PATENT DOCUMENT

Publication Number: US-9239489-B2
Application Number: US-201213568949-A
Country: US
Kind Code: B2

Title: Display backlight with closely spaced light-emitting diode packages

Abstract:
An electronic device may be provided with a display having backlight structures that include a light guide plate formed from a clear polymer film. The polymer film may have an edge into which light is emitted from an adjacent array of light-emitting diodes. The light emitting diodes may each include a semiconductor device that emits light. The semiconductor device in each diode may be mounted on lead frame structures and wirebonded to the lead frame structures with first and second wire bonds. To improve backlight homogeneity and thereby reduce the mixing distance for light in the light guide plate, the diodes may be spaced closely together using diode packages having end faces that are free of lead frame structures. Exposed lead frame structures for soldering the light-emitting diodes to a substrate may be formed under the light-emitting diodes and on rear surfaces of the light-emitting diodes.

Claims:
What is claimed is: 
     
       1. Display backlight structures, comprising:
 a light guide plate having an edge; 
 a dielectric substrate layer having solder pads; and 
 light-emitting diodes arranged in a row along the edge that emit light into the edge, wherein each light-emitting diode includes a package having opposing front and rear surfaces, wherein each light-emitting diode includes first and second lead frame structures and a semiconductor device mounted to and wirebonded to one of the lead frame structures, wherein the package comprises a plastic body that covers at least some of the first and second lead frame structures, wherein the front surface comprises a window through which the light is emitted, wherein the plastic body further comprises a lower surface on which portions of the first and second lead frame structures are formed, wherein the plastic body further comprises opposing end faces that are free of lead frame structures, wherein the light-emitting diodes are soldered to the dielectric substrate layer by soldering the portions of the first and second lead frame structures on the lower surface to the solder pads with solder, and wherein the lead frame structures include lead frame terminals on the rear surface. 
 
     
     
       2. The display backlight structures defined in  claim 1  wherein the light guide plate comprises a polymer film. 
     
     
       3. The display backlight structures defined in  claim 2  wherein the polymer film comprises a rectangular sheet of roll-to-roll film having an edge that forms a planar surface that is free of light mixing feature and wherein the edge runs along substantially all of the light guide plate. 
     
     
       4. The display backlight structures defined in  claim 1  wherein the light guide plate has a planar portion with a thickness of less than 0.3 mm. 
     
     
       5. The display backlight structures defined in  claim 1  wherein the window has a window surface area, wherein the front surface has a front surface area, and wherein the window surface area divided by the front surface area is greater than 0.7. 
     
     
       6. The display backlight structures defined in  claim 5  wherein the semiconductor device is coupled to the first lead frame structure with a first wire bond and is coupled to the second lead frame structure with a second wire bond. 
     
     
       7. The display backlight structures defined in  claim 6  wherein the first lead frame terminal comprises a planar surface to which the semiconductor device is attached and wherein the plastic body is molded over at least some of the lead frame structures. 
     
     
       8. The display backlight structures defined in  claim 7  wherein the window structures includes phosphorescent filler material. 
     
     
       9. The display backlight structures defined in  claim 8  wherein portions of solder that have wicked up at least part of the lead frame terminals on the rear surface of each package solders the lead terminals on the rear surface to the solder pad traces. 
     
     
       10. Display backlight structures, comprising:
 a light guide plate having an edge; and 
 light-emitting diodes arranged in a row along the edge that emit light into the edge, wherein each light-emitting diode includes a package having opposing front and rear surfaces, wherein each light-emitting diode includes lead frame structures, wherein the lead frame structures include lead frame terminals on the rear surface, and wherein each package has at least one end face that is free of lead frame structures and that is separated from an end face of an adjacent one of the packages by a gap of less than 1 mm. 
 
     
     
       11. The display backlight structures defined in  claim 10  wherein the printed circuit substrate comprises a flexible printed circuit substrate. 
     
     
       12. Display backlight structures, comprising:
 a light guide plate having an edge; 
 a printed circuit substrate; and 
 an array of light-emitting diodes that are mounted to the printed circuit substrate along the edge of the light guide plate and that emit light into the edge of the light guide plate, wherein each light-emitting diode includes a semiconductor device and a package having a plastic body with a front surface with a window from which the light is emitted and a rear surface, wherein each package has lead frame structures on the rear surface, and wherein the semiconductor device is mounted to and electrically connected to at least one of the lead frame structures, and wherein each package has at least one end face that is free of lead frame structures and that is separated from an end face of an adjacent one of the packages by a gap of less than 1 mm. 
 
     
     
       13. The display backlight structures defined in  claim 12  further comprising solder pads on the printed circuit substrate, wherein the light-emitting diodes are soldered to the solder pads with solder. 
     
     
       14. The display backlight structures defined in  claim 13  wherein the plastic body of each light-emitting diode includes a lower surface, wherein the lead-frame structures comprise lead frame portions on the lower surface that are soldered to the solder pads with the solder. 
     
     
       15. The display backlight structures defined in  claim 14  wherein the lead frame structures for each light-emitting diode include a first lead frame structure having a planar portion on the lower surface and a planar portion on the rear surface and include a second lead frame structure having a planar portion on the lower surface and a planar portion on the rear surface, wherein the planar portions of the first and second lead frame structures are soldered to the solder pads with the solder. 
     
     
       16. A display, comprising:
 upper and lower polarizers; 
 a thin-film-transistor layer between the upper and lower polarizers; 
 a color filter layer interposed between the first and second polarizers; 
 a layer of liquid crystal material interposed between the color filter layer and the thin-film-transistor layer; 
 a light guide plate for providing backlight that travels through the upper and lower polarizers, the thin-film transistor layer, the color filter layer, and the layer of liquid crystal material, wherein the light guide plate has an edge; 
 a dielectric substrate layer having solder pads; and 
 an array of light-emitting diodes that emit light into the edge, wherein each light-emitting diode has first and second lead frame structures, a semiconductor device mounted to the first lead frame structure and wirebonded to the first and second lead frame structures, and a plastic body that covers at least some of the first and second lead frame structures, wherein the plastic body has a front surface with a window through which the light is emitted, an opposing rear surface, a lower surface on which portions of the first and second lead frame structures are formed, and opposing end faces that are free of lead frame structures, and wherein the array of light-emitting diodes are soldered to the dielectric substrate layer by soldering the portions of the first and second lead frame structures on the lower surface to the solder pads with solder. 
 
     
     
       17. The display defined in  claim 16  wherein the first and second lead frame structures of each light-emitting diode comprise first and second respective lead frame terminal portions on the rear face of that light-emitting diode that are soldered to the solder pads with the solder. 
     
     
       18. The display defined in  claim 17  wherein the window comprises a phosphorescent window material having a window surface, wherein the front surface has a front surface area, wherein the window surface occupies more than 80% of the front surface area, and wherein the light guide plate comprises a die cut sheet of roll-to-roll polymer film. 
     
     
       19. Display backlight structures, comprising:
 a light guide plate having an edge; and 
 light-emitting diodes arranged in a row along the edge that emit light into the edge, wherein each light-emitting diode includes a package having opposing front and rear surfaces and having a printed circuit substrate that forms at least part of the rear surface of the package, wherein light is emitted into the edge of the light guide plate from the front surface, and wherein each light-emitting diode includes electrical contacts on the rear surface. 
 
     
     
       20. The display backlight structures defined in  claim 19  wherein each package includes light-redirecting structures attached to the printed circuit substrate. 
     
     
       21. The display backlight structures defined in  claim 20  wherein the electrical contacts on the rear surface comprise electrical contacts on the printed circuit substrate. 
     
     
       22. The display backlight structures defined in  claim 20  wherein the light-redirecting structures comprise a window that extends to the front surface through which the light passes and a wall structure and wherein the window is formed from material that fills a cavity formed by the wall structure and the printed circuit substrate. 
     
     
       23. The display backlight structures defined in  claim 19  wherein the printed circuit substrate has opposing first and second surfaces, wherein the first surface faces towards the edge of the light guide plate, wherein the second surface faces away from the edge of the light guide plate, and wherein the electrical contacts are formed on the second surface. 
     
     
       24. The display backlight structures defined in  claim 23  wherein the light guide plate comprises opposing front and rear surfaces connected by the edge. 
     
     
       25. The display backlight structures defined in  claim 24 , further comprising a reflector positioned along the rear surface of the light guide plate.

Description:
This application claims the benefit of provisional patent application No. 61/678,062, filed Jul. 31, 2012, which is hereby incorporated by reference herein in its entirety. 
    
    
     BACKGROUND 
     This relates generally to electronic devices, and more particularly, to electronic devices with displays. 
     Electronic devices often include displays. For example, cellular telephones and portable computers often include displays for presenting information to a user. An electronic device may have a housing such as a housing formed from plastic or metal. Components for the electronic device such as display components may be mounted in the housing. 
     It can be challenging to incorporate a display into the housing of an electronic device. Size and weight are often important considerations in designing electronic devices. If care is not taken, displays may be bulky or may be surrounded by overly large borders. 
     It would therefore be desirable to be able to provide improved backlit displays for electronic devices. 
     SUMMARY 
     An electronic device may be provided with a display that has display layers such as a layer of liquid crystal material interposed between a color filter layer and a thin-film-transistor layer. The color filter layer, liquid crystal layer, and thin-film transistor layer may be interposed between upper and lower polarizers. 
     Backlight structures may be used to provide backlight that passes through the display layers. The backlight structures may include a light guide plate formed from injection molded plastic or a clear polymer film. The light guide plate may have an edge into which light is emitted from an adjacent array of light-emitting diodes. 
     The light emitting diodes may each include a semiconductor device that emits light. The semiconductor device in each diode may be mounted on lead frame structures in a molded plastic package or may be formed as a portion of a chip-on-board package with attached light redirecting structures. 
     In configurations in which the semiconductor device is mounted on lead frame structures, the semiconductor device may be electrically connected to the lead frame structures using first and second wire bonds or using solder bumps on a bottom surface of the semiconductor device. 
     In configurations in which the semiconductor device is formed as a portion of a chip-on-board package with attached light redirecting structures, the attached light redirecting structures may be attached to a substrate using adhesive. The attached light redirecting structures may include ceramic, metal, plastic, or resin wall structures that at least partially surround the semiconductor device. In this type of configuration, the semiconductor device may be electrically connected to the electrical contacts on the substrate using wire bonds. 
     Light from each diode may be emitted through a window in the molded plastic package or the chip-on-board package. The window may occupy a relatively large portion of the front face of a package and may contain a phosphorescent filler material. 
     To improve backlight homogeneity and thereby reduce the mixing distance for light in the light guide plate, the diodes may be spaced closely together using diode packages having end faces that are free of lead frame structures. Lead frame structures for the light-emitting diodes may be formed under the light-emitting diodes and on rear surfaces of the light-emitting diodes. The exposed lead frame structures may be soldered to solder pads on a dielectric substrate. 
     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 perspective view of an illustrative electronic device such as a laptop computer with a display in accordance with an embodiment of the present invention. 
         FIG. 2  is a perspective view of an illustrative electronic device such as a handheld electronic device with a display in accordance with an embodiment of the present invention. 
         FIG. 3  is a perspective view of an illustrative electronic device such as a tablet computer with a display in accordance with an embodiment of the present invention. 
         FIG. 4  is a schematic diagram of an illustrative electronic device with a display in accordance with an embodiment of the present invention. 
         FIG. 5  is a cross-sectional side view of an illustrative display in accordance with an embodiment of the present invention. 
         FIG. 6  is a top view of backlight structures for a display in accordance with an embodiment of the present invention. 
         FIG. 7  is a cross-sectional side view of an illustrative light guide plate and an associated light-emitting diode that is injecting light into an edge of the light guide plate in accordance with an embodiment of the present invention. 
         FIG. 8  is a front view of an illustrative packaged light-emitting diode in accordance with an embodiment of the present invention. 
         FIG. 9  is a top view of an illustrative light guide plate and an associated array of light-emitting diodes for providing backlight in a display in accordance with an embodiment of the present invention. 
         FIG. 10  is a top view of a portion of a light guide plate showing how reduced diode-to-diode spacing in an array of light-emitting diodes may help minimize mixing distance in the light guide plate in accordance with an embodiment of the present invention. 
         FIG. 11  is a rear perspective view of an illustrative packaged light-emitting diode showing how lead frame terminals may be wrapped under and around the back of a light-emitting diode to allow the light-emitting diode to be mounted close to another light-emitting diode in an array of light-emitting diodes of the type shown in  FIG. 10  in accordance with an embodiment of the present invention. 
         FIGS. 12 ,  13 , and  14  are perspective views of illustrative lead frame configurations that may be used in forming a packaged light-emitting diode of the type shown in  FIG. 11  in accordance with an embodiment of the present invention. 
         FIG. 15  is a flow chart of illustrative steps involved in forming an electronic device having display backlight structures in accordance with an embodiment of the present invention. 
         FIG. 16  is a perspective view of an illustrative lead frame configuration that may be used in forming a packaged light-emitting diode of the type shown in  FIG. 11  in accordance with an embodiment of the present invention. 
         FIG. 17  is a rear perspective view of an illustrative packaged light-emitting diode showing how electrical contacts may be formed on a rear surface of a chip-on-board package to allow the light-emitting diode to be mounted close to another light-emitting diode in an array of light-emitting diodes of the type shown in  FIG. 10  in accordance with an embodiment of the present invention. 
         FIG. 18  is a cross-sectional side view of a chip-on-board package of the type shown in  FIG. 17  in accordance with an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Electronic devices may include displays. The displays may be used to display images to a user. Illustrative electronic devices that may be provided with displays are shown in  FIGS. 1 ,  2 , and  3 . 
       FIG. 1  shows how electronic device  10  may have the shape of a laptop computer having upper housing  12 A and lower housing  12 B with components such as keyboard  16  and touchpad  18 . Device  10  may have hinge structures  20  that allow upper housing  12 A to rotate in directions  22  about rotational axis  24  relative to lower housing  12 B. Display  14  may be mounted in upper housing  12 A. Upper housing  12 A, which may sometimes referred to as a display housing or lid, may be placed in a closed position by rotating upper housing  12 A towards lower housing  12 B about rotational axis  24 . 
       FIG. 2  shows how electronic device  10  may be a handheld device such as a cellular telephone, music player, gaming device, navigation unit, or other compact device. In this type of configuration for device  10 , housing  12  may have opposing front and rear surfaces. Display  14  may be mounted on a front face of housing  12 . Display  14  may, if desired, have a display cover layer or other exterior layer that includes openings for components such as button  26 . Openings may also be formed in a display cover layer or other display layer to accommodate a speaker port (see, e.g., speaker port  28  of  FIG. 2 ). 
       FIG. 3  shows how electronic device  10  may be a tablet computer. In electronic device  10  of  FIG. 3 , housing  12  may have opposing planar front and rear surfaces. Display  14  may be mounted on the front surface of housing  12 . As shown in  FIG. 3 , display  14  may have a cover layer or other external layer (e.g., a color filter layer or thin-film-transistor layer) with an opening to accommodate button  26  (as an example). 
     The illustrative configurations for device  10  that are shown in  FIGS. 1 ,  2 , and  3  are merely illustrative. In general, electronic device  10  may be a laptop computer, a computer monitor containing an embedded computer, a tablet computer, a cellular telephone, a media player, or other handheld or portable electronic device, a smaller device such as a wrist-watch device, a pendant device, a headphone or earpiece device, or other wearable or miniature device, a television, a computer display that does not contain an embedded computer, a gaming device, a navigation device, an embedded system such as a system in which electronic equipment with a display is mounted in a kiosk or automobile, equipment that implements the functionality of two or more of these devices, or other electronic equipment. 
     Housing  12  of device  10 , which is sometimes referred to as a case, may be formed of materials such as plastic, glass, ceramics, carbon-fiber composites and other fiber-based composites, metal (e.g., machined aluminum, stainless steel, or other metals), other materials, or a combination of these materials. Device  10  may be formed using a unibody construction in which most or all of housing  12  is formed from a single structural element (e.g., a piece of machined metal or a piece of molded plastic) or may be formed from multiple housing structures (e.g., outer housing structures that have been mounted to internal frame elements or other internal housing structures). 
     Display  14  may be a touch sensitive display that includes a touch sensor or may be insensitive to touch. Touch sensors for display  14  may be formed from an array of capacitive touch sensor electrodes, a resistive touch array, touch sensor structures based on acoustic touch, optical touch, or force-based touch technologies, or other suitable touch sensor components. 
     Displays for device  10  may, in general, include image pixels formed from light-emitting diodes (LEDs), organic LEDs (OLEDs), plasma cells, electrowetting pixels, electrophoretic pixels, liquid crystal display (LCD) components, or other suitable image pixel structures. In some situations, it may be desirable to use LCD components to form display  14 , so configurations for display  14  in which display  14  is a liquid crystal display are sometimes described herein as an example. It may also be desirable to provide displays such as display  14  with backlight structures, so configurations for display  14  that include a backlight unit may sometimes be described herein as an example. Other types of display technology may be used in device  10  if desired. The use of liquid crystal display structures and backlight structures in device  10  is merely illustrative. 
     A display cover layer may cover the surface of display  14  or a display layer such as a color filter layer, thin-film transistor layer or other portion of a display may be used as the outermost (or nearly outermost) layer in display  14 . For example, a color filter layer or thin-film transistor layer that is covered by a polarizer layer may form the outermost layer for device  10 . A display cover layer or other outer display layer may be formed from a transparent glass sheet, a clear plastic layer, or other transparent member. 
     Touch sensor components such as an array of capacitive touch sensor electrodes formed from transparent materials such as indium tin oxide may be formed on the underside of a display cover layer, may be formed on a separate display layer such as a glass or polymer touch sensor substrate, or may be integrated into other display layers (e.g., substrate layers such as a thin-film transistor layer). 
     A schematic diagram of an illustrative configuration that may be used for electronic device  10  is shown in  FIG. 4 . As shown in  FIG. 4 , electronic device  10  may include control circuitry  28 . Control circuitry  28  may include storage and processing circuitry for controlling the operation of device  10 . Control circuitry  28  may, for example, include storage such as hard disk drive storage, nonvolatile memory (e.g., flash memory or other electrically-programmable-read-only memory configured to form a solid state drive), volatile memory (e.g., static or dynamic random-access-memory), etc. Control circuitry  28  may include processing circuitry based on one or more microprocessors, microcontrollers, digital signal processors, baseband processors, power management units, audio codec chips, application specific integrated circuits, etc. 
     Control circuitry  28  may be used to run software on device  10 , such as operating system software and application software. Using this software, control circuitry  28  may present information to a user of electronic device  10  on display  14 . When presenting information to a user on display  14 , sensor signals and other information may be used by control circuitry  28  in making adjustments to the strength of backlight illumination that is used for display  14 . 
     Input-output circuitry  30  may be used to allow data to be supplied to device  10  and to allow data to be provided from device  10  to external devices. Input-output circuitry  30  may include communications circuitry  32 . Communications circuitry  32  may include wired communications circuitry for supporting communications using data ports in device  10 . Communications circuitry  32  may also include wireless communications circuits (e.g., circuitry for transmitting and receiving wireless radio-frequency signals using antennas). 
     Input-output circuitry  30  may also include input-output devices  34 . A user can control the operation of device  10  by supplying commands through input-output devices  34  and may receive status information and other output from device  10  using the output resources of input-output devices  34 . 
     Input-output devices  34  may include sensors and status indicators  36  such as an ambient light sensor, a proximity sensor, a temperature sensor, a pressure sensor, a magnetic sensor, an accelerometer, and light-emitting diodes and other components for gathering information about the environment in which device  10  is operating and providing information to a user of device  10  about the status of device  10 . 
     Audio components  38  may include speakers and tone generators for presenting sound to a user of device  10  and microphones for gathering user audio input. 
     Display  14  may be used to present images for a user such as text, video, and still images. Sensors  36  may include a touch sensor array that is formed as one of the layers in display  14 . 
     User input may be gathered using buttons and other input-output components  40  such as touch pad sensors, buttons, joysticks, click wheels, scrolling wheels, touch sensors such as sensors  36  in display  14 , key pads, keyboards, vibrators, cameras, and other input-output components. 
     A cross-sectional side view of an illustrative configuration that may be used for display  14  of device  10  (e.g., for display  14  of the devices of  FIG. 1 ,  FIG. 2 , or  FIG. 3  or other suitable electronic devices) is shown in  FIG. 5 . As shown in  FIG. 5 , display  14  may include backlight structures such as backlight unit  42  for producing light such as backlight  44 . During operation, backlight  44  travels outwards (vertically upwards in the orientation of  FIG. 5 ) and passes through display pixel structures in display layers  46 . This illuminates any images that are being produced by the display pixels for viewing by a user. For example, backlight  44  may illuminate images in display layers  46  that are being viewed by viewer  48  in direction  50 . 
     Display layers  46  and/or backlight structures  42  may be mounted in chassis structures such as a plastic chassis structure and/or a metal chassis structure to form a display module for mounting in housing  12  or display layers  46  and/or backlight structures  42  may be mounted directly in housing  12  (e.g., by stacking display layers  46  and/or backlight structures  42  into a recessed portion in housing  12 ). Display layers  46  and/or backlight structures  42  may form a liquid crystal display or may be used in forming displays of other types (e.g., field sequential color (FSC) displays that pulse colored light of various colors from the backlight structures into individual, fast pixels). 
     In a configuration in which display layers  46  are used in forming a liquid crystal display, display layers  46  may include a liquid crystal layer such a liquid crystal layer  52 . Liquid crystal layer  52  may be sandwiched between display layers such as display layers  58  and  56 . Layers  56  and  58  may be interposed between lower polarizer layer  60  and upper polarizer layer  54 . 
     Layers  58  and  56  may be formed from transparent substrate layers such as clear layers of glass or plastic. Layers  56  and  58  may be layers such as a thin-film transistor layer and/or a color filter layer. Conductive traces, color filter elements, transistors, and other circuits and structures may be formed on the substrates of layers  58  and  56  (e.g., to form a thin-film transistor layer and/or a color filter layer). Touch sensor electrodes may also be incorporated into layers such as layers  58  and  56  and/or touch sensor electrodes may be formed on other substrates. 
     With one illustrative configuration, layer  58  may be a thin-film transistor layer that includes an array of thin-film transistors and associated electrodes (display pixel electrodes) for applying electric fields to liquid crystal layer  52  and thereby displaying images on display  14 . Layer  56  may be a color filter layer that includes an array of color filter elements for providing display  14  with the ability to display color images. If desired, the positions of color filter layer  56  and thin-film-transistor layer  58  may be inverted so that the thin-film-transistor layer is located above the color filter layer. 
     During operation of display  14  in device  10 , control circuitry  28  (e.g., one or more integrated circuits such as components  68  on printed circuit  66  of  FIG. 5 ) may be used to generate information to be displayed on display  14  (e.g., display data). The information to be displayed may be conveyed from circuitry  68  to display driver integrated circuit  62  using a signal path such as a signal path formed from conductive metal traces in flexible printed circuit  64  (as an example). 
     Display driver integrated circuit  62  may be mounted on thin-film-transistor layer driver ledge  82  or elsewhere in device  10 . A flexible printed circuit cable such as flexible printed circuit  64  may be used in routing signals between printed circuit  66  and thin-film-transistor layer  60 . If desired, display driver integrated circuit  62  may be mounted on printed circuit  66  or flexible printed circuit  64 . Printed circuit  66  may be formed from a rigid printed circuit board (e.g., a layer of fiberglass-filled epoxy) or a flexible printed circuit (e.g., a flexible sheet of polyimide or other flexible polymer layer). 
     Backlight structures  42  may include a light guide plate such as light guide plate  78 . Light guide plate  78  may be formed from a transparent material such as clear glass or plastic. During operation of backlight structures  42 , a light source such as light source  72  may generate light  74 . Light source  72  may be, for example, an array of light-emitting diodes. 
     Light  74  from light source  72  may be coupled into edge surface  76  of light guide plate  78  and may be distributed laterally throughout light guide plate  78  due to the principal of total internal reflection. Light guide plate  78  may include light-extracting features such as pits or bumps. The light-extracting features such as light-scattering features may be located on an upper surface and/or on an opposing lower surface of light guide plate  78 . 
     Light  74  that is extracted from light guide plate  78  toward a viewer such as viewer  48  may serve as backlight  44  for display  14 . Light  74  that scatters downwards may be reflected back in the upwards direction by reflector  80 . Reflector  80  may be formed from a reflective material such as a layer of white plastic, other shiny materials, or from stacked layers of dielectric materials having relatively different indices of refraction (e.g., high and low indices of refraction) that cause reflection of light. 
     To enhance backlight performance for backlight structures  42 , backlight structures  42  may include optical films  70 . Optical films  70  may include diffuser layers for helping to homogenize backlight  44  and thereby reduce spatial non-uniformities/defects or hot spots, compensation films for enhancing off-axis viewing, and prism films (also sometimes referred to as turning films) for collimating backlight  44 . Optical films  70  may overlap the other structures in backlight unit  42  such as light guide plate  78  and reflector  80 . For example, if light guide plate  78  has a rectangular footprint when viewed in direction  50  of  FIG. 5  (i.e., when viewed as a top view), optical films  70  and reflector  80  may have a matching rectangular footprint. 
     As shown in the top view of display  14  of  FIG. 6 , display  14  may be characterized by a central rectangular active area AA (defined by dashed rectangular line  90 ) and an inactive border region (inactive area IA) that surrounds the periphery of active area AA. In active region AA, thin-film transistor layer  58  may contain an array of display pixels. Each display pixel may include electrode structures. Each display pixel may also include a thin-film transistor for controlling the amount of electric field that is applied to liquid crystal layer  52  by the electrode structures. During operation, gate line signals and data lines signals that are provided to the array of display pixels may be used to display images on display  14  for viewer  48 . 
     Backlight for display  14  may be generated by an array of light-emitting diodes  72 . Light-emitting diodes  72  may run along the upper edge of light guide plate  78  as shown in  FIG. 6 . If desired, more than one array of light-emitting diodes may be used to generate backlight. For example, in larger displays, it may be desirable to inject light from opposing edges of light guide plate  78 . With this type of configuration, light  74  may be generated using opposing upper and lower rows of light-emitting diodes or light-emitting diode arrays that run along the opposing left and right edges of light guide plate  78  (as examples). Backlight configurations for display  14  in which there are more than two arrays of light-emitting diodes may also be used. The configuration of  FIG. 6  in which light  74  is provided by a single row of light-emitting diodes that runs along the upper edge of light guide plate  78  is merely illustrative. 
     It may be desirable to minimize the size of inactive area IA. For example, it may be desirable to minimize the size of inactive area IA on the right and left of display  14  of  FIG. 6  to allow display  14  to fill more of the front face of device  10 . It may also be desirable to minimize the size of inactive area IA on the top and bottom edges of display  14  to provide more interior space for internal components in device  10  (e.g., antennas, buttons, speakers, integrated circuits, etc.). Reductions in the thickness of backlight structures such as light guide plate (i.e., in vertical direction  50  of  FIG. 5 ) may also be helpful. For example, thin backlight structures may allow the overall thickness of device  10  and display  14  to be minimized, thereby reducing device weight and bulk for a user or may allow additional space for other electronic components such as a battery. 
     If desired, light guide plate  78  may be implemented using a structure of the type shown in  FIG. 7 . With this type of configuration, light guide plate  78  may have a relatively thin planar portion  104  that overlaps active area AA. Light extracting structures such as bumps or pits may, if desired, be formed on the upper and/or lower surfaces of planar light guide portion  104  to enhance scattering of light  74  upwards as backlight  44 . 
     Light guide plate  78  may, in general, be formed from a clear substance such as glass or plastic. Plastic molding techniques, roll-to-roll film fabrication techniques, or other fabrication techniques may be used in forming polymer light guide plates. To help minimize the size of thickness T 1  (e.g., to a value of less than 0.4 mm, less than 0.3 mm, less than 0.25 mm, less than 0.2 mm, or less than 0.1 mm) it may be desirable to form light guide plate  78  from a polymer film (e.g., a flexible thin sheet of plastic that is formed using a roll-to-roll manufacturing process rather than a molding process). Configurations in which display  14  is formed using roll-to-roll film-based light guide plates are sometimes described herein as an example. This is, however, merely illustrative. Display  14  may use light guide plates formed by plastic molding, glass fabrication techniques, or other fabrication methods, if desired. 
     Locally thickened portion  100  of light guide plate  78  may be formed along the edge of light guide plate that is adjacent to light-emitting diodes  72 . Portion  102  may be formed by stamping, molding, embossing, adding thermoplastic material to portion  104  using heat, laminating additional film material, adding clear adhesive to light guide plate portion  104 , or by otherwise enhancing the thickness of the light guide plate (e.g., to produce a thickness T 2  that is greater than thickness T 1 ). Thickness T 2  may be, for example, 0.1 to 0.6 mm, less than 1 mm, or more than 0.2 mm. 
     As shown in  FIG. 7 , light-emitting diode  72  may have a package body such as package body  108 . Package body  108 , which may sometimes be referred to as a package, may be formed from a molded thermoplastic, ceramic, or resin (as examples). Light-emitting diode die  120  (e.g., a semiconductor device) may be used in producing light  74  that propagates in direction Z through package window  92 . Window  92  may be formed from a material such as epoxy or other polymer that includes phosphorescent filler material (e.g., phosphor). The phosphorescent material may help turn relatively bluish output light from device  120  into white light for backlighting display  14 . Window structures  92  may be formed on front edge  106  of light-emitting diode package  108 . Windows such as window structures  92  may encapsulate the die and phosphor, thereby allowing light generated from those two components to propagate outside of package  108  into air. Light  74  may be emitted from front edge (face)  106  and may be injected into opposing edge (face)  94  of light guide plate  78 . 
     Particularly in scenarios in which light guide plate  78  is characterized by a relatively small thickness, challenges may arise in ensuring that backlight  44  is sufficiently bright. To help ensure adequate brightness for backlight  44 , it may be desirable to use a relatively large window size for window  92 . As shown in  FIG. 8 , window  92  may have a rectangular shape and may occupy an area AS on front face  106  of package  108 . The total area of front face  106  (i.e., front face area AB) is equal to central window area AS plus the area of the front face portion of plastic package  108  that surrounds window  92 . To help enhance backlight power, it may be desirable to ensure that area AS divided by area AB is greater than 0.5, greater than 0.7, greater than 0.8, or greater than 0.9 (as examples). Other configurations for light-emitting diode package  108  may also be used, if desired (e.g., arrangements in which window area AS divided by total area AB is less than 0.5. The use of configurations for light-emitting diode  72  in which area AS divided by area AB is greater than 0.5 are merely illustrative. 
     The width of inactive border region IA in the vicinity of light-emitting diodes  72  is affected by the homogeneity of light  74  within light guide plate  78  in the vicinity of edge  94 . When first launched into light guide plate  78  from light-emitting diodes  72 , light  74  is concentrated. Unless light  74  is allowed to propagate for a distance in light guide plate  78  before entering under active area AA of display  14 , undesirable hotspots may be visible to viewer  48  (e.g., bright or dark zones (or alternating bright and dark zones) around the exit of each light-emitting diode). 
     One way in which to enhance light mixing within light guide plate  78  involves the use of light mixing features  110  along edge  94  of light guide plate  78 . Light mixing features  110  may be formed by molding features  110  into plate  78  (e.g., using a mold having features complimentary to features  110  for producing many similar plates  78 ), by patterning clear adhesive along edge  94 , by cutting grooves or other features into edge  94  (e.g., using a corrugated die), by laser trimming edge  94 , or by patterning edge  94  using other suitable manufacturing processes. Features  110  may, for example, take the form of vertical grooves running parallel to dimension Y. 
     In film-based light guide plates, it may be challenging to form light mixing features that are sufficiently clean (i.e., optically smooth) to avoid lossy backscattering. Accordingly, it may be desirable to form edge  94  using a straight die cut, laser cut, or slit cut, so that edge  94  forms a planar surface lying in the X-Y plane of  FIG. 9 . 
     When injecting light into light guide plate  78  in a configuration in which edge  94  is planar (and even in other configurations), care should be taken that there is a sufficient mixing distance L for propagating light  74  within light guide plate  78  before using the scattered backlight from light guide plate  78  to serve as backlight for active area AA. The minimum suitable distance before light  74  has mixed sufficiently to be used as display backlight  44  is sometimes referred to as the mixing distance for light guide plate  78 . As shown in  FIG. 9 , mixing distance L is the distance between edge  94  and the edge of active area AA. 
     Diodes  72  emit light with an angular spread (in the X-Z plane). For example, light  74  may be emitted within a cone that is bounded on the left by light ray  74 L and on the right by light ray  74 R. Light rays  74 L and  74 R may be determined by the critical angle of the material for a flat input face of edge  94 . Under this assumption, all light emitted from diodes  72  will be emitted within the angles indicated by rays  74 L and  74 R. (Angular spread in the Y dimension is limited by the relatively narrow separation between the upper and lower surfaces of light guide plate  78 ). 
     The value of mixing distance L is affected by the angular spread of light  74  and light-emitting diode center-to-center spacing S. When there are relatively large gaps WA between adjacent diodes  72 , the center-to-center spacing S of diodes  72  will be relatively large and (for a given angular spread of emitted light  74 ), a relatively larger value of L will be needed to adequately mix light  74  before using light  74  as backlight for active area AA. Larger values of WA will therefore lead to larger values of L and more inactive border for display  14 . To reduce the size of mixing length L and thereby reduce the size of the inactive border region IA for the portion of display  14  bordered by light-emitting diodes  72 , the minimum value of diode-to-diode separation WA for the diodes in the array of diodes that runs along the length of edge  94  may be minimized. 
     To help minimize diode-to-diode separation WA between the packages of adjacent diodes  72 , diodes  72  may be provided with terminals  112  that extend out of the rear of diodes  72  (in direction −Z), as shown in  FIG. 10 , rather than terminals that extend out of the sides of the diodes (in directions +/−X) as with conventional light-emitting diode packages for backlights. As shown in  FIG. 10 , small values of WA tend to reduce the diode center-to-center spacing S and thereby help light cones  74  from adjacent diodes  72  overlap and produce homogeneous light  74  within a relatively small mixing distance L. 
     Terminals  112  may include positive and negative power supply terminals. A power supply signal may be supplied across the positive and negative terminals for each diode  72  to adjust the power of the emitted light from that diode. Light-emitting diodes  72  may be mounted on a substrate such as substrate  122 . Substrate  122  may be a dielectric material. For example, substrate  122  may be a rigid printed circuit board (e.g., a printed circuit board formed from fiberglass-filled epoxy such as an FR4 board) or may be a flexible printed circuit (e.g., a printed circuit formed from a flexible sheet of polyimide or a layer of other flexible polymer). Metal traces such as illustrative trace  124  of  FIG. 10  may be formed on substrate  122  to provide power to terminals  112  of light-emitting diodes  72 . 
     Terminals  112  may be formed from bent metal structures of the type that are sometimes referred to as lead frames. A lead frame structure may be formed from a stenciled metal sheet. A lead frame package for diodes  72  may be formed by bending a sheet of patterned lead frame metal into a desired shape, overmolding a thermoplastic to form housing  108 , attaching die  120  to the lead frame structures using epoxy or a eutectic die attach material (e.g., solder), forming electrical connections between die  120  and respective lead frame structures (e.g., by using positive and ground wire bond wires or solder bumps on die  120  to form electrical paths to respective positive and ground terminal lead frame structures), and by forming window structure  92  in package  108  by filling a window-shaped recess within package  108  with a phosphorescent material such as epoxy with a phosphor filler. 
     A perspective view showing an illustrative configuration for light-emitting diodes  72  is shown in  FIG. 11 . As shown in the rear perspective view of  FIG. 11 , diode  72  may have lead frame structures  112  with rear portions  142  that lie flush with rear surface  114  of package  108 . There may, for example, be a pair of portions  142  that serve as positive and ground terminals for light-emitting diode  72 . Portions of lead frame structures  112  may also be formed on the lower surface of package  108 . Solder  128  may be interposed between these lower portions of lead frame structures  112  and may be used in soldering light-emitting diode  72  to substrate  122  (e.g., to solder lead frame structures  112  to solder pads  126  and other traces  124  on substrate  122 ). Solder  128  may extend between the portions of lead frame structures  112  that lie on the lower surface of package  108  and solder pad traces  126  on substrate  122 , thereby forming an electrical and mechanical connection between light-emitting diode  72  and substrate  122 . Some solder  128  may wick up the rear face of diode  72  (e.g., by wicking up and covering some or all of rear terminal portions  142  of lead frame structures  112  to form solder fillets of the type shown in  FIG. 11 ). Because end faces  200  of diode package  108  may be maintained partly or completely free of lead frame structures  112 , no lead frame structures need be interposed between diode  72  and adjacent diodes in the array of light-emitting diodes that is being used to provide light  74  to light guide plate  78 . As a result, the size of gap WA between the opposing end faces of diodes  72  in the array can be minimized to reduce mixing distance L. 
     Any suitable shape may be used for lead frame structures  112  so long as rear terminals  142  are formed to maintain some or all of the end faces of diodes  72  free of undesired bulky lead frame structures.  FIGS. 12 ,  13 , and  14  show illustrative configurations that may be used in forming a pair of lead terminal structures. As shown in  FIG. 12 , lead frame structures  112  may have a first structure  112 A and a second structure  112 B. Diode die  120  may be mounted on horizontal mounting surface  140  of structure  112 B. Wire bond  132  may electrically couple one of the diode&#39;s terminals to lead frame structure  112 B. Wire bond  130  may be used to connect another of the diode&#39;s terminals to surface  140  of lead frame structure  112 A. Structures  112 A and  112 B may be bent outwards and backwards as shown in  FIG. 12  to form planar rear surface terminal structures  142 , lying in the X-Z plane (e.g., on rear face  114  of diode package  108  in a configuration of the type shown in FIG.  11 ). 
     In the illustrative arrangement of  FIG. 13 , lead frame structures  112  have been bent inward and backwards to form rear terminals  142 . 
     In the illustrative arrangement of  FIG. 14 , lead frame structures  112  have been bent backwards to form lead frame rear terminals  142 . 
     Other types of lead frame configurations may be used in forming a package for light-emitting diodes  72  if desired. The configurations of  FIGS. 12 ,  13 , and  14  are merely illustrative. 
       FIG. 15  is a flow chart of illustrative steps involved in forming display  14  and device  10 . During the operations of step  160 , a thin layer of transparent material for light guide plate  78  may be formed (e.g., using a roll-to-roll process). Light guide plate  78  may be, for example, a thin polymer film having a thickness of less than 0.4 mm, less than 0.3 mm, less than 0.25 mm, less than 0.2 mm, greater than 0.1 mm, or other suitable thickness. 
     During the operations of step  162 , wedge portion  110  (e.g., a thickened edge portion) of light guide plate  78  may be formed. For example, additional polymer may be formed on an edge portion of the film of polymer that was produced using the roll-to-roll process of step  160 . Features for scattering light such as light mixing features  110  may be incorporated into the edge of light guide plate  78  or may be omitted (e.g., to minimize light scattering of the type that may reduce backlight efficiency). 
     At step  164 , light-emitting diodes  72  (e.g., light-emitting diodes with terminal-free end faces and rear and lower surfaces having exposed portions of lead frame structures) may be mounted on substrate  122  using solder  128 . The spacing (gap) WA between respective adjacent diodes (e.g., between opposing end faces  200 ) may be minimized (e.g., to less than 2 mm, less than 1 mm, less than 0.5 mm, less than 0.25 mm, or less than 0.125 mm, as examples). The width of each package (along dimension X) may be, as an example, 1-8 mm, more than 1 mm, less than 8 mm, 3-5 mm, or other suitable size. The lateral dimensions of the solder fillet formed by solder  128  at the rear of package  108  may be about 0.5 to 3 mm or less than 3 mm (as examples). The spacing between solder pads  126  may be about 1-10 mm, less than 2 mm, less than 1 mm, less than 0.2 mm, less than 10 mm, or more than 1 mm (as examples). 
     Although lead frame terminals  142  have been depicted in  FIG. 11  as being formed in the left rear and right rear corners of package  108 , terminals  142  may, if desired, be formed at intermediate locations along the rear surface of package  108 , if desired (e.g., to provide more lateral spacing between the solder connections associated with adjacent light-emitting diodes. 
       FIG. 16  shows an illustrative configuration that may be used in forming a pair of lead terminal structures that connect to a diode die such as die  120  that has been implemented using a flip-chip arrangement. As shown in  FIG. 16 , first lead frame structure  112 A and a second lead frame structure  112 B may each be coupled to solder bumps  131  on a bottom surface of die  120 . Diode die  120  may be mounted on horizontal mounting surfaces  140  of structures  112 A and  112 B. Solder bumps  131  may electrically couple one of the diode&#39;s terminals to lead frame structure  112 B and another of the diode&#39;s terminals to surface  140  of lead frame structure  112 A. Structures  112 A and  112 B may be bent outwards and backwards as shown in  FIGS. 12 and 16 , may be bent as shown in  FIG. 13  or  14 , or may be bent into other suitable configurations for forming planar rear surface terminal structures  142 , lying in the X-Z plane (e.g., on rear face  114  of diode package  108  in a configuration of the type shown in  FIG. 11 ). 
     The examples of  FIGS. 11 ,  12 ,  13 ,  14 , and  16  in which light-emitting diode  72  is formed from a semiconductor device that is mounted to lead frame structures in a plastic package are merely illustrative. If desired, light-emitting diode  72  may be formed as a chip-on-board package such as a chip-on-ceramic package with attached light-redirecting structures as in the example of  FIG. 17 . As shown in  FIG. 17 , light-emitting diode  72  may be formed on a substrate such as substrate  300  (e.g., a ceramic substrate having electrical interconnects and other electrical components in a layered circuit stack). 
     Substrate  300  may, for example, be a printed circuit board having a light-emitting component attached to the printed circuit board or formed as an integrated component of the printed circuit board. 
     Substrate  300  may include electrical contacts  306  that lie flush with rear surface  114  of package  108 . There may, for example, be a pair of contacts  306  that serve as positive and ground terminals for light-emitting diode  72 . 
     Wire bonds  307  may be coupled between contacts  306  on substrate  300  and electrical contacts  304  on substrate  122 , thereby forming an electrical connection between light-emitting diode  72  and substrate  122 . 
     In configurations in which diode  72  includes a chip-on-board semiconductor device such as substrate  300 , substrate  300  may be attached to light-redirecting structures  302 . Light-redirecting structures  302  may include an opaque plastic, metal, resin, or ceramic wall that at least partially surrounds a light-emitting portion of substrate  300 . Structures  302  may be partially or completely filled with material that forms a window such as window  92  ( FIG. 8 ) on front face  106  of diode  72  that allows light  74  to escape from diode  72 . However, this is merely illustrative. If desired, structures  302  may be formed from a molded encapsulant dome over the light emitting-portion of substrate  300  that allows light  74  to escape from front face  106  of diode  72 . 
     In configurations in which diode  72  includes a chip-on-board semiconductor device such as substrate  300 , diode  72  may be mechanically attached to substrate  72  using adhesive  320 . Adhesive  320  may be a pressure-sensitive adhesive, a light-cured adhesive, or other suitable adhesive. 
     In the example of  FIG. 17 , because end faces  200  of diode package  108  may be maintained partly or completely free of lead frame structures  112  or electrical contacts  306 , no structures need be interposed between diode  72  and adjacent diodes in the array of light-emitting diodes that is being used to provide light  74  to light guide plate  78 . As a result, the size of gap WA between the opposing end faces of diodes  72  in the array can be minimized to reduce mixing distance L. 
       FIG. 18  shows a cross-sectional side view of diode  72  of  FIG. 17  showing how substrate  300  may include semiconductor device  120  that is attached to substrate  300  (e.g., using solder) or that is formed as an integrated portion of chip-on-board substrate  300 . Substrate  300  may be formed from ceramic, metal, layers of dielectric and conductive materials, etc. Substrate  300  may be a printed circuit substrate that includes conductive interconnects  322  that couple contacts  306  to semiconductor device  120 . Semiconductor device  120  may emit light  314 . Light  314  may be released directly from device  120  or may interact with filler material  310  prior to exiting diode  72  from front face  106 . 
     Filler material  310  may be formed between wall structures  308  that are attached to substrate  300  to form window (also see, e.g.,  FIG. 8 ). Wall structures  308  may be formed from plastic, metal, resin, ceramic or other materials. Wall structures  308  and printed circuit substrate  300  may form a cavity. Filler material  310  may be formed in the cavity so that wall structures  308  at least partially surround window  92 . Filler material  310  may include epoxy or other polymer that includes phosphorescent filler material (e.g., phosphor). The phosphorescent material may help turn relatively bluish output light from device  120  into white light for backlighting display  14 . Window structures  92  may extend from substrate  300  to front edge  106  of light-emitting diode package  108 . Light  74  may be emitted from front edge (face)  106  and may be injected into an opposing edge of light guide plate  78  (see, e.g.,  FIG. 7 ). 
     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.

Metadata:
Filing Date: 20120807
Publication Date: 20160119
Grant Date: 20160119
Priority Date: 20120731
Inventors: DOYLE DAVID A.
GETTEMY SHAWN R.
Assignee: APPLE INC
CPC Classifications: [{"code": "G02B6/002", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02B6/0035", "inventive": false, "first": false, "tree": "[]"}, {"code": "G02B6/0055", "inventive": false, "first": false, "tree": "[]"}, {"code": "G02F1/133608", "inventive": true, "first": true, "tree": "[]"}, {"code": "G02B6/0035", "inventive": false, "first": false, "tree": "[]"}, {"code": "G02B6/0055", "inventive": false, "first": false, "tree": "[]"}, {"code": "G02B6/002", "inventive": true, "first": false, "tree": "[]"}, {"code": "H01L2224/48257", "inventive": false, "first": false, "tree": "[]"}, {"code": "G02F1/133608", "inventive": true, "first": true, "tree": "[]"}, {"code": "H01L2224/48247", "inventive": false, "first": false, "tree": "[]"}, {"code": "G02B6/0035", "inventive": false, "first": false, "tree": "[]"}, {"code": "G02B6/0055", "inventive": false, "first": false, "tree": "[]"}, {"code": "G02F2001/133612", "inventive": false, "first": false, "tree": "[]"}, {"code": "G02F1/133608", "inventive": true, "first": true, "tree": "[]"}, {"code": "G02B6/002", "inventive": true, "first": false, "tree": "[]"}, {"code": "H01L2224/48257", "inventive": false, "first": false, "tree": "[]"}, {"code": "G02F1/133612", "inventive": false, "first": false, "tree": "[]"}, {"code": "G02F1/133612", "inventive": false, "first": false, "tree": "[]"}, {"code": "H01L2224/48247", "inventive": false, "first": false, "tree": "[]"}]
Family ID: 50025163