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Choosing the Right Optical Inspection System
Opticon inspection system.
By Patrick Schuchard, Goepel electronic, Jena, Germany
Picking the right AOI system means a lot of work based on the many different criteria and options. Every AOI company will tell you there's no need to look further; their machine is as good as it gets.
It is important that you know which features are needed now and in the near future. And that basically depends on the PCB itself and the test requirements for the components used. It also includes questions about inline or offline, required speed, accuracy and available options. As a guideline, it is always good to know how much this system will cost 5 years from now.
One of the most important questions regarding the machine itself is: should it run in-line or off-line? In-line systems are integrated into the existing production process and therefore must be able to keep up with the production line's beat rate. Low-cost in-line AOI systems can often become the bottleneck of the whole production cycle. Off-line machines are stand-alone, and do not have to keep up with the actual production cycle. Off-line machines generally cost less, since they do not need conveyer systems and automatic PCB handling.
A system's inspection speed depends on several features and capabilities:
The speed of the positioning system; how fast does the camera move across the test area?
How many images from different positions, angles or cameras are required by the application?
How fast is the software, are the algorithms applied in real-time or does the systems need additional "processing time" after all images are taken?
Additional elements that impact the overall inspection time include PCB load/unload speed, top/bottom inspection capability and the imaging hardware. As a guideline, vendors often advertise the inspection speeds in the form of surface area examined per unit of time — cm
/sec or in
/sec. These should be considered average values, because they depend how densely populated the board is, the area and the test being performed. As a result, each manufacturer might specify different inspection times for the same effective performance. The best way to determine the right value is to perform benchmark tests with your own boards.
In the end, any speed rating is meaningless if the machine cannot provide accurate and consistent results. The "false calls rate" is a well-known term for a machine's ability to find real failures with a low percentage of false calls — incorrectly reported errors which should be passes. The false calls rate depends on many different factors, such as the recognition algorithm, the quality of images or the manufacturing process itself. A perfectly adjusted high-end AOI machine will produce many false calls if the production of the PCB is not consistent.
Shorts on IC pins.
For example, if the solder process is fluctuating, the pick-and-place Machine is faulty or the components vary in their appearance. An AOI system with a low camera resolution will have problems inspecting small components, such as 01005 packages, or an incorrect algorithm for solder joints will not be able to find shorts. Any of these factors can result in a high false calls rate. It is therefore not easy to say where a current high false calls rate comes from. Today's AOI systems, properly chosen and adjusted for the task at hand provides not only an additional test step, but a very good indicator of the quality of the production process. To make this information usable, today's AOI systems should be able to create failure statistics, store results and offer interfaces to existing process control software (SPC).
Lighting is All-Important
An often overlooked key feature is the illumination system. At least as important as the camera and lenses themselves, a proper light source is the basis for a high rate of fault detection on PCBs. Flexible illumination is necessary to cover specifications for the huge range of electronic components and their fault variables. Because of their inherent long-term stability and illumination power, LEDs in various configurations and colors have become widely accepted as the norm in recent years. Illumination can be arranged or configured in a wide variety.
There are several lighting setups for AOI, and each one meets specific needs:
Ring illumination from above, mostly arranged around the lens. This is a useful configuration for checking the presence of components and the quality of solder joints.
However, in case of shorts between IC pins, false calls may occur because of fluxes or solder masks.
Angled illumination from various directions and different incidence angles is useful for solder short checks, and reduces the number of false calls. The angled lighting enables a high-contrast display of laser labeling, a precondition for the effective utilization of true-OCR functions as well as polarity check.
Illumination with different colors enables high-contrast display of colored polarity marks and helps to distinguish components from the PCB background. Colored lighting can also fault slips and false calls rate.
In addition, for different board materials or component colors, a brightness control becomes necessary for certain test tasks. Because of the huge number of permutations possible for illumination settings, there are numerous parameters which must be provided and the AOI software must set the lighting quickly and easily.
Goepel electronics' OptiCon system series unites all of the possible lighting variations, and provides easy access to these features using predefined settings. For example, a special illumination arrangement was developed that allows a safe solder-short check at minimum angles. The user can select between different color varieties that provide safe detection of incorrect polarity, such as colored marks.
Rotating camera can provide views from many angles.
A typical optical inspection system has at least one top camera with a main illumination module, but often test tasks require additional features such as angled view cameras to inspect from a 45° angle. One common application is inspecting solder joints of IC Pins, especially J-leads. Sometimes obstacles such as PTH components or heat sinks block the view of certain areas of the board. AOI system designers try to solve this problem by adding more than one angled view camera to the system with the result that there are as many as 8 fixed cameras set up in some systems. Goepel electronic has developed a better way by letting the angled view camera rotate around the object. The "Chameleon" module can rotate with a step size of 1° through a full 360° of rotation.
Standard camera resolutions range from 1 up to 16 Megapixel, but focusing on the only camera itself would be a mistake. Increasing the resolution of the camera alone will not necessarily lead to more detail in captured images. One possible solution for these increasing requirements is adapting the lens to the camera being used. Specifically, the lens has to be designed to the pixel size of the applied camera in terms of its optical resolution capability.
Pixel Adapted Lenses
Another important refinement found in today's AOI systems is the use of telecentric lenses, which avoid the typical distortion of conventional lenses.
Use of telecentric lenses produces extremely high quality and dramatically reduced distortion found in the vertical view at each position of the inspected area, resulting in a significantly reduced debug effort during test program creation, but also a reduced false call rate in production tests. This system characteristic proves to be a decisive benefit particularly for higher components but also multi-pin and fine-pitch ICs.
Storing Good Sample Profiles
AOI vendors typically use two different ways for detecting failures. The first approach is to store images of good samples and simply compare the current camera image with the recordings. The user can set certain quality thresholds and adjust the system in order to define the pass/fail threshold.
The far superior approach is the use of neuronal networks. Attributes and test parameters are stored and the system learns with each new teaching process. This technology is more flexible and faster over time.
In order to provide a fast and easy start for a new user, vendors typically provide component libraries in which test parameters, dimensions and variations of the most common components are predefined.
Goepel electronic's OptiCon systems also include such ready-to-use libraries. Despite the huge variety, test programs can be created and optimized very quickly. All inspection parameters can be changed on four different levels: for a single component on the PCB, a model type in the current test program, in all new test programs or in all new and existing test programs. The OptiCon concept is based on the important goal of obtaining a stable, reliable test program within the shortest time, and to provide flexible adaptation possibilities to allow for specific quality requirements and supplier conditions.
Flexibility and Options
A flexible system configuration is not just about the system software; it also includes the capability to integrate additional modules. Because of the huge variety of electronic assemblies likely to be seen in a production environment, add-on modules can be very helpful in increasing fault coverage. The OptiCon systems provide these additional options:
Camera for THT with an enhanced clearance of 50mm for safe inspection of tall components (e.g. encased-electrolytic capacitors).
Laser height measurement system that allows co-planarity inspection with micrometer range accuracy — such as for BGA components.
Camera with angled view that enables inspection of critical components — for shorts and solder joint inspection at PLCC and SOJ components.
Boundary Scan add-on that consists of the 1149.x controller and the software to perform Boundary Scan test on the PCB.
Solder paste inspection camera that can be added next to the main camera to increase the test coverage before the pick-and-place step.
Integrated Barcode Reader for board tracing and quality control.
A typical approach begins with an online search and/or trade show visit, continues with contacting the vendors. This is where the customer gets to ask certain questions about technical
specifications, prices, maintenance and support. These questions are probably best answered in a face-to-face at a trade show, where you have the opportunity to see and compare several different AOI systems.
A real-time benchmark could follow these first steps where the customer visits the vendor with his own board, complete with faults. This way, programming, false call rates and accuracy can be assessed and it becomes possible to form a general impression of the machine. A visit to the vendor can also provide an impression of the company and people involved.
As a final step in the decision-making process, it is not uncommon to rent or borrow an AOI system for a certain amount of time, typically 4 weeks, before actually cutting the purchase order.
Contact: Goepel electronic LLC, 9737 Great Hills Trail, Suite 170, Austin, TX 78759
888-446-3735 or 512-782-2500 fax: 734-471-1444 E-mail: info-AT-goepelusa.com Web:
See at electronica Booth #A1.351.
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