PATENT DOCUMENT

Publication Number: US-9190720-B2
Application Number: US-201213629218-A
Country: US
Kind Code: B2

Title: Flexible printed circuit structures

Abstract:
Flexible printed circuit structures may be provided that have regions with different electrical and mechanical properties. A flexible printed circuit substrate may be formed from a sheet of polymer having different regions with different thicknesses. The flexible printed circuit substrate may be bent in a thin region of the substrate. Additional flexible printed circuit substrate portions may be coupled to the flexible printed circuit substrate. The additional portions may have different substrate thicknesses. A groove or other recess may be formed in a flexible printed circuit substrate to promote bending. Openings may also be formed in the substrate to promote bending.

Claims:
What is claimed is: 
     
       1. Flexible printed circuit structures, comprising:
 a polymer sheet having a first region with a first thickness and a second region with a second thickness that is thinner than the first thickness; and 
 patterned metal traces on the polymer sheet, wherein the polymer sheet is bent within the second region, and wherein the polymer sheet is bent along an axis and wherein the polymer sheet comprises a groove along the axis. 
 
     
     
       2. The flexible printed circuit structures defined in  claim 1  wherein the patterned metal traces are configured to form a radio-frequency transmission line in the first region. 
     
     
       3. The flexible printed circuit structures defined in  claim 2  wherein the patterned metal traces are configured to form antenna structures in the second region. 
     
     
       4. The flexible printed circuit structures defined in  claim 3 , wherein the transmission line comprises first and second metal traces on opposing sides of the polymer sheet in the first region. 
     
     
       5. The flexible printed circuit structures defined in  claim 3 , wherein the antenna structures are electrically connected to the second metal trace through a via in the polymer sheet. 
     
     
       6. The flexible printed circuit structures defined in  claim 3  wherein the polymer sheet is bent at a right angle. 
     
     
       7. The flexible printed circuit structures defined in  claim 3 , wherein the polymer sheet comprises at least two layers of polyimide joined by an adhesive. 
     
     
       8. Flexible printed circuit structures, comprising:
 a polymer sheet; 
 metal traces on the polymer sheet, wherein the metal traces have a first thickness in a first region of the polymer sheet and have a second thickness that is thinner than the first thickness in a second region of the polymer sheet and wherein the polymer sheet is bent in the second region; 
 a layer of adhesive on the metal traces, wherein the layer of adhesive is present in the first region and is absent from the second region; and 
 a solder mask layer on the adhesive in the first region, wherein the solder mask is absent from the second region. 
 
     
     
       9. The flexible printed circuit structures defined in  claim 8  wherein the metal traces are configured to form a radio-frequency transmission line in the first region. 
     
     
       10. The flexible printed circuit structured defined in  claim 8 , wherein the metal traces are configured to form a microstrip radio-frequency transmission line in the first region. 
     
     
       11. Flexible printed circuit structures, comprising:
 a polymer sheet; and 
 metal traces on the polymer sheet, wherein the metal traces have a first thickness in a first region of the polymer sheet and have a second thickness that is thinner than the first thickness in a second region of the polymer sheet, wherein the polymer sheet is bent in the second region, wherein the metal traces are configured to form a radio-frequency transmission line in the first region, and wherein the radio-frequency transmission line comprises first and second metal traces separated by the polymer sheet. 
 
     
     
       12. A method for forming a flexible printed circuit structures, comprising:
 forming a polymer sheet having a first region of a first thickness and a second region of a second thickness, wherein the second thickness is thinner than the first thickness; 
 depositing metal traces that form an antenna resonating element in the second region and a radio-frequency transmission line in the first region; and 
 bending the polymer sheet along an axis in the second region, wherein forming the polymer sheet having the first region of the first thickness and the second region of the second thickness comprises:
 providing a polymer sheet of the first thickness; and 
 selectively thinning the polymer sheet to the second thickness in the second region. 
 
 
     
     
       13. A method for forming a flexible printed circuit structures, comprising:
 forming a polymer sheet having a first region of a first thickness and a second region of a second thickness, wherein the second thickness is thinner than the first thickness; 
 depositing metal traces that form an antenna resonating element in the second region and a radio-frequency transmission line in the first region; and 
 bending the polymer sheet along an axis in the second region, wherein forming the polymer sheet having the first region of the first thickness and the second region of the second thickness comprises:
 providing first and second polymer layers; 
 dispensing liquid dielectric material between the first and second polymer layers; and 
 rolling the liquid dielectric material between the first and second polymer layers to form the polymer sheet. 
 
 
     
     
       14. A method for forming a flexible printed circuit structures, comprising:
 forming a polymer sheet having a first region of a first thickness and a second region of a second thickness, wherein the second thickness is thinner than the first thickness; 
 depositing metal traces that form an antenna resonating element in the second region and a radio-frequency transmission line in the first region; and 
 bending the polymer sheet along an axis in the second region, wherein bending the polymer sheet along an axis in the second region comprises: 
 forming a plurality of holes in the polymer sheet along the axis; and 
 bending the polymer sheet along the axis.

Description:
This application claims the benefit of provisional patent application No. 61/615,156 filed Mar. 23, 2012, which is hereby incorporated by reference herein in its entirety. 
    
    
     BACKGROUND 
     This relates generally to printed circuit structures and, more particularly, to flexible printed circuit structures. 
     Electronic devices such as computers, cellular telephones, and other equipment contain electrical components. Electrical components may be mounted on printed circuits. Printed circuits may also be used to form structures such as antennas, transmission lines, and signal buses. 
     Some printed circuits are formed using flexible substrate materials. For example, flexible printed circuits may be formed using sheets of a flexible polymer such as polyimide. Layers of patterned metal traces may be formed on the flexible polyimide substrate material. 
     In some devices, flexible printed circuits are mounted in a planar configuration. In other devices, flexible printed circuits are bent. For example, a flexible printed circuit antenna or flexible printed circuit signal bus may be bent. It may be desirable to form a bend in a flexible printed circuit to allow the flexible printed circuit to be incorporated within the potentially tight confines of an electronic device housing or to better utilize available volume within the device housing. 
     It can be challenging to form bends in flexible printed circuits, particularly when the flexible printed circuits include relatively thick substrate material. 
     It would therefore be desirable to be able to provide improved flexible printed circuit structures. 
     SUMMARY 
     Flexible printed circuit structures may be provided that have regions with different electrical and mechanical properties. In one region of a flexible printed circuit structure, for example, the structure may be configured to provide satisfactory performance for a transmission line or other electrical structure. In another region, the flexible printed circuit may be configured to facilitate bending. A bent flexible printed circuit structure may be mounted on a support structure and installed in an electronic device housing. 
     A flexible printed circuit substrate may be formed from a sheet of polymer having different regions with different thicknesses. The flexible printed circuit substrate may be bent in a portion of the substrate that is relatively thin. Additional flexible printed circuit substrate portions may be coupled to the flexible printed circuit substrate using conductive material such as solder or conductive adhesive. An additional portion may have a different substrate thickness than the substrate portion to which it is connected. 
     A groove or other recess may be formed in a flexible printed circuit substrate to promote bending. Grooves and portions of a substrate with different thicknesses may be formed using a die, using a mold, using extruding equipment, using roller-based equipment, using machining equipment, using light-based processing equipment, using scribing equipment, by selectively depositing polymers, by selectively removing polymers, or by otherwise processing the substrate. Openings may also be formed in the substrate to promote bending. 
     Further features of the invention, its nature and various advantages will be more apparent from the accompanying drawings and the following detailed description of the preferred embodiments. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of illustrative flexible printed circuit structures in accordance with an embodiment of the present invention. 
         FIG. 2  is a side view of illustrative flexible printed circuit structures having a polymer substrate with different thicknesses in different regions in accordance with an embodiment of the present invention. 
         FIG. 3  is a side view of illustrative flexible printed circuit structures mounted on a support structure in accordance with an embodiment of the present invention. 
         FIG. 4  is a side view of illustrative flexible printed circuit structures with regions of different thicknesses in accordance with an embodiment of the present invention. 
         FIG. 5  is a diagram showing how flexible printed circuit structures may be processed to selectively thin a portion of a substrate in accordance with an embodiment of the present invention. 
         FIG. 6  is a diagram showing how roller-based fabrication equipment may be used to generate a flexible printed circuit substrate having multiple thicknesses in accordance with an embodiment of the present invention. 
         FIG. 7  is a diagram showing how roller-based fabrication equipment may be used to roll multiple sheets of dielectric material and a liquid material together to form a flexible printed circuit substrate having multiple thicknesses in accordance with an embodiment of the present invention. 
         FIG. 8  is a perspective view of roller-based fabrication equipment being used to roll together flexible substrate layers to produce a flexible substrate with multiple thicknesses in accordance with an embodiment of the present invention. 
         FIG. 9  is a perspective view of roller-based fabrication equipment being used to roll together flexible substrate layers including a patterned layer with holes and a solid layer to produce a flexible substrate with multiple thicknesses in accordance with an embodiment of the present invention. 
         FIG. 10  is a perspective view of an extrusion tool of the type that may be used to produce a flexible substrate having multiple thicknesses or other features such as grooves in accordance with an embodiment of the present invention. 
         FIG. 11  is a cross-sectional side view of a tool such as a mold or die being used to form a flexible substrate having multiple thicknesses in accordance with an embodiment of the present invention. 
         FIG. 12  is a side view of illustrative equipment being used to selectively thicken a flexible substrate by depositing and curing liquid polymeric material in accordance with an embodiment of the present invention. 
         FIG. 13  is a diagram showing how a groove may be formed in a flexible substrate to promote bending of the flexible substrate in accordance with an embodiment of the present invention. 
         FIG. 14  is a side view of illustrative light-based equipment such as a laser-based tool that may be used to remove material from a substrate to form a groove to promote bending of the flexible substrate in accordance with an embodiment of the present invention. 
         FIG. 15  is a cross-sectional side view of a tool having an opening into which a flexible substrate may be pressed to form a groove in the flexible substrate to promote bending of the flexible substrate in accordance with an embodiment of the present invention. 
         FIG. 16  is a cross-sectional side view of the tool of  FIG. 15  following the pressing of the flexible substrate into the opening in accordance with an embodiment of the present invention. 
         FIG. 17  is a cross-sectional side view of the tool of  FIG. 16  following removal of protruding material from the flexible substrate to form a groove in the flexible substrate in accordance with an embodiment of the present invention. 
         FIG. 18  is a cross-sectional side view of a flexible substrate that has been provided with a groove to facilitate bending using equipment of the type shown in  FIGS. 15 ,  16 , and  17  in accordance with an embodiment of the present invention. 
         FIG. 19  is a cross-sectional side view of flexible printed circuit structures having a metal layer or other layer with a locally thinned region to promote bending in accordance with an embodiment of the present invention. 
         FIG. 20  is a diagrams showing how holes may be formed in a substrate to promote bending in accordance with an embodiment of the present invention. 
         FIG. 21  is a cross-sectional side view of a die or mold of the type that may be used to create a groove in a flexible substrate to promote bending in accordance with an embodiment of the present invention. 
         FIG. 22  is a cross-sectional side view of a flexible substrate following formation of a groove using equipment of the type shown in  FIG. 21  and following the formation of patterned metal traces in accordance with an embodiment of the present invention. 
         FIG. 23  is an exploded perspective view of illustrative flexible printed circuit structures in which flexible polymer layers of different sizes are being attached using adhesive to form a substrate with multiple thicknesses in accordance with an embodiment of the present invention. 
         FIG. 24  is a diagram showing how flexible printed circuit substrates may be joined to form a flexible printed circuit structure with multiple substrate thicknesses in accordance with an embodiment of the present invention. 
         FIG. 25  is a top view of flexible printed circuit substrates of different thicknesses that have been joined using connections along edge portions of the substrates in accordance with an embodiment of the present invention. 
         FIG. 26  is a flow chart of illustrative steps involved in forming flexible printed circuit structures with different regions that have different mechanical and electrical properties such as portions for forming bends and portions for forming electrical structures in accordance with an embodiment of the present invention. 
         FIG. 27  is a side view of illustrative flexible printed circuit structures having a polymer substrate with different thicknesses in different regions including a stiffening region in accordance with an embodiment of the present invention. 
         FIG. 28  is a perspective view of illustrative flexible printed circuit structures showing how bends in the flexible printed circuit structures may include bends and warps that conform to a 2-dimensional or 3-dimensional splined surface in accordance with an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Electronic devices such as computers, tablet computers, desktop computers, televisions, media players, cellular telephones, wireless access points, set-top boxes, and other electronic equipment may include printed circuits. Printed circuits may be formed from patterned conductive layers and dielectric layers. Patterned conductive layers may be formed from metal (e.g., metal traces). In rigid printed circuit boards, dielectric layers may be formed form fiberglass-filled epoxy (e.g., FR4) or other rigid dielectric materials. In flexible printed circuits (“flex circuits”), dielectric layers may be formed from flexible sheets of polyimide or other flexible polymer layers. 
     Printed circuit structures such as flexible printed circuit structures may be used as substrates for electrical components such as microprocessors, memory chips, application-specific integrated circuits, and other integrated circuits, discrete components such as capacitors, resistors, and inductors, connectors, switches, filters, and other circuitry. Printed circuit structures such as flexible printed circuit structures may also be used in forming flex circuit signal buses (e.g., parallel paths for analog and/or digital signals), antenna structures (e.g., ground plane structures, antenna resonating element structures, and parasitic antenna resonating element structures), radio-frequency transmission line structures, and other printed circuit structures. 
     In some situations, such as when forming transmission line structures, it may be desirable to ensure that a minimum thickness is used for the flexible printed circuit substrate. In a microstrip transmission line structure, for example, it may be desirable to separate a positive signal line on an upper surface of the flexible printed circuit from a ground signal line on the lower surface of the flexible printed circuit by a minimum separation of T 1 . The value of T 1  may, for example, be 50 microns, a value in the range of 10-100 microns, a value less than 70 microns, a value more than 10 microns, or a value more than 40 microns. 
     The thickness of the flexible printed circuit substrate that is used in forming the transmission line structure and associated antenna structures may be thicker than desired for forming a bend in the flexible printed circuit to accommodate mounting constraints. As an example, satisfactory bends may be formed in a printed circuit when the thickness of the printed circuit substrate is a maximum of thickness T 2 . The value of T 2  may be, for example, 25 microns, 50 microns, a value in the range of 10-100 microns, less than 100 microns, less than 40 microns, or other suitable value. 
     In an illustrative arrangement, satisfactory microstrip transmission line structures may be formed using a flexible printed circuit substrate thickness of 50 microns, whereas satisfactory flexible printed circuit bending may be achieved by using a flexible printed circuit substrate thickness of 25 microns (as an example). 
     To accommodate diverging thickness requirements such as these, flexible printed circuit structures may be formed that have different regions, each of which has different corresponding flexibility and/or thickness characteristics. As an example, one portion of a flexible printed circuit substrate may have a thickness of T 1  that is satisfactory for forming microstrip transmission lines, whereas another portion of the flexible printed circuit substrate may have a thickness of T 2  that is satisfactory for forming one or more bends. The bends that are formed may include a 90° bend (i.e., a right-angle bend), a bend characterized by an angle of less than 120°, a bend characterized by an angle of less than 180° and greater than 120°, a bend characterized by an angle of less than 100°, a bend characterized by an angle of 90° or less, a bend characterized by an angle of less than 50°, a bend about a 2-dimensional or 3-dimensional spline, or a bend with a different bend angle. Regions of different flexibility and suitability for forming structures such as transmission line structures may also be implemented by selectively thinning metal layer structures and/or other printed circuit layers, by forming holes along a bend axis, or by forming a groove to promote bending. 
     Illustrative flexible printed circuit structures of the type that may have at least a first region for supporting transmission line structures or other structures that operate satisfactorily with a relatively thick flexible printed circuit substrate and that have a second region for supporting the formation of a right-angle bend to accommodate mounting in an electronic device housing are shown in  FIG. 1 . 
     As shown in  FIG. 1 , flexible printed circuit structures  10  may have a substrate such as substrate  26 . Substrate  26  may be formed from a sheet of polyimide or other flexible polymer. Conductive structures such as metal trace  18  may be formed on the upper surface of substrate  26  and conductive structures such as metal trace  20  may be formed on the opposing lower surface of substrate  26 . Flexible printed circuit structures  10  may, if desired, have three or more metal layers or only a single metal layer. The use of a two-metal layer configuration for structures  10  of  FIG. 1  is merely illustrative. 
     Metal trace  18  may include portions  18 - 1 ,  18 - 2 , and  18 - 3  (as examples). Flexible printed circuit structures  10  may include a first region such as region  12 , a second region such as region  14 , and a third region such as region  16 . Region  12  may be used to form a transmission line. For example, a microstrip transmission line may be formed using a positive signal line formed from trace  18 - 1  and ground signal line formed from trace  20 . Trace portions  18 - 2  and  18 - 3  may form an antenna resonating element for an antenna. Via structure  22  may couple ground signal trace  20  to trace portion  18 - 2 . Terminals  28  and  30  may be coupled to radio-frequency transceiver circuitry. 
     To ensure satisfactory operation of the transmission line in region  12 , it may be desirable to ensure that thickness T 1  of substrate  26  in region  12  has a minimum value (e.g., about 50 microns). To ensure that portion  16  of structures  10  can be bent about bend axis  24  to fit structures  10  within an electronic device housing structure, it may be desirable to ensure that thickness T 2  of portion  16  of substrate  26  (or at least the region of substrate  26  that overlaps bend axis  24 ) has a maximum value that is less than T 1  (e.g., 25 microns). In regions  12  and  14 , flexible printed circuit structures  10  may be planar or may be bent. In region  16 , substrate  26  may be bent as shown in  FIG. 1 , so that substrate  26  exhibits a bend radius R about bend axis  24 . The value of bend radius R may be, for example, less than 5 mm, less than 2 mm, less than 1 mm, or less than 0.4 mm (e.g., substrate  26  may bend more in region  16  than in regions  12  and  14 ). The value of thickness T 1  in regions  12  and  14  may be selected to ensure that printed circuit structures  10  satisfy desired mechanical and/or electrical design criteria (e.g., transmission line loss criteria). The value of thickness T 2  in region  16  may be selected to ensure that substrate  26  can bend about axis  24  with a desired maximum bend radius R and a desired bend angle. Other types of flexible printed circuit structures may use substrates with different regions having different electrical and mechanical properties if desired. The example of  FIG. 1  is merely illustrative. 
       FIG. 2  is a cross-sectional side view of illustrative flexible printed circuit structures  10  having substrate  26  of thickness T 1  in region  32  (e.g., region  12  and/or  14  of  FIG. 1 ) and having substrate  26  of thickness T 2  in region  16 . If desired, some or all of metal traces  18  and  20  may be covered by adhesive layers and/or insulating layers such as coverlay layer  19 . However, this is merely illustrative. If desired, flexible printed circuit structures  10  may be provided without any coverlay layer or a portion such as portion  21  of metal trace  20  (or metal trace  18 ) may be exposed (e.g., may remain uncovered by coverlay layer  19 ) in order to provide a conductive contact on flexible printed circuit structures  10 . 
       FIG. 3  is a cross-sectional side view of flexible printed circuit structures  10  in a configuration in which adhesive layer  34  has been used to attach flexible printed circuit structures  10  to a support structure such as dielectric support structure  36  (e.g., a plastic support, a portion of a housing member, a printed circuit substrate, or other support structure). The structures of  FIG. 3  may be mounted within an electronic device housing (e.g., a housing formed from metal housing walls, plastic housing walls, glass or ceramic structures, etc.). Adhesive layer  34  may be formed from a liquid adhesive, a film adhesive such as a pressure sensitive adhesive, a light-cured adhesive or other suitable adhesive. If desired, other engagement members (e.g., snaps, clips, screws, or other structures) may be used in combination with adhesive layer  34  or in place of adhesive layer  34  for attaching flexible printed circuit structures  10  to a support structure such as dielectric support structure  36 . 
     As shown in  FIG. 4 , patterned metal trace  18  on the upper surface of polyimide substrate  26  may be covered with a layer of adhesive such as layer  42  and a layer of patterned coverlay (i.e., a solder mask formed from patterned polyimide or other suitable material) such as layer  44 . Patterned metal trace  20  on the lower surface of substrate  26  may be covered with a layer of adhesive such as layer  38  and a layer of patterned coverlay such as layer  40 . To promote bending of substrate  26  about bend axis  24  in region  16 , substrate  26  in region  16  may be maintained free of adhesive  38  and coverlay  40 , as shown in  FIG. 4 . Coverlay  44  and adhesive  42  may also be excluded from region  16  to promote flexibility, if desired. Some or all of metal traces  18  and  20  may also be excluded from certain portions of structures  10  to promote bending. In the  FIG. 4  configuration, for example, metal layer  20  has been excluded from region  16 . Part of the thickness of a given metal layer may also be removed to promote flexibility, if desired. 
       FIG. 5  is a diagram showing how a flexible printed circuit substrate such as substrate  26  may be provided with multiple thicknesses. 
     Initially, substrate  26  may be formed from a sheet of polyimide or other flexible polymer having a single uniform thickness such as thickness T 1 . 
     Using selective thinning tool  46 , region  16  of substrate  26  may be thinned until exhibiting a thickness T 2  that is less than T 1 . Thinning equipment  46  may include a die that presses down more in region  16  than in region  32 , may be etching equipment, may be machining equipment, may be light-based equipment such as laser-based equipment, or may be any other equipment for selectively removing part of substrate layer  26 . The example of  FIG. 5  shows how two distinct regions (regions  16  and  32 ) may be formed each having a different respective thickness. If desired, multi-thickness substrates may be formed that have three or more different thicknesses. 
     Following formation of multi-thickness substrate  26 , substrate  26  may be coated with patterned metal layers such as layers  18  and  20  using metal deposition and patterning equipment  48 . Equipment  48  may include physical vapor deposition equipment, screen printing equipment, spraying equipment for spraying conductive inks, pad printing equipment, foil lamination equipment, electrochemical deposition equipment, photolithographic equipment, or other equipment for forming patterned metal traces on the surfaces of substrate  26 . 
     If desired, roller-based equipment such as the equipment of  FIG. 6  may be used in forming multi-thickness substrate  26 . With this type of arrangement, a flexible polymer substrate such as substrate  26 ′ (e.g., a layer of polyimide) may be fed into rollers  52  and  50 . As layer  26 ′ moves between rollers  52  and  50  in direction  58 , computer-controlled positioners  54  and  56  may be used to control the vertical separation between the opposing surfaces of rollers  50  and  52 . By alternating the spacing between rollers  50  and  52  so that the spacing is sometimes equal to distance T 1  and is sometimes equal to distance T 2  as the substrate moves in direction  58 , substrate  26  may be produced having corresponding thicknesses T 1  (in regions such as region  32 ) and T 2  (in regions such as region  16 ). Substrate  26  may be die cut, laser cut, or cut using other cutting equipment to produce individual flexible printed circuit structures. 
     In the illustrative configuration of  FIG. 7 , liquid dielectric material  60  is being dispensed between polymer sheet  64  and polymer sheet  68 . Polymer sheet  64  may be dispensed from a roll of polymer material such as roll  66 . Polymer sheet  68  may be dispensed from a roll of polymer material such as roll  70 . Polymer sheets  64  and  68  may be, for example, polyimide layers. Liquid material  60  may be dispensed by dispensing equipment  62  (e.g., a liquid dispensing nozzle, spraying equipment, etc.). Material  60  may be, for example, liquid (uncured) polyimide or adhesive. After passing through rollers  50  and  52 , heat (i.e., infrared light  74 ) may be applied by heat source  72  to cure material  60 . Heat source  72  may be an infrared lamp or other source of heat. If desired, rollers  50  may be heated to cure material  60 . 
     During the process of rolling layers  64  and  68  with liquid material  60 , computer-controlled positioners such as positioners  54  and  56  may be used to control the relative position between rollers  52  and  50 , thereby producing substrate  26  with thicker regions such as region  32  with thickness T 1  and thinner regions such as region  16  with thickness T 2 . If desired, substrate thickness variations such as these may be produced by providing rollers  50  and  52  with raised portions. For example, roller  50  of  FIG. 7  may be provided with raised portions such as raised portion  76  and roller  52  of  FIG. 7  may be provided with raised portion  78 . When the substrate is compressed between elevated portions  76  and  78 , thinner regions  16  may be formed. If desired, rollers  50  and  52  of  FIG. 6  may be provided with raised portions such as portions  76  and  78  to form thicker and thinner substrate regions. 
       FIG. 8  shows how multi-thickness substrate  26  may be formed by attaching (laminating) a first polymer layer such as polyimide layer  26 - 1  to a second polymer layer such as polyimide layer  26 - 2 . A dispenser such as dispenser  86  may be used to spray or otherwise dispense an optional liquid material such as material  88  between polyimide layer  26 - 1  and polyimide layer  26 - 2 . Material  88  may be liquid adhesive, liquid polyimide, or other liquid material for attaching layer  26 - 1  to layer  26 - 2  (e.g., a liquid polymer). 
     With an arrangement of the type shown in  FIG. 8 , layer  26 - 1  may have a first width W 1  and layer  26 - 2  may have a second width W 2  that is greater than W 1 , thereby creating multi-thickness substrate  26  of thickness T 1  (in region  32 ) and thickness T 2  (in region  16 ) at the exit of rollers  80  and  82 , as substrate  26  moves in direction  84 . As shown in  FIG. 8 , rollers  80  may, if desired, be provided with portions having a radius that is smaller than the radius of other portions of rollers  80 . For example, a roller such as roller  80  may have portion with a radius suitable for accommodating the additional thickness of layer  26 - 1  during laminating operations. 
     As shown in  FIG. 9 , upper polymer layer  26 - 1  (e.g., a first polyimide layer) may be patterned (e.g., by incorporating openings such as openings  92  into layer  26 - 1 ). Lower polymer layer  26 - 2  (e.g., a second polyimide layer) may be a solid layer of polyimide. When pressed together by rollers  90  and  94  (with or without using optional liquid adhesive, liquid polyimide, or other material such as material  88  of  FIG. 8  between layers  26 - 1  and  26 - 2 ), layers  26 - 1  and  26 - 2  may form multi-thickness substrate  26 . As shown in  FIG. 9 , substrate  26  may have thicker regions such as regions  32  of thickness T 1  and thinner regions such as regions  16  (associated with holes  92  in layer  26 - 1 ) of thickness T 2 . 
     If desired, multi-thickness substrate  26  may be extruded using an extrusion tool. As shown in  FIG. 10 , for example, substrate  26  may be extruded in direction  94  from extrusion head  96  of an extrusion tool. The extrusion tool openings may be configured so that substrate  26  has a first thickness such as thickness T 1  in thicker region  32  and a second thickness such as thickness  16  in thinner region  32 . 
       FIG. 11  shows how a die or mold may be used to form multi-thickness substrate  26 . Tool  98  may have a first portion such as upper portion  98 - 1  and a second portion (or more portions) such as lower portion  98 - 2 . The material for substrate  26  (which may be cured or uncured material in the form of a sheet of polymer, polymer pellets, liquid polymer, or other material), may be compressed between upper portion  98 - 1  and lower portion  98 - 2 . During compression, heat may optionally be applied to the material of substrate  26  to cure the material (e.g., when tool  98  is a mold for molding and curing polymer substrate  26 ) or to soften the material (e.g., when tool  98  is a die that is compressing a uniform sheet of polymer to produce a multi-thickness substrate such as substrate  26 ). 
     With the illustrative arrangement of  FIG. 12 , substrate  26  may be formed by moving uniform substrate layer  26 ′ past material dispensing equipment  102  in direction  100 . Equipment  102  may apply material  104  to the upper surface of substrate layer  26 ′. Material  104  may be polyimide or other polymer material. Following curing with infrared light  108  from infrared lamp  106  or other heat source, material  104  may cure to form polyimide (or other suitable polymer). Following curing, substrate  26  may be characterized by portions with thickness T 1  such as region  32  and portions with thickness T 2  such as region  16 . 
       FIG. 13  is a diagram showing how bending of substrate layer  26  may be promoted by localized thinning of layer  26 . As shown in  FIG. 13 , flexible printed circuit structures  10  may include metal traces such as traces  18  and  20  on a polymer layer such as polyimide substrate layer  26 . Layer  26  in the example of  FIG. 13  may initially have a uniform thickness T. 
     Using material removal equipment  110 , some of the material of layer  26  may be locally removed. For example, material may be removed from layer  26  to form groove  114 . Groove formation tool  110  may be a laser, a machining tool, a knife blade, scribing tool, or other cutting tool or scoring equipment. As shown in  FIG. 13 , following formation of groove  114  in substrate layer  26 , bending tool  116  (e.g., a computer-controlled arm or other equipment) may be used to bend substrate  26  to form bend  118  along groove  114 . Bend  118  may also be formed manually, if desired. The presence of groove  114  locally decreases the thickness of substrate layer  26 , thereby facilitating bending. 
       FIG. 14  shows how light-based equipment such as laser  120  may be used to apply a beam of laser light such as light  122  to substrate  26 . Laser light  122  may be, for example, an ultraviolet, visible, or infrared laser beam. Computer-controlled equipment may be used to scan beam  122  across the surface of substrate  26  to form groove  114 . 
     If desired, groove  14  may be formed by removing a portion of the material that makes up substrate layer  26  using a machining tool, knife edge, or other cutting equipment. Initially, substrate  26  may be placed in equipment such as the equipment of  FIG. 15 . Biasing member  124  may be moved in direction  126  to push a portion (e.g., a groove-shaped portion) of substrate  26  through opening  130  in tool  128 . Opening  130  may be an elongated slot in tool  128  that has a longitudinal axis extending into the page in the orientation of  FIG. 15 . Tool  128  may be a metal plate (as an example). 
     Using biasing member  124  in this way, portion  132  of substrate  26  may be caused to protrude through opening  130  in plate  128 . By moving cutting tool  134  (e.g., a knife edge, milling head, or other cutting equipment) in direction  136  (or other suitable directions) as shown in  FIG. 16 , portion  132  may be removed from substrate  26  to form cut surface  132 ′ on substrate  26  ( FIG. 17 ). 
     Following removal of biasing tool  124 , substrate  26  may exhibit a recess such as groove  114  to facilitate subsequent bending of substrate  26 . 
     If desired, layers of material on substrate  26  such as layers of adhesive, layers of metal, layers of coverlay, and other material layers may be selectively thinned in certain regions and/or may be selectively omitted in certain regions to promote bending of layer  26  about bend axis. As shown in  FIG. 19 , for example, flexible printed circuit structures  10  may have layers on polyimide substrate  26  such as lower layer  20  and upper layer  138 . Layer  20  may be, for example, a metal trace. Upper and/or lower layers of material such as layer  138  may also include an adhesive layer, a layer of coverlay, or a combination of two or more or three or more of these layers and other optional layers. For example, layer  138  may be a metal trace layer such as layer  18 . In region  140 , portion  138 - 1  of layer  138  may have a first thickness (T 3 ). In region  142 , portion  138 - 2  of layer  138  may have a second thickness (T 4 ), where T 4  is less than T 3 . Fabrication tools such as selective thinning equipment  46 , material removal equipment such as equipment  110 , or selective deposition equipment (see, e.g., dispensing tool  102  of  FIG. 12 ) may be used in thinning layer  138  in region  142  to form layer  138 - 2 . 
       FIG. 20  is a diagram showing how openings may be formed in flexible printed circuit structures  10  along bend axis  24  to facilitate bending flexible printed circuit structures  10 . As shown in  FIG. 20 , structures  10  may initially be formed from polyimide substrate  26  and metal traces such as metal traces  18  (and if desired, traces  20 ). 
     Hole formation equipment  144  such as a laser, die press, or other cutting equipment may be used in forming openings  146  in structures  10 . Openings  146  may, if desired, be formed at a relatively small size. For example, in a situation in which trace  18  is being used to form an antenna, openings  146  may be formed that have lateral dimensions that are less than a tenth of a wavelength at a desired frequency of operation for the antenna to avoid adversely affecting antenna operation. As shown in  FIG. 20 , openings  146  may include openings that pass through traces  18  and polymer substrate layer  26 . 
     If desired, openings  146  may be elongated, slotted, or otherwise shaped, sized, and positioned to avoid adversely affecting electrical functioning of traces  20  while facilitating bending around bend axis  24 . All openings  146  may have a common size and shape or some openings  146  may have a size and/or shape that is different from the size and/or shape respectively of other openings  146 . Openings  146  may be evenly spaced or may be arranged in other configurations. 
     After forming one or more openings such as openings  146  (e.g., openings aligned along bend axis  24 ), structures  10  may be bent along bend axis  24 , as shown in  FIG. 20 . Because material was removed from openings  146  prior to bending, less material will be available in structures  10  along axis  24  than would otherwise be present, thereby facilitating bending. 
       FIG. 21  shows how a tool such as a die or mold may be used to form a groove in substrate  26 . Initially, as shown in  FIG. 21 , the material for forming substrate  26  (e.g., cured polyimide sheet, polymer precursor material, or other material) may be compressed within the inner cavity of tool  146 . Tool  146  may be a die or a mold. Tool  146  may have two or more pieces such as upper member  146 - 1  and lower member  146 - 2 . During operation, members  146 - 1  and  146 - 2  may be pressed towards each other. Material  148  may be compressed and deformed by a die or molded into shape by members  146 - 1  and  146 - 2  (e.g., when tool  146  is a mold). Due to the presence of protrusion  150  (e.g., a rib that extends into the page in the orientation of  FIG. 21 ), groove  114  of  FIG. 22  may be formed in substrate  26  by tool  146  to facilitate bending. 
     As shown in the exploded perspective view of substrate  26  of  FIG. 23 , it may be desirable to use a layer of adhesive such as adhesive  88  when assembling substrate  26  from multiple layers such as polyimide layer  26 - 1  and polyimide layer  26 - 2 . To prevent adhesive  88  from touching the sidewalls of via  152 , adhesive  88  may be omitted from within adhesive keep-out area  154  during lamination. 
     If desired, flexible printed circuit structures  10  with multiple thicknesses may be formed by electrically and mechanically attaching a thicker piece of flexible printed circuit material to a thinner piece of flexible printed circuit material. Consider, as an example, the arrangement of  FIG. 24 . As shown in  FIG. 24 , flexible printed circuit structures  10  may be formed from first portion  10 - 1  with substrate portion  26 ′ and second portion  10 - 2  with substrate portion  26 ″. Substrate portion  26 ′ may have a smaller thickness (e.g., thickness T 2 ) and substrate portion  26 ″ may have a larger thickness (e.g., thickness T 1 , where T 1  is greater than T 2 ). Patterned metal traces such as traces  160  and vias such as via  162  may be formed on substrates  26 ′ and  26 ″ before assembly of portions  10 - 1  and  10 - 2  to form structures  10 . 
     Patterned metal traces such as traces  160  and exposed portions of vias such as via  162  may be provided with a surface plating such as gold plating using a plating process such as Hot Air Solder Leveling (HASL), Organic Solder Protective (OSP) coating, Electroless Nickel Immersion Gold (ENIG) or any other plating process before assembly of portions  10 - 1  and  10 - 2  to form structures  10 . 
     If desired, conductive surfaces on portion  10 - 1  may be plated using a plating process that is different from the plating processes used to plate conductive surfaces on portion  10 - 2 . 
     As an example, portion  10 - 1  may have conductive surfaces that are plated with nickel and gold using an ENIG process while portion  10 - 2  may have conductive surfaces plated with solder or other materials using an OSP coating process. Providing portions  10 - 1  and  10 - 2  with coatings formed using different plating processes may help reduce production costs, increase reliability, or otherwise facilitate production of flexible printed circuit structures  10 . 
     During assembly of structures  10 - 1  and  10 - 2 , a heat source such as heat source  156  (e.g., an infrared lamp, a laser, a hot bar, a reflow oven, or other heat source) may be used to emit infrared radiation (heat  158 ) that reflows solder  164 , thereby mechanically and electrically the metal traces on substrate  26 ″ with the metal traces on substrate  26 ′. If desired, other conductive materials (e.g., conductive adhesive) may be used in joining structures  10 - 1  and  10 - 2 . Because the thickness of layer  26 ′ is thinner than the thickness of layer  26 ″, bending of layer  26 ′ about bend axis  24  may be facilitated. 
       FIG. 25  is a top view of flexible printed circuit structures  10  that have been formed from first and second portions  10 - 1  and  10 - 2  with different respective polyimide substrate thicknesses. Conductive material  164  may, in general, be solder, conductive adhesive, or other conductive material for coupling portions  10 - 1  and  10 - 2 . 
     A flow chart of illustrative steps involved in forming flexible printed circuit structures  10  is shown in  FIG. 26 . 
     At step  166 , substrate layer  26  may, if desired, be formed with multiple thicknesses. For example, a layer of polymer such as a sheet of polyimide may be provided with a first portion that has a thickness T 1  and a second portion that has a thickness T 2  that is less than T 2 . The substrate may also have additional portions with additional thicknesses, if desired. 
     Following formation of the polymer substrate, metal traces such as traces  18  and  20  may be formed on the substrate (step  168 ). During the operations of step  168 , optional features such as through-substrate vias, grooves to facilitate bending, and holes to facilitate bending may be incorporated into the substrate. 
     Additional structures such as additional portions of polyimide substrate with different thicknesses may, if desired, be incorporated during the operations of step  170  (e.g., by forming solder connections between layers, by forming connections using conductive adhesive, or otherwise forming conductive paths that attach the additional layer or layers of polyimide substrate material to the substrate). 
     During the operations of step  172 , the flexible printed circuit structures that have been formed may be bent and, if desired, attached to a support structure. 
       FIG. 27  is a side view of flexible printed circuit substrate  10  showing how, if desired, a stiffening structure such as stiffening structure  200  may be included on substrate  26  in order to provide a desired amount of resistance to bending. Stiffening structure  200  may be a portion of region  16  of substrate  26  that is relatively wider than other portions of region  16  (e.g., a region having an additional layer of polyimide) or may include a stiffener that is attached to, or embedded in, substrate  26 . 
     Region  16  of substrate  26  may be bent into any suitable shape during assembly and manufacturing operations. Stiffening structure  200  may be configured to provide region  16  with a restoring force that resists further bending of region  16  after assembly and manufacturing operations have been completed (e.g., for providing additional strength to region  16  or for using region  16  as a substitute for a torsion spring). 
       FIG. 28  is a perspective view of flexible printed circuit substrate  10  showing how region  16  of substrate  26  may be bent into more complex shapes than a pure radial bend about a single bend radius such as bends and warps that conform to a 2-dimensional or 3-dimensional splined surface. As shown in  FIG. 28 , in region  16 , substrate  26  may be bent so that substrate  26  exhibits a complex bend in multiple dimensions. 
     A complex bend such as that shown in  FIG. 28  may be used to conform portion  16  of substrate  26  to a surface such as a surface of a device housing or an internal support structure that is curved in one or more dimensions. 
     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: 20120927
Publication Date: 20151117
Grant Date: 20151117
Priority Date: 20120323
Inventors: SHEDLETSKY ANNA-KATRINA
Assignee: APPLE INC
CPC Classifications: [{"code": "H05K2201/10098", "inventive": false, "first": false, "tree": "[]"}, {"code": "H05K1/028", "inventive": true, "first": false, "tree": "[]"}, {"code": "H01Q1/38", "inventive": true, "first": true, "tree": "[]"}, {"code": "H05K2201/0191", "inventive": false, "first": false, "tree": "[]"}, {"code": "Y10T29/49016", "inventive": false, "first": false, "tree": "[]"}, {"code": "Y10T29/49016", "inventive": false, "first": false, "tree": "[]"}, {"code": "H01Q1/38", "inventive": true, "first": true, "tree": "[]"}, {"code": "H05K2201/0191", "inventive": false, "first": false, "tree": "[]"}, {"code": "H05K1/028", "inventive": true, "first": false, "tree": "[]"}, {"code": "H05K2201/10098", "inventive": false, "first": false, "tree": "[]"}]
Family ID: 49211273