Poor vision: the great potential of machine inspection can be undone by poor planning.

Ed.: For the full article, please see circuitsassembly.com/cms/content/view/1819/

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The director of purchasing at a major EMS company summarized one of the key issues of automatic optical inspection: "Inspection adds no value in the SMD line ... It does not place parts ... It does not reflow parts."

Coupled with this attitude has been the failure of many AOI systems to meet customer expectations, for a range of reasons. Logic would seem to dictate that a system that can cost as much as a pick-and-place machine should deliver similar value. The initial promise of AOI to improve yields and reduce manufacturing costs was bright, and yet today it provokes much cynicism. Why this has occurred? And how does one gain greater value from an AOI solution?

Automatic optical inspection originated during World War II, when the interpretation of aerial reconnaissance photography meant hours of painstaking inspection of freshly developed film in darkrooms in the Mediterranean and Pacific. Photographs taken from 30' to 20,000' required the interpretation of highly skilled experts to detect gun emplacements, troop movements and ship identification as quickly as possible since thousands of lives were at stake. The first computers were entering use, and were taken up solely for cryptology or weapons development. Thus, simple pattern-matching overlays and optical comparators allowed users to eliminate much of the clutter and focus on what was important. This evolved into simple computer algorithms, most of which continued to develop for intelligence applications. During the Space Age, the first reconnaissance satellites used more sophisticated versions of these algorithms to achieve the same goals. The first AOI systems for inspection of PCBs were based on these technologies, and today's systems are their evolutionary cousins.

As AOI is widely used in a number of industries and applications, many of the technologies associated with it are better suited to different applications. A misprint on a pharmaceutical label may look similar to that on a component, but the way it is identified and reported can be entirely different. Most AOI systems in our industry are compromises, and by understanding their limitations and capabilities we can better devise a successful inspection strategy. AOI can also be used at multiple locations in the assembly process--on the solder paste printer, after the printer, post-placement and post-reflow--making selection of the best value and performance solution an even greater challenge. One size does not fit all.

This article details some of the issues that have frustrated, foiled and frightened many users.

Matt Holzmann is president of Christopher Group (christopherweb.com); [e-mail].