Flexible circuitry with heat and pressure spreading layers

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.

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.

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 inFIGS. 1,2,3and4.

An illustrative electronic device of the type that may be provided with flexible circuitry having a heat and pressure spreading layer is shown inFIG. 1. Electronic device10may 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 inFIG. 1, device10may include a display such as display14. Display14may be a touch screen that incorporates capacitive touch electrodes or other touch sensor components or may be a display that is not touch sensitive. Display14may include image pixels formed from liquid crystal display (LCD) components or other suitable display pixel structures. Arrangements in which display14is 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 display14if desired.

Display14may 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 display14may 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.

Device10may have a housing such as housing12. Housing12, 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.

Housing12may be formed using a unibody configuration in which some or all of housing12is 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 inFIG. 1, housing12may have multiple parts. For example, housing12may have upper portion12A and lower portion12B. Upper portion12A may be coupled to lower portion12B using a hinge that allows portion12A to rotate about rotational axis16relative to portion12B. A keyboard such as keyboard18and a touch pad such as touch pad20may be mounted in housing portion12B.

In the example ofFIG. 2, device10has been implemented using a housing that is sufficiently small to fit within a user's hand (i.e., device10ofFIG. 2may be a handheld electronic device such as a cellular telephone). As show inFIG. 2, device10may include a display such as display14mounted on the front of housing12. Display14may have openings (e.g., openings in the inactive or active portions of display14) such as an opening to accommodate button22and an opening to accommodate speaker port24.

FIG. 3is a perspective view of electronic device10in a configuration in which electronic device10has been implemented in the form of a tablet computer. As shown inFIG. 3, display14may be mounted on the upper (front) surface of housing12. An opening may be formed in display14to accommodate button22.

FIG. 4is a perspective view of electronic device10in a configuration in which electronic device10has been implemented in the form of a computer integrated into a computer monitor. As shown inFIG. 4, display14may be mounted on the front surface of housing12. Stand26may be used to support housing12.

Display14may 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 circuitry30to a rigid circuit member such as display14of device10(e.g., device10ofFIG. 1,FIG. 2,FIG. 3, orFIG. 4or other suitable electronic devices) is shown inFIG. 5. As shown inFIG. 5, display14may include one or more display layers such as color filter glass layer52, liquid crystal layer54, and thin-film transistor (TFT) glass layer58. In some configurations, color filter glass layer52, liquid crystal layer54, and thin-film transistor (TFT) glass layer58are interposed between polarizer layers.

Color filter glass layer52may contain an array of colored filters that provide display14with the ability to represent different colors. Liquid crystal material in liquid crystal layer54may be controlled electrically (e.g., selectively polarized) by thin-film transistor array56. Thin-film transistors in array56may be formed on the upper surface of thin-film transistor (TFT) glass layer58.

Thin-film transistors56may be controlled by drivers contained in driver circuit60. Color filter layer52may be horizontally (laterally) recessed with respect to TFT layer58to form a ledge such as ledge50. Driver circuit60and flexible circuitry30may be attached to ledge50.

Display14may include a number of driver chips such as circuit60that are mounted around the periphery of the display. Conductive traces on the upper surface of TFT layer58interconnect driver circuit60with thin-film transistors56. The use of LCD technology is merely illustrative.

Flexible circuit substrate30(also referred to herein as flexible circuitry or a flexible printed circuit) and driver circuit60may be attached to ledge50using anisotropic conductive film (ACF) adhesive such as ACF adhesive48. Flexible circuitry30may 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 circuit30may route signals to circuitry such as circuit60from an additional rigid circuit member such as printed circuit board44. Flexible printed circuit substrate30may include electrical contacts such as bond pads63and bond pads67. Metal lines in flexible printed circuit30may interconnect bond pads63and bond pads67. ACF adhesive48may be interposed between bond pads63of flexible printed circuit30and corresponding electrical contacts such as bond pads61on ledge50of TFT layer58.

With one suitable arrangement, flexible circuitry30is connected to a main printed circuit board such as printed circuit board (PCB)44using ACF adhesive42. Bond pads67on flexible printed circuit30may be aligned with electrical contacts such as bond pads65on PCB44. ACF adhesive42may be interposed between bond pads67of flexible circuitry30and bond pads65of PCB44.

Circuitry for device10may be mounted on printed circuit boards such as board44and/or may be coupled to the circuitry on printed circuit board44through 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, components46.

Components46may 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). Components46may 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 device10by a cable and a connector in device10and/or internal circuits in device10.

Discrete components may be mounted on board44with other components. Examples of discrete components are inductors, capacitors, and resistors. Other components46that may be mounted on board44or elsewhere in device10include switches, connectors, cameras, camera flash circuits (e.g., light-emitting diodes or other light sources that serve as a camera flash), and audio circuits.

Components46may include video chips such as one or more display driver integrated circuits for displaying images on display14and a video driver integrated circuit or circuits for driving video signals onto a monitor or other external display that is coupled to device10. Accessory interface circuitry such as circuitry that is associated with an external component that is controlled by device10and/or that provides input to device10and other circuits and devices may also be included in components46if desired.

During assembly of device10, a bonding tool such as a heat bar may be used to bond flexible printed circuit30to TFT layer58and/or to PCB44. For example, a heat bar may be heated and pressed against flexible printed circuit30in order to generate heat and pressure that causes ACF48to form conductive paths between electrical contacts63and61.

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 circuit30may 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 ledge50thereby reducing the size of an inactive portion of display14.

As shown inFIG. 6, a flexible printed circuit (“flex circuit”) such as flexible printed circuit30may be provided with an insulating layer such as heat and pressure spreading layer70on a flex circuit layer such as flex circuit layer31. Heat and pressure spreading layer70(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 layer70may be attached to a first surface of a flexible sheet of polymer such as polyimide layer74of flex circuit layer31using, for example, an adhesive such as adhesive72.

As shown inFIG. 6, flex circuit layer31of flexible printed circuit30may include a conductive layer such as conductive layer76on an opposing second surface of polyimide layer74. Conductive layer76may be a patterned conductive layer formed from printed conductive traces (e.g., copper, aluminum or other metal traces) on polyimide layer74.

A portion of patterned conductive layer76may be covered by an insulating layer such as coverlay layer78. A portion such as portion79of patterned conductive layer76may be exposed (e.g., uncovered by coverlay layer76). Exposed portion79of conductive layer76may include electrical contacts such as bond pads63for electrically coupling flexible printed circuit30to a rigid circuit member such as TFT layer58of display14or PCB44.

During assembly of device10, bond pads63in portion79of flexible printed circuit30may be aligned with bond pads such as bond pads61(or bond pads65) ofFIG. 5. ACF adhesive may be provided between bond pads63and bond pads61. A heated bonding tool such as heat bar71may then be heated and pressed against pressure-spreading layer70of flexible printed circuit30in order to bond flexible printed circuit30to, for example, TFT layer58.

Heat-spreading layer70may be configured to spread heat and pressure from heated ACF bonding tool71evenly (uniformly) over surface80of flexible printed circuit30.

During manufacturing of flexible printed circuit30, a sheet of flexible printed circuitry having a layer of flexible polymer (e.g., polyimide layer74), a patterned conductive layer (e.g., patterned conductive layer76) on a first surface of the layer of flexible polymer, and an insulating layer (e.g., coverlay layer78) on a portion of the layer of flexible polymer over a corresponding portion (e.g., portion79) 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 layer70) 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 layer70may 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 circuit30having 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 ofFIG. 6, heat-spreading layer70extends along substantially the entire length of flexible printed circuit30. However, this is merely illustrative. If desired, head-spreading layer70may be formed only on portion79of flexible printed circuit30as shown inFIG. 7.

Heat-spreading layer70may be formed only on portion79of flexible printed circuit30by forming heat-spreading layer70along the entire length of a sheet of flexible polymer substrate and removing a portion of heat-spreading layer70from the sheet of flexible polymer substrate before or after die cutting the sheet of flexible printed circuitry (as described above in connection withFIG. 6). If desired, heat-spreading layer70may be formed on portion79of flexible printed circuit30by attaching a strip of heat-spreading material70that has a width equal to the width of portion79to top surface82of flex circuit layer31using adhesive72.

In the examples ofFIGS. 6 and 7, flexible printed circuit30includes a single flex circuit layer31. However, this is merely illustrative. As shown inFIG. 8, flexible printed circuit30may include a second flex circuit layer31attached to coverlay layer78using an adhesive such as adhesive84. The second flex circuit layer31may include a conductive layer such as patterned conductive layer86(e.g., copper, aluminum or other metal traces) that is covered by a coverlay layer such as coverlay layer88. Flexible printed circuit30may include one or more conductive vias such as plated through-hole90that interconnect conductive layer76with conductive layer86.

As shown inFIG. 9, flexible printed circuit30may, if desired, include a third flex circuit layer31attached to surface80of heat and pressure spreading layer70using adhesive92. Third flex circuit layer31may include a conductive layer such as patterned conductive layer94(e.g., copper, aluminum or other metal traces) that is covered by a coverlay layer such as coverlay layer96.

A flex circuit layer31that is attached to top surface80of pressure-spreading layer70may cover a portion of pressure-spreading layer70that is different from portion79of pressure-spreading layer70so that surface80of heat-spreading layer70remains exposed for pressing a bonding tool during assembly of device10.

During manufacturing of flexible printed circuit30, one or more additional sheets of flexible circuitry such as flex circuit layers31may be attached to coverlay layer78and/or pressure-spreading layer70using 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 inFIG. 9, a conductive via such as plated through-hole98may interconnect conductive layers through flexible polymer layer74, pressure-spreading layer70, and coverlay layer78. However, this is merely illustrative. Some conductive vias may pass through coverlay layer78without passing through polymer layer74and pressure-spreading layer70. Some conductive vias may pass through polymer layer74and pressure-spreading layer70without passing through coverlay layer78.

If desired, flexible printed circuit30may 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 inFIG. 10, during assembly of device10, a component such as driver circuit60may be attached to a rigid circuit member such as TFT layer58of display14. A bonding tool such as heat bar71may then be pressed against top surface80of heat-spreading layer70of flexible printed circuit30(e.g., in direction73) in order to bond flexible printed circuit30to bond pads61of TFT layer58using ACF adhesive48. Providing flexible printed circuit30with a heat and pressure spreading layer such as pressure-spreading layer (heat-spreading layer)70that has been die cut with flexible printed circuit30may enable precise positioning of bonding tool71during bonding of flexible printed circuit30to TFT layer58. In this way, flexible printed circuit30may be mounted to TFT layer58at a distance D from component60that 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. 11is a flow chart of illustrative steps that may be involved in assembling device10using a flexible printed circuit having a heat and pressure spreading layer. At step100, 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 step102, 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 step104, bond pads on the flexible printed circuit may be aligned with corresponding bond pads on the rigid circuit member.

At step106, 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. 12is a flow chart of illustrative steps that may be involved in manufacturing a flexible printed circuit having a heat and pressure spreading layer. At step110, 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 step112, 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 step114, one or more additional sheets of flexible printed circuitry (e.g., flex circuit layers31) 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 step116, 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 step118, 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 device10.

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.