GB2221297A - Vision system for inspecting objects - Google Patents

Abstract

A vision system comprises linear array detectors, such as CCD array detectors 12, arranged in a pattern corresponding to areas of the object 10 to be viewed. When the object is a rectangular TAB package the legs of which are to be viewed to ensure accurate placement of the package, four linear CCD arrays are arranged in a rectangle to match the four sides of the package. The package may be illuminated from a laser 14, and imaging optics 16 are interposed between the chip and the detector to alter magnification eg by means of a zoom lens. <IMAGE>

Description

Title: Vision System Field of the Invention This invention relates to a vision system comprising a detector for viewing an object to be inspected.

Background to the Invention Vision systems for inspection and feedback control have been known for many years and used in numerous applications. For inspection tasks, such systems are commonly designed around array detectors such as vidicons and area CCD's (charge coupled devices). Access times of these systems are, however, generally tied to TV frame rates which may not be adequate for fast real time control. Furthermore, CCD area arrays are currently available in sizes of up to 512 x 512 elements, setting an upper limit to the resolution which may be achieved using such a device. Should large areas, or higher resolution be required, conventional solutions currently involve imaging a fraction of the object and building up a full pattern by translating the object in X or X and Y directions.This places high tolerance requirements on the linearity and accuracy of the translational device and introduces additional complexity in the matching of data in successive 'quadrants' of the area viewed. In addition, for certain applications, where imaging of a reduced portion of the object is required, a vast amount of redundant information has to be processed by such area devices.

Summary of the Invention According to the invention a vision system comprises a linear array detector having a configuration corresponding to areas of an object to be viewed, and imaging optics interposed between the detector and the object to image said areas of the object onto the detector.

The invention overcomes some of the prior art limitations, notably processing speed, resolution and superfluous information processing, by utilising linear arrays orientated in a configuration similar to the 'target' object. This mode of operation has in-built advantages in all the aforementioned respects, namely; - increased resolution: for example linear CCD's are readily available with 4096 elements - reduced data redundancy: with a match between the detector configuration and object geometry, only useful information will be collected - increased processing speed; arising from processing only valuable information - stationary 'snap shot' of the object allowing the useful information to be gathered by imaging the relevant object areas without requiring to translate the object According to other preferred features of this invention, facilities including autofocus and zoom may be incorporated in the imaging optics. Such a system would provide the flexibility to accommodate different object sizes and aspect ratios, and to provide X, Y and theta information.

According to another preferred feature of the invention, the system may be used with an independent illumination system. This may project light within a limited part of the spectrum while the detector is fitted with a filter to pass only light of the desired part of the spectrum.

Such a system would address all areas of inspection and alignment and may be coupled to a feedback mechanism for automatic control of any device. Although the system may be used for virtually any task, it is perceived that it will be of most benefit in applications involving accurate inspection and alignment of components having a large number of small area locations. In the electronics industry, for example, this may be in the placement and attachment of TAB components, quad packs, or bonding LCD screens. Inspection of fine toothed gear wheels may also be particularly suited.

The invention will now be described with reference to the accompanying drawing, in relation to a particular example: inspection, positioning and laser soldering of TAB components, although it should be stressed that this is only one of a number of possible applications. The drawing shows the preferred vision system in schematic form. The object to be viewed is a TAB package 10, the detector (formed by four linear CCD arrays) is shown at 12, an illumination system is illustrated at 14 and imaging optics at 16.

Illumination System 14 If required, then the illumination system should provide a controlled lighting environment, independent of ambient lighting conditions, to facilitate imaging analysis. In order to render the system insensitive to changes in ambient light it is necessary that the system be capable of discriminating between illumination and the ambient light. This is achieved by illuminating the package 10 using a specific spectral band and detecting in the detector 12 only within that spectral band. Ideally the spectral band should be one that is not present in the ambient light; if this is not possible, then it should be arranged that the power per unit spectral bandwidth of the illumination is much greater than that of the ambient light.

This latter approach is preferred since some form of wavelength-specific filtration (such as the optical filter 18 illustrated) may be required to protect the detector 12 from any high-intensity laser light should soldering and cropping operations be carried out on the package 10 using such a technique. In addition, the soldering laser beam must be coincident with a tracer beam at a different wavelength. The tracer beam may be derived from a laser diode, it is therefore convenient to choose that wavelength. The illumination system 14 may then consist of an array of high power infra-red LEDs whose intensity per unit bandwidth is much greater than that of the ambient light.

Detector 12 Consider for example a TAB package or chip, being square or rectangular and having, say, 200 legs per side at a lead pitch of 3 thousands of an inch. An area array detector (512 x 512) imaged over the whole chip would only allow a single resolution element to address one lead.

For a higher number of leads, this limit would be exceeded. In addition, because of the shape of the package, an area array detector collects a considerable amount of useless information. In this respect a detector format of 4 linear arrays, arranged in the configuration shown in the drawing is more appropriate, where each array is imaged on one of the four sides of the chip.

The important information regarding chip position in X, Y and theta can be obtained from the four linear arrays oriented along images of the four sides of the chip and in this case the amount of data handled is directly proportional to the number of leads on the chips. X and Y position information can be obtained from referencing a given lead to a given pixel, eg in the X direction, lead number 1 is referenced against pixel number 20. Theta information can be extracted from the pixel numbers of opposing leads. Linear CCDs are readily available with 4096 elements, which is useful in both increasing the resolution and allowing autofocus control of the imaging optics. The information is read out of the linear CCDs serially, the state of each pixel in the CCD's being representative of the position of a corresponding lead on the chip.

Since a useful feature of the imaging system is that it is capable of autofocus, then oversampling (ie viewing a single feature with more than one pixel element in the CCD) is particularly desirable. Focussing may then be achieved by maximising the image contrast between say lead and space, requiring that the detector resolution is in excess of the lead spacing.

Since TAB packages cover a range of sizes and aspect ratios, four fixed CCD arrays with a constant magnification optical system is a relatively inflexible configuration and cannot address different chip geometries. In another aspect of the invention it is possible to accommodate different chip sizes by changing the magnification of the optical system, this is achieved by providing the imaging optics 16 with a "zoom lens" type arrangement. In order to cope with different aspect ratios it is necessary to perform two imaging operations, the first to gain X and theta information, and the second at a different magnification, to gain Y information.

This system can then cope with differing sizes and aspect ratios.

For TAB package placement, the system is required to image both the TAB package and the printed circuit board on which it is to be placed. The necessarily close correlation between the TAB device and the board contacts ensures that the vision system is matched to both objects.

In order to view both the PCB and TAB package, precise vertical movement of the entire system would therefore be incorporated to overcome the limited depth of focus of the optical system.

The overall vision system would therefore be capable of autofocus and changing magnification. In this example it would be capable of aligning the component on the board and checking for the presence and straightness of the leads on the TAB package. The system would also be capable of providing an active sensing mechanism for laser soldering TAB devices. In this configuration, the viewing system would read an alignment laser beam colinear with the soldering laser and activate the soldering laser when its output was coincident with the desired lead. In this example optical filtering of the high power soldering laser may be required to prevent damage or 'flooding' of the detector.

According to a further optional feature of the invention, an alternative CCD array could be used to monitor the thermal profile of the bonded leads.

Imaging Optics The optical system which transfers the chip image to the detector plane must satisfy the following requirements: - A magnification which may vary to suit different chip geometries. This is achieved by a zoom lens.

- A throw (defined as the sum of the object and image distances and the length of the optics) which can be accepted in the production environment. Although it may be easier to design an optical system with a long throw as compared with a short throw, it may not be possible or desirable to physically accommodate this in the workspace of the overall system. This requirement, together with that of the size of the field of view, dictates the field angle of the lens system.

- A resolution sufficient to allow vision correction capability to 0.3 thou (8 microns) over the field of view. For typical wavelengths this damands a numerical aperture of at least O .1.

- Low distortion to allow each lead to be imaged onto the CCD elements. If the distortion were large then it is conceivable that, with a cropped, formed lead, the images of the leads near the corners of the package may be distorted such that they lie outside the field of view of the CCD elements.

- A depth of focus sufficient to accommodate the lead thickness.

- A field of view which is flat, to within the depth of focus tolerance, over the area of the chip.

In addition to these requirements it would be preferable if the lateral magnification of the system were insensitive to small changes in focus, this dictates that a telecentric optical system must be used. In these systems, the principal ray through the system is parallel to the optical axis and the magnification is invariant with respect to focus position; this dictates that the lens system must have a physical aperture greater than the size of the CCDs. A conventional optical system could be designed to be consistent with these requirements.

Vision Control System The vision system should be a static system, it must be capable of refocussing and changing magnification. These changes must be effected without altering the position of the centreline of the optics with respect to the chip or board, ie the mechanics of the system must be independent of those used to position the system. It is envisaged that, within the sub-assembly, there are two vertical position translators 20; these will permit refocussing and changes in magnification.

Advantages The above example illustrates the advantages offered by this invention, namely: increased resolution increased speed reduced redundancy high flexibility in terms of: geometry magnification autofocus

Claims (9)

1. Claims 1. A vision system comprising a linear array detector having a configuration corresponding to areas of an object to be viewed, and imaging optics interposed between the detector and the object to image said areas of the object onto the detector.
2. 2. A vision system according to claim 1, wherein the linear array detector is a linear CCD array.
3. 3. A vision system according to claims 1 and 2, wherein the array comprises four linear CCDs respectively aligned alone the sides of a rectangle, said areas being four marginal edges of the object.
4. 4. A vision system according to any of the preceding claims and including illuminating means projecting onto the object light within a predetermined part of the spectrum, the detector being sensitive to light within said predetermined part of the spectrum.
5. 5. A vision system according to any of the preceding claims, wherein an optical filter is interposed between the detector and the object to protect the detector.
6. 6. A vision system according to any of the preceding claims, wherein the magnification afforded by the imaging optics is variable to accommodate objects of differing sizes.
7. 7. A vision system according to claim 6, wherein the imaging optics include a zoom lens to alter the magnification.
8. 8. A vision system according to any of the preceding claims, wherein the imaging optics are adjustable in position in the direction spanning the detector and object, to permit refocusing and changes in magnification.
9. 9. A vision system constructed and arranged substantially as herein particularly described with reference to the accompanying drawings.