CC BY
36
УДК 681.3 Kontishev V.N.
Toronto, Canada
ANALYSISOFOPTICALIMAGINGMETHODS
Abstract. The problem of filtering and image processing systems, optical inspection of printed circuit boards are described.
Keywords: optical monitoring, Fourier transform, filter, coherent methods, printed circuit board.
Optical monitoring techniques are widely used in industry. They allow you to control the composition and properties of materials and conduct research, operational control structures and analysis process by recording the intensity, phase, spectral composition, polarization, spatial distribution of the optical radiation interacting with the test objects and emitted by them. Optical methods are based on phenomena such as reflection, absorption, interference and diffraction of light. In the manufacture of printed circuits and photo templates they used a variety of materials (metals, semiconductors, dielectrics), which interact differently with optical radiation. This interaction is determined by the properties of materials, their geometry, environmental conditions, as well as the spectral composition, polarization and phase of the radiation used [1].
Optical methods can be classified as visual-optical (microscopic), interference, spectrum, polarization, nephelometric, photometric and others, phase and polarization of the radiation used.
Optical devices includes microscopes and image analyzers. To establish control systems are widely used optical devices on a combination of the reference images and the controlled object and the allocation of optically differential image. Devices that implement the methods of this group, called the image analyzer. Image analyzer have two channels, one of which carries the sample image from the controlled and in the second - from the reference. To contrast the difference of optical image using three methods [2].
The first is the "coloring" of each of the optical channels by means of different color filters. Both colored controlled and reference images of objects optically aligned. In places where the images coincide completely (identical), seen unpainted grey drawing pictures, and where there are differences, adding violated any discrepancy appears as colored red or blue region depending on which of the image belongs to one colored a site that is not in the other. Separate fields are visible as colored spots, variations in the spatial dimensions of the image as color fringing. In the second method, one of the optical channel with a certain frequency bridged valve. Then in the difference image different parts of flash at a frequency of functioning valve[3]. The third way, which is, in fact, is a special case of the first, is the combination of positive and negative images processing and model objects, respectively.
Devices based on the visual comparison of images printed circuit boards, can detect the defects of various kinds (increase or decrease the size of the conductor areas, stains and rips tracks, etc.). However, they are not free from drawbacks. First, for the detection of defects of small size have to consider the increase. To control the whole board turns to consider some of its parts, which increases the time it takes to control.
Second, these methods require a large voltage on the operators, resulting in increased probability of skipping defects. Therefore there is a need to automate these processes.
Analysis of the literature showed that the more advanced photovoltaic methods. In this case, optical signals that carry information about the images being compared samples are converted into electrical, which are then analyzed. In a device implementing this method generates two laser beams simultaneously scanning the reference and controlled the board. Reflected light incident on the two photodiodes, electric signals which are then subtracted. If there are defects in the controlled circuit board on the oscilloscope screen is fixed pulse rejection.
A significant drawback of the described photovoltaic devices is that they do not allow the board and simultaneously to observe the detected defect to check the reliability of the testing results and the degree of unsuitability of the defective board.
Using optical and TV devices eliminates this drawback. In this case, the optical signals from the comparison as original images are converted into electrical, which are to be analyzed. Then, the resultant optical signal corresponding to the difference image is displayed on the control device and the operator can visually observe the degree of deviation.
Currently, there are several schemes for topology control with the use of optical-TV systems. Advantages of such systems lies in the fact that if the board is controlled by defects (different from the standard), they are visible on the screen. The resolution and accuracy of the test results on such a device is determined by the accuracy of combining the image signal identity videopath cameras and optical systems.
The advantage of this scheme is to use a common channel for generating video signals, which eliminates the possibility of errors due to differences in video path. The disadvantages include the possibility of a false alarm in a non-uniform illumination or lighting differences compared boards and observation of defects on a light box, making it difficult to detect.
Perhaps the most advanced existing methods of optical quality control of television devices PCB layout is an apparatus in which disadvantages of the above schemes, it eliminated by introducing a television transceiver transmitting path amplitude selector, which allows to select a signal corresponding to the difference between two images. In case of discrepancy between drawings (if any defects) there is an intermediate scale (image overlay substrate and track). Each gradation of brightness corresponds to the value of the electrical signal.
Using coherent optical means can be quite simply and effectively control the spectrum of the spatial frequencies of the object image is formed in the back focal plane of the lens if the object itself or a transparency located at the front focal plane. In the simplest case this filter reduces to control the spatial frequency spectrum. Methods of coherent optics solved the object recognizing problem.
No less promising holographic techniques that allow to document the results of monitoring, visualizing the location of the defect. Information recorded holography has a large redundancy, making it the noise immune to dust, scratches, etc.
The following discusses some of the methods of coherent optics and holography, which can be used for nondestructive testing of topology components of electronic equipment.
Fourier analysis and the theory of linear systems form the foundation on which to build the theory of image formation, optical information processing and holography.
It is based on the ability of simple optical systems to carry out transformations similar to the two-dimensional Fourier transforms. If the input optical system (in front of the focal plane) is the amplitude transmittance on the banner with the (x, y) plane illuminated by the light wave, the optical field distribution in the frequency (back focal plane) may be expressed by the equation
Uf(A q) = ——: \ \ 0(x, y) exp
2n
--— (Px+qy) X .
dxdy
(1)
x fS
wherex,y - coordinates in the image plane; p,q- coordinates in the frequency plane; A - amplitude of a plane light wave; f - the focal length of the optical system; X - the wavelength of the radiation. In equation (1) the integration within the surface S defined by the optical aperture of the system.
The resulting intensity distribution in the plane I Ap,q) = \Uf(p,q)] in the plane of spatial
frequency for the simplest case of a periodic sequence of transparent and opaque portions described by
(2n pq
I I A2 l X 2
If \A\ 2n pq
X 2
whereN - number
X - the period
(2n pb
il----—
l X 2
2ж pb
~X' ~2
elements
sin N •
2n pX X * 2
. T 2n pX
N--—
X2
(2)
(gaps);
By placing the frequency plane light-sensitive medium, you can register the Fourier spectrum of the object. The changing nature of the object will cause a change of the intensity distribution in the frequency plane.
These analysis makes it easy to understand the operation of schemes based on the spatial frequency filtering. In the back focal plane of the lens performing the Fourier transform, the filter is placed, resulting in the formation of the Fourier spectrum of the reference sample and representing the structure of opaque dots on a transparent substrate. In the front focal plane of the controlled object is established. The presence of a defect in a controlled microtiter plate causes - the appearance of additional diffraction peaks in the spatial-frequency plane. Filtering reduces the delay of the flow of light coming from repeating the desired temperature and the step interval of the object. The light beams corresponding to the defects G (p, q), are skipped filter with weakening those frequencies which correspond to the reference frequency. After filtration, the light passes through the second lens performs an inverse Fourier transform. As a result, a defect image that is always a bright field (image) irrespective of whether it is a defect in the object light or dark.
The Fourier transforms have the property that the Fourier-image is not dependent on the position of the object. This property can be used to sequence through large object.
In addition to the above-described filtering techniques on spatial frequency filter can be accommodated in the input image plane. However, in practice the spatial synthesis requires the scanning plane, and hence more complicated.
Attempts to use a spatial filtering method for detecting defects in integrated circuits have made it possible to create an experimental setup in the frequency domain filtering, which allows to detect defects as small as 5 microns using a filter simplified cross shape. The apparatus is also used as a filter in the reflective plate with the holes in the field maxima of the diffraction pattern. This form of filters allows not only to identify the defects, but also play a defect-free image of the object, focusing lens, the light transmitted through the filter hole.
Methods based on spatial filtering, have a number of significant shortcomings. Firstly, the defects of which sizes are closed to the sizes of the individual elements of a reference sample are not distinguishable in the frequency plane, are blocked by the filter and cannot be visualized. Secondly, it requires very high accuracy in the orientation of the filter object relative to the axis of an optical system and relative to the coordinate axes perpendicular to it. Finally it should be noted that the discussion relates to the amplitude images compared objects. In a real situation, at least one of them (continuous sample) is the phase.
Indeed, the surface of a microplate is a real structure, sputtered or etched on the substrate. This leads to a complicated structure of the reflected or transmitted wavefront containing information about the image. Having the phase information will cause additional structure in the frequency plane, and that structure is unique in the spatial filter. As a result, the image plane image after filtering any "false defects."
Thus, the scope of application of the method of spatial filtering is limited ability to control the amplitude of objects and simple periodic structure.
Interference image comparison method is based on the possibility of interference between the two images when coherent illumination. The differences in the images appear to change the visibility of the interference pattern of bands. The principal advantage of this method over the above is that the interference pattern is observed visually, and the system bands help automate the process of finding the defect.
Pretty simple method is implemented in the scheme of the Mach-Zehnder interferometer. Compared objects set each arm of the interferometer and, by varying the phase in one arm, shift
2 i-
2 i-
2
the interference pattern as a whole, and the change in the frequency bands and their direction is achieved by tilting svetootdelitel at the interferometer output.
Comfort of the method is that the interference pattern can be designed with a corresponding increase in the photocathode optoelectronic device. At the coordination of frequency bands of the interference pattern with a resolution of the optical system and the matched filter on the TV can only image the differences between the compared objects.
The literature describes various schemes interference subtraction images using a combination of images as elements of the objects being compared diffraction grating beamsplitters or polarizing prism.
The general lack of interference methods is the inability to control their phase objects, as these greatly complicate the emerging interference pattern.
Holographic methods for quality control of printed circuit boards on the principle of action is a multistep process, initially recorded hologram of the object, then restores his image, and quantitative information is obtained by processing the image. Method for recording and restoring the image of the object based on the interference of two waves, the wave reflected or transmitted through the product, and coherent with the reference wave with a known distribution of phases. The resulting interference pattern is recorded on a photographic plate (or other recording medium). Manifested plate with a recorded interference pattern called a hologram. To restore the studied three-dimensional image on the hologram should send a wave coincides with a reference wave to record. The reconstructed image, which is an exact copy of the product has all the properties of the image, which are inherent in the original. The described methods of recording and reconstruction of the wavefront can operate direct noncontact measurements of spatial and spectral intensity of the pixels on the surface of an object of any shape, its geometrical dimensions, to study the shape of objects and their surface topography.
The disadvantage of this method is the presence of the chemical treatment that significantly retards the operation control and makes it low-tech.
REFERENCES
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2. Zuwairie Ibrahim. Wavelet-Based Printed Circuit Board Inspection System / Zuwairie Ibrahim, Syed Abd, Rahman Al-Attas // International Journal of Information and Communication Engineering. - 2005. - № 1:2. - P.73-79.
3. Информационные технологии проектирования РЭС. Единое информационное пространство предприятия : учеб. пособие / В. Б. Алмаметов, В. Я. Баннов, И. И. Кочегаров. - Пенза : Изд-во
ПГУ, 2013. - 108 с.
