PATENT DOCUMENT

Publication Number: US-8946561-B2
Application Number: US-201213353203-A
Country: US
Kind Code: B2

Title: Flexible circuitry with heat and pressure spreading layers

Abstract:
A flexible printed circuit may be provided with an integrated heat and pressure spreading layer. The heat and pressure spreading layer may be configured to uniformly spread heat and pressure from a bonding tool across a portion of the flexible printed circuit during bonding of the flexible printed circuit to additional circuitry. During manufacturing of the flexible printed circuit, a sheet of heat and pressure spreading material may be attached to a sheet of flexible printed circuitry and the heat and pressure spreading material and the sheet of flexible printed circuitry may be die cut to form multiple flexible printed circuits each with a heat and pressure spreading layer. An electronic device may be provided with a flexible printed circuit with a heat and pressure spreading layer coupled to a component such as a display.

Claims:
What is claimed is: 
     
       1. A method of manufacturing flexible printed circuits, comprising:
 providing a sheet of flexible printed circuitry having a layer of flexible polymer, a patterned conductive layer on a first surface of the layer of flexible polymer, and an insulating layer over a portion of the patterned conductive layer; 
 attaching a layer of material to an opposing second surface of the layer of flexible polymer, wherein the layer of material is configured to spread heat and pressure across the second surface of the layer of flexible polymer when pressed by a bonding tool; and 
 die cutting the sheet of flexible printed circuitry that has the layer of material formed on the opposing second surface. 
 
     
     
       2. The method defined in  claim 1  wherein the layer of material comprises a sheet of polytetrafluoroethylene and wherein attaching the layer of material to the opposing second surface comprises attaching the sheet of polytetrafluoroethylene to the opposing second surface using a layer of adhesive. 
     
     
       3. The method defined in  claim 2 , further comprising:
 providing at least one additional sheet of flexible circuitry having a patterned conductive layer; 
 attaching the at least one additional sheet of flexible circuitry to the layer of material on the opposing second surface of the layer of flexible polymer; and 
 forming at least one conductive via through the layer of material on the opposing second surface, wherein the at least one conductive via electrically couples the patterned conductive layer of the sheet of flexible circuitry to the patterned conductive layer of the at least one additional sheet of flexible circuitry. 
 
     
     
       4. The method defined in  claim 1  wherein attaching the layer of material to the opposing second surface of the layer of flexible polymer comprises roll-laminating a sheet of the material onto the opposing second surface of the layer of flexible polymer. 
     
     
       5. A flexible printed circuit, comprising:
 a flexible polymer substrate having opposing first and second surfaces; 
 a patterned conductive layer formed on the second surface of the flexible polymer substrate; and 
 a layer of insulating material on the first surface of the flexible polymer substrate that is configured to distribute pressure across the first surface of the flexible polymer substrate when pressed by a bonding tool. 
 
     
     
       6. The flexible printed circuit defined in  claim 5  wherein the layer of insulating material is configured to spread heat and pressure applied by a heated anisotropic-conductive-film bonding tool. 
     
     
       7. The flexible printed circuit defined in  claim 6  wherein the layer of insulating material comprises a layer of polytetrafluoroethylene. 
     
     
       8. The flexible printed circuit defined in  claim 6  wherein the layer of insulating material comprises silicone. 
     
     
       9. The flexible printed circuit defined in  claim 5  wherein the flexible polymer substrate comprises a flexible polyimide layer. 
     
     
       10. The flexible printed circuit defined in  claim 5 , further comprising an insulating coverlay over a portion of the patterned conductive layer on the second surface of the flexible polymer substrate. 
     
     
       11. The flexible printed circuit defined in  claim 10  wherein the patterned conductive layer comprises a first patterned conductive layer, the flexible printed circuit further comprising:
 an adhesive layer formed on a portion of the insulating coverlay; 
 a second patterned conductive layer attached to the insulating coverlay using the adhesive layer; and 
 a conductive via that electrically couples the first patterned conductive layer to the second patterned conductive layer. 
 
     
     
       12. The flexible printed circuit defined in  claim 11 , further comprising:
 an second adhesive layer formed on a portion of the layer of insulating material; 
 a third patterned conductive layer attached to the portion of the layer of insulating material using the second adhesive layer; and 
 a conductive via that electrically couples the second patterned conductive layer to the third patterned conductive layer through the layer of insulating material. 
 
     
     
       13. The flexible printed circuit defined in  claim 5 , wherein the layer of insulating material comprises a die cut layer of insulating material. 
     
     
       14. The flexible printed circuit board defined in  claim 5 , wherein the flexible polymer substrate has a length and the layer of insulating material extends across the length of the flexible printed circuit. 
     
     
       15. The flexible printed circuit board defined in  claim 5 , wherein the flexible polymer substrate has a length and the layer of insulating material extends only along a portion of the length of the flexible polymer substrate. 
     
     
       16. The flexible printed circuit board defined in  claim 15 , further comprising:
 a layer of adhesive formed along the portion of the length of the flexible polymer substrate, wherein the layer of adhesive is interposed between the flexible polymer substrate and the layer of insulating material and attaches the layer of insulating material to the flexible polymer substrate. 
 
     
     
       17. The flexible printed circuit board defined in  claim 16 , wherein the layer of insulating material is further configured to distribute heat across the first surface of the flexible polymer substrate when pressed by the bonding tool. 
     
     
       18. The flexible printed circuit board defined in  claim 17 , wherein the flexible polymer substrate comprises a flexible polyimide layer and the layer of insulating material comprises a layer of polytetrafluoroethylene. 
     
     
       19. The flexible printed circuit board defined in  claim 18 , wherein the patterned conductive layer comprises bond pads for electrically coupling the printed circuit board to a rigid circuit member. 
     
     
       20. The flexible printed circuit board defined in  claim 5 , wherein the layer of insulating material on the first surface of the flexible polymer substrate is further configured to distribute heat and the pressure uniformly across the first surface of the flexible polymer substrate when pressed by the bonding tool.

Description:
BACKGROUND 
     This relates generally to electronic devices, and more particularly, to electronic devices having flexible circuitry. 
     Electronic devices often include displays, printed circuit boards and flexible printed circuits. A display is often coupled to a printed circuit board using an interposing flexible printed circuit that is attached to both the display and the printed circuit board. 
     A flexible printed circuit is often attached to the printed circuit board and the display using an anisotropic conductive film adhesive. The anisotropic conductive film adhesive is commonly interposed between the flexible printed circuit and the printed circuit board and/or between the flexible printed circuit and the display. During manufacturing, heat and pressure are commonly applied to the flexible printed circuit in order to form conductive bonds in the interposed anisotropic conductive film adhesive. 
     Heat and pressure are commonly applied using a heat bar that is pressed against a top surface of the flexible printed circuit. 
     Non-uniformities on the flexible printed circuit can cause heat and pressure to be applied unevenly to the flexible printed circuit. 
     It would therefore be desirable to be able to provide improved flexible circuitry for attachment to rigid circuitry in electronic devices. 
     SUMMARY 
     An electronic device may have circuitry such as flexible circuitry. Flexible circuitry such as a flexible printed circuit may be attached to a rigid circuit member such as a rigid layer of a device display, a rigid printed circuit board, or any other suitable rigid printed circuit substrate. 
     An electronic device may include an electronic component mounted to the rigid circuit member. For example, a driver integrated circuit may be mounted to a thin-film transistor (TFT) glass layer of a display. Flexible circuitry such as a flexible printed circuit may be attached to the rigid circuit member using, for example, an anisotropic conductive film (ACF) adhesive in the vicinity of the electronic component. 
     Flexible circuitry may be provided with one or more heat and pressure spreading layers (spreader layers). A flexible printed circuit having a heat and pressure spreading layer may be die cut from one or more sheets of flexible circuitry to which a sheet of heat and pressure spreading material has been attached. The heat and pressure spreading material may form a top layer of the flexible printed circuit or may form an integrated interior layer of the flexible printed circuit. 
     During device assembly, bonding equipment such as a heat bar may be used to attach the flexible printed circuit to the rigid circuit member. Heat and pressure may be applied by the heat bar to the spreader layer of the flexible circuitry in order to generate conductive portions in an ACF adhesive that is interposed between the flexible printed circuit and the rigid circuit member. Conductive portions of the ACF adhesive may couple electrical contacts on the flexible printed circuit to electrical contacts on the rigid circuit member. 
     A spreader layer that is die cut along with the flexible circuit may facilitate precise positioning of bonding equipment during attachment of the flexible circuit to the rigid circuit member. 
     In this way, tolerances may be reduced for spacing between the location of attachment of the flexible circuit and the location of attachment of an electronic component on a rigid circuit member. In this way, the size of one or more lateral dimensions of a rigid circuit member such as a TFT display layer may be reduced, thereby helping to reduce the overall size of an electronic device. 
     Further features of the invention, its nature and various advantages will be more apparent from the accompanying drawings and the following detailed description of the preferred embodiments. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram of an illustrative electronic device with a flexible circuit having a spreader layer such as a portable computer in accordance with an embodiment of the present invention. 
         FIG. 2  is a diagram of an illustrative electronic device with a flexible circuit having a spreader layer such as a cellular telephone or other handheld device in accordance with an embodiment of the present invention. 
         FIG. 3  is a diagram of an illustrative electronic device with a flexible circuit having a spreader layer such as a tablet computer in accordance with an embodiment of the present invention. 
         FIG. 4  is a diagram of an illustrative electronic device with a flexible circuit having a spreader layer such as a computer monitor with a built-in computer in accordance with an embodiment of the present invention. 
         FIG. 5  is a cross-sectional side view of illustrative circuitry in an electronic device such as a display and a printed circuit board coupled using a flexible printed circuit with a spreader layer in accordance with an embodiment of the present invention. 
         FIG. 6  is cross-sectional side view of a portion of an illustrative flexible circuit having a spreader layer in accordance with an embodiment of the present invention. 
         FIG. 7  is cross-sectional side view of a portion of an illustrative flexible circuit having a partial spreader layer in accordance with an embodiment of the present invention. 
         FIG. 8  is cross-sectional side view of a portion of an illustrative flexible circuit having multiple flex circuit layers and spreader layer in accordance with an embodiment of the present invention. 
         FIG. 9  is cross-sectional side view of a portion of an illustrative flexible circuit having a conductive via through a spreader layer in accordance with an embodiment of the present invention. 
         FIG. 10  is a diagram of an illustrative assembly system for attaching a flexible circuit having a spreader layer to a rigid circuit member in accordance with an embodiment of the present invention. 
         FIG. 11  is a flow chart of illustrative steps involved in attaching a flexible circuit having a spreader layer to a rigid circuit member in accordance with an embodiment of the present invention. 
         FIG. 12  is a flow chart of illustrative steps involved forming a flexible printed circuit having a spreader layer in accordance with an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Electronic devices may include flexible circuitry. Flexible circuitry may include a flexible printed circuit substrate connected to one or more rigid circuit members. A rigid circuit member may be any rigid or semi-rigid circuit substrate such as a printed circuit board, a rigid layer of a display, a rigid portion of a rigid-flex circuit or any other suitable substrate that is configured to receive an electrical attachment to a flexible printed circuit. A display having a rigid layer or other rigid portion may be used to display visual information such as text and images to users. 
     An electronic device may include an electronic component mounted to the rigid circuit member. For example, a driver integrated circuit may be mounted to a thin-film transistor (TFT) layer of a display. Flexible circuitry may be attached to the rigid circuit member using, for example, an anisotropic conductive film (ACF) adhesive. 
     A flexible printed circuit may be provided with one or more insulating layers such as heat and pressure spreading layers (sometimes referred to herein as a spreader layer, a heat-spreading layer, or a pressure-spreading layer). A flexible printed circuit having a heat and pressure spreading layer may be die cut from a sheet of flexible circuitry. 
     A spreader layer that is die cut along with attached layers of flexible circuitry may help enable precise positioning of assembly equipment such as a heated bonding tool during attachment of the flexible circuitry to the rigid circuit member by allowing the heated bonding tool to be applied directly to the heat and pressure spreading layer of the flexible circuitry. In this way, manufacturing tolerances may be reduced for spacing between the location of an attachment of a flexible circuit and an electronic component on a rigid circuit member. 
     In this way, the size of one or more lateral dimensions of a rigid circuit member such as a TFT display layer may be reduced, thereby helping to reduce the overall size of an electronic device. 
     During assembly of the device, a heated bonding tool such as a hot bar (heat bar) may be moved into contact with the heat and pressure spreading layer so that the hot bar applies heat and pressure to the flexible circuitry for bonding to the rigid circuit member. The heat and pressure spreading layer may help spread heat and pressure evenly from a hot bar across a surface of the flexible circuitry. 
     Heat and pressure applied to the flexible circuitry through the spreader layer may cause portions of an ACF adhesive interposed between the flexible circuit and the rigid circuit member to become conductive. Conductive portions of the ACF adhesive may couple electrical contacts on the flexible circuitry to electrical contacts on the rigid circuit member. 
     Illustrative electronic devices that may be provided with flexible circuitry that includes a heat and pressure spreading layer are shown in  FIGS. 1 ,  2 ,  3  and  4 . 
     An illustrative electronic device of the type that may be provided with flexible circuitry having a heat and pressure spreading layer is shown in  FIG. 1 . Electronic device  10  may be a computer such as a computer that is integrated into a display such as a computer monitor, a laptop computer, a tablet computer, a somewhat smaller portable device such as a wrist-watch device, pendant device, or other wearable or miniature device, a cellular telephone, a media player, a tablet computer, a gaming device, a navigation device, a computer monitor, a television, or other electronic equipment. 
     As shown in  FIG. 1 , device  10  may include a display such as display  14 . Display  14  may be a touch screen that incorporates capacitive touch electrodes or other touch sensor components or may be a display that is not touch sensitive. Display  14  may include image pixels formed from liquid crystal display (LCD) components or other suitable display pixel structures. Arrangements in which display  14  is formed using liquid crystal display pixels are sometimes described herein as an example. This is, however, merely illustrative. Any suitable type of display technology may be used in forming display  14  if desired. 
     Display  14  may be substantially filled with active display pixels or may have an active portion and an inactive portion. An inactive portion of the display may include display circuitry for delivering electrical signals to display pixels. For example, a flexible printed circuit having a heat and pressure spreading layer may be attached to a display layer such as a TFT glass layer in an inactive region of the display. Display circuitry in an inactive portion of display  14  may be hidden from view by, for example, an opaque coating on a display cover layer. The size of the inactive portion of a display may be reduced by providing a flexible printed circuit with a heat and pressure spreading layer that enables precise positioning of a bonding tool during bonding of the flexible printed circuit to the TFT glass layer. 
     Device  10  may have a housing such as housing  12 . Housing  12 , which may sometimes be referred to as a case, may be formed of plastic, glass, ceramics, fiber composites, metal (e.g., stainless steel, aluminum, etc.), other suitable materials, or a combination of any two or more of these materials. 
     Housing  12  may be formed using a unibody configuration in which some or all of housing  12  is machined or molded as a single structure or may be formed using multiple structures (e.g., an internal frame structure, one or more structures that form exterior housing surfaces, etc.). 
     As shown in  FIG. 1 , housing  12  may have multiple parts. For example, housing  12  may have upper portion  12 A and lower portion  12 B. Upper portion  12 A may be coupled to lower portion  12 B using a hinge that allows portion  12 A to rotate about rotational axis  16  relative to portion  12 B. A keyboard such as keyboard  18  and a touch pad such as touch pad  20  may be mounted in housing portion  12 B. 
     In the example of  FIG. 2 , device  10  has been implemented using a housing that is sufficiently small to fit within a user&#39;s hand (i.e., device  10  of  FIG. 2  may be a handheld electronic device such as a cellular telephone). As show in  FIG. 2 , device  10  may include a display such as display  14  mounted on the front of housing  12 . Display  14  may have openings (e.g., openings in the inactive or active portions of display  14 ) such as an opening to accommodate button  22  and an opening to accommodate speaker port  24 . 
       FIG. 3  is a perspective view of electronic device  10  in a configuration in which electronic device  10  has been implemented in the form of a tablet computer. As shown in  FIG. 3 , display  14  may be mounted on the upper (front) surface of housing  12 . An opening may be formed in display  14  to accommodate button  22 . 
       FIG. 4  is a perspective view of electronic device  10  in a configuration in which electronic device  10  has been implemented in the form of a computer integrated into a computer monitor. As shown in  FIG. 4 , display  14  may be mounted on the front surface of housing  12 . Stand  26  may be used to support housing  12 . 
     Display  14  may include an array of display pixels. Each display pixel may be used to control the light intensity associated with a portion of the display. 
     A cross-sectional side view of an illustrative configuration that may be used for connecting flexible circuitry such as flexible circuitry  30  to a rigid circuit member such as display  14  of device  10  (e.g., device  10  of  FIG. 1 ,  FIG. 2 ,  FIG. 3 , or  FIG. 4  or other suitable electronic devices) is shown in  FIG. 5 . As shown in  FIG. 5 , display  14  may include one or more display layers such as color filter glass layer  52 , liquid crystal layer  54 , and thin-film transistor (TFT) glass layer  58 . In some configurations, color filter glass layer  52 , liquid crystal layer  54 , and thin-film transistor (TFT) glass layer  58  are interposed between polarizer layers. 
     Color filter glass layer  52  may contain an array of colored filters that provide display  14  with the ability to represent different colors. Liquid crystal material in liquid crystal layer  54  may be controlled electrically (e.g., selectively polarized) by thin-film transistor array  56 . Thin-film transistors in array  56  may be formed on the upper surface of thin-film transistor (TFT) glass layer  58 . 
     Thin-film transistors  56  may be controlled by drivers contained in driver circuit  60 . Color filter layer  52  may be horizontally (laterally) recessed with respect to TFT layer  58  to form a ledge such as ledge  50 . Driver circuit  60  and flexible circuitry  30  may be attached to ledge  50 . 
     Display  14  may include a number of driver chips such as circuit  60  that are mounted around the periphery of the display. Conductive traces on the upper surface of TFT layer  58  interconnect driver circuit  60  with thin-film transistors  56 . The use of LCD technology is merely illustrative. 
     Flexible circuit substrate  30  (also referred to herein as flexible circuitry or a flexible printed circuit) and driver circuit  60  may be attached to ledge  50  using anisotropic conductive film (ACF) adhesive such as ACF adhesive  48 . Flexible circuitry  30  may be a flexible printed circuit substrate (“flex circuit”) such as a sheet of polyimide or other flexible dielectric having conductive traces (e.g., metal lines). 
     Conductive traces in flexible printed circuit  30  may route signals to circuitry such as circuit  60  from an additional rigid circuit member such as printed circuit board  44 . Flexible printed circuit substrate  30  may include electrical contacts such as bond pads  63  and bond pads  67 . Metal lines in flexible printed circuit  30  may interconnect bond pads  63  and bond pads  67 . ACF adhesive  48  may be interposed between bond pads  63  of flexible printed circuit  30  and corresponding electrical contacts such as bond pads  61  on ledge  50  of TFT layer  58 . 
     With one suitable arrangement, flexible circuitry  30  is connected to a main printed circuit board such as printed circuit board (PCB)  44  using ACF adhesive  42 . Bond pads  67  on flexible printed circuit  30  may be aligned with electrical contacts such as bond pads  65  on PCB  44 . ACF adhesive  42  may be interposed between bond pads  67  of flexible circuitry  30  and bond pads  65  of PCB  44 . 
     Circuitry for device  10  may be mounted on printed circuit boards such as board  44  and/or may be coupled to the circuitry on printed circuit board  44  through additional signal lines (e.g., signal lines on additional flex circuits, signal lines on additional rigid printed circuit boards, etc). This circuitry may include, for example, components  46 . 
     Components  46  may include control circuitry such as control circuitry based on one or more processing integrated circuits (e.g., microprocessors) and storage (e.g., volatile and non-volatile memory). Components  46  may include communications circuits such as integrated circuits for communicating over serial buses and parallel buses with internal components and external equipment that is connected to device  10  by a cable and a connector in device  10  and/or internal circuits in device  10 . 
     Discrete components may be mounted on board  44  with other components. Examples of discrete components are inductors, capacitors, and resistors. Other components  46  that may be mounted on board  44  or elsewhere in device  10  include switches, connectors, cameras, camera flash circuits (e.g., light-emitting diodes or other light sources that serve as a camera flash), and audio circuits. 
     Components  46  may include video chips such as one or more display driver integrated circuits for displaying images on display  14  and a video driver integrated circuit or circuits for driving video signals onto a monitor or other external display that is coupled to device  10 . Accessory interface circuitry such as circuitry that is associated with an external component that is controlled by device  10  and/or that provides input to device  10  and other circuits and devices may also be included in components  46  if desired. 
     During assembly of device  10 , a bonding tool such as a heat bar may be used to bond flexible printed circuit  30  to TFT layer  58  and/or to PCB  44 . For example, a heat bar may be heated and pressed against flexible printed circuit  30  in order to generate heat and pressure that causes ACF  48  to form conductive paths between electrical contacts  63  and  61 . 
     Common flexible printed circuit substrates may be heat sensitive or may have non-uniformities that can cause heat and pressure from a heat bar to be applied non-uniformly to the substrate. This can have undesirable consequences for the robustness of the ACF bond to a rigid circuit member. In some situations an additional bonding sheet may be placed between a heat bar and a flexible printed circuit during bonding to spread heat and pressure across the flexible printed circuit. However, an additional bonding sheet that is inserted during bonding may sometimes contact and damage or dislodge a component that is already attached to the rigid circuit member. 
     Additional unused space is sometimes therefore required in a device (e.g., additional inactive display area on a ledge of a TFT layer) in order to avoid this type of situation. For this reason, flexible printed circuit  30  may be provided with a layer of heat and pressure spreading material so that an additional bonding sheet is not necessary during the bonding process. A flexible printed circuit having a heat and pressure spreading layer may reduce the need for additional margin on TFT ledge  50  thereby reducing the size of an inactive portion of display  14 . 
     As shown in  FIG. 6 , a flexible printed circuit (“flex circuit”) such as flexible printed circuit  30  may be provided with an insulating layer such as heat and pressure spreading layer  70  on a flex circuit layer such as flex circuit layer  31 . Heat and pressure spreading layer  70  (sometimes referred to herein as simply heat-spreading layer or pressure-spreading layer) may be formed from a suitable insulating material such as polytetrafluoroethylene, silicone or other suitable material for spreading heat and pressure. Heat-spreading layer  70  may be attached to a first surface of a flexible sheet of polymer such as polyimide layer  74  of flex circuit layer  31  using, for example, an adhesive such as adhesive  72 . 
     As shown in  FIG. 6 , flex circuit layer  31  of flexible printed circuit  30  may include a conductive layer such as conductive layer  76  on an opposing second surface of polyimide layer  74 . Conductive layer  76  may be a patterned conductive layer formed from printed conductive traces (e.g., copper, aluminum or other metal traces) on polyimide layer  74 . 
     A portion of patterned conductive layer  76  may be covered by an insulating layer such as coverlay layer  78 . A portion such as portion  79  of patterned conductive layer  76  may be exposed (e.g., uncovered by coverlay layer  76 ). Exposed portion  79  of conductive layer  76  may include electrical contacts such as bond pads  63  for electrically coupling flexible printed circuit  30  to a rigid circuit member such as TFT layer  58  of display  14  or PCB  44 . 
     During assembly of device  10 , bond pads  63  in portion  79  of flexible printed circuit  30  may be aligned with bond pads such as bond pads  61  (or bond pads  65 ) of  FIG. 5 . ACF adhesive may be provided between bond pads  63  and bond pads  61 . A heated bonding tool such as heat bar  71  may then be heated and pressed against pressure-spreading layer  70  of flexible printed circuit  30  in order to bond flexible printed circuit  30  to, for example, TFT layer  58 . 
     Heat-spreading layer  70  may be configured to spread heat and pressure from heated ACF bonding tool  71  evenly (uniformly) over surface  80  of flexible printed circuit  30 . 
     During manufacturing of flexible printed circuit  30 , a sheet of flexible printed circuitry having a layer of flexible polymer (e.g., polyimide layer  74 ), a patterned conductive layer (e.g., patterned conductive layer  76 ) on a first surface of the layer of flexible polymer, and an insulating layer (e.g., coverlay layer  78 ) on a portion of the layer of flexible polymer over a corresponding portion (e.g., portion  79 ) of the patterned conductive layer may be provided. 
     A layer of insulating material (e.g., a polytetrafluoroethylene or silicone heat and pressure spreading layer such as layer  70 ) may be formed on an opposing second surface of the flexible polymer substrate in order to spread heat and pressure across the second surface of the flexible polymer substrate when pressed by a bonding tool. As an example, heat-spreading layer  70  may be formed on the opposing second surface of the flexible polymer substrate by roll-laminating a sheet of heat-spreading material (e.g., polytetrafluoroethylene, silicone, etc.) onto the flexible polymer substrate. Multiple flexible printed circuits such as flexible printed circuit  30  having heat and pressure spreading layers may then be formed by die cutting the sheet of flexible printed circuitry that has the layer of insulating material formed on the opposing second surface. 
     In the example of  FIG. 6 , heat-spreading layer  70  extends along substantially the entire length of flexible printed circuit  30 . However, this is merely illustrative. If desired, head-spreading layer  70  may be formed only on portion  79  of flexible printed circuit  30  as shown in  FIG. 7 . 
     Heat-spreading layer  70  may be formed only on portion  79  of flexible printed circuit  30  by forming heat-spreading layer  70  along the entire length of a sheet of flexible polymer substrate and removing a portion of heat-spreading layer  70  from the sheet of flexible polymer substrate before or after die cutting the sheet of flexible printed circuitry (as described above in connection with  FIG. 6 ). If desired, heat-spreading layer  70  may be formed on portion  79  of flexible printed circuit  30  by attaching a strip of heat-spreading material  70  that has a width equal to the width of portion  79  to top surface  82  of flex circuit layer  31  using adhesive  72 . 
     In the examples of  FIGS. 6 and 7 , flexible printed circuit  30  includes a single flex circuit layer  31 . However, this is merely illustrative. As shown in  FIG. 8 , flexible printed circuit  30  may include a second flex circuit layer  31  attached to coverlay layer  78  using an adhesive such as adhesive  84 . The second flex circuit layer  31  may include a conductive layer such as patterned conductive layer  86  (e.g., copper, aluminum or other metal traces) that is covered by a coverlay layer such as coverlay layer  88 . Flexible printed circuit  30  may include one or more conductive vias such as plated through-hole  90  that interconnect conductive layer  76  with conductive layer  86 . 
     As shown in  FIG. 9 , flexible printed circuit  30  may, if desired, include a third flex circuit layer  31  attached to surface  80  of heat and pressure spreading layer  70  using adhesive  92 . Third flex circuit layer  31  may include a conductive layer such as patterned conductive layer  94  (e.g., copper, aluminum or other metal traces) that is covered by a coverlay layer such as coverlay layer  96 . 
     A flex circuit layer  31  that is attached to top surface  80  of pressure-spreading layer  70  may cover a portion of pressure-spreading layer  70  that is different from portion  79  of pressure-spreading layer  70  so that surface  80  of heat-spreading layer  70  remains exposed for pressing a bonding tool during assembly of device  10 . 
     During manufacturing of flexible printed circuit  30 , one or more additional sheets of flexible circuitry such as flex circuit layers  31  may be attached to coverlay layer  78  and/or pressure-spreading layer  70  using adhesive and one or more conductive vias (e.g., plated through-holes) may be formed between conductive layers of the flex circuit layers. As shown in  FIG. 9 , a conductive via such as plated through-hole  98  may interconnect conductive layers through flexible polymer layer  74 , pressure-spreading layer  70 , and coverlay layer  78 . However, this is merely illustrative. Some conductive vias may pass through coverlay layer  78  without passing through polymer layer  74  and pressure-spreading layer  70 . Some conductive vias may pass through polymer layer  74  and pressure-spreading layer  70  without passing through coverlay layer  78 . 
     If desired, flexible printed circuit  30  may include two flex circuit layers, three flex circuit layers, four flex circuit layers or more than four flex circuit layers. Each flex circuit layer may include one or more flexible polymer layers, one or more coverlay layers, one or more adhesive layers, one or more conductive layers and may include conductive vias that interconnect conductive layers within the flex circuit layer and/or conductive vias that interconnect conductive layers of multiple flex circuit layers. 
     As shown in  FIG. 10 , during assembly of device  10 , a component such as driver circuit  60  may be attached to a rigid circuit member such as TFT layer  58  of display  14 . A bonding tool such as heat bar  71  may then be pressed against top surface  80  of heat-spreading layer  70  of flexible printed circuit  30  (e.g., in direction  73 ) in order to bond flexible printed circuit  30  to bond pads  61  of TFT layer  58  using ACF adhesive  48 . Providing flexible printed circuit  30  with a heat and pressure spreading layer such as pressure-spreading layer (heat-spreading layer)  70  that has been die cut with flexible printed circuit  30  may enable precise positioning of bonding tool  71  during bonding of flexible printed circuit  30  to TFT layer  58 . In this way, flexible printed circuit  30  may be mounted to TFT layer  58  at a distance D from component  60  that is less than the distance that would be required when bonding a conventional flex circuit to a TFT layer. As examples, distance D may be less than 0.3 mm, less than 0.2 mm, less than 0.1 mm, less than 0.5 mm, 0.1-0.2 mm, 0.1-0.3 mm, 0.1-0.2 mm or more than 0.3 mm. 
       FIG. 11  is a flow chart of illustrative steps that may be involved in assembling device  10  using a flexible printed circuit having a heat and pressure spreading layer. At step  100 , an electronic component such as a driver integrated circuit may be attached to a rigid circuit member such as a thin-film-transistor (TFT) glass layer of a display. 
     At step  102 , an anisotropic conductive film (ACF) adhesive may be provided between the rigid circuit member and a flexible printed circuit. The ACF adhesive may be applied to the rigid circuit member, to the flexible printed circuit, or to both the rigid circuit member and the flexible printed circuit, 
     At step  104 , bond pads on the flexible printed circuit may be aligned with corresponding bond pads on the rigid circuit member. 
     At step  106 , a bonding tool such as a heat bar may be pressed against a heat and pressure spreading layer (e.g., a layer of polytetrafluoroethylene, silicone or other suitable heat-spreading material) of the flexible printed circuit. Pressing the heat bar against the heat and pressure spreading layer of the flexible printed circuit may electrically couple the flexible printed circuit to the rigid circuit member by forming conductive portions in the ACF adhesive between the bond pads of the flexible printed circuit and the bond pads of the rigid circuit member. 
       FIG. 12  is a flow chart of illustrative steps that may be involved in manufacturing a flexible printed circuit having a heat and pressure spreading layer. At step  110 , a sheet of flexible printed circuitry may be provided. The sheet of flexible printed circuitry may be a single-sided, double-sided, multilayer, or other suitable sheet of flexible printed circuitry. 
     At step  112 , a layer of heat and pressure spreading material (e.g., a sheet of polytetrafluoroethylene, silicone or other suitable heat-spreading material) may be attached to the sheet of heat and pressure spreading material. The sheet of heat and pressure spreading material may, for example, be roll-laminated onto the sheet of flexible printed circuitry. 
     At optional step  114 , one or more additional sheets of flexible printed circuitry (e.g., flex circuit layers  31 ) may be attached to the sheet of flexible circuitry and/or the layer of heat and pressure spreading material. In situations in which an additional sheet of flexible circuitry is attached to the sheet of heat and pressure spreading material, a portion of the sheet of heat and pressure spreading material may remain uncovered by the additional sheet of flexible circuitry to provide space for a bonding tool to press against the heat and pressure spreading material during assembly of an electronic device. 
     At optional step  116 , one or more conductive vias such as plated through-holes may be formed between conductive layers in the sheets of flexible circuitry. Conductive vias may pass through layers of coverlay, flexible polymer layers, and/or the sheet of heat and pressure spreading material. 
     At step  118 , the sheets of flexible circuitry having the attached sheet of heat and pressure spreading material may be die cut to form multiple flexible printed circuits each having a layer of heat and pressure spreading material for spreading heat and pressure when pressed by a bonding tool during assembly of an electronic device such as device  10 . 
     Each sheet of flexible circuitry may include one or more flexible polymer sheets, one or more coverlay sheets, one or more adhesive layers, one or more conductive layers and may include conductive vias that interconnect conductive layers within the sheet of flexible circuitry and/or conductive vias that interconnect conductive layers of multiple sheets of flexible circuitry. 
     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: 20120118
Publication Date: 20150203
Grant Date: 20150203
Priority Date: 20120118
Inventors: WURZEL JOSHUA G.
FEINSTEIN CASEY J.
Assignee: APPLE INC
CPC Classifications: [{"code": "H05K1/02", "inventive": true, "first": true, "tree": "[]"}, {"code": "H05K1/02", "inventive": true, "first": true, "tree": "[]"}, {"code": "H05K3/361", "inventive": true, "first": false, "tree": "[]"}, {"code": "H05K2201/015", "inventive": false, "first": false, "tree": "[]"}, {"code": "H05K2201/0162", "inventive": false, "first": false, "tree": "[]"}, {"code": "H05K2201/2009", "inventive": false, "first": false, "tree": "[]"}, {"code": "Y10T29/49126", "inventive": false, "first": false, "tree": "[]"}, {"code": "Y10T29/49126", "inventive": false, "first": false, "tree": "[]"}, {"code": "H05K2201/0162", "inventive": false, "first": false, "tree": "[]"}, {"code": "H05K2201/015", "inventive": false, "first": false, "tree": "[]"}, {"code": "H05K3/361", "inventive": true, "first": false, "tree": "[]"}, {"code": "H05K2201/2009", "inventive": false, "first": false, "tree": "[]"}]
Family ID: 48779198