Abstract:
A new heat sink apparatus and method that simplify the assembly of the heat sink and thermal stud. The new heat sink assembly uses a spring retainer that, in most cases, can use existing socket mounting screws. A spring clip ( 202 ) presses a thermal stud ( 204 ) against the back of an electrical device package ( 206 ). The present invention is especially useful for attaching a spatial light modulator to a printed circuit board ( 106 ) since it provides a simple, reliable heat sink without blocking the light path to and from the device. The preceding abstract is submitted with the understanding that it only will be used to assist in determining, from a cursory inspection, the nature and gist of the technical disclosure as described in 37 C.F.R. § 1.72(b). In no case should this abstract be used for interpreting the scope of any patent claims.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is a Divisional of application Ser. No. 10/210,681, filed Aug. 1, 2002 now U.S. Pat. No. 6,816,375 and Provisional Application No. 60/310,439, filed Aug. 3, 2001. 
   The following patents, and/or commonly assigned patent applications are hereby incorporated herein by reference: 
   
     
       
             
             
             
             
             
             
           
             
             
             
             
             
           
         
             
                 
             
             
               Patent No./ 
               Filing 
               Publi- 
               Publication 
               Issue 
                 
             
             
               Serial No. 
               Date 
               cation No. 
               Date 
               Date 
               Title 
             
             
                 
             
           
           
             
                 
             
           
        
         
             
               10/186,410 
               Jul. 01, 2002 
               2003- 
               Jan. 30, 
               DMD 
             
             
                 
                 
               0020882 
               2003 
               Heat Sink 
             
             
                 
                 
                 
                 
               Socket 
             
             
                 
                 
                 
                 
               Assembly 
             
             
                 
             
           
        
       
     
   

   FIELD OF THE INVENTION 
   This invention relates to the field of electronic assemblies, more particularly to thermal management in spatial light modulator display systems. 
   BACKGROUND OF THE INVENTION 
   Proper thermal dissipation is necessary to ensure reliable operation of many electronic devices. Many devices use heat sinks to allow thermal energy to be exhausted to either a cold wall or a cooling stream of air. Spatial light modulators, such as the digital micromirror device or DMD, provide unique thermal challenges. Not only does the electrical operation of the DMD create heat, the micromirrors absorb a portion of the intense beam of light focused on them, dramatically increasing the temperature of the micromirror device. Furthermore, the necessity of a free optical path to the surface of the micromirror array prevents the use of conventional heat sink methods. 
   A conventional DMD package and heat sink is shown in  FIG. 1 . In  FIG. 1 , the micromirror array typically is packaged in a ceramic or plastic substrate  102  sealed with a glass lid  104 . Electrical connections between contacts on the rear surface of the micromirror package and a printed circuit board  106  are provided by an interposer  108 . Several types of interposers  108  may be used. The interposer  108  may be an insulative resilient material having conductive portions in contact with the contacts on the micromirror package and electrical contacts on the printed circuit board. Alternatively, a plastic interposer  108  with metal spring contacts may be used. The DMD and interposer  108  are held in contact by a socket, not shown in  FIG. 1 , mounted over the DMD. 
   To remove heat from the DMD, an opening through the printed circuit board is used to allow a thermal stud  110  to contact the micromirror package substrate  102  from the back. A heat sink  112  is attached to the end of the thermal stud. The thermal stud  110  is epoxied to the back of the micromirror device using an epoxy patch  114 . Another epoxy patch  116 , or mechanical fasteners such as machine screws, are use to attach the heat sink  112  to the thermal stud  110 . While this arrangement is quite effective to remove the heat from the micromirror package, it is difficult and expensive to produce. 
   The method used to attach the thermal stud to the package begins by inspecting the thermal stud  110  for defects, indentations, and sharp corners. The electrostatic integrity of the stud attachment fixture is then verified. The fixture pocket is cleaned, and then the micromirror package is loaded into the fixture pocket with the bottom surface up. The bonding surfaces of both the package and the stud are then wiped clean. An alignment plate is laced over the device in the stud attachment fixture. The thermal stud is placed on a hot plate to warm it. An epoxy patch is attached to the bonding surface of the thermal stud and the thermal stud is placed through the opening of the alignment plate and onto the micromirror device. A weight is then placed on the thermal stud and the epoxy patch is allowed to cure. When the epoxy patch has cured, the attachment is subjected to a shear force to verify the integrity of the attachment. 
   The process is not only labor intensive, it is also difficult to control. Due to inconsistencies from batch to batch of the epoxy patches and other process variations, the thermal stud attachment process often fails to achieve sufficient bond strength. If the bond fails the device must be scrapped because the thermal stud cannot be epoxied onto the micromirror a second time. An improved apparatus and method of extracting heat from the micromirror package is needed. 
   SUMMARY OF THE INVENTION 
   Objects and advantages will be obvious, and will in part appear hereinafter and will be accomplished by the present invention which provides a method and system for attaching a thermal stud to an electronic package. One embodiment of the present invention provides a method and apparatus for attaching a heat sink. 
   Another embodiment of the present invention provides a method of attaching a heat sink to an electronic device. The method comprises: providing an anchor surface having a first side and a second side; positioning an electronic device on the first side; placing a heat sink against the electronic device, the heat sink extending to the second side of the anchor surface; and retaining the heat sink against the electronic device using a spring clip. 
   Another embodiment of the present invention provides an electronic assembly comprising: an anchor surface having a first side and a second side; an electronic device on the first side; a heat sink in thermal communication with the electronic device, the heat sink extending to the second side of the anchor surface; and a spring clip holding the heat sink against the electronic device. 
   Another embodiment of the present invention provides an electronic assembly comprising: an anchor surface having a first side and a second side; an electronic device on the first side; means for heat sinking the electronic device in thermal communication with the electronic device and extending to the second side of the anchor surface; and means for holding the heat sink against the electronic device. 
   Another embodiment of the present invention provides a display system comprising: a light source for providing a beam of light along a light path; a display controller for providing image data; an anchor surface having a first side and a second side; spatial light modulator on the first side, the spatial light modulator on the light path and operable to modulate the beam of light in response to the image data; a heat sink in thermal communication with the spatial light modulator, the heat sink extending to the second side of the anchor surface; and a spring clip holding the heat sink against the spatial light modulator. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which: 
       FIG. 1  is a side view of a thermal stud and heat sink attached to a digital micromirror package according to the prior art. 
       FIG. 2  is side view of a thermal stud attached to an electronic device using a spring clip of the present invention. 
       FIG. 3  is a side view of a thermal stud and heat sink attached to an electronic device using the spring clip of the present invention present invention anchored to a heat sink socket. 
       FIG. 4  is plan view of the heat sink attached to the electronic device using the spring clip of  FIG. 3 . 
       FIG. 5  is a side view of a clip according to one embodiment of the present invention. 
       FIG. 6  is a plan view of a clip according to one embodiment of the present invention. 
       FIG. 7  is a side view of a clip according to another embodiment of the present invention. 
       FIG. 8  is a plan view of a clip according to another embodiment of the present invention. 
       FIG. 9  is a cross section side view of a spring clip retainer according to another embodiment of the present invention. 
       FIG. 10  is a cross section side view of a one piece thermal stud and heat sink held against an electronic package by a spring clip according to one embodiment of the present invention. 
       FIG. 11  is a cross section side view of a one piece thermal stud and heat sink held against an electronic package by a spring clip according to yet another embodiment of the present invention. 
       FIG. 12  is a schematic view of a micromirror-based projection system utilizing a thermal stud and heat sink attached to a micromirror device according to the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   A new heat sink apparatus and method have been developed that greatly simplify the assembly of the heat sink and thermal stud. The new assembly process eliminates many assembly steps and their associated problems and opportunities for failure. The resulting apparatus is mechanically robust and exceeds the thermal performance requirements. The new heat sink assembly uses a spring retainer that, in most cases, can use existing spatial light modulator socket mounting screws. The new retainer satisfies electromagnetic interference, electrostatic discharge, grounding and insulation requirements. Additionally, the new heat sink mount is extremely reliable, eliminating part failures, product rejections, and returns. 
     FIG. 2  is a cross section side view of a thermal stud attached to a digital micromirror illustrating one embodiment of the present invention. In  FIG. 2 , a spring clip  202  presses a thermal stud  204  against the back of an electrical device package  206 . The thermal stud  204  may be any heat conductive material, and typically is aluminum. The electrical device package  102  typically houses a spatial light modulator such as a DMD or liquid crystal on silicon device. The package  206  shown in  FIG. 2  uses an interposer  108  to provide electrical connection between the package  102  and the printed circuit board  106 . 
   The spring clip  202  of  FIG. 2  rests in a groove formed in the top surface of the thermal stud  204 . The groove is not necessary to retain the thermal stud  204  since the printed circuit board  106  and interposer  108  restrain the thermal stud in two dimensions while the spring clip  202  and device package  102  restrain the thermal stud  204  in the third direction. Alternatively, screws or other fastening means such as adhesives, retaining fingers, slots, or channels, or mechanical fasteners may be used to attach the thermal stud  204  to the spring clip  202 . The spring clip  202  of  FIG. 2  is attached directly to the printed circuit board  106  by means of screws  208 . A heat sink, not shown, may be attached to the thermal stud  204  by a variety of means including mechanical restraints and fasteners such as screws. 
   While the present invention will be described in terms of attaching the electronic device  102  to a printed circuit board  106 , it should be understood that the term printed circuit board is intended to include any many other structures or types of anchor surfaces. For example, the electronic device  102  may be attached to a plastic or metal sheet, perhaps forming a reference plane to locate various other components or to provide additional thermal dissipation. If a metal sheet is used in place of the printed circuit board the interposer typically would be insulative or merely would serve to ground the electronic device. The only requirement is that the heat sink or thermal stud extend from the electrical component on one side of the printed circuit board through the printed circuit board to the thermal heat sink on the other side of the printed circuit board. 
     FIG. 3  is a cross section side view of a thermal stud  304  and heat sink  312  attached to a electrical device, shown as a digital micromirror device, using the spring clip  302  of the present invention present invention anchored to the heat sink socket  312 . In  FIG. 3 , a socket  314  is shown on the front side of the printed circuit board  106 . The screws  206  retaining the spring clip  302  extend through the printed circuit board  106  and engage the socket  314 . The socket  314  has an open region through which light is allowed to enter and exit the window  104  on the top of the spatial light modulator package  102 . Tension from the spring clip  302  is sufficient to provide good thermal contact between the thermal stud  304  and the package  102 . Thermal grease, or thermally conductive tape may be used to improve the thermal conductivity between the package  102 , thermal stud  304 , and heat sink  312  in any of the embodiments of the present invention. 
     FIG. 4  is plan view of the heat sink and spring clip attached to the electronic device of  FIG. 3 . In  FIG. 4 , the two screws  206  used to attach the spring clip  302  to the heat sink  312  are optionally offset from the centerline of the spring clip  302 . 
     FIG. 5  is a side view of a spring clip  206  according to one embodiment of the present invention.  FIG. 6  is a plan view of the spring clip  206 . Assuming the flat spring load is calculated based on a standard beam formula and that the system can be assumed to be a cantilever load, the force exerted by the spring clip  206  is: 
           Force   =         Ebt   3     ⁢   F       4   ⁢     L   3               
where: E=Young&#39;s Modulus
         F=Deflection   b=Width   L=Length   t=Thickness       
   The spring clip  206  of  FIGS. 5 and 6  is 1075/1095 spring steel with a Young&#39;s Modulus of 11.6*10 6 . The spring clip  206  is 0.015 inches thick, 0.280 inches wide, and 0.390 inches long. When installed, there is a 0.133 inch deflection, resulting in a force of approximately 6.08 pounds. The force may be varied over a fairly wide range by changing the dimensions and materials used. Table 1 lists the force achieved by various combinations of deflection and spring thickness. 
   
     
       
             
             
           
             
             
             
             
           
         
             
                 
             
             
               Material Thickness 
               Deflection (Force) 
             
             
                 
             
           
           
             
                 
             
           
        
         
             
               0.015″ 
               0.100″ (4.6#)  
               0.133″ (6.08#) 
               0.150″ (6.87#) 
             
             
               0.018″ 
               0.100″ (7.9#)  
               0.133″ (10.5#) 
               0.150″ (11.8#) 
             
             
               0.020″ 
               0.100″ (10.9#) 
               0.133″ (14.4#) 
               0.150″ (16.3#) 
             
             
                 
             
           
        
       
     
   
     FIG. 7  is a side view of an alternative spring clip  702  according to another embodiment of the present invention.  FIG. 8  is a plan view of the spring clip  702  of  FIG. 7 . Spring clip  702  is formed using music wire or other materials. 
     FIG. 9  is a cross section side view of a spring clip  902  retainer according to another embodiment of the present invention. In  FIG. 9 , a spring clip  902  has hooks  904  on each end of the spring clip  902 . The hooks  904  engage similar hooks  906  attached to the printed circuit board  106  or other reference structure. 
     FIG. 10  is a cross section side view of a one piece thermal stud and heat sink  1004  held against an electronic package  102  by a spring clip  1002  according to one embodiment of the present invention. In  FIG. 10 , hooks  1006  formed in the ends of the spring clip engage the printed circuit board  106 , or other reference structure, to hold the spring clip  1002  in place and to apply a force to the heat sink  1004 . 
     FIG. 11  is a cross section side view of a one piece thermal stud and heat sink  1004  held against an electronic package  102  by a spring clip  1102  according to yet another embodiment of the present invention. In  FIG. 11 , hooks  1106  formed in the ends of the spring clip engage the socket  1114  used to hold and align the electronic device  102 . Depending on the application, retaining screws  206  may be eliminated and the entire assembly comprising the electronic device  102 , the socket  1114 , and the heat sink  1004  held in place by compression provided by the retaining spring clip  1102  and the interposer  108 . In  FIG. 11 , some restraint would be required to prevent the socket from moving away from the printed circuit board  106  if the retaining screws were removed. This restraint could be provided by an additional bend in the retaining spring clip  1102  that would apply pressure to the heat sink side of the printed circuit board  106 . Alternatively, the heat sink  1004  could be shaped to contact the printed circuit board, or a resilient member could be placed between the heat sink  1004  and the printed circuit board  106  to keep the socket  1114  pressed against the printed circuit board. 
     FIG. 12  is a schematic view of an image projection system  1200  using spatial light modulator  1202  in thermal communication with a heat sink as described above. In  FIG. 12 , light from light source  1204  is focused on the spatial light modulator  1202  by lens  1206 . Although shown as a single lens, lens  1206  is typically a group of lenses and mirrors which together focus and direct light from the light source  1204  onto the surface of the spatial light modulator  1202 . Image data and control signals from controller  1214  cause the modulator to selectively modulate in incident beam of light. In the case of a micromirror device, the image data and control signals cause some mirrors to rotate to an on position and others to rotate to an off position. Mirrors on the micromirror device that are rotated to an off position reflect light to a light trap  1208  while mirrors rotated to an on position reflect light to projection lens  1210 , which is shown as a single lens for simplicity. Projection lens  1210  focuses the light modulated by the spatial light modulator  1202  onto an image plane or screen  1212 . 
   Thus, although there has been disclosed to this point a particular embodiment for a heat dissipation attachment device and method therefore, it is not intended that such specific references be considered as limitations upon the scope of this invention except insofar as set forth in the following claims. It is intended that the features of the various embodiments, such as materials, structures, attachment means, types of thermal studs and heat sinks, be interchangeable to the greatest extent possible without departing from the express teachings of this disclosure. Furthermore, having described the invention in connection with certain specific embodiments thereof, it is to be understood that further modifications may now suggest themselves to those skilled in the art, it is intended to cover all such modifications as fall within the scope of the appended claims. In the following claims, only elements denoted by the words “means for” are intended to be interpreted as means plus function claims under 35 U.S.C. § 112, paragraph six.