Abstract:
A display having improved thermal management and a method for producing the display are disclosed. The display includes a pixel structure adjacent a front panel with thermo-mechanical elements extending between a back panel and the pixel structure to dissipate heat generated by the pixel structure.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is related to and claims the benefit of U.S. Provisional Application No. 60/379,456, filed May 10, 2002, for “Array Electrical Interconnections.” 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to the field of electronic displays and, more particularly, to thermal management of these electronic displays. 
     BACKGROUND OF THE INVENTION 
     Many presently available electronic displays employ one or more arrays of picture elements (hereinafter “pixels”) to display an image. Each pixel typically includes a light emitting material that emits light when a current is passed therethrough to illuminate the pixel. The current passing through the light emitting material of an illuminated pixel and through current supply lines supplying current thereto generates heat in the electronic display. 
     Generally, the output characteristic of each pixel within an array is thermally sensitive. When the heat generated from illuminating the pixels within the array is not properly managed (i.e., dissipated), the array may develop localized “hot spots,” which are small areas of an array that are significantly hotter than surrounding areas. These hot spots may lead to changes in the output characteristics of individual pixels or groups of pixels within the array, thereby causing different output characteristics to develop in individual pixels and groups of pixels within the array. These hotspots may also reduce the image quality of an electronic display and reduce its useful life. 
     Accordingly, displays with improved thermal management and methods for producing such displays are needed. The present invention fulfills this need among others. 
     SUMMARY OF THE INVENTION 
     A display in accordance with the present invention includes a front panel and a back panel spaced from the front panel to define a space therebetween. At least one pixel structure is adjacent the front panel in the space between the front and back panels and a plurality of electrical connections extend between the back panel and the at least one pixel structure. A plurality of thermo-mechanical elements extend between the back panel and the at least one pixel structure to dissipate heat from the at least one pixel structure toward the back panel. At least a portion of at least one of the thermo-mechanical elements is positioned between adjacent pixel structures of the at least one pixel structure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a front plan drawing of a tiled display (with two tiles removed) in accordance with the present invention; 
         FIG. 2  is a perspective drawing of the back side of a tile suitable for use in the tiled display of  FIG. 1 ; 
         FIG. 3  is an exploded perspective drawing of a tile in accordance with the present invention; 
         FIG. 4  is a pixel diagram of an exemplary pixel and connection layout for portions of four tiles in accordance with the present invention; 
         FIG. 5  is a top view of a display material formed upon a column electrode in accordance with the present invention; 
         FIG. 5A  is a cross-sectional view taken along line A—A of  FIG. 5  depicting column and row electrodes along a column electrode; 
         FIG. 5B  is a cross-sectional view taken along line B—B of  FIG. 5  depicting column and row electrodes along a row electrode; 
         FIG. 5C  is a cross-sectional view similar to the cross-sectional view of  FIG. 5B  with an additional insulating layer; 
         FIG. 6A  is a cross-sectional view of an assembled tile along a column electrode in accordance with the present invention; and 
         FIG. 6B  is a cross-sectional view of an assembled tile along a row electrode in accordance with the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The invention is described in terms of exemplary embodiments, which are illustrated in the drawing figures. The drawing figures are not to scale and may be exaggerated to aid in the description of the invention. Although the invention is described in terms of an organic light emitting diode (OLED) display device, it is contemplated that it may be practiced with other emissive display technologies employing elements such as electroluminescent elements, light emitting diodes, field emissive elements, plasma elements, or cathodoluminescent elements; or with reflective display technologies employing elements such as bistable, reflective cholesteric (BRC) liquid crystal elements. 
       FIG. 1  is a front plan view of a partially assembled exemplary display  100  according to one aspect of the present invention. The display  100  is a tiled display in which emissive or reflective elements that form pixel structures (represented by pixel  102 ) are built in relatively small arrays to form tiles  104 . The illustrated tiles  104  each include sixteen pixels, however, each tile may contain fewer pixels or more pixels, e.g. tens, hundreds, or even thousands of pixels. The tiles  104  are then assembled into a frame  106  to produce the display  100 . Alternatively, the tiles  104  may be assembled side-to-side in rows and columns without a frame. In this instance, the individual tiles may be held together by millions. The display  100  is shown in  FIG. 1  with two tiles  104   a  and  104   b  missing. These tiles are inserted into the display in a first position  108  and a second position  110  to complete the display  100 . Although the invention is described in terms of a tiled display that includes a plurality of tiles, those of skill in the art will recognize that the invention can be used in non-tiled displays as well. 
       FIG. 2  is a back plan view of a tile  104  suitable for use in the display  100  of FIG.  1 . The tile  104  includes at least one integrated circuit  200  mounted on a circuit board  202 . Conductive traces  204  coupled to vias (not shown) extend through the circuit board  202  to connect the integrated circuit  200  to the pixel structures  102  ( FIG. 1 ) on the front of the tile  104 . 
       FIG. 3  is an exploded perspective diagram that shows an exemplary display tile  300 . The exemplary display tile  300  is formed in two parts: the display module  302  and the circuit module  304 . In an exemplary embodiment, these two parts are formed separately and then joined to form a complete tile. 
     The display module  302  includes a transparent front panel  306 , e.g., a float glass plate. A plurality of pixel structures are formed adjacent the front panel  306 . Each pixel structure includes a column electrode  308 , display material  310 , and a row electrode  312 . The column electrodes  308  are formed on the front panel  306 . In an exemplary embodiment, the column electrodes  308  are formed by depositing thin bands of a transparent conductor, e.g., indium-tin oxide (ITO), using well known processes. 
     The display materials  310  are then deposited on the column electrodes  308  to define the active area of the pixel structure, which is described in further detail below. In an exemplary embodiment, the display materials  310  are red, green, and blue OLED materials that are selectively deposited on top of the column electrodes  308  to form a “color” display tile  300 . 
     The row electrodes  312  are then formed on the display materials  310 . In the illustrated embodiment, the row electrodes  312  are substantially perpendicular to the column electrodes  308  and together form a grid pattern that allows each of the active pixel areas to be addressed by specifying a column number and a row number. In an exemplary embodiment, the row electrodes  312  are formed from a polysilicon material or from a metal such as aluminum using standard deposition techniques. 
     An insulating layer  314  is formed on top of the row electrodes  312 . The exemplary insulating layer  314  may be formed from any of a number of insulating materials. In an exemplary embodiment, the insulating layer  314  is desirably formed using low-temperature processes to protect the display materials  310 . Exemplary insulating layers  314  known low-temperature inorganic materials, that can be formed using low-temperature processes. The insulating layer  314  may be applied using thick film or thin film deposition techniques. The insulating layer  314  includes a plurality of openings  316  enabling electrical connection with the row electrodes  312  or column electrodes  308  of the pixel structures and enabling thermo-mechanical connections to one or more locations within the display module  302 . The formation of electrical connections and thermo-mechanical connections are described in further detail below. 
     On top of the insulating layer  314  are deposited a plurality of conductor traces  318 . In an exemplary embodiment, the conductor traces  318  are formed using vapor deposited aluminum or a metallic ink or paste, such as silver combined with a solvent, which is deposited using thick film processes. Each of the conductor traces  318  is electrically coupled to one of the column electrodes  308  or one of the row electrodes  312 , and/or thermo-mechanically connected to one or more positions within the display module  302 , by vias (not shown) that extend through the openings  316  in the insulating layers  314 . Via is used in the broadest sense and includes conductors that go through openings in the layer(s) and those that go around the edge of a layer(s). 
     Each of the exemplary conductor traces- 318  makes electrical contact with only one row electrode  312  or one column electrode  308 . To ensure that a good connection is made, however, each conductor trace  318  may connect to its corresponding row or column electrode  312 ,  308  at several locations. Because each conductor trace  318  makes electrical contact with only one row or column electrode, the number of conductor traces  318  is greater than or equal to the sum of the number of column electrodes  308  and the number of row electrodes  312  in the tile  300 . 
     The circuit module  304  includes image processing and display driving circuitry  200  (FIG.  2 ), a circuit board  202 , conductive traces  204 , and connecting pads  320 . The circuit board  202  is a back panel that is spaced from the front panel  306  to accommodate the pixel structures in a space therebetween. 
     Vias  322  electrically connect the conductive traces  204  to the connecting pads  320  through the circuit board  202 . In an exemplary embodiment, the conductive traces  204 , vias  322 , and connecting pads  320  are formed using thick film deposition processes to apply a metallic ink or paste. In an alternative exemplary embodiment, the connecting pads  320  are formed from vapor-deposited aluminum. In an exemplary embodiment, each connecting pad  320  of the circuit module  304  corresponds to a conductor trace  318  of the display module  302 . 
     The display module  302  and the circuit module  304  are combined to form the display tile  300 . In an exemplary embodiment, the connecting pads  320  are electrically connected to the corresponding conductor traces  318  by applying an anisotropically conductive adhesive between the display module  302  and the circuit module  304 . Alternative methods for electrically connecting the connecting pads  320  to the conductor traces  318  will be readily apparent to those of skill in the art. 
       FIG. 4  shows a pixel structure layout  400  suitable for use in a display such as that shown in FIG.  1 .  FIG. 4  illustrates portions of 4 tiles  402 ,  404 ,  406 ,  408 . In the layout shown in  FIG. 4 , active portions of the pixel structures (represented by active portions  410 ) are positioned within respective pixel structure regions (represented by pixel region  412 ). Row electrodes (see  FIG. 3 ) and column electrodes (see  FIG. 3 ) may be electrically coupled by electrical connections  414  and  416 , respectively, to corresponding vias on the circuit module  304  (FIG.  1 ). In certain exemplary embodiments, the electrical connections  414  and  416  are vias formed from a conductive material such as indium-tin (InSn) solder or a silver-filled epoxy adhesive. 
     In an exemplary embodiment of the present invention, thermo-mechanical elements  418  are provided to thermally couple the pixel structures to the circuit module  304  (FIG.  1 ). The thermo-mechanical elements  418  may be positioned throughout each pixel structure and, in certain embodiments, are positioned between adjacent pixel structures within the display module  302 . For example, thermo-mechanical elements  418  may be placed between each active pixel area not having an electrical connection via  414 ,  416 ; next to the electrical connections; under the active pixel area (element shown in phantom); or essentially anywhere on and in the vicinity of the display module  302  to dissipate heat from the display module  302  to the circuit module  304 . 
     In certain exemplary embodiments, the thermo-mechanical elements  418  provide a redundant electrical connection between the row and column electrodes and the circuit module  302 . In these embodiments, the thermo-mechanical elements  418  may be vias formed from the same materials as the electrical connections  414 ,  416 , e.g., InSn solder or a silver filled epoxy adhesive. In certain other exemplary embodiments, one or more of the thermo-mechanical elements  418  are electrically non-functional. In accordance with this embodiment, the thermo-mechanical elements  418  may be separated from conductors on one or both ends of the via  418  with a passivation layer or may be formed from a dielectric material such as an epoxy filled with materials having suitable thermal conduction properties, e.g., diamond, BN, AlN, and/or SiC. The selection of suitable material for forming the thermo-mechanical elements  418  will be readily apparent to those of skill in the related arts. 
       FIG. 5  shows a view of a portion of a display module of an electronic pixel structure according to the present invention.  FIGS. 5A and 5B  illustrate cross-sectional views taken along lines A—A and B—B, respectively, of  FIG. 5  to illustrate an exemplary pixel structure according to the present invention. 
     A transparent column electrode  308 , e.g., ITO is formed on the front panel  306 . A display material  310  formed upon the column electrode  308  defines the active portion  410  ( FIG. 4 ) of the pixel structure. As shown in  FIGS. 5A and 5B , an insulator  502  such as SiO 2  is then deposited on the ends of the display material  310 . A row electrode  312  is then formed upon the display material  310  and the insulator  502 . The insulator  502  allows the row electrode  312  to be formed wide enough to completely encapsulate the display material without shorting the row electrode  312  to the column electrode  308 . Thus, the row electrode  312 , the insulator  502 , the column electrode  308 , and possibly the front plate  306  encapsulate the display material  310 . This encapsulation seals the display materials  310  to help prevent exposure of the display materials  310  to conditions including oxygen and water vapor to provide more predictable performance over a longer lifetime. In another exemplary embodiment of the present invention, the insulator  502  covers the display material  310  and the row electrode  312  contacts the display material  310  through a via (not shown) formed in the insulator  502 . 
       FIG. 5  illustrates exemplary positions of electrical connections  414  and  416  for electrically coupling row electrodes and column electrodes, respectively, to the circuit module  304 . In addition, thermo-mechanical elements  418  for thermally connecting to the pixel structure are illustrated. In  FIG. 5B , an electrical connection  414  is shown as formed upon the row electrode  312  passing between adjacent pixel structures. The conductor  414  shown in  FIG. 5B  is illustrated as a conductive bump. In an exemplary embodiment, one or more additional electrical connections  414  and/or thermo-mechanical elements  418  are formed on the row electrode  312  in a similar manner. In certain exemplary embodiments, the thermo-mechanical elements  418  provide redundant electrical connections. In alternative embodiments, a passivation layer (not shown) is positioned on at least one end of the thermo-mechanical elements  418  or the thermo-mechanical elements  418  are formed from a dielectric material. Thus, in accordance with these embodiments, the thermo-mechanical via is electrically non-functional. 
       FIG. 5C  is a cross-sectional view taken along line B—B of  FIG. 5  illustrating an insulating pad  504  formed upon the row electrodes  312 , column electrodes  308 , and the display materials  310 . In an exemplary embodiment, the electrical connections  414  and the thermo-mechanical elements  418  are formed in apertures in the insulating pad  504 . In certain exemplary embodiments, electrically non-functional thermo-mechanical elements may be placed directly over or adjacent to the active elements and on top of the insulating pad  504 . 
     In the embodiment illustrated in  FIGS. 5-5C , the electrical connections  414 ,  416  are connected directly to the row and column electrodes  312 ,  308 , respectively. In an alternative embodiment, one or more of the electrical connections  414 ,  416  may be connected to a corresponding row and/or column electrode through a conductive trace (not shown) to allow greater flexibility in the placement of the connections  414 ,  416 . The thermo-mechanical element  418  is illustrated as connected directly to the row and/or column electrodes  312 / 308 . In alternative exemplary embodiments, the thermo-mechanical elements  418  may be connected to various other locations within the pixel structure to dissipate heat formed in that structure. In certain exemplary embodiments, one or more of the thermo-mechanical elements  418  may be connected to row or column electrodes and/or various other locations within the pixel structure through conductive traces (not shown) to allow greater flexibility in the placement of the elements  418 . The formation of a suitable conductive trace will be readily apparent to those of skill in the art. 
       FIGS. 6A and 6B  are cross-sectional views of an assembled electronic display tile  600  viewed, respectively, along a column electrode  308  and along a row electrode  312  in accordance with the present invention. The tile  600  includes a display module  302  and a circuit module  304 , each comprised of multiple layers. The display module  302  includes a front panel  306  and a pixel structure (represented by pixel structure  602 ) adjacent the front panel  306 . Each pixel structure includes a column electrode  308 , a display material  310 , and a row electrode  312 . The circuit module  304  is a back panel positioned substantially parallel to the front panel  306 . The circuit module  304  is spaced from the front panel  306  to define a space therebetween in which the pixel structure  602  is formed. Integrated circuits (represented by integrated circuit  604 ) are positioned upon the circuit module  304  on a side of the circuit module opposite the display module. The integrated circuit  604  is connected to the circuit module  304  in a conventional manner, e.g., with solder  605 . Many tile components depicted in  FIG. 3  are omitted to facilitate description of the present invention, e.g., circuit module vias  322 , circuit module conductive traces  204 , insulating layers  314 , conductor traces  318 , and connecting pads  320 . 
       FIG. 6A  illustrates a plurality of connections  606  that extend from the back panel  304  toward the front panel  306 . One or more of the connection  606  are electrical connections, e.g., connections  606   a . The electrical connections  606   a  extend between the circuit module  304  and the pixel structure  602 . Specifically, the electrical connections  606   a  extend from the circuit module  304  to a column electrode  308  of the pixel structure  602 . In addition, one or more of the connections are thermo-mechanical elements, e.g., connections  606   b , that dissipate heat generated in the pixel structure  602  toward the circuit module  304 . In certain exemplary embodiments, the thermo-mechanical elements  606   b  also provide redundant electrical connections. 
     In an exemplary embodiment, the thermo-mechanical elements  606   b  have a larger cross-sectional area than the electrical connections  606   a  to improve the thermal transfer capabilities of the thermo-mechanical elements. In certain exemplary embodiments, the thermo-mechanical elements  606   b  are sized to maximize thermal transfer capabilities without adversely affecting the operation of the display. Although the thermo-mechanical elements  606   b  in  FIG. 6A  are shown as extending from the circuit module  304  to the column electrode  308  near the front panel  306 , the thermo-mechanical elements  606   b  may extend to essentially any depth and may be positioned at essentially any location on the circuit module  304 . Those of skill in the art will recognize that the connections  606  may include conductor traces  318  and connecting pads  320  ( FIG. 3 ) present between the circuit module  304  and the depth to which the connection  606  extends. 
       FIG. 6B  illustrates a plurality of connections  608  that extend from the back panel  304  toward the front panel  306 . One or more of the connection  608  are electrical connections, e.g., connections  608   a . The electrical connections  608   a  extend between the circuit module  304  and the pixel structure  602 . Specifically, the electrical connections  608   a  extend from the circuit module  304  to a row electrode  312  of the pixel structure  602 . In addition, one or more of the connections are thermo-mechanical elements, e.g., connections  608   b , that dissipate heat generated in the pixel structure  602  toward the circuit module  304 . In certain exemplary embodiments, the thermo-mechanical elements  606   b  also provide a redundant electrical connection. 
     Although the thermo-mechanical elements  608   b  in  FIG. 6B  are shown as extending from the circuit module  304  to the row electrode  312  near the front panel  306 , the thermo-mechanical elements  608   b  may extend to essentially any depth and may be positioned at essentially any location on the circuit module  304 . For example, a thermo-mechanical element  607  may be positioned directly under a pixel structure. Those of skill in the art will recognize that the connections  608  may include conductor traces  318  and connecting pads  320  ( FIG. 3 ) present between the circuit module  304  and the depth to which the connection  608  extends. 
     Referring to  FIGS. 6A and 6B , in certain exemplary embodiments, an underfill encapsulant  610  encapsulates the area surrounding the components in the space defined by the circuit module  304  and the front panel  306 . In an exemplary embodiment, the underfill encapsulant  610  is selected based on its heat dissipation properties. When positioned within the space defined by the circuit module  304  and the front panel  306 , the underfill encapsulant  610  dissipates heat generated by the pixel structure to the circuit module  304 . In certain exemplary embodiments, the underfill encapsulant is an alumina filled epoxy such as EPO-TEK H77 supplied by Epoxy Technoloy, Inc. of Billerica, Mass., USA. In certain other exemplary embodiments, the underfill encapsulant includes a filler material including diamond, BN, AlN, BeO and/or SiC. In certain other exemplary embodiments, the filler material includes small concentrations of highly conductive metal particles such as Al and Cu by themselves and in combination with non-metallic fillers. 
     In certain exemplary embodiments, the circuit module  304  includes at least one layer selected based on its heat dispersion properties. In an exemplary embodiment, the at least one layer dissipates at least the heat received from the pixel structure  602  at the circuit module  304  via the thermo-mechanical elements  606   b ,  608   b . In certain exemplary embodiments, the at least one layer dissipates heat received from the pixel structure  602  at the circuit module  304  via the electrical connections  606   a ,  608   a  and/or the underfill encapsulant  610  as well. In an exemplary embodiment, the at least one layer is formed from a material selected from alumina or aluminum nitride. 
     In certain exemplary embodiments, reflections from the row electrodes  312  and the connections  606 ,  608  are minimized by minimizing their surface area or by coating the “viewer side” of these components black. The column electrodes  308 , which in an exemplary embodiment are transparent, are not an issue since they reflect only a small amount of light. Coating the viewer side of the row electrodes  312  and the connections  606 ,  608  black can be accomplished by first depositing a conductive black coating (e.g. carbon black) in all areas where viewable metal electrodes or connections will be later deposited. In certain embodiments, the shape of the connections may be such that reflections are minimized, e.g, having an oval cross-section with the widest portion perpendicular to the nearest active pixel area  410  (FIG.  4 ). Various other techniques and shapes for minimizing reflections will be readily apparent to those of skill in the art and are considered within the scope of the present invention. 
     In certain exemplary embodiments, the connections  606 ,  608  are designed to reflect light from the display material  310  toward the viewer side of the display. In accordance with this embodiment, stray light (i.e., light emitted from the display material in a direction that is not viewable on the display side) is reflected toward the display side to increase the amount of visible light emitted by the display material. In this manner, the connections  606 ,  608  contribute to the efficiency of the pixel structure. Accordingly, displays with increased light output or displays with similar light output emitted from smaller sized display materials are achievable. In addition, reflecting the light toward the viewer side reduces the amount of light absorbed by the pixel structure, thereby preventing this stray light from generating heat within the pixel structure. 
     Although the invention is illustrated and described herein with reference to specific embodiments, the invention is not intended to be limited to the details shown. Rather, various modifications may be made in the details within the scope and range of equivalents of the claims and without departing from the invention.