PATENT DOCUMENT

Publication Number: US-9769920-B2
Application Number: US-201414226593-A
Country: US
Kind Code: B2

Title: Flexible printed circuits with bend retention structures

Abstract:
An electronic device may be provided with printed circuits. Electrical components may be interconnected using signal paths formed from metal traces in the printed circuits. The printed circuits may include flexible printed circuits with bent configurations. The flexible printed circuits may be provided with integral bend retention structures. A bend retention structure may be formed from a polymer layer, a solder layer, a stiffener formed from metal or polymer that is attached to flexible printed circuit layers with adhesive, a conformal plastic coating that covers exposed metal traces at a bend, a metal stiffener with screw holes, a shape memory alloy, a portion of a flexible printed circuit dielectric substrate layer with a reduced elongation at yield value, or combinations of these structures. The bend retention structure maintains a bend in a bent flexible printed circuit.

Claims:
What is claimed is: 
     
       1. A flexible printed circuit having a bend, comprising:
 flexible printed circuit layers including metal traces; and 
 a bend retention structure that is attached to the flexible printed circuit layers at the bend and that retains the flexible printed circuit layers in a bent configuration to maintain the bend, wherein the bend retention structure includes a polymer layer with a flattened wrinkle that is attached to the flexible printed circuit layers with a layer of adhesive. 
 
     
     
       2. An electronic device, comprising:
 a display; 
 a printed circuit; and 
 a flexible printed circuit with a bend, wherein the flexible printed circuit is coupled between the display and the printed circuit and wherein the flexible printed circuit includes a bend retention structure that maintains the bend, the flexible printed circuit includes a polyimide substrate layer, and the flexible printed circuit includes an opening in the polyimide substrate layer that is filled with a polymer that has a lower elongation at yield value than the polyimide substrate layer. 
 
     
     
       3. The electronic device defined in  claim 2  wherein the flexible printed circuit includes metal traces on the polyimide substrate layer and the bend retention structure comprises a layer of cured adhesive on an outer surface of the flexible printed circuit.

Description:
BACKGROUND 
     This relates generally to printed circuits and, more particularly, to printed circuit structures with bends for use in electronic devices. 
     Electronic devices often include printed circuits. Flexible printed circuits can serve as substrates for electrical components and other devices and may be used to create signal cables that interconnect circuitry in an electronic device. 
     Flexible printed circuits are formed from patterned metal traces supported by layers of dielectric substrate material such as sheets of polyimide. It can be challenging to form bends in flexible printed circuits, because polyimide resists bending and has a springiness that attempts to restore a bent flexible printed circuit to its original unbent state. The restoring forces generated by a bent flexible printed circuit can create assembly difficulties and can impact reliability. 
     It would therefore be desirable to be able to provide improved arrangements for providing flexible printed circuits with bends for use in an electronic device. 
     SUMMARY 
     An electronic device may be provided with printed circuits. Electrical components may be interconnected using signal paths formed from metal traces in the printed circuits. The printed circuits may include flexible printed circuits with bent configurations. A bent flexible printed circuit may be coupled between a printed circuit board and a device component such as a display or other electrical device or may be coupled between other circuitry. 
     A flexible printed circuit may be provided with an integral bend retention structure. A bend retention structure may be formed from a polymer layer, a solder layer, a stiffener formed from metal or plastic that is attached to flexible printed circuit layers with adhesive, a conformal plastic coating that covers exposed bent metal traces at the bend in the flexible printed circuit, a metal stiffener with screw holes, a planar shape memory alloy structure that has been returned to a bent configuration by heating the shape memory alloy after attaching the shape memory alloy to flexible printed circuit layers in the flexible printed circuit, a localized portion of a flexible printed circuit dielectric substrate layer with a reduced elongation at yield value, or combinations of these structures. The bend retention structure maintains a bend in the flexible printed circuit or at least reduces the restoring force generated by bending the dielectric substrate layer in the flexible printed circuit. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of an illustrative electronic device such as a laptop computer in accordance with an embodiment. 
         FIG. 2  is a perspective view of an illustrative electronic device such as a handheld electronic device in accordance with an embodiment. 
         FIG. 3  is a perspective view of an illustrative electronic device such as a tablet computer in accordance with an embodiment. 
         FIG. 4  is a perspective view of an illustrative electronic device such as a computer or other equipment with a display in accordance with an embodiment. 
         FIG. 5  is a schematic diagram of illustrative circuitry in an electronic device in accordance with an embodiment. 
         FIG. 6  is a cross-sectional side view of an illustrative electronic device in accordance with an embodiment. 
         FIG. 7  is a cross-sectional side view of a flexible printed circuit with a bend in accordance with an embodiment. 
         FIG. 8  is a cross-sectional side view of a portion of a flexible printed circuit to which an electrical component has been mounted in accordance with an embodiment. 
         FIG. 9  is a cross-sectional side view of a flexible printed circuit having a single layer of patterned metal traces in accordance with an embodiment. 
         FIG. 10  is a cross-sectional side view of a flexible printed circuit having patterned metal traces formed on opposing upper and lower surfaces of a polymer substrate layer in accordance with an embodiment. 
         FIG. 11  is a cross-sectional side view of an illustrative flexible printed circuit in accordance with an embodiment. 
         FIG. 12  is a cross-sectional side view of an illustrative conductive via in a flexible printed circuit in accordance with an embodiment. 
         FIG. 13  is a schematic diagram of illustrative equipment that may be used in processing flexible printed circuit structures in accordance with an embodiment. 
         FIG. 14  is a cross-sectional side view of an illustrative flexible printed circuit following bending of the flexible printed circuit about a bend axis in accordance with an embodiment. 
         FIG. 15  is a flow chart of illustrative steps involved in forming a flexible printed circuit with a bend and a bend retention structure for use in an electronic device in accordance with an embodiment. 
         FIG. 16  is a cross-sectional side view of an illustrative flexible printed circuit having a substrate with a flexible region to facilitate bending in accordance with an embodiment. 
         FIG. 17  is a cross-sectional side view of the illustrative flexible printed circuit of  FIG. 16  following bending in accordance with an embodiment. 
         FIG. 18  is a diagram showing how a flexible printed circuit may have exposed metal traces that are coated with a conformal dielectric coating in a bend in accordance with an embodiment. 
         FIG. 19  is a cross-sectional side view of an illustrative flexible printed circuit to which stiffener structures have been used to help hold the flexible printed circuit in a bent configuration in accordance with an embodiment. 
         FIG. 20  is a cross-sectional side view of the illustrative flexible printed circuit of  FIG. 19  following bending of the flexible printed circuit in accordance with an embodiment. 
         FIG. 21  is a cross-sectional side view of an illustrative flexible printed circuit and associated structures for helping the flexible printed circuit to hold a bend in accordance with an embodiment. 
         FIG. 22  is a cross-sectional side view of the structures of  FIG. 21  during bending operations in accordance with an embodiment. 
         FIG. 23  is a cross-sectional side view of the structures of  FIG. 22  showing how an integral bend retention structure maintains a bend in a flexible printed circuit in accordance with an embodiment. 
         FIG. 24  is a diagram showing how a flexible printed circuit may be coated with a layer of material and processed to help the flexible printed circuit hold a bend in accordance with an embodiment. 
         FIG. 25  is a diagram showing how a flexible printed circuit may be coated with a material following bending to help the flexible printed circuit hold a bend in accordance with an embodiment. 
         FIG. 26  is a top view of a flexible printed circuit structure and associated structures for forming a wrinkle in a layer of material to be placed on top of the flexible printed circuit structure in accordance with an embodiment. 
         FIG. 27  is a cross-sectional side view of an illustrative flexible printed circuit with a wrinkled upper layer prior to bending the flexible printed circuit in accordance with an embodiment. 
         FIG. 28  is a cross-sectional side view of the flexible printed circuit of  FIG. 27  following bending to flatten the wrinkle and form a bend retention structure by attaching the flattened wrinkle to flexible printed circuit layers using adhesive in accordance with an embodiment. 
         FIG. 29  is a perspective view of an illustrative bend retention structure for a flexible printed circuit in accordance with an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Electronic devices may be provided with printed circuits. The printed circuits may include rigid printed circuit boards (e.g., printed circuits formed from rigid printed circuit board material such as fiberglass-filled epoxy) and flexible printed circuits (e.g., printed circuits that include one or more sheets of polyimide substrate material or other flexible polymer layers). The flexible printed circuits may be provided with bends. The bends may be used to route the flexible printed circuits between different areas of interest in an electronic device. Illustrative electronic devices that may be provided with flexible printed circuits are shown in  FIGS. 1, 2, 3 , and  4 . 
     Electronic device  10  of  FIG. 1  has the shape of a laptop computer and has upper housing  12 A and lower housing  12 B with components such as keyboard  16  and touchpad  18 . Device  10  has hinge structures  20  (sometimes referred to as a clutch barrel) to allow upper housing  12 A to rotate in directions  22  about rotational axis  24  relative to lower housing  12 B. Display  14  is mounted in housing  12 A. Upper housing  12 A, which may sometimes referred to as a display housing or lid, is placed in a closed position by rotating upper housing  12 A towards lower housing  12 B about rotational axis  24 . 
       FIG. 2  shows an illustrative configuration for electronic device  10  based on 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 , device  10  has opposing front and rear surfaces. The rear surface of device  10  may be formed from a planar portion of housing  12 . Display  14  forms the front surface of device  10 . Display  14  may have an outermost layer that includes openings for components such as button  26  and speaker port  28 . 
     In the example of  FIG. 3 , electronic device  10  is a tablet computer. In electronic device  10  of  FIG. 3 , device  10  has opposing planar front and rear surfaces. The rear surface of device  10  is formed from a planar rear wall portion of housing  12 . Curved or planar sidewalls may run around the periphery of the planar rear wall and may extend vertically upwards. Display  14  is mounted on the front surface of device  10  in housing  12 . As shown in  FIG. 3 , display  14  has an outermost layer with an opening to accommodate button  26 . 
       FIG. 4  shows an illustrative configuration for electronic device  10  in which device  10  is a computer display, a computer that has an integrated computer display, or a television. Display  14  is mounted on a front face of device  10  in housing  12 . With this type of arrangement, housing  12  for device  10  may be mounted on a wall or may have an optional structure such as support stand  30  to support device  10  on a flat surface such as a table top or desk. 
     An electronic device such as electronic device  10  of  FIGS. 1, 2, 3, and 4 , may, in general, be a computing device such as 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. The examples of  FIGS. 1, 2, 3, and 4  are merely illustrative. 
     Device  10  may include a display such as display  14 . Display  14  may be mounted in housing  12 . Housing  12 , which may sometimes be referred to as an enclosure or 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.). 
     Display  14  may be a touch screen display that incorporates a layer of conductive capacitive touch sensor electrodes or other touch sensor components (e.g., resistive touch sensor components, acoustic touch sensor components, force-based touch sensor components, light-based touch sensor components, etc.) or may be a display that is not touch-sensitive. Capacitive touch screen electrodes may be formed from an array of indium tin oxide pads or other transparent conductive structures. 
     Display  14  may include an array of display pixels formed from liquid crystal display (LCD) components, an array of electrophoretic display pixels, an array of plasma display pixels, an array of organic light-emitting diode display pixels, an array of electrowetting display pixels, or display pixels based on other display technologies. 
     Display  14  may be protected using a display cover layer such as a layer of transparent glass or clear plastic. Openings may be formed in the display cover layer. For example, an opening may be formed in the display cover layer to accommodate a button, an opening may be formed in the display cover layer to accommodate a speaker port, etc. 
     A schematic diagram of an illustrative device such as devices  10  of  FIGS. 1, 2, 3, and 4  is shown in  FIG. 5 . As shown in  FIG. 5 , electronic device  10  may include control circuitry such as storage and processing circuitry  38 . Storage and processing circuitry  38  may include one or more different types of storage such as hard disk drive storage, nonvolatile memory (e.g., flash memory or other electrically-programmable-read-only memory), volatile memory (e.g., static or dynamic random-access-memory), etc. Processing circuitry in storage and processing circuitry  38  may be used in controlling the operation of device  10 . The processing circuitry may be based on a processor such as a microprocessor and other suitable integrated circuits. With one suitable arrangement, storage and processing circuitry  38  may be used to run software on device  10 , such as internet browsing applications, email applications, media playback applications, operating system functions, software for capturing and processing images, software implementing functions associated with gathering and processing sensor data such as stress data, etc. 
     Input-output circuitry  32  may be used to allow data to be supplied to device  10  and to allow data to be provided from device  10  to external devices. Input-output circuitry  32  may include wired and wireless communications circuitry  34 . Communications circuitry  34  may include radio-frequency (RF) transceiver circuitry formed from one or more integrated circuits, power amplifier circuitry, low-noise input amplifiers, passive RF components, one or more antennas, and other circuitry for handling RF wireless signals. Wireless signals can also be sent using light (e.g., using infrared communications). 
     Input-output circuitry  32  may include input-output devices  36 . Input-output devices  36  may include devices such as buttons (see, e.g., button  26  of  FIGS. 2 and 3 ), joysticks, click wheels, scrolling wheels, a touch screen (see, e.g., display  14 ), other touch sensors such as track pads (see, e.g., track pad  18  of  FIG. 1 ), touch-sensor-based buttons, vibrators, audio components such as microphones and speakers, image capture devices such as a camera module having an image sensor and a corresponding lens system, keyboards, status-indicator lights, tone generators, key pads, strain gauges (e.g., a button based on a strain gauge), proximity sensors, ambient light sensors, capacitive proximity sensors, light-based proximity sensors, gyroscopes, accelerometers, magnetic sensors, temperature sensors, fingerprint sensors, and other equipment for gathering input from a user or other external source and/or generating output for a user. 
     A cross-sectional side view of an illustrative electronic device of the type that may be provided with one or more flexible printed circuits is shown in  FIG. 6 . As shown in the illustrative configuration of  FIG. 6 , device  10  may have a display such as display  14  that is mounted on the front face of device  10 . Display  14  may have a display cover layer such as cover layer  52  and a display module such as display module  50 . Display cover layer  52  may be formed from a glass or plastic layer. Display module  50  may be, for example, a liquid crystal display module or an organic light-emitting diode display layer (as examples). Display module  50  may have a rectangular outline when viewed from the front of device  10  and may be mounted in a central rectangular active area AA on the front of device  10 . An inactive area IA that forms a border for display  14  may surround active area AA. Opaque masking material such as black ink  54  may be used to coat the underside of cover layer  52  in inactive area IA. 
     Device  10  may include components such as components  62  that are mounted on one or more printed circuit boards such as printed circuit board  60 . Printed circuit board  60  may have one or more layers of dielectric material and one or more layers of metal traces. Printed circuit board  60  of  FIG. 6  may be a rigid printed circuit board or a flexible printed circuit board. Components  62  may be, for example, integrated circuits, discrete components such as capacitors, resistors, and inductors, switches, connectors, sensors, input-output devices such as status indicators lights, audio components, or other electrical and/or mechanical components for device  10 . Components  62  may be attached to printed circuit  54  using solder, welds, anisotropic conductive film or other conductive adhesives, or other conductive connections. One or more layers of patterned metal interconnects (i.e., copper traces or metal traces formed from other materials) may be formed within one or more dielectric layers in printed circuit board  60  to form signal lines that route signals between components  62 . 
     If desired, device  10  may have components mounted on the underside of display cover layer  52  such as illustrative component  56  on opaque masking layer  54  in inactive area IA of device  10  of  FIG. 6 . Component  56  may be a touch sensor, a fingerprint sensor, a strain gauge sensor, a button, or other input-output device  36  (as examples). 
     Flexible printed circuits  58  may have layers of dielectric and layers of metal traces. The metal traces of flexible printed circuits  58  may be used to form signal paths to interconnect the circuitry of device  10 . For example, flexible printed circuits  58  may have signal paths that interconnect component  56  to the circuitry of components  62  on printed circuit  60 , signal path that couple display module  50  to components  62  on printed circuit  60 , or signal paths for interconnecting other components in device  10 . 
     Flexible printed circuits such as illustrative flexible printed circuits  58  of  FIG. 6  are often bent. The ability to bend flexible printed circuits in device  10  helps a device designer to route signals in tight spaces and in portions of a device where a planar printed circuit would be ineffective or cumbersome. 
     A cross-sectional side view of an illustrative flexible printed circuit is shown in  FIG. 7 . As shown in  FIG. 7 , flexible printed circuit  58  may have a bend such as bend  66 . Flexible printed circuit  58  may include multiple layers of material such as layers  64 . Layers  64  may include one or more metal layers, one or more dielectric layers, and one or more adhesive layers (or no adhesive layers). Metal traces formed from the metal layers may be used to carry electrical signals. Examples of metals that may be used in the metal layers of layers  64  in flexible printed circuit  58  include copper, nickel, gold, and aluminum. Examples of dielectric materials that may be used in forming the dielectric layers of layers  64  in flexible printed circuit  58  include polyimide, acrylic, and other polymers. Examples of adhesives that may be used in forming the adhesive layers of layers  64  in flexible printed circuit  58  include acrylic adhesives and epoxy adhesives. Other types of metal, dielectric, and adhesive may be used in forming layers  60  if desired. These are merely illustrative examples. Moreover, additional structures may be added to the dielectric, metal, and adhesive layers of flexible printed circuit  58  to help hold flexible printed circuit  58  in a bent configuration. 
     Electrical components such as illustrative electrical component  68  of  FIG. 8  may be attached to flexible printed circuit  58 . Components that may be attached to flexible printed circuit  58  in this way include connectors (e.g., all or part of a board-to-board connector, a zero insertion force connector, or other connector), integrated circuits, discrete components such as resistors, capacitors, and inductors, switching circuitry, and other circuitry (see, e.g., circuitry  38  and  32  of  FIG. 5 ). Electrical and physical connections between component  68  and flexible printed circuit  58  may be made using solder, conductive adhesive, welds, or other conductive coupling mechanisms. In the illustrative configuration of  FIG. 8 , component  68  has metal contacts (solder pads)  70  and flexible printed circuit  58  has corresponding metal contacts (solder pads  72 ). A patterned dielectric layer such as a layer of polyimide or other polymer (sometimes referred to as a solder mask or cover layer) such as layer  76  may serve as the outermost layer of flexible printed circuit  58  (e.g., layer  76  may be formed on top of other layers in flexible printed circuit  58  such as the metal layer used in forming solder pads  72  and other layers  74  of metal, dielectric, and adhesive). If desired, a dielectric cover layer (e.g., a polyimide cover layer) may be formed on both the upper and lower surfaces of the layers of flexible printed circuit  58  (e.g., in a configuration in which metal traces are formed on upper and lower surfaces of an internal polyimide substrate layer). As shown in  FIG. 8 , openings in layer  76  may be formed to accommodate solder pads  72  and to help control the lateral spread of solder  70  when using solder  70  to solder component  68  to flexible printed circuit  58 . 
       FIG. 9  shows how flexible printed circuit  58  may have signal paths formed from a patterned metal layer on a dielectric substrate. In the example of  FIG. 9 , flexible printed circuit  58  has a flexible dielectric substrate such as substrate  80  (e.g., a flexible polyimide layer) that has been covered with a patterned layer of metal traces  82  formed directly on the surface of substrate  80 . If desired, additional layers of material (e.g., an adhesive layer, a polymer cover layer, etc.) may be formed on top of the flexible printed circuit  58  of  FIG. 9  and/or below substrate  80 . The  FIG. 9  arrangement is a single-metal-layer flexible printed circuit. Flexible printed circuit configurations with two or more layers of metal may also be used. 
       FIG. 10  is a cross-sectional side view of flexible printed circuit  58  in a configuration in which flexible printed circuit  58  has been provided with two layers of patterned metal. As shown in  FIG. 10 , flexible printed circuit  58  has a polymer substrate such as a polyimide substrate (substrate  80 ). Substrate  80  has opposing upper and lower surfaces. Metal traces  84  of  FIG. 10  are formed directly on the upper surface of substrate  80 . Metal traces  86  are formed directly on the lower surface of substrate  80 . A polymer cover layer such a layer  90  may be used to cover the upper metal layer used in forming metal traces  84 . A polymer cover layer or other dielectric material  92  may be used to cover the lower metal layer used in forming metal traces  86 . Openings may be formed in insulating layers such as polymer layers  90  and  92  (e.g., to allow components to be soldered to traces  84  and/or  86 ). A patterned dielectric layer such as a polymer layer with openings may also be formed over traces  82  of flexible printed circuit  58  of  FIG. 9 . 
     The outermost dielectric layers of flexible printed circuit  58  (i.e., the cover layers for flexible printed circuit  58 ) may be formed from a laminated polymer film (e.g., a polyimide film attached to flexible printed circuit  58  with a layer of adhesive), may be formed from a cured liquid polymer (e.g., photoimageable polymer formed directly on underlying layers without adhesive), or may be formed from other dielectric materials formed directly on underlying metal traces or other structures on the surface of printed circuit  58  and/or attached to underlying metal traces or other structures on the surface of printed circuit  58  using adhesive. Metal traces  82  may be formed directly on the surface of substrate  80  as shown in the examples of  FIGS. 9 and 10  or may be laminated to substrate  80  using adhesive. For example, traces  82  in  FIG. 9  may be formed by laminating a metal foil layer to substrate  80  with an interposed layer of adhesive). If desired, three or more metal layers may be formed in flexible printed circuit  58 , as described in connection with  FIG. 7 . In configurations for printed circuit  58  that contain multiple metal layers, multiple intervening substrate layers may, if desired, be used to separate metal layers. For example, there may be two or more polyimide substrate layers in printed circuit  58 . Adhesive layers, metal layers, substrate layers, and polymer cover layers (sometimes referred to as solder mask layers or coverlay) may be arranged in a stack in a desired pattern to form flexible printed circuit  58 . The use of a single-layer design for flexible printed circuit  58  of  FIG. 9  and a two-layer design for flexible printed circuit  58  of  FIG. 10  is merely illustrative. 
       FIG. 11  is a cross-sectional side view of an illustrative two-layer flexible printed circuit showing how both the upper and lower surfaces of substrate  80  may be covered with layers of material that are attached to substrate  80  using adhesive. As shown in  FIG. 11 , flexible printed circuit  58  is formed using a substrate layer such as substrate  80  (e.g., a polyimide layer or other suitable layer). Substrate  80  has upper surface  94  and opposing lower surface  96 . Layer  98  may be formed on upper surface  94 . Layer  98  may include metal layer  100  and adhesive layer  102 . Adhesive layer  102  may be used to laminate metal layer  100  to upper surface  94  of substrate  80 . Layer  104  may be formed on top of layer  98 . Layer  104  may include polymer layer  106  such as a polyimide layer (sometimes referred to as a cover layer, coverlay, or solder mask). Adhesive layer  108  in layer  104  may be used to attach polymer layer  106  to layer  98 . The underside of flexible printed circuit substrate  80  may be provided with layers  110  and  116 . Layer  110  may include metal layer  114 . Adhesive layer  112  in layer  110  may be used to attach metal layer  114  to lower surface  96  of substrate  80 . Layer  116  may include dielectric layer  120  (e.g., a polymer cover layer such as a polyimide layer) and adhesive layer  118  for attaching layer  120  to layer  110 . Metal layers in flexible printed circuit  58  such as metal layer  114  and metal layer  100  of  FIG. 11  may be patterned using photolithography, laser cutting, die cutting (e.g., foil stamping techniques), or other patterning techniques. Dielectric layers  106  and  120  and/or the adhesive layers in flexible printed circuit  58  may also be patterned using these techniques. 
     If desired, through vias, blind vias, and buried vias may be used to interconnect metal traces on different layers of flexible printed circuit  58 . Holes or other openings may be formed in flexible printed circuit  58  using laser drilling, stamping, machining, or other hole formation techniques. The holes may be filled with metal using electroplating, electroless deposition, or other metal deposition techniques. Plated holes may form tubular vias that form conductive signal paths between the metal layers of flexible printed circuit  58 . As shown in  FIG. 12 , for example, the layers of flexible printed circuit  58  may be provided with holes such as hole  122 . Metal  124  may be deposited on the inner surface of hole  122  using electrochemical deposition (e.g., electroplating and/or electroless deposition), thereby forming via  126 . Via  126  can form a signal path between metal layer  100  and metal layer  114 . Vias with other configurations (e.g., blind vias and buried vias) can likewise interconnect different metal layers in flexible printed circuit  58 . 
       FIG. 13  is a diagram of illustrative processing equipment that may be used in forming flexible printed circuit  58  and in mounting electrical components to flexible printed circuit  58  or otherwise coupling flexible printed circuit  58  into the circuitry of device  10 . 
     The equipment of  FIG. 13  may include printing equipment  130 . Printing equipment  130  may include ink-jet printing equipment, pad printing equipment, screen printing equipment, and other equipment for printing blanket layers and/or patterned layers of material. Examples of structures that may be formed using equipment  130  include printed layers of dielectric, strips of dielectric, metal lines (e.g., metal traces formed from metallic paint or other liquid conductive material), blanket layers of metal, etc. 
     Hole formation equipment  132  may include tools such as laser drilling tools, machining tools, and other equipment for forming openings in one or more layers of material for flexible printed circuit  58 . For example, hole formation equipment  132  may use a laser or other tool to drill holes for vias such as via  126  of  FIG. 12 . 
     Lamination equipment  134  may include rollers and other equipment for laminating layers of material together (e.g., using heat and pressure to cause adhesive to attach layers of flexible printed circuit  58  together or to otherwise attach layers together). 
     Global layer deposition equipment  142  may include equipment for depositing layers of material by blanket spray coating, by spinning, by physical vapor deposition (e.g., sputtering), or other deposition techniques. 
     Patterning equipment  140  may be used to pattern layers of material such as blanket layers of metal and/or dielectric. Equipment  140  may include photolithographic equipment such as equipment for depositing photoresist or other photoimageable materials, equipment for exposing photoresist or other photoimageable materials to patterned light associated with a photomask, developing equipment to use in developing photoresist or other photoimageable materials, etching equipment for etching the structures of flexible printed circuit  58  after deposited photoresist has been patterned by exposure and development, etc. 
     Electrochemical deposition tools  144  such as tools for electroplating metal in a via, tools for electroless deposition, and other electrochemical deposition equipment may be used in forming flexible printed circuit  58 . 
     One or more of the layers of flexible printed circuit  58  and/or other structures may be bent using bending tools  146 . Bending tools  146  may be formed from stand-alone equipment or equipment that is integrated into other equipment of  FIG. 13 . Examples of bending equipment that may be used in forming bends in flexible printed circuit  58  include mandrels, presses, grippers, and other bending machines. 
     If desired, other tools  136  may be used in processing the structures of flexible printed circuit  58  such as lasers for cutting, machining tools for trimming or cutting, heated presses, die cutting equipment, injection molding equipment, heating equipment such as infrared lamps and ovens, light-emitting diodes, or other light sources for adhesive curing (e.g., ultraviolet light-emitting diodes), and other equipment for depositing, patterning, processing, and removing layers of dielectric and metal for structures  58 . 
     Soldering tools  138  and other equipment may be used in mounting electrical components to flexible printed circuit  58  and/or may be used in coupling flexible printed circuit  58  to other circuitry in device  10 . 
     Materials such as polyimide are desirable in forming flexible insulating substrates for flexible printed circuit  58 . However, when a planar polyimide substrate layer is bent, the polyimide substrate layer will attempt to spring back into its original planar shape. This gives rise to a restoring force. Consider, as an example, flexible printed circuit  58  of  FIG. 14 , which has been bent around bend axis  150  to form bend  152 . In the absence of processing or structures in region  156 , flexible printed circuit  58  will exhibit a restoring force (spring force) F in upwards direction  154  due to the presence of bend  152 . Force F can be significant due to the relatively high elongation at yield value of polyimide. Force F can tend to press apart structures in device  10 , leading to reliability concerns if force F is too great. 
     To address the concerns raised when force F is more than a negligible amount, flexible printed circuit  58  can be processed in region  156  and/or can be provided with structures in region  156  (and, if desired, elsewhere in printed circuit  58 ) to overcome force F. In particular, flexible printed circuit  58  can be configured so as to reduce F to zero or to at least reduce F to a fraction of a fraction (e.g., 50% or less, 20% or less, or 10% or less, as examples) of the original restoring force that would have been exhibited without the use of the processing and/or structures in region  156 . When provided in this type of configuration, flexible printed circuit  58  is said to be bend-restoring-force compensated or is said to have been provided with a bend retention structure. The bend retention structure may be implemented by incorporating one or more additional layers of material and/or other supplemental structures into flexible printed circuit  58 , by processing one or more existing layers of flexible printed circuit  58  to reduce or eliminate restoring force F, or by otherwise configuring flexible printed circuit  58  so that it fully or at least partially retains its desired bent shape. The bend retention structure may be formed as an integral portion of flexible printed circuit  58 , so that external structures such as brackets need not be relied on as the sole structures for holding flexible printed circuit  58  in a desired bent shape. The bend retention structure is preferably formed without adding significant bulk to the layers of the flexible printed circuit. 
     Flexible printed circuit  58  may be provided with one or more bends. Configurations in which flexible printed circuit  58  is provided with a single bend (e.g., a bend of about 90° or 180°) are sometimes described herein as an example. This is, however, merely illustrative. Flexible printed circuit  58  may be provided with two or more bends, may be provided with bends of less than 90°, of 90-180°, of 180°, of more than 180°, or of less than 180°, if desired. 
       FIG. 15  is a flow chart of illustrative steps involved in forming a flexible printed circuit with a bend retention structure. At step  160 , layers  64  ( FIG. 7 ) may be formed (e.g., by creating one or more sheets of polymer such as polyimide substrate layers, polyimide cover layers, adhesive layers, metal layers, etc.). The layers of flexible printed circuit  58  that are formed at step  160  may be attached to each other and/or may have portions that are not attached to each other. Layers  64  may, if desired, be processed at step  162  by adding additional structures such as one or more additional layers of dielectric, adhesive, metal, or other materials and/or by applying light, heat, or other energy to modify the properties of existing layers. Processing operations may also be performed to cut and otherwise pattern layers  64 . 
     At step  164 , the flexible printed circuit structures of step  160  may be bent. For example, computer-controlled or manually controlled bending equipment  146  ( FIG. 13 ) may form one or more bends in the layers of flexible printed circuit  58 . After bending flexible printed circuit  164 , flexible printed circuit  58  may be installed in device  10  at step  168 , as shown by line  170 . For example, if the processing operations of step  162  and/or the materials selected when forming the flexible printed circuit layers of step  160  are chosen to avoid creating an excessive restoring force F upon bending at step  164  (i.e., if a bend retention structure is formed as an integral portion of flexible printed circuit  58  during the operations of steps  160 ,  162 , and/or  164 ), the bent version of flexible printed circuit  58  can be installed directly in device  10  without further processing. In some situations, it may be desirable to add additional structures, to apply heat, light, or other energy, or to otherwise process the bent flexible printed circuit so as to ensure that the bent flexible printed circuit holds its desired bent shape (i.e., to complete formation of a bent flexible printed circuit with an integral bend retention structure). These additional processing operations may be performed at step  166 . Examples of operations that may be performed at step  166  include adhesive curing, solder reflow operations, coating, etc. After the operations involved in forming a bent flexible printed circuit with a bend retention structure have been completed, flexible printed circuit  58  may be installed in device  10  (step  168 ). 
       FIG. 16  is a cross-sectional side view of an illustrative flexible printed circuit that has been provided with a structure that helps retain a bend. As shown in  FIG. 16 , unbent flexible printed circuit  58  has regions such as end regions in which dielectric substrate  80 A is formed from a springy material such as polyimide (i.e., a material with a high elongation at yield value). The dielectric substrate of flexible printed circuit  58  also has a portion such as central portion  80 B that is formed from a material with a lower elongation at yield value than the springy polyimide of the end regions. Central portion  80 B may be formed by filling an opening in polyimide substrate layer  80 A with a material that is softer and less springy than polyimide (e.g., a soft acrylic) or may be formed by processing the central portion of a polyimide layer with heat, light, other energy, chemicals, and/or mechanical operations to locally reduce the springiness of the polyimide. In the example of  FIG. 16 , flexible printed circuit  58  has a single dielectric substrate layer and upper and lower metal layers  98  and  110  covered with polymer cover layers  104  and  116 . If desired, flexible printed circuit  58  may have one or more additional stacked dielectric layers (e.g., layers formed from polyimide  80 A and less springy material  80 B) and/or may have fewer or more metal layers. 
       FIG. 17  shows how flexible printed circuit  58  of  FIG. 16  may be bent about bend axis  174  to form bend  172 . The presence of less springy material  80 B in the portion of flexible printed circuit  58  that overlaps bend axis  174  forms a bend retention structure that helps hold flexible printed circuit  58  in its bent shape. If desired, material  80 B (i.e., a material or modified polyimide layer that exhibits a springiness less than polyimide) may be extended in size (i.e., some or all of material  80 A may be replaced with material  80 B). The use of a dielectric substrate that includes both polyimide portion  80 A and dielectric portion  80 B is merely illustrative. 
       FIG. 18  shows how flexible printed circuit  58  may be provided with a bend retention structure formed from a conformal coating that is used to coat bare metal traces. As shown in the upper portion of  FIG. 18 , flexible printed circuit  58  has one or more metal layers such as metal layer  178 . Metal layer  178  may be patterned to form one or more signal lines (e.g., lines running across the page in the orientation of  FIG. 18 ). Metal layer  178  may be sandwiched between layers  176 . Layers  176  may include a polyimide layer or other dielectric substrate material, outer polymer cover layer(s), layers of adhesive, etc.) In central region  180 , an opening is formed in layers  176  (e.g., using etching, by cutting an opening prior in these layers prior to lamination to form flexible printed circuit  58 , etc.). After forming the structures of the upper portion of  FIG. 18 , flexible printed circuit  58  is bent about bend axis  182  using bending tool  186  to form bend  184  in the exposed (uncoated) portion of metal  178 . Coating equipment  188  is the used to apply a conformal dielectric coating such as coating  190  to the exposed surfaces of metal  178  (and, if desired, portions of layers  176 ). Equipment  188  may include spraying equipment, painting equipment, adhesive-dispensing equipment such as a nozzle or needle dispenser, ink jet printing equipment, plastic injection-molding equipment, or other equipment for applying dielectric coating  190  to exposed metal layer  178 . After the metal traces of layer  178  have been covered with dielectric, bending tool  186  may be removed. The presence of coating  190 , which is stiff, stiffens the bent portion of flexible printed circuit  58  and helps hold flexible printed circuit  58  in its bent shape (i.e., conformal coating  190  serves as an integral bend retention structure for flexible printed circuit  58 ). Conformal coating  190  may be formed on both sides of the metal traces in layer  178  or may be formed on only the inner or outer surface of layer  178  in exposed region  180 . 
       FIG. 19  shows how layers of material such as layers  200  may be attached to flexible printed circuit  58 ′ using attachment layers  202  to form a flexible printed circuit with an integral bend retention structure (i.e., flexible printed circuit  58 ). Layers  200  may, if desired, be stiff thin members that serve as stiffening layers. Stiffening layers  200  may, once attached to the flexible printed circuit layers of flexible printed circuit portion  58 ′, help retain the flexible printed circuit layers in a bent configuration to retain a desired flexible printed circuit bend. The thickness of layers  200  and the thicknesses of the other bend retention structures may be comparable to the thickness of flexible printed circuit layers  58 ′ (i.e., the bend retention structure is preferably thinner than the other layers in flexible printed circuit  58  or at least does not add appreciable bulk to flexible printed circuit  58 ). 
     Layers  200  and  202  may be formed in the portion of flexible printed circuit  58  in which it is desired to form a bend (i.e., in region  204  of  FIG. 19 ). Layers  200  may be formed from metal (e.g., metal strips such as strips of brass, steel, stainless steel, nickel, other metals, etc.), plastic (e.g., a sheet of polymer), or other material. Attachment layer  202  may be formed from solder, adhesive, or other materials. 
     Following bending, flexible printed circuit  58  may be processed in region  204  (e.g., by application of light, heat, pressure, etc.). The processing that is performed in region  204  may reflow any solder that is present so that metal layers  200  may be soldered to exposed metal traces in the layers of flexible printed circuit  58 ′, may cure any adhesive that is present to attach structures  200  to the layers of flexible printed circuit  58 ′, or may otherwise complete the attachment of layers  200  to flexible printed circuit  58 . As shown in  FIG. 20 , this forms a bend retention structure for flexible printed circuit  58  that may hold flexible printed circuit  58  in its bent configuration. Flexible printed circuit  58  may include metal traces in metal layer  206 . The thicknesses of layers  200  and  202  on the upper and lower surfaces of flexible printed circuit  58  may be selected to ensure that bent metal layer  206  in the bend of flexible printed circuit  58  lies in the neutral stress plane of flexible printed circuit  58 . 
     In the illustrative configuration of  FIG. 21 , attachment layer  202  has been restricted to ends  210  of layers  200  and has been omitted from central portion  212  of layers  202 . This type of arrangement may help accommodate movement of layers  200  relative to the surfaces of flexible printed circuit  58 ′ during bending (i.e., to allow the ends of upper layer  200  to move inwardly relative to the upper surface of flexible printed circuit  58 ′ when flexible printed circuit  58 ′ is bent upward in direction  208  and to allow the ends of lower layer  200  to move outwardly relative to the lower surface of flexible printed circuit  58 ′ at the same time).  FIG. 22  shows how heated press  214  or other equipment may be used in bending flexible printed circuit  58 ′. While in the bent configuration, heat from an oven or heat from heated press  214  may cause layers  202  at the opposing ends of layers  200  to attach layers  200  to flexible printed circuit  58 ′ (i.e., the heat may reflowing solder in layer  202  or may cure adhesive in layer  202 ). After removing equipment  214 , layers  200  will form a bend retention structure that allows flexible printed circuit  58  to retain its bent shape (e.g., a configuration with a 90° bend in the example of  FIG. 23 ). 
       FIG. 24  shows how layers such as layer  202  may be placed directly on the layers of flexible printed circuit  58  to form a bend retention structure without requiring the use of additional stiffening layers  200 . Layer  202  may be formed from a thermoset polymer (e.g., a liquid adhesive such as epoxy), a thermoplastic polymer, solder paste, or other suitable material that is stiffened during processing to form a stiffening layer structure. Initially, flexible printed circuit  58 ′ is free of material  202 , as shown in the uppermost portion of  FIG. 24 . Layer  202  may then be applied using equipment of the type shown in  FIG. 13  (e.g., printing equipment  130 , equipment  140  and/or  142 , etc.). For example, liquid adhesive may be screen printed onto the surface of flexible printed circuit  58 ′ or solder paste may be screen printed onto the surface of flexible printed circuit  58 ′. Equipment  218  may then be used to bend flexible printed circuit  58 ′ and layer  202 . Once flexible printed circuit  58 ′ has been bent, equipment  216  may apply energy  218  to layer  202  to stiffen layer  202 . Energy  218  may be heat to reflow solder paste and thereby form a solid layer of rigid solder, may be heat to cure thermally cured adhesive, may be ultraviolet light or other light to cure light-cured adhesive, or may be other energy. After layer  202  has been stiffened (e.g., by reflowing solder, by converting liquid adhesive into a rigid layer of cured adhesive, etc.), layer  202  will serve as a stiffening layer in a bend retention structure for flexible printed circuit  58  that holds flexible printed circuit  58  in its bent configuration, as shown at the bottom of  FIG. 24 . 
     In the illustrative arrangement of  FIG. 25 , flexible printed circuit  58 ′ is bent using bending equipment  220 . While bent, coating equipment  222  applies layer  202 . While flexible printed circuit  58 ′ is bent, equipment  216  may apply energy  218  to stiffen layer  202  as described in connection with  FIG. 24  (e.g., to cure adhesive, to reflow solder paste to form a stiff layer of solder, etc.). This creates a rigid structure from layer  202  that serves as an integral bend retention structure for flexible printed circuit  58 . 
     If desired, a layer of polymer (e.g., a cover layer or other polymer layer) may be provided with a wrinkle overlapping a location where flexible printed circuit  58 ′ is to be bent. As shown in the top view of  FIG. 26 , for example, flexible printed circuit  58 ′ may include metal trace  222 . Structures  224  (e.g., plastic members or other supports) may be placed on the surface of the substrate layer on which trace  222  is formed. Structures  224  may be placed on opposing sides of trace  222  along future bend axis  226 . A layer of polymer  230  (e.g., a cover layer or an additional polyimide substrate layer in a multilayer flexible printed circuit) may then be placed on top of the structures of  FIG. 26 , forming wrinkle  228  in polymer layer  230 , as shown in  FIG. 27 . Adhesive layer  232  may be used to attach polymer layer  230  to trace  222  and other flexible printed circuit layers  234  (e.g., a polyimide substrate layer, optional additional adhesive and metal layers, a lower coverlay, etc.). Upon bending of flexible printed circuit  58 ′, wrinkled (buckled) portion  228  of layer  230  flattens out to form a flattened wrinkle that is attached by adhesive layer  232  to underlying layers such as metal layer  232  and other layers  222 , thereby forming an integral bend retention structure for flexible printed circuit  58 , as shown in  FIG. 28 . 
     In the illustrative example of  FIG. 29 , bend retention structure  236  in flexible printed circuit  58  has been provided with attachment features such as protrusions  240  with holes  242 . Bend retention structure  236  may be formed from plastic, metal, or other materials and may be attached to the surface of flexible printed circuit portion  58 ′ of flexible printed circuit  58  using adhesive, using lamination under heat and pressure without adhesive, using soldering, or using other attachment mechanisms. Holes  242  allow bent flexible printed circuit  58  to be attached to structures in device  10  such as structure  244  using fasteners such screw  238 . Structure  244  may be a portion of housing  12  or other support structure in device  10 . If desired, adhesive may be used in attaching protrusions  240  or other portions of structure  236  to housing structures. 
     If desired, structures  236  or other bend retention structures such as bend retention structures  200  may be formed from a shape memory alloy that relaxes into a bent shape upon heating. The shape memory alloy may serve as a stiffening layer and may be attached to flexible printed circuit layers  58 ′ using adhesive  202 . After attaching the shape memory alloy (e.g., in a planar configuration), the shape memory alloy may be heated using an oven or heated with a lamp or other heating equipment, thereby causing the shape memory alloy to move into its bent configuration in which the shape memory alloy serves as an integral bend retention structure for flexible printed circuit  58 . 
     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: 20140326
Publication Date: 20170919
Grant Date: 20170919
Priority Date: 20140326
Inventors: ELY COLIN M.
SHEDLETSKY ANNA-KATRINA
ROTHKOPF FLETCHER R.
LYNCH STEPHEN BRIAN
Assignee: APPLE INC
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Family ID: 54192454