DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0030] Referring to FIG. 1 of the drawing, an embodiment of the present invention for visualization of parts having metallic objects against a dielectric or nonmetallic poorly contrasting background, is explained in conjunction with a conventional computer vision system 10 recognizing a usually two dimensional Ball Grid Array (BGA) 12 of small high melt solder balls 14 carried by a ceramic dielectric substrate 16.

[0031] BGA 12 is held by a gripper or manipulator 18 of a pick and place machine (not shown) over an upwardly looking camera 20 of computer vision system 10. The camera 20 forms and provides an electronic image to computer vision system 10, which system utilizes the electronic image to recognize and inspect the solder balls 14 of the BGA 12, as to existence, size and spacing. If the BGA 12 passes inspection, the vision system 10 steers the translation and placement of BGA 12 by manipulator 18 on a circuit board or card 22, with the recognized solder balls 14 of the BGA in registration with contact pads 24 of the board or card 22.

[0032] As is conventional, prior to placement of the BGA 12 and other surface mount components on the circuit board or card 22 the contact pads 24 are coated with a eutectic solder paste, and after the placement of the BGA 12 and other surface mount components on circuit board or card 22, heat is applied to card with placed BGA and other components to fuse the solder paste into fillets (not shown) firmly surface mounting the BGA to the circuit board or card.

[0033] In the poorest contrast situation when the solder balls 14 are silver colored and the substrate 16 is white or light colored, the light box 26 is used to provide generally upwardly and inwardly directed polarized illumination I of BGA 12, in conjunction with a polarization filter 28 in front of the lens 30 of camera 20, in order to allow for proper visualization by camera 20, and proper recognition by vision system 10 of solder balls 14.

[0034] As shown in FIG. 2, a preferred embodiment of light box 26 has a central aperture 32 through which camera 20 looks upward via polarization filter 28, and four upwardly and outwardly inclined trapezoidal faces 34, 36, 38, 40 of a frusto-pyramid, each face forming a separate source of polarized light. The faces 34, 36, 38, and 40 comprise polarization filters 40, 42, 44, and 46 over respective milk glass diffuser panels 50, over LED arrays 52.

[0035] With the direction of linear polarization passed by polarization filter 28 taken as 0°, polarization filters 40 and 44 are oriented to pass linear polarization directed at 90° and polarization filters 42 and 46 are oriented to pass linear polarization directed at 180°. The result is that the downwardly directed component of light from the four sources reflected from the solder balls 14 has a polarization at 90°, which is, or is nearly, orthogonal with the polarization direction that would be passed by polarization filter 28. Therefore no light reflected by solder balls 14 reach the lens 30 of camera 20.

[0036] On the other hand, the downwardly directed component of light reflected from the white or light colored substrate 16 is substantially uniformly distributed in polarization and a substantial amount of light reflected downward from the substrate 16 passes through polarization filter 28 and reaches the lens 30 of camera 20. As a result, in the electronic image formed by camera 20, the solder balls 14 appear substantially black and the substrate 16 appears substantially white, providing good contrast for reliable recognition of solder balls 14. This contrast producing effect is thought to be due to a preservation of polarization in reflection of electromagnetic radiation from shiny metal objects versus a the smearing of polarization in reflection of electromagnetic radiation from dielectric objects.

[0037] A printed circuit board assembly machine will inspect parts of different types during the assembly of a single circuit board. If some of the parts being inspected have a light colored substrate, while others have a black substrate, an additional problem arises. Because the effect of the linear polarizing filter 28 is to cause the solder balls 14 appear substantially black in the electronic image formed by camera 20, the solder balls may not be easily recognized on parts having a black substrate, when viewed through such a filter. In this situation, it is desirable to deploy the filter 28 in front of the lens 30 of camera 20 when visualizing parts with a white substrate, and retract the filter when visualizing parts with a black substrate.

[0038] A similar problem arises when the circuit board assembly machine handles printed circuit boards or cards. Some circuit boards are manufactured with a fiberglass substrate, which is typically green in color. Such a substrate provides good contrast with the silver or gold colored fiducials used to register the parts installed on the circuit board. However, other circuit boards or cards handled by the same assembly machine may have a white, or other light colored, ceramic substrate, which provides poor contrast with the metallic fiducials. The technique described above for deploying a filter when appropriate to achieve reliable recognition of solder balls on parts having either light colored or dark colored substrates can also be used to reliably recognize metallic fiducials on either light colored or dark colored printed circuit boards and cards.

[0039] The control system of a circuit board assembly machine typically utilizes a specification file for each type of item (printed circuit board, card or part) it handles. This file contains information on the characteristics of each type of item. Information from that file, such as the number and location of physical features (fiducials and contact pads on boards and cards, or contacts on parts), is provided to the computer vision system for the inspection process. In an assembly machine employing the techniques of the invention, the specification file would also contain information indicating whether a filter should be used when inspecting an item of that type and, for the embodiment shown in FIG. 5, specifying which type of filter is to be used.

[0040] Referring now to FIG. 3 of the drawing, an embodiment of the invention employing a linear filter deployment apparatus 60 is shown with the filter retracted. A filter 61 is attached by linkage 64 to linear actuator 65. When energized by the circuit board assembly machine control system, the linear actuator 65 will adopt the configuration shown, with the filter retracted from the optical axis of the camera 20. When de-energized by the control system, the linear actuator 65 will adopt the configuration shown in FIG. 4 of the drawing, with the linkage extended and the filter 61 deployed in front of the lens 30 of camera 20.

[0041] For some types of linear actuators, the action of the actuator 65 will be the opposite of that described above: when the actuator 65 is energized, the filter 61 will be deployed and when the actuator 65 is de-energized, the filter 61 will be retracted. While the above description discusses the use of a linear actuator to deploy filter 61, it should be understood that a solenoid could also be employed in an embodiment of the invention.

[0042] Filter 61 in FIGS. 3 and 4 could be the linear polarization filter 28 shown in FIG. 1, employed in combination with linearly polarized illumination, as described above, to correct the visualization problem associated with metallic physical features on a light colored background. Or, filter 61 could be one of several other types of filter chosen to correct other characteristic problems, as will now be described. These other types of filters can be used to equal effectiveness with polarized or non-polarized illumination.

[0043] If the item being visualized has, for example, red physical features on a green background, filter 61 could be chosen to be a red color filter. Such a filter would pass the red light reflecting from the red physical features while blocking the green light reflecting from the background, causing the physical features to appear substantially white and the background to appear substantially black in the image from a monochrome camera.

[0044] If camera 20 employs a charge coupled device (CCD) to form the electronic image, items that are especially reflective in the infrared spectrum may present a different characteristic visualization problem. Because a CCD is very sensitive to radiation in the infrared spectrum, the electronic image of such an item may be washed out, with both the physical features and the background of the item appearing as light gray objects in the image, preventing the computer vision system from being able to recognize the physical features on the item. For this type of item, filter 61 could be chosen to be an infrared filter, which blocks infrared radiation and passes visible light. Blocking the infrared radiation would allow the CCD to form an electronic image only from the visible light reflected from the item, resulting in better contrast between the physical features and background of the item in the filtered image.

[0045] If glare is characteristically encountered when visualizing certain items, the image produced by camera 20 will again be washed out. In these situations, filter 61 could be chosen to be a circular polarizing filter. Such a filter restricts the rotation component of the illumination reflected from the item, thereby reducing glare from the item and allowing correct recognition of the physical features on the item.

[0046] While the preceding discussion has described recognizing contacts on electronic parts or fiducials and contact pads on printed circuit boards and cards, it should be understood that the physical features being recognized on other items may not be contacts or fiducials. For example, when a lens is to be mounted over a light emitting diode, the lens will be inspected for mounting features that allow it to be attached to the circuit board or card. Similarly, a circuit board or card may be inspected for the presence and accurate placement of thermoset adhesive glue dots, which serve to mount parts that are not soldered to the board.

[0047] If the items being handled by a circuit board assembly machine each exhibit a different characteristic problem, then one of several different filters must be deployed, depending on the item being inspected, to allow reliable recognition of the physical features of that item. An embodiment of the invention allowing one of several filters to be deployed is illustrated in FIG. 5 of the drawing. A rotational filter deployment apparatus 70 is shown, which is a filter wheel 75 mounted on the shaft of stepper motor 76. Also visible in FIG. 5 are filters 61 and 63 mounted on the filter wheel 75 in registration with apertures 72 and 74. In response to signals from the assembly machine control system, stepper motor 76 will rotate to a specified position, thereby deploying the appropriate filter for the part being visualized.

[0048] The rotational filter deployment apparatus 70 is shown in more detail in FIG. 6. Filter wheel 75 can be seen to have, in this embodiment of the invention, four apertures 71, 72, 73, and 74. Apertures 72, 73, and 74 are covered by filters 61, 62, and 63, respectively, while aperture 71 is not covered with a filter. The hub of filter wheel 75 is mounted to the shaft of stepper motor 76. Lens 30 is seen through aperture 74.

[0049] While motor 76 is described above as a stepper motor, it should be understood that servo motors could also be used in an embodiment of the invention to position filter wheel 75 in response to signals from the assembly machine control system.

[0050] While FIGS. 3, 4 and 5 show embodiments of the invention being used for visualization of a BGA package, it should be appreciated that the techniques of the invention can be used to improve visualization of other types of packages and of printed circuit boards and cards.

[0051] While the embodiments discussed above show only a single filter deployment apparatus in use, it should be understood that, in yet another embodiment of the invention, multiple filters could be deployed together by multiple filter deployment apparatuses, to improve visualization of items that present multiple characteristic problems.

[0052] It should now be appreciated that the objects of the invention have been satisfied. While the invention has been described in particular detail, it should also be appreciated that numerous modifications are possible within the intended spirit and scope of the invention. In interpreting the appended claims it should be understood that:

[0053] a) the word “comprising” does not exclude the presence of other elements or steps than those listed in a claim;

[0054] b) the word “a” or “an” preceding an element does not exclude the presence of a plurality of such elements.

[0055] c) any reference signs in the claims do not limit their scope; and

[0056] d) several “means” may be represented by the same item of hardware or software implemented structure or function.