This patent application is assigned to
The following quote was obtained by the news editors from the background information supplied by the inventors: "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.times.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 FIG. 1, one approach to mitigate the triangulation shadow effect is to use multiple pattern projection sources with a normally incident camera. Each of the sources projects a structured pattern onto the circuit board from different incident angles. If one pattern projection source is occluded, or otherwise blocked, from an area of the test surface, there is a high probability that the other pattern projection source will be able to illuminate that area. To acquire a non-occluded height image, the camera acquires images from each of the pattern projection sources serially and then combines the results of the multiple height images to ensure all areas of the image contain valid height data. Typically, the height image sensor is held stationary while acquiring multiple images from each of the sources. One disadvantage to this approach is that it requires multiple image acquisition cycles of one field of view (FOV) to generate a single height image which slows down the overall acquisition process when compared to a sensor that uses a single source. Implementation of multiple source white light phase triangulation sensors requires the pattern projection sources to be turned on separately so that the image from one source, followed by acquisition of an image from another source, can be acquired in sequence by the camera. This operation will typically require two or more image acquisition cycles of the sensor in order to acquire height image data.
"In the sensor shown in FIG. 1, the structured light is characteristically generated by imaging a reticle consisting of a fixed chrome-on-glass pattern onto the circuit board. To acquire a height image, a sequence of patterned images are required, each of the images being a shifted version of the previous image. Typically, the structured pattern is a sinusoidal intensity pattern and the sequence of images are the same sinusoidal pattern; each image of the sequence shifted relative to the other images of the sequence some known fraction of the sinusoidal period. Usually, the phase shift in the sequence of images is created by physically moving the reticle within the sensor. One disadvantage to utilizing a chrome-on-glass reticle is that changing the frequency or orientation of the structured light requires replacing the reticle, changing the magnification of the pattern projection optics or both. Additionally, physically moving a glass reticle within the sensor requires expensive mechanical motion components.
"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."
In addition to the background information obtained for this patent application, NewsRx journalists also obtained the inventors' summary information for this patent application: "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.
BRIEF DESCRIPTION OF THE DRAWINGS
"FIG. 1 is a diagrammatic view of a height image sensor used to inspect circuit boards in accordance with the prior art.
"FIG. 2 is a diagrammatic image of a structured light phase pattern projection system that is typically used to illuminate the circuit board under test.
"FIG. 3 is a diagrammatic view of a multi-camera height image sensor using phased structured light in accordance with an embodiment of the present invention.
"FIG. 4 is a diagrammatic view of multi-camera height image sensor for three-dimensional imaging using phase structured light generated by a spatial light modulator (SLM) in accordance with an embodiment of the present invention
"FIG. 5 is a diagrammatic view of a four-camera sensing system for a height image sensor using phase structured light generated by a spatial light modulator in accordance with an embodiment of the present invention.
"FIG. 6 is flow diagram of a method of acquiring images and generating height maps in accordance with an embodiment of the present invention.
"FIG. 7 is a diagrammatic view of a four-camera sensing system for a height image sensor using phase structured light generated by a spatial light modulator where one pair of cameras provides black and white images and a second pair of cameras provides color images in accordance with an embodiment of the present invention
"FIG. 8 is a diagrammatic view of a four-camera sensing system for a height image sensor using phase structured light generated by a spatial light modulator where each pair of cameras is configured with a different optical magnification in accordance with an embodiment of the present invention
"FIG. 9 is a diagrammatic view of a four-camera sensing system for a height image sensor using phase structured light generated by a spatial light modulator where each pair of cameras provides a separate triangulation angle in accordance with an embodiment of the present invention."
URL and more information on this patent application, see: Haugen, Paul R.; Rudd,
Keywords for this news article include: Circuit Board, Microtechnology, Electronic Components,
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