integrated into the production chain-has become an established part of printed-circuit board production in the electronics industry. A 100% optical inspection is used to regulate individual production steps and to ensure product quality. In MID production too there are now high-performance AOI solutions available for detecting 

FIGURE 6.4 Left challenges for the automation of optical inspection for MID right AOI system from the Viscom company [10] 

Another MID speciflc besides geometry that should be mentioned in this context is that there can be a great deal of variation in the range of colors and the optical behavior of the substrate plastics and the conductor layout. Consequently, the software and the camera system for AOI need image correction capabilities, and users have to take these discrepancies into account when configuring the limits for production tolerances. 

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AOlFiles. The CAD reference files, if required, are generally created at the PCB manufacturing site. These files prepare automated optical inspection (AOI) systems with data that confirm that manufactured pieces match, within tolerances, the PCB design. 

Defects are most often detected by visual inspection or automated optical inspection (AOI). Other means of nondestructive evaluation (NDE) include electrical testing, x-ray inspection, and ultrasonic inspection. The preferred NDE inspection technique for BGA and CSP solder joints is x-ray inspection. Automated x-ray inspection equipment is often placed directly into the assembly process line for circuit board products having a large number of area-array components. 

Unimpeded inspection PCAs that have gone throngh a proper aqneous cleaning process are easy to inspect because there is no flux residue on the surface to interfere with solder-joint inspection by eye or automated optical inspection devices. 

There may be defects that do not show up as faults. Examples are insufficient solder, a misaligned component, a missing bypass capacitor, and an open power pin. Inspection systems such as automated optical inspection (AOI) and automated x-ray inspection (AXI) detect many of the defects and also some of the same faults as electrical test. The definition includes the phrase at the end of the manufacturing process, which is important when compared with potential defect. ... 

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-fme-pitch deposits, can require the same level of magnification and therefore views as small as the 3-D solder paste inspection systems. 

There should be no silk-screened ontUnes around components. Although such outlines may be useful for visual inspection, they just confuse automated optical inspection systems. 

Lead-free solder has a rougher surface finish and generates a different-shaped fillet. It also is more prone to voids and tombstoning. These and other deviations can require adjustments to commonly used inspection techniques, such as automated optical inspection (AOI). While the results of a National Physical Laboratory (NPL) study confirm that AOI systems can be used to inspect lead-free surface mount assemblies, many defects created by lead-free processes are not visible. The added loss of visual and electrical access due to the growing complexity of PCBAs compounds the problem. 

Figure 6.5, left, shows a typical X-ray image of an MID by way of example. Suitable control and systems technology can extend X-ray processing to automated X-ray inspection (AXI), which can also be combined with automated optical inspection (AOI). 

The key point about assessing and defining process and product state similar to the machine operators way, is having objective information about the product quality. In the presented approach, the information from the optical inspection system was used to define characteristic situations based on the profile quality (the kind, distribution and quantity of defects) and the process parameters measured and stored by the automation system. A situation or case is thus characterized, among other things, by the aforementioned profile quality, the kind of profile that is produced, the used rubber-mixture, environmental data like air pressure or humidity, the values and latest progression of physical process parameters hke extruder-temperature, power of microwave heating or speed of conveyor-belts and the countermeasures that are taken by the machine operators. 

Typical dye bath tests begin by placing the ampoules under test in a chamber, evacuating the chamber to about 0.85 bar, and holding for 10 to 15 min. The dye is then admitted to the chamber and the chamber pressure increased to about 2 bar and held for a further 10 to 15 min. The ampoules are then washed, dried, and inspected. Inspection is most often visual but may be done by automated optical detection methods. 

The conveyor speed of automated print-and-etch lines ranges from 30 to 45 ft/min. Some lines are equipped with an online optical inspection which enables the elimination of the final electrical open/short test. 

It may well be necessary to adapt an optical inspection system to accommodate the three-dimensional layout of MID process surfaces. Class 2l D MID can be inspected without additional kinematics and adaptation of the optical inspection system. This holds true only if the process surfaces are plane-parallel in the inspection plane of the sensor array. Classes n x 2D and 3D call for changes to the AOI system if the electronic components are widely spaced. This could well mean integrating an extra handling and positioning unit for MID, for example an automated workpiece carrier. 

Solder joints for optical and structured light automated inspection systems must be clearly visible. 

Implementation of the MID application in series production necessitated the development of a fully automated assembly solution by a manufacturer of special-purpose machines. The 3D placement of the SMDs, the switch elements, and the contact pins is only one of the functions discharged by the system. Others include incoming-goods inspection, electrical testing of the conductor tracks, dispensing the solder paste with optical process monitoring, and final inspection. [116]... 

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