High-speed sensors address inspection
Stephen F. Scheiber, Contributing Technical Editor -- Test & Measurement World, 5/1/2005
People who aren't experienced with machine vision may think that measuring a linewidth on a printed-circuit board and determining the presence or absence of a component on that same board are essentially the same task. But the equipment and techniques used for each application will be very different. Similarly, equipment used to inspect an expensive, high-margin product like a cellular-network base station requires features quite different from those used to inspect a high-volume, low-margin (or zero-margin) product such as a cellular phone. According to Terry Guy, product marketing manager for Eastman Kodak's Image Sensor Solutions Group in Rochester, NY, success in any vision application requires keeping the needs of that application firmly in mind.
At the "heart" of the systemConsider the image sensor, what you might call the "film" part of a camera, computer, or imaging system. Light-sensitive sites (pixels) on the sensor capture the image in much the same way that the retina of the eye captures its image. The sensor then converts the pixels into electrical signals and transmits those signals out of the sensor and eventually to the computer system for analysis. Figure 1 shows a block diagram for a typical machine-vision camera.
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| Fig. 1 A block diagram of a typical camera system. The sensor serves the same purpose as the retina of the eye. Courtesy of Eastman Kodak Image Sensor Solutions. |
The sensors capture an image using either progressive-scan or interlaced techniques. In progressive-scan, the sensor exports the entire image in a single read-out cycle. An interlaced imager reads out the even-numbered lines of the image in one cycle and the odd-numbered lines in the next. The use of two read cycles for each image might suggest that this technique can provide increased image resolution. In many situations, however, interlaced images suffer from motion-based blurring because of the time delay between the two cycles.
Guy suggests that selecting a camera with the right sensor becomes critical to an application's success, because the sensor defines the maximum speed and resolution of the imaging system as a whole. Assuming comparable processing software and comparable computer capability, the time required to analyze the image remains relatively constant. Capturing the image is the only real variable in the process.
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| Fig. 2 A high-speed, high-resolution sensor. Courtesy of Eastman Kodak Image Sensor Solutions. |
Some higher-resolution applications demand the ability to capture even more information in each frame. One new sensor manages up to 11 Mpixels—4008x2672 pixels. Higher resolution also permits users to increase the field of view captured in each frame. With twice the resolution, each image contains four times the area at the same level of detail. In that case, the camera can cover a large surface with only a quarter as many images. In some situations, that might mean fewer images required of each camera. For high throughput, it can mean that the system includes only a quarter as many cameras—substantially reducing system cost.
On the factory floorTwo of the largest users of high-speed cameras are manufacturers of electronics and manufacturers of semiconductors. Vision-based inspection systems look at patterns on silicon wafers in real-time production to identify shorts and opens. The goal is to eliminate defective material and provide process information to increase future yields. On diced chips, a system may look at lead-bonding operations, including examining the edge of the die. On both packaged components and finished boards, a system looks for bent or broken leads; on boards, it also looks at the interface between device leads and the board surface, including the quality of the solder joints.
Production-rate requirements have increased dramatically over the past few years. Until a few years ago, for example, a wafer inspection system at full speed could examine 100 wafers/hr. With higher production rates and demands to reduce factory floor space, new systems must handle 150 wafers/hr with no compromise in frame rate or image resolution. A manufacturer must choose a camera that can achieve the necessary resolution even at the higher speeds.
Let it be a challengeImperx designs and manufactures imaging products that produce high-resolution black-and-white and color still images and full-rate motion images for high-demand applications. President and CEO Petko Dinev underscores the importance of attaining the highest possible frame rate and resolution, including high-speed sensors, in his camera designs.
As an example of a high-demand application, he cites the explosive growth in the manufacture of flat-panel liquid-crystal displays (LCDs)—relatively small ones for notebook computers and computer monitors as well as the large panels that have appeared for presenting high-definition television (HDTV). Dinev notes that supplying high-speed cameras for LCD inspection represents a substantial portion of Imperx's business.
LCD manufacture is a complicated multi-step process that requires testing at every step to weed out bad panels. As with any product aimed at consumers, manufacturers must drive costs down—which requires increasing factory throughput as much as possible—while keeping yields and quality up. A 21-in. flat-panel monitor with 1600x1200 resolution, which only a few years ago would have sold for thousands of dollars, can cost less than $700 today.
Flat-panel displays consist of two pieces of glass spaced 0.8 mm apart. Companies manufacture the panels in sheets larger than 2 m2 and then cut individual displays from the sheets. As a first step, inspection must verify that the glass is perfectly flat and defect-free. Then, tiny spacers are placed between the two pieces. Another inspection step ensures that the spacer positions are correct and that the glass panels are parallel, with the distance between them remaining strictly constant. Any spatial variation will cause pixel colors to bleed and therefore cause the panel to be rejected.
Once the crystal cells are in place, the displays must be inspected again. The size of the panels and the necessary throughput require manufacturers to use many cameras simultaneously for each inspection. The use of high-speed imaging sensors with high-resolution can reduce the number of cameras needed to acquire information in real time.
Flexibility is keyDinev says that there is another advantage to designing a camera around a high-speed sensor: flexibility. For example, his company's 11-Mpixel camera inspects the LCD panels in high resolution at 210 frames/s. With that same camera, NASA can inspect jets from the Space Shuttle at more than 1000 frames/s. In extreme cases, you might even look at a small field of view at 3000 frames/s.
In addition, building a camera around a high-speed sensor dramatically reduces the cost of that level of performance. With a tweak of the software, the same camera hardware can offer lower-resolution images even at slower speeds, when that is what an application demands. Because they contain fewer bits of information, such images permit faster analysis than their high-resolution counterparts and take up less storage space.
Imaging flexibility can offer enormous advantages to users. Consider the plight of the contract manufacturer. For vertical manufacturers, the next product generally resembles the last one, at least to some extent. Manufacturing and inspection and test strategies remain relatively constant as well. But a contract manufacturer never knows what the next project might be. Since inspection has become such an integral part of the contract manufacturer's arsenal, the freedom to specify a single camera that will meet needs ranging from low speed with low resolution to the most demanding high speed with high resolution has enormous appeal.
Time marches onLike all electronics technologies, the capability of image sensors has grown dramatically in the past few years, while prices have fallen. Incorporating such sensors into camera designs permits a single system to perform a plethora of inspection tasks, from the relatively mundane to the most demanding. While one size does not fit all, this type of versatility simplifies the task of specifying a camera that will meet uncertain and ever-changing needs.
