IES20000277A2 - Machine vision - Google Patents

Abstract

An annular support (1) has rings of LEDs (3,4,5,6) in order from the top down. One of the rings (5) has R, G, and B LEDs arranged in a pleated configuration. Each colour is sequentially activated so that a sequence of monochrome images at the same angle is obtained by a monochrome camera. These images are then superimposed by an image processor to provide the equivalent image to a direct colour image, but at the higher resolution of the monochrome camera. <Figure 1>

Description

The invention relates to illumination of targets and capture of images for inspection of the targets. The targets may, for example, be circuit boards in a surface mount production line, either pre-reflow or post-reflow. £ *3# Ir ί I? Irs ov , >.5jjS£t9!SSBiS£$ pw«) .Ϊί.-.ΐ j» It is well known that colour images provide more detailed information than monochrome images. For example, colour images are generally required for prereflow component inspection. 4° S 5 Unites States Patent Specification No. US 5245671 (Omron) describes one .4 '*‘i i technique. This involves use of three rings of light emitters. Each ring emits one of red, green, and blue colours. The light emitters are mounted so that the light is emitted at different angles so that they converge and combine to provide white light.

It appears that this technique is effective at generating good quality colour images for analysis. However, it suffers from the need to use a colour camera. This imposes restrictions on the available resolution and it is also quite expensive. Japanese Patent -- Specification No. JP07012748 (Omron) describes a system having multiple cameras and optical filtering to separate images. This arrangement appears to be complex because of the need for multiple cameras and for optical filtering.

Therefore, the invention is directed towards providing an illumination unit and a machine vision inspection head incorporating the unit to address these problems.

It is another object of the invention to provide for improved image quality whereby more information may be gleaned than has heretofore been the case.

According to the invention, there is provided an illumination unit for a machine vision system, the illumination unit comprising: IE000277 -2means for illuminating a target with monochrome illumination for sufficient time for a camera to capture a monochrome image, and means for subsequently illuminating the target with a different monochrome illumination for sufficient time for a camera to capture a different monochrome image of the same target whereby the sequentially-captured monochrome images may be superimposed by an image processor to provide a colour or near-colour image.

In one embodiment, the unit further comprises means for sequentially illuminating a target with a third monochrome illumination.

In one embodiment, the monochrome illumination wavelengths are red, green, and blue.

In one embodiment, each monochrome illumination means comprises a plurality of light emitting diodes of the same wavelength.

In one embodiment, the diodes are arranged in a ring.

In one embodiment, the diodes of different monochrome illumination means are distributed to provide a similar illumination intensity and distribution for each image.

In another embodiment, the unit comprises a plurality of rings, each ring comprising diodes of all illumination means arranged in a uniform pattern.

In one embodiment, the overall pattern is a pleated pattern when viewed from the target.

IE000277 -3In one embodiment, the unit further comprises an additional illumination means.

In one embodiment, the additional illumination means comprises a ring of light emitting diodes mounted to direct light at an angle onto a target which is different from that of the monochrome illumination means.

In a further embodiment, the unit comprises an annular support supporting the monochrome illumination means, and supporting a plurality of additional illumination means for illumination at different angle ranges.

In one embodiment, each additional illumination means emits monochrome illumination.

According to another aspect, the invention provides a machine vision head 15 comprising an illumination unit as defined above, a monochrome camera, and a camera controller comprising means for controlling sequential capture of the successive monochrome images.

According to another aspect, the invention provides a machine vision system 20 comprising a machine vision head as defined above and an image processor comprising means for superimposing die monochrome images.

According to a still further aspect, the invention provides a method of inspecting a target, the method comprising the steps of: (a) illuminating the target with monochrome illumination; (b) capturing an image when the target is illuminated with the illumination of step (a); IE000277 -4(c) repeating steps (a) and (b) for each of at least one different monochrome illumination until a plurality of images are captured; (d) combining the images to provide a combined image; and (e) processing the combined image to determine information about the target.

In one embodiment, the target includes wet solder paste and a monochrome illumination is of ultraviolet wavelength.

In another embodiment, the target includes copper leads or tracks and a monochrome illumination is of near infra red wavelength.

In one embodiment, the target includes component leads and solder paste, and the monochrome illuminations include green and blue illuminations.

In the latter embodiment, the method may comprise the further steps of illuminating the target with red wavelength illumination at a different angle and processing the images by measuring the overlap between leads, paste, and pads.

The invention will be more clearly understood from the following description of some embodiments thereof given by way of example only with reference to the accompanying drawings in which: Fig. 1 is a diagrammatic cross-sectional elevational view of an illumination unit of the invention; and Fig. 2 is an underneath plan view of the unit showing two sections of LEDs.

IE000277 -5Referring to the drawings, an illumination unit 1 of the invention is shown. The unit 1 comprises a support which is generally annular in shape extending in the vertical direction to provide different angles of illumination onto a target. The support 2 comprises, in order from the top down, a first section 3, a second section 4, a third section 5, and a fourth section 6. Each section comprises a wall extending at a different angle to the vertical so that light emitting diodes (LEDs) mounted on the wall emit at a particular range of angles associated with the section.

In this embodiment, the first section 3 comprises UV LEDs 10 which emit at an angle range approximating to 75°, the second section 4 comprises I.R. LEDs 11 which emit at an angle range approximating to 60°, the third section 5 comprises LEDs which emit at an angle range approximating to 45°, and the fourth section 6 comprises red LEDs 15 which emit at an angle range approximating to 30°.

The LEDs in the different sections are driven so that a target is illuminated with light at different angles so that different inspection information is obtained. The light wavelength is not important and the camera used is a monochrome camera. Such cameras provide excellent resolution for relatively little expense.

The unit 1 also provides for generation of colour images where this is important for inspection analysis. The third section 5 supports three rings of LEDs in close vertical proximity. Each ring comprises blue LEDs 12, red LEDs 13, and green LEDs 14 arranged so that they are interspersed in a uniform pattern around the ring. The pattern is very simple and may be expressed as follows: block position 1: red, block position 2: green, block position 3: blue, block position 4: red, -6IE000277 There may be any suitable number of LEDs in each block, typically ranging from one to ten. This pattern is present in each ring, and it is implemented in each ring so that the colours are offset in adjoining rings in an overall pleated pattern for illumination uniformity.

The unit 1 also comprises a drive means, not shown, which activates all blue LEDs 12 in the section 5 for a time period sufficient for the monochrome camera to capture a blue monochrome image. It then activates all of the red LEDs 13 in the section 5 for sufficient time for the camera to capture a red monochrome image while the target is at the same position. Finally, the drive activates all of the green LEDs 14 in the section 5 for sufficient time for the camera to capture a green monochrome image with the target at the same position.

Thus, a single high-speed, high-resolution camera obtains an image for each colour of light by time sequencing the lighting ON with the exposure of the camera for each wavelength of light for which an image is required. Also, further information may be obtained by using a near-infrared lighting ring or any wavelength ring within the sensor response wavelength range. UV LED lighting may be used to stimulate fluorescence or emission at a longer wavelength light that is in the detection band of the sensor. This may further improve classification of scenes.

The camera captures three monochrome images of the same target. These are then superimposed by an image processor to provide a colour image. Because the LEDs 12, 13, and 14 are distributed uniformly around the section 5, the illumination intensity and distribution is the same for each monochrome image. Therefore, the colour image generated by the image processor is of excellent quality and at least matches that of a colour camera of the same resolution captured with white light illumination. Indeed, because monochrome cameras are generally available in higher resolution than colour cameras the image quality would be typically better.

IE000277 -7Indeed, it has been found that by using a multiple monochrome images more information may be obtained than with colour illumination. There are various advantages to use of monochrome illumination and the invention achieves these benefits and the benefits of colour images to obtain the “best of both worlds”. In more detail, the resolution, sensor size and speed on monochrome cameras is better. Using a monochrome camera and lighting with different wavelength sources allows sharper and larger images to be obtained as up to 25mm sensors are available, the pixels are smaller, there is no bleeding of colour, the sensors are higher quality, and there is no issue with registration of images as all images are taken with the camera/sensor in the same position relative to the object field. This is not true of Colour CCD cameras or multiple monochrome cameras.

Also, an RGB camera is designed to work with white light and to output colour information on the scene. Accordingly, the capability to detect some features is limited as the optimum contrast between these features may occur at a wavelength (or multiple wavelengths) outside the area allowable by the white light/colour sensor combination. The presence of light at a wavelength other than the optimum wavelength for viewing the feature of interest prevents useful information on the scene from being extracted.

Where optical filtering is used to select the image there is a limit on the number of wavelengths that can be used and the registration between cameras is a problem. On monochrome cameras made from Silicon the CCD sensor has sensitivity for imaging in the 400nm to lOOOnm wavelength range. Any LED wavelength falling within this range may be used with a monochrome camera and sequential lighting. Using LED switching in the time domain gives greater versatility as some circuit boards may have components that are particularly reflective/transmissive at one wavelength. Using LEDs at this wavelength of interest allows the contrast of the combined images to be maximised by using the spectral reflective and transmissive properties of the region under inspection to be maximised.

IE000277 -8Example 1: Wet paste inspection Using ultraviolet LEDs (e.g. Roithner lasertechnik™) may be used to stimulate fluorescence on particular features so that they may also be viewed without distortion from other reflective components. For example, wet solder paste fluoresces visible light when Ultraviolet is used as illumination. In this case the scene appears as highly bright objects (fluorescent paste) on a black background (UV absorbing solder mask etc). Although metal objects (copper) may reflect the UV this is outside the wavelength selection range of the sensor and so these objects also appear black.

Example 2: Copper inspection Near Infra red light penetrates the solder mask layer of circuit boards. To inspect copper tracks below the solder mask layer it is possible to switch to an Infrared LED. Infrared may then be used to confirm no defects in copper under solder mask. Switching to the Red LED lighting in the next sequence allows the metal outside the solder mask to be viewed and to confirm registration to the area of bare copper visible under RED (or other) lighting. Further inspection of paste on the pads is then possible using the UV leds as described in Example 1.

In effect, the images under each illumination condition can be used to determine spectral information on the item to be inspected. This may then be used to program the lighting sequence to highlight the contrasts of interest in the identification of particular defects. There is no advantage to using a colour camera to achieve this. Monochrome cameras offer significant advantages to colour cameras for this process.

Example 3: Lead to paste direct measurement.

Using red LED lighting at angles and using green and blue lighting at one angle it is possible to segment the lead and the paste directly. The overlap between the lead, paste and pad is measured much more accurately if the part will reflow properly.

IE000277 -9The number of correct but unnecessary false calls when a part is offset is reduced. This may be possible using a colour CCD also but would require the time sequencing of the lighting to get the contrast required. As a monochrome camera has a bigger sensor, better resolution and a higher number of frames per second, it offers significant advantages over a colour camera.

Example 4: Paste under lead measurement.

Using colour, the system detects the presence of paste around leads on pads. If the paste is not present over several adjacent deposits, an error is flagged. The capability to inspect paste when the lead is on top of it is possible because the reflective properties of the lead and paste differ due to selection of lighting conditions that allow specific detection and segmentation of each material. Also, for inspection of a SMT component on a circuit board it is possible to inspect several deposits near each other, only calling an error if a certain number of them are bad. Finally, solder bridges between leads may be found using the same lighting configuration.

Example 5: Loaded board paste and component inspection.

Using the monochrome camera and multiwavelength /angled lighting described it is possible to achieve simultaneous 100% component AND 100% or nearly 100% paste inspection.

This may be used to report paste position as well as presence / absence, to report offsets to the screen printer and to monitor the forward and backward strokes of the printer.

Finally, unpopulated deposits may be used to perform 3D paste measurement if required.

IE000277 -10Colour cameras alone would not provide the capability to perform the inspections outlined above. To achieve this it is necessary to time sequence the lighting at each wavelength of interest. In this regard a colour camera offers no advantage to using a monochrome camera as each image is effectively monochrome and the monochrome images are identified by the lighting source which formed them and the features they contain relative to other monochrome images taken with other lighting on the same camera.

The invention is not limited to the embodiments described, but may be varied in construction and detail. For example, it is not essential that the unit have additional illumination means to provide illumination at different angles. However, the arrangement described is particularly versatile as it may be used very effectively for both pre-reflow and post-reflow inspection. Also, the LEDs in the section 5 may be distributed in a different pattern which achieves sufficiently uniform illumination for all of the sequential images. It is also envisaged that the unit may include only the light emitters to provide a colour image. The benefits from capturing successive monochrome images may be obtained by using more than one camera. If multiple cameras are used, they may be mounted at different angles with respect to the target.

IE000277

Claims (5)

1. An illumination unit for a machine vision system, the illumination unit comprising: means for illuminating a target with monochrome illumination for sufficient time for a camera to capture a monochrome image, and means for subsequently illuminating the target with a different monochrome illumination for sufficient time for a camera to capture a different monochrome image of the same target whereby the sequentially-captured monochrome images may be superimposed by an image processor to provide a colour or near-colour image.

2. An illumination unit as claimed in claim 1, wherein the unit further comprises means for sequentially illuminating a target with a third monochrome illumination, and wherein the monochrome illumination wavelengths are red, green, and blue, and wherein each monochrome illumination means comprises a plurality of light emitting diodes of the same wavelength, and wherein the diodes are arranged in a ring.

3. An illumination unit substantially as described with reference to the drawings.

4. A machine vision head comprising an illumination unit as claimed in any preceding claim, a monochrome camera, and a camera controller comprising means for controlling sequential capture of the successive monochrome images.

5. A machine vision system comprising a machine vision head as claimed in claim 4 and an image processor comprising means for superimposing the monochrome images to provide a colour image. IE000277 «r ΤΟ tn U.V. LEDs 10 a: tn Q 111 rO) Ll co co Q LLJ CN IE000277 CM CT> Ll ? !