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Automated Optical Inspection Technology Continued from previous page
with a smaller FOV, although camera sensors with a larger number of imaging sites or camera pixels require more time to acquire each image, resulting in slower imaging speeds or lower number of frames/second.
type of camera requires less time to capture a new frame. A higher frame rate makes it possible to scan a large PCB using dynamic imaging without stopping to process each frame.
Counting Cameras The most common AOI systems employ a sin-
3D switch inspection result with
reconstruction image and 2D cross-section. Although cameras with lower pixel counts
have smaller FOVs, the individual pixels for such cameras are larger and an AOI system with this
gle-camera architecture. Single-camera AOI sys- tems are effective for finding missing or mis- aligned components on a PCB and for inspecting most solder joints. Multiple-camera AOI systems include side-view cameras for front, rear, left, and right viewing angles in addition to the views from the top camera. Angled cameras are effective for inspecting solder joints and package leads. Traditional two-dimensional cameras in
many AOI systems may be inadequate for some inspection requirements, such as examining lifted components, lifted leads, oversized or tall compo-
March, 2015
nents, and applications in inspecting high-density PCBs and PCBAs. In such cases, three-dimension- al (3D) AOI technology can overcome the limita- tions of camera-based AOI systems, providing high dynamic range and 3D reconstructed models of components for review. Such 3D inspection sys- tems are ideal for post-reflow PCB inspections. Of course, optical cameras offer a limited
Reflective and black opaque components are difficult to inspect reliably by means of conventional optical inspection.
depth of field of several millimeters, so that tall components will be out of focus. Laser 3D inspec- tion systems can examine compo- nents to 25mm high, with 1µm reso- lution for close examination of some problematic components, such as switches, coils, and connectors. These 3D inspection systems offer an alter- native to more costly x-ray inspection systems, and effective 3D reconstruc- tion models can help to quickly spot defects. Some components can pose
problems for 3D inspection systems because of their optical properties, such as black or reflective compo- nents which can overload the dynam- ic range of an inspection camera. Fortunately, a 3D inspection system employing a laser light source (model TR7730 from TRI) can deliver clear images over a wide dynamic range even with such difficult components. Data management is an impor-
tant part of using any AOI system. By working with a computer-aided- design (CAD) file, an AOI system can properly identify the locations of a PCB's components and establish ref- erence points. The software for many AOI systems includes a database of standard component libraries to help identify various components on a PCB. Components not included in a library and that must be part of a PCB inspection can be added to the library file manually. Some inspection tasks may
include multiple-PCB panels, with many PCBAs on one board. An AOI system can learn the component placement for one PCBA and then duplicate it for the remaining PCBAs in the panel. These different PCBAs may be placed at 90° or 180° angles relative to the first board, so that an AOI system must be capable of mak- ing adjustments when examining one
AOI systems are available as desktop for batch operations.
of these multiple PCB panels. An inspection system is typically opti- mized by fine-tuning on production PCB samples to achieve the best results and highest inspection yields. Selecting an AOI system is also
a matter of size, since such systems are available as compact desktop sys- tems or much larger in-line produc-
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