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SMT & Inspection | December 15, 2008

Sharp Eyed AOI

Because of the continual miniaturisation of electronic components there are challenging requirements in the production process as well as quality assurance. Since electric test technologies are harder and harder to utilise because of the increasing packaging density, Automated Optical Inspection becomes more important as qualitative test method.
But due to the structures getting smaller and smaller, the highest requirements are required for the image capturing and processing within such a system. In the following, the necessary preconditions for the recognition of smallest characteristics, e.g. for the inspection of 01005 components or solder joints with 0.3 pitch, are described.

Size comparisons of 1206 to 01005 components


Basics of Detail Recognition
The application of CCD-matrix cameras within an AOI system has established as quasi-standard. Compared to scanner solutions, AOI systems with area scan cameras are more flexible in terms of image capture with various illumination variants, and offer a lower optical distortion at a higher resolution.

AOI systems’ recognition output is often evaluated in pixel resolution with the unit “µm/Pixel” – regardless how the image was captured. But this size is rather a theoretical value, for which the impact of critical lens parameters such as diffraction or aberration it is not considered. A considerable proportion for detail recognition lays in the design of the optical system as well as its ideal combination to the applied CCD-matrix. The basics are explained in the following:

One of the most important parameters for the optical resolution ability of lenses is their numeric aperture. It can be understood as a scale for the light collection ability of an optical system. Typically, it is determined by the lens aperture. Because of the physical optics theory they are responsible for occurring diffractions. One of the possible results, e.g. is the projection of an object spot as more or less highly “blurred disc”. The mathematical function of such a light spot before entering and after leaving the lens is shown in image 2.

Ideal light spot and its mathematical function before and after transmission through a lens


If there’s a CCD-matrix behind the lens, an additional light spot enlargement based on the pixel size is the result.

“Blurred” light spot on the CCD-Matrix


When looking at the entire image capturing chain one finds that in the captured camera image an area of 3 X 3 pixels has emerged from the original light spot with minimal diameter. Now, at the latest, it becomes obvious that in such a case an increase in pixel resolution (µm/pixel), e.g. by a higher pixel number of the CCD-camera, is of no benefit in regard to improving the detail recognition. In contrast, it is obvious that the resolution ability of the applied image sensor has not fully been used.

An analogue situation occurs in projecting a light/dark transition (optical edge). After leaving the lens, such an edge is also “blurred” by a higher number of pixels. This situation is shown in image 4.

Ideal light/dark transition before and after transmission by the lens


It comes out that a detail resolution is mainly determined by the quality of the lens used. An increase in pixel numbers would not result in improvement. On the contrary, it would lead to an increase in data amount with its well-known consequences. Knowing these physical basics and considering commercial photography, it emerges that even on this field the quality of the applied lens is actually responsible for high-quality images rather than the number of pixels.

Increased Resolution by pixel adapted Lens Design
As already mentioned in the chapter before, the optimisation of the applied optics is the only useful way to significantly increase the image capturing resolution. Considering the CCD-matrix to be utilised, the lens must be designed in a way that the optics’ blur is smaller than the pixel size of the applied CCD-matrix. An effective resolution increase in terms of detail recognition is possible by such a pixel adapted lens at constant pixel number of the camera. This situation is displayed in image 5.

Ideal light spot after transmission by a pixel adapted lens


The utilisation of a lens built on this basis, does naturally not only result in improvements in spot-wise projection but also for image capturing of optical edges (image 6).

Ideal light/dark transition on a CCD-matrix after transmission through a pixel-adapted lens


Utilisation of pixel-adapted Lenses in the OptiCon Systems’ Camera Modules
01005 ICs are at the moment the smallest applied components that have a dimension of 0.4mm x 0.2mm. Typically, the solder joints are approx. 0.15mm x 0.08mm. At a pixel resolution of 21µm/pixel, this means that the solder meniscus is displayed at ca. 28 pixel (approx. 4 X 4 pixel) - the entire component only covers 180 pixel, which is a sufficient number for a safe component detection and solder joint inspection.

In reality, these pixels are not available in the required quality for sufficient feature recognition – limited by the applied optics’ resolution ability. The usage of a lens, which wasn’t ideally designed to the camera’s pixel geometry, leads to a “blur” of the relevant characteristic areas. Just increasing pixel resolution (e.g. to 10µm/pixel) doesn’t give an improvement of the recognition results. In contrast, a lens design to the camera’s pixel geometry allows an inspection without increasing the pixel number or pixel resolution.

Based on this physical regularity, the image capturing concept of the OptiCon AOI system family has been consistently developed further. By utilising a pixel-adapted lens, detail recognition is possible, that guarantees a safe inspection of 01005 components solder joints on 0.3 pitch. As a matter of course, the well-proven telecentric view for image capturing without any parallax errors is kept.

For an improvement in statistical safety during processing by the respective recognition algorithms, the resolution was increased to 10.5µm/pixel, based on a feature optimised image transformation. Additionally, this higher pixel number enables a better visualisation; and it is operation beneficial, e.g. for manual adjustment of test areas.

01005 resistors captured with the camera module of the OptiCon AOI systems, resolution 10.5µm/pixel


Conclusion
For the inspection of the smallest components and solder joints, the exclusive increase in pixel resolution (e.g. by CCD-cameras with higher pixel number) does not give the suggested increase in respect of the required detail recognition. The utilised lens is the typically limiting component in such an optical system. In order to achieve the maximum detail capturing opportunity, it has to be designed to the pixel size of the applied camera in terms of its optical resolution ability. Subsequent transformation processes increase the statistic safety of the used algorithms, and contribute to an ideal visual representation and operability.

The text was contributed to evertiq's website by Jens Kokott, André Hacke (GOEPEL electronic GmbH)

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