Abstract:
The present invention involves forming wiring sections on a substrate capable of UV-ray transmission, and fixing a component to be electrically connected to the wiring sections onto the substrate using a UV-curable ACF. When doing so, fluidity in the UV-curable ACF is produced by applying pressure to the component, and UV rays are directly projected from the surface of the substrate even onto the UV-curable ACF areas shielded by the wiring sections. The UV-curable ACF is made to be fluid by applying pressure to the component after increasing fluidity of the UV-curable ACF by heating.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a method of fixing a component, a circuit substrate including a component fixed by the method, and a display panel including the circuit substrate. 
       BACKGROUND ART 
       [0002]    A display panel such as a liquid crystal display panel or an organic EL display panel is assembled into a module along with components such as a driver IC for driving it and a flexible printed circuit (FPC). Generally, a display panel is assembled into a module by a method in which, to an electrode formed on a substrate surface of a display panel, a component is electrically connected and simultaneously physically fixed by an anisotropic conductive film (hereinafter referred to as ACF in the present specification and the appended claims). One example is disclosed in Patent Document 1. 
         [0003]    According to the production method of a liquid crystal display device described in Patent Document 1, an ACF that includes as adhesive a resin that cures with either ultraviolet rays (UV light) or heat is used for connecting a TCP (tape carrier package) or a flexible printed circuit to a liquid crystal display element. First, connection electrodes of the liquid crystal display element and electrodes of the TCP or the flexible printed circuit are placed opposite each other; then, a pressure is applied; then, an adhesive layer of the ACF is irradiated for a predetermined time with a predetermined amount of UV light from the liquid crystal display element side. Thus, the adhesive of the ACF in an area where the connection electrodes of the liquid crystal display element do not shield light hardens through a photo-curing reaction. Then, the TCP or the flexible printed circuit is subjected to permanent thermocompression; here, the cured adhesive suppresses a flow in the conductive film, and this ensures that the electrodes of the TCP or the flexible printed circuit conduct to the connection electrodes of the liquid crystal display element. 
       LIST OF CITATIONS 
     Patent Literature 
       [0004]    Patent Document 1: JP-A-2000-105388 
       SUMMARY OF THE INVENTION 
     Technical Problem 
       [0005]    With the production method of a liquid crystal display device described in Patent Document 1, the ACF is cured with both UV light and heat and, inconveniently, the heat causes a component and a substrate to warp. Nowadays, liquid crystal display panels are in a trend toward an ever smaller area outside a display portion so as to attain a narrow frame, and thus the distances between the display portion and a driver IC and between the driver IC and a FPC are becoming increasingly small. The smaller distances cause the heat for curing the ACF to produce adverse effects. For example, display quality can deteriorate, and the driver IC and the FPC can have lower connection reliability. 
         [0006]    Thus, recently, an UV-curable ACF, with which curing progresses only with UV light, is increasingly used. Using an UV-curable ACF makes it possible to connect and fix a component at comparatively low temperature, and thus makes it less likely for heat to damage a component or a substrate. Moreover, it is possible to reduce the time required to raise temperature to a target temperature, and thus, advantageously, it is possible to improve production efficiency. 
         [0007]    On the other hand, an UV-curable ACF causes the following inconveniences. If part of an ACF is located at where a conductor (in the present specification, the term “conductor” is used to cover both electrodes for electrically connecting a component and those for connecting electrodes together) shields light, that part, if it is a part of the ACF described in Patent Document 1, can be cured by heating but, if it is a part of an UV-curable ACF, cannot be cured by a process of heating. The part of the ACF located at where light is shielded does cure to some degree with UV light propagating inside the ACF by reflection, but it cures so incompletely as to be evaluated as uncured. 
         [0008]    An uncured ACF does not function properly and rather produces adverse effects, such as low adhesion between a component and a substrate, high electric resistance due to small curing shrinkage, and change in hygroscopicity. The uncured ACF readily absorbs moisture, inconveniently leading to corrosion of a metal conductor on a substrate surface, and high electric resistance resulting from the substrate swelling by absorbing moisture. To achieve component mounting with high reliability, an ACF needs to be cured with a rate of reaction higher than a given rate in every part of it. The given rate, though depending on the type of ACF, is generally eighty percent or more. 
         [0009]    Devised against the background discussed above, the present invention is directed to the fixing of a component by use of an UV-curable ACF, and aims to leave no part of the ACF uncured by irradiating even a part of the UV-curable ACF located at where a conductor shields light directly with UV light. 
       Means for Solving the Problem 
       [0010]    According to one aspect of the present invention, a method of fixing a component, involving forming a conductor on a substrate that transmits UV light, and electrically connecting the component to the conductor and simultaneously fixing the component to the substrate with a UV-curable ACF, includes the steps of: producing a flow in the UV-curable ACF by applying a pressure to the component, and irradiating the UV-curable ACF with UV light from the reverse side of the substrate such that even part of the UV-curable ACF located at where the conductor shields the UV light is directly irradiated by the UV light. 
         [0011]    The above-described method of fixing a component preferably further includes a step of: applying a pressure to the component after increasing the fluidity of the UV-curable ACF by heating. 
         [0012]    The above-described method of fixing a component preferably further includes a step of: performing irradiation with the UV light before the UV-curable ACF starts to flow. 
         [0013]    The above-described method of fixing a component preferably further includes a step of: performing irradiation with the UV light while the UV-curable ACF is flowing. 
         [0014]    According to another aspect of the present invention, a circuit substrate includes a component fixed by any of the methods described above. 
         [0015]    In the circuit substrate described above, preferably, the conductor to which the component is connected is arranged displaced from the center of a mounting portion of the component. 
         [0016]    In the circuit substrate described above, preferably, the conductor to which the component is connected has an opening formed at a place aligned with a center of a mounting portion of the component. 
         [0017]    In the circuit substrate described above, preferably, the conductor to which the component is connected has a plurality of openings formed in a distributed fashion in an area that includes the center of a mounting portion of the component. 
         [0018]    According to yet another aspect of the present invention, a display panel includes any of the circuit substrates described above. 
       Advantageous Effects of the Invention 
       [0019]    According to the present invention, a flow is produced in a UV-curable ACF by applying a pressure to a component, and even a part of the UV-curable ACF located at where a conductor shields light is directly irradiated with UV light from the reverse side of a substrate. It is thus possible to leave no part of the ACF unirradiated with UV and hence uncured, and thereby to overcome inconveniences resulting from part of the UV-curable ACF remaining uncured. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0020]    [ FIG. 1 ] A first diagram illustrating a method of fixing a component according to the present invention. 
           [0021]    [ FIG. 2 ] A second diagram illustrating a method of fixing a component according to the present invention. 
           [0022]    [ FIG. 3 ] A third diagram illustrating a method of fixing a component according to the present invention. 
           [0023]    [ FIG. 4 ] A fourth diagram illustrating a method of fixing a component according to the present invention. 
           [0024]    [ FIG. 5 ] A diagram illustrating an electrical connection by an UV-curable ACF. 
           [0025]    [ FIG. 6 ] A first Gantt chart illustrating a timing of irradiation with UV light. 
           [0026]    [ FIG. 7 ] A second Gantt chart illustrating a timing of irradiation with UV light. 
           [0027]    [ FIG. 8 ] A third Gantt chart illustrating timing of irradiation with UV light. 
           [0028]    [ FIG. 9 ] A diagram illustrating a flow in an UV-curable ACF. 
           [0029]    [ FIG. 10 ] A plan view showing a first configuration example of a conductor. 
           [0030]    [ FIG. 11 ] A plan view showing a second configuration example of a conductor. 
           [0031]    [ FIG. 12 ] A plan view showing a third configuration example of a conductor. 
           [0032]    [ FIG. 13 ] A plan view showing an undesirable configuration example of a conductor. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0033]    First, with reference to  FIGS. 1 to 4 , an outline of a component-fixing method according to the present invention will be described. 
         [0034]    A component-fixing method according to the present invention is performed by use of a component-fixing device  1 . A main component of the component-fixing device  1  is a horizontal stage  2 . The stage  2  has a structure in which a stage central portion  2   a  made of a material that transmits UV light, such as glass, is supported by a stage peripheral portion  2   b  made of metal. 
         [0035]    As shown in  FIG. 1 , on the stage central portion  2   a,  a substrate  10  that transmits UV light is placed. As an example of the substrate  10 , a TFT glass substrate of a liquid crystal display panel is shown. On a surface of the substrate  10 , a conductor  11  made of a metal with low electric resistance is formed, and a UV-curable ACF  12  is adhered so as to cover the conductor  11 . The conductor  11  does not transmit light, and thus serves as a light-shielding portion for UV light. 
         [0036]    The UV-curable ACF  12  is supplied in a form, like double-sided adhesive tape, adhered to an unillustrated separator and wound into a reel. Whereas the separator is in a form of continuous tape, the UV-curable ACF  12  has splits at predetermined intervals. A portion of the UV-curable ACF  12  with a predetermined length from one split to the next is put in contact with the substrate  10 , and heat and pressure are applied to the portion from above across the separator. In this way, while the UV-curable ACF  12  adheres to the substrate  10 , the separator separates from the UV-curable ACF  12 , leaving only the UV-curable ACF  12  neatly transferred to the substrate  10 . 
         [0037]    As shown in  FIG. 2 , a component is mounted on a top surface of the UV-curable ACF  12 . Although the component may be an FPC or a TCP, here, an IC  13  is shown as an example. The IC  13  is mounted on the conductor  11  by a COG (chip on glass) process. At this stage, the IC  13  is simply positioned within a horizontal plane and placed lightly on the UV-curable ACF  12 . Bumps  13   a  are formed on a bottom surface of the IC  13  to serve as terminals. The UV-curable ACF  12  serves to electrically connect the bumps  13   a  to the conductor  11  and to physically fix the IC  13  to the substrate  10 . 
         [0038]    As shown in  FIG. 3 , the IC  13  is heated by a heater tool  14 . The heating reduces the viscosity of the UV-curable ACF  12 , which thus liquefies; that is, the fluidity of the UV-curable ACF  12  increases. After so heating the UV-curable ACF  12  and increasing its fluidity, a pressure is applied to the UV-curable ACF  12  by the heater tool  14  to make the UV-curable ACF  12  flow. It is preferable that the IC  13  be heated so as to raise the temperature of the UV-curable ACF  12  to 70° C. to 100° C. The pressure applied to the UV-curable ACF  12  can be approximately equal to the pressure during thermocompression bonding using a thermosetting ACF. 
         [0039]    The heat from the heater tool  14  is expended not for curing the UV-curable ACF  12  but only for making it flow. This requires an amount of heat smaller than that required for curing the UV-curable ACF  12 . This allows the IC  13  to be mounted at lower temperature, and mounting it at lower temperature helps alleviate warping of the IC  13  and of the substrate  10 . In a case where the substrate  10  is one to be incorporated in a display panel, improved display quality results. 
         [0040]    The part of the fluidized UV-curable ACF  12  located at where the conductor  11  shields light is pushed out of the place by being pressed by the bumps  13   a.  As the pressing of the IC  13  progresses, the IC  13  itself begins to press the UV-curable ACF  12 , producing a large-scale flow inside the UV-curable ACF  12 . Also by this large-scale flow, the part of the UV-curable ACF  12  located at where the conductor  11  shields light is pushed out of the place. 
         [0041]    While the bumps  13   a  are approaching the conductor  11  under the pressure from the heater tool  14 , an unillustrated UV light source arranged under the stage  2  emits UV light and irradiates the UV-curable ACF  12  with the UV light from the reverse side of the substrate  10 . 
         [0042]    Not only the part of the UV-curable ACF  12  located at where the conductor  11  does not shield light, but also the part of the UV-curable ACF  12  which would stay at where the conductor  11  shields light without the flow is irradiated directly with UV light by being pushed out of the place where the conductor  11  shields light as the result of the flow. Here, “direct irradiation” refers not to irradiation with UV light propagating inside the UV-curable ACF  12  by reflection, but to irradiation with UV light from the UV light source with no interception on the way. 
         [0043]    The UV-curable ACF  12  begins to cure by being irradiated directly with UV light. Although part of the UV-curable ACF  12  is moved by the flow to where the conductor  11  shields light, this is the part of the UV-curable ACF  12  that has been located at where the conductor  11  does not shield light and it has already been irradiated directly with UV light, so that it also begins to cure. 
         [0044]    As described above, the phrase “the UV-curable ACF located at where the conductor shields light” has two meanings: it means, first, the part of the UV-curable ACF which, without the flow, would stay at where the conductor shields light but which, because of the flow, is pushed out of the place where the conductor does not shield light; second, the part of the UV-curable ACF which is located at where the conductor does not shield light but which is moved, by the flow, to where the conductor shields light. Irrespective of which part it means, the UV-curable ACF  12  is irradiated directly with UV light, and thus such part of the UV-curable ACF  12  as would otherwise be left uncured is removed. In this way, it is possible to overcome the inconvenience that could result from part of the uncured UV-curable ACF  12  remaining uncured. 
         [0045]      FIG. 4  shows a stage after completion of the heating and pressing of the IC  13  and the irradiation of the UV-curable ACF  12  with UV light. The thickness of the UV-curable ACF  12  is designed to be larger than the height of the bumps  13   a  so as to prevent the space between the IC  13  and the substrate  10  from being incompletely filled with the UV-curable ACF  12 . Thus, when the IC  13  is pressed until the bumps  13   a  approach the conductor  11 , part of the UV-curable ACF  12  becomes surplus, which has to be removed. The surplus part of the UV-curable ACF  12  is removed outside the IC  13 , and the bumps  13   a  and the conductor  11  are connected together electrically via the UV-curable ACF  12 . 
         [0046]      FIG. 5  conceptually shows how conductive particles  15  inside the UV-curable ACF  12  are pressed and flattened between the conductor  11  and the bumps  13  to establish conduction between the conductor  11  and the bumps  13 . This state is maintained owing to the UV-curable ACF  12  curing through irradiation with UV light. 
         [0047]    Now, how the setting of the timing of heating the IC  13  and irradiating it with UV light influences the fixing of the IC  13  will be described with reference to Gantt charts in  FIGS. 6 to 8 . 
         [0048]    The setting of timing shown in  FIG. 6  will be referred to as a first embodiment, the setting of timing shown in  FIG. 7  will be referred to as a second embodiment, and the setting of timing shown in  FIG. 8  will be referred to as a first comparative example. 
       First Embodiment  
       [0049]    In  FIG. 6 , irradiation with UV light starts before the IC  13  is heated. Previously irradiated with UV light, the UV-curable ACF  12  increases its fluidity through the subsequent heating, and thus flows under pressure. 
         [0050]    By irradiating the UV-curable ACF  12  with UV light before it starts to flow, it is possible to move the UV-curable ACF  12  that has absorbed sufficient UV light. However, if the UV-curable ACF  12  absorbs an excessive amount of UV light, its curing progresses and its viscosity increases. This prevents it from fluidizing through the subsequent heating. Though depending on the resin material of the UV-curable ACF  12 , the time-lag between the start of irradiation with UV light and the start of heating is preferably one second or less. 
       Second Embodiment  
       [0051]    In  FIG. 7 , irradiation with UV light starts after the IC  13  is heated. Having started to flow under pressure after heating, the UV-curable ACF  12  continues to flow while being irradiated with UV light, and absorbs UV light. When the UV-curable ACF  12  cures through irradiation with UV light, it stops flowing even before completion of heating and pressing. 
         [0052]    Though depending on the resin material of the UV-curable ACF  12 , preferably, the time of irradiation with UV light is about three to ten seconds, and the time-lag between the start of heating and the start of irradiation with UV light is one second or less. 
       First Comparative Example  
       [0053]    In  FIG. 8 , irradiation with UV light starts when the heating and pressing of the IC  13  are almost over and after the UV-curable ACF  12  stops flowing. In this case, since the flow of the UV-curable ACF  12  has stopped, the UV-curable ACF  12  irradiated with UV light does not move. 
         [0054]    If any part of the UV-curable ACF  12  cures incompletely, that is because the conductor portion  11  shields UV light and prevents it from reaching the UV-curable ACF  12 . If irradiation with UV light is performed after the UV-curable ACF  12  stops flowing as shown in  FIG. 8 , the part of the UV-curable ACF  12  located at where the conductor portion  11  shields light ends up never being irradiated directly with UV light. 
         [0055]    On the other hand, when irradiation with UV light is timed as in the first embodiment ( FIG. 6 ) and the second embodiment ( FIG. 7 ), since part of the UV-curable ACF  12  located at where the conductor  11  shields light moves, every part of the UV-curable ACF  12  passes through where it is directly irradiated with UV light. Thus, irrespective of whether irradiation with UV light is performed before the UV-curable ACF  12  starts to flow as in the first embodiment or irradiation with UV light is performed while the UV-curable ACF  12  is flowing as in the second embodiment, the part of the UV-curable ACF  12  located at where the conductor  11  shields light can be directly irradiated with UV light. 
         [0056]    When the IC  13  is heated and then pressed, in the space between the IC  13  and the substrate  10 , the UV-curable ACF  12  flows in directions indicated by arrows in  FIG. 9  and the surplus part of the UV-curable ACF  12  is removed. However, in a central part of the IC  13 , while a surplus part of the UV-curable ACF  12  moves out, very little of it moves in from elsewhere; as a result, part  12   a  of the UV-curable ACF  12  (indicated by hatching in  FIG. 9 ) is left behind from the flow. A problem associated with part  12   a  of the UV-curable ACF  12  being left behind from the flow will be described below by way of a second comparative example with reference to  FIG. 13 . Solutions to the problem will be described below by way of third, fourth, and fifth embodiments with reference to  FIGS. 10 ,  11 , and  12  respectively. 
       Second Comparative Example  
       [0057]    In  FIG. 13 , a conductor  11  that passes through a central portion of the IC  13  is so configured as to connect together the electrodes arranged on the right and left, and a part of the conductor  11  is located right under the part  12   a  of the UV-curable ACF  12  that is left behind from the flow. With this configuration, the part  12   a  of the UV-curable ACF  12  left behind from the flow remains unirradiated with UV light because of the conductor  11  shielding it, and thus ends up uncured. To avoid that, the conductor  11  can be configured ingeniously as shown in any of  FIGS. 10 to 12 . In  FIG. 13  and in  FIGS. 10 to 12 , the reference sign  11   a  identifies a signal input electrode portion which is connected to an unillustrated FPC to receive signals, and the reference sign  1  lb identifies a signal output electrode portion which is connected to an unillustrated display area of a liquid crystal display panel. Only the position of the IC  13 , which is a component to be fixed by the UV-curable ACF  12 , is shown by a dashed-line, and the area surrounded by the dashed-line is the mounting portion (COG mounting portion) of the IC  13 . 
       Third Embodiment  
       [0058]    In the configuration shown in  FIG. 10 , no conductor  11  passes through the center of the mounting portion of the IC  13 . Even a conductor  11  that passes closest to the center of the mounting portion of the IC  13  is displaced by a small distance from the central mounting portion. Thus, the part  12   a  of the UV-curable ACF  12  left behind from the flow is directly irradiated with UV light without the conductor  11  shielding it, and thus cures. 
       Fourth Embodiment  
       [0059]    In the configuration shown in  FIG. 11 , one opening  11   c  is formed in a wide conductor  11  which passes through the center of the mounting portion of the IC  13 , at a place aligned with the center of mounting portion. The opening  11   c  is in a rectangular shape and its longitudinal direction is aligned with the longitudinal direction of the conductor  11 . Thus, even the part  12   a  of the UV-curable ACF  12  left behind from the flow cures by being irradiated directly with UV light through the opening  11   c.    
       Fifth Embodiment  
       [0060]    In the configuration shown in  FIG. 12 , a plurality of openings  11   c  are arrayed in a distributed fashion in a wide conductor  11  which passes through the center of the mounting portion of the IC  13 . The plurality of the openings  11   c  are arrayed in a distributed fashion in an area that includes the center of the mounting portion. The part of the UV-curable ACF left behind from the flow cures by being irradiated directly with UV light through one of the plural of the openings  11   c  arrayed in a distributed fashion. 
         [0061]    In  FIG. 12 , rectangular openings  11   c  of which the longitudinal direction is aligned with the longitudinal direction of the conductor  11  are arrayed in three rows and in three columns, in a matrix-like formation. This arrangement, however, is only illustrative and is not meant to limit the invention. The intervals between the openings  11   c  are, both in the up/down and left/right directions in  FIG. 12 , preferably 0.5 mm or less, and more preferably 0.2 mm or less. 
         [0062]    Although in the above description a TFT glass substrate of a liquid crystal display panel is taken up as an example of a substrate  10  which becomes a circuit substrate when mounted with components, this is not meant as any limitation; the present invention may be applied to a glass substrate of an organic EL display panel. The present invention may be applied also to circuit substrates in general which are not intended for incorporation in display panels. 
         [0063]    The embodiments by way of which the present invention is described above are in no way meant to limit the scope of the present invention, which thus allows for many modifications and variations within the spirit of the present disclosure. 
       INDUSTRIAL APPLICABILITY 
       [0064]    The present invention finds wide application in display panels and in circuit substrates in general. 
       LIST OF REFERENCE SIGNS 
       [0000]    
       
           1  component-fixing device 
           2  stage 
           10  substrate 
       
     
         [0068]      11  conductor
     11   c  opening     12  UV-curable ACF     12   a  UV-curable ACF left behind from flow     13  IC     13   a  bump     14  heater tool