Thorsten Niermeyer, Agilent Technologies
Testing has always had a major role to play on the shop floor, and not only as a sorter of good products from bad. Electrical test and inspection (the verification tools) provide the means to continually monitor process quality and to provide the feedback necessary to constantly optimize the manufacturing process parameters. As technology becomes more complex, electronics packaging become ever tighter as product density continues to escalate, the verification challenge increases. And, in particular, how to achieve the test accuracy required maximizing product reliability without compromising the throughput needed for high-volume production?
Automated optical inspection (AOI) is the primary in-line verification method used in high-volume manufacturing. In such applications, the throughput of the AOI equipment has to keep pace with the rest of the manufacturing equipment in order to achieve a balanced production flow. At the same time, the accuracy of the inspection has to be such as to minimize the probability of either a good board being declared bad or a bad board being declared good (false calls). For AOI, this accuracy is often referred to as the call accuracy.
This is a recurring problem in all methods of test and verification: thresholds have to be optimally set in order to balance the probability of either situation occurring. Conventionally, those thresholds are set to ensure that the chance of passing a bad board is at an absolute minimum. This is done to minimize failures in the field and so minimize the costs of maintaining quality and reliability. However, this strategy can increase the chance of good boards being failed during manufacture, the so-called false failures. False calls also involve penalties, the time of a repair technician to determine whether a declared fault is true and possibly unnecessary rework. The accuracy of a verification system has a direct and potentially very significant impact on the overall cost of the manufacturing process.
AOI systems use imaging techniques to examine the structure of a board assembly. Optical inspection involves capturing images of a device or feature (e.g. solder joint) on a board and then making a determination of whether the device or feature is correct. Depending upon this decision, the AOI system may declare, for example, a miss-ing device, reversed polarity device, solder bridge or suspect solder joint. There are two fundamental aspects to this determination: one is the image capture and the other is the analysis to produce a decision. Both of these affect the ultimate accuracy that can be achieved with an AOI system.
Over the development of AOI there have been many efforts made to refine the imaging system by varying the configuration of the lighting system or the camera. Typically, these have involved either using different angles of illumination and an orthogonal positioned camera, or moving the camera position to slightly ”off-angle” (around 200 from vertical) and using orthogonal lighting. The geometry of optics suggests that the most information from an object (say a solder joint) can be captured when the angle of illumination is close to 90° from that of the camera. This is the reason why the majority of AOI systems use an orthogonal positioned camera and some form of domed lighting to illuminate the board from low angles (around 600 from the vertical is possible). This law of optics is slightly at odds with the ”intuition” of most people who first look at AOI systems and think that moving the camera to an off-angle would give a better view of solder joints. In addition, of course, the continuing developments in imaging technology with the increased resolution of megapixel digital cameras have greatly benefited the AOI industry.
