Abstract:
A thermally transferable electrically conductive ribbon includes a carrier web having first and second sides and an electrically conductive layer disposed on the first side of the carrier web. A portion of the electrically conductive layer is transferable to an associated object to form an electrically conductive circuit thereon. A method for making and using the ribbon are also disclosed.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The present invention is directed to a transferable conductive ribbon and a method of making a conductive pathway. More particularly, the present invention is directed to a conductive ribbon that is thermally applied or printed to a substrate.  
         [0002]     There are many known processes for fabricating circuitry. One such process that is particularly useful in the fabrication of flexible or bendable circuitry is a silkscreen method. Such circuitry is found in, for example, automobile dashboards, appliance control panels, aircraft backlit panels, computers and the like. The circuitry is printed on to a flexible substrate such as a polyester film.  
         [0003]     The silkscreen process, however, can be quite complex. First, a screen is fabricated to meet the particular, desired circuit by producing a photographic negative of the circuit. A frame is made and silk is stretched over the frame. A photo resist (negative) is applied to the silk, and the screen is exposed to the negative. The screen is then developed to produce a “picture” of the circuit on the screen.  
         [0004]     A panel is then fabricated by using a substrate that can accept the screen print inks, such as polyester, and mixing and applying conductive inks. Typically, the inks are applied in layers. After the ink is applied, the screen is cured to harden or dry the ink on the substrate.  
         [0005]     Although the silkscreen process works to provide flexible circuitry, there are drawbacks. One drawback, generally, to silk-screening is that it uses flammable and toxic chemicals. The chemicals presently known and used for fabricating the screens are volatile and in some instances harmful. In addition, the chemical waste that is generated requires disposal. Depending upon the types of inks and/or chemicals, special handling may be required for disposal. It is also a relatively expensive process.  
         [0006]     Moreover, there is limited flexibility (in design) using silkscreen processes. Prototyping is difficult and, once a screen is made, it cannot be easily changed, if at all.  
         [0007]     Alternative methods for fabricating conductive circuits have used inkjet printing technologies. However, in such a technology, the ink is formulated with conductive nano-particles and then printed with a modified inkjet printer. The printed circuits are sintered (heat treated) to fully fuse the conductive particles in the ink to achieve a continuous conductive pathway to create the circuit. Drawbacks to this method are the high cost of the conductive nano-particles, the difficulty formulating a jettable ink with desired end properties, special design features that are required for the inkjet printer to handle the conductive ink and the additional sintering step required for the “printed” circuit to achieve the desired conductivity.  
         [0008]     Accordingly, there is a need for a flexible electrically conductive circuit that is formed by a non-silkscreen process or non-inkjet process. Desirably, such a process permits flexibility in circuit design. More desirably, in such a process, the circuit is formed using a ribbon applied method. More desirably still, the process is a thermal printing process in which the circuit is readily design and created using computer-aided circuit design tools and transferred to an object using known thermal transfer processes.  
       BRIEF SUMMARY OF THE INVENTION  
       [0009]     A thermally transferable electrically conductive ribbon includes a carrier web having first and second sides and an electrically conductive layer disposed on the first side of the carrier web. A portion of the electrically conductive layer is transferable to an associated object to form an electrically conductive circuit thereon.  
         [0010]     To facilitate release of the conductive layer from the web, a release coat is disposed on the first side of the carrier web between the carrier web and the electrically conductive layer. An adhesive layer is disposed on the electrically conductive layer to provide adhesion between the portion of the electrically conductive layer that is transferred to the associated object and the associated object.  
         [0011]     The present ribbon and method for making and using the ribbon, avoid the time and expense of the silkscreen process. The present method forms the circuit using a ribbon applied thermal transfer process. Using the present process, an electrical circuit is readily designed, created and transferred to an object, and advantageously a flexible object such as a polyester film, using computer-aided circuit design tools and known thermal transfer or printing technologies.  
         [0012]     These and other features and advantages of the present invention will be apparent from the following detailed description and drawings in conjunction with the appended claims. 
     
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS  
       [0013]     The benefits and advantages of the present invention will become more readily apparent to those of ordinary skill in the relevant art after reviewing the following detailed description and accompanying drawings, wherein:  
         [0014]      FIG. 1  is a plan view of an exemplary flexible circuit formed in accordance with the principles of the present invention;  
         [0015]      FIG. 2  is a cross-sectional view of a portion of the circuit of  FIG. 1  taken along line  2 - 2  of  FIG. 1 ; and  
         [0016]      FIG. 3  is a perspective illustration of a thermally printable electrically conductive ribbon in accordance with the principles of the present invention;  
         [0017]      FIG. 4  is a cross-sectional view of the ribbon of taken along line  4 - 4  of  FIG. 3 ; and  
         [0018]      FIG. 5  is a flow diagram illustrating one exemplary method for fabricating the flexible circuit of  FIG. 1 . 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0019]     While the present invention is susceptible of embodiment in various forms, there is shown in the figures and will hereinafter be described a presently preferred embodiment with the understanding that the present disclosure is to be considered an exemplification of the invention and is not intended to limit the invention to the specific embodiment illustrated.  
         [0020]     It should be further understood that the title of this section of this specification, namely, “Detailed Description Of The Invention”, relates to a requirement of the United States Patent Office, and does not imply, nor should be inferred to limit the subject matter disclosed herein.  
         [0021]     The present invention permits the fabrication of flexible circuitry using thermal transfer processes. Advantageously, the present invention eliminates the need for expensive silkscreen processes, and their attendant drawbacks.  
         [0022]     Referring to the  FIG. 1 , there is shown an exemplary flexible circuit  10  formed in accordance with the principles of the present invention. The circuit  10  is formed on a flexible base film or substrate  12 , such as mylar, acrylic, polyester film, vinyl film, paper, paper board or most any printable substrate. It will be appreciated that the substrate need not be a flexible medium, that is, it can be a rigid medium; however, the advantages of the present invention are well appreciated in a flexible substrate  12  environment. Such flexible circuits can, for example, be used in automobile dashboards, appliance control panels, aircraft backlit panels, computers and the like.  
         [0023]     A cross-section of the flexible circuit  10  is illustrated in  FIG. 2 . The substrate  12  supports and provides structure for the electrically conductive material  14 . The conductive material  14  is held to the substrate  12  by an adhesive  16 . An optional protective coat  18  can be applied over the conductive material  14  (layer).  
         [0024]     A cross-section of a film  20  for use in thermally transferring the conductive material  14  (layer) to the substrate  12  is illustrated in  FIG. 4 . In a present form, the film  20  is formed as a ribbon R as seen in  FIG. 3 . Referring to  FIGS. 3 and 4 , the ribbon-formed film  20  includes a carrier web  22  and a release coat  24  formed on the carrier web  22 . A conductive layer  26  is applied to the release coat  24  and an adhesive layer  28  is applied to the conductive layer  26 . A backcoat  30  can be applied to the opposite side of the web  22  (see  FIG. 3 ) to facilitate thermal transfer from the web  22 . The backcoat  30  can be formulated to allow greater heat application (for thermal transfer) through the web  22 . Those skilled in the art will recognize that as applied to the flexible circuit substrate  12 , the adhesive layer  28  (forms the adhesive  16  that) adheres the electrically conductive layer  26  (to form the conductive material  14 ) to the substrate  12 . The release coat  24  may remain on the thermal transfer film  20  (i.e. with the carrier  22 ) subsequent to transfer.  
         [0025]     The carrier web  22  can be formed from any of a wide variety of materials. One known material for use in thermal printing webs  22  (carriers) is a polyester film. In commonly used thermal printing processes, a polyester film of about 4 to about 20 microns is used. The back side of the web  22  can be treated, as with the backcoat  30 , to protect the film  22  as it is used in a thermal transfer process.  
         [0026]     The release coat  24  is formulated to respond to the heat applied to the web during the thermal transfer process to “release” the subsequent layers  26 ,  28 . One type of release coat  24  that releases with the subsequent layers  26 ,  28  (transfers with the conductive and adhesive layers) is an alkali-soluble thermoplastic polymer that is removed from the conductive layer (from the flexible circuit) after transfer. Removing the release coat  24  reduces the likelihood of interference with the conductive layer. The release coat  24  can be removed with an alkaline solution such as an ammonia and water mixture. Other materials that may be used that transfer with the conductive layer  26  include various waxes such as paraffin, microcrystalline or polyethylene glycol. Modifiers such as cross-linking agents or coupling agents may be added to the release layer to improve print performance.  
         [0027]     Alternately, the release coat  24  can be of the type that remains on the web  22  and does not transfer with the subsequent layers  26 ,  28 . These types of coatings include, for example, cross-linked silicone based materials and the like. Modifiers can be included to facilitate release of the subsequent layers  26 ,  28 .  
         [0028]     The electrically conductive layer  26  is applied to the web  22 , over the release coat  24 . The layer  26  can be formed from a wide variety of metals, such as aluminum, copper, silver, gold, platinum, molybdenum, tungsten, titanium, tantalum, germanium, silicon and silicon-containing materials, indium tin oxide (ITO), aluminum tin oxide (ATO), aluminum zinc oxide (AZO), carbon, nickel, and the like. The conductive layer  26  can be applied using processes such as spraying, coating, ion vapor deposition, vacuum metallization, sputter coating and the like. Those skilled in the art will recognize the various methods by which the conductive layer  26  can be applied to or embedded into the film. It is also contemplated that the conductive material is mixed with (e.g., formulated within) a coating such as a resin, that is applied to the web  22 . In such cases the coating may be formulated to release from the carrier when printed, without the need for a release layer (such as layer  24 ). Optionally, the adhesive layer  28  can be applied to the substrate  12 , creating a print receptive substrate, thus eliminating the need for an adhesive layer applied to the ribbon R.  
         [0029]     The adhesive  16  (applied as layer  28 ) provides the necessary adhesion between the conductive material  14  and the circuit substrate  12  to assure good bonding of the conductive material  14 . A preferred adhesive  16  (applied over the conductive layer  26 , as adhesive layer  28 ) is a thermoplastic resin, such as vinyl chloride acrylic, polyester or chlorinated polyolefin resin or mixtures thereof, and is responsive (e.g., softens and fuses) at the desired transfer temperatures. Coupling agents such as silanes can be added to the adhesive layer  28  to promote adhesion of the conductive material  14  to the substrate  12 .  
         [0030]     One method  110  for fabricating the flexible circuit  10  is illustrated in the flow diagram of  FIG. 5 . The method  110  includes the steps of providing a substrate  112 , providing a thermally printable electrically conductive ribbon  114  having an electrically conductive layer thereon, and transferring a portion of the electrically conductive layer onto a flexible substrate  116 . The transferred portion defines a desired electrical circuit or portion of an electrical circuit  10 .  
         [0031]     If necessary, any remaining release coat material is removed  118  from the now formed electrical circuit or portion of an electrical circuit  10 . An optional protective coating (e.g., an over coating) can be applied  120  to the transferred electrical circuit  10 .  
         [0032]     One of the advantages of the present invention is that when used in conjunction with presently available circuit design tools, circuits can be designed, prototypes created and tested, in far less time and with far less effort than previously used silk-screening applications. For example, using CAD circuit design tools, a circuit can be designed, and by entering a print command, with the requisite thermally printable electrically conductive ribbon and substrate in a printer, the circuit can be printed and tested. Adjustments and/or changes can be made to a design and subsequent prototype circuits printed. Once a final design is made, production runs of the circuit can be made using the same thermal printing or transfer methods and technology.  
         [0033]     All patents referred to herein, are hereby incorporated herein by reference, whether or not specifically done so within the text of this disclosure.  
         [0034]     In the present disclosure, the words “a” or “an” are to be taken to include both the singular and the plural. Conversely, any reference to plural items shall, where appropriate, include the singular.  
         [0035]     From the foregoing it will be observed that numerous modifications and variations can be effectuated without departing from the true spirit and scope of the novel concepts of the present invention. It is to be understood that no limitation with respect to the specific embodiments illustrated is intended or should be inferred. The disclosure is intended to cover all such modifications as fall within the scope of the claims.