Patent Publication Number: US-2022217850-A1

Title: Production method of wiring board and wiring board

Description:
TECHNICAL FIELD 
     The present invention relates to a production method of a wiring board including a conductor pattern formed on a board and an insulating film that is formed by flexographic printing and covers at least a part of the conductor pattern, and more particularly, to a wiring board produced by the production method. 
     BACKGROUND ART 
     Japanese Patent Application Laid Open No. 2001-51259 (referred to below as Patent Literature 1) describes the formation of a surface protective film (insulating film) covering an electrode pattern formed on a board using flexographic printing in the board used for liquid crystal apparatuses.  FIGS. 1A, 1B, 2A, and 2B  illustrate how an insulating film is formed by the flexographic printing, which is described in Patent Literature 1, reference numeral  11  represents the board and reference numeral  12  represents the electrode pattern in  FIGS. 1A, 1B, 2A, and 2B . In addition, reference numerals  13 ,  13   a , and  13   b  respectively represent a coating film, a lower coating film, and an upper coating film which are formed by transferring and applying a liquid precursor for forming an insulating film. 
     Patent Literature 1 describes the following with reference to  FIGS. 1A, 1B, 2A, and 2B . 
     1) When the liquid precursor is applied by flexographic printing, the portion of the coating film  13  near an border segment  14  becomes a protuberance with a height approximately twice the film thickness of the inner area as illustrated in  FIG. 1A  and, when this protuberance  15  is approximately 1500 Å, cracks occur in cleaning or rubbing treatment. Such cracks cause peeling of the coating film  13 . In  FIG. 1A , the upper limit of the thickness of the surface protective film is 750 Å. 
     2) When the lower coating film  13   a  is formed by flexographic printing, the upper coating film  13   b  is formed by flexographic printing again, and then the upper coating film  13   b  and the lower coating film  13   a  are cured as illustrated in  FIG. 1B , cracks occur in the subsequent cleaning and rubbing treatment when a protuberance  16  near the border segment  14  reaches approximately 2000 Å. In  FIG. 1B , the upper limit of the thickness of the surface protective film is 1000 Å. 
     3) The lower coating film  13   a  having a film thickness of approximately 650 Å is formed as illustrated in  FIG. 2A  and then the upper coating film  13   b  having a film thickness of 650 Å is formed as illustrated in  FIG. 2B . At this time, the upper coating film  13   b  is formed so that an border segment  18  of the upper coating film  13   b  is located approximately 200 μm inward of the border segment  14  of the lower coating film  13   a . A protuberance  19  of the upper coating film  13   b  does not overlap a protuberance  17  of the lower coating film  13   a  and the sum of the film thicknesses of the lower coating film  13   a  and the film thicknesses of the upper coating film  13   b  is approximately 1950 Å, which is less than 2000 Å, even at the position (position at which the protuberance  19  is generated) at which the sum of the film thicknesses is maximum. Accordingly, since a surface protective film having a film thickness of 1300 Å can be formed and the film thickness of the portion at which the protuberance  19  is generated is only 1950 Å in  FIGS. 2A and 2B , no cracks occur. 
     As described above, Patent Literature 1 describes that, in the formation of an insulating film by flexographic printing, cracks occur when a protuberance generated near the border segment of an insulating film is thick, and the insulating film is formed via double-coating by displacing the border segments from each other to form a thick insulating film while avoiding the occurrence of cracks. 
     By forming the insulating film by superposition printing of two layers as described above, it is possible to form an insulating film having a film thickness thicker than the maximum film thickness that can be formed by single printing and prevent the occurrence of a problem such as communication between both sides (upper and lower sides) of the insulating film through pinholes penetrating in the film thickness direction that may be generated in the insulating film having been printed and formed. 
     However, when the insulating film is formed by superposition printing of two layers, superposition printing is performed in a form as described in Patent Literature 1, that is, in a form in which the border segment of the second layer, which is formed on the first layer being a lower layer, is recessed into the area of the film from the border segment of the first layer, a valley with a remarkable film thickness is formed between the protuberance  17  near the border segment of the first layer and the protuberance  19  near the border segment of the second layer as illustrated in  FIG. 2B . 
     This valley causes a problem in that when, for example, a layer of an optical clear adhesive (OCA) is disposed on the insulating film and an optical member such as a cover is mounted on this layer, air bubbles are generated (air bubbles are trapped) between the insulating film and the optical clear adhesive layer in the valley, and the presence of such air bubbles along, for example, the periphery of the insulating film disables proper adhesion, thereby making the mounting strength insufficient. 
     In addition, for example, when the valley of the insulating film is present in a visible area of the device, the surface of this portion looks distorted, interference fringes are generated, or other visual problems occur. 
     As described above,  FIGS. 3 and 4  illustrate how air bubbles are generated when a cover  24  is mounted on first and second insulating films  21  and  22  having the forms illustrated in  FIG. 2B  via an optical clear adhesive layer  23 . In  FIGS. 3 and 4 , reference character  21   a  represents the protuberance near the border segment of the first insulating film  21  and reference character  22   a  represents the protuberance near the border segment of the second insulating film  22 . Reference numeral  25  represents the air bubble generated (remaining) in the valley between the protuberances  21   a  and  22   a . In addition, reference numeral  26  represents a board and reference numeral  27  represents a conductor pattern. It should be noted that  FIG. 4  illustrates the state in which the air bubbles  25  are generated in the valleys between the protuberances  21   a  and  22   a  around a through-hole  28  when the through-hole  28  is provided in the insulating film. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention addresses such situations with an object of providing a production method that prevents valleys from being formed on the surface of an insulating film in the production of a wiring board including the insulating film formed by superposition printing of flexographic printing, and further providing a wiring board that includes an insulating film with the two-layer structure and has no valleys on the surface of the insulating film. 
     The technical matters described herein are not intended to expressly or implicitly limit the invention described in the claims and express the possibility of accepting such limitation by persons other than those who benefit from the present invention (for example, the applicant and the right holder), but are merely provided for easy understanding of the gist of the present invention. The outline of the present invention from another point of view can be understood from, for example, the claims at the time of filing of this patent application. 
     Although the term “wiring board” is used in this specification, “wiring board” is paraphrased as “printed board” in this section. The printed board according to the present invention includes a board and a printed structure on the board. The printed structure includes an insulating layer on the board or the printed structure includes a conductive layer having a pattern formed of a conductor and an insulating layer on the conductive layer. When the printed structure includes a conductive layer, the conductive layer is formed directly or indirectly on the board, in other words, the conductive layer is formed on the board or the base layer. The insulating layer has the following characteristics. 
     The insulating layer includes a first cured coating layer made of first insulating ink and a second cured coating layer made of second insulating ink. The material of the second insulating ink may be the same as or different from that of the first insulating ink. The ratio of the thickness of the first cured coating layer to the area of the first cured coating layer is generally sufficiently small. That is, the first cured coating layer generally has a thin film shape. The ratio of the thickness of the second cured coating layer to the area of the second cured coating layer is generally sufficiently small. That is, the second cured coating layer generally has a thin film shape. The first cured coating layer is located below the second cured coating layer and the first cured coating layer and the second cured coating layer are in contact with each other. The two-layer structure of the first cured coating layer and the second cured coating layer included in the insulating layer may be observed from the cross section of the insulating layer. The edge of the first cured coating layer is covered with the second cured coating layer. In the area between the edge of the first cured coating layer and the edge of the second cured coating layer, the second cured coating layer is located on the board, the base layer, or the conductive layer. The shortest distance from any point on the edge of the second cured coating layer to the edge of the first cured coating layer is more than 0 μm and not more than 400 μm, preferably not less than 50 μm and not more than 400 μm, more preferably not less than 100 μm and not more than 400 μm, and most preferably not less than 200 μm and not more than 400 μm. 
     When the printed structure includes an insulating layer on the board, the method of manufacturing a printed board includes: applying the first insulating ink onto the board; forming the first cured coating layer by curing the first insulating ink; applying the second insulating ink onto the first cured coating layer in which the second insulating ink is applied beyond the edge of the first cured coating layer by a distance more than 0 μm and not more than 400 μm—in other words, the second insulating ink covers the edge of the first cured coating layer —; and forming the second cured coating layer by curing the second insulating ink. 
     When the printed structure includes a conductive layer and an insulating layer, the method of manufacturing a printed board includes: applying the first insulating ink onto the conductive layer; forming the first cured coating layer by curing the first insulating ink; applying the second insulating ink onto the first cured coating layer in which the second insulating ink is applied beyond the edge of the first cured coating layer by a distance more than 0 μm and not more than 400 μm—in other words, the second insulating ink covers the edge of the first cured coating layer —; and forming the second cured coating layer by curing the second insulating ink. 
     The application of the insulating ink is performed by printing—preferably, flexographic printing —. 
     Effects of the Invention 
     According to the present invention, in the production of a wiring board including an insulating film formed by superposition printing of flexographic printing, it is possible to provide a production method that prevents valleys from being formed on a surface near the border segment of the insulating film including the first insulating film and the second insulating film which undergo superposition printing. 
     In addition, in the wiring board according to the present invention, no valleys are present on a surface near the border segment of the insulating film having the two-layer structure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a sectional view illustrating a portion near an border segment of an insulating film formed by conventional flexographic printing. 
         FIG. 1B  is a sectional view illustrating a portion near border segments of insulating films formed via double-coating (superposition printing) by conventional flexographic printing. 
         FIG. 2A  is a sectional view illustrating a portion near an border segment of an insulating film formed by conventional flexographic printing. 
         FIG. 2B  is a sectional view illustrating the state in which an insulating film undergoes superposition printing on the insulating film in  FIG. 2A  by flexographic printing with the border segments thereof deviated from each other. 
         FIG. 3  is a sectional view illustrating the state in which a cover is mounted on the same structure as in  FIG. 2B  via an optical clear adhesive layer. 
         FIG. 4  is a sectional view illustrating the state in which a cover is mounted via an optical clear adhesive layer when the border segment of the insulating film around a through-hole has the same shape as in  FIG. 3 . 
         FIG. 5A  is a sectional view illustrating the shapes of the main portions of the insulating films in a wiring board according to the present invention. 
         FIG. 5B  is a sectional view illustrating shapes that are different from those in  FIG. 5A , which are formed by a portion near the border segments of the insulating films. 
         FIG. 6  is a sectional view illustrating the state in which a cover is mounted on the structure illustrated in  FIG. 5A  via an optical clear adhesive layer. 
         FIG. 7  is a plan view illustrating an outline of a touch panel to which a production method of the wiring board according to the present invention is applied. 
         FIG. 8A  is an enlarged plan view of a main portion in  FIG. 7 . 
         FIG. 8B  is a sectional view of the portion illustrated in  FIG. 8A  when a cover is mounted via an optical clear adhesive layer. 
     
    
    
     LIST OF REFERENCE NUMERALS 
     
         
         
           
               11 : board 
               12 : electrode pattern 
               13 : coating film 
               13   a : lower coating film 
               13   b : upper coating film 
               14 : border segment 
               15 ,  16 ,  17 ,  19 : protuberance 
               18 : border segment 
               21 : first insulating film 
               21   a : protuberance 
               22 : second insulating film 
               22   a : protuberance 
               23 : optical clear adhesive layer 
               24 : cover 
               25 : air bubble 
               26 : board 
               27 : conductor pattern 
               28 : through-hole 
               31 : board 
               32 : conductor pattern 
               40 : insulating film 
               41 : first insulating film 
               41   a : protuberance 
               42 : second insulating film 
               50 : clear board 
               61 : first sensor electrode row 
               61   a : island electrode 
               61   b : coupling portion 
               62 : second sensor electrode row 
               62   a : island electrode 
               62   b : coupling portion 
               62   c : connection portion 
               71 ,  72 : lead-out wire 
               72   a : connection portion 
               73 : terminal portion 
               81 : first conductor pattern 
               82 : second conductor pattern 
               83 : optical clear adhesive layer 
               84 : cover 
               90 : insulating film 
               91 : first insulating film 
               92 : second insulating film 
               93 : through-hole 
           
         
       
    
     DETAILED DESCRIPTION 
     An embodiment of the present invention will be described using examples with reference to the drawings. 
       FIG. 5A  illustrates a portion near an border segment of an insulating film  40  of a wiring board according to the present invention. The wiring board includes a conductor pattern  32  formed on a board  31  and the insulating film  40  that covers at least a part of the conductor pattern  32 , in which the insulating film  40  includes a first insulating film  41  and a second insulating film  42 . 
     Although the overall structure is not illustrated in  FIG. 5A , the first insulating film  41  is provided in a first region f 1  on the board  31  that covers at least a part of the conductor pattern  32 . The second insulating film  42  is provided in a second region f 2  on the board  31  that covers at least a part of the first region f 1 . 
     In  FIG. 5A , reference character e 1  represents the border segment (first border segment) of the first region f 1  and reference character e 2  represents the border segment (second border segment) of the second region f 2 . In this example, the border segment e 2  is disposed outside the first region f 1 , and the shortest distance d from any point belonging to the border segment e 2  to the border segment e 1  is not more than 400 μm. 
     The insulating film  40  including the first insulating film  41  and the second insulating film  42  described above is formed by flexographic printing, and the forming steps of the insulating film  40  will be sequentially described below. 
     (1) First Printing Step 
     The first printing step transfers the first ink of insulating film material to the first region f 1  from a first flexographic plate. 
     (2) First Curing Step 
     The first curing step cures the first ink of insulating film material transferred to the first region f 1 . This forms the first insulating film  41  and forms a protuberance  41   a  of the film thickness extending along the border segment e 1  on the first insulating film  41 . 
     (3) Second Printing Step 
     The second printing step transfers the second ink of insulating film material to the second region f 2  from a second flexographic plate. 
     (4) Second Curing Step 
     The second curing step cures the second ink of insulating film material transferred to the second region f 2 . This forms the second insulating film  42 , that is, completes the insulating film  40  including the first insulating film  41  and the second insulating film  42 . 
     In the steps described above, the first insulating film material and the second insulating film material have the same composition in this example and are insulating film materials such as, for example, polyimide, epoxy resin, and acrylic resin. The first and second inks of insulating film materials are cured only by heat treatment. Specifically, both the first curing step and the second curing step include the following two processes. 
     1) Temporal drying for three minutes at 60° C. 
     2) Final drying for ten minutes at 160° C. 
     When the positional relationship between the border segments e 1  and e 2  of the first and second regions f 1  and f 2  in which the first and second insulating films  41  and  42  are respectively formed is defined and the second ink of insulating film material is transferred to the area that covers the border segment e 1  from above the cured first insulating film  41  and extends to the outside as described above, the second ink of insulating film material compensates for the height difference of the film thickness of the first insulating film  41  due to the fluidity thereof, completely cancels the protuberance  41   a  of the film thickness existing near the border segment e 1  of the first insulating film  41 , and buries the protuberance  41   a.    
     Such an action of the second ink of insulating film material is obtained because the surface of the cured first insulating film  41  has an affinity for the second ink of insulating film material while the surface of the board  31  has no affinity for the ink. 
     That is, because of the noticeable contrast between good wettability to the surface of the cured first insulating film  41  and good repelling to the surface of the board  31  which forms a large contact angle at the border segment e 2 , the second ink of insulating film material climbs over the protuberance  41   a  of the first insulating film  41  with a low resistance from above the first insulating film  41  and flows well to and stays in the area on the surface of the board  31  between the border segments e 1  and e 2 . 
     As described above, in this example, the second insulating film  42  exerts a high leveling effect, and the surface shape near the border segment of the insulating film  40  eventually descends gently, becomes substantially horizontal once in the middle, and descends gently again as illustrated in  FIG. 5A , or the surface shape descends while drawing a monotonous slope as illustrated in  FIG. 5B . A protuberance of the film thickness is not formed basically near the border segment e 2  of the second insulating film  42  and does not have a significant size even if a protuberance is formed. Accordingly, no valleys are formed on the surface of the insulating film  40  in this example. It should be noted that the maximum value of the protuberance  41   a  of the film thickness formed on the first insulating film  41  is generally not less than one and half times the minimum value of the film thickness of the first insulating film  41 . 
     The distance (shortest distance) d between the border segments e 1  and e 2  is not more than 400 μm in the example described above. When the distance d exceeds 400 μm, the influence of the second ink of insulating film material that has climbed over the protuberance  41   a  of the first insulating film  41  from above the first insulating film  41  and flowed does not reach the portion near the border segment e 2  and the protuberance of the film thickness along the border segment e 2  of the second insulating film  42  becomes apparent. Accordingly, the insulating film  40  having no valleys on the surface described above can be obtained satisfactorily under the condition that the distance d is not more than 400 μm. It should be noted that the distance d between the border segments e 1  and e 2  is preferably set to be not too small and rather large within the range not more than 400 μm. 
       FIG. 6  illustrates the state in which the optical clear adhesive layer  23  is provided by pasting an optical clear adhesive sheet to the area on the board  31  including the border segments e 1  and e 2 , and the cover  24  is mounted via the optical clear adhesive layer  23  as in  FIG. 3  described above in a wiring board having the structure illustrated in  FIG. 5A . The optical clear adhesive sheet is a sheet formed by sandwiching an optical clear adhesive film between two separators (two pieces of release paper) and is attached by peeling off the separators. Since no valleys are present on the surface of the insulating film  40 , air bubbles are not generated in valleys unlike conventional cases and a good adhesive state can be obtained. 
       FIG. 7  illustrates a capacitance type touch panel as a specific example of the wiring board produced by the production method of the wiring board according to the present invention. The touch panel has a rectangular clear board  50  on which a plurality of first sensor electrode rows  61  and a plurality of second sensor electrode rows  62  are formed. The plurality of first sensor electrode rows  61  extend in the X-direction parallel to the short sides of the clear board  50  and are disposed in parallel in the Y-direction parallel to the long sides of the clear board  50 . The plurality of second sensor electrode rows  62  extend in the Y-direction and are disposed in parallel in the X-direction. 
     A lead-out wire  71  is drawn from one end of each of the first sensor electrode rows  61  and a lead-out wire  72  is drawn from one end of each of the second sensor electrode rows  62 . These lead-out wires  71  and  72  extend to terminal portions  73  formed near the middle of one short side of the clear board  50 . 
     Each of the first sensor electrode rows  61  includes a plurality of island electrodes  61   a  arranged in the X-direction and coupling portions  61   b  connecting adjacent island electrodes  61   a  and each of the second sensor electrode rows  62  includes a plurality of island electrodes  62   a  arranged in the Y-direction and coupling portions  62   b  connecting adjacent island electrodes  62   a . Although  FIG. 7  illustrates only the outer shapes of these components, in the touch panel according to the embodiment, the first and second sensor electrode rows  61  and  62  (specifically, the island electrodes  61   a  and the coupling portions  61   b , and the island electrodes  62   a  and the coupling portions  62   b ) are configured by a conductive thin wire mesh having been printed and formed. 
       FIG. 8A  illustrates an enlarged view of the portion in which the lead-out wire  72  is drawn from the second sensor electrode row  62  and  FIG. 8B  illustrates the sectional structure of the touch panel so as to correspond to  FIG. 8A . 
     As illustrated in  FIG. 8B , the touch panel has the structure in which a first conductor pattern  81 , an insulating film  90 , and a second conductor pattern  82  are stacked in sequence on the clear board  50 . In addition, although not illustrated in  FIGS. 7 and 8A , a cover  84  is mounted on the second conductor pattern  82  via an optical clear adhesive layer  83 . The plurality of first sensor electrode rows  61 , the plurality of lead-out wires  71  and  72 , and the plurality of terminal portions  73  belong to the first conductor pattern  81 , and the plurality of second sensor electrode rows  62  belong to the second conductor pattern  82  that is insulated from the first conductor pattern  81  by the insulating film  90 . The first sensor electrode rows  61  and the second sensor electrode rows  62  intersect each other so as to be insulated from each other, and the coupling portions  61   b  and  62   b  are located so as to overlap each other. 
     The insulating film  90  includes a first insulating film  91  and a second insulating film  92 , and the positional relationship between the border segment e 1  of the first region in which the first insulating film  91  is provided and the border segment e 2  of the second region in which the second insulating film  92  is provided is the same as the positional relationship described with reference to  FIG. 5A . 
     The connection between the second sensor electrode rows  62  and the lead-out wires  72  is performed via portions of through-holes  93  formed in the insulating film  90 , and the through-holes  93  are provided at the lower ends in the Y direction of the second sensor electrode rows  62  as illustrated in  FIG. 7 . The details on the portions of the through-holes  93  will be described below with reference to  FIGS. 8A and 8B . 
     The border segment e 1  of the first region in which flexographic printing of the first insulating film  91  is performed forms the first closed curve as illustrated in  FIG. 8A  and the border segment e 2  of the second region in which flexographic printing of the second insulating film  92  is performed forms the second closed curve located inside the first closed curve, so the insulating film  90  including the first insulating film  91  and the second insulating film  92  is provided with the through-hole  93  defined by the second closed curve. 
     A connection portion  72   a  of the lead-out wire  72  belonging to the first conductor pattern  81  is located in the through-hole  93 . The second conductor pattern  82  is printed and formed on the insulating film  90  and in the through-hole  93 , and a connection portion  62   c  extending from the second sensor electrode row  62  belonging to the second conductor pattern  82  is located in the through-hole  93 . The connection portion  72   a  and the connection portion  62   c  are directly superposed in the through-hole  93  and connected to each other, thereby connecting the second sensor electrode row  62  and the lead-out wire  72  to each other. Although  FIGS. 7 and 8A  illustrate only the outer shapes of these components, in the touch panel according to the embodiment, the connection portion  72   a  and the connection portion  62   c  are configured by a conductive thin wire mesh having been printed and formed. 
     As described above, the touch panel has the structure in which the through-hole  93  is provided in the insulating film  90 , the connection portion  72   a  belonging to the first conductor pattern  81  is located in the through-hole  93 , the second conductor pattern  82  is formed on the insulating film  90  and in the through-hole  93 , the connection portion  62   c  belonging to the second conductor pattern  82  is located in the through-hole  93 , and the connection portion  72   a  and the connection portion  62   c  are directly superposed in the through-hole  93  and connected to each other. In this touch panel, the border segment e 1  of the first region f 1  is the first closed curve in which the connection portion  72   a  is located in the above-described first printing step and the border segment e 2  of the second region f 2  is the second closed curve located inside the first closed curve in the above-described second printing step. 
     Then, after the second curing step described above, the touch panel is produced by printing and forming the second conductor pattern  82  in which the connection portion  62   c  is located inside the second closed curve in the step of forming the second conductor pattern  82 . 
     Furthermore, in this touch panel, after the step of forming the second conductor pattern  82 , the optical clear adhesive layer  83  is provided in the area on the clear board  50  including the border segment e 1  and the border segment e 2  by pasting an optical clear adhesive sheet as in  FIG. 6  described above and the cover  84  is mounted via the optical clear adhesive layer  83 . 
     The details on the portion of the through-hole  93  provided in the insulating film  90  has been described above using a touch panel as an example. Since no valleys are present on the surface near the border segment surrounding the through-hole  93  of the insulating film  90 , the insulating film  90  being formed by superposition printing of flexographic printing and including the first insulating film  91  and the second insulating film  92 , the second conductor pattern  82  can be printed and formed successfully, and a problem such as generation of air bubbles around the through-hole  93  does not occur even when the optical clear adhesive layer  83  is disposed. 
     It should be noted that the cover  84  as an optical member is mounted via the optical clear adhesive layer  83  in the touch panel described above, but a display device may be mounted via the optical clear adhesive layer  83  depending on the disposition form of the touch panel. 
     The foregoing description of the embodiment of the invention has been presented for the purpose of illustration and description. It is not intended to be exhaustive and to limit the invention to the precise form disclosed. Modifications or variations are possible in light of the above teaching. The embodiment was chosen and described to provide the best illustration of the principles of the invention and its practical application, and to enable one of ordinary skill in the art to utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. All such modifications and variations are within the scope of the invention as determined by the appended claims when interpreted in accordance with the breadth to which they are fairly, legally, and equitably entitled.