Abstract:
A process for manufacturing a flexible wiring board according to the present invention includes growing metal bumps ( 16 ) using a mask film patterned by photolithography. Fine openings are formed in a polyimide film with good precision allowing fine metal bumps ( 16 ) to be formed with good precision. After metal bumps ( 16 ) have been formed, the mask film is removed and a liquid resin material is applied and dried to form a coating, which is then cured into a resin film. The coating can be etched at surface portions during coating stage to expose the tops of metal bumps ( 16 ).

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to the field of flexible wiring boards, particularly to a process for manufacturing a flexible wiring board capable of forming fine metal bumps and the flexible wiring board manufactured thereby. 
     2. Description of Related Art 
     Recently, there is an increasing demand for miniaturized semiconductor devices and a great importance is placed on flexible wiring boards on which a bare-chip semiconductor can be mounted. 
     FIGS.  4 ( a )-( d ) is a processing diagram showing a process for manufacturing a flexible wiring board of the related art. Referring to the processing diagram, the process is explained in order. At first, a copper foil is applied on a polyimide film  111  and then the copper foil is patterned into a copper wiring  112  (FIG.  4 ( a )). 
     Then, the surface of polyimide film  111  is irradiated with laser beam  114  (FIG.  4 ( b )) to form openings  115  having a predetermined diameter (FIG.  4 ( c )). At this stage, the top surface of copper wiring  112  is exposed at the bottoms of openings  115 , and then copper wiring  112  is plated with copper while the bottom surface is protected with a resin film  117  so that copper grows in openings  115  to form metal bumps  116 . 
     When a bare-chip semiconductor device is to be mounted on such a flexible wiring board  110 , an anisotropic conductive film is applied on metal bumps  116  and bonding pads of the semiconductor device are brought into contact with metal bumps  116  via the anisotropic conductive film and pressure is applied. Then, circuits within the semiconductor device contact copper wiring  112  via the anisotropic conductive film and metal bumps  116 . 
     Flexible wiring boards of this type  110  are recently much used because they are thin and light and freely foldable to provide a high mounting flexibility. 
     However, residues of polyimide film  111  remain on the top surface of metal wiring  112  exposed at the bottoms of openings  115  when openings  115  are formed with laser beam  114  as described above. Residues are removed by immersing the assembly in a chemical solution after openings  115  have been formed. However, it becomes more difficult for the chemical solution to penetrate into openings  115  as openings  115  become finer, and therefore more difficult to remove residues. 
     If residues cannot be removed, copper deposition speed varies from opening  115  to opening  115 , whereby homogeneous metal bumps  116  cannot be formed. 
     Another problem is variation in the diameter of fine openings  115  (about 40 μm to 50 μm) formed by irradiating a rigid polyimide film  111  with laser beam  114 , resulting in variation in the diameter and height of metal bumps  116  which causes failure of connection with semiconductor chips. 
     Still another problem is that it is difficult to reduce the spot diameter of high power laser beam  114 , which makes it impossible to form openings  115  having a diameter smaller than 40 μm, contrary to the recent demand for finer openings  115 . 
     SUMMARY OF THE INVENTION 
     An object of the invention is to provide a technique capable of forming fine metal bumps with good precision to overcome the above disadvantages of the related art. 
     To attain the above object, the invention provides a process comprising the steps of forming a mask film, patterned by exposure and development, on a metal foil and growing metal bumps on the metal foil exposed at the bottoms of openings in the mask film. 
     In the invention, the step of growing metal bumps is followed by the steps of removing the mask film, applying a liquid resin material to form a resin material coating on the surface of the metal foil on which the metal bumps have been formed, and then curing the resin material coating into a resin film. 
     In the invention, the resin material coating may consist of a plurality of layered coatings. 
     When the resin material coating consists of a plurality of layered coatings, at least the uppermost coating may be a thermoplastic coating. 
     In the invention, the surface of the resin material coating on the metal foil may be located below the height of the metal bumps. 
     In the invention, the height of said metal bumps from the surface of the resin film may be 35 μm or less. 
     In the invention, the curing step may be preceded by the step of etching surface portions of the resin material coating. 
     In the invention, the resin material may be a liquid containing a polyimide precursor to form the resin film from a polyimide. 
     In the invention, the step of forming a resin film may be followed by the step of partially etching the metal foil from the bottom surface to form a patterned metal wiring. 
     In this case, a support film may be formed on the bottom surface of the metal wiring. 
     In the invention, the support film may be partially etched to expose desired regions of the metal wiring. 
     Said process may further comprise the steps of bringing bonding lands of a semiconductor chip into contact with the metal bumps and applying heat and pressure to allow the resin film to develop adhesiveness, whereby the semiconductor chip is bonded to flexible wiring board. 
     The invention also provides a flexible wiring board manufactured by the process as defined above. 
     Flexible wiring boards of the invention include those having a semiconductor device connected to the metal bumps. 
     As defined above, the invention relates to a process for manufacturing a flexible wiring board having metal bumps and the flexible wiring board manufactured thereby. 
     In the invention, an exposable dry film or resist film is applied or deposited on a metal foil and patterned by exposure and development to form a mask film. 
     The metal foil is exposed at the bottoms of openings in the mask film, so that metal bumps grow at exposed regions of the metal foil when the metal foil is immersed in a plating solution while its bottom surface is protected. 
     The openings in the mask film can be formed in a fine size with high precision by photolithography. Therefore, the metal bumps can also be homogeneously grown both in width and height. 
     Then, a liquid resin material is applied and dried or otherwise treated to form a resin material coating on the surface of the metal foil on which the metal bumps have been formed, after which the resin material coating is heated or otherwise cured into a resin film, whereby the surface of the metal foil on which the fine metal bumps have been formed can be covered with the resin film. If the resin material coating has a thickness smaller than the height of metal bumps, the tops of the metal bumps may project from the surface of the resin film without post-treatment. 
     If the resin material cover the tops of the metal bumps, the resin film is also formed by curing on the surfaces of the metal bumps, which can be, however, exposed by polishing or etching. 
     If etching is used, an uncured resin material coating can be etched to form a resin film with the tops of the metal bumps being exposed. 
     The resin film may be a thermosetting or thermoplastic film or a laminate of such films as far as it is flexible. From the viewpoint of durability or reliability, it is preferable that the resin material is a polyimide precursor to be cured into a polyimide film. 
     After the resin film has been formed, the bottom surface of the metal foil can be exposed and etched using a dry film or photoresist as a mask to give a copper wiring. Then, a support film can be formed on the bottom to protect the copper wiring, whereby a flexible wiring board having reliable insulating properties is obtained. 
     The resin film can be formed to have a multilayer structure by layering resin material coatings. If the uppermost layer of the resin film consists of a thermoplastic resin, the thermoplastic resin film develops adhesiveness upon heating to ensure bonding to a semiconductor device or the like without using anisotropic conductive film. 
     The support film may be formed by applying a sheet-like film or coating a resin material solution as defined above and curing it. The support film may be patterned to partially expose desired regions of the metal wiring for forming contact regions for connection with another flexible wiring board or contact regions for wire bonding. 
     Variation in the height of metal bumps grown by electroplating increases with size. Experiments show that the variation is limited to ±3 μm when the height above the surface of the resin film is 35 μm or less in contrast to ±5 to ±7 μm observed when said height is 40 μm. When a non-flexible material such as a semiconductor chip is to be connected to metal bumps, the yield is more influenced by variation than bump height. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS.  1 ( a )- 1 ( p ) are a processing diagram illustrating an example of process of the present invention. 
     FIG. 2 is a perspective view of a metal bump and a metal wiring. 
     FIG.  3 ( a ) is a surface microphotograph of metal bumps and their vicinities on which a resin material coating has been formed. 
     FIG.  3 ( b ) is a sectional microphotograph of one of the metal bumps. 
     FIG.  3 ( c ) is a surface microphotograph of metal bumps and their vicinities on which a resin film has been formed. 
     FIG.  3 ( d ) is a sectional microphotograph of one of the metal bumps. 
     FIGS.  4 ( a )- 4 ( d ) are a processing diagram illustrating a process for manufacturing a flexible wiring board of the related art. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The invention will now be described with reference to the attached drawings. 
     FIGS.  1 ( a )- 1 ( p ) are a processing diagram illustrating a process of the invention. The reference  2  in FIG.  1 ( n ) represents an example of flexible wiring board of the invention manufactured by the process, and the reference  30  in FIG.  1 ( p ) represents the flexible wiring board  2  having a semiconductor chip  31  connected thereto. 
     Referring to FIG.  1 ( a ), a metal foil  11  (a rolled copper foil having a thickness of 18 μm here) is initially prepared, and a protective film  12  is applied on the bottom surface and a UV-exposable mask film  13  (dry film SPG- 152  made by Asahi Chemical Industry Co., Ltd.) is applied on the top surface (at a temperature of 130° C. and a line speed of 2 m/min here) (FIG.  1 ( b )). 
     Then, mask film  13  is exposed to light (exposure light intensity 100 mJ) through a glass mask having a predetermined pattern and developed with a chemical solution to form openings  15  at locations corresponding to a plurality of metal bumps  16  described below (FIG.  1 ( c )). Openings  15  can be formed with a precision within ±2.5 μm in diameter and a precision within ±2 μm in height using a mask having a circular pattern of 30 to 50 μm in diameter. 
     Then, the assembly is immersed in an electrolyte for copper plating and electric current is applied to grow copper into metal bumps  16  on the top surface of metal foil  11  exposed at the bottoms of openings  15  (FIG.  1 ( d )). Metal bumps  16  standing on a plurality of openings  15  have a homogeneous height with good precision because no residues remain on the top surface of metal foil  11  exposed at the bottoms of openings  15  after development. Instead, a clean surface is exposed. 
     Then, mask film  13  and protective film  12  are removed with an alkali (FIG.  1 ( e )). At this stage, a plurality of mushroom-like metal bumps  16  are upright on the top surface of metal foil  11 . A carrier film  18  is applied on the bottom of metal foil  11  (FIG.  1 ( f )), and then a resin material consisting of a polyimide precursor is applied on the top surface of metal foil  11  and dried to form a resin material coating  20  consisting of the polyimide precursor (FIG.  1 ( g )). 
     This resin material coating  20  is convex on metal bumps  16  and their vicinities, but flat away from metal bumps  16 . The thickness of flat regions is smaller than the height of metal bumps  16  so that the tops of metal bumps  16  may project from flat regions on resin material coating  20 . 
     If resin material coating  20  is too thin with a single application, an additional resin material consisting of a polyimide precursor may be applied on the previously formed resin material coating  20  and dried to layer a second resin material coating thereon. The reference  21  in FIG.  1 ( h ) represents such a second resin material coating layered on resin material coating  20 . The upper resin material coating  21  here is thermoplastic, contrary to the lower resin material coating  20 . 
     A surface microphotograph of vicinities of metal bumps  16  at this stage is shown in FIG.  3 ( a ). A sectional microphotograph is shown in FIG.  3 ( b ). The tops of metal bumps  16  are covered with resin material coatings  20 ,  21 . 
     Then, an alkaline solution is sprayed on resin material coatings  20 ,  21  to etch the surface. Here, a depth of 2-5 μm from the surface is etched by spraying at 25° C. for 20 seconds to expose the tops of metal bumps  16  (FIG.  1 ( i )). A plasma cleaner may be =used for etching instead of spraying an alkaline solution. 
     Then, carrier film  18  on the bottom is removed and then resin material coatings  20 ,  21  are cured by heating (280° C. for 10 minutes) to form a resin film  23  consisting of two polyimide film layers on the top surface of metal foil  11 (FIG.  1 ( j )). 
     A surface microphotograph of metal bumps  16  at this stage is shown in FIG.  3 ( c ) and a sectional microphotograph is shown in FIG.  3 ( d ). The surfaces of the tops of metal bumps  16  are exposed, though indiscernible from FIG.  3 ( c ) and FIG.  3 ( d ). The upper layer of resin film  23  is thermoplastic so that it is not necessary to use an anisotropic conductive film for connecting a semiconductor device or the like. 
     A photosensitive resin film is applied on the bottom surface of metal foil  11  and patterned by exposure and development into a predetermined configuration to form a mask film  24  (FIG.  1 ( k )). Then, the pattern of mask film  24  is transferred to metal foil  11  by etching, to form a metal wiring  25  (FIG.  1 ( l )). 
     This metal wiring  25  has line-shaped wiring regions  25   a  and large-area contact regions  25   b  located at the bottoms of metal bumps  16 , so that metal bumps  16  can be connected to outer terminals or ICs via contact regions  25   b  and wiring regions  25   a  . 
     Mask film  24  is removed (FIG.  1 ( m )) and a polyimide precursor is applied on the exposed bottom surface of metal wiring  25  and dried and then patterned using a photosensitive resist to expose contact regions  25   b . Then, the assembly is heated and a support film  26  consisting of a polyimide is formed on the bottom of metal wiring  25  to give a flexible wiring board  2  (FIG.  1 ( n )). The height of metal bumps  16  of this flexible wiring board  2  from the surface of resin film  23  is 35 μm or less. 
     In flexible wiring board  2 , the top and bottom surface of metal wiring  25  are protected with resin film  23  and support film  26 , respectively, and the tops of metal bumps  16  project from the surface of resin film  23 . The bottoms  27  of contact regions  25   b  are exposed. 
     FIG. 2 is a perspective view of metal wiring  25  and metal bump  16 , in which polyimide films  23 ,  26  are not shown. 
     Next, a process for mounting a semiconductor chip on flexible wiring board  2 , having the structure described above, is explained. 
     FIG.  1 ( o ) shows the state in which semiconductor chip  31  is ready to be mounted on flexible wiring board  2 . A plurality of bonding pads  32  consisting of an aluminium thin film are exposed on the surface of this semiconductor chip  31 , and metal bumps  16  formed on flexible wiring board  2  are provided to face bonding pads  32 . 
     Semiconductor chip  31  is pressed against flexible wiring board  2  via each bonding pad  32  of this semiconductor chip  31  in contact with the counterpart metal bump  16 , whereby resin film  23  exposed between metal bumps  16  tightly contact the surface of semiconductor chip  31 . 
     When semiconductor chip  31  or flexible wiring board  2  is heated during the pressing step, resin film  23  develops adhesiveness to bond semiconductor chip  31  to flexible wiring board  2 . 
     When the assembly is cooled as such, semiconductor chip  31  is fixed to flexible wiring board  2  while maintaining electric connection between bonding pads  32  and metal bumps  16 . The reference  30  in FIG.  1 ( p ) represents a flexible wiring board on which semiconductor chip  31  is fixed. 
     The tops of metal bumps  16  of another flexible wiring board  2  having a similar structure may be brought into contact with contact regions  25   b  of the former flexible wiring board  2 , and the flexible wiring boards  2  are connected together by means of adhesiveness of resin film  23  of the former flexible wiring board  2 . 
     Table 1 below shows the relation between bump height and connection failure when an IC chip (a kind of semiconductor chip) is connected to bumps  16  of flexible wiring board  2  or when flexible wiring boards  2  are connected together (connection between bumps  16  and bottoms  27  of contact regions  25   b ) PCT (Pressure Cooker Test) was performed under conditions of 121° C., 2 atm. for 24 hours. All heights of 35 μm or less passed PCT without showing any failure point even after PCT. 
     
       
         
               
             
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Bump height and connection results 
               
             
          
           
               
                   
                   
                   
                   
                   
                 Comparative 
                 Comparative 
                 Comparative 
               
               
                 Description 
                 Example 1 
                 Example 2 
                 Example 3 
                 Example 4 
                 example 1 
                 example 2 
                 example 3 
               
               
                   
               
             
          
           
               
                 Bump height (μm) 
                 0 
                 10 
                 10 
                 35 
                 37 
                 40 
                 55 
               
               
                 Range of variation 
                 1 
                 1 
                 1 
                 2 
                 3 
                 4 
                 6 
               
               
                 in bump height (μm) 
               
               
                 Device bonded 
                 Wiring 
                 Wiring 
                 IC chip 
                 Wiring 
                 Wiring 
                 IC chip 
                 Wiring 
               
               
                   
                 board 
                 board 
                   
                 board 
                 board 
                   
                 board 
               
               
                 Number of success 
               
               
                 points among 25 
               
               
                 connection points 
               
               
                 Before PCT 
                 25 
                 25 
                 25 
                 25 
                 25 
                 25 
                 18 
               
               
                 After PCT 
                 25 
                 25 
                 25 
                 25 
                 24 
                 16 
                 0 
               
               
                 Connection result 
                 Pass 
                 Pass 
                 Pass 
                 Pass 
                 Fail 
                 Fail 
                 Fail 
               
               
                   
               
             
          
         
       
     
     In flexible wiring board  2  of the invention as described above, a resin film is formed after metal bumps  16  are formed, therefore, it is not necessary to form openings in the resin film with laser beam. Thus, fine metal bumps can be formed with good precision. 
     Although copper was grown by plating to form metal bumps  16  in the above example, other metals may also be used. Metal foil  11  is not limited to copper, either. Resin coatings  23 ,  26  may have a monolayer structure or a two-layer structure and may not be formed from a polyimide. 
     It is preferable to form a gold coating (thickness of about 1-2 μm) by plating or other means on the surfaces of metal bumps  16  consisting of copper. A chip-like semiconductor can be connected to such metal bumps  16  via an anisotropic conductive film or the like to prepare a circuit component. 
     Metal bumps formed on another flexible wiring board can also be connected to contact regions  25   b  to connect flexible wiring boards together. Therefore, a plurality of flexible wiring boards of the invention can be layered. 
     In the invention, fine metal bumps can be formed with good precision. 
     A desired shape of opening (for example, square or hexagonal) can be formed because laser beam is not used. 
     The selection of bump height of 35 μm or less decreases variation in bump height to reduce failure of connection with non-flexible semiconductor chips such as IC chips.