Abstract:
A method for forming a bump structure and a bump structure for conductive interconnection with another element having at least one of microelectronic devices or wiring thereon, used as an electric connection in an electronic circuit, includes the steps of forming a mandrel by steps including forming at least one opening extending through a bump-forming die body in the thickness direction thereof and positioning a bump-forming die lid on a surface of the bump-forming die body so as to cover one end of the opening and to thereby define a bump-forming recess. The bump-forming die body may be comprised of a metal sheet. A metal layer is formed at least on an inner surface of the bump-forming die lid exposed within the bump-forming recess. The mandrel is removed so as to expose the metal layer and form a bump structure.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims the benefit of Japanese Application Ser. No. 2007-295299, filed Nov. 14, 2007, entitled A PROCESS FOR FORMING A BUMP STRUCTURE AND A BUMP STRUCTURE, the disclosure of which is hereby incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     The present invention relates to a process for forming a bump structure in an electronic circuit implementation and the bump structure and, more particularly, relates to a process for forming a bump structure for a circuit implementation in an electronic device such as IC for use in an electronic apparatus, and relates to the bump structure. Further, the present invention relates to a bump-structure forming member for forming the bump-structure, a process for manufacturing thereof, a circuit board having bumps formed thereon and a process for manufacturing thereof. 
     Conventionally, as a method for connecting an IC chip or the like having a plurality of connecting terminals to a printed wiring board (PWB) using the bumps in order to intensify the implementation density of an electronic circuit, land grid array (LGA) type connecting style has been known. If one surface of a chip or an intermediate board called an interposer or a substrate is loaded with an IC in order to connect a terminal or a wiring on the PWB to a wiring of a chip like the IC, the bumps for securing electrical connection with the PWB are formed on one surface of that interposer. 
     To construct the PWB in a high density, a multilayered board, such as a build-up board has been known. Using this, an electronic circuit component is mounted on each of both faces of a circuit board according to, for example, surface mount technology. Alternatively, in order to make conductive electric wirings of respective layers between adjacent layers or between desired layers, an interlayer connecting portion is provided via holes called via-hole provided for securing conductivity. A method of using the bumps as this interlayer connecting portion has been known (for example, Japanese Patent Application Laid-Open No. 2002-359471). 
     As a process for forming the bumps in these cases, for example, a process by electric plating using plating resist such as dry film resist (DFR) has been known. According to an example of this process, first, plating resist having an opening is provided on a conductive layer of copper foil or the like. Next, metal such as copper may be deposited in this opening by electric plating. By removing the plating resist after the electric plating treatment, the bumps projecting from the conductive layer are obtained. 
     As well as the process by the electric plating using plating resist described above, a process for forming the bump by etching has been known (for example, Japanese Patent Application Laid-Open No. 2003-309370). 
     An example of the process for forming the bump structure by etching will be described briefly with reference to  FIG. 19 . 
     As shown in  FIG. 19(   a ), a multilayer metal sheet  200  is prepared. The multilayer metal sheet  200  is obtained by overlaying a wiring film forming metal layer  203  on a main surface of a bump-forming metal layer  201  via an etching barrier layer  202 . As an example, the bump-forming metal layer  201  is a copper foil having a thickness of 100 μm, the etching barrier layer  202  is nickel having a thickness of 2 μm and the wiring film forming metal layer  203  is a copper foil having a thickness of 18 μm. 
     As shown in  FIG. 19(   b ), an etching resist  204  is formed selectively on the surface of the bump-forming metal layer  201 . As shown in  FIG. 19(   c ), the bump-forming metal layer  201  is etched with the etching resist  204  used as a mask. After the etching, the etching resist  204  is removed as shown in  FIG. 19(   d ). Then, the etching barrier layer  202  is etched as shown in  FIG. 19(   e ). Consequently, a bump  205  projecting from the wiring film forming metal layer  203  is formed. 
     However, when a plurality of bumps is formed according to the aforementioned method by electric plating using plating resist, the height of the formed bumps may disperse because metal is deposited within each opening of the plating resist. According to this method, the height of the bump depends on the thickness of the plating resist. Some implementations of the electronic circuit need bumps having a large height. If a semiconductor chip is mounted on a printed board using, for example, a flip-chip method, the bump having a large height is desirable. However, there is a limitation in the thickness of material which constitutes the plating resist, such as DFR on market. Thus, it is difficult to raise the height of the bump. 
     On the other hand, the aforementioned method by etching can suppress the dispersion of the bump height more effectively than the method by electric plating, because the metal layer is etched with the etching resist used as a mask. Additionally, the above-mentioned problem originating from the height of the plating resist never occurs. However, if the bump is formed by etching, the surface of the metal layer, that is, a portion near a surface in a contact with the etching resist is etched more for the reason of the character of etching technology, and as the bottom portion of the metal layer is approached, the degree of the etching makes smaller. Thus, as shown in  FIG. 19(   c ) to  19 ( e ), the configuration of the bump may turn to substantially frusto-conical. As an example, if a bump about 0.1 mm in height is produced by the method by etching, the diameter of its top face is about 0.1 mm and the diameter of its bottom portion is about 0.15 mm. Therefore, if it is desired to form a bump having a larger height with respect to the diameter of the bottom portion, that is, a bump having a high aspect ratio, the method by etching has a limitation. 
     Additionally, there is a limitation by the etching resist. For, example, if it is intended to create a plurality of bumps in which the diameter of the top face is 0.1 mm, the height thereof is 0.1 mm and the diameter of the bottom portion is 0.15 mm according to the method by etching, it is necessary to use etching resist having a diameter of about 0.3 mm for each bump. If a plurality of the bumps is produced, it is necessary, to provide a gap which osmoses etching solution between an etching resist for forming a bump and an etching resist for forming an adjacent bump. At this time, it comes that respective bumps are located apart at least about 0.33 mm because this gap needs to be about 0.03 mm if considering the character of the DFR or the like. Therefore, the interval of the bumps, that is, the accuracy of a fine pitch is limited by the etching resist. 
     SUMMARY OF THE INVENTION 
     The embodiments disclosed herein provide solutions to the problems of the related art and provide a process for forming a bump structure and the bump structure which can achieve a bump having a high aspect ratio. Even if a plurality of bumps is formed, dispersion of the height of the bumps is small and a fine pitch between the bumps can be achieved. 
     The bump-forming process of the present invention is a forming process for the bump structure for use as an electric connection in electronic circuit implementation. The process for forming the bump structure of the claimed embodiments includes the steps of: forming a mandrel from a die body and a die lid by forming at least one opening extending through a bump-forming die body in the thickness direction thereof, the bump-forming die body including a metal sheet; and positioning a bump-forming die lid against the bump-forming, die body so as to cover one end of the opening, thereby defining a bump-forming recess. Additional steps for forming the bump structure include forming a metal layer at least on an inner surface of the bump-forming lid exposed within the bump-forming recess; and removing the mandrel so as to expose the metal layer. 
     The bump-forming die body may have a uniform thickness and the process for forming the bump structure may include the steps of: forming a plurality of openings extending through the bump-forming die body in the thickness direction thereof; positioning a bump-forming die lid against the bump-forming die body so as to cover one end of each of the opening, thereby defining a plurality of bump-forming recesses; forming a metal layer at least on the inner surface of each bump-forming recess; and forming a bump structure which includes protrusions formed of the metal layer, by removing the mandrel so as to expose the metal layer. 
     The bump structure having the recesses can be used such that it is joined to various kinds of supporting members, depending on a fashion at the time of implementation of the circuit or applications. The supporting members mentioned herein refer to generally a matter provided with the bump structure, including hips such as IC, a semiconductor loading intermediate board comprised of polyimide film or glass fiber reinforced resin substrate usually called an interposer (or a substrate) in an electronic component loading technology field, element board which constitutes a multilayer printed wiring board, an inspection jig for securing temporary conduction for a socket for loading a semiconductor chip or inspection, and a resin filled section for sealing the semiconductor chip of a circuit board for use for improvement of implementation density by burying a semiconductor chip internally. 
     The opening is formed by, for example, using an NC drill, punching, or laser machining. As another example, the hole is opened by etching the bump-forming die body from opposite sides thereof. 
     For example, the metal layer may be formed by plating a metal from a surface of the bump-forming die body on which an opening of the bump-forming recess is formed, onto the inner surface of the bump-forming recess. The metal layer may be formed by pattern-plating, and at this time a wiring pattern may be formed. As another example, the wiring pattern may be formed by etching the metal layer after the metal layer is formed. A gold-plated layer may be formed at least on the inner surface of the bump-forming recess, and then the metal layer may be disposed upon the gold-plated layer. 
     The bump-forming die lid positioned against the bump-forming die body may be press-welded to the bump-forming die body by high-temperature pressing. 
     The bump-forming die body and/or the bump-forming die lid may be copper-based metal members, wherein the metal layer is formed as a copper-based metal layer by forming a nickel-based metal layer as a etching barrier layer on the inner surface of the bump-forming recess and then plating a copper-based metal upon the etching barrier layer, wherein the bump-forming die body and the bump-forming die lid are removed using an alkaline etching solution, and then the etching barrier layer is removed. 
     Furthermore, the bump-forming die body may include a metal sheet formed by stacking a plurality of metallic foils on top of one another, wherein the step of forming an opening may include forming an opening in each of a plurality of metallic foils such that the opening extending through the metallic foil in the thickness direction thereof, and stacking a plurality of metallic foils on top of one another such that the openings are aligned with each other, whereby an opening extending through the bump-forming die body in the thickness direction thereof is defined. In this case, for example, at least two of the openings in a plurality of foils may differ in diameter. As an example, the step of forming an opening includes forming another opening in at least one of a plurality of metallic foils other than that contacting with the bump-forming die lid. 
     An alternative bump structure formed according to the process for forming the bump structure includes a flat upper surface of the protrusion formed of the metal layer, and an edge defined by the intersection of the upper surface and a side surface of the protrusion. 
     A method for manufacturing a bump-surface forming member used for forming a bump structure used as an electric connection in an electronic circuit implementation is also disclosed. The method for manufacturing the bump-structure forming member includes the steps of: forming a mandrel from a bump-forming die body and a bump-forming die lid by forming an opening extending through a bump-forming die body in the thickness direction thereof, the bump-forming die body including a metal sheet; and positioning a bump-forming die lid against the bump-forming die body so as to cover one end of the hole, thereby defining a bump-forming recess, the bump-forming die lid including a metal sheet; and forming a metal layer at least on the inner surface of the bump-forming recess. 
     Further, a bump-structure forming member used for forming a bump structure used as an electric connection in an electronic circuit implementation is disclosed. The bump-structure forming member includes: a bump-forming die body having at least one hole extending through the bump-forming die body in the thickness direction thereof, the bump-forming die body including a metal sheet; a bump-forming die lid which is positioned against the bump-forming die body so as to cover one end of the hole, thereby defining a bump-forming recess, the bump-forming die lid including a metal sheet; and a metal layer formed at least on an inner surface of the bump-forming recess. 
     The bump-structure forming member may include a member in a state in which the metal layer formed in the bump-forming recess remains formed in the bump-forming recess without being removed from the bump-forming die body and the bump-forming die lid. For example, by distributing the members in this state through the market, a purchaser can form a wiring pattern on that member or overlay a circuit board. Removal of the bump-forming die body and the bump-forming die lid at a stage in which a desired processing is completed, a desired circuit board in which the bump structure is formed can be achieved. Because the bump-structure forming member is in the state in which the bump-forming die body and the bump-forming die lid are not removed, the bump-forming die body and the bump-forming die lid take a role of a supporting body, which is convenient for distribution as a product and subsequent processing. 
     The present embodiments also disclose a method for manufacturing a circuit board having a bump formed thereon. The process for manufacturing a circuit board having a bump formed thereon includes the steps of: forming a mandrel from a bump-forming die body and a bump-forming die lid by steps including forming an opening extending through a bump-forming die body in the thickness direction thereof, the bump-forming die body including a metal sheet; and positioning a bump-forming die lid against the bump-forming die body so as to cover one end of the opening, thereby defining a bump-forming recess, the bump-forming die lid including a metal sheet; forming a metal layer, from a surface of the bump-forming die body on which an opening of the bump-forming recess is formed, onto the inner surface of the bump-forming recess, the metal layer including a predetermined wiring pattern; placing a circuit board upon the metal layer; and forming a bump structure including a protrusion formed of the metal layer, by removing the mandrel so as to expose the metal layer. 
     In each of the above-described steps, after the circuit board having the bump structure is obtained, the solder mask may be formed at a desired position of both main surfaces of the circuit board, that is, a surface having the protrusions of the circuit board and a surface on a side opposing the surface. At this time, the solder mask is formed such that the surface of the protrusion is exposed. Further, after the solder mask is formed, the nickel plating layer and the gold plating layer may be formed on the surface or the top face of the protrusion to prepare for soldering work at the time of circuit implementation. 
     Further, the circuit board to be overlaid on the metal layer may be a multilayered circuit board such as a build-up board. 
     According to another embodiment, a circuit board having a bump formed thereon is manufactured according to the process for manufacturing the circuit board having the bump formed thereon. In this embodiment, an upper surface of the protrusion formed of the metal layer is flat, and the upper surface and a side surface of the protrusion intersect to define an edge. 
     According to another embodiment for a process for forming a bump structure, the opening is formed in the thickness direction of the bump-forming die body, one end of the opening is covered by the bump-forming die lid so as to form the bump-forming recess, and the metal layer formed on the inner surface of the bump-forming recess is removed so as to obtain the bump structure having the protrusion formed by the metal layer. 
     Thus, there is no restriction by plating resist unlike the electric plating method using the plating resist. Further, because the above-mentioned process enables the height of the formed protrusion, that is, the height of the bump to be adjusted by changing the thickness of the bump-forming die body, the bump having a larger height than by a method by the electric plating using the plating resist or a method by etching can be formed. Because the diameter of the bump to be formed can be adjusted by changing the diameter of the opening of the bump-forming recess, a bump having a high aspect ratio can be formed. 
     If a plurality of the bumps is formed according to this process, the height of each bump is defined by the thickness of the bump-forming die body, thereby suppressing the dispersion of the height. Further, because there is no restriction by etching resist unlike by the method by etching, a fine pitch between the bumps can be achieved. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1(   a )-( d ) are schematic sectional views showing a first embodiment of the process for forming the bump structure according to one embodiment in a sequence of steps; 
         FIGS. 2(   a )-( d ) are a schematic sectional views showing a second embodiment of the process for forming the bump structure in a sequence of steps; 
         FIGS. 3(   e )-( h ) are a schematic sectional views showing the second embodiment of the process for forming a bump structure in a sequence of steps; 
         FIGS. 4(   a )-( d ) are a schematic sectional views showing a third embodiment of the process for forming a bump structure in a sequence of steps; 
         FIGS. 5(   e )-( h ) are a schematic sectional views showing the third embodiment of the process for forming a bump structure in a sequence of steps; 
         FIGS. 6(   a )-( f ) are a schematic sectional views showing a fourth embodiment of the process for forming a bump structure in a sequence of steps; 
         FIGS. 7(   g )-( k ) are a schematic sectional views showing the fourth embodiment of the process for forming a bump structure in a sequence of steps; 
         FIGS. 8(   a )-( d ) are a schematic sectional views showing a fifth embodiment of the process for forming a bump structure in a sequence of steps; 
         FIGS. 9(   a )-( d ) are a schematic sectional views showing a sixth embodiment of the process for forming a bump structure in a sequence of steps; 
         FIGS. 10(   a )-( e ) are a schematic sectional views showing a seventh embodiment of the process for forming a bump structure in a sequence of steps; 
         FIG. 11  is a sectional view showing schematically a circuit board in which a plurality of the bumps of an eighth embodiment is formed; 
         FIGS. 12(   a )-( e ) are a schematic sectional views showing the eighth embodiment of the process for forming a bump structure in a sequence of steps; 
         FIGS. 13(   f )-( i ) are a schematic sectional views showing the eighth embodiment of the process for forming a bump structure in a sequence of steps; 
         FIGS. 14(   j )-( l ) are a schematic sectional views showing the eighth embodiment of the process for forming a bump structure in a sequence of steps; 
         FIG. 15  is a sectional view showing schematically a circuit board in which a plurality of the bumps of a ninth embodiment is formed; 
         FIGS. 16(   a )-( e ) are a schematic sectional views showing the ninth embodiment of the process for forming a bump structure in a sequence of steps; 
         FIGS. 17(   f )-( h ) are a schematic sectional views showing the ninth embodiment of the process for forming a bump structure in a sequence of steps; 
         FIGS. 18(   i )-( k ) are a schematic sectional views showing the ninth embodiment of the process for forming a bump structure in a sequence of steps; and 
         FIGS. 19(   a )-( e ) are a schematic sectional views showing a conventional process for forming the bump structure in a sequence of steps. 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, the embodiments disclosed herein will be described with reference to the accompanying drawings. 
       FIG. 1  is a schematic sectional view showing an embodiment of the process for forming a bump structure in a sequence of steps. A mandrel is defined by a bump-forming die body  1  and a bump-forming die lid joined thereto. Referring to  FIG. 1(   a ), a bump-forming die body  1  is prepared. According to this embodiment, the bump-forming die body  1  is, for example, a rolled copper foil having a thickness of 200 μm. A plurality of through holes  2  are formed in the bump-forming die body. The through holes  2  may be formed in the thickness direction of the rolled copper foil using an NC drill. In this embodiment, the walls of the through holes  2  can be perpendicular to one of the top surface and bottom surface of the bump-forming die body  1 . The walls of the through holes  2  can be parallel to each other. This is in contrast to other embodiments, for example, wherein the opening can be tapered so as to decrease in diameter towards the bottom surface. An opening of each through hole  2  may be circular and the diameter of the opening is, for example, 200 μm. The end of the opening at the top surface of the bump-forming die body  1  and the end of the opening at the bottom surface of the bump-forming die body  1  may also have the same or substantially the same diameter. 
     As shown in  FIG. 1(   b ), a bump-forming die lid  3  formed of a metal sheet may be overlaid on the main surface of the bump-forming die body  1  having through holes  2  and press-welded to the main surface. In this embodiment, the bump-forming die lid  3  may be, for example, a rolled copper foil having a thickness of 70 μm, although other metals may suffice. The opening on one side of the through hole  2  is covered by the bump-forming die lid  3 , so as to form a bump-forming recess  4  which is defined by the inner surface of the through hole  2  and the bump-forming die lid  3 . No adhesive layer exists between the bump-forming die body  1  and the bump-forming die lid  3 . The contact surfaces of the bump-forming die body  1  and the bump-forming die lid  3  are treated by wet soft etching. The bump-forming die body  1  and the bump-forming die lid  3  are pressed against each other at a high-temperature (for example, about 350° C.), so that the bump-forming die body  1  and the bump-forming die lid  3  are press-welded together. 
     Because the mandrel, in this embodiment, is defined by the bump-forming die body  1  including the rolled copper foil and the bump-forming die lid  3 , the inner wall of the recess  4  is conductive. Once the mandrel is formed, as shown in  FIG. 1(   c ), a continuous surface from a surface la of the bump-forming die body  1  in which the opening of the bump-forming recess  4  is formed toward the interior of the bump-forming recess  4 . The continuous surface may be plated with electrolytic nickel, so as to form a nickel plating layer  6  of, for example, 20 μm in a thickness. According to this embodiment, the surface  1   a  in which the openings of the bump-forming recess  4  are formed is plated with electrolytic nickel continuously. Therefore, in adjacent bump-forming recesses  4 , the nickel plating layer  6  is formed continuously from the interior of one of the bump-forming recesses up to the interior of the other bump-forming recess. 
     The bump-forming die body  1  and the bump-forming die lid  3  may be removed by etching with alkaline etching solution (hereinafter referred to as alkaline etching). As shown in  FIG. 1(   d ), by removing the mandrel, comprised of the bump-forming die body  1  and the bump-forming die lid  3 , a bump structure having a plurality of the bumps  7  comprised of the nickel plating layer  6  can be obtained. The height and the top face diameter and bottom portion diameter of the formed bump  7  are 200 μm, respectively. 
     The bump  7  may be constructed in a cylindrical configuration depending on the configuration of the bump-forming recess  4 . Because the bump-forming recess  4  is defined by the bump-forming die body  1  in which the through holes  2  are formed and the metal bump-forming die lid  3  which closes the openings on one side of the through holes  2 , the top face of the bump  7  is flat while an intersecting portion between the top face and the side face of the bump  7  forms an angular edge. 
     According to the embodiment of  FIGS. 1(   a )-( d ), a bump structure in which a ratio between the height, top face diameter and bottom portion diameter of each bump is 1:1:1 while the dispersion of each bump height is 10% or less can be obtained. Further, the ratio between the height, top face diameter and bottom portion diameter of each bump can be adjusted by changing the thickness of a metal member which constitutes the bump-forming die body  1  and/or by changing the diameter of the through hole  2 . Thus, a bump having a high aspect ratio can be achieved. 
     Because the height of the bump  7  depends on the thickness of the bump-forming die body  1 , a larger bump height can be achieved by increasing the thickness of the bump-forming die body  1 . Additionally, because there is no restriction by the etching resist described above unlike the conventional etching method, a fine pitch between the bumps can be realized by shortening a distance between the adjacent through holes  2  when the through holes  2  are formed. 
     It is to be appreciated that the formation of a bump structure is not limited to the aforementioned description of a bump structure. Various embodiments are possible depending on the compatibility with a circuit or the purpose of the embodiment. For example, other embodiments will be described herein. 
       FIGS. 2 and 3  are schematic sectional views showing the process for forming the bump structure according to another embodiment of the present invention in a sequence of steps. The step of forming a mandrel from a bump-forming die body  10  and bump-forming die lid  12  attached thereto as shown in  FIGS. 2(   a )-( b ). As shown, through holes  11  may be formed in the bump-forming die body  10  shown and the bump-forming recess  13  is formed when the bump-forming die lid  12  shown in  FIG. 2(   b ) is positioned adjacent the bump-forming die body. The steps forming the mandrel are identical to the embodiment of  FIGS. 1(   a )-( d ). 
     Once the mandrel is formed, a continuous surface from a surface  10   a  of the bump-forming die body  10  in which the openings of the bump-forming recess  13  are formed up to the interior of the bump-forming recess  13  is plated with electrolytic nickel so as to form a nickel plating layer  15 . Electrolytic copper plating may be implemented on the nickel plating layer  15  so as to form a copper plating layer  16 . Because the nickel plating layer  15  and the copper plating layer  16  may be formed by pattern plating using plating resist (not shown), no continuous nickel plating layer  15  or copper plating layer  16  is formed on the surface  10   a  located between the adjacent bump-forming recesses  13 , different from the embodiment of  FIGS. 1(   a )-( d ). That is, according to this embodiment, a wiring pattern is formed at the same time when the copper plating layer  16  is formed. 
     As shown in  FIG. 2(   d ), dielectric member such as glass cloth epoxy prepreg may be pressed to the bump-forming die body  10  in which the nickel plating layer  15  and the copper plating layer  16  are formed so as to form an insulation layer  17  which fills the interior of the bump-forming recess  13  while covering the surface  10   a  of the bump-forming die body  10  and the surface of the copper plating layer  16 . 
     As shown in  FIG. 3(   e ), the mandrel, comprised of the bump-forming die body  10  comprised of a rolled copper foil and the bump-forming die lid  12 , may be removed by alkaline etching. At this time, the nickel plating layer  15  takes a role as the etching barrier layer. The nickel plating layer  15  may be removed by stripping the nickel. Consequently, a plurality of bumps  18  made of the copper plating layer  16  is produced. 
     As shown in  FIG. 3(   f ), a surface  19  which is a plane from which the bumps  18  may project and comprised of the insulation layer  17  and the copper plating layer  16  may be coated with photosensitive material for a solder mask so as to form a photosensitive material layer  20 . As shown in  FIG. 3(   g ), the photosensitive material layer  20  may be exposed using a photo mask (not shown) and developed so as to form a solder mask  21  between the adjacent bumps  18 . 
     As shown in  FIG. 3(   h ), electroless gold plating may be implemented to the surface of the bump  18  so as to form a gold plating layer  22 . The gold plating layer  22  is provided to prevent the copper of the copper plating layer  16  from diffusing to the solder in soldering at the time of the circuit implementation. 
     When forming the gold plating layer  22 , electroless nickel plating is implemented to the surface of the bump  18  before the gold plating is executed, so as to form a nickel plating layer (not shown). Further, in soldering at the time of the circuit implementation, oxide film may be formed on the nickel plating layer. Thus, soldering cannot be made directly to the nickel plating layer. 
     The bump  18  formed according to this embodiment can be used as, for example, a connecting means for mounting a semiconductor chip to a mother board (printed board). 
       FIGS. 4 and 5  are schematic sectional views showing another embodiment of the process for forming a bump structure in a sequence of steps. The step of forming a mandrel from a bump-forming die body  25  (with openings  26  extending therethrough) and a bump-forming die lid  27  disposed adjacent the bump-forming die body  25  is shown in  FIGS. 4(   a ) and  4 ( b ). The bump-forming recess  28  formed from the mandrel is shown in  FIG. 4(   b ). The process may be identical to the embodiment of  FIGS. 1(   a )-( d ). 
     Electrolytic nickel plating may be implemented to a continuous surface from a surface  25   a  of a bump-forming die body  25  in which openings of the bump-forming recesses  28  are formed up to the inner surface of the bump-forming recess  28 , so as to form a nickel plating layer  30 . Electrolytic copper plating may be implemented to the nickel plating layer  30  so as to form a copper plating layer  31 , as shown in  FIG. 4(   c ). In the adjacent bump-forming recesses  28  of this embodiment, the nickel plating layer  30  and the copper plating layer  31  are formed continuously from the interior of one of the bump-forming recesses up to the interior of the other bump-forming recess. 
     Referring to  FIG. 4(   d ), the mandrel, comprised of the bump-forming die body  25  and the bump-forming die lid  27  which are comprised of the rolled copper foil, may be removed by alkaline etching with the nickel plating layer  30  used as an etching barrier layer. The nickel plating layer  30  is removed by stripping the nickel. Consequently, a plurality of the bumps  32  made of the copper plating layer  31  is produced. 
     As shown in  FIG. 5(   e ), a dielectric member such as glass cloth epoxy prepreg is pressed to a plane  31   a  from which the bumps  32  are projected and which is formed of the copper plating layer  31  so as to form an insulation layer  34 . 
     The copper plating layer  31  may be pattern-etched as shown in  FIG. 5(   f ). As shown in  FIG. 5(   g ), a solder mask layer  35  which fills the interior of each bump  32  and covers the surfaces of the insulation layer  34  and the copper plating layer  31  may be formed. 
     As shown in  FIG. 5(   h ), electroless gold plating is implemented to the surface of the bumps  32  so as to form a gold plating layer  36 . To prevent diffusion of gold, a nickel plating layer (not shown) is formed by applying the electroless nickel plating to the surface of the bumps  32  before gold plating is executed. 
     The bumps  32  formed in this embodiment can be used as, for example, a connecting means for mounting a semiconductor chip onto a mother board (printed board). 
       FIGS. 6 and 7  are schematic sectional views showing another embodiment of the process for forming the bump structure in the order of steps. First, as shown in  FIG. 6(   a ), a dry film  41  is affixed to both main surfaces of a bump-forming die body  40  comprised of a tabular rolled copper foil. 
     Referring to  FIG. 6(   b ), the dry film  41  may be exposed using a photo mask (not shown) and developed so as to remove the dry film  41  selectively, thereby forming a dry film resist  42 . At this time, of the dry film  41  affixed to the main surface on one side of the bump-forming die body and the dry film  41  affixed to the main surface on the other side, dry films at corresponding positions via the bump-forming die body  40  are removed, respectively. 
     With the dry film resist  42  used as etching resist, the bump-forming die body  40  may be etched in the thickness direction of the bump-forming die body  40  from both main surfaces of the bump-forming die body  40 . Consequently, as shown in  FIG. 6(   c ), a plurality of through holes  43  is formed in the thickness direction of the bump-forming die body  40 . Because the through hole  43  may be etched from the both main surfaces of the bump-forming die body  40 , the diameter near the center of the through hole  43  is smaller than the diameter of both openings of the through hole  43 . 
     As shown in  FIG. 6(   d ), the dry film resist  42  may be stripped. Referring to  FIG. 6  ( e ), a bump-forming die lid  44  may be press-welded to the main surface on one side of the bump-forming die body  40  by high-temperature pressing so as to form a bump-forming recess  45 . 
     As shown in  FIG. 6(   f ), electrolytic nickel plating may be implemented to a continuous surface from a surface  40   a  of the bump-forming die body  40  in which the bump-forming recesses  45  are formed up to the inner surface of the bump-forming recess  44  so as to form a nickel plating layer  47 . Electrolytic copper plating may be implemented on the nickel plating layer  47  so as to form a copper plating layer  48 . Here, in the adjacent bump-forming recesses  45 , the nickel plating layer  47  and the copper plating layer  48  are formed continuously from the interior of one of the bump-forming recesses up to the interior of the other bump-forming recess. 
     As shown in  FIG. 7(   g ), the copper plating layer  48  may be pattern-etched with the nickel plating layer  47  used as an etching barrier layer. Consequently, part of the nickel plating layer  47  covering the plane  40   a  between the adjacent bump-forming recesses  44  is exposed. As shown in  FIG. 7(   h ), an insulation layer  49  may be comprised of dielectric material that is formed such that it fills the interior of the bump-forming recess  45  covered with the copper plating layer  48  and covers the surface of the exposed nickel plating layer  47  and the surface of the copper plating layer  48 . 
     As shown in  FIG. 7(   i ), the bump-forming die body  40  and the bump-forming die lid  44  comprised, for example, of a rolled copper foil may be removed by alkaline etching with the nickel plating layer  47  used as an etching barrier layer. The nickel plating layer  47  may be removed by stripping the nickel. Consequently, a plurality of bumps  50  comprised of the copper plating layer  48  is produced. Here, the bumps  50  have a cylindrical configuration whose side face is constricted slightly near the center in the height direction, depending on the configuration of the through hole  43 . 
     Referring to  FIG. 7(   j ), a solder mask layer  52  may be formed on a plane  51  from which the bumps  50  are projected and which is comprised of the insulation layer  49  and the copper plating layer  48 . As shown in  FIG. 7(   k ), electroless gold plating may be implemented to the surface of the bump  50  so as to form a gold plating layer  53 . Further, to prevent diffusion of gold, the electroless nickel plating may be implemented to the surface of the bump  50  so as to form the nickel plating layer (not shown) before the gold plating is executed. 
     The bump  50  formed according to this embodiment can be used as a connecting means for mounting a semiconductor chip onto a mother board (printed board) using, for example, flip-chip method. 
     Referring now to  FIG. 8 , a schematic sectional view shows another embodiment of the process for forming the bump structure of the present invention in the order of steps. First, a mandrel may be formed from a bump-forming die body and a bump-forming die lid  64 . The bump-forming die body  60  is comprised of a plurality of metal sheets as shown in  FIG. 8(   a ). In this embodiment, the bump-forming die body  60  may be comprised of a first metal member  60   a  comprised of the rolled copper foil and a second metal member  60   b  comprised of the rolled copper foil thinner than the first metal member  60   a.    
     Through holes  61  may be formed in the first metal member  60   a  in the thickness direction using an NC drill. Here, the through holes  61  are formed at two positions of the first metal member  60   a . Through holes  62 ,  63  are formed in the second metal member  60   b  in the thickness direction using the NC drill. The through hole  62  is formed at a position communicating with the through hole  61  formed in the first metal member  60   a  when the first metal member  60   a  and the second metal member  60   b  are disposed such that they overlap each other and the diameter of the through hole  62  is larger than that of the through hole  61 . The through holes  63  are formed at two positions between the two through holes  62 . Because no through hole is formed at a position corresponding to the through hole  63  in the first metal member  60   a , when the first metal member  60   a  is overlaid on the second metal member  60   b , the opening on one side of the through hole  63  is covered by the first metal member  60   a.    
     The first metal member  60   a  may be overlaid on the second metal member  60   b  and a bump-forming die lid  64  comprised of the tabular rolled copper foil is overlaid on the first metal member  60   a  and those members are press-welded by high-temperature pressing. Consequently, as shown in  FIG. 8(   b ), a bump-forming recess  65  and a bump-forming recess  66  having a smaller depth than the bump-forming recess  65  are formed. Because the diameter of the through hole  62  formed in the second metal member  60   b  may be larger than the diameter of the through hole  61  formed in the first metal member  60   a , the bump-forming recess  65  is formed in a stepped recess having a large opening. 
     As shown in  FIG. 8(   c ), electrolytic nickel plating is implemented to a continuous surface from a surface  60   d  of the bump-forming die body  60  in which openings of the bump-forming recesses  65 ,  66  are formed up to the inner surfaces of the bump-forming recesses  65 ,  66  so as to form a nickel plating layer  68 . Electrolytic copper plating is implemented on the nickel plating layer  68  so as to form a copper plating layer  69 . In the adjacent bump-forming recesses  65 ,  66  of this embodiment, the continuous nickel plating layer  68  and copper plating layer  69  are formed from the interior of one of the bump-forming recesses  65 ,  66  up to the interior of the other bump-forming recess  65 ,  66 . 
     After that, as shown in  FIG. 8(   d ), the mandrel, comprised of the bump-forming die body  60  and the bump-forming die lid  64  comprised of the rolled copper foil, may be removed by alkaline etching with the nickel plating layer  68  used as an etching barrier layer. The nickel plating layer  68  may be removed by stripping the nickel. Consequently, a plurality of bumps  70 ,  71  comprised of the copper plating layer  69  is produced. According to this embodiment, the bump  70  formed from the bump-forming recess  65  has a stepped configuration in which the diameter of its apex is smaller than the diameter of the bottom portion in a section taken along a height direction and is configured into a cylinder having steps as if cylinders having different outside diameters are stacked in a staircase pattern. The height of the bump  70  is larger than that of the bump  71  formed from the bump-forming recess  66 . 
     As described above, in this embodiment, a plurality of the bumps having different heights can be formed. Thus, the bumps different in height can be used separately for applications, for example, the higher bump  70  is used for connecting to a mother board (printed board) while the lower bump  71  is used for connecting to a semiconductor chip. 
       FIG. 9  is a schematic sectional view showing another embodiment of the process for forming a bump structure in a sequence of steps. First, a mandrel may be formed from a bump-forming die body  75  and bump-forming die lid  79 . The bump-forming die body  75  comprised of a first metal member  75   a , a second metal member  75   b  and a third metal member  75   c  as shown in  FIG. 9(   a ) is prepared. In this embodiment, the respective metal members are rolled copper foils having the same thickness, although other metals known in the art may be used. 
     Through holes  76 ,  77 ,  78  may be formed in the first metal member  75   a , the second metal member  75   b  and the third metal member  75   c , respectively, using an NC drill. At this time, the respective through holes  76 ,  77 ,  78  are formed at positions communicating with each other when the metal members are overlaid and of the through holes  76 ,  77 ,  78 , the diameter of the through hole  76  is the smallest while the diameter of the through hole  78  is the largest. 
     Of the respective metal members in which each through hole is formed, the second metal member  75   b  is overlaid on the third metal member  75   c , and the first metal member  75   a  is overlaid on the second metal member  75   b . Further, a bump-forming die lid  79  comprised of the tabular rolled copper foil is overlaid on the first metal member  75   a  and press-welded thereto by high-temperature pressing. 
     Once the mandrel is formed, as shown in  FIG. 9(   b ), a bump-forming recess  80  is formed. The bump-forming recess  80  may be formed in a stepped recess configuration whose opening is the largest because the diameter of the through hole  76  is the smallest while the diameter of the through hole  78  is the largest. 
     As shown in  FIG. 9(   c ), electrolytic nickel plating may be implemented to a continuous surface from a surface  75   d  of the bump-forming die body  75  in which the bump-forming recesses  80  are formed up to the inner surface of the bump-forming recess  80  so as to form a nickel plating layer  82 . Electrolytic copper plating may be implemented to the nickel plating layer  82  so as to form a copper plating layer  83 . In the adjacent bump-forming recesses  80  of this embodiment, the nickel plating layer  82  and the copper plating layer  83  may be formed such that they are continuous from the interior of one of the bump-forming recesses up to the interior of the other bump-forming recess. 
     After that, as shown in  FIG. 9(   d ), the bump-forming die body  75  and the bump-forming die lid  79  comprised of a rolled copper foil may be removed by alkaline etching with the nickel plating layer  82  used as an etching barrier layer. Next, the nickel plating layer  82  is removed by stripping the nickel. Consequently, a plurality of the bumps  84  comprised of the copper plating layer  83  may be produced. According to this embodiment, the bump  84  has a stepped configuration in which the diameter of its apex is smaller than the diameter of the bottom portion in a section taken along a height direction and is configured into a cylinder having steps as if cylinders having different outside diameters are stacked in a staircase pattern. 
     According to this embodiment, a plurality of the metal members having each through hole is stacked so as to realize a larger bump height. 
       FIG. 10  is a schematic sectional view showing another embodiment of the process for forming the bump structure of the present invention in a sequence of steps As shown in  FIGS. 10(   a )-( b ), the process for forming a bump-forming recess  93  includes forming a mandrel from a bump-forming die body  90  and a bump-forming die lid  91 . Referring to  FIG. 10(   a ), through holes  92  are formed in a bump-forming die body  90  comprised of the rolled copper foil and then press-welding a bump-forming die lid  91  comprised of the rolled copper foil onto the bump-forming die body  90  having through holes  92  is the same as the embodiment of  FIGS. 1(   a )-( d ). 
     Referring to  FIG. 10(   b ), plating resist  95  is attached to a surface  90   a  of the bump-forming die body  90  having openings of the bump-forming recesses  93 . On the surface  90   a , the plating resist  95  is attached to expose the periphery of the opening of each bump-forming recess  93 . 
     After that, as shown in  FIG. 10(   c ), electrolytic nickel plating may be implemented to a continuous surface from the surface  90   a  up to the inner surface of the bump-forming recess  93  so as to form a nickel plating layer  96 . Subsequently, electrolytic gold plating is implemented to the nickel plating layer  96  so as to form a gold plating layer  97 . The plating resist  95  is removed after the nickel plating layer  96  and the gold plating layer  97  are formed. 
     Referring to  FIG. 10(   d ), electrolytic nickel plating may be implemented to a continuous surface from the surface  90   a  of the bump-forming die body  90  up to the inner surface of the bump-forming recess  93  so as to form a nickel plating layer  98 . Here, the nickel plating layer  98  may be stacked to cover the gold plating layer  97  and the surface  90   a . In the adjacent bump-forming recess  93 , the nickel plating layer  98  is formed continuously from the interior of one of the bump-forming recesses up to the interior of the other bump-forming recess. The nickel plating layers  96 ,  98  take a role as the etching barrier layer described later and also a role of preventing diffusion of gold from the nickel plating layer  97 . Electrolytic copper plating may be implemented on the nickel plating layer  98  so as to form a copper plating layer  99 . In the adjacent bump-forming recesses  93 , the copper plating layer  99  may be formed continuously from the interior of one of the bump-forming recesses up to the interior of the other bump-forming recess. 
     After that, as shown in  FIG. 10(   e ), the mandrel, comprised of the bump-forming die body  90  and the bump-forming die lid  91  comprised of a rolled copper foil, may be removed by alkaline etching with part of the nickel plating layer  96  and the nickel plating layer  98  used as an etching barrier layer. Part of the nickel plating layer  96  and the nickel plating layer  98  used as the etching barrier layer may be removed by stripping the nickel. A plurality of bumps  100  whose surface is covered with the gold plating layer  97  is obtained by the removal of the mandrel, comprised of the bump-forming die body  90  and the bump-forming die lid  91 , described above and stripping of the nickel. 
     In other embodiments, the nickel plating layer and the gold plating layer covering the surface of the bumps may be formed by electroless plating. The reason is that a substrate having the bumps cannot be provided with any bus bar necessary for electrolytic plating because its wiring pattern is at a high density. According to this embodiment, contrary to this, the nickel plating layer  98  and the gold plating layer  97  can be formed by electrolytic plating without use of any bus bar because they are applied to the inner surface of the bump-forming recess  93  defined by the rolled copper foil. 
     By forming the gold plating layer  97  by plating the bump-forming recess  93 , the dispersion of a height of the bumps after the gold plating layer  97  is formed between the bumps can be reduced and the surface of the gold plating layer  97  covering the top face of the bump  100  can be made more flat. 
     Referring now to  FIG. 11 , a sectional view showing schematically a circuit board having bumps of another embodiment is shown. In a circuit board  110 , insulation layers  112 ,  113  comprised of dielectric material such as glass cloth epoxy prepreg are overlaid on both main surfaces of a substrate  111  comprised of high Tg epoxy multilayer material or the like. A plurality of bumps  114  is projected outward from the surface of the insulation layer  112  overlaid on the main surface on one side of the substrate  111 . The interior of the bump  114  is filled with a copper plating layer  115  and a nickel plating layer  116  and a gold plating layer  117  are formed on the surface of the copper plating layer  115 . A solder mask layer  119  is formed on a surface  112   a  of the insulation layer  112  from which the bumps  114  are projected. Further, a solder mask layer  120  is formed on the surface of the insulation layer  113  on the other side. 
     A formation process of the bump  114  shown in  FIG. 11  will be described with reference to  FIGS. 12 to 14 . Because a plurality of the bumps  114  is formed in the same way, respectively, one bump  114  will be picked up for explanation. 
     First, as shown in  FIGS. 12(   a ) and  12 ( b ), a mandrel may be formed from a bump-forming die body  130  and a bump-forming die lid  132 . Referring to  FIG. 12(   a ), a through hole  131  may be formed in the bump-forming die body  130  using a laser drill. The bump-forming die body  130  may be comprised of a flat rolled copper foil having a thickness, for example, of 75 μm. In this embodiment, the through hole  131  is configured so that its inner side face is inclined such that the diameter of an opening on one side is smaller than the diameter of an opening on the other side. Further, according to this embodiment, as an example, the diameter of the opening having a larger diameter is set to about 85 μm while the diameter of the opening having a smaller diameter is set to about 35 μm. 
     A bump-forming die lid  132  may be comprised of a flat rolled copper foil overlaid on the main surface of the bump-forming die body  130  having a smaller opening of the through hole,  131  and press-welded thereto by high-temperature pressing as shown in  FIG. 12(   b ) so as to form a bump-forming recess  133 . A flat rolled copper foil having a thickness, for example, of 50 μm, may comprise the bump-forming die lid. 
     As shown in  FIG. 12(   c ), electroless copper plating may be implemented to a continuous surface from a surface  130   a  of the bump-forming die body  130  having openings of the bump-forming recesses  133  up to the inner surface of the bump-forming recess  133  so as to form a copper plating layer  135  having a thickness of about 2 μm. The copper plating layer  135  is formed to seal a gap generated on a corner where the bump-forming die body  130  and the bump-forming die lid  132  make a contact with each other. As shown in  FIG. 12(   d ), electrolytic nickel plating may be implemented on the copper plating layer  135  so as to form a nickel plating layer  136  having a thickness of about 3 to 5 μm. By implementing electrolytic copper plating as shown in  FIG. 12(   e ), a copper plating layer  115 , which fills the interior of the bump-forming recess  133  and covers the nickel plating layer  136  overlaid on the surface  130   a  via the copper plating layer  135 , is formed. 
     As shown in  FIG. 13(   f ), the copper plating layer  115  may be alkaline-etched selectively so as to form a wiring pattern. After that, as shown in  FIG. 13(   g ), the insulation layer  112  and the substrate  111  may be laid together and press-welded. As an example, the thickness of the substrate  111  may be set to about 0.8 mm and the thickness of the insulation layers  112 ,  113  is set to about 0.1 mm. 
     After that, as shown in  FIG. 13(   h ), the bump-forming die body  130  comprised of a rolled copper foil and the bump-forming die lid  132  may be removed by alkaline etching with the nickel plating layer  136  used as an etching barrier layer. At this time, the copper plating layer  135  formed for sealing the gap may also be removed. 
     Referring to  FIG. 13(   i ), the nickel plating layer  136  may be removed by stripping the nickel. Consequently, the bumps  114  can be obtained. The bump  114  is configured into frusto-conical depending on the configuration of the bump-forming recess  133 . Because  FIG. 13(   i ) represents the bump  114  such that it is projected downward, the inverted frusto-conical section is indicated as the bump  114 . As shown in  FIG. 14(   j ), photosensitive material such as photo resist may be applied to the surface of the bump  114  and the insulation layer  112  exposed outside by spraying, so as to form a photosensitive material layer  137  having a thickness of about 25 μm. As shown in  FIG. 14(   k ), the photosensitive material layer  137  is exposed to light selectively so as to expose the bump  114  and developed so as to form a solder mask layer  119  on a plane  112   a  from which the bump  114  is projected. After that, as shown in  FIG. 14(   l ), electroless nickel plating may be implemented to the entire surface of the bump  114  so as to form a nickel plating layer  116  having a thickness of about 3 μm. Electroless gold plating may be implemented on the nickel plating layer  116  so as to form a gold plating layer  117  having a thickness of about 0.05 μm. 
     According to this embodiment, by setting a distance between the centers of the adjacent through holes  131  to 0.13 mm, the pitch Lp of the adjacent bumps  114  in  FIG. 11  is set to 0.13 mm. The diameter Db of the bottom portion of the bump  114  may be 85 μm, the diameter Dt of the top face may be 3.5 μm and the height H may be 75 μm. However, as described above, the height, the top face diameter and bottom portion diameter of each bump can be adjusted to each desired value by changing the thickness of the metal member which constitutes the bump-forming die body  130  and/or the diameter of the through hole  131 . Thus, a bump having a larger height and/or a higher aspect ratio can be achieved. Additionally, the distance between the adjacent bumps can be decreased further by shortening the distance between the adjacent through holes  131  when the through holes  131  are formed. 
     According to this embodiment, by setting a distance between the centers of the adjacent through holes  131  to 0.13 mm, the pitch Lp of the adjacent bumps  114  in  FIG. 11  may be set to 0.13 mm. The diameter Db of the bottom portion of the bump  114  may be 85 μm, the diameter Dt of the top face may be 35 μm and the height H may be 75 μm. However, as described above, the height, the top face diameter and bottom portion diameter of each bump can be adjusted to each desired value by changing the thickness of the metal member which constitutes the bump-forming die body  130  and/or the diameter of the through hole  131 . Thus, a bump having a larger height and/or a higher aspect ratio can be achieved. Additionally, the distance between the adjacent bumps can be decreased further by shortening the distance between the adjacent through holes  131  when the through holes  131  are formed. 
       FIG. 15  is a sectional view showing schematically a multilayered circuit board having bumps according to another embodiment. A build-up layer  141  of a multilayered wiring board  140  has an insulation layer  142  comprised of three layers formed of dielectric material such as glass cloth epoxy prepreg. Each insulation layer comprising the insulation layer  142  may have a via  143  filled with copper plating and wiring patterns  144 . 
     A plurality of bumps  145  may project outward from a main surface  141   a  on one side of the build-up layer  141 . The interior of the bump  145  may be filled with a copper plating layer  146  and a nickel plating layer  147  and a gold plating layer  148  may be formed on the surface of the copper plating layer  146 . A solder mask layer  150  may be formed on a surface  141   a  from which the bumps  145  are projected. The surface  141   a  may be comprised of the insulation layer  142  and the wiring pattern  144 . On a main surface  141   b  on the other side of the build-up layer  141 , a nickel plating layer  152  and a gold plating layer  153  are formed for covering the wiring pattern  144  and a solder mask layer  154  is provided between the respective wiring patterns  144  formed on the main surface  141   b.    
     A formation process of the bump  145  shown in  FIG. 15  will be described with reference to  FIGS. 16 to 18 . Because a plurality of the bumps  145  may be formed in the same way, one bump  145  will be picked up for explanation. The steps shown in  FIGS. 16(   a ) to  16 ( e ) are the same as the embodiment shown in  FIGS. 11-14(   j )-( l ). At a step shown in  FIG. 16(   e ), a copper plating layer  164  and a nickel plating layer  165  may be formed on the interior of a bump-forming recess  162  comprised of a bump-forming die body  160  and a bump-forming die lid  161 , and a surface  160   a  of the bump-forming die body  160 . Additionally, the copper plating layer  146  may be formed on the nickel plating layer  165  and the copper plating layer  146  fills the interior of the bump-forming recess  162 . The copper plating layer  146  is alkaline-etched selectively so as to form a wiring pattern  144   a.    
     After  FIG. 16(   e ), the build-up layer  141  is overlaid on the copper plating layer  146  and the nickel plating layer  165  as shown in  FIG. 17  ( f ). The build-up layer  141  may be formed by forming a wiring pattern and filled via in a first insulation layer overlaid on the copper plating layer  146  and the nickel plating layer  165  according to a semi-additive method, and, implementing the same processing on a second layer and a third layer. A wiring pattern  144   b  may be formed on the surface of the third insulation layer, that is, the surface of the build-up layer  141 . 
     As shown in  FIG. 17(   g ), an etching resist layer  166  may be formed by covering the surface  141   b  of the build-up layer  141  and the wiring pattern  144   b  with etching resist. After that, the bump-forming die body  160  and the bump-forming die lid  161  comprised of the rolled copper foil may be removed by alkaline etching with the etching resist layer  166  and the nickel plating layer  165  used as etching resist. The copper plating layer  164  formed for sealing the gap may also be removed. 
     As shown in  FIG. 17(   h ), the nickel plating layer  165  can be removed by stripping the nickel. Consequently, the bumps  145  can be obtained. As shown in  FIG. 18(   i ), a photosensitive material layer  167  may be formed on the bumps  145  exposed outside and the surface  141   a  of the build-up layer  141  by spraying so as to form the photosensitive material such as photo resist. Further, a photosensitive material layer  168  may be formed on the main surface  141   b  on the other side of the build-up layer  141  such that the wiring pattern  144   b  is covered by spraying. 
     As shown in  FIG. 18(   j ), the photosensitive material layer  167  may be exposed to light selectively and developed so as to expose the bump  145  so as to form a solder mask layer  150  on the plane  141   a  from which the bumps  145  are projected. Additionally, the photosensitive material layer  168  is exposed to light selectively and developed so as to expose the surface of the wiring pattern  144   b  so as to form the solder mask layer  154 . 
     As shown in  FIG. 18(   k ), electroless nickel plating may be implemented on the entire surface of the bump  145  so as to form the nickel plating layer  147 . Electroless gold plating may be implemented on the nickel plating layer  147  so as to form the gold plating layer  148 . 
     The bump-structure forming member in the above-described respective embodiments refers to a member in a state in which the metal layer formed in the bump-forming recess is not picked out from the mandrel, comprised of the bump-forming die body and the bump-forming die lid, but remains in the bump-forming recess. For example, in the embodiment of  FIGS. 1(   a )-( d ), it is the member in a state shown in  FIG. 1(   c ) and in the embodiment of  FIGS. 11-14(   j )-( l ), it is the member in the state shown in  FIG. 12(   e ) or  FIG. 13(   f ). The bump-structure forming member may be in a state in which a wiring pattern has been already formed in the metal layer or in a state in which the wiring pattern has not been yet formed. 
     As described above, for example, a purchaser of the bump-structure forming member can form a wiring pattern on that member or overlay a circuit board. By removing the mandrel, comprised of the bump-forming die body and the bump-forming die lid, at a stage when a desired processing is completed, a desired circuit board having the bump structure can be achieved. 
     Although in the respective embodiments, use of an NC drill or laser drill or etching processing has been mentioned as a means for forming the through holes in the bump-forming die body, the through holes may be formed by punching or other means. Although in the respective embodiments, the rolled copper foils are used as metal materials which comprise the mandrel formed from the bump-forming die body and the bump-forming die lid, the embodiments should not be limited to such materials. 
     Additionally, although the respective embodiments have adopted a through hole having a circular opening as the through hole to be formed in the bump-forming die body, the present embodiments are not so limited. In the respective embodiments, by forming the through holes whose opening has other shape, the bumps having various shapes can be formed. For example, by forming the through holes whose opening has a rectangular shape, bumps having a rectangular column or truncated pyramid may be formed. 
     Although the respective embodiments have disclosed specific steps for forming the wiring pattern, the embodiments should not be limited to such steps. For example, the embodiment of  FIGS. 15-18(   i )-( k ) adopts a subtractive method for forming the wiring pattern by alkaline etching after the copper plating layer  146  is formed. However, it is permissible to form the wiring pattern at the same time as the formation of the copper plating layer  146  which is overlaid in the interior of the bump-forming recess according to the semi additive method. In the semi additive method, a pattern is formed by implementing electrolytic or electroless plating to a portion coated with no resist while plating resist is formed at a portion in which no pattern is desired to be formed. In this case, from the state shown in  FIG. 16(   d ), the copper plating layer  146 , which may fill the interior of the bump-forming recess  162  and comprise a desired wiring pattern  144   a  can be formed by implementing electrolytic copper plating on the nickel plating layer  165 , by forming the plating resist on that copper plating layer and by implementing the copper pattern plating. Further, from the state shown in  FIG. 16(   c ), the copper plating layer  146 , which fills the interior of the bump-forming recess  162  and constitutes a desired wiring pattern  144   a , can be formed by forming plating resist on the copper plating layer  164  and after that, by implementing the electrolytic nickel plating and the electrolytic copper plating. 
     Although the invention herein has been described with reference to particular embodiments, it is to be understood that these embodiments are merely illustrative of the principles and applications of the present invention. It is therefore to be understood that numerous modifications may be made to the illustrative embodiments and that other arrangements may be devised without departing from the spirit and scope of the present invention as defined by the appended claims.