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Advances in AOI Camera Technology
December 31, 1969 |Estimated reading time: 5 minutes
By Joseph L. Vilella
During the development of an AOI system based on a single-shot, high-resolution digital color camera, a related aggressive evolutionary process also was carried out. Developed when AOI technologies were based on grayscale image analysis, the camera that supported the technology was based on a four-million-pixel digital color sensor that displayed a 4 x 4" image format providing 16 million tones of color contrast. Though slow (compared to its successor's image acquisition time), it was adequate for its day — mid- to late-90s — relative to inspection cycle time. It could resolve chips down to the 0402 level with its standard field of view and it could have handled 0201 chips, if required, with a slightly reduced image. However, despite its benefits, it became apparent that this state-of-the-art camera would have to continue evolving to address the future printed circuit board (PCB) processing challenges that loomed on the horizon.
Camera Evolution
Today, 0201 chip inspections are not a theoretical possibility; they are a reality. Other areas of surface mount board processing also have achieved high levels of complexity. Additionally, there is a pressing demand to close the loop on component placement machines by offering accurate X/Y position- and rotation-gauging data.
Hand-in-hand with the evolutionary requirements of the inspection process was the elevation of the digital color AOI camera to its next level. The new camera made a quantum leap in development to address many of the typical difficulties and challenges that have become almost routine in the inspection of populated PCBs.
However, it is key to realize that the SMT process is in a constant state of flux. It is in the dynamic nature of populated PCB inspection to demand the most flexibility from an AOI technology. This demand has spurred an effort resulting in the most advanced AOI imaging technology available today.
Figure 1. With AOI technology, the more data available per pixel, the better. This image illustrates a board fiducial referenced measurement.
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The SMT Process Challenge
Although there has been a tendency to underestimate the inspection challenge of populated PCBs, users today are realizing what should and should not be expected of an AOI system. Actually, this challenge poses such a high level of complexity and variability as to require sophisticated state-of-the-art solutions. A close look at the process demonstrates
the following:
- That the challenge of performing accurate and reliable AOI board inspection stands as one of the two most technically demanding processes faced by the industry today. The other is the high-speed placement of very small components such as the 0201 chips and ultra-fine-pitch devices.
- That the technical demands and uncertainties of AOI inspection of PCBs are ever changing, stemming from the considerable variability of the boards within single and multiple lots. For example, multiple types of substrate color values within a lot may have to be identified in a single production day. Similarly, substrate regions with ground planes can be so dark that they provide little or no contrast between the board and the chips it carries. In addition, some polarity marks are so inadequate that they resist detection. And same-value components such as resistors or capacitors can have different identifying colors, depending on availability. Lastly, boards that are perfectly flat before the first run through the oven can enter it with varying degrees of warpage due to components having been placed on their second side. At times, paste also can induce variability (too much, not enough, smeared, etc.).
Figure 2. Expanded image analysis is key in managing AOI complexity levels. Advanced image analysis is shown for a complete feature extraction.
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A Fundamental Solution
To manage the level of AOI complexity effectively, it was important to develop a camera that would provide the short- and long-term resolution and contrast identification required. The result is a camera* system featuring the following advances:
- A Six-million-pixels Sensor, which provides up to a 4 x 6" field of view and more pixels per feature — fundamental to achieving a reliable AOI technology. (With pixel count, more is always better.)
- More Data Per Pixel: Also fundamental to a successful AOI strategy. With the amount of variability expected of populated PCB inspection, having as much data as possible per pixel is essential (Figure 1). Present sensors possess the capability of providing 16 million tones of digital color, permitting a robust inspection environment. The more color contrast definition that can be performed, the greater the quality and accuracy of the inspection analysis, e.g., that it is able to measure the positional location of the chips.
- A Single-shot Image: Not based on video capture or frame grabbing, the AOI image produced is a single and accurate digital color image. The data received are not second-generation information. Rather, they are derived from a first-generation digital process and, therefore, are cleaner and more faithful to the subject under examination.
- A Large-format Image: This feature is advantageous in reducing the production-cycle time as well as parametric analysis. The larger image promotes faster inspections but also can be reduced to a specific size for inspecting miniscule parts. The quality of the image and the type of data it provides also will allow for the accurate measuring of component X/Y positions and rotation.
- Expanded Image Analysis: With a digital image one can perform sophisticated analyses as well as boundary studies (Figure 2). This is because the digital image permits testing not only the acquired image but those variations that can be altered digitally to prove the effectiveness of the inspection algorithms. This capability is critical for the development of increasingly robust inspection processes.
- Image Archiving: This is one area in which the digital image can become extremely valuable "after the fact." This new camera technology not only takes images but also can save them for archival purposes for possible retrieval weeks, months or years later, should that be necessary. This feature can be useful when dealing with defense and medical clients who require positive proof of board integrity in case of an operational fault.
- Going Microscopic: The camera technology permits the user to go down as much as necessary — even microscopic — to inspect a site, a capability highly feasible in the semiconductor-fabrication realm. It also is reassuring to know that 0201, 0101 (in the future) or even hybrid chips are not an issue when it comes to inspection detection.
- Speed: The ability to acquire the images rapidly, in combination with the large-format footprint, has reduced inspection cycle times significantly. This reduced image transfer time feature is essential to further increase overall inspection speed.
Conclusion
There is much to be gained from the use of digital imaging technology as it is applied to AOI. The development of more powerful sensors will cause even more to be gained in the future as the technology continues to retain or improve the processing speeds achieved today. One thing is certain, sophisticated AOI technology is key to successful process challenges.
Joseph L. Vilella, president and CEO of Vectron Inc.,10109 Carroll Canyon Rd., San Diego, CA 92131, may be contacted at (858) 621-2400; E-mail: joev@vectroninc.com.