Automatic optical inspection
The automated optical inspection ( English automated optical inspection AOI) describes systems using image processing methods detect errors in production and in other goods and can report. They are a further development of machine vision .
AOI systems, including vision systems called (. English machine vision), are used in the meantime almost all lines of industrial production of goods - from food and pharmaceutical manufacturing to electronics, plastics, automotive, aviation and space industry. Among other things, they are also used to control the manufacture of printed circuit boards and the production of electronic assemblies (assembled printed circuit boards ).
tasks
AOI systems are additional investments in production. Against these costs one calculates the advantages, which lie in the following potentials:
- higher quality level
- Analysis of disturbance variables and process improvement
- less scrap
- Securing the supply chain
- Monitoring of highly dynamic production processes
- Cost optimization.
Example circuit boards
In the production of electronic assemblies, checks are usually carried out after the circuit boards have been assembled and soldered to ensure that they are manufactured without errors. It does happen that errors occur during assembly by assembly machines or in the subsequent soldering process (e.g. reflow, wave, selective soldering, vapor phase). Errors can e.g. B. missing or incorrectly assembled components , twisted or offset components, unsoldered components, short-circuited components or contamination of the circuit board. These errors must be caught before the next step in production and, if possible, repaired; otherwise, if repairs are not worthwhile or impossible, the circuit board will be scrapped.
There are two ways to check the printed circuit boards for defects:
- through man
- by machines
Here, human control has some serious disadvantages:
- People can overlook mistakes
- People get tired and need breaks
- People can no longer inspect the fine structures and components on today's printed circuit boards without tools such as microscopes
- People classify errors differently (important for traceability)
AOI systems do not have all of the disadvantages mentioned above (whereby the failure to overlook errors results from the working principle and the quality of the machine).
With these systems, however, you have to distinguish a few things:
Image acquisition
The images of the circuit boards can be taken with scanners or cameras. If the images are recorded with a scanner, it only has to move once over the printed circuit board to be checked. In the case of camera systems, there are systems with a camera that is aimed at the plate from above, systems with several cameras and systems in which there are also cameras attached at an angle. In contrast to scanners, the limited field of view means that the camera must be moved over the circuit board using an XY positioning unit. Software controls where the slide has to be moved and when pictures are to be taken.
So that the machine knows which component must be in which position, it needs the CAD data of the corresponding circuit board, in which the position, the designation, the angle of rotation and, if necessary, the shape of the housing are recorded.
Stand-Alone or Inline
With stand-alone devices, a circuit board is always inserted into the system by hand and removed again by hand after the inspection.
With the inline system, the circuit boards are brought into the machine via a conveyor belt. The transport is monitored and controlled via SMEMA or Siemens interfaces and light barriers and after the control, the circuit board is moved out on the other side of the machine.
AOI or combination device only
Normal devices only offer the option of performing a visual visual inspection. However, some manufacturers also have devices in their range that can also perform an X-ray test, for example, and thus offer two-in-one functionality (AOI and AXI combined).
lighting
So that the components to be checked on a circuit board can be recognized sufficiently well with the cameras or scanners, the circuit board must be artificially illuminated. The light sources are arranged differently in the machine. Depending on the system, there is the option of illuminating the current image section with direct light from above or with indirect light from the side ( side light ). In addition, different colored light sources can be used depending on the system. In the devices of earlier days, fluorescent tubes were sometimes used for lighting. However, since the lights have to be changed constantly, the fluorescent tubes are quickly destroyed by constantly switching them on and off. Modern devices therefore use LEDs. These darken a little over time, but this can be compensated for by a calibration (gray value comparison) that is carried out at certain intervals.
Open or closed
Open systems do not have any shielding against the incidence of stray light. If sunlight or the light of a fluorescent lamp falls on the circuit board, the systems can run into difficulties.
Closed systems are protected against the incidence of interference light sources by an almost completely closed housing and special panes. In them, the circuit boards are almost only illuminated by artificial light. This allows the lighting situations to be reproduced as required and the machines to be optimally set.
Analysis method and type of image evaluation
The pictures made of a printed circuit board cutout with components are examined to see whether the component shown has been properly assembled and soldered.
With the bitmap comparison, the recorded images are compared with previously recorded comparison images of good components. If a previously defined deviation occurs, the component is considered defective.
When comparing vectors, an attempt is made to find the component based on light-dark transitions. From this you can then see how much the component is offset and / or twisted. The same analysis is used for the pins. Gray value analyzes are then used for the solder joint control, in which the mean gray value of the solder joint is measured under a certain lighting in a measuring window that is placed at the end of the pin.
In addition, there are systems that do not fit into either of the above two categories and that use a mixture of 2-color image and gray value analysis to find components and soldering points.
Use of the AOI systems
The AOI systems can be used in the field of electronics production after the following production steps:
- Solder paste printing in the SMD process
- Assembly control during reflow soldering in the SMD process
- Solder joint control during reflow soldering in the SMD process
- Assembly control in the wave soldering or selective soldering process
- Solder joint control in the wave soldering or selective soldering process
Optical inspection of printed circuit boards
If a circuit board has no electrical test points (approx. <0.8 mm), the circuit board must be measured optically. Two or four camera points are approached and the displacement in X and Y, the shrinkage / expansion and the rotation in theta are determined. With these factors, the printed circuit board or the contacting unit can then be positioned accordingly. If circuit boards have fine test structures on both sides, the circuit board can be measured from both sides so that the contacting units can be corrected independently of one another.
Camera points
When it comes to camera points, points, circles and crosses appear most frequently, or structures are simply taken from the circuit board itself. The use of crosses is ideal, as these provide a large number of edges for recognition.
Camera calculations
The more camera points are made available for calculation, the more you can say about the circuit board. With two camera points, good statements about the position can be made with the corresponding algorithm, but only with four camera points can an exact statement be made about the position with shrinkage and expansion of the circuit board.
| 2 camera points | 4 camera points | 8 camera points |
|---|---|---|
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Camera lighting
The lighting plays an essential role in capturing the camera point. Depending on the camera point and background, different lighting can deliver the best results for each application. Therefore, appropriate camera tests should be carried out in advance. In practice, the ring light in particular has proven itself, as this minimizes the formation of shadows on the product and thus ensures sharp contours.
Other testing techniques
The following are a few more test techniques that are often used in the manufacturing process of electronics production:
- AXI automatic X-ray inspection
- ICT in-circuit test
- FKT function test / final test
- Boundary Scan technology