Patent Application
20140198185
A portion of the disclosure of this patent document contains material which is subject to copyright protection. The copyright owner has no objection to the facsimile reproduction by anyone of the patent document or the patent disclosure, as it appears in the Patent and Trademark Office patent file or records, but otherwise reserves all copyright rights whatsoever.
Circuit boards that carry electronic integrated circuits and discrete electronic components are well known. A circuit board substrate is prepared with predetermined conductor paths and pads for receiving the leads of electronic components such as integrated circuit chips, resistors or capacitors. During the circuit board assembly process, solder paste deposits are placed onto the board substrate at appropriate positions. The solder paste deposits are usually applied by placing a stencil screen onto the substrate, applying solder paste through the stencil openings and removing the stencil from the substrate. The circuit board electronic components are then positioned onto the substrate, preferably with a pick and place machine, with leads of the electronic components placed on the respective solder paste deposits. The circuit board is passed through an oven after all of the components are positioned on the substrate to melt the solder paste deposits thus creating an electrical as well as mechanical connection between the components and the substrate.
The size of the solder paste deposits and electronic components and the accuracy with which they must be placed on the substrate has become increasingly smaller and tighter with the increased emphasis on miniaturization in the electronics industry. Solder paste deposit heights can be as small as 50 microns and the height of the solder paste brick must often be measured to within 1 percent of the designed height and size. The center-to-center spacing between solder bricks is sometimes as little as 200 microns. Too little solder paste can result in no electrical connection between the lead of an electronic component and the pad of the circuit board substrate. Too much paste can result in bridging and short-circuiting between the leads of a component. Discrete electronic components such as resistors and capacitors can be as small as 200×400 microns and leads on micro ball grid array components can have a center-to-center spacing less than 300 microns.
A single circuit board can cost thousands and even tens of thousands of dollars to manufacture. Testing of a circuit board after the fabrication process is complete can detect errors in solder paste placement and component placement and lead connection, but often the only remedy for a faulty board is rejection of the entire board. In addition, with the miniaturization of components, visual inspection of the circuit board, even with optical magnification, is unreliable. It is accordingly imperative that a circuit board be inspected during the fabrication process so that improper solder paste deposits can be detected prior to the placement of the electronic components onto the substrate. Such in-process solder inspection reduces the cost of failure since expensive components have not yet been placed onto the circuit board.
After placement, it is also important to inspect the components to ensure proper placement of the components. Improperly placed components, missing components or poor solder joints are typical defects introduced during the placement of the components and reflow of the solder paste. After reflow, proper placement of the components and the quality of the reflowed solder junctions can be inspected using an automated optical inspection system to ensure that all components are properly soldered and connected to the circuit board. Current optical inspection systems use 2D video images of the circuit board to detect defects. However, optical inspection systems that detect 3D height images of the circuit board make possible or otherwise improve the detection of placement defects such as lifted leads, package coplanarity, and component tombstones and billboards.
The use of white light phased profilometry is a well-known technique for optically acquiring topological surface height images of circuit boards. However, current circuit board inspection sensors that employ phased profilometry have some limitations. Typical phase profilometers used to acquire topological surface height images of circuit boards generally use triangulation principles combined with structured light to determine the height of the surface at every pixel defined by the sensor's camera. One limitation of using triangulation sensing to produce a height image of a circuit board is that the incident angle of the pattern projection optical axis and image sensing optical axis are different. If the circuit board has height features that have an edge slope large enough that they occlude either the pattern projection optical axis or image sensing optical axis relative to some area on the surface, the sensor will not be able to measure those areas of the circuit board.
Referring to the diagram of the height image sensor in
In the sensor shown in
Providing a multiple viewpoint triangulation sensor for generating height images of a circuit board using phased structured light that does not have the associated cost or speed penalty that is present in the current state of the art for multiple source phase height image sensors would represent a useful advance to high-speed three-dimensional inspection of circuit boards.
Additionally, coupled with the multiple viewpoint triangulation sensor, providing a way to change the frequency, orientation and type of the structured light pattern in real time without physically moving the reticle would allow the sensor to change characteristics without modifying the sensor hardware and would increase the reliability of the sensor.
A system for sensing a three-dimensional topology of a circuit board is provided. An illumination source projects an illumination pattern from a first angle of incidence. A first camera acquires an image of the structured light pattern on the circuit board from a second angle of incidence. A second camera simultaneously acquires an image of the structured light pattern on the circuit board from a third angle of incidence, the third angle of incidence differing from the second angle of incidence. A controller is coupled to the illumination source and to the at least two camera devices. The controller generates a height topology of the circuit board based on images acquired from the at least two camera devices of the structure light illuminator.
In the system described with respect to
In operation, structured light source 12a projects a sinusoidal structured light pattern 30 onto the circuit board 18 and an image is acquired by camera 16. Reticle 20 is then shifted an equivalent distance of a fractional phase distance of the sinusoidal pattern by a linear actuator 28 and camera 16 acquires a second image. A similar sequence of image acquisitions and reticle shifts then occurs to collect images generated from structured light source 12b by camera 16. In all, the number of images required to generate a height image by multiple projection source height image sensor 10 is n×m where n is the number of structured light sources and m is the required number of phase images. Since the number of phase images required for a reliable height image is typically three or four, the number of the images captured by the camera 16 per generated height image is six to eight. To improve measurement performance, it is also typical to increase the number of pattern projection sources 12 to four which increases the required number of images to twelve to sixteen images acquired per height image by camera 16. Because the multiple projection source height image sensor 10 shown in
The method of converting the intensity information from the multiple sinusoidal intensity pattern images to actual height images can be in accordance with any known techniques, such as those described in U.S. Pat. No. 6,750,899.
In operation, light source 24 backlights reticle 20. Imaging lens 22 projects the reticle onto circuit board 18. Simultaneously, cameras 52a, 52b acquire images of the circuit board 18 during the illumination period. Reticle 20 is then shifted an equivalent distance of a fractional phase distance of the sinusoidal pattern by linear actuator 28 and the cameras 52a, 52b acquire a second image. Since cameras 52a, 52b acquire images of the projected structured light pattern 30, only one image acquisition time is required to generate height images from two different triangulation angles.
In all, the number of images required to be acquired by multiple camera height image sensor 50 is n×m where n is the number of image sensors 52 and m is the number of phase images. However, the number of patterns that are projected is only m. Since the number of phase images required for a reliable height image is typically three, the number of images acquired by each of cameras 52a, 52b remains constant at three. To improve performance, it is possible to increase the number of cameras to four which increases the number of images per capture to twelve. However, since the four cameras are acquiring images in parallel, the time to acquire all twelve images is only the time required to project and image three images. With parallel acquisition, the time required to acquire a single height image is the same for a height image sensor comprised of a single camera as a sensor comprised of multiple cameras. Since adding multiple cameras greatly improves the quality of the height image without increase time of generating the height image, this embodiment of the invention is a major advantage over prior art techniques, reducing the overall time to inspect circuit board 18.
In operation, light source 24 illuminates SLM 64 and pixels that are programmed with high brightness values reflect light through imaging lens 22. Imaging lens 22 projects the light from SLM 64 onto the circuit board 18. Simultaneously, both cameras 52a, 52b acquire a first image of the circuit board 18 during the illumination period. The projection pattern programmed into SLM 64 is then changed to a second sinusoidal pattern with a relative phase shift of an equivalent distance of a fractional phase distance of the first sinusoidal pattern and cameras 52a, 52b acquire a second image. Finally, the projection pattern programmed into SLM 64 is then changed to a third sinusoidal pattern with a relative phase shift of an equivalent distance of a fractional phase distance of the first and second sinusoidal patterns and cameras 52a, 52b acquire a third image.
Using SLM 64 to generate a sequence of structured light images has advantages over using a mechanically shifted chrome-on-glass reticle. With a chrome-on-glass reticle, structured light pattern 30 is fixed with the chrome-on-glass pattern and sequences of images with differing phases are generated by physically moving the reticle. Physically moving the reticle is costly and requires motion components that are prone to mechanical wear and ultimately failure. In addition, it is often required to change the sinusoidal pattern's period. By changing the sinusoidal pattern's period, the height range and height resolution of the height image sensor can be adjusted. Changing the height range of the sensor is particularly important when inspecting a circuit board after components have been placed since the height of the placed components can be higher than the height range of the sensor which is determined by the reticle pattern. Changing the chrome-on-glass reticle pattern requires physically replacing one reticle with another which typically cannot be accomplished during operation of the sensor.
With SLM 64, various patterns can be projected unto circuit board 18 simply by programming an array of numbers into the controller 66. Projecting an image sequence with varying phases is simply accomplished by programming successive images to controller 66. By addressing the successive images from controller 66 memory, a sequence of phase images is projected without physically moving the reticle. In addition, by changing the phase period of the pattern programmed to controller 66, the height resolution and height range of height imaging sensor 62 can be changed during the operation of the sensor.
The images acquired by cameras 52a, 52b, 52c, 52d are sent to a controller, not shown, which processes the images sets into a height image. Using four cameras improves the quality of the height map by decreasing imager noise effects and further eliminating the chance of an area of circuit board 18 to be in shadow or otherwise false height data. Since the images are acquired by cameras 52a, 52b, 52c, 52d simultaneously, there is no impact on multiple imaging device height image sensor's 70 acquisition speed.
In addition to generating height images using multiple cameras and structured light as described above, the functionality of all embodiments can be extended by using pairs of angled cameras present in these embodiments to generate an additional height image using stereo image pair. Producing height images based on a stereo pair cameras with different points of view is a well-known technique. Prior art height image sensor 10 shown in
In some embodiments, the performance of the height image sensor is further enhanced by configuring each or combinations of the multiple cameras with different operating characteristics. For one embodiment, at least one of the cameras is configured as black and white (B/W) monochrome camera and at least one of the cameras is configured as a color camera. Acquiring a color image of the circuit board is desired to enhance the user's visualization of the circuit board and to enhance 2D images that are used to recognize features on the circuit board. However, cameras that are typically used to acquire color images employ Bayer color filters over the semiconductor detector array, which when combined into a color image, effectively reduces the spatial resolution of the camera. Using and combining the images generated from each of the B/W camera and color camera, a high resolution height map can be generated with images from the B/W cameras and a lower resolution height image and color image of the circuit board can be generated with images from the color cameras. By combining these height images, high performance height image and a color image of the circuit board is generated during one height image acquisition cycle.
In another embodiment, each of the cameras of the height image sensor is configured to use a different exposure time. Using multiple exposure times is a technique used in some machine vision applications to improve the dynamic range of a single camera. However, for a single camera, images based on multiple exposure times requires multiple image acquisition cycles which increases the total time required to acquire the image. Using multiple cameras, each using a different exposure time, the resulting height and video images have increased dynamic range without incurring a time penalty.
The height sensor 70 in
In another embodiment, at least one of the cameras is configured with a large field of view and at least one of the cameras is configured with higher magnification optics creating a higher resolution image. For measurements that require high performance, the height images generated by the high magnification cameras can be used. For areas of the circuit board that do not require high resolution height maps and in applications where high speed is required, the cameras that are configured with a larger FOV are used. In practice, switching between the high resolution cameras and the large field of view cameras is equivalent to adding zoom functionality to the height sensor without using moving optical components found in typical optical zoom systems. In addition, both high resolution and large FOV images are acquired at the same time.
In another embodiment, the triangulation angle between the structured light source and each of the cameras is varied. For a given structured light pattern, the range and resolution of the resulting height map is determined, in part, by the triangulation angle between the structured light source's optical axis and cameras' optical axis. By configuring each camera with a different triangulation angle, the combined height images will have a higher height resolution over a larger measurement range.
In each embodiment, a controller is coupled to the illumination source and to the cameras. The controller preferably generates a height topology of the circuit board based on images of the structured light acquired from the cameras. The controller can be configured to program the structured light source to project a light pattern onto a target, acquire images of the projected light pattern from the each of the cameras, generate a height image and a video image from images acquired from each of the cameras, and combine separated height and video images into composite height and video images.
Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention. For example, while embodiments of the present invention generally describe the utilization of a CMOS detector, any suitable image acquisition device including a CCD array can be used. Also, while embodiments of the present invention generally describe the utilization of DVM device, other SLM technologies, such as Liquid Crystal Display Devices (LCD) and Liquid Crystal on Silicon (LCOS) SLM can also be used to produce programmable structured light patterns. In the present invention, these programmable structured light patterns were described as sinusoidal intensity patterns. However, there are several other suitable patterns such as binary gray code patterns and pseudo random structured patterns.
The present application is based on and claims the benefit of U.S. Provisional Patent Application Ser. No. 61/753,496, filed Jan. 17, 2013 and U.S. Provisional Patent Application Ser. No. 61/765,399, filed Feb. 15, 2013, the content of which applications is hereby incorporated by reference in their entireties.
| Number | Date | Country | |
|---|---|---|---|
| 61753496 | Jan 2013 | US | |
| 61765399 | Feb 2013 | US |
