AOI: Off-Line Inspection

September 1, 2002
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Automated optical inspection does not have to be performed in-line. In some cases, off-line inspection may be more cost-effective. We asked Todd LaMarche, sales manager with Scienscope International Corp.'s Capital Div., for his thoughts on off-line AOI technology.

Q: When would you use AOI off-line instead of in-line? LaMarche: /b>Off-line AOI is excellent for first-article inspection and optimizing the manufacturing process. Because it’s off-line, it does not disrupt manufacturing flow, which maximizes utilization of the production line. In addition, off-line AOI is perfect for low-volume or prototype manufacturing. In-line AOI equipment is typically not available for this level of production. However, the need to ensure that all parts are present still exists. The key concern for most manufacturers is whether all of the components have been placed. Are they actually there? This cannot be determined through visual inspection by an operator. The off-line AOI can provide this.

Q: Where on the line is the best place to use AOI? LaMarche: /b>AOI is a powerful technology for checking the presence or absence of solder paste; for checking the presence or absence of components prior to reflow; for performing optical character recognition; and for inspecting solder attachments after reflow. I believe AOI is best-utilized as a pre-reflow inspection tool. It is at this point that the key quality issues will arise. Most manufacturers have a high level of confidence in their pick-and-place systems. They just need to know if all of the parts placed correctly before they begin full-out production.

Q: Is there any reason to have more than one AOI machine on the line? LaMarche: /b>For high-volume applications, it may be necessary to have three AOI systems on-line. However, this is not generally seen in high-mix production, because the time needed to change programs would most likely be prohibitive.

Q: Does an AOI machine require different features or programming depending on where on the line it's used? LaMarche: /b>Typically, yes. Solder paste, components in paste, and reflowed boards have very different features, and the programs for inspecting them will have different parameters.

Q: How does AOI complement in-circuit and functional test? LaMarche: /b>AOI can identify small component defects that may not have any affect on testing. In-circuit test requires a certain amount of real estate on the board to be effective, and many designs today are so congested that points on the board may not be accessible. It is possible to have a board pass the initial functional test station and still be missing some components. These components may not be important to the initial test, but will have a long-term effect on the performance of the board and will ultimately need to be present.

Q: Can AOI obviate either test process? LaMarche: /b>No. It is performs a visual inspection only and cannot determine if the board actually works. It truly is a great compliment to in-line testing.

Q: When would you use automated X-ray inspection rather than AOI? Would you ever use both? LaMarche: /b>Automated X-ray is best-suited to post-reflow inspection only. The contrasts that are generated by X-ray vs. AOI in solder joint analysis are much better. As a result, the accuracy of solder joint inspection is much better with X-ray than AOI. Often, AOI will have difficulty with 0402s and smaller chip components. It may not effectively and consistently see the difference between the end cap and the solder. Reflections, camera resolution and lighting can make it difficult for AOI to perform well on this application. For the most part, automated X-ray inspection does not encounter these problems in post-reflow and can more repeatably perform this function. The ideal line would have AOI for solder paste and pre-reflow inspection, and automated X-ray inspection for post-reflow.

Q: Some AOI machines offer color imaging. Why is that an advantage? LaMarche: /b>The logical answer would be to identify color-coded components. However, it is possible that color imaging offers better overall contrast during inspection.

Q: Some AOI machines are equipped with more than one camera. Why is that important? LaMarche: /b>Throughput and resolution. More cameras enable the user to perform more functions in a shorter period of time and position the cameras so they can perform large field of view (FOV) inspection (lower magnification and resolution requirements) and small FOV inspection (high magnification and resolution requirements) without having to change the camera position.

Q: If an AOI machine identifies too many false accepts and rejects, what might be causing the problem? LaMarche: /b>There are many answers to this question. It could be instability of the XY positioning system, lighting instability, or camera resolution limitations. In addition, the finer the operator sets the acceptance parameters, the more potential for false calls. If the tolerance window for acceptance is widened, the false calls are reduced. There is a definite trade-off.

Q: How can it be corrected? LaMarche: /b>The best in-line AOI systems have sound motion control platforms. If this is done, most false calls can then be attributed to lighting, camera, or the defect parameters.

Q: More than 25 companies offer AOI equipment. How do you choose an AOI machine that's best for your line? LaMarche: /b>High-mix manufacturers should use off-line AOI. If you are doing prototype work, one-off work, or simply want first-article inspection and ongoing process monitoring, off-line AOI will offer the best return on investment. If you have higher volumes, or your customer demands 100 percent inspection, then in-line AOI is the best choice.

Q: What specs do you really need to pay attention to? LaMarche: /b>The time needed to generate an inspection routine, and the actual changeover time from one job to the next. Can the machine inspect 0402 and 0201 chips? If so, how does that affect throughput? Do you need to 100 percent inspection? Or do you just want to ensure that all of your parts are initially placed? I am not a big believer in the false calls ratio specification, because it is so directly connected to how you set up the inspection program. It’s very subjective. Most AOI companies promote the defect call accuracy, but there are so many variables in confirming this, that most any system can operate to the highest level with the right amount of tweaking.

For more information about off-line AOI equipment, contact:

Todd LaMarche
Sales Manager, Capital Div.
Scienscope International Corp.
(909) 590-7273
(909) 590-7020, fax
ToddLaMarche@cs.com
www.scienscope.com

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