Abstract:
Method of forming a flexible circuit laminate for use in the production of flexible circuits, comprising the steps of electrodepositing a continuous layer of copper on a first side of a generally continuous strip of polyimide having a layer of metal on the first side, modifying a second side of the polyimide strip to increase the surface energy thereof, applying a preformed adhesive film on the second side of the generally continuous strip of polyimide, the adhesive strip being formed of a substantially uncured polymeric material, and curing the adhesive film wherein at least the outmost layer of the adhesive film is only partially cured.

Description:
This application is a divisional application of 09/266,952, Mar. 12, 1999, U.S. Pat. No. 6,146,480 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to flexible circuits, and more particularly, to a flexible laminate for use in forming a flexible circuit. 
     BACKGROUND OF THE INVENTION 
     Flexible circuits find advantageous application where an electrical connector/conductor is subject to vibration or movement. Flexible circuits are generally comprised of a polymeric substrate having a copper circuit formed thereon. The electric circuit is generally formed from a continuous layer of copper electrodeposited onto one side of the polymeric substrate. In some instances, it may be desirable to be able to adhere the flexible circuit onto a support surface, or to attach two flexible circuits together with an intermediate insulating layer between the two circuits. 
     The present invention provides a flexible circuit component for use in forming flexible circuits and a flexible circuit that may be attached to another surface. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, there is provided a method of forming a flexible circuit laminate for use in the production of flexible circuits, comprising the steps of: a) depositing a continuous layer of copper on a first side of a generally continuous strip of polyimide having at least one layer of metal on one side thereof; b) modifying a second side of the polyimide strip to increase the surface energy thereof; c) applying an adhesive onto the second side of the generally continuous strip of polyimide, the adhesive being formed of a substantially uncured, polymeric material, the polymeric material having properties in its uncured state wherein it will not flow under pressure exerted along its planar surface; and d) curing the adhesive wherein at least an outermost layer of the adhesive is only partially cured. 
     In accordance with another aspect of the present invention, there is provided a method of forming a flexible circuit laminate for use in the production of flexible circuits, comprising the steps of: a) depositing a continuous layer of copper on a first side of a generally continuous strip of polyimide film having a layer of metal on the first side; b) exposing a second side of the polyimide film to a chemical plasma at sufficient levels to modify the surface energy of the polyimide film; c) applying at least one layer of metal to the second side of the polyimide film; d) applying an adhesive onto the at least one layer of metal on the second side of the polyimide film, the adhesive being formed of a substantially uncured, polymeric material, the polymeric material having properties in its uncured state wherein it will not flow under pressure exerted along its planar surface; and e) inductively heating the polyimide film to cure the adhesive, wherein at least an outermost region of the adhesive is only partially cured. 
     In accordance with a further aspect of the present invention, there is provided a method of forming a flexible circuit laminate for use in the production of flexible circuits, comprising the steps of: modifying a first side and a second side of a generally continuous polyimide strip to increase the surface energy thereof; depositing a continuous layer of copper on the first side of the generally continuous strip of polyimide film, the first side having at least one layer of metal thereon; applying an adhesive onto the second side of the generally continuous strip of polyimide, the adhesive being formed of a substantially uncured, polymeric material, the polymeric material having properties in its uncured state wherein it will not flow under pressure exerted along its planar surface; and curing the adhesive wherein at least an outermost region of the adhesive is only partially cured. 
     It is an object of the present invention to provide a flexible laminate for use in forming flexible circuits. 
     Another object of the present invention is to provide a flexible laminate as described above wherein one side of the flexible laminate includes an at least partially uncured adhesive film. 
     Another object of the present invention is to provide a flexible laminate as described above wherein the laminate is comprised of a polymeric layer having a layer of copper adhered to one side of the polymeric substrate, and a layer of a polymeric adhesive applied to a second side of the polymeric substrate. 
     A still further object of the present invention is to provide a flexible laminate as described above wherein the polymeric adhesive is a dimensionally stable film of an uncured polymeric adhesive. 
     A still further object of the present invention is to provide a method of adhering a polymeric adhesive film to a polymeric substrate. 
     An even further object of the present invention is to provide a method as described above wherein the method includes the step of surface-treating the polymeric substrate to increase the surface energy thereof. 
     These and other objects and advantages will become apparent from the following description of preferred embodiments of the invention, taken together with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention may take physical form in certain parts and arrangement of parts, embodiments of which are described in detail in the specification and illustrated in the accompanying drawings, wherein: 
     FIG. 1 is an elevational, schematic view of a process line for forming a flexible circuit laminate illustrating a preferred embodiment of the present invention; 
     FIG. 2 is an enlarged, perspective view of a copper-coated polymeric strip prior to undergoing the process shown in FIG. 1; and 
     FIG. 3 is a cross-sectional view taken along the line  3 — 3  of FIG. 1, showing a flexible circuit laminate formed in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Referring now to the drawings wherein the showings are for the purpose of illustrating preferred embodiments of the invention, and not for the purpose of limiting same, FIG. 1 shows a schematic view of a process line  10  for forming a flexible circuit laminate in accordance with the present invention. In the embodiment shown, a generally continuous strip or web  20  of a polymeric substrate having a layer of copper formed thereon is conveyed along a predetermined path. FIG. 2 shows an enlarged, perspective view of a portion of web  20 . In FIG. 2, the polymeric substrate is designated  22  and the continuous layer of copper is designated  24 . Substrate  22  has a first surface, designated  22   a , and a second surface, designated  22   b . The copper-coated, polymeric strip is preferably formed by applying a layer of metal  23  onto surface  22   a  of polymeric substrate  22 . Layer  23  may be applied by a metal sputtering technique, or by a chemical vapor deposition process. Copper layer  24  is then electrodeposited onto metal layer  23 . 
     In accordance with the present invention, the generally continuous web  20  of the copper-coated polymeric material is conveyed along a path wherein an adhesive  44  of a generally uncured resin material is applied thereto. Web  20  is preferably formed of a polyimide material, and more specifically, a biaxially oriented polyimide such as DuPont&#39;s kapton (KAPTON is a trademark of DuPont). 
     According to the present invention, adhesive  44  is an uncured, or substantially uncured, resin material that is generally dimensionally stable under the exertion of forces along its surface. As used herein, the term “dimensionally stable” as applied to the uncured resin material shall mean the resin has properties wherein it will not significantly alter its shape or flow under a pressure exerted along its planar surface as a result of stacking pressure. Basically, it is intended that the term “dimensionally stable,” as used to describe the resin film used in the present invention, should distinguish such film from uncured resins that flow when under the exertion of planar pressure. 
     A product manufactured and sold by Minnesota Mining and Manufacturing (3M) under the name “High Performance Epoxy Adhesive Bonding Film” finds advantageous use as Adhesive  44  in the production of flexible laminate  20  according to the present invention. This product is comprised of an epoxy resin and is available in thicknesses of about 1 or 2 mils under 3M designations “9901” and “9902, ” respectively. The materials are provided by the manufacturer with removable protective polymer films on both surfaces thereof. The material has the following physical properties as disclosed by the manufacturer. 
     
       
         
               
               
               
               
             
               
               
               
               
             
           
               
                   
               
               
                 Property 
                 Units 
                   
                 IPC Test 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Glass Transition* 
                 ° C. 
                 180 
                   
               
               
                 CTE 20°-110° C. 
                 ppm/°C. 
                 60 
               
               
                       110°-180° C. 
                   
                 87 
               
               
                       180°-250° C. 
                   
                 213 
               
               
                 Peel Strength 
                 Lbs./inch 
                 8 
                 2.4.9 
               
               
                 Volatile Content 
                 % 
                 1 
                 2.3.37 
               
               
                 Moisture Absorption 
                 % 
                 1.7 
                 2.6.2B 
               
               
                 Chemical Resistance 
                 % 
                 &gt;90 
                 2.3.2 
               
               
                 Dielectric Constant* 
                   
                 3.4 
                 2.5.5.3 
               
               
                 Dissipation Factor* 
                   
                 .022 
                 2.5.5.3 
               
               
                 Dielectric Strength 
                 Volts/mil 
                 2200 
                 D-149 
               
               
                 Insulation Resistance 
                 Megohms 
                 1.00E + 06 
                 2.6.3.2 
               
               
                 Volume Resistivity 
                 Megohms-cm 
                 5.00E + 07 
                 2.5.17 
               
               
                 Surface Resistivity 
                 Megohms 
                 7.00E + 05 
                 2.5.17 
               
               
                 Solder Float 
                   
                 Pass 
                 2.4.13 
               
               
                 Low Temperature Flexibility 
                   
                 Pass 
                 2.6.18 
               
               
                 Flexural Endurance 
                 Cycles 
                 1787 
                 3.7.4 
               
               
                 Fracture Toughness 
                 Mpa*m ½   
                 0.65 
               
               
                 Modulus 
                 GPa 
                 3.5 
               
               
                   
               
             
          
         
       
     
     In accordance with the present invention, the exposed surface of polymeric strip  22  undergoes a surface treatment, schematically represented by a box  30  in FIG. 1, to modify the surface energy of the polymeric strip to insure appropriate adhesion. In this respect, the sine-qua-non of adhesion is wetting. Absence wetting, good adhesion between two materials will not occur. Specifically, if the surface energy of a surface is higher than the surface energy of an adhesive applied thereto, the applied adhesive will spread and wet the surface, thereby lowering the total energy of the surface. Good wetting is therefore required for good adhesion. 
     In accordance with the present invention, the exposed surface  22   b  of polymeric substrate  22  is treated to increase the surface energy thereof. Treatment of surface  22   b  may be accomplished by heating or by ion bombardment, plasma treatment, electron etching, heat or other types of particle bombardment or electromagnetic wave radiation etching. Surface  22   b  of polymeric substrate  22  may also be exposed to a plasma of a chemical that adheres to substrate  22  and raises the surface energy thereof, for example, exposing substrate  22  to an oxygen plasma, a low molecular weight silane plasma, a plasma of a halogen gas such as a chlorine plasma, a bomine plasma, etc. 
     In accordance with a preferred embodiment of the present invention, surface  22   b  of substrate  22  undergoes a two-step surface treatment process to increase the surface adhesion properties thereof. The adhesion properties of surface  22   b  may be increased through physically altering surface  22   b  to increase the surface are thereof, by chemically altering surface  22   b  to increase the surface energy thereof, or a combination of both. In a preferred embodiment, surface  22   b  undergoes a first step of surface treatment to physical or chemical modify surface  22   b , followed by a second step wherein a layer of metal is deposited on the modified surface  22   b.    
     The first step preferably physical roughens or chemical modifies surface  22   b  of substrate  22 . Laser etching or electromagnetic radiation may be used to physical roughen surface  22   b . Ion beam bombardment or plasma may be used to chemically alter surface  22   b . In a preferred embodiment oxygen (O 2 ) plasma is applied to surface  22   b  prior to applying a metal layer in step two. 
     Referring now to step two, at least one thin layer of metal  32  is preferably applied to the previously treated surface  22   b . Metal layer  32  that is applied to surface  22   b  may be selected from the group consisting of chromium, titanium, aluminum, nickel, copper, iron vanadium, silicon or alloys thereof. Metal layer  32  is preferably applied by a conventional metal sputtering technique. In a preferred embodiment, chromium (Cr) is sputtered onto surface  22   b  following surface preparation of surface  22   b  by oxygen (O 2 ) plasma as discussed in step one. 
     Chromium layer  32  preferably has a thickness of between about 50 Angstroms (Å) to about 300 Angstroms (Å). Chromium layer  32  provides a metal surface having a surface energy higher than the surface energy of polyimide substrate  22 , thereby improving the adhesion between substrate  22  and adhesive  44  to be applied thereto. In this respect, chromium layer  32  further enhances the adhesion properties of the roughened surface  22   b . It will of course be appreciated that in some applications the further adhesion enhancement of metal  32  may not be required, and that the adhesion enhancement provided by the surface treatment of step one may alone be sufficient to provide satisfactory adhesion between surface  22  and adhesive  44 . 
     Following surface treatment process  30 , the generally continuous web  20  moves past an adhesive feed assembly  40 . Adhesive feed assembly  40  is comprised of a roll  42  having an adhesive  44  in film form wound thereon. Adhesive film  44  is preferably of the type manufactured by 3M as described above. Removable, protective layers  46  are typically provided on both surfaces of adhesive film  44 . In the process shown, the protective layer  46  on the surface of adhesive film  44  that is facing substrate  22  is removed by a film take-up roller  52 . Specifically, protective layer  46  is directed over a nip roller  54  onto take-up roller  52 . The removal of inner protective layer  46  exposes adhesive film  44  to chromium layer  32  on surface  22   b  of substrate  22 . Heated nip rollers  56  force adhesive film  44  and layer  32  on surface  22   b  into engagement with each other as exposed adhesive film  44  engages treated layer  32  on surface  22   b . Preferably, nip rollers  56  are heated sufficiently to warm adhesive film  44  to insure positive contact with layer  32  on substrate  22 . 
     In accordance with one aspect of the present invention, web  20 , with adhesive film  44  thereon, is preferably conveyed, in a continuous fashion, past a heating element, designated  60  in the drawings. Heating element  60  is operable to heat the uncured resin forming adhesive film  44 . The heating of adhesive film  44  may be undertaken by conventional gas-fired electric-fired heating means or induction heating. 
     With respect to the preferred embodiment heretofore described, wherein chromium layer  32  is deposited onto surface  22   b , web  20  is preferably heated by induction heating, wherein the induction heating of metallic layer  32  heats adhesive film  44  through radiation and conduction as the heat generated in metal layer  32  adjacent adhesive film  44  radiates and is conducted into adhesive film  44 . Heating element  60  is preferably controlled such that adhesive film  44  is cured to what is conventionally referred to in the art as “B-staged.” In this respect, it is conventionally understood that an “A-staged” resin refers to a resin that is substantially uncured. A “B-staged” resin refers to a resin that is partially cured, although not fully cured. A “C-staged” resin refers to a resin that is substantially fully cured. 
     Thus, in accordance with the present invention, adhesive film  44  on web  20  is only partially cured to a B-stage. Web  20  with adhesive film  44  thereon then preferably passes between two nip rollers  62 . A continuous laminate, designated  70 , is thus formed in accordance with the aforementioned process. Laminate  70  may then be cut into sheets  72 , as schematically illustrated in FIG.  1 . 
     FIG. 3 shows a cross-sectional view of a flexible laminate  70  formed in accordance with the process shown in FIG. 1, wherein a partially cured adhesive layer  44  having a protective, removable strip  46  is attached to polymeric substrate  22  that has a copper layer  24  on surface  22   a  thereof. 
     The foregoing description is a specific embodiment of the present invention. It should be appreciated that this embodiment is described for purposes of illustration only, and that numerous alterations and modifications may be practiced by those skilled in the art without departing from the spirit and scope of the invention. According to another aspect of the present invention, the induction heating of web  20  may be controlled such that the temperature of the region of adhesive film  44  in contact with the layer reaches a temperature whereby this region of adhesive is fully cured to a C-stage, but the outer region of adhesive film  44 , and specifically, at outer surfaces of adhesive film does not attain a temperature for a sufficient length of time to fully cure such region, and therefore, the outer surface of adhesive film remains at least partially uncured, i.e., at an A-stage, a B-stage or a mixture of both. It is intended that all such modifications and alterations be included insofar as they come within the scope of the invention as claimed or the equivalents thereof.