Abstract:
An optical system for camera-based 3D inspection of semiconductor devices that provides two different perspectives of a device by switching the color of the light source is disclosed. The system presents a camera with a first view when one color of light is on, and a different perspective view when a different color of light is on.

Description:
BACKGROUND—FIELD OF THE INVENTION  
       [0001]     Electronic devices are commonly inspected in three-dimensions by machine vision systems to ensure that their electrical contacts are within sufficient tolerances to properly solder to a printed circuit board. The present invention is a convenient optical system for inspecting electronic devices such as BGA (Ball Grid Array) devices, LCC (Leadless chip carrier) devices or leaded devices using stereoscopic imaging.  
       BACKGROUND—PRIOR ART  
       [0002]     Camera-based machine vision 3D inspection systems of electronic semiconductor devices have been in use for years. Such systems measure the contact locations of semiconductors in 3D by imaging two different perspectives of the BGA device.  
         [0003]     Typically two cameras are used to create stereo images. U.S. Pat. No. 6,778,282 to Smets (2004) discloses such a system. One camera is positioned for a plan-view perspective and another camera is positioned at an angle to provide an oblique perspective. The main problem with this approach is that using two cameras is expensive because of the redundant equipment required. Two cameras, two lenses, two sets of cables and often two frame grabbers are needed. Additionally the setup or maintenance required is doubled, as both lenses need focusing and both cameras need aligning. Finally, the space requirements for two cameras is often difficult to accommodate on a semiconductor handling machine.  
         [0004]      FIG. 1  illustrates this prior art method in which two cameras are used to inspect a BGA device. The first camera  1  has a lens  2  and views the device  7  via an on-axis beam path  3 . A ringlight comprising Light Emitting Diodes (LEDs)  9  (just 2 LEDs are depicted, however an complete circle of LEDs surrounds the device) emit rays of light  10  toward the device. A second camera  4  has a lens  5  and views the device via an oblique beam path  6 . The resulting camera  1  on-axis image of a typical BGA device looks like  FIG. 2 . Each properly shaped BGA ball reflects the ringlight to produce a ring or circle  31  on its surface. Thus, nine circles appear, one for each ball on the BGA device. The resulting oblique image produced by camera  4  looks like  FIG. 3 . Each ball reflects only a portion of the ringlight and thus a semicircular ring  32  appears on each ball.  
         [0005]     Another approach of the prior art is to use one camera with a Field Of View (FOV) large enough that it includes a mirror in addition to the BGA device. In the mirror an oblique view of the device is seen. U.S. Pat. Nos. 6,862,365, 6,915,006, 6,915,006, 7,079,678, and 7,085,411 to Beaty (2005-2006) utilize this method as illustrated in  FIG. 4 . Camera  1  has a lens  2  and views the device  7  via an on-axis beam path  3 . A mirror  11  reflects ray  6  from an oblique perspective to be directed as ray  12  toward the camera. The resulting image is depicted in  FIG. 5 . This approach diminishes the resolution of the camera on the device and therefore the measurement accuracy. Additionally, as inspection requirements change to inspect larger or smaller devices, the mirror location must be adjusted to remain close to the device and the centerline adjustment of the camera often requires adjustment in order to minimize the FOV to optimize measurement accuracy with this method.  
       SUMMARY OF THE INVENTION  
       [0006]     The present invention is a color-dependent optical switch for consecutively imaging 2 perspectives of a BGA device with a single camera. By illuminating the device with one color of light the on-axis image is seen by the camera. Utilizing a different color of light presents an oblique image to the camera.  
         [0007]     To attain this, the present invention generally comprises a light source of a limited spectral range, a second light source of a limited spectral range that is different than the range of the first light source, an optical element that transmits light of substantially the first spectral range and reflects light of substantially the second spectral range, and two reflecting surfaces. These optical elements are arranged such that when only the first light source is turned on, a camera positioned to view a device illuminated by the light source sees the device from a first perspective. Additionally, when only the second light source is turned on the camera sees the device from a second perspective. Further, the beampath length from the camera to the device being inspected is roughly identical regardless of the spectrum of light used so that the device is in focus for both perspectives.  
         [0008]     A primary object of the present invention is to provide 2 switchable perspectives of a BGA device that will overcome the shortcomings of the prior art devices, notably to utilize a single camera to save on equipment cost and to optimize the usage of the resolution to increase measurement accuracy. Another object is to minimize changeover time and adjustments required to change the system to inspect other smaller or larger BGA devices with changing the FOV. The invention additionally provides for both optical paths to be roughly equivalent in length so that both perspectives are in focus. Both perspectives can also be substantially centered in the FOV for convenience. A final object is to provide an alternate method of measuring other electronic semiconductor devices such as LCC devices and leaded devices.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]      FIG. 1  is a side section view of prior art utilizing two cameras to produce 2 perspectives.  
         [0010]      FIG. 2  is a camera image of the prior art depicted in  FIG. 1 .  
         [0011]      FIG. 3  is a camera image of the prior art depicted in  FIG. 1 .  
         [0012]      FIG. 4  is a side section view of prior art utilizing one camera and one mirror to produce two perspectives.  
         [0013]      FIG. 5  depicts a single camera image of the prior art shown in  FIG. 4 .  
         [0014]      FIG. 6  is a side section view of the invention with red light turned on.  
         [0015]      FIG. 7  is a camera image of the invention in the state depicted in  FIG. 6 .  
         [0016]      FIG. 8  is a side section view of the invention with blue light turned on.  
         [0017]      FIG. 9  is a camera image of the invention in the state depicted in  FIG. 8 .  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0018]      FIG. 6  illustrates a preferred embodiment of the invention. A ringlight of red LEDs  9  encircles the device  7  in the inspection area. A semicircle of blue LEDs  29  partially encircles the device. This semicircle of blue LEDs is opposite the camera. A mirror  17  is positioned below the device at a 45 degree angle to fold the beampath by 90 degrees. A camera  1  and lens  2  are positioned to view the image reflected off of this reflective surface. Between the camera  1  and the reflective surface  17  is a dichroic mirror  18  that allows substantially red light to pass thru while reflecting substantially blue light.  
         [0019]     To the right of the inspection area is a dichroic filter  15  which only transmits substantially blue light while reflecting other frequencies of light. Further to the right is a mirror  16  which reflects light downwards toward dichroic mirror  18 .  
         [0020]     When the red LEDs  9  are energized and the blue LEDs  29  are off, light rays  10  illuminate device  7 . Light reflects off of the device in various directions. Considering ball  8  on device  7 , there are two possible beam paths that could allow light to be seen by the camera, but only one that allows the red light to be seen. Light ray  13  reflecting from ball  8  toward the blue filter  15  is not allowed to pass thru the filter. However, light ray  14  reflecting from ball  8  is incident on mirror  17 , reflects as ray  19  and passes thru dichroic mirror  18  into camera lens  2  producing an image similar to  FIG. 7 .  
         [0021]     When the blue LEDs  29  are energized and the red LEDs  9  are off, light ray  20  illuminates device  7  as shown in  FIG. 8 . Considering ball  8  on device  7 , light reflects in various directions but only one of two optical paths allows light to be seen by the camera. Light ray  21  reflecting downward is incident on mirror  17  and reflects as ray  22 . Ray  22  passes thru the nonreflective surface of the dichroic mirror, but when it is incident upon the dichroic coating it cannot pass thru but is reflected out of the system as ray  23 . However, ray  24  that is reflected off of ball  8  passes thru blue filter  15  to be incident on mirror  16  which reflects the light as ray  25 . Ray  25  is incident on dichroic mirror  18  as ray  26  which then enters into the camera lens producing an image similar to  FIG. 9 . The angles of all the reflecting surfaces are angled such that ray  26  is substantially coaxial with ray  19  of  FIG. 6 . Also, the reflecting surfaces are positioned such that the distance traveled by the blue ray that enters the camera lens in  FIG. 8  is substantially identical to the distance traveled by the red ray that enters the lens in  FIG. 6 . Therefore ball  8  can be in focus when illuminated with either light source. And finally, by fixing mirror  17 , filter  15 , and dichroic mirror  18 , the perspective angle and beam path length can be finely and conveniently adjusted by tilting and moving only mirror  16 .  
         [0022]     The red LED ringlight  9  consists of a 360 degree circle of red LEDs. The ringlight could consist of other light sources such as filament bulbs or gas bulbs or some other light source. The ringlight could utilize a fiber optics or a light pipe or other means to deliver light to the circle around the device. The ringlight need not be red, but it must be of limited range so that it can be separated from the other light source. The light source could comprise a broadband white light source with a color filter to limit its frequency range so that it substantially produces at least one unique frequency of light not produced by the other light source and substantially doesn&#39;t produce at least one frequency of light that is produced by the other light source.  
         [0023]     The blue LEDs  29  need not be blue, but must be differentiated from light source  9  by a unique color spectrum. Also, this semicircular light source need not subtend exactly 180 degrees, but could be as little as 1 degree to still create a spot of light on the bottom of the ball. The light source could be nonLED and it could use fiber optics or lightpipes or other optical means to deliver a band of limited frequency to the device. In the preferred embodiment the center of the semicircle is roughly opposite ray  24  so that the light seen in the oblique view reflects off of the bottom or nearly the bottom of the ball. The light source could comprise a broadband white light source with a color filter to limit its frequency range.  
         [0024]     Mirrors  16  &amp;  17  could be any reflecting surface such as a prism, mirror or holographic reflector, so long as they reflect a coherent image. They could be frequency specific reflectors. Mirror  17  need not be exactly below device and need not reflect light by exactly 90 degrees. Other angles can be used and still embody the invention.  
         [0025]     Filter  15  could be a dichroic mirror or a gel filter or any other type of optical filter that can selectively prohibit light of specific frequencies from passing. This filter is not required but is preferred in order to correct for non-desired frequencies that may reflect off of dichroic mirror  18 .  
         [0026]     Dichroic mirror  18  is a clear polished substrate with a dichroic coating on one surface. The coating could transmit and reflect different colors than mentioned herein so long as it is matched with the colors of light sources used.  
         [0027]     The camera  1  could be any of a variety of electronic machine vision cameras such as a Sony XC-ST50 or a Basler a202k or any other make and model that can electronically image in real-time.  
         [0028]     Another embodiment of the invention substitues a color camera for the black and white camera. Both light sources are energized simultaneously and the color camera takes a snapshot. The color camera provides a red image that yields one perspective, and a blue image that yields the second desired perspective. This embodiment is advantageous in that the device can be inspected with one snapshot. With a very short shutter time or by strobing the lights the device can be inspected on the fly (as it moves). This is helpful for high speed inspection.