PATENT DOCUMENT

Publication Number: US-8690412-B2
Application Number: US-201213421703-A
Country: US
Kind Code: B2

Title: Backlight structures and backlight assemblies for electronic device displays

Abstract:
An electronic device may have a liquid crystal display with backlight structures. The backlight structures may produce backlight that passes through the display layers in the display. The display layers may include a layer of liquid crystal material interposed between a color filter layer and a thin-film transistor layer. The backlight structures may include a light guide plate. A plurality of light-emitting diodes mounted on a flexible printed circuit may be coupled to an edge of the light guide plate. The flexible printed circuit may be curled into a spring element to bias the light-emitting diodes against the edge of the light guide plate. A plurality of gaps may be formed in the flexible printed circuit and may be used to separate and mechanically decouple adjacent light-emitting diodes. Individual light-emitting diodes may independently register to the light guide plate to maximize optical efficiency in the display.

Claims:
What is claimed is: 
     
       1. A backlight assembly configured to provide backlight illumination for a display, comprising:
 a light guide plate having a surface from which the backlight illumination is provided to the display and having an edge into which light is launched to form the backlight illumination; 
 a substrate; and 
 a plurality of light-emitting diodes each having opposing first and second surfaces, wherein each of the light-emitting diodes is interposed between the edge and the substrate, wherein each of the light-emitting diodes launches a portion of the light from its first surface into the edge, wherein the second surface of each of the light-emitting diodes is mounted to the substrate, and wherein the substrate comprises a flexible sheet of polymer that is curled to form a spring that biases the first surfaces of the light-emitting diodes against the edge of the light guide plate. 
 
     
     
       2. The backlight assembly defined in  claim 1  wherein the flexible sheet of polymer includes gaps and wherein each gap is interposed between two adjacent light-emitting diodes within the plurality of light-emitting diodes to mechanically decouple the two adjacent light-emitting diodes. 
     
     
       3. The backlight assembly defined in  claim 1  further comprising a bend guiding structure around which the substrate is bent. 
     
     
       4. A backlight assembly configured to provide backlight illumination for a display, comprising:
 a light guide plate having a surface from which the backlight illumination is provided to the display and having an edge into which light is launched to form the backlight illumination; 
 a flexible printed circuit; and 
 a plurality of light-emitting diodes mounted on the flexible printed circuit and coupled to the edge of the light guide plate, wherein the flexible printed circuit is curled to form a spring and is configured to bias the plurality of light-emitting diodes against the edge of the light guide plate. 
 
     
     
       5. The backlight assembly defined in  claim 4  wherein each of the light-emitting diodes has a base surface with which the light-emitting diode is mounted to the flexible printed circuit and wherein each of the light-emitting diodes has an edge surface that is perpendicular to the base surface and that emits light into the edge of the light guide plate. 
     
     
       6. The backlight assembly defined in  claim 5  wherein the flexible printed circuit comprises a plurality of gaps, wherein each gap is interposed between two adjacent light-emitting diodes within the plurality of light-emitting diodes to mechanically decouple the two adjacent light-emitting diodes. 
     
     
       7. The backlight assembly defined in  claim 4  wherein the flexible printed circuit comprises a plurality of gaps, wherein each gap is interposed between two adjacent light-emitting diodes within the plurality of light-emitting diodes to mechanically decouple the two adjacent light-emitting diodes. 
     
     
       8. The backlight assembly defined in  claim 4  wherein the curled flexible printed circuit comprises a bent portion having portions forming perforations. 
     
     
       9. The backlight assembly defined in  claim 4  wherein the curled flexible printed circuit comprises a bent portion having locally widened traces. 
     
     
       10. A backlight assembly configured to provide backlight illumination for a display, comprising:
 a light guide plate having a surface from which the backlight illumination is provided to the display and having an edge into which light is launched to form the backlight illumination; 
 a flexible printed circuit; and 
 a plurality of light-emitting diodes mounted on the flexible printed circuit and coupled to the edge of the light guide plate, wherein the flexible printed circuit has gaps, wherein each gap is interposed between two adjacent light-emitting diodes within the plurality of light-emitting diodes to mechanically decouple the two adjacent light-emitting diodes. 
 
     
     
       11. The backlight assembly defined in  claim 10  wherein each gap has straight edges that form a rectangular slot. 
     
     
       12. The backlight assembly defined in  claim 10  wherein each gap has edges that form a slot and wherein each slot has a locally widened portion. 
     
     
       13. The backlight assembly defined in  claim 10  wherein the gaps are formed along an edge of the flexible printed circuit and wherein additional gaps are formed on an opposing edge of the flexible printed circuit so that the flexible printed circuit has a serpentine shape. 
     
     
       14. A backlight assembly configured to provide backlight illumination for a display, comprising:
 a light guide plate having a surface from which the backlight illumination is provided to the display and having an edge into which light is launched to form the backlight illumination; 
 a flexible substrate; 
 a plurality of light-emitting diodes mounted on the flexible substrate and coupled to the edge of the light guide plate; and 
 a biasing structure that presses the light-emitting diodes against the edge of the light guide plate. 
 
     
     
       15. The backlight assembly defined in  claim 14  wherein the biasing structure comprises foam. 
     
     
       16. The backlight assembly defined in  claim 15  wherein the foam comprises thermally conductive foam that serves as a heat sink for the light-emitting diodes. 
     
     
       17. The backlight assembly defined in  claim 14  further comprising:
 a support structure; and 
 adhesive interposed between the support structure and the flexible substrate that attaches the flexible substrate to the support structure, wherein the adhesive is configured to allow sufficient lateral motion of the flexible substrate and the light-emitting diodes on the flexible substrate to laterally align each of the light-emitting diodes against the edge of the light guide plate. 
 
     
     
       18. The backlight assembly defined in  claim 14  wherein the flexible substrate comprises rail holes to accommodate lateral movement of the flexible substrate and the light-emitting diodes relative to the light guide plate. 
     
     
       19. The backlight assembly defined in  claim 14  wherein the flexible substrate has gaps and wherein each gap is interposed between two adjacent light-emitting diodes within the plurality of light-emitting diodes to mechanically decouple the two adjacent light-emitting diodes. 
     
     
       20. A backlight assembly, comprising:
 a light guide plate having a surface from which the backlight illumination is provided to the display and having a plurality of holes from which light is launched to form the backlight illumination, wherein each hole extends from an upper surface of the light guide plate to a lower surface of the light guide plate; 
 a plurality of light-emitting diodes each of which is mounted in an associated one of the holes and is separated from an edge of the light guide plate by a respective gap; and 
 an index-of-refraction-matching material that fills the gaps. 
 
     
     
       21. The backlight assembly defined in  claim 20  wherein the light guide plate has reservoirs configured to receive excess portions of the index-of-refraction-matching material and wherein each of the reservoirs is formed as an extension to a respective one of the holes.

Description:
BACKGROUND 
     This relates generally to electronic devices and, more particularly, to electronic devices with displays and associated backlight structures. 
     Electronic devices such as computers and cellular telephones have displays. Some displays such as plasma displays and organic light-emitting diode displays have arrays of display pixels that generate light. In displays of this type, backlighting is not necessary because the display pixels themselves produce light. Other displays contain passive display pixels that can alter the amount of light that is transmitted through the display to display information for a user but do not produce light themselves. As a result, it is often desirable to provide backlight for a display with passive display pixels. 
     In a typical backlight assembly for a display, a light guide plate is used to distribute backlight generated by a light source such as a light-emitting diode light source. Optical films such as a diffuser layer and brightness enhancing film may be placed on top of the light guide plate. A reflector may be formed under the light guide plate to improve backlight efficiency. 
     To provide satisfactory backlighting, it may be desirable to locate one or more strips of light-emitting diodes on the edges of a light guide plate. A light strip of light-emitting diodes may be formed by mounting a row of light-emitting diodes onto a flex circuit. Light strips are typically attached at the edges of the light guide plate so that the light-emitting diodes can direct light into the light guide plate. 
     In an ideal light strip, the light-emitting diodes are aligned with each other so that each light-emitting diode can physically contact the light guide plate. However, there are often placement variations within a row of light-emitting diodes that result in misalignment. If care is not taken, this type of misalignment can result in air gaps between the light-emitting diodes and the light guide plate. The presence of air gaps can have an adverse impact on backlight efficiency. Poor backlight efficiency may in turn decrease power consumption efficiency and can reduce battery life in an electronic device. 
     It would therefore be desirable to be able to provide electronic devices with improved arrangements for backlighting displays. 
     SUMMARY 
     A backlight assembly may be provided for producing backlight illumination for a display. The backlight assembly may have light sources such as light-emitting diodes. The light-emitting diodes may be edge-emitting diodes that emit light through edges that are perpendicular to a base surface or may emit light through a surface that opposes the base surface. 
     The backlight assembly may include a light guide plate. The light guide plate may have an upper surface through which backlight is provided to the underside of the display. The light guide plate may also have edge portions into which light may be launched from the light-emitting diodes. 
     The light-emitting diodes may be mounted on a flexible substrate such as a flexible printed circuit formed form a flexible sheet of polymer. The flexible printed circuit may be wrapped around a bend guiding structure to form a spring. The spring may press the light-emitting diodes against the edge portions of the light guide plate. 
     Slots or other decoupling features may be provided within the flexible printed circuit to mechanically decouple adjacent light-emitting diodes from each other. The slots may be rectangular in shape and may have locally widened portions. Slots may be formed along one edge of a flexible printed circuit or may be formed on opposing edges of the flexible printed circuit so that the flexible printed circuit has a serpentine shape. 
     Perforations may be formed within the bent portion of a spring-shaped curled flexible printed circuit. Locally widened traces may be formed on the bent portion of a flexible printed circuit to enhance trace strength. 
     A foam structure such as a thermally conductive foam that serves as a heat sink, a bent metal structure, or other biasing structure may be used to bias the flexible printed circuit and attached light-emitting diodes against the edge portions of the light guide plate. The flexible printed circuit may be attached to a support structure using an adhesive that allows the flexible printed circuit and the light-emitting diodes to laterally move relative to the edge portions of the light guide plate. This aligns the light-emitting diodes to the edge portions of the light guide plate and minimizes gaps between the light-emitting diodes and the light guide plate. Rail holes within a flexible printed circuit may be used to allow the flexible printed circuit and light-emitting diodes to be laterally aligned with the light guide plate. 
     The light guide plate may have holes into which the light-emitting diodes are placed. Index-of-refraction-matching material that matches the index-of-refraction of the light guide plate may be used to fill gaps between the light-emitting diodes and the light guide plate to improve coupling efficiency. Reservoirs may be coupled to the holes to accommodate excess index-of-refraction-material material. 
     Further features of the invention, its nature and various advantages will be more apparent from the accompanying drawings and the following detailed description of the preferred embodiments. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram of an illustrative electronic device such as a portable computer having a backlit display in accordance with an embodiment of the present invention. 
         FIG. 2  is a diagram of an illustrative electronic device such as a cellular telephone or other handheld device having a backlit display in accordance with an embodiment of the present invention. 
         FIG. 3  is a diagram of an illustrative electronic device such as a tablet computer having a backlit display in accordance with an embodiment of the present invention. 
         FIG. 4  is a diagram of an illustrative electronic device such as a computer monitor with a built-in computer having a backlit display in accordance with an embodiment of the present invention. 
         FIG. 5  is a cross-sectional side view of an illustrative backlit display in accordance with an embodiment of the present invention. 
         FIG. 6  is a cross-sectional side view of a conventional backlight arrangement having air gaps between the light-emitting diodes and the light guide plate. 
         FIG. 7  is a top view of a conventional backlight arrangement having air gaps between the light-emitting diodes and the light guide plate. 
         FIG. 8A  is a cross-sectional side view of a backlight arrangement in which light-emitting diodes are mounted on a flexible substrate that is curled into a spring element that biases the top of each light-emitting diode against a light guide plate in accordance with an embodiment of the present invention. 
         FIG. 8B  is a top view of a flexible substrate that may be used to form the spring element of  FIG. 8A  in which the flexible substrate is provided with gaps that separate and mechanically decouple adjacent light-emitting diodes in accordance with an embodiment of the present invention. 
         FIG. 8C  is a top view of a flexible substrate that may be used to form the spring element of  FIG. 8A  in which the flexible substrate is free of gaps in accordance with an embodiment of the present invention. 
         FIG. 9A  is a cross-sectional side view of a backlight arrangement in which light-emitting diodes are mounted on a flexible substrate that is curled into a spring element that biases the side of each light-emitting diode against a light guide plate in accordance with an embodiment of the present invention. 
         FIG. 9B  is a top view of a flexible substrate that may be used to form the spring element of  FIG. 9A  in which the flexible substrate is provided with gaps that separate and mechanically decouple adjacent light-emitting diodes in accordance with an embodiment of the present invention. 
         FIG. 9C  is a top view of a flexible substrate that may be used to form the spring element of  FIG. 9A  in which the flexible substrate is provided with locally widened gaps in accordance with an embodiment of the present invention. 
         FIG. 10A  is a cross-sectional side view of a backlight arrangement in which a high shear adhesive is used to attach the flexible substrate to a support structure in accordance with an embodiment of the present invention. 
         FIG. 10B  is a top view of a flexible substrate that may be used in the arrangement of  FIG. 10A  in which the flexible substrate has a serpentine shape that mechanically decouples adjacent light-emitting diodes in accordance with an embodiment of the present invention. 
         FIG. 11A  is a cross-sectional side view of a backlight arrangement in which a biasing structure is used to press light-emitting diodes against a light guide plate in accordance with an embodiment of the present invention. 
         FIG. 11B  is a top view of a flexible substrate that may be used in the arrangement of  FIG. 11A  in which the flexible substrate is provided with rail holes to attach the flexible substrate to a support structure in accordance with an embodiment of the present invention. 
         FIG. 11C  is a top view of a flexible substrate that may be used in the arrangement of  FIG. 11A  in which the flexible substrate has a serpentine shape that mechanically decouples adjacent light-emitting diodes in accordance with an embodiment of the present invention. 
         FIG. 12A  is a top view of a backlight arrangement in which an index-matching material is used to fill gaps between light-emitting diodes and a light guide plate in accordance with an embodiment of the present invention. 
         FIG. 12B  is a cross-sectional side view of the backlight arrangement of  FIG. 12A  in which an index-matching material is used to fill gaps between light-emitting diodes and a light guide plate in accordance with an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     A display may be provided with backlight structures. The backlight structures may produce backlight for the display that helps a user of a device view images on the display in a variety of ambient lighting conditions. Displays with backlights may be provided in any suitable type of electronic equipment. 
     An illustrative electronic device of the type that may be provided with a backlit display is shown in  FIG. 1 . Electronic device  10  may be a computer such as a computer that is integrated into a display such as a computer monitor, a laptop computer, a tablet computer, a somewhat smaller portable device such as a wrist-watch device, pendant device, or other wearable or miniature device, a cellular telephone, a media player, a tablet computer, a gaming device, a navigation device, a computer monitor, a television, or other electronic equipment. 
     As shown in  FIG. 1 , device  10  may include a backlit display such as display  14 . Display  14  may be a touch screen that incorporates capacitive touch electrodes or other touch sensor components or may be a display that is not touch-sensitive. Display  14  may include image pixels formed from liquid crystal display (LCD) components or other suitable display pixel structures. Arrangements in which display  14  is formed using liquid crystal display pixels are sometimes described herein as an example. This is, however, merely illustrative. Any suitable type of display technology may be used in forming display  14  if desired. 
     Device  10  may have a housing such as housing  12 . Housing  12 , which may sometimes be referred to as a case, may be formed of plastic, glass, ceramics, fiber composites, metal (e.g., stainless steel, aluminum, etc.), other suitable materials, or a combination of any two or more of these materials. 
     Housing  12  may be formed using a unibody configuration in which some or all of housing  12  is machined or molded as a single structure or may be formed using multiple structures (e.g., an internal frame structure, one or more structures that form exterior housing surfaces, etc.). 
     As shown in  FIG. 1 , housing  12  may have multiple parts. For example, housing  12  may have upper portion  12 A and lower portion  12 B. Upper portion  12 A may be coupled to lower portion  12 B using a hinge that allows portion  12 A to rotate about rotational axis  16  relative to portion  12 B. A keyboard such as keyboard  18  and a touch pad such as touch pad  20  may be mounted in housing portion  12 B. 
     In the example of  FIG. 2 , device  10  has been implemented using a housing that is sufficiently small to fit within a user&#39;s hand (i.e., device  10  of  FIG. 2  may be a handheld electronic device such as a cellular telephone). As show in  FIG. 2 , device  10  may include a backlit display such as display  14  mounted on the front of housing  12 . Display  14  may be substantially filled with active display pixels or may have an inactive portion and an inactive portion. Display  14  may have openings (e.g., openings in the inactive or active portions of display  14 ) such as an opening to accommodate button  22  and an opening to accommodate speaker port  24 . 
       FIG. 3  is a perspective view of electronic device  10  in a configuration in which electronic device  10  has been implemented in the form of a tablet computer. As shown in  FIG. 3 , backlit display  14  may be mounted on the upper (front) surface of housing  12 . An opening may be formed in display  14  to accommodate button  22 . 
       FIG. 4  is a perspective view of electronic device  10  in a configuration in which electronic device  10  has been implemented in the form of a computer integrated into a computer monitor. As shown in  FIG. 4 , backlit display  14  may be mounted on the front surface of housing  12 . Stand  26  may be used to support housing  12 . 
     A cross-sectional side view of display  14  is shown in  FIG. 5 . As shown in  FIG. 5 , display  14  may include backlight structures  30 . Backlight structures  30  may include a light source such as light-emitting diode light source  38 , a light guide plate such as light guide plate  32 , optical films  34 , and a reflector such as reflector  36 . During operation, light-emitting diode light source  38  may emit light  44  into light guide plate  32 . Light guide plate  32  may be formed from a rectangular sheet of clear plastic. Light  44  may travel within light guide plate  32  by means of total internal reflection. Light that escapes through the upper surface of light guide plate  32  may pass through overlying display layers in direction z and may serve as backlight for display  14 . Light that escapes through the lower surface of light guide plate  32  may be reflected by reflector  36  and redirected upwards in direction z. Reflector  36  may be formed from a reflective material such as white plastic (as an example). Optical films  34  may include brightness enhancing film layers, diffusing film layers, and compensating film layers (as examples). 
     Display  14  may have an upper polarizer layer such as polarizer layer  40  and a lower polarizer layer such as polarizer layer  42 . Polarizer layer  42  may polarize backlight  44 . Thin-film transistor (TFT) layer  46  may include a layer of thin-film transistor circuitry and an array of associated pixel electrodes. Pixel structures such as thin-film transistor structures and associated pixel electrodes in the array of pixel electrodes on thin-film transistor layer  46  may produce electric fields corresponding to image data to be displayed. The electric field produced by each electrode on thin-film transistor layer  46  adjusts the orientation of liquid crystals in an associated portion of liquid crystal layer  48  by a corresponding amount. As light travels through display  14 , the adjustment of the orientation of the liquid crystals adjusts the polarization of the light that passes through layer  48 . When this light reaches upper polarizer  40 , the polarization state of each pixel of light is attenuated by an amount that is proportional to its polarization, thereby creating visible images for a user. 
     Color filter layer  50  may contain an array of colored pixels (e.g., an array of red, blue, and green color filter elements) for providing display  14  with the ability to form color images. Sealant  52  may be used to seal color filter layer  50  to thin-film transistor layer  46  and to retain liquid crystal material  48  within display  14 . 
     Display  14  may include a touch-sensitive layer such as touch-sensitive layer  54  for receiving touch input from a user of device  10 . Touch-sensitive layer  54  may include a pattern of indium tin oxide (ITO) electrodes or other suitable transparent electrodes that have been deposited to form a capacitive touch sensor array. Touch-sensitive layer  54  may, in general, be configured to detect the location of one or more touches or near touches on touch-sensitive layer  54  based on capacitive, resistive, optical, acoustic, inductive, or mechanical measurements, or any phenomena that can be measured with respect to the occurrence of the one or more touches or near touches in proximity to touch-sensitive layer  54 . If desired, touch-sensitive layer  54  may be incorporated into thin-film transistor layer  46  (e.g., display pixel electrodes and capacitive touch electrodes may be formed on a common substrate). The example of  FIG. 5  in which touch-sensitive layer  54  is separate from thin-film transistor layer  46  is merely illustrative. 
     If desired, additional layers may be included in display  14 . An optional layer of transparent glass or plastic may be used to provide a protective cover for display  14 , as illustrated by cover layer  56  of  FIG. 5 . 
     A cross-sectional side view of a conventional backlight arrangement is shown in  FIG. 6 . As shown in  FIG. 6 , backlight illumination is provided by a strip of light-emitting diodes  202  located along the edge of light guide plate  204 . Light-emitting diodes  202  are mounted to flexible printed circuit substrate  206 , typically using solder. Due to placement variation during the mounting process, light-emitting diodes  202  are often misaligned. As a result, an air gap G forms between light-emitting diodes  202  and the edge of light guide plate  204 . Such air gaps G can have an adverse impact on backlight efficiency. For example, light  208  will experience a change in refractive index as it travels from air gap G to light guide plate  204 . This in turn will alter the angle at which light  208  enters light guide plate  204 , possibly inhibiting the ability of light guide plate  204  to evenly distribute backlight to the entire display. 
     A top view of a conventional backlight arrangement with air gaps is shown in  FIG. 7 . As shown in  FIG. 7 , light-emitting diodes  202  are misaligned, resulting in air gaps G between light-emitting diodes  202  and light guide plate  204 . Because light-emitting diodes  202  are mechanically coupled together by the solid strip of flexible printed circuit substrate  206 , individual light-emitting diodes  202  are not able to independently register to light guide plate  204 . As a result, backlight structures  200  will have low optical efficiency. 
       FIG. 8A  is a cross-sectional side view of a portion of display  14  illustrating how the optical efficiency of display  14  may be maximized. As shown in  FIG. 8A , backlight structures  30  may include light guide plate  32 , reflector  36 , and a plurality of light-emitting diodes  38 . Light-emitting diodes  38  may be mounted on a strip of flexible printed circuit (sometimes referred to as a “flex circuit” or “flex tail”) such as flex circuit  60 . Flex circuit  60  and other flexible printed circuits in device  10  may be formed from sheets of polyimide and/or other layers of polymer. Flex circuit  60  may include patterned metal traces to which packaged light-emitting diodes  38  are soldered. Patterned metal traces on flex circuit  60  may be used to distribute power to conductive terminals of light-emitting diodes  38 . The strip of flex circuit  60  on which the plurality of light-emitting diodes  38  is mounted is sometimes referred to as a “light strip” or a “light bar.” 
     Backlight structures  30  may be mounted within an optional support structure such as support structure  62 . Support structure  62  (sometimes referred to as a chassis or mechanical chassis) may be formed from materials such as plastic, ceramic, fiber composites, metal, or other suitable materials. If desired, display  14  may be formed by mounting backlight structures  30  directly within housing  12  or by mounting backlight structures  30  in support structures of other shapes. In the illustrative configuration of  FIG. 8A , mechanical chassis  62  is used in forming a backlight assembly for display  14  that may be mounted within housing  12  under a display cover layer such as display cover layer  56  of  FIG. 5 . Other mounting configurations may be used, if desired. 
     As shown in  FIG. 8A , light-emitting diodes  38  may be interposed between flex tail  60  and light guide plate  32 . Each light-emitting diode  38  may have a base surface that is mounted (e.g., soldered) to flex tail  60  and a top surface  38 T opposing the base surface that emits light into light guide plate  32 . To ensure that light-emitting diodes  38  press against light guide plate  32 , flex tail  60  may be curled and/or bent to form a spring element such as spring element  60 P. Spring element  60 P may exert a force on light-emitting diodes  38  in direction  66  (e.g., towards light guide plate  32 ). To form flex tail  60  into this type of spring element, flex tail  60  may be curled inside of support structure  62 . In an attempt to return to its equilibrium position (e.g., uncurled), flex tail  60  will naturally exert a force in direction  66 , thereby pressing top surface  38 T of light-emitting diodes  38  against light guide plate  32 . Light may be emitted from top surface  38 T of light-emitting diode  38  directly into the edge of light guide plate  32 . Forming flex tail  60  into a spring element that biases light-emitting diodes  38  against light guide plate  32  may help reduce or eliminate air gaps between light-emitting diodes  38  and light guide plate  32 . 
     If desired, a bend-guiding structure such as bend-guiding structure  35  may optionally be used to form and shape flex tail  60  into spring element  60 P. Flex tail  60  may be bent around bend-guiding structure  35  to form the desired bend in flex tail  60 . Bend-guiding structure  35  (sometimes referred to as a mandrel) may be a compliant or undersized structure and may be formed from materials such as foam, rubber, plastic, or other suitable materials. For example, bend-guiding structure  35  may have an elongated rod shape that runs parallel to an edge of electronic device  10 . A curved surface on bend-guiding structure  35  may be used in forming a bent portion on flex tail  60  as flex tail  60  curls around bend-guiding structure  35 . If desired, bend-guiding structure  35  may be heated while manipulating flex tail  60  into spring element  60 P. Bend-guiding structure  35  may be formed as an integral part of housing  12 , may be formed as an integral part of support structure  62 , or may be a separate structure used to form flex tail  60  into spring element  60 P. 
     Some areas such as bend region  61  of spring element  60 P may have a smaller bend radius than other areas of spring element  60 P. Measures may be taken to minimize the stress on flex tail  60  in regions such as region  61 . For example, region  61  of flex tail  60  may be provided with perforations, may be preformed (e.g., using a heated forming process or a cold forming process), may have reduced layers (e.g., copper plating in the bend region of flex circuit  60  may be reduced to one layer to increase flexibility in the bend region), etc. Patterned traces may be strengthened in portions of flex tail  60  that have a small bend radius by increasing the width of the traces in the bend region. 
     Light-emitting diodes  38  may be mechanically decoupled from one another so that each individual light-emitting diode  38  may independently register to light guide plate  32 . To decouple light-emitting diodes  38 , gaps may be formed in flex tail  60  to separate adjacent light-emitting diodes  38 . A top view of flex tail  60  in which gaps are used to separate adjacent light-emitting diodes  38  is shown in  FIG. 8B . If desired, flex tail  60  of  FIG. 8B  may be used to form the spring element of  FIG. 8A . 
     As shown in  FIG. 8B , a plurality of gaps such as gaps  68  may be formed in flex tail  60 , thereby creating a plurality of separated flexible “tabs.” Each light-emitting diode  38  may be mounted on an associated flexible tab. Gaps (sometimes referred to as slots) may separate and mechanically decouple light-emitting diodes  38  from one another, allowing each to independently register to light guide plate  32 . If desired, traces such as trace  67  may be locally widened in the bent portions of flex tail  60  to enhance the strength of the traces in the bent portions. 
     In  FIG. 8B , flex tail  60  is shown in a flat position (e.g., “uncurled”). When flex tail  60  is curled into the shape of spring element  60 P shown in  FIG. 8A , top surface  38 T of each light-emitting diode  38  will be pressed against the edge of light guide plate  32 . The force provided by spring element  60 P may push top surfaces  38 T of light-emitting diodes  38  up against light guide plate  32 , and each individual light-emitting diode  38  may independently register to light guide plate  32 . 
     In the example of  FIG. 8B , gaps  68  are used to isolate each individual light-emitting diode  38  (e.g., a single light-emitting diode  38  is mounted on each flexible tab). This is, however, merely illustrative. If desired, gaps  68  may be used to isolate groups of light-emitting diodes  38 . For example, there may be two, three, or more than three light-emitting diodes on an associated flexible tab, if desired. In general, any number of gaps  68  may be used to separate any number of light-emitting diodes  38 . 
       FIG. 8C  is a top view of another possible configuration of flex tail  60  that may be used to form spring element  60 P of  FIG. 8A . As shown in  FIG. 8C , flex tail  60  on which light-emitting diodes  38  are mounted may be a solid strip of flexible printed circuit substrate. In  FIG. 8C , flex tail  60  is shown in a flat position (e.g., “uncurled”). When flex tail  60  is curled into the shape of spring element  60 P shown in  FIG. 8A , the force provided by spring element  60 P will push top surface  38 T of each light-emitting diode  38  up against light guide plate  32 . Light may be emitted from top surface  38 T of light-emitting diodes  38  directly into the edge of light guide plate  32 . 
     Since top surface  38 T of each light-emitting diode  38  registers to light guide plate  32 , any placement variation in light-emitting diodes  38  on flex tail  60  (e.g., variation in location on the surface of flex tail  60 ) will not affect the light-emitting diodes&#39; ability to physically contact light guide plate  32 . The force provided by spring element  60 P will push top surface  38 T in direct contact with light guide plate  32  regardless of any lateral misalignment on the surface of flex tail  60 . 
       FIG. 9A  is a cross-sectional side view of a portion of display  14  illustrating another possible backlight assembly that may be used to optimize the optical efficiency of display  14 . As shown in  FIG. 9A , flex tail  60  may be curled and/or bent to form spring element  60 P. Spring element  60 P of  FIG. 9A  may exert a force on light-emitting diodes  38  in direction  72  (e.g., towards light guide plate  32 ), thereby pressing side surface  38 S of light-emitting diodes  38  against light guide plate  32 . Light may be emitted from side surface  38 S of each light-emitting diode  38  (e.g., a surface that is perpendicular to the base surface of light-emitting diode  38 ) directly into the edge of light guide plate  32 . 
     If desired, bend-guiding structure  35  may optionally be used to form and shape flex tail  60  into spring element  60 P. Flex tail  60  may be wrapped around bend-guiding structure  35  to form the desired bend in flex tail  60 . To minimize the stress on flex tail  60  in areas of small bend radius, portions of flex tail  60  may be perforated, may be preformed (e.g., using a heated forming process or a cold forming process), may have reduced layers (e.g., copper plating in bend regions of flex circuit  60  may be reduced to one layer to increase flexibility in the bend region), etc. Patterned traces may be strengthened in portions of flex tail  60  that have a small bend radius by increasing the width of the traces in the bend region. 
     If desired, light-emitting diodes  38  may be mechanically decoupled from one another so that each individual light-emitting diode  38  may independently register to light guide plate  32 . A top view of flex tail  60  in which gaps are used to separate adjacent light-emitting diodes  38  is shown in  FIG. 9B . If desired, flex tail  60  of  FIG. 9B  may be used to form the spring element of  FIG. 9A . 
     As shown in  FIG. 9B , a plurality of gaps  68  (e.g., rectangular slots) may be formed in flex tail  60 , thereby creating a plurality of separated flexible tabs. Each light-emitting diode  38  may be mounted on an associated flexible tab. Gaps  68  may separate and mechanically decouple light-emitting diodes  38  from one another, allowing each to independently register to light guide plate  32 . 
     In  FIG. 9B , flex tail  60  is shown in a flat position (e.g., “uncurled”). When flex tail  60  is curled into the shape of spring element  60 P shown in  FIG. 9A , side surface  38 S of light-emitting diode  38  may be in direct contact with light guide plate  32 . The force provided by spring element  60 P may push side surface  38 S of each light-emitting diode  38  against light guide plate  32 . Because light-emitting diodes  38  are mechanically decoupled from one another, misalignment in light-emitting diodes  38  will not affect the ability of individual light-emitting diodes  38  to physically contact the edge of light guide plate  32 . 
     During manufacturing, a light-emitting diode may be unintentionally soldered to a flex tail at a slight angle. If care is not taken, this type of angled position may lead to an air gap between the light-emitting diode and the light guide plate. To ensure that light-emitting diodes  38  are flush with the edge of light guide plate  32 , light-emitting diodes  38  may be provided with rotational capabilities.  FIG. 9C  is a top view of flex tail  60  in which light-emitting diodes  38  are provided with rotational capabilities. If desired, flex tail  60  of  FIG. 9C  may be used to form spring element  60 P of  FIG. 9A . 
     As shown in  FIG. 9C , a plurality of slots  68  may be interposed between adjacent light-emitting diodes  38 . Slots  68  may have locally widened portions such as widened portions  74 . Having slots with locally widened portions may allow each light-emitting diode  38  to rotate slightly, as indicated by arrows  76  in  FIG. 9C . Providing light-emitting diodes  38  with rotational capabilities may ensure that the entire side surface  38 S of each light-emitting diode  38  is in direct contact with light guide plate  32 . 
       FIG. 10A  is a cross-sectional side view of a portion of display  14  illustrating another possible backlight arrangement that may optimize the optical efficiency of display  14 . As shown in  FIG. 10A , flex tail  60  may lie flat along the edge of light guide plate  32 . 
     A layer of adhesive such as adhesive  82  may be interposed between flex tail  60  and support structure  62 . Adhesive  82  may be a high shear adhesive that attaches flex tail  60  to support structure  62  while allowing some movement of flex tail  60  along the surface of support structure  62 . A high shear adhesive such as adhesive  82  may provide a means of securing flex tail  60  to the interior of support structure  62  without constricting its lateral movement on the surface of support structure  62  (e.g., without inhibiting registration between light guide plate  32  and light-emitting diodes  38 ). Adhesive  82  may be formed from pressure sensitive adhesive, UV-curable adhesive, air-curable adhesive, moisture-curable adhesive, or other suitable type of adhesive. If desired, adhesive  82  may be used as a heat sink. For example, adhesive  82  may be formed from a material with high thermal conductivity and may be configured to transfer heat from backlight structures  30  to support structure  62 , housing  12 , or other suitable heat spreader in the vicinity of backlight structures  30 . 
     The plurality of light-emitting diodes  38  that are mounted on flex tail  60  may be mechanically decoupled from one another so that each individual light-emitting diode  38  may independently register to light guide plate  32 . A top view of a flex tail of the type that may be used in the configuration of  FIG. 10A  is shown in  FIG. 10B . 
     As shown in  FIG. 10B , flex tail  60  may have a serpentine shape in which gaps are formed on both sides of flex tail  60 . For example, a gap such as gap  68 A may be formed on side A of flex tail  60 , between adjacent light-emitting diodes  38 . The next closest gap such as gap  68 B may be formed on side B of flex tail  60  (e.g., the opposite side of flex tail  60 ). The gaps may alternate sides along the length of flex tail  60  to create a serpentine-shaped flexible substrate. 
     When backlight structures  30  are inserted into support structure  62 A, a force may be applied in direction  84  ( FIG. 10A ). This may push flex tail  60  into rear wall  62 R of support structure  62  and reduce or eliminate gaps between light-emitting diodes  38  and light guide plate  32 . Due to the serpentine-shape of flex tail  60  ( FIG. 10B ), light-emitting diodes  38  may be mechanically decoupled from one another so that side surface  38 S of each light-emitting diode  38  may independently register to light guide plate  32 . 
       FIG. 11A  is a cross-sectional side view of a portion of display  14  illustrating another possible backlight assembly that may be used to optimize the optical efficiency of display  14 . As shown in  FIG. 11A , flex tail  60  may lie flat along the edge of light guide plate  32 . A biasing structure such as biasing structure  92  may be used to bias light-emitting diodes  38  against light guide plate  32  to help reduce or eliminate air gaps between light-emitting diodes  38  and light guide plate  32 . Biasing structure  92  may be interposed between light-emitting diodes  38  and rear wall  62 R of support structure  62 . 
     If desired, biasing structure  92  may be formed from a conformable, thermally conductive foam (e.g., a foam formed from Gap Pad® material or other suitable material). Using a thermally conductive material to form biasing structure  92  may allow biasing structure  92  to transfer heat from backlight structures  30  (e.g., from light-emitting diodes  38 ) to support structure  62 , housing  12 , or other suitable heat spreader in the vicinity of backlight structures  30 . Other structures that may be used to bias light-emitting diodes  38  against light guide plate  32  include metal-filled foam, a V-shaped structure (e.g., a V-shaped metal spring member), a spring structure, other suitable structures, etc. 
     If desired, an optional adhesive such as high shear adhesive  82  may be used to attach flex tail  60  to support structure  62  without constricting its lateral movement on the surface of support structure  62  (e.g., without resisting the biasing force provided by biasing structure  92 ). This is, however, merely illustrative. Other methods may be used to attach flex circuit  60  to support structure  62 . For example, opposing ends of flex tail  60  may be provided with rail holes. Screws or pins may be used to secure flex tail  60  to support structure  62  at the rail holes.  FIG. 11B  is a top view of flex tail  60  illustrating how rail holes may be used to attach flex tail  60  to support structure  62 . 
     As shown in  FIG. 11B , holes such as rail holes  98  (sometimes referred to as openings or slots) may be formed in opposing ends of flex tail  60 . A pin such as pin  96  may be inserted through each rail hole  98 . Pins  96  may be used to fasten flex tail  60  to support structure  62 . Pins  96  may be mushroom pins, straight pins, or any other suitable type of pin or screw. Rail holes  98  may have an elongated “rail” shape. Movement of flex tail  60  in the y and z-directions may be restricted, whereas movement in the x-direction may be permitted (e.g., movement along the length of elongated rail holes  98  may be permitted). This type of fastening method may provide a means of securing flex tail  60  to the interior of support structure  62  without constricting its lateral movement relative to light guide plate  32  (e.g., without resisting the biasing force provided by biasing structure  92 ). 
     When backlight structures  30  are inserted into support structure  62 , biasing structure  92  may exert a force in direction  94  (e.g., biasing structure  92  may bias light-emitting diodes  38  against light guide plate  32 ). The biasing force provided by biasing structure  92  may reduce or eliminate gaps between light-emitting diodes  38  and light guide plate  32 . If desired, flex tail  60  may be formed with a serpentine-shape as shown in  FIG. 11C . The serpentine-shape of flex tail  60  may be used to mechanically decouple light-emitting diodes  38  from one another so that side surface  38 S of each light-emitting diode  38  may independently register to light guide plate  32 . 
       FIG. 12A  is a top view of a portion of display  14  illustrating another possible backlight assembly that may be used to optimize the optical efficiency of display  14 . As shown in  FIG. 12A , light guide plate  32  may have a row of holes  32 P that extends parallel to one of the edges of light guide plate  32 . Each hole  32 P (sometimes referred to as an opening or light guide plate opening) may be enclosed and surrounded by light guide plate material. 
     Backlight structures  30  may include a row of light-emitting diodes  38  mounted on flex tail  60 . An edge of light guide plate  32  may overlap flex tail  60  such that the row of light-emitting diodes  38  aligns with the row of light guide plate openings  32 P. Light-emitting diodes  38  may each be positioned within an associated hole  32 P. Holes  32 P may have any suitable shape that accommodates light-emitting diodes  38  when light-emitting diodes  38  are inserted into light guide plate openings  32 P. If desired, more than one light-emitting diode  38  may be mounted into an associated light guide plate opening  32 P. The example of  FIG. 12A  in which a single light-emitting diode  38  is mounted in each opening  32 P is merely illustrative. 
     To eliminate air gaps between light-emitting diodes  38  and light guide plate  32 , an index-matching material such as index-of-refraction-matching material  104  may be used to fill holes  32 P around each light-emitting diode  38 . The refractive index of index-matching material  104  may be matched to the refractive index of light guide plate  32 . In this type of configuration, the angle at which light from light-emitting diode  38  enters light guide plate  32  will not be effected by a change in refractive index as it passes from index-matching material  104  to light guide plate  32 . Index-matching material  104  may be optically clear and may be formed from UV-curable adhesive, air-curable adhesive, moisture-curable adhesive, gel, or other suitable materials. 
     If desired, an optional reservoir such as reservoir  102  may be formed in light guide plate  32  adjacent to an associated opening  32 P. Reservoir  102  may be formed as an extension to opening  32 P (e.g., a recess or cavity adjacent to opening  32 P). Reservoir  102  may be configured to receive excess index-matching material  104  in opening  32 P. Reservoir  102  may be formed on a side of light-emitting diode  38  that does not emit light (e.g., a rear side of light-emitting diode  38  as shown in  FIG. 12A ). If desired, light guide plate openings  32 P may not be provided with reservoirs  102 . The example of  FIG. 12A  in which light guide plate openings  32 P are provided with reservoirs  102  for receiving excess index-matching material  104  is merely illustrative. 
     A cross-section of backlight structures  30  taken along axis  106  is shown in  FIG. 12B . As shown in  FIG. 12B , light  44  may be emitted from light-emitting diode  38  and may travel through index-matching material  104  and into light guide plate  32 . Index-matching material  104  may ensure that the angle at which light enters light guide plate  32  is not affected by a change refractive index at interface  108 . Light  44  may travel within light guide plate  32  by means of total internal reflection. Light that escapes through the upper surface of light guide plate  32  may pass through overlying display layers in direction z and may serve as backlight for display  14 . Light that escapes through the lower surface of light guide plate  32  may be reflected by reflector  36  and redirected upwards in direction z. 
     If desired, light may be launched into light guide plate  32  from more than one edge of plate  32 . For example, a strip of light-emitting diodes  38  may be placed along one edge, two edges, three edges, or all four edges of light guide plate  32 . The example of  FIG. 12A  in which light-emitting diodes  38  are located along one edge of light guide plate  32  is merely illustrative. 
     The foregoing is merely illustrative of the principles of this invention and various modifications can be made by those skilled in the art without departing from the scope and spirit of the invention. The foregoing embodiments may be implemented individually or in any combination.

Metadata:
Filing Date: 20120315
Publication Date: 20140408
Grant Date: 20140408
Priority Date: 20120315
Inventors: FRANKLIN JEREMY C.
WRIGHT DEREK
ZHU WENYONG
Assignee: APPLE INC
CPC Classifications: [{"code": "G02B6/0091", "inventive": true, "first": true, "tree": "[]"}, {"code": "G02B6/0068", "inventive": false, "first": false, "tree": "[]"}, {"code": "G02B6/0088", "inventive": true, "first": false, "tree": "[]"}, {"code": "H05K1/189", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02B6/0068", "inventive": false, "first": false, "tree": "[]"}, {"code": "G06Q20/3223", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02B6/0073", "inventive": true, "first": false, "tree": "[]"}, {"code": "G06Q20/204", "inventive": true, "first": true, "tree": "[]"}, {"code": "G02B6/0091", "inventive": true, "first": false, "tree": "[]"}]
Family ID: 49157443