No. 29 05/2013 Viscom Extremely fast and with the highest inspection depth XM: The AOI camera technology of the future The demands on systems for the automatic optical inspection (AOI) of soldered electronic assemblies are becoming ever higher. While formerly reliable detection of soldering and placement defects was the most essential, today 3-D and color features may also need be evaluated and to avoid human error, additional image information furnished during rework at the verification station. All this is coupled with ever-increasing throughput requirements. Therefore new technical concepts are required in order to satisfy the demands stated here in a manner compatible with cycle times. During the automated assembly and soldering of electronic assemblies in printed circuit board manufacture, production errors are difficult to avoid. To detect these defects and secure quality, systems for automatic optical inspection (AOI) are well established. Frequently, these systems are positioned after the solder oven for post-reflow inspection, because the greatest defect coverage is obtained at this inspection stage. In series manufacturing, systems which allow very short placement times and so raise throughput requirements on the AOI are increasingly employed in the placement area. In parallel, due to the miniaturization of components and smaller pads the demands on resolution and inspection depth for the AOI are ever higher. The task for further development of the AOI is, therefore: in order to be equipped for the challenges of the coming years, both inspection depth and throughput must be raised at the same time. In the following, not only will the concrete demands on the AOI and the effect on the camera technology be described, but a solution concept will be introduced as well. Requirements on a modern AOI The AOI as end-of-line control must offer the possibility to recognize defect types which may have different causes. Some of the significant influencing factors are: the solder paste print the component placement the different components the soldering process material properties of printed circuit board, components and solder paste Each influencing factor can in turn have diverse repercussions on defects, so that at the end the AOI must recognize the widely varying defect characteristics in the acquired camera images. The miniaturization of components and pads is a relentlessly continuing trend. While earlier 0402 components were typical for chips, today multiple 01005 chips are used in serial placement. For the QFP components, a trend from 0.65 pitch through 0.5 pitch to 0.4 pitch is observed. A typical example for a solder defect is a Lifted Lead on a QFP, as shown below.
Solder defect Lifted Lead In the meantime, the angled view analysis has generally come to be seen as a necessary feature of high performance modern AOIs. According to the dominant market opinion, AOI systems with exclusively orthogonal cameras are no longer considered adequate to guarantee a solder joint inspection with no defect escape. An especially conspicuous example is the continuously used PLCCs; their soldered connections are hidden orthogonally and therefore can only be reliably inspected with an angled view. More and more often the need to evaluate color as a feature arises. This may be the case with color ring analysis for resistors, and also for specific polarity inspections and tests to differentiate components before the further inspection. Coplanarity of components comprises an indicator for possible solder joint defects. On ICs, especially those with hidden soldered connections, the parallelism of the component surface to the printed circuit board surface and the correct distance to the printed circuit board is an essential prerequisite for good soldered connections. On chip elements, the 3-D measurement of the component surface can be used as supplemental information in, for example, the detection of flat lifted leads. This applies especially to unfavorable pad design. The lifted lead inspection for IC leads, for which the 3-D information can supplement the classical solder joint inspection with angled view, is also improved. During verification of the inspection results it is desirable to make further image information available to the viewer in addition to the camera image relied upon for evaluation on the AOI. Helpful, for example, are the angled views of the soldered connection and component from up to eight directions, or so-called color gradient images in which the inclination of the solder surface is color-coded. The goal of these supplementary images is always to keep human error the erroneous classification of a real defect as a pseudo defect by the operator Angled view additional image in verification of a diode as low as possible. Further inspections and tasks which should increasingly be covered by the AOI are: optical character recognition (OCR) reading data matrix codes open area inspection (e. g. stray components ) recognition of solder beads external substitute inspection for ICs with hidden soldered connections (e. g. QFN) Therefore the highest possible quality and flexibility of the camera images are needed to facilitate optimum preconditions for an automatic good/bad classification of the images by the analysis software. Technical solution concepts The following elements are crucial for the flexibility and quality of the camera images: high resolution multicolored illumination from all spatial directions orthogonal and angled camera views possibilities for color image acquisition for orthogonal and angled cameras possibility of 3-D measurement For the optical resolution, a value as good as can be is desirable. True, additional resolution can be achieved computationally through corresponding mathematical processing (subpixel 2
process) during the evaluation. Yet the human eye is largely incapable of recognizing the resulting effects. Therefore this method can only serve as supplement to real optical resolution. AOIs available on the market generally work with optical resolutions on the printed circuit board of from 10 µm to 20 µm per camera pixel. To ensure reliable defect recognition, the optical resolution for 01005 chip elements should be 10 µm per pixel. An additional improvement is thus eminently desirable: the pads on the printed circuit board for a 01005 chip are so tiny that with poor resolutions, only a few camera pixels still detect the soldered connection. In principle, improving the optical resolution is no problem. However, unless other conditions are changed the usable field of view is restricted, and therefore the AOI throughput is significantly restricted as well. Doubling the resolution from 20 µm to 10 µm reduces the field of view and the throughput by a fourth! On the other hand the number of camera pixels per surface area is multiplied fourfold on, for example, the same soldered connection, therefore increasing the reliability of recognition. Thus the art lies in combining outstanding resolution with outstanding throughput. An obvious approach is to select cameras having as many pixels on the camera chip as possible. In industrial 400 µm 400 µm 01005 component 8 µm resolution 01005 component 16 µm resolution Camera pixel on a 01005 chip at 8 µm and 16 µm pixel resolution applications by now cameras which have one to fifteen megapixels, depending on the available budget, are in use. However, the number of pixels alone is not decisive. Quite often reasonably priced cameras with a high number of pixels only have a low frame rate, i. e. the number of pictures per second the camera can deliver. Yet a high area performance during inspection of the printed circuit boards only comes into play when a high pixel count is also combined with a high frame rate. Typical frame rates for megapixel cameras are 10 fps to 30 fps (frames per second, images per second). A high frame rate is also desirable for other reasons: frequently the user would like when the system technology makes 200 µm 200 µm this possibility available at all to draw on additional images with different illuminations or views for the analysis in order to improve the reliability of the defect detection. Yet each image acquisition costs time. The higher the camera frame rate, the less dependent is the AOI throughput on the number of different image acquisitions. In addition to camera technology, illumination also plays a decisive role in industrial image processing. The illumination ensures that the defect characteristics on the printed circuit board are clearly visible in the camera image. This is an essential prerequisite for the automatic image processing to be able to recognize defects with the greatest possible reliability and without pseudo defects. As described, thus the available view and of course the direction of illumination as well play a major role. In addition to the direction, however, the color of the illumination also plays an important role. Certain features emerge much more distinctly when color (red, green, blue) is used instead of white. Thus a system achieves the greatest possible flexibility and range of application when it offers the possibility to freely define and select illuminations and their intensity, color and directions. To attain a quality which offers sufficiently good repetitive accuracy, additional elementary requirements are indispensable. Changing the illumination -> Short circuit defect with good and poor definition 3
The camera and illumination technology must permit a highly accurate calibration of the camera grayscale values through the software. Only through this possibility are complete program transferability and use of the same inspection library for several systems and production lines made possible. One significant decisive feature of the AOIs available on the market is the camera view. A distinction is made between orthogonal camera view (top view) and angled camera view (angled view). In principle, many of the above defect types can be covered by an orthogonal camera view. One critical type of defect for the orthogonal camera view is the lifted lead. Only under favorable external boundary conditions (e. g. suitable pad design) can lifted leads be located with a top view. But especially where demands on quality are high, the use of angled camera views is unavoidable. This employment of angled views requires several competences, particularly where calibration and grayscale value calibration are concerned. If only one (orthogonal) camera is deployed in the system, a continuous calibration monitoring does not necessarily have to be mastered, because within certain limits a camera recalibrates itself by means of the fiducials. On a multicamera system, the software must enable calibration of the camera positions to each other and for logical reasons, to continuously monitor the cameras even beyond the basic calibration. If the software does not enable a simple, accurate and fast calibration, the AOI supplier will face problems with integration of an angled view. The usefulness of this performance feature is then significantly diminished. It is also necessary for the angled view cameras to acquire color images and to use a good resolution. Often, due to cost reasons lower quality cameras are used for the angled view; such cameras have either a low frame rate or do not facilitate color acquisitions. As an option, angled cameras should not only cover the four main directions, but also be capable of augmentation by four more angled view cameras in the corners of the surrounding area. The new XM sensor concept At Viscom, the above boundary conditions and hardware newly available on the market have confirmed the decision to provide an entirely new sensor module (XM module), which extends the current 8M sensor technology upwards as equipment variant for the high end AOI S6056 based on the latest basic technology. The 8M camera technology does fulfill all specified conditions pertaining to inspection depth. However, for particularly high speed requirements, customer have expressed the desire for an additional throughput increase. The previous designs have clearly demonstrated that a high performance and high quality camera technology is indispensable for AOI sensoring that will meet modern and future demands. Therefore Viscom has decided to develop the camera technology for the new XM module completely by themselves, so maximum performance can be provided. The following performance features for the camera technology have been accomplished: number of pixels: 25 megapixel orthogonal view and 5 megapixel angled view resolution: 8/16 µm/pixel orthogonal and 16 µm/pixel angled frame rate: 45/80 fps orthogonal and 70 fps angled Particularly interesting is the feature by which the optical resolution of the orthogonal view can be switched. Through distortion-free switching between 8 µm and 16µm pixel resolutions, only a portion of the 25 megapixels can be used where the maximum resolution is not required, giving an additional and significant boost to inspection speed. The high camera frame rates require a correspondingly high performance transmission path from the cameras via the frame grabber and to the computer. Because not only the cameras but also the actual frame grabber in the computer and the transmission path between cameras and frame grabber represent a completely Viscom development, four cameras can take images in parallel and transfer them to the computer. This enables a genuine inspection speed of up to 50 cm²/s with full inspection depth and intensive use of the angled view cameras. At the same time, the high frame rate allows the greatest possible independence of inspection speed from the number of image acquisitions used. For example, adding the complete angled view inspection changes the inspection time over many printed circuit boards 4
by only 10-15 %. The effect of activating individual, special inspection features on the inspection time is barely measurable and liberates the user from the double bind between inspection depth and inspection time. With the no-compromise optimization of the XM module, this question is no longer raised. Highest inspection depth and greatest throughput the XM standard. Naturally this high frame rate also places particular requirements on the intensity of the illumination. High frame rates mean low exposure times. In order to nevertheless attain a good basic illumination, SMD high performance LEDs are employed; compared to conventional LEDs, these furnish a 10-fold intensity increase. Summary These remarks make it clear: the demands on a modern and future-proof AOI are always on the rise. Immense throughput requirements must be met; yet at the same time reliable defect recognition with as few pseudo defects as possible must also be secured. As well, more and more additional inspections which must not negatively influence inspection speed and therefore cycle time are imposed on the AOI. Only a high performance camera technology can cover these requirements. Investment in finished components, even when of top quality, is no longer sufficient. Rather, all components (cameras, illumination, transmission path, frame grabber) must be optimally tuned to each other and be optimally adapted to the given task. The XM sensor module for AOIs from Viscom fulfills these criteria. This completely Viscom proprietary development delivers high pixel counts and a high frame rate and therefore ensures an enormous data rate for the image acquisition. The Viscom transmission path and the Viscom frame grabber are matched to the camera data rate and can transfer up to about 20 Gigabit/s. The illumination with four colors from all spatial directions achieves optimum contrast and is ready for all conceivable future demands on the inspection. Expansion of the angled view, acquisition of additional images for the verification station or for additional illuminations are therefore nearly cycle time-neutral. This increases inspection depth and first-pass yield without a significant influence on cycle time. Thus in every respect, the XM AOIs take their place at the pinnacle of optical inspection systems and are more than ready to meet the growing demands of SMD inspection for years to come. 5
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