Patent Publication Number: US-6341067-B1

Title: Printed circuit board for a CCD camera head

Description:
This application is a division of U.S. application Ser. No. 08/683,644, filed Jul. 15, 1996, now U.S. Pat. No. 6,078,359. 
    
    
     The United States Government has rights in this invention pursuant to Contract No. W-7405-ENG48 between the United States Department of Energy and the University of California for the operation of Lawrence Livermore National Laboratory. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to imaging devices, particularly to imaging of visible light, ultraviolet radiation, and soft to penetrating x-rays, a more particular to a charge-coupled device (CCD) camera head for such imaging wherein the camera head is small, capable of operating in and out of a vacuum environment, and is versatile. 
     Cameras of various types have been used for imaging of visible light, ultraviolet radiation, and soft to penetrating x-rays within a target chamber where laser produced plasma images are studied. This imaging, in general, has been carried out using film, which can be damaged, particularly involving high energy density plasmas, such as generated by pulse-power, ICF, and other high powered laser-plasma interaction studies. In many of these studies the imaging apparatus must be located with a target or reaction chamber and thus must be capable of withstanding harsh environments, and well as being capable of operating in and out of a vacuum environment. 
     The present invention provides a charge-coupled device (CCD) camera head which can replace film for the above-identified imaging applications. The camera head is small, capable of operating both in and out of a vacuum environment, and is versatile. This is accomplished by utilizing printed circuit (PC) boards having an internal heat sink which are secured to the camera chassis to form a thermally conductive path out of the camera, and enable close stacking of the PC boards for miniaturization of the camera head. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a charge-coupled device (CCD) camera head. 
     A further object of the invention is to provide a vacuum compatible CCD camera head. 
     A further object of the invention is to provide a miniature CCD camera head capable of operating both in and out of a vacuum environment. 
     Another object of the invention is to provide a CCD camera head for digital imaging of visible light, ultraviolet radiation, and soft to penetrating x-rays. 
     Another object of the invention is to provide a miniature CCD camera head for high energy density plasmas studies. 
     Another object of the invention is to provide a miniature CCD camera head for imaging applications and which is capable of withstanding hard environmental conditions, such as high powered laser-plasma interactions. 
     Another object of the invention is to provide a miniature CCD camera head capable of dissipating the heat generated by the electronics therein, while enabling close stacking of the electronic components. 
     Another object of the invention is to provide a miniature CCD camera head which utilizes PC boards having internal heat sinks which are connected to the camera chassis to form a thermally conductive path out of the camera head to the camera chassis where the heat can be removed using conventional methods. 
     Another object of the invention is to provide a CCD camera head using closely stacked PC boards which include an integral heat sink having edges or end extending beyond the PC board for connection to the camera chassis. 
     Another object of the invention is to provide a CCD camera head using PC boards with internal heat sinks and a heat sink/PC board forming a vacuum barrier, and which utilizes a thermal electric cooler, and a direct connection between the PC board assembly and a heat sink located in the camera chassis. 
     Other objects and advantages of the present invention will become apparent from the following description and accompanying drawings. Basically, the invention is a vacuum compatible miniature charge-coupled device (CCD) camera head. By the use of printed circuit (PC) boards having an integral heat sink, the PC boards can be closely stacked, whereby the electronic components or elements between the opposing PC boards are within 0.04-0.25 inch of each other, thus allowing miniaturization of the camera head. The camera head also includes a thermal electric cooler to enable the CCD to cool to −40° C. or less and provides for a vacuum barrier while allowing the electrical signals are brought through cutouts in the heat sink and passed to the CCD. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are incorporated into and form a part of the disclosure, illustrate an embodiment of the invention and, together with the description, serve to explain the principles of the invention. 
     FIG. 1 illustrates an embodiment of a PC board arrangement in accordance with the present invention, wherein each PC board has an integral internal heat sink which is connected to the chassis of the camera. 
     FIG. 2 is an enlarged partial cross-sectional view of a portion of a PC board of FIG. 1, as indicated at  2 , illustrating the heat flow therein. 
     FIG. 3 is a partial cross-sectional view of an embodiment of stacked PC boards using the integral heat sink arrangement in accordance with the invention. 
     FIGS. 4,  5  and  6  illustrate an embodiment of a heat sink/PC board forming a vacuum barrier in accordance with the invention, with FIG. 6 being an enlargement of a portion of FIG. 5, as indicated at  6 . 
     FIGS. 7A,  7 B and  7 C illustrate an embodiment of a vacuum pump-down port arrangement that may be incorporated into the CCD camera head. 
     FIGS. 8 and 9 illustrate an embodiment of a chassis for retaining the PC boards, whereby the internal core on heat sink of each PC board is connected to camera chassis to provide the needed heat transfer. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention is a compact, large active area, high spatial resolution, high dynamic range, charge-coupled device (CCD) camera used to replace film for digital imaging of visible light, ultraviolet radiation, and soft to penetrating x-rays. Integration of such a CCD camera to existing instrumentation provides a substantial enhancement of diagnostic capabilities for studying high energy density plasma, such as generated by pulse-power, inertial confinement fusion (ICF), and other high powered laser systems. Laser-plasma interaction studies will have applications in a variety of defense, energy, industrial, and medical imaging arenas. Many of these applications require the CCD camera to be located within a vacuum chamber and often very dose to the actual target or source, and such imposes very tight size and vacuum compatibility restrictions on the CCD camera. The CCD camera has a high level of flexibility, can use CCD chips of various types, with various CCD formats, and sensitivities to a wide range of radiation, as referenced above, as well as charged particles. The CCD camera will operate both in vacuum and at standard atmospheric pressure. Digital output from the camera can be accessed by a wide range of computer platforms and image processing software packages. The CCD camera utilizes several major elements that together provide the utility for use in many applications. The major elements or components of this invention include: 1) printed circuit (PC boards with built in heat sinks, and 2) heat sink/PC board forming a vacuum barrier. In addition, the CCD camera may utilize: 1) a vacuum pump-down port, and 2) a camera chassis assembly which incorporated donut or ring configured members for retaining the PC boards. Each of the four (4) above-referenced major components or elements of an embodiment of the CCD camera will be described separately hereinafter. 
     1. PC Board With Built In Heat Sink 
     The camera head housing must be capable of dissipating the heat generated by the electronics within the package. Within a vacuum environment there can be no convective cooling, and the size constraint on the camera head eliminates the possibility of radiative cooling, and thus only conductive cooling can be employed to remove excess heat from the camera electronics. FIGS. 1 and 2 illustrate a unique PC board and chassis construction which provides a method to remove this heat from the camera electronics, while at the same time maintaining a small volume, and increasing the overall structural strength of the package. 
     Referring now to FIGS. 1 and 2, with FIG. 2 being an enlargement of the section  2  of FIG. 1, the camera head is composed of a plurality of PC boards (three shown for simplicity), indicated at  10 ,  11  and  12 , and which are provided with internal integral heat conductive cores, members or heat sinks, constructed for example of copper, indicated at  13 ,  14  and  15 , each having edges or ends  16  and  17  which extend beyond the PC boards  10 ,  11  and  12 . The PC boards  10 - 12  are provided with electronic components indicated at  18  and the PC boards  10 - 12  are separated by spacers or insulator members  19 . The edges or ends  16  and  17  of the cores or heat sinks  13 ,  14  and  15  are bolted or otherwise secured in a camera chassis generally indicated at  20  and which may be composed of heat conductive ring sections  21  and  22 . In this embodiment, the edges or ends  16  and  17  are secured in grooves or cut-away sections  23  of the chassis  20  by screws  24 , as shown in FIG. 2, via a washer  25 , and which may extend through the ring sections  21 - 22  of chassis  20 . The screws  24  and washer  25  are constructed of a heat conductive material. Thus, as shown in FIGS. 1 and 2, each PC board has an integral internal heat sink that is secured to the chassis and which forms a thermally conductive path from the electronic components out of the camera head to the camera chassis, as shown by the arrows in FIG. 2, where the heat can be removed using conventional methods. The internal members or heat sinks  13 - 15  may be in addition to copper constructed of Ag; with the chassis rings  21  and  22  constructed of Al, Cu, Ag, or brass, and the screws  24  constructed of brass, such that maximum heat dissipation from the electronic components  18  can be obtained. The PC board material may be composed of polyimide or fiberglass, as known in the art. By the edges or ends  16  and  17  extending beyond the PC boards, such enables easy and direct securing of the internal heat sinks  13 - 15  to the camera chassis  20 , adding strength to the overall package. 
     As shown in FIG. 1, the PC boards are stacked, forcing the electronic components between adjacent PC boards to be closely located, for example with 0.04 inch of each other. This is done to reduce the size of the overall package. Thus, conventional heat sinks that sit on the electronic components themselves will not fit in this small space between components. Even conforming liquid heat sinks require too much space. FIG. 3 illustrates a cut-away view of an embodiment of a stacked PC board using an internal integral core or heat sink whose edges extend beyond the PC material, as in the FIGS. 1 and 2 embodiment. 
     As shown in FIG. 3, the embodiment of a stacked PC board, generally indicated at  30  includes a plated-thru hole or member  31 , having at the center thereof a layer of insulating material  32  extending around the member  31 , with a conductive metal core or heat sink  33 , of 100% soft copper for example, or Al or Ag, extending around the insulating material  32 , and which forms the center of the PC board  30 . Secured to the core  33  on opposite sides are layers  34  of prepeg material, to which are secured layers  35  of a conductive metal, layers  36  of a double sided laminate, layers  37  of a conductive metal, layers  38  of prepeg, layers  39  of a conductive metal, layers  40  of a double sided laminate, and outer layers or foils  41 . Note that the edge of the core  33  extends beyond the layers  34 - 41 . The layers  35 ,  37 ,  39  and  41  may, for example be composed of  1  oz. copper, the layers  34  and  38  of prepeg material may be composed of polyimide or fiberglass, with the double sided laminate layers  36  and  40  being composed of polyimide or fiberglass, and the insulating material  32  composed of epoxy rosin, with the plated-thru hole  31  being formed by materials such as copper. Note that the PC board of FIG. 3 is a cut-away view of any configuration with heat sink  33  centrally located. 
     By incorporating a conductive metal core in a stacked PC board, whose edge extends beyond the PC material, not only forms a heat sink that is an integral part of the PC board, but also enables securing same to the camera chassis, which results in the PC board and chassis becoming a single structural element, thereby adding strength to the PC board and chassis, while enabling a thermally conductive material path out of the PC board to the camera chassis, where the heat can be removed using conventional methods. 
     Other applications for this type of PC board include avionics where power dissipation, compact size, and mechanical ruggedness are required; as well as the generation computers, i.e., Pentium class computers, that require large quantities of heat to be transferred to the chassis or heat exchanger. 
     2. Heat Sink/PC Board Forming A Vacuum Barrier 
     To obtain high quality images from the CCD within the camera head it is necessary to cool the CCD to −40° C. or less. This requires that a thermal electric (TE) cooler be installed in the camera head. Unfortunately, up to 30 W of exhaust heat is dissipated from these devices when cooling the CCD. This heat must be transferred to the camera chassis to avoid overheating the camera head. 
     In many applications the CCD camera will be used in standard atmosphere. If the CCD is not surrounded by a vacuum, then at −40° C. water could condense onto the CCD and damage it. To avoid this the CCD must be maintained within a vacuum environment. At the same time, the CCD requires and generates electrical signals that must pass out into the associated equipment. The present invention removes the heat from the TE cooling element, provides for a vacuum barrier at the CCD while allowing the electrical signals to pass through, adds structural strength to the camera head, and offers all this in a small volume, as shown in FIGS. 4-6, with FIG. 6 illustrated as an enlargement of section  6  of FIG.  5 . 
     As shown in FIG. 4, which incorporates the PC boards  10 ,  11  and  12  and the camera chassis  20  of FIG. 1 additionally includes an additional chassis ring  43 , similar to chassis rings  21  and  22  of FIG. 1, and to which is connected a member or heat sink  44  as by screws  24  of FIG. 1, and on one side of which is directly mounted a PC board  45 . A cap  46 , constructed of material such as stainless steel, is secured to heat sink  44  via a gasket  47 , such as rubber, and includes an opening  48  having a groove or cut-away edge  49  in which a transparent member  50 , constructed for example, of glass or other material, depending on the application of the camera head. A CCD chamber  51  is defined by heat sink  44 , cap  46  and member  50 , within which is located a TE cooler  52  and a CCD  53 , with the TE cooler  52  being secured as indicated at  52 ′ to heat sink  44 . CCD  53  is supported within chamber  51  via a support member or plate  54  which is secured in chamber  51  via connected members  55 , such as bolts or screws (only two shown), which extend through cap  46  into heat sink or member  44 , screws or bolts  55  include a threaded section  56  and a smooth smaller diameter section  57 , with support member or plate  54  being retained by springs  58  at the point of intersection of sections  56  and  57  of bolts  55 . While not shown, the CCD  53  is secured to or integral with support plate  54 , as indicated at  54 ′. The member or heat sink  44  is provided with a plurality off cutouts or openings  59  (only two shown) through which electrical wiring  60  and  61  extend (see FIG. 6) and interconnects the plate  54  of CCD  53  with the PC board  45 . As seen in FIG. 6, the PC board  45  includes a substrate  62  composed, for example, of polyimide, and a conductive layer  63 , composed, for example, of copper, with the conductive layer  63  being secured to member or heat sink  44  via a solder layer  64 , and the electrical wire  60  is secured in substrate  62  and in conductive layer  63  via a solder joint  65 . 
     The heat from the TE cooler  52  passes directly into the member or heat sink  44 , which may be constructed of copper, Ag, Au, or Al, and out to the camera chassis  20 . The CCD chamber  51  can be evacuated to provide a vacuum for the CCD  53 , and the PC board  45  where the electrical wires  60  and  61  are soldered to the PC board to form a vacuum tight barrier. The electrical signals generated in the PC boards are brought out through the cutouts  59  in the heat sink  44  and passed to the CCD. The solder  64  at the PC board to heat sink interface and at the electrical wire to PC board interface form a vacuum tight seal. 
     Standard electrical/vacuum feedthrus are too large to be used in this small camera head and the concept of soldering the PC board  45  directly to the heat sink/vacuum barrier to form a vacuum seal is a unique approach. 
     3. Vacuum Pump-Down Port 
     In order to use the CCD camera in standard atmosphere, the CCD chamber of the head shown in FIG. 4 has to be evacuated, for reasons set forth above. This may be done using a vacuum pump-down port. Unfortunately, standard pump-down port arrangements are too bulky for this small camera head, and thus a very small ball-valve and gasketed screw arrangement in the cap  46  that surrounds the CCD chamber of FIG. 4 has been developed, as illustrated in FIGS. 7A,  7 B, and  7 C. Basically, the cap  46  is provided with an opening  70  of different diameter sections  71 ,  72 , and  73 , with section  72  being threaded. A  0 -ring  74 , for example, is secured at the opening section  72 - 73  interface. A small ball  75  is positioned in opening section  72 . During pump-down of CCD chamber  51  a removably pump-down valve stem  76  is threadedly secured in threaded section  72  of opening  70 , as shown in FIG.  7 A. During pump-down the ball  75  moves away from O-ring  74  allowing air to pass from the chamber  51  through valve stem  76 , as indicated by the arrow in FIG.  7 A. When the chamber  51  is evacuated, the valve stem  76  is removed and exterior air pressure forces the ball  75  against the O-ring  74 , sealing off the opening  70 , as shown in FIG.  7 B. To assure that the chamber  51  remains at the desired vacuum, a gasketed screw  77  is then inserted into the threaded section  72  of opening  70  to hold the ball  75  in place against O-ring  74 , as shown in FIG. 7C. A separate gasketed screw, not shown, mounted in cap  46  can be loosened to allow air back into the chamber  51 . A miniature pump-down port arrangement, such as illustrated in FIGS. 7A-7C, greatly enhances the utility of the CCD camera by allowing it to remain very compact while providing vacuum pump-down capability. Other types of pump-down arrangement may be effectively utilized to obtain and retain a vacuum condition within the CCD chamber of the camera head. 
     The camera head chassis, as illustrated in FIGS. 1,  7 ,  8  and  9  may be made up of a series of ring members or“donuts” that can be clamped together, as described above with respect to FIG.  1 . FIG. 8 illustrates a CCD camera head generally similar to FIG. 7, but additionally includes an end cap or cover member, whereby the PC boards are enclosed. The arrangement of the chassis  20  as shown in FIG. 8 is unique in that the ring members or “donuts”  80 ,  81  and  82 , together with an end cap or member  83 , form a tight press fill against the ends  16  and  17  of the internal cores or heat sinks  13 ,  14  and  15  which extend from the ends of PC boards  10 ,  11  and  12 , as described above with respect to FIGS. 1 and 2. The end cover  83  and ring members  80 ,  81  and  82  are tightly secured together and to member or heat sink  44  by screws  24 ′, as seen in FIG. 8, whereby the ends  16  and  17  of internal heat sinks  13 - 15  secured in cut-aways  23 ′, and produces a thermal path for the heat to escape from within the camera head. To remove the heat from the camera chassis  20 , a conventional heat exchanger arrangement may be used. 
     The chassis  20  is designed to be placed in a vacuum where outgasing from the camera head can be a problem. The chassis assembly illustrated in either FIG. 8 or FIG. 9 is not air tight and has been purposely designed to allow air to pass through the chassis. This will speed up the pump-down rate in the vacuum chamber and not cause the camera head to appear as a virtual leak. The“donut” shape of the chassis rings is more evident from FIG. 9 where a ring member  80 ′ and an end cover  83 ′ secure a PC board  10 ′ therebetween, such that the integral internal core  13 ′ of PC board  10 ′ is secured in cut-aways  23 ′ in the members  80 ′ and  83 ′, when the members are  83 ′ and  80 ′ are secured together via screws  24 ′ which pass through openings  85  in cover member  83 ′ (four openings  85  shown) and through cut-aways  86  and  87  in PC board  10 ′ and heat sink  13 ′, and terminate in threaded openings  88  in heat sink  44 , as shown in FIG.  8 . Note that the screws  24 ′ extend within the outer walls of the ring members  80 ,  81 , and  82  of FIG. 8, and not openings in members  80 - 82 , as more readily seen in FIG.  9 . 
     The “donut” or ring member assembly of the chassis will accept production tolerances in the manufacture of the thermal core or internal heat sink of the PC boards, allowing good thermal contact between the PC board and the chassis, while additionally providing strength to the overall package due to the interconnection of adjacent “donuts”. 
     It has thus been shown that the present invention provides a vacuum compatible miniature CCD camera head. By the use of PC boards with integral internal heat sinks which have ends or edges extending from the PC material and secured in the chassis, good thermal conduction from the electronic components of the PC boards to the chassis is provided, thereby the size of the camera head can be reduced and has sufficient cooling thereof. By providing the camera head with a vacuum chamber, the camera can be used in either vacuum or atmosphere environments. 
     While particular embodiments, materials, and parameters have been set forth to exemplify and describe the principles of the invention, such are not intended to be limiting. Modifications and changes may become apparent to those skilled in the art, and it is intended that the invention be limited only by the scope of the appended claims.