Wednesday, October 26, 2016
Publication Date: 12/1/2008
Archive >  December 2008 Issue >  Special Feature: Test and Measurement > 

AOI Makes All the Difference
Offline debug program editor main window.

AOI systems were developed around 1985 to satisfy the need to automate manual (visual) inspection of printed circuit boards. The forces that pushed for automation were familiar: labor costs, smaller boards, smaller components, and problems with the inconsistency of human inspection. The first AOI system was used off-line, and its purpose was to provide a more reliable and repeatable inspection process.

Before AOI was developed, most PC board manufacturers manually selected "good" and "no good" boards after production. Then workers visually inspected the boards, taking a lot of time to inspect them. From this, a support system for repair resulted, and all this time, the stability and accuracy of visual inspection improved.

This inspection process improvement was not sufficient to reduce the large number of defects generated by equipment set up for mass production. Repair requirements continued to increase because of wrong or missed components, insufficient or cold solder, and bridges between leads. Visual inspection systems just were not practical, and selecting "good" PC boards remained a problem. There was also a crying need to avoid false rejects and to reduce labor costs and such problems as worker fatigue, eyestrain, and related physical problems.
Debug program editor component editor and OK/NG images.

At that time, visual inspection systems did not generate a good return on investment. The most significant issue was real-time inspection. Manufacturers desperately needed to synchronize production and inspection.

In-line Inspection
In-line AOI was developed to resolve these problems. In-line inspection can give real-time feedback of defect patterns and trends to the production line and process engineers. This real-time feedback can be used to stabilize the process quickly and improve quality. The result is a higher yield and better overall quality. Throughout the SMD Process each line can be examined separately where AOI has been installed; inspection after the solder paste printing process reveals printing defects. Both printing and component placement can be inspected directly and accurately just prior to reflow, where PC board dimension and shape, components, and paste are still undistorted by heat. Almost no "escapes" and false rejects occur here, and inspection at this point in the line is extremely accurate.
Extra component detection setup procedure.
The inspection repeat ratio is nearly 100 percent. For example, one user moved a vision system from post-reflow to pre-reflow. The result was a 10 percent reduction of the total defect ratio. Of course the decision to place visual inspection before or after reflow depends on the user's situation.

Easier and Cheaper Repairs
Another benefit is that defect repair is far easier and cheaper to perform before reflow. The printer or placement machine operator can do in-line PC board repair. This gives the operator excellent feedback on machine operation because defects caused by printing or component placement are known immediately. Shifted, missed, wrong or reversed components, potential tombstones, bent leads, contamination, insufficient or excess solder, and bridged leads can all be found at this inspection stage.

On the other side of the coin, stable quality control is assured by post-reflow inspection. Reflow creates lifted leads and tombstones, and because these components are fixed by solder, repair must be done off-line. With in-line AOI after reflow, "no good" selection is processed automatically by a "no good" stacker, and the machine operator usually is not involved in the selection process. This allows him or her to focus on the actual process. Operators who are trained for this specific task are usually also required to perform the repair so they are better suited for overseeing the process. On the negative side, performing AOI after reflow has its own problems because the inspection accuracy and quality at this stage depend on PC board design and component accuracy, so the false reject ratio is usually higher, and false rejects mean unnecessary extra work.

When it comes to debugging, the best approach has been to take known "good" and "no good" PC boards and improve the detection libraries on a machine that could be set aside for such a purpose or to take a machine out of the production line to troubleshoot the programs. This is a very ineffective way of solving the problem.

Personnel involved in such a task have been requesting a better way for a long time. Saki recognized this and developed a new software tool that could take defect images and allow someone to debug the inspection program offline, but still using defects as they happen (nearly in real time). This software allows the AOI technician to set up the in-line machine to store the defect images as they happen so that he or she can "tune" the inspection program without interfering with ongoing production.

Saki's Offline Debug mode is easy to set up and allows full library editing of the program based on the actual images. Once you turn on the function, the system starts to collect the images and they can start being used for debugging the component libraries used by the program in question.

Editing Algorithm Settings
As the user edits the settings used by the algorithms used in the inspection, the system immediately shows how the changes affect the inspection of other components.

  • Offline Debug allows for verification of parameter change and the effects on other components.
  • The saved images of defects as well as OK samples can be quickly verified.
  • All parameters, lighting conditions and algorithms can be checked and quickly determine the effects that any changes can have in the library.
  • It is a powerful tool to do library tuning of current running products (saved defects) or new product generation using existing libraries.

Once the editing is finished, there is the option of updating inspection machine data. This will allow the user to begin using the modified library immediately. This process may be repeated as often as necessary to improve inspection yields thereby reducing costs. Other improvements have been made to the program editing suite such as a "component library tree" and "module copy" which simplifies program and library creation, thus reducing the time it takes to prepare programs.

Extra Component Inspection
The method previously used to inspect "other" areas of the PC board for dropped components, PC board damage, solder balls, etc. has been to add another inspection window. This method is not effective as the location of these defects is always different and the inspection time keeps increasing little by little. To correct this, the company has introduced ECD (Extra Component Detection) to its inspection software, which is an effective way of easily adding this kind of inspection to the whole board or large areas of the board. Since the whole PC board is already scanned by the line scan technology, this method can reduce inspection time as compared to adding more windows for inspection.
Extra component detection test result.

After the initial setup to gather images to create the template (Good sample boards are used) it is possible to begin using the inspection system to detect foreign objects such as stray solder balls, extra components, and other metallic objects.

This is an invaluable tool to improve final inspection yield and reduce returns from the field, since foreign objects may not be detected by standard methods of inspection, including ICT and functional testing since these objects can move during packaging and shipping.

Contact: Saki America Inc., 970 W. 190th St., Suite 880, Torrance, CA 90502 310-329-6762 fax: 310-329-6276 E-mail: Web:

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