PATENT DOCUMENT

Publication Number: US-9939672-B2
Application Number: US-201414498814-A
Country: US
Kind Code: B2

Title: Electronic device with heat spreading film

Abstract:
An electronic device may have a housing in which components are mounted that produce heat. The heat producing components may be light-emitting diodes mounted on a flexible printed circuit in a display backlight, may be integrated circuits, or may be other devices that generate heat during operation. A heat spreading layer such as a layer of graphite may be attached to the backlight unit or other structures in the electronic device using adhesive. The adhesive may be patterned to form an unbonded area between at least some of the backlight unit or other structures to which the heat spreading layer is being attached and the heat spreading layer. The heat spreading layer may be mounted adjacent to a housing structure such as a metal midplate member that is attached to housing walls in the housing.

Claims:
What is claimed is: 
     
       1. A display, comprising:
 display layers including a layer of liquid crystal material; 
 backlight structures that supply backlight that passes through the display layers; and 
 a heat spreading layer that is attached to the backlight structures, wherein the heat spreading layer includes patterned adhesive that attaches first portions of the heat spreading layer to the backlight structures and that does not attach second portions of the heat spreading layer to the backlight structures, wherein the heat spreading layer is positioned in a first position at a first temperature and a second position at a second temperature, wherein in the first position the first and second portions of the heat spreading layer are separated from a metal midplate by an air gap, wherein in the second position the first portions of the heat spreading layer are separated from the metal midplate by the air gap and the second portions of the heat spreading layer directly contact the metal midplate, and wherein the second temperature is greater than the first temperature. 
 
     
     
       2. The display defined in  claim 1  wherein the heat spreading layer comprises a layer of graphite. 
     
     
       3. The display defined in  claim 1  wherein the heat spreading layer comprises a layer of copper. 
     
     
       4. The display defined in  claim 1  wherein the backlight structures include a light guide plate and a reflector and wherein the patterned adhesive attaches the heat spreading layer to the backlight structures so that the reflector is interposed between the heat spreading layer and the light guide plate. 
     
     
       5. The display defined in  claim 4  further comprising:
 light-emitting diodes that supply light to the light guide plate; and 
 a flexible printed circuit on which the light-emitting diodes are mounted. 
 
     
     
       6. The display defined in  claim 5  further comprising light blocking tape that is attached to the flexible printed circuit and the reflector. 
     
     
       7. The display defined in  claim 6  wherein the light blocking tape is interposed between the heat spreading structures and the flexible printed circuit. 
     
     
       8. The display defined in  claim 7  wherein the patterned adhesive includes patterned adhesive selected from the group consisting of: a C-shape strip of adhesive, an array of adhesive patches, a cross-shaped region of adhesive, and parallel strips of adhesive. 
     
     
       9. An electronic device, comprising:
 a housing that includes a metal midplate member; 
 display layers including a layer of liquid crystal material; 
 backlight structures that supply backlight that passes through the display layers; and 
 a flexible heat spreading layer that is attached to the backlight structures, wherein at first temperature the entire flexible heat spreading layer is separated from the metal midplate member by an air gap, wherein at a second temperature at least a portion of the flexible heat spreading layer expands across the air gap to contact the metal midplate member, wherein the second temperature is greater than the first temperature, and wherein the flexible heat spreading layer is heated from the first temperature to the second temperature by heat generated by the backlight structures. 
 
     
     
       10. The electronic device defined in  claim 9  wherein the heat spreading layer comprises:
 at least one polymer layer; 
 a layer of graphite on the polymer layer; 
 adhesive that attaches the heat spreading layer to the backlight structures. 
 
     
     
       11. The electronic device defined in  claim 10  wherein the backlight structures comprise:
 light-emitting diodes on a flexible printed circuit; and 
 a reflector. 
 
     
     
       12. The electronic device defined in claim  11  wherein the adhesive forms a strip that runs along at least one edge of the backlight structures and creates an unbonded area between the heat spreading layer and the backlight structures. 
     
     
       13. The electronic device defined in  claim 12  wherein the backlight structures comprise a chassis, wherein the backlight structures include a light guide plate that receives light from the light-emitting diodes, wherein the light guide plate and the reflector are mounted in the chassis, and wherein at least some of the adhesive is attached to the chassis. 
     
     
       14. An apparatus, comprising:
 a flexible structure that is supplied with heat in an electronic device, wherein the flexible structure is a flexible structure selected from the group consisting of: an organic light-emitting diode display and a flexible printed circuit in a backlight unit; and 
 a flexible heat spreading layer having adhesive that is patterned to attach the flexible heat spreading layer to the flexible structure so that there is at least one unbonded region of the flexible heat spreading layer that is free of adhesive, wherein the unbonded region is interposed between first and second regions of the flexible heat spreading layer that are attached to the flexible structure with the adhesive, wherein at a first temperature the entire flexible heat spreading layer is separated from a metal structure by and air gap, wherein at a second temperature at least a portion of the unbonded region of the flexible heat spreading layer extends across the air gap to directly contact the metal structure, and wherein the second temperature is greater than the first temperature. 
 
     
     
       15. The apparatus defined in  claim 14  wherein the flexible structure comprises the organic light-emitting diode display. 
     
     
       16. The apparatus defined in  claim 15  wherein the flexible heat spreading layer includes a layer of graphite. 
     
     
       17. The apparatus defined in  claim 14  wherein the flexible structure comprises the flexible printed circuit in the backlight unit, wherein the backlight unit includes light-emitting diodes on the flexible printed circuit, a light guide plate, and a reflector, and wherein the apparatus further comprises light-blocking tape attached to the flexible printed circuit and the reflector. 
     
     
       18. The apparatus defined in  claim 14  wherein the metal structure is a stainless steel plate. 
     
     
       19. The apparatus defined in  claim 14 , wherein the flexible heat spreading layer is parallel to the metal structure at the first temperature. 
     
     
       20. The apparatus defined in  claim 14 , wherein the heat spreading layer has at least one bend at the second temperature.

Description:
This application claims the benefit of provisional patent application No. 62/011,850, filed Jun. 13, 2014, which is hereby incorporated by reference herein in its entirety. 
    
    
     BACKGROUND 
     This relates generally to electronic devices, and more particularly, to electronic devices with components that generate heat. 
     Electronic devices often include components that generate heat. For example, a display may have a backlight unit that provides backlight illumination. The backlight unit may have a light guide plate that distributes backlight from a row of light-emitting diodes. The light-emitting diodes and other electrical components in a device may generate waste heat. Heat can be dissipated through portions of a device housing, such as metal housing structures. If care is not taken, however, hotspots can develop due to concentrations of heat-producing components. To minimize the formation of thermal hotspots when the light-emitting diodes in a backlight unit are used, control circuitry may restrict the amount of light produced by the light-emitting diodes whenever the temperature rise associated with use of the light-emitting diodes has become too large. This can result in undesirably dim output levels from the backlight unit, making images on the display difficult to view in bright ambient lighting conditions. 
     It would therefore be desirable to be able to provide improved arrangements for managing heat in electronic devices such as devices with displays. 
     SUMMARY 
     An electronic device may have a housing in which components are mounted that produce heat. The heat-producing components may be light-emitting diodes mounted on a flexible printed circuit in a display backlight, may be integrated circuits, or may be other devices that generate heat during operation. A heat spreading layer such as a layer of graphite, copper, or other material with an elevated thermal conductivity level, may be attached to the backlight unit or other structures in the electronic device. The light-emitting diodes or other heat generating components may overlap the heat spreading layer. During operation, heat that is produced by the heat generating components may be spread laterally thorough the heat spreading layer, which helps dissipate heat efficiently. 
     Adhesive may be used in attaching the heat spreading layer to the backlight unit. The adhesive that is used in attaching the heat spreading layer to the backlight unit may be patterned to ensure that at least some portion of the heat spreading layer is not bonded to the backlight unit and can therefore move relative to the backlight unit to accommodate expansion and contraction of the heat spreading layer and other display layers during operation of the backlight unit. 
     The heat spreading layer may be mounted adjacent to a housing structure such as a metal midplate member that is attached to housing walls in the housing of the electronic device. Heat may flow between the heat spreading layer and the metal midplate member across an air gap. In some configurations, the heat spreading layer may expand against the metal midplate member during operation acting as a thermal “switch.” 
     The layers of material in the heat spreading layer may be sufficiently flexible to bend with other flexible layers in an electronic device. As an example, the heat spreading layer may be attached to a flexible structure such as a flexible printed circuit or a flexible organic light-emitting diode display. The adhesive that is used to attaching the heat spreading layer to the flexible structure may be formed only along the edges of the structure or may be arranged in other patterns that ensure that there are at least some unbonded regions between portions of the flexible structure and the heat spreading layer. The presence of the unbounded regions helps accommodate bending of the flexible structure during operation. 
    
    
     
       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. 
         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. 
         FIG. 3  is a perspective view of an illustrative electronic device such as a tablet computer with a display in accordance with an embodiment. 
         FIG. 4  is a perspective view of an illustrative electronic device such as a computer display with display structures in accordance with an embodiment. 
         FIG. 5  is a cross-sectional side view of an illustrative display in accordance with an embodiment. 
         FIG. 6  is a cross-sectional side view of an illustrative heat producing component and an associated heat spreading structure in accordance with an embodiment. 
         FIG. 7  is a cross-sectional side view of a heat spreading layer in accordance with an embodiment. 
         FIG. 8  is a perspective view of an illustrative backlight unit in accordance with an embodiment. 
         FIG. 9  is a perspective view of an illustrative flexible printed circuit that has been populated with components such as light-emitting diodes in accordance with an embodiment. 
         FIG. 10  is a top view of an end portion of a backlight unit having an array of light-emitting diodes in accordance with an embodiment. 
         FIG. 11  is an exploded perspective view of an illustrative backlight unit and an associated heat spreading layer in accordance with an embodiment. 
         FIG. 12  is a cross-sectional side view of a portion of a display in which a heat spreading layer is being used to spread heat away from a light-emitting diode in accordance with an embodiment. 
         FIG. 13  is a top view of an illustrative configuration for a display in which a heat spreading layer is attached to a reflector or other layer in the display using a rectangular patch of adhesive in accordance with an embodiment. 
         FIG. 14  is a top view of an illustrative configuration for a display in which a heat spreading layer is attached to a reflector or other layer in the display using an array of rectangular adhesive patches in accordance with an embodiment. 
         FIG. 15  is a top view of an illustrative configuration for a display in which a heat spreading layer is attached to a reflector or other layer in the display using an array of circular adhesive patches in accordance with an embodiment. 
         FIG. 16  is a top view of an illustrative configuration for a display in which a heat spreading layer is attached to a reflector or other layer in the display using cross-shaped adhesive areas in accordance with an embodiment. 
         FIG. 17  is a top view of an illustrative configuration for a display in which a heat spreading layer is attached to a reflector or other layer in the display using elongated strips of adhesive in accordance with an embodiment. 
         FIG. 18  is a top view of an illustrative configuration for a display in which a heat spreading layer is attached to a reflector or other layer in the display using a C-shaped strip of adhesive that runs along the peripheral edges of a backlight unit in accordance with an embodiment. 
         FIG. 19  is a cross-sectional side view of a device structure such as a display layer or other flexible layer to which a heat spreading layer has been attached in accordance with an embodiment. 
         FIG. 20  is a cross-sectional side view of the device structure of  FIG. 19  during operation at an elevated temperature that has caused the heat spreading layer to expand and buckle in an unbonded (adhesive-free) region in accordance with an embodiment. 
         FIG. 21  is a cross-sectional side view of the device structure of  FIG. 20  following additional heating that has caused the heat spreading layer to expand sufficiently press against a housing structure or other heat sinking structure in accordance with an embodiment. 
         FIG. 22  is a cross-sectional side view of an illustrative flexible structure to which a heat spreading layer has been attached with adhesive in a configuration that leaves a portion of the heat spreading layer unbonded to the flexible structure to accommodate bending of the flexible structure in accordance with an embodiment. 
         FIG. 23  is a cross-sectional side view of the illustrative structure of  FIG. 22  following bending of the middle of the structure in a downward direction in accordance with an embodiment. 
         FIG. 24  is a cross-sectional side view of the illustrative structure of  FIG. 23  following bending of the middle of the structure in an upward direction in accordance with an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Electronic devices may include components that produce heat. For example, an electronic device may have a display with a backlight unit. The backlight unit may have light-emitting diodes that produce backlight illumination for the display. The light-emitting diodes produce heat during operation. Integrated circuits and other components in an electronic device may also produce heat. To avoid excessive concentrations of heat (hotspots), an electronic device may be provided with a heat spreading layer. The heat spreading layer may be formed from a layer of graphite, carbon nanotubes, copper, and/or other layers of material with high thermal conductivity values. The heat spreading layer distributes heat so that excessive concentrations of heat within the electronic device are avoided. 
     The heat spreading layer may be attached to the backlight unit of a display. To prevent undesired wrinkling of layers in the display such as a reflector layer in the backlight unit, the heat spreading layer may be attached to the backlight unit using a C-shaped strip of adhesive or adhesive that has been patterned using other sparse patterns. With this type of arrangement, the adhesive bonds some of the heat spreading layer to the backlight unit, but is absent in other areas. The areas in the heat spreading layer in which the adhesive is absent remain unbonded to the backlight unit and can therefore shift in position to accommodate expansion and contraction during thermal cycling without disrupting satisfactory operation of the backlight unit. 
     Illustrative electronic devices that may be provided with displays and other structures that include heat spreading layers for spreading heat produced by light-emitting diodes and other heat generating components are shown in  FIGS. 1, 2, 3, and 4 . 
     Illustrative electronic device  10  of  FIG. 1  has 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 be 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 openings for components such as button  26 . Openings may also be formed in display  14  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 an opening to accommodate button  26  (as an example). 
       FIG. 4  shows how electronic device  10  may be a computer display, a computer that has been integrated into a computer display, or a display for other electronic equipment. With this type of arrangement, housing  12  for device  10  may be mounted on a support structure such as stand  30  or stand  30  may be omitted (e.g., stand  30  can be omitted when mounting device  10  on a wall). Display  14  may be mounted on a front face of housing  12 . 
     The illustrative configurations for device  10  that are shown in  FIGS. 1, 2, 3, and 4  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, metal midplate members, 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. 
     Display  14  for device  10  may include display pixels formed from liquid crystal display (LCD) components or other suitable image pixel structures. 
     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 . The outermost display layer may be formed from a transparent glass substrate, a clear plastic layer, or other transparent substrate member. 
     A cross-sectional side view of an illustrative configuration for display  14  of device  10  (e.g., for display  14  of the devices of  FIG. 1 ,  FIG. 2 ,  FIG. 3 ,  FIG. 4  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 backlight  44 . During operation, backlight  44  travels outwards (vertically upwards in dimension Z 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 on display layers  46  that are being viewed by viewer  48  in direction  50 . 
     Display layers  46  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  may be mounted directly in housing  12  (e.g., by stacking display layers  46  into a recessed portion in housing  12 ). Display layers  46  may form a liquid crystal display or may be used in forming displays of other types. 
     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 as 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, lower layer  58  may be a color filter layer and upper layer  56  may be a thin-film transistor layer. Another illustrative configuration involves forming color filter elements and thin-film transistor circuits with associated pixel electrodes on a common substrate. This common substrate may be the upper substrate or may be the lower substrate and may be used in conjunction with an opposing glass or plastic layer (e.g., a layer with or without any color filter elements, thin-film transistors, etc.) to contain liquid crystal layer  52 . 
     During operation of display  14  in device  10 , control circuitry (e.g., one or more integrated circuits on a printed circuit) may be used to generate information to be displayed on display  14  (e.g., display data). The information to be displayed may be conveyed to a display driver integrated circuit such as circuit  62 A or  62 B using a signal path such as a signal path formed from conductive metal traces in a rigid or flexible printed circuit such as printed circuit  64  (as an example). 
     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. If desired, light sources such as light source  72  may be located along multiple edges of light guide plate  78 . 
     Light  74  from light source  72  may be coupled into edge surface  76  of light guide plate  78  and may be distributed in dimensions X and Y throughout light guide plate  78  due to the principal of total internal reflection. Light guide plate  78  may include light-scattering features such as pits or bumps. The 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 scatters upwards in direction Z from light guide plate  78  may serve as backlight  44  for display  14 . Light  74  that scatters downwards may be reflected back in the upwards direction by a reflective film such as reflector  80 . Reflector  80  may be formed from a reflective material such as a reflective polymer layer, a metalized layer, a layer of white plastic, or other reflective materials. Reflector  80  may, as an example, have multiple dielectric layers with alternating high and low index of refraction values that form a dielectric stack that efficiently reflects light  74 . 
     To enhance backlight performance for backlight structures  42 , backlight structures  42  may include optical films  70 . Optical films  70  may include one or more diffuser layers for helping to homogenize backlight  44  and thereby reduce undesired concentrations of backlight and one or more prism films (also sometimes referred to as turning films or brightness enhancement films) for collimating backlight  44 . Compensation films for enhancing off-axis viewing may be included in optical films  70  or may be incorporated into other portions of display  14  (e.g., compensation films may be incorporated into one or more polarizer layers). 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 in the X-Y plane of  FIG. 5 , optical films  70  and reflector  80  may have a matching rectangular footprint. 
     During operation of device  10 , integrated circuits, light-emitting diodes, and other components produce heat. A heat spreading layer may spread the heat within device  10  to avoid creating thermal hotspots. An illustrative configuration in which heat is being spread using a heat spreading layer is shown in  FIG. 6 .  FIG. 6  is a cross-sectional side view of illustrative device structures in device  10 . Component  90  is an electrical component that produces heat. Component  90  may, for example, include an integrated circuit or a light-emitting diode (as examples). Heat spreading layer  92  may spread heat  96  that is emitted from component  90 . In the example of  FIG. 6 , heat spreading layer  92  lies in the X-Y plane and spreads heat  96  laterally in dimensions X and Y. Heat  96  also flows vertically (in the −Z direction of  FIG. 6 ). In particular, heat  96  that has passed from component  90  to heat spreading layer  92  and that has been laterally distributed using heat spreading layer  92  may pass to structure  94 . Structure  94  may be a metal structure or other structure that helps to dissipate heat  96 . Structure  94  may be, for example, a metal layer such as a metal housing midplate (e.g., a stainless steel sheet metal layer or other planar housing member in device  10 ). A housing midplate may be located in the middle of device housing  12 . The midplate (i.e., structure  94 ) may, as an example, span the interior of device  10  between opposing housing edges in housing  12 . The midplate may be welded or otherwise attached to housing  12  around the periphery of housing  12 . 
     In the absence of heat spreading layer  92 , heat  96  may remain concentrated in the vicinity of component  90 , creating an undesired thermal hotspot on device  12 . When heat spreading layer  92  is present in a location of the type shown in  FIG. 6 , heat  96  from component  90  spreads outwardly in dimensions X and Y within layer  92 . Heat  96  can then be transferred to structure  94 , which serves as a heat sink and helps further dissipate the heat. The enlarged surface area between heat spreader  92  and structure  94  that results from using heat spreader  92  to laterally spread heat  96  may help to improve heat dissipation efficiency. Heat spreader  92  may contact structure  94  at one or more locations or may be separated from structure  94  by an air gap. The air gap may serve as a buffer that allows the layers of a display or other structures in device  10  to be assembled without unnecessary risk of consuming more vertical space than is available within device (i.e., the air gap may serve to satisfy vertical alignment tolerance requirements). 
     Heat spreading layer  92  may be formed from any suitable material that has a high thermal conductivity and can therefore serve to spread heat  96 . Examples of materials that may be used for forming heat spreading layer  92  include metal (e.g., copper, other metals, or combinations of copper and other metals), carbon nanotubes, graphite, or other materials that exhibit high thermal conductivity. If desired, heat spreading layer  92  may be formed from two or more thermally conductive layers of different types (e.g., a layer of copper attached to a layer of graphite, etc.). Polymer carrier films may also be incorporated in layer  92  (e.g., to support a layer of graphite). 
     In the example of  FIG. 7 , heat spreading layer  92  is a graphite heat spreading layer having a graphite layer such as graphite layer  102  interposed between upper and lower polymer carrier layers  100  and  104 . An adhesive layer such as adhesive layer  98  may be applied to upper polymer layer  100  (as an example). Adhesive  98  may be pressure sensitive adhesive, liquid adhesive, adhesive that is cured by application of light (e.g., ultraviolet-light-cured adhesive), may be temperature sensitive adhesive, may be other types of adhesive, or may include two or more of these types of adhesive. If desired, one of layers  100  and  104  may be omitted (e.g., layer  100  may be omitted). Graphite layer  102  may have a thickness of 5-20 microns, more than 10 microns, less than 30 microns, or other suitable thickness. Polymer layers  100  and  104  may be formed from sheets of flexible material such as polyethylene terephthalate or other polymer material. 
       FIG. 8  is a perspective view of an illustrative backlight unit for device  10 . Backlight unit  42  may have backlight layers  106  (e.g., optical films  70 , light guide plate  78 , reflector  80 , etc.) in chassis  108 . Chassis  108  may be a plastic frame, a plastic frame molded over a metal frame, a metal frame, or other support structure. Backlight layers  106  and light-emitting diodes  72  may be assembled in chassis  108  to form backlight unit  42 . Display  14  may be formed by mounting backlight unit  42  under display layers  46 , as described in connection with  FIG. 5 . 
     As shown in the perspective view of  FIG. 9 , light-emitting diodes  72  may be mounted in a row (e.g., a linear array) on a substrate such as substrate  110 . Substrate  110  may be a plastic carrier, a printed circuit, or other suitable substrate structure. As an example, substrate  110  may be a printed circuit (e.g., a rigid printed circuit formed from a rigid printed circuit board material such as fiberglass-filled epoxy or a flexible printed circuit formed from a flexible printed circuit material such as a sheet of polyimide or other flexible polymer layer). The row of light-emitting diodes  72  may be mounted along one of the edges of light guide plate  78 . If desired, arrays of light-emitting diodes  72  may be formed along multiple edges of light guide plate  78  (e.g., opposing upper and lower edges, opposing right and left edges, etc.). 
       FIG. 10  is a top view of a portion of an illustrative backlight unit showing how chassis  108  may have notches such as notches  112  to accommodate light-emitting diodes  72 . Configurations for display  14  in which backlight unit  42  does not have notches  112  in chassis  108  may also be used. 
     In some situations, it may be desirable to prevent heat spreading layer  92  from being too widely and firmly attached to other structures in display backlight unit  42 , as mismatch in the coefficients of thermal expansion between the heat spreading layer and the structures to which the heat spreading layer is attached may lead to undesired artifacts (e.g., wrinkles in flexible layers). This concern can be addressed by attaching heat spreading layer  92  to structures in backlight unit  42  using adhesive that covers only a portion of heat spreading layer  92 , so that a portion of heat spreading layer  92  remains unbonded to backlight unit  42  by adhesive. 
     As an example, consider the scenario of  FIG. 11 .  FIG. 11  is an exploded perspective view of backlight unit  42  and associated layers within heat spreading layer  92 . As shown in  FIG. 11 , heat spreading layer  92  may include layers such as graphite layer  102  (or other heat spreading material or layers of multiple materials). Layer  102  may be supported by one or more supporting layers such as polymer layers  100  and  104 . Adhesive layer  98  may be used to attach heat spreading layer  92  to the underside of backlight unit  42 . During operation, light-emitting diodes  72  produce light  74  for backlight  44 . As a byproduct of the process of producing light  74 , light-emitting diodes  72  also produce heat. The heat is spread laterally in dimensions X and Y through heat spreading layer  92 . To facilitate the transfer of heat from light-emitting diodes  72  to heat spreading layer  92 , heat spreading layer  92  preferably overlaps light-emitting diodes  72  (i.e., light-emitting diodes  72  are located at lateral positions in dimensions X and Y that overlap underlying heat-spreading layer  92 ). 
     The heat that is produced by light-emitting diodes  72  causes the structures of  FIG. 11  to rise in temperature. Due to mismatches in the coefficients of thermal expansion for the structures of  FIG. 11 , the rise in temperature of layers  92  can give rise for a potential for one or more of the structures of  FIG. 11  to buckle. For example, one or more of layers  106  (e.g., reflector  80  on the lower surface of backlight unit  42 ) may wrinkle due to mismatches between the lateral expansion of the reflector or other layers in backlight unit  42  relative to the lateral expansion of heat spreading layer  92 . The tendency of rises in temperature to cause wrinkles can be minimized by configuring adhesive  98  to reduce the extent of the attachment between heat spreading layer  92  and backlight unit  42 . With one suitable arrangement, the thickness of adhesive layer  98  may be increased and/or the adhesive layer  98  may be formed using an adhesive material with an enhanced softness to give layer  98  an ability to shear slightly when transverse loads are applied during heating. With another suitable arrangement, which is illustrated in  FIG. 11 , adhesive  98  may cover only part of the surface area of heat spreading layer  92  (e.g., layers  100 ,  102 , and  104 ). In the  FIG. 11  example, adhesive  98  has been formed in a C-shaped strip that runs along the edges of backlight unit  42 . Adhesive  98  may overlap light-emitting diodes  72  and may attach heat spreading layer  92  to reflective layer  80  (on the bottom layer of backlight unit  42 ) or other portions of backlight unit  42  (e.g., chassis  108 ). Central opening  114  is formed in adhesive  98 , so that the portion of heat spreading layer  92  in area  114  is unbonded to backlight unit  42 . Opening  114  may extend across most of the width of display backlight unit  42  in dimension X (as an example). The C-shape of the strip of adhesive forming adhesive  98  allows adhesive  98  to surround unbonded area  114  on three sides (i.e., the strip of adhesive  98  in  FIG. 11  runs along three edges of opening  114 ). Due to the presence of unbonded region  114 , adhesive  98  is weakly attached to heat spreading layer  92 , thereby accommodating expansion and contraction of display structures during heating and cooling that results from use of light-emitting diodes  72 . 
       FIG. 12  is a cross-sectional side view of a portion of display backlight  42  and heat spreading layer  92  of  FIG. 11  mounted in device housing  12 . As shown in  FIG. 12 , device  10  may have a housing with an outer sidewall (wall  12 ) and an internal housing member such as midplate  122 . Housing wall  12  may be formed from aluminum, stainless steel, other metals, plastic, or other materials. Midplate  122  may be formed from metal and/or plastic molded over sheet metal parts (as an example). For example, midplate  122  may be formed from stainless steel or other metal that is attached to housing wall  12 . Welds  117  or other attachment mechanisms may be used in mounting midplate  122  to housing  12 . Midplate  122  may be formed from one or more sheet metal members that span housing  12  and can help provide device  10  with structural rigidity. If desired, midplate  122  can be omitted and a rear wall portion of housing  12  or other device structures can be used to provide device  10  with structural rigidity. The use of midplate  122  in device  10  is merely illustrative. 
     Light-emitting diodes  72  may be mounted on a printed circuit such as printed circuit  123 . Printed circuit  123  may be, for example, a flexible printed circuit. Reflector  80  may be used to reflect light that is traveling in light guide plate  78  upwards in direction Z to serve as backlight illumination  44  for display  14 . 
     To reduce stray light, a light blocking structure such as black tape  116  or other light blocking tape may be attached to backlight unit  42  under light-emitting diodes  42 . Black tape  116  may include a layer of adhesive such as pressure sensitive adhesive  118  (or liquid adhesive, temperature-sensitive adhesive, etc.) and a layer of opaque material such as black plastic layer  120 . As shown in  FIG. 12 , light-blocking tape  116  may overlap light-emitting diodes  72  (and flexible printed circuit  123 ) and an adjoining portion of reflector  80 . 
     Heat spreading layer  92  may be attached to display  14  by using adhesive  98  to attach one or more regions of layer  92  to the lower surface of backlight unit  42 . With the illustrative configuration of  FIG. 12 , adhesive  98  attaches heat spreading layers  100 ,  102 , and  104  of heat spreading layer  92  to the lower surface of black tape  116 . Heat is produced while using light-emitting diodes  72  to produce backlight. The heat is spread laterally in dimensions X and Y by heat spreading layer  92 . Heat spreading layer  92  may be separated (at least initially) from midplate  122  by an air gap H. Because heat from light-emitting diodes  72  is spread laterally over a relatively large area (i.e., the entire area of heat spreading layer  92 ), the heat can efficiently travel downwards across any air gap H that lies between heat spreading layer  92  and midplate  122  into midplate  122 . Once the heat has reached midplate  122 , midplate  122  can dissipate the heat through other device structures and/or the surrounding environment of device  10  (e.g., heat can pass from midplate  122  to the walls of housing  12  and other portions of device 10  and can pass from housing  12  and the other portions of device  10  to the air surrounding device  10 ). 
     Tape  116  is attached to backlight unit structures such as chassis  108 , flexible printed circuit  123 , and reflector  80  and therefore forms part of backlight unit  42 . Adhesive  98  preferably attaches heat spreading layer  92  to backlight unit  42  using an arrangement for adhesive  98  that accommodates thermal expansion and contraction without causing undesired wrinkles in reflector  80  or other structures of backlight unit  42  (e.g., flexible printed circuit  123 ). Illustrative configurations for adhesive  98  are shown in  FIGS. 13, 14, 15, 16, 17, and 18 . 
     In the configuration of  FIG. 13 , adhesive  98  has a blanket rectangular shape that covers substantially all of heat spreading layer  92 . In this type of configuration (and, if desired, in the configurations of  FIGS. 14, 15, 16, 17, and 18 ), adhesive  98  may be formed using a layer of material that is relatively thick and/or soft. With this approach, adhesive  98  is loosely attached to backlight unit  42  so that when shearing forces are generated due to thermal expansion, the adhesive bond between backlight unit  42  and heat spreading layer can give somewhat to accommodate the shearing forces and resulting lateral movement between adjacent layers. 
     In the illustrative configuration of  FIG. 14 , adhesive  98  is provided in an array of rectangular areas, so that portions of heat spreading layer  92  remain unbonded to backlight unit  42 . An array of adhesive  98  having circular or oval shapes is used in the illustrative configuration of  FIG. 15 . If desired, adhesive patches such as the rectangular pads of adhesive of  FIG. 14  and/or adhesive pads having curved edges such as the pads of adhesive of  FIG. 15  or other isolated regions of adhesive may be provided in non-uniform arrays (e.g., distributed in a pseudo random pattern or a pattern that includes regions of adhesive that are not all in regular rows and columns).  FIG. 16  shows how regions of adhesive  98  may be provided using cross shapes. The cross-shapes may be symmetrical (e.g., with branches that are all of the same length) or asymmetrical (e.g., having branches of uneven lengths). In the  FIG. 17  example, adhesive  98  has been provided in strips that run along dimension X. Adhesive  98  may also be provide in strip-shaped regions that run diagonally, that run in dimension Y, using zigzag patterns, or using any other suitable shapes. In the example of  FIG. 18 , adhesive  98  has been provided in a C-shaped strip that runs along all of one edge of backlight  42  and parts of two other edges of backlight  42 . Configurations in which adhesive  98  has been patterned to form other shapes (e.g., other shapes that form an unbonded region between layer  92  and structures  42 ) may be used if desired. The examples of  FIGS. 13, 14, 15, 16, 17, and 18  are merely illustrative. 
       FIGS. 19, 20, and 21  show how heat spreading layer  92  may move into contact with device structures such as midplate  122  to accommodate thermal expansion during operation of device  10 . As shown in  FIG. 19 , before heat is produced by light-emitting diodes or other components in device  10 , heat spreading layer  92  may have a relatively flat configuration (as an example). Heat spreading layer  92  may include heat spreading sheet  92 ′ (e.g., layers such as layers  100 ,  102 , and  104 ) and adhesive  98 . Adhesive  98  may be formed at opposing ends of heat spreading layer  92  and may attach heat spreading layer  92  to structure  124  while leaving an unbonded region in the center of layer  92  (as an example). Structure  124  may be a structure such as backlight unit  42  (e.g., a reflector, black tape coupled to an array of light-emitting diodes on a flexible printed circuit, a light guide plate, etc.), a flexible printed circuit, a housing structure, or other component or structure associated with device  10 . An air gap H may separate heat spreading layer  92  from midplate  122  or other structure in device  10 . Air gap H may be used to ensure that alignment tolerance requirements are satisfied (e.g., vertical tolerances). 
     Following heating of layer  92 ′ (e.g., due to heat from light-emitting diodes, integrated circuits, or other heat generating components), layer  92 ′ may expand and buckle until portions of layer  92 ′ contact midplate  122 , as shown in  FIG. 20 . In the configuration of  FIG. 20 , additional heat may be transferred from heat spreading layer  92  to midplate  122 , because direct contact between layer  92 ′ and midplate  122  creates a more efficient heat transfer path than air gap H. 
     Following additional heating (e.g., after heating layer  92 ′ of  FIG. 20  to an elevated temperature), layer  92 ′ may expand sufficiently to spread downwardly into contact with an enlarged area on the upper surface of midplate  122 , as shown in  FIG. 21 . In the location shown in  FIG. 21 , heat transfer from heat spreading layer  92  may be increased beyond the heat transfer in  FIG. 20 , because heat spreading layer  92  is in contact with midplate  122  over a larger area in  FIG. 21  than in  FIG. 20 . Due to the increased heat transfer between layer  92 ′ and midplate  122 , the rate at which layer  92 ′ is raised in temperature due to heating from the heat generating components may be slowed, preventing further, potentially excessive, expansion of layer  92 ′. 
     If desired, the expansion of layer  92 ′ in the examples of  FIGS. 19, 20, and 21  may be temporary. Initially, when layer  92 ′ is unheated, layer  92 ′ will have a shape of the type shown in  FIG. 19 . Following heating, layer  92 ′ may take on the form shown in  FIGS. 20 and 21 . Upon cooling, layer  92 ′ will contract back to the original shape shown in  FIG. 19 . 
     The expansion of layer  92 ′ may also be permanent. During initial display assembly operations, layer  92 ′ and structure  124  may be assembled into device housing  12 . When display  14  is used for the first time or when other heat-producing component is used for the first time, the heat will cause layer  92 ′ to expand (e.g., into the shape shown in  FIG. 20 or 21 ). Material in layer  92 ′ (e.g., polymer support layers, etc.) may remain expanded following heating. This type of approach may allow air gaps such as air gap H to be present during initial assembly operations so that display backlight unit  42 , heat spreading layer  92 , and other structures can be properly assembled within device housing  12 . After assembly, the presence of persistent air gaps could have the potential to hinder heat dissipation. The use of permanently deformed heat spreading layers such as layer  92 ′ of  FIG. 20  or  FIG. 21  may therefore help enhance heat dissipation. 
       FIGS. 22, 23, and 24  show how heat spreading layer may be attached to a flexible structure such as flexible structure  124 . Structure  124  may be a display (e.g., a flexible organic light-emitting diode display), may be a flexible printed circuit, or other flexible structure in device  10 . Heat generating components in structure  124  (e.g., diodes and other circuitry in an organic light-emitting diode display or integrated circuits, light-emitting diodes on a flexible printed circuit, other components on a flexible printed circuit, or other heat generating components) may generate heat. Heat spreading layer  92  may spread the heat that is generated. Heat spreading layer  92  may have heat spreading material layer  92 ′ (e.g., a layer of graphite, copper, and/or other material with optional plastic carrier layers). Adhesive  98  in heat spreading layer  92  may be used to attach heat spreading layer to flexible structure  124 . Adhesive  98  may be located at the edges of structure  124  or may be patterned in other shapes that create unbonded areas (e.g., thin air gaps or other areas not directly attached by adhesive) between heat spreading layer  92  and flexible structure  124  (see, e.g., the adhesive configurations of  FIGS. 13, 14, 15, 16, 17, and 18 ). If desired, layer  92  may be provided with excess material and may be attached to flexible structure  124  so that a wrinkle is present such as wrinkle  126 . When flexible structure  124  is bent so that the center of structure  124  moves downwards with respect to the opposing edges of structure  124 , layer  92 ′ may be pulled taut and wrinkle  126  may expand to allow heat spreading layer  92  to bend with flexible structure  124 . When flexible structure  124  is bent in the opposite direction so that the center of structure  124  moves upwards with respect to the edges of structure  124 , layer  92 ′ can move away from structure  124  and form wrinkles, as shown in  FIG. 24 . If the center of layer  92  were to be attached to the center of flexible structure  124 , the formation of wrinkles in layer  92  might wrinkle structure  124 . The use of adhesive  98  in a pattern that allows the unbonded center of heat spreading layer  92  to move relative to flexible structure  124  therefore prevents wrinkles from being formed. The configuration of  FIGS. 22, 23, and 24  may be used to allow the use of a heat spreading layer on a flexible display, on the underside of a flexible printed circuit, or other flexible component for device  10 . 
     The foregoing is merely illustrative and various modifications can be made by those skilled in the art without departing from the scope and spirit of the described embodiments. The foregoing embodiments may be implemented individually or in any combination.

Metadata:
Filing Date: 20140926
Publication Date: 20180410
Grant Date: 20180410
Priority Date: 20140613
Inventors: KAKUDA TYLER R.
GETTEMY SHAWN R.
SULLIVAN MARK T.
CHOWDHURY IHTESHAM H.
Assignee: APPLE INC
CPC Classifications: [{"code": "H01L23/40", "inventive": false, "first": false, "tree": "[]"}, {"code": "G02F1/133385", "inventive": true, "first": true, "tree": "[]"}, {"code": "G02F2001/133628", "inventive": false, "first": false, "tree": "[]"}, {"code": "H01L23/34", "inventive": false, "first": false, "tree": "[]"}, {"code": "G02F1/133308", "inventive": false, "first": false, "tree": "[]"}, {"code": "G02F2001/133317", "inventive": false, "first": false, "tree": "[]"}, {"code": "H01L2023/4068", "inventive": false, "first": false, "tree": "[]"}, {"code": "F28F2013/006", "inventive": false, "first": false, "tree": "[]"}, {"code": "H01L23/40", "inventive": false, "first": false, "tree": "[]"}, {"code": "G02F1/133385", "inventive": true, "first": true, "tree": "[]"}, {"code": "F28F2013/006", "inventive": false, "first": false, "tree": "[]"}, {"code": "G02F1/133628", "inventive": false, "first": false, "tree": "[]"}, {"code": "G02F1/133308", "inventive": false, "first": false, "tree": "[]"}, {"code": "H05K7/20954", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02F1/133628", "inventive": false, "first": false, "tree": "[]"}, {"code": "G02F1/133385", "inventive": true, "first": true, "tree": "[]"}, {"code": "G02F1/133317", "inventive": false, "first": false, "tree": "[]"}, {"code": "H01L2023/4068", "inventive": false, "first": false, "tree": "[]"}, {"code": "H01L23/34", "inventive": false, "first": false, "tree": "[]"}, {"code": "G02F1/133603", "inventive": true, "first": true, "tree": "[]"}, {"code": "G02F1/133628", "inventive": false, "first": false, "tree": "[]"}]
Family ID: 53730649