Optical inspection systems

   Automated Optical Inspection (AOI) systems have reached a price/performance point at which they can be used in-line to detect several important assembly defects. Systems using visible light are among the fastest and least expensive. They can replace human inspection of circuit assemblies, and provide data to improve control over assembly processes.

    Automatic Optical technique been in use in PCB manufacturing have existed for almost two decades, and these techniques hold out the promise of a non-contact detection alternative for some classes of process defects. Historically, however, such automatic systems have been plagued with long inspection times, high fault escape rates and higher false flag rates, factors that precluded broad deployment into modern high-speed production lines and lights out factories.

   New generation AOI systems having sophisticated vision algorithms, multiple, broad optical data pathways and accurate mechanical and optical systems are just now emerging, and are finally able to deliver on the simultaneous promises of high speed and high reliability detection.

Vision System Alternatives

Just as with electronic test equipment, there are several classes of vision inspection equipment.

The machine-vision industry has a wide range of equipment vendors and systems integrators for ‘rack and stack’ vision systems. Software tool-kits provide an extensive suite of image-processing algorithms and graphical tools to simplify the programming of inspection tasks. They are suitable for low-cost, very specific inspections of a device or assembly. But users must be well-versed in vision systems design to build an effective inspection system. Just like rack-and-stack test systems, these inspection systems tend to be customized for an individual application and require considerable re-programming for each new board design.
General-purpose PCB inspection and gauging systems are fully-integrated systems containing XY stage, cameras, flexible lighting and board transport. They are capable of operating in-line to measure and inspect components on circuit boards, using a variety of inspection techniques. These types of systems offer precise measurements, flexibility and speed.
Specialized PCB inspection systems, designed to be operated in-line immediately after a solder paste or part placement operation, that are optimized for detecting assembly defects from that operation at the highest throughput possible. These types of systems offers low cost and fast, simple, CAD-driven programming for new circuit board designs. However their measurement capabilities may not be as precise or as flexible as general-purpose machines. These systems are the ‘Manufacturing Defect Analyzers’  (MDAs) of the vision inspection world.

Defect Coverage

   Vision systems used after a placement process, are capable of detecting the following defects:

Missing part.
Non-resident part present by mistake.
Mis-oriented (backwards) part.
Part offset and rotation measurement.
Location of odd-form components and hardware.

   When used after a solder process, vision systems can detect solder bridges. However, attention is shifting to vision inspection of solder paste immediately after the paste is printed, since many solder defects can be attributed directly to printing defects and are more easily detected at this stage.

Economics

For a vision inspection system to be economically justifiable in a high-volume PCB assembly operation, it must:

Operate in-line at line beat rates (30 seconds or less).
Offer a high degree of defect coverage for the targeted defects (90% or more).
Provide measurement data to data-collection systems.
Have low initial purchase cost.

Offer fast, simple programming for new board designs (a few hours to generate an initial inspection plan), and allow fast re-programming for design changes. Ideally the programmer should not require any specialized vision knowledge.
Be tolerant of normal manufacturing variations (for example, component color and marking changes caused by components from different vendors). It should not be necessary for the programmer to ‘tweak’ the inspection plan to cope with such changes.
Have very low false-accept and false-fail rates.

   False-accepts can lead to potentially bad product being shipped. Failures are diverted to a repair loop, so a high proportion of false failures wastes resources and brings the system’s trustworthiness into doubt. Some parts just cannot be inspected reliably. Ideally the system should detect this to prevent false calls on such parts.

What makes a good AOI system?

   What are the key inspection challenges that must be overcome by modern AOI systems to make them competent, consistent, and reliable? They fall into three main areas of technology, the lighting system, the imaging system, and the motion system. Any solution must excel in all three areas to produce a system that is reliable, repeatable, and can properly detect defects without false calls and/or escaped defects.

The Lighting system

   Lighting is extremely important. One example: when inspecting a solder joint, it is not sufficient to merely “light up” the scene in a generic way. The lighting must be flexible and configurable enough to “bring out”, or highlight one or more characteristics of the solder joint, and these must be characteristics that uniquely indicate that the joint has the proper shape and volume. That same lighting configuration must also definitively produce enough variation on a bad joint to obtain reliable differentiation. To isolate the proper characteristics and also differentiate good from bad, a highly structured, precise and programmable lighting tool is required. This tool must be able to properly illuminate the target from all directions, relative to the board and relative to any of cameras. It must also be able to provide precise directional control and intensity to deal with shadowing and obstructions created by nearby components.

   A best approach is an array of high precision point-source light emitters such as LEDs. The LED array provides a large number of precision lighting angles, and permits fine control of the intensity and direction of the light. Additional benefits of LED technology are consistency and reliability. The intensity of a LED emitter however is constant over the life of the equipment. LED arrays require no service at all for the life of the equipment.

The Imaging system

   The second core competency in a high-performance AOI system is the imaging system, sometimes referred to as the vision system. There are two classes of imaging systems: vertical camera only systems, (called 2D or two-dimensional), and 3D or three-dimensional systems. The 3D systems incorporate angled cameras. 2D systems are of course cheaper to build, and simpler to use, but they have some inherent limitations in fault coverage. Imagine that your own eye is looking down a tube while you are inspecting a board.

   Certain defects such as lifted gull-wing leads are very difficult to see with a single, straight-PCB down satellite view of the board. Angled cameras are much more advantageous for detecting this very important fault category. Some 2D systems attempt to overcome their lack of angled cameras by using color. One approach is to use a vertical camera with colored lights at various angles; these lights produce a color banding profile on the top of a gull-wing lead.

   From that banding profile one can indirectly infer that perhaps the lead is lifted and not making contact. Angled cameras provide a more direct, positive sensing capability in that by nature, they can “see” the position of the lifted lead directly, and without inference.

   Four angled cameras are typically employed in a 3D system – one each to cover north, south, east, and west – in addition to a single vertical camera. When using angled cameras at high magnification, board wrap page becomes a very significant issue. A small amount of warp can move the target image completely out of the inspection window. Therefore, a modern AOI system must have a competent warp correction system. Some systems make only a single height correction for the entire board, but warp correction should be more comprehensive, compensating at every field of view (FOV). Comprehensive warp correction ensures accurate placement of the inspection window and eliminates yet another source of false calls.

   A common question is whether color or monochrome imaging is best. When considering all types of imaging systems, the answer is both. In certain applications such as metrology, color is valuable, but for the mainstream post-reflow use, which is the area of concern for this discussion, color is not a requirement, and actually can be a hindrance. As mentioned earlier, it is important that the lighting configuration be able to highlight the solder joint characteristic that uniquely indicates that the joint has the proper shape and volume. In other words, it is desirable to bring out the indicative qualities of the image so that we attain the highest possible “signal-to-noise ratio”. The same concept is true for the imaging components. By using monochrome lighting and cameras, the resulting image data is spectrally pure, and it is relatively easy to analyze the data and differentiate out the salient components of the image. Color systems generate much more data and yield more spectral “clutter” that obscures the salient characteristics.

   The optics employed in the front end of a vision system must be able to deliver meaningful images to the rest of the imaging system. Generally speaking, to inspect today’s printed circuit assemblies including 0201 packages a minimum pixel resolution of 20 to 30 microns is required to have enough pixels in the image to reasonably map the target. A high-resolution image generates abundant data. In fact, in a single inspection pass, hundreds of Megabytes of image data can be generated. A high bandwidth image capture system is required to maintain a high basic scan rate. To maintain throughput, these systems must have an extremely high-bandwidth data pathway, which processes image data in parallel. The required level of performance cannot be achieved by using off-the-shelf components; the cameras, synchronization system, frame grabbers and memory transfer mechanisms must be optimized for processing rates on the order of hundreds of frames per second. The resulting images must be either analysed on the fly, or stored for later analysis out of the critical timing path.

The Motion system

   The third crucial element of a world-class AOI system is the mechanics, or motion system. The requirement here is to move the camera relative to the board quickly, yet be able to capture very stable images on the fly. Very few systems attempt move-and-stop type operation - it is far too problematic from a physics standpoint, and greatly reduces throughput. Similarly, moving the board rather than the camera is a concept fraught with trouble as the board being inspected is an uncontrolled variable to the AOI motion system, and can cause unwanted vibration amongst other undesirable effects.

   To capture stable images on the fly, the camera head must be completely rigid on its mount, and must be driven through its center of gravity to eliminate any vibration. Similarly, the XY camera positioning system that moves the head must be cleanly designed and solid to minimize positional tolerances. The XY camera positioning system must be powerful and precise and have active feedback, which can read actual position data to the micron resolution level. The high-resolution encoders are also the key to synchronizing the camera exposures… the “firing” of the cameras on the fly at high speed as they traverse the board. All of these characteristics of a high-performance motion system are necessary to ensure that the cameras are positioned correctly to deliver the correct images to the software system so that the image analysis algorithms can operate on the intended data.

Bringing it all together

   How do these the lighting, imaging and motion systems work together during board inspection? After the board has been conveyed into place, the motion system and imaging systems activate to locate the fiducials marks so that all inspection windows can be registered properly to the board. A warp scan occurs either prior to or integrated with the actual optical inspection pass, correcting some angled camera window positions as required by the curvature of the current board being inspected. Warp must be sampled and corrected for uniquely at every field of view across the board.

   As the inspection scan progresses, the motion system smoothly drives the camera head across the board and images are captured on the fly during this continuous, non-stop motion. The lighting and imaging systems are synchronized to the motion system through the use of high precision encoders that report the actual position of the camera head in real time. Accurate positional feedback is critical to ensure that the correct lighting mode fires at the right time, and that the electronic camera shutter captures the image at that same moment. Once a particular field of view (FOV) image has been captured by the frame grabber, it is block transferred at high speed into memory so that image analysis can begin as scanning continues. Image analysis is the quintessential activity that is the “inspecting” of the devices on the board. Many types of inspections are done on each component, and each inspection is typically performed by individual inspection windows that were laid out over the image of the device by the generic AOI device model when the inspection program was originally created.

   All of the above performance requirements in the areas of lighting, Imaging, and motion control systems (“Lights, Cameras and Action”) are crucial in creating a viable high performance AOI system. All three categories are mandatory and must work together to make the overall technology competent just as a tripod must have all three legs to stand.

   We offer a broad line of products for building your own sophisticated visual inspection system. Our programmers will write specialized tailor software to make your inspection process best suitable for your requirements. We provide flexible integration of our systems into your technological process. If you look for powerful and cheap inspection system, don’t hesitate to contact us!

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