Abstract:
A method of forming a flexible conductive strip includes: molding a continuous, flexible base of an electrically insulating thermoplastic resin, while forming channels in a surface of the base; at least partially filling the formed channels with a flowable, electrically conductive composition; and then stabilizing the flowable composition in the channels to form a pattern of stable, electrically conductive traces within the channels. A method of forming a flexible circuit board having loop-engageable touch fastener elements includes: molding a continuous, flexible base from an electrically insulating thermoplastic resin, while forming a field of stems integrally molded with and extending from a first side of the base; applying a conductive material to the base to form a pattern of electrically conductive traces in accordance with a circuit design; and forming loop-engageable heads on the stems.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims the benefit of U.S. Provisional Patent Application No. 60/703,331, filed Jul. 28, 2005, which is incorporated herein by reference in its entirety. 
     
    
     TECHNICAL FIELD  
       [0002]     This invention relates to flexible circuits, and more particularly to methods of forming flexible circuits.  
       BACKGROUND  
       [0003]     The increased use of electrical wires, cables and circuits has resulted in an increased need for efficient and inexpensive means for production of flexible substrates carrying conductive circuit traces, and controllably directing and securing such circuits to avoid, damage, wear, and inadvertent disconnection. Touch fasteners have been suggested as one means of securing such flexible conductive regions on a substrate having circuits, for example.  
         [0004]     One approach to producing flexible substrates with conductive circuit traces features using printing technologies to apply conductive material to a flexible substrate.  
         [0005]     One approach to forming conductive regions on a substrate having touch fasteners features selectively metallizing portions of a surface covered with touch fasteners. Another approach features feeding continuous conductors into a roll molding apparatus with molten resin, such that the conductors become encapsulated in a resin base molded to have touch fastener elements extending from its outer surface.  
       SUMMARY  
       [0006]     In one aspect of the invention, a method of forming a flexible conductive strip includes: molding a continuous, flexible base of an electrically insulating thermoplastic resin, while forming channels in a surface of the base; at least partially filling the formed channels with a flowable, electrically conductive composition; and then stabilizing the flowable composition in the channels to form a pattern of stable, electrically conductive traces within the channels.  
         [0007]     In another aspect of the invention, a method of forming a releasably securable, flexible conductive strip includes: molding a continuous, flexible base of an electrically insulating thermoplastic resin, while forming channels in a surface of the base; at least partially filling the formed channels with a flowable, electrically conductive composition; stabilizing the composition in the channels to form a pattern of stable, electrically conductive traces within the channels; and providing a field of loop-engageable fastener elements on the base and exposed to releasably secure the base to a loop-bearing support.  
         [0008]     In another aspect of the invention, a method of forming a flexible circuit includes: molding a continuous, flexible base of an electrically insulating thermoplastic resin, while forming channels in a surface of the base; at least partially filling the formed channels with a flowable, electrically conductive composition; stabilizing the composition in the channels to form a pattern of stable, electrically conductive traces within the channels; providing a field of loop-engageable fastener elements on the base and exposed to releasably secured the base to loop-bearing support; and securing at least one discrete electrical component to the surface of the base, such that the electrical components electrically interconnect a plurality of the traces.  
         [0009]     In another aspect of the invention, a method of forming a flexible circuit board having loop-engageable touch fastener elements includes: molding a continuous, flexible base from an electrically insulating thermoplastic resin, while forming a field of stems integrally molded with and extending from a first side of the base; applying a conductive material to the base to form a pattern of electrically conductive traces in accordance with the circuit design; and forming loop-engageable heads on the stems.  
         [0010]     In some embodiments, at least partially filling the formed channels comprises using printing techniques to dispense conductive ink into the channels. In some other embodiments, at least partially filling the formed channels comprises dispensing the flowable composition onto the surface of the base, and then substantially removing the flowable composition from non-channel regions of the surface. In some cases, removing the flowable composition comprises wiping the surface.  
         [0011]     In some embodiments, the flowable composition is in powder form prior to stabilization. In some other embodiments, the flowable composition comprises a liquid carrier solution containing metal ions. In some cases, the flowable composition comprises a suspension of metal particles.  
         [0012]     In some embodiments, the composition is stabilized in the channels by evaporating a solvent from the composition. In some other embodiments, the composition is stabilized by radiating the composition in the channels with radiation selected from a group consisting of heat, ultraviolet radiation, and microwave radiation. In some cases, the flowable composition is stabilized by subjecting the composition to reducing conditions. In some embodiments, the flowable composition is stabilized by releasing reducing agents from capsules contained within the flowable composition.  
         [0013]     In some embodiments, molding the base comprises feeding the thermoplastic resin in a moldable form into a gap adjacent a mold roll. In some cases, the gap is defined between the mold roll and a counter-rotating roll. In some cases, methods also include forming a field of loop-engageable fastener elements extending from the base by: introducing the resin into the gap such that the resin fills a field of fixed cavities defined in the mold roll to form a field of molded stems; solidifying the molded stems; stripping the stems from the mold roll; and forming loop-engageable heads on the molded stems.  
         [0014]     In some embodiments, molding the channels comprises employing a mold roll that defines headed features in the surface of the channels for mechanically locking the flowable composition in the channels when it stabilizes. In some cases, the channels are formed with varying depths such that the resulting conductive traces are of varying thicknesses. Similarly, in some cases, the channels are formed with varying widths such that the resulting conductive traces are of varying widths.  
         [0015]     In some embodiments, methods also include surface-treating the channels to promote adhesion of the flowable composition prior to filling the channels.  
         [0016]     In some embodiments, methods also include providing a field of loop-engageable fastener elements on the base exposed to releasably secure the base to a loop-bearing support. In some cases, providing the fastener elements comprises integrally molding the fastener elements with the base such that the fastener elements extend outwards from a surface of the base. In some other cases, providing the fastener elements comprises attaching fastener elements to the base.  
         [0017]     In some embodiments, forming the channels comprises forming the channels with at least a portion whose width decreases with increasing distance from the resin base.  
         [0018]     In some embodiments, the pattern of electrically conductive traces is longitudinally continuous and arranged such that, when the base is severed to create individual strips of a desired, finite length between severed ends, the electrically conductive traces provide an electrical connection between the severed ends. In some cases, methods also include forming touch fastener elements exposed along the length of the base and arranged such that the individual strips each have some of the touch fastener elements exposed for releasably mounting the strip to a support surface.  
         [0019]     In some embodiments, the pattern of electrically conductive traces form interconnected path segments arranged in accordance with a desired circuit pattern.  
         [0020]     In some embodiments, methods also include electroplating a second conductive material onto the conductive traces.  
         [0021]     In some embodiments, methods also include attaching an electrically insulating cover over the conductive traces, the cover attached to the base. In some cases, attaching the insulative layer comprises passing the sheet-form base through a gap adjacent a mold roll in the presence of moldable resin to encapsulate the conductive traces. In some other cases, attaching the insulative cover comprises spraying an insulating composition onto the base, such that the insulating composition encapsulates the conductive traces.  
         [0022]     In some embodiments, the flowable composition contains silver. In some cases, the silver composition is a reducible silver composition.  
         [0023]     Methods of the present invention provide an efficient approach to forming conductive traces on a flexible backing. Such methods can rapidly produce large amounts of longitudinally continuous substrate carrying flexible circuits. In addition, by focusing the application of conductive material to desired locations on the substrate, these methods can limit the use of conductive material.  
         [0024]     Forming channels in the substrate allows for more control in the placement of the conductive traces. It also provides a convenient means of varying the thickness as well as the width of the conductive traces. As the current carrying ability of the conductor is proportional to its cross-section, this provides an efficient method of varying the current carrying ability of the conductive traces while conserving surface space on the substrate. This approach also can save time and avoided registration problems because, in some configurations, it only requires one pass, rather than multiple passes, of the device dispensing the conductive material.  
         [0025]     Flexible conductive hook fastener substrates can be efficiently and continuously formed with integral hook fastener elements according to certain methods and apparatus of the invention. These techniques allow for electrical conductivity along the substrate in a patterned arrangement, on one or more surfaces, and/or on the hook fastener members themselves, as desired. Furthermore, the resulting conductive hook fastener substrates provide a surface on which other electrical components can be attached to process, relay, or modify electrical signals carried along the substrate.  
         [0026]     The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims. 
     
    
     DESCRIPTION OF DRAWINGS  
       [0027]      FIG. 1  is a schematic side view of the manufacturing system used to produce a flexible circuit.  
         [0028]      FIG. 1A  is a cross-sectional view of the nip of the manufacturing system shown in  FIG. 1 .  
         [0029]      FIG. 1B  is a cross-sectional view of the flexible circuit shown in  FIG. 1 , taken along the circuit&#39;s centerline, before conductive traces are added.  
         [0030]      FIG. 1C  is a cross-sectional view of the flexible circuit shown in  FIG. 1 , taken along the circuit&#39;s centerline, after conductive traces are added.  
         [0031]      FIG. 1D  is a cross-sectional view taken into the nip of the manufacturing system shown in  FIG. 1 .  
         [0032]      FIGS. 2A and 2B  are perspective views of alternate embodiments of circuit patterns formed by the manufacturing system shown in  FIG. 1 .  
         [0033]      FIGS. 3-5  are schematic views of alternate embodiments of the manufacturing system shown in  FIG. 1 .  
         [0034]      FIG. 5A  is a cross-sectional view of the flexible circuit shown in  FIG. 5 , taken along the circuit&#39;s centerline, before and after the head of the stem is deformed.  
         [0035]      FIG. 6  is a schematic view of another alternate embodiment of the manufacturing system shown in  FIG. 1 . 
     
    
       [0036]     Like reference symbols in the various drawings indicate like elements. The drawings are not to scale as the dimensions of various features shown in the drawings have been adjusted for clarity of illustration.  
       DETAILED DESCRIPTION  
       [0037]     Referring to  FIGS. 1-1D , a manufacturing method and system  10  produces a flexible circuit  12  with a thermoplastic resin base  14  that carries a pattern of conductive traces  16 . Manufacturing system  10  includes a forming or roll molding apparatus  18  of the general type shown in U.S. Pat. No. 4,872,243 issued to Fisher. An extruder  20  feeds molten resin  22  into a nip  24  defined between a mold roll  26  and a counter-rotating second mold roll  28 . An outer surface  30  of second mold roll  28  includes structural features  32  configured to shape shallow channels  34  in resin base  14 . Mold roll  26  has a field of small mold cavities  36  extending into its peripheral surface. Each mold cavity  36  is shaped to form a loop-engageable hook  38 .  
         [0038]     In this embodiment, structural features  32  that form channels  34  are configured to form heads  116  extending from resin base  14  into the channels. Heads  116  are symmetrical stems whose cylindrical outer surface has a circumference that increases with increasing distance from resin base  14 . This tapering effect allows flowable conductive material filling channels  34  to surround heads  116  while providing a mechanical resistance to the removal of conductive traces  16  from resin base  14  after the conductive material is stabilized to form the conductive traces. In other embodiments, heads  116  are configured as hooks or as longitudinally-extending ridges. In still other embodiments, no heads are present in channels  34 .  
         [0039]     Structural features  32  are also configured to form channels  34  whose opening is narrower than the width of the base of the channel. Some other embodiments form channels  34  with different shapes. However, channels  34  with at least a portion whose width decreases with increasing distance from resin base  14  provide additional mechanical resistance to the removal of conductive traces  16  from the resin base after stabilization.  
         [0040]     Channels  34  are formed with varying widths and thicknesses. Consequently, conductive traces  16  also have varying widths and thicknesses whose dimensions are selected based on the desired current carrying ability of specific regions of the conductive traces. As the current carrying ability of conductors is proportional to their cross-sections, this provides an efficient method of varying the current carrying ability of the conductive traces while conserving surface space on the substrate. This approach also can save time and avoided registration problems because it only requires one pass, rather than multiple passes, of the device dispensing the conductive material.  
         [0041]     In this embodiment, second mold roll  28  is formed of a roller sleeve whose surface is etched to form structural features  32 . Alternatively, second mold roll  28  can be assembled from multiple rings, each ring including structural features  32  configured to shape shallow channels  34 . The use of roll molding produces channels  34  in longitudinally extending repeating patterns. Multiple flexible circuits  12  with longitudinally-extending patterns of channels  34  can be produced side-by-side on a single roll molding apparatus  18 . In some embodiments, these multiple flexible circuits  12  are separated from each other as part of manufacturing process. In other embodiments, these multiple flexible circuits  12  are produced in a longitudinally-extending sheet for later separation.  
         [0042]     As molten resin  22  enters nip  24 , pressure in the nip forces the resin into mold cavities  36  and around structural features  32 . After passing through nip  24 , resin  22  continues on the surface of rotating temperature-controlled (cooled) mold roll  26  until the resin is sufficiently cooled to enable removal from the mold roll by a stripping roll  40 . In this embodiment, hooks  38  are integrally molded with base  14  and extend in a longitudinally extending band from a side opposite the side of the base which defines channels  34 . In use, hooks  38  can be used to releasably secure base  14  to a loop-bearing support  39  (see  FIG. 1C ).  
         [0043]     In other embodiments, other loop-engageable or self-engageable fastener elements may be molded on resin base  14 . Hooks  38  or other fastener elements may be arranged in discrete islands of fastener elements rather than in longitudinally extending bands.  
         [0044]     Manufacturing system  10  also includes a filling station  42  and a sealing station  44 . Filling station  42  includes an inkjet  46  which dispenses ultraviolet curable conductive ink into channels  34 . Ultraviolet emitter  48  radiates ultraviolet light which cures and solidifies the conductive ink in channels  34  to form conductive traces  16 . Optionally, a second inkjet  50  dispenses a surface treatment (e.g., a solvent pre-wash, or an adhesive) into channels  34  to prepare the channels to receive the conductive ink.  
         [0045]     After conductive traces  16  are formed, sealing station  44  sprays a cover  52  (e.g., an epoxy, an acrylate, or an epoxy-acrylate) on the upper surface of resin base  14 . Cover  52  is selected at least in part for its compatibility with and ability to bond to the resin of base  14  and for its insulative properties. Cover  52  and resin base  14  cooperate to substantially insulate conductive traces  16  from each other and from the surrounding environment. The resulting flexible circuit  12  is spooled for storage on storage roll  54 .  
         [0046]     Manufacturing system  10  can form conductive traces  16  in a variety of configurations. In one example, an embodiment of mold roll  28  includes structural features  32  arranged to form conductive traces  16  as interconnected path segments arranged in accordance with a desired circuit pattern, as shown in  FIG. 2A , for receiving six-pin light emitting diodes. In another example, another embodiment of mold roll  28  includes structural features  32  arranged to form conductive traces  16  as two parallel strips, as shown in  FIG. 2B . The pattern shown in  FIG. 2B  also illustrates the flexibility resulting from use of an appropriate thermoplastic resin to form base  14  of flexible circuit  12 . Because the conductive traces are arranged in a repeating pattern, the base can be severed between adjacent iterations of the pattern at multiple locations to create circuit strips of a desired finite length. In such embodiments, the conductive traces electrically connect the severed ends of the finite strip to each other and to electrical devices mounted along the length of the strip.  
         [0047]     Referring to  FIG. 3 , in an alternate manufacturing method and system  56 , extruder  20  feeds molten resin  22  into nip  24  defined between mold roll  28  and a support roll  58 . Resin base  14  is formed in nip  24  and passes to filling station  42 A. It is not necessary for the resin  22  to continue on the surface of mold roll  28  or support roll  58  because no hooks are being formed. Consequently, it is not necessary to allow time for roll induced cooling to occur to solidify molded stems or hooks.  
         [0048]     Filling station  42 A includes a print roll  60  and a doctor blade  62 . As base  14  passes between print roll  60  and a second support roll  58 , the print roll applies a quick-drying conductive ink  64  to the upper surface of resin base  14 . Conductive ink  64  fills channels  34  and accumulates on the face of resin base  14 . Doctor blade  62  wipes accumulated ink  64  from the face of resin base  14  while leaving ink in channels  34  where the ink dries and solidifies to form conductive traces on the resin base as the resin base proceeds past tensioning roll  66  to lamination rolls  68 . Optionally, filling station  42 A also includes a hot air blower  68  which hastens the stabilization process by heating and ventilating conductive ink  64  to encourage the evaporation of the solvents which keep the ink in liquid form.  
         [0049]     Resin base  14  and preformed fastener tape  72  are fed into lamination nip  78  defined between lamination rolls  68 . Heater  74  heats fastener tape  72  as the fastener tape proceeds from feed roll  76  into lamination nip  78 . Fastener tape  72  is selected from fastener tapes which are compatible with the resin of base  14 . Thus, when heated fastener tape  72  proceeds through lamination nip  78  with base  14 , the fastener tape and the base cooperate in sealing and insulating conductive traces  16  within the flexible circuit  12 ′. In other embodiments, an adhesive is applied to fastener tape  72  before it enters lamination nip  78  rather than heating the fastener tape before it enters the lamination nip.  
         [0050]     Referring to  FIG. 4 , another alternate manufacturing method and system  80  forms resin base  14  using a similar approach to that described for manufacturing system  56 . However, manufacturing system  80  includes a filling station  42 B which fills channels  34  with particles of metallic powder and forms conductive traces  16  by bonding these particles together. In filling station  42 B, spray dispenser  82  sprays or otherwise dispenses particles of metallic powder on the upper surface of resin base  14 . The particles of metallic powder fill channels  34  and accumulate on the face of resin base  14 . Doctor blade  62  wipes accumulated particles from the face of resin base  14  while leaving particles in channels  34 . The particles can have various geometries (e.g., angular or spherical) and fill channels  34  with adjacent particles touching at contact points while otherwise leaving interstitial voids between the particles. As resin base  14  passes through a sintering device  84 , the sintering device emanates radio-frequency (RF) energy that causes eddy currents to develop within the particles in the channels. These currents cause the contact points between adjacent particles to heat up such that surface melting fuses the adjacent particles together at the contact points and locally melts resin of the channel walls touching the particles, but does not generally increase the density of the powder matrix. The result is an electrically conductive matrix extending along the channel as a trace. The metallic powder is preferably selected from a material (e.g., a tin-bismuth alloy) that has a high electrical conductivity and a low melting point and/or specific heat. Resin base  14  with the stabilized metal forming conductive traces  16  passes through a chiller  86  to cool the metal and, thus, limit melting of the thermoplastic resin base.  
         [0051]     In some embodiments, system  80  also includes an electroplating station used to electroplated a second conductive material onto conductive traces  16 . This can increase the uniformity of the conductivity along the surface of conductive traces  16  which can be important in some applications including, for example, radio-frequency identification tags.  
         [0052]     Manufacturing system  80  installs electrical components (e.g., light emitting diodes) on resin base  14 . A component feed roll  88  places light emitting diode devices  90  into receptacles  92  on a placement roll  94 , with diode pins  95  directed radially outwards. Optionally, a pin heater  96  is placed to heat pins  95  of light emitting diode devices  90  as placement roll  94  rotates to bring the light emitting diode devices into contact with resin base  14 . Pins  95  contact and pierce conductive traces  16  and resin base  14 . This provides both electrical connection and mechanical fastening for light emitting diode devices  90 . In other embodiments, similar manufacturing systems include mechanisms for forming mounting receptacles on a flexible circuit as is discussed in more detail in “Mounting Electrical Components,” U.S. Patent App. Ser. No. 60/703,330 filed on Jul. 28, 2005, the entire contents of which are incorporated herein by reference.  
         [0053]     It can be difficult to spool circuits with electrical components attached. Therefore, manufacturing system  80  includes a cutting roll  98 . As circuit  12 ″ is pulled between cutting roll  98  and support roll  58 ; ridges  100  arranged on the peripheral surface of the cutting roll cut the longitudinally extending circuit into multiple circuit strips of discrete length. Although this illustrative embodiment does not include fastener elements, some embodiments of cutting rolls  98  include fastener elements. When the fastener elements are formed or provided as a continuous strip extending longitudinally along resin base  14 , each discrete circuit strip necessarily includes fastener elements. However, if the fastener elements are formed or provided in islands along resin base  14 , the spacing of the islands and the spacing of ridges  100  on cutting roll  98  are chosen such that each discrete circuit strip includes the desired amount of fastener elements.  
         [0054]     Referring to  FIG. 5 , another alternate manufacturing method and system  102  forms resin base  14  in a gap  104  defined between extruder  20  and mold roll  28 , molding channels in a surface of the base. After stripping roll  40  removes resin base  14  from mold roll  28 , dispenser  82  sprays a liquid silver composition  106  (e.g., a binding agent such as ethylenediaminetetraacetic acid (EDTA) or citric acid containing silver ions) on the resin base. The liquid silver composition contains reducing agents (e.g., ascorbic acid or ferrous ammonium sulfate) encapsulated in micro-bubbles. After doctor blade  62  wipes accumulated silver composition from non-channel regions of resin base  14 , energy radiated by ultrasonic emitter  108  releases the reducing agents initially contained by the micro-bubbles and solidifies the silver composition. In other embodiments, other liquid compositions of similar properties, including for example compositions with other metals such as copper or aluminum, are used to fill channels  34  and to form conductive traces  16  on resin base  14 .  
         [0055]     Resin base  14  with conductive traces  16  passes tensioning roll  66  and is fed into nip  24  defined between mold roll  26  and pressure roll  29  with molten resin  22  from a second extruder  20 . Mold roll  26  includes fields of mold cavities (not shown) into which molten resin  22  is forced. Resin  22  is selected to be compatible with the resin of base  14  such that passage through nip  24  laminates a resin layer  109  to the base to seal conductive traces  16 . Although shown in  FIG. 5A  as distinct for purposes of illustration, the resin of layer  109  and base  14  can be joined together under conditions that cause the resins to so intimately bond as to become unitary.  
         [0056]     The mold cavities in roll  26  form longitudinally-extending bands of molded stems integrally molded with and extending outward from resin layer  109 . After stripping roll  40  removes circuit  12  from mold roll  26 , stem heater  110  softens stems  38 ′ such that the application of pressure by flat-topping roll  112  deforms the end of the stems to form loop-engageable heads  114  ( FIG. 5A ).  
         [0057]     Referring to  FIG. 6 , in another alternate manufacturing method and system  118 , extruder  20  feeds molten resin  22  into nip  24  defined between pressure roll  29  and a support roll  58 . Resin base  14 , formed in nip  24 , does not include channels. Resin base  14  passes from nip  24  to printing station  43  which, like filling station  42 , includes inkjet  46 , ultraviolet emitter  48 , and, optionally, second inkjet  50 . Because resin base  14  is channel-less, inkjet  46  dispenses ultraviolet curable conductive ink directly onto the upper surface of the resin base in the pattern of the desired conductive traces. Ultraviolet emitter  48  radiates ultraviolet light which cures and solidifies the conductive ink to form conductive traces (not shown) on the surface of resin base  14 . Optionally, a second inkjet  50  dispenses a surface treatment to predispose portions of the surface of resin base  14  to receive the conductive ink. Sealing station  44  and storage roll  54  cover the conductive traces and store on the flexible circuit as described in more detail in the discussion of  FIG. 3  above.  
         [0058]     The various features and components of the above-described systems may be combined in other ways. For example, another manufacturing system (not shown) features roll-molding apparatus  18  of manufacturing system  10  and filling station  42 A and preformed fastener strip sealing of manufacturing system  56  and forms a flexible circuit with fastener elements extending from both opposing sides.  
         [0059]     A number of embodiments of the invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. For example, other printing techniques including, for example, spraying conductive material through a mask, could be used for initial formation of the conductive traces. Accordingly, other embodiments are within the scope of the following claims.