Patent Publication Number: US-2016242283-A1

Title: Wiring board, and mounting structure and laminated sheet using the same

Description:
TECHNICAL FIELD 
     The present invention relates to a wiring board used for electronic apparatuses (for example, various kinds of audiovisual apparatuses, home electrical appliances, communication apparatuses, and computer apparatuses and peripherals thereof), and to a mounting structure and a laminated sheet using the same. 
     BACKGROUND ART 
     Conventionally, a mounting structure in which electronic components are mounted on a wiring board has been used for electronic apparatuses. 
     As this wiring board, for example, Patent Literature 1 describes a structure provided with an inorganic insulating layer (ceramic layer) and a conductive layer (nickel thin layer) disposed on the inorganic insulating layer. 
     CITATION LIST 
     Patent Literature 
     Patent Literature 1: Japanese Unexamined Patent Publication JP-A 04-122087(1992) 
     SUMMARY OF INVENTION 
     Technical Problem 
     However, according to Patent Literature 1, for example, when heat is applied to the mounting structure while an electronic component is being mounted or operating, since the thermal expansion coefficients of the wiring board and the electronic component are different, stress is applied to the wiring board, and this sometimes causes a crack in the inorganic insulating layer. If this crack extends to reach the conductive layer, disconnection occurs in the conductive layer. This sometimes reduces the electrical reliability of the wiring board. 
     An object of the invention is to provide a wiring board excellent in electrical reliability, and a mounting structure and a laminated sheet using the same. 
     Solution to Problem 
     According to one embodiment of the invention, a wiring board includes a first resin layer; an inorganic insulating layer disposed on the first resin layer; a second resin layer disposed on the inorganic insulating layer; and a conductive layer disposed on the second resin layer, the inorganic insulating layer containing a plurality of first inorganic insulating particles partly connected to each other and having a particle diameter of not less than 3 nm and not more than 15 nm, a plurality of second inorganic insulating particles existing with the first inorganic insulating particles in between and having a particle diameter of not less than 35 nm and not more than 110 nm, a resin portion disposed in a gap between the plurality of first inorganic insulating particles, the inorganic insulating layer having a first region located in a vicinity of the second resin layer and a second region located on a side opposite to a second resin layer side of the first region, and a content ratio of the second inorganic insulating particles in the first region being lower than a content ratio of the second inorganic insulating particles in the second region. 
     According to one embodiment of the invention, a mounting structure includes the above-described wiring board; and an electronic component mounted on the wiring board and electrically connected to the conductive layer. 
     According to one embodiment of the invention, a laminated sheet includes a support sheet; an uncured resin layer disposed on the support sheet; and an inorganic resin layer disposed on the uncured resin layer, the inorganic insulating layer containing a plurality of first inorganic insulating particles partly connected to each other and having a particle diameter of not less than 3 nm and not more than 15 nm, and a plurality of second inorganic insulating particles existing with the first inorganic insulating particles in between and having a particle diameter of not less than 35 nm and not more than 110 nm, the inorganic insulating layer having a first region located in a vicinity of the uncured resin layer and a second region located on a side opposite to a uncured resin layer side of the first region, and a content ratio of the second inorganic insulating particles in the first region being lower than a content ratio of the second inorganic insulating particles in the second region. 
     Advantageous Effects of Invention 
     According to the wiring board of the invention, since the content ratio of the second inorganic insulating particles in the first region is lower than the content ratio of the second inorganic insulating particles in the second region, crack occurrence in the first region of the inorganic insulating layer located in the vicinity of the second resin layer can be reduced. Thereby, a wiring board excellent in electrical reliability can be obtained. 
     According to the mounting structure of the invention, since the above-described wiring board is provided, a mounting structure using a wiring board excellent in electrical reliability can be obtained. 
     According to the laminated sheet of the invention, since the above-described wiring board can be produced by using this laminated sheet, a wiring board excellent in electrical reliability can be produced. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1( a )  is a cross-sectional view obtained by cutting a mounting structure according to an embodiment of the invention in a thickness direction thereof, and  FIG. 1( b )  is an enlarged cross-sectional view showing a part R 1  in  FIG. 1( a ) ; 
         FIG. 2( a )  is an enlarged cross-sectional view showing a part R 2  in  FIG. 1( b )  and  FIG. 2( b )  is an enlarged cross-sectional view showing a part R 3  in  FIG. 1( b ) ; 
         FIG. 3( a )  is an enlarged cross-sectional view showing a part R 4  in  FIG. 2( a )  and  FIG. 3( b )  is an enlarged cross-sectional view showing a part R 5  in  FIG. 2( a ) ; 
         FIGS. 4( a ) to ( c )  are cross-sectional views explaining a method of producing the mounting structure shown in  FIG. 1( a ) , and  FIG. 4( d )  is an enlarged cross-sectional view showing a part, in  FIG. 4( c ) , corresponding to the part R 4  of  FIG. 2( a ) ; 
         FIG. 5 ( a )  is a cross-sectional view explaining a method of producing the mounting structure shown in  FIG. 1( a ) ,  FIG. 5( b )  is an enlarged cross-sectional view showing a part, in FIG.  5 ( a ), corresponding to the part R 4  of  FIG. 2( a ) ,  FIG. 5( c )  is a cross-sectional view explaining a method of producing the mounting structure shown in  FIG. 1( a ) , and  FIG. 5( d )  is an enlarged cross-sectional view showing a part, in  FIG. 5( c ) , corresponding to the part R 4  of  FIG. 2( a ) ; and 
         FIGS. 6 ( a ) to ( d )  are cross-sectional views explaining a method of producing the mounting structure shown in  FIG. 1( a ) . 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, a mounting structure provided with a wiring board according to an embodiment of the invention will be described in detail with reference to the drawings. 
     A mounting structure  1  shown in  FIG. 1( a )  is used in electronic apparatuses such as various kinds of audiovisual apparatuses, home electrical appliances, communication apparatuses, computer apparatuses or peripherals thereof. This mounting structure  1  includes an electronic component  2  and a wiring board  3  on which the electronic component  2  is mounted. 
     The electronic component  2  is, for example, a semiconductor element such as an IC or an LSI, or an elastic wave device such as a surface acoustic wave (SAW) device or a film bulk acoustic resonator (FBAR). This electronic component  2  is flip-chip mounted on the wiring board  3  through a bump  4  formed of a conductive material such as solder. 
     The wiring board  3  has a function of supporting the electronic component  2  and supplying the electronic component  2  with power and signals for driving or controlling the electronic component  2 . This wiring board  3  includes a core substrate  5  and a pair of buildup layers  6  formed on the upper and lower surfaces of the core substrate  5 . 
     The core substrate  5  provides electrical continuity between the pair of buildup layers  6  while enhancing the rigidity of the wiring board  3 . This core substrate  5  includes a substrate  7  supporting the buildup layers  6 , a tubular through hole conductor  8  disposed in a through hole passing through the substrate  7  in the thickness direction thereof, and a columnar insulator  9  surrounded by the through hole conductor  8 . 
     The substrate  7  makes the wiring board  3  high in rigidity and low in thermal expansion coefficient. This substrate  7  contains, for example, a resin such as epoxy resin, a base material such as glass cloth covered with the resin and filler particles formed of silicon oxide or the like dispersed in the resin. 
     The through hole conductor  8  electrically connects the pair of buildup layers  6 . This through hole conductor  8  contains a conductive material such as copper. 
     The insulator  9  is filled in the space surrounded by the through hole conductor  8 . This insulator  9  contains a resin such as epoxy resin. 
     On the upper and lower surfaces of the core substrate  5 , the pair of buildup layers  6  are formed as mentioned above. Of the pair of buildup layers  6 , one buildup layer  6  connects with the electronic component  2  through the bump  4 , and the other buildup layer  6  connects with an external circuit, for example, through a solder ball (not shown). 
     The buildup layers  6  include a plurality of insulating layers  10  having via holes passing therethrough in the thickness direction (Z direction) thereof, a plurality of conductive layers  11  disposed partially on the substrate  7  or on the insulating layers  10 , and a plurality of via conductors  12  adhering to the inner walls of the via holes and connecting with the conductive layers  11 . 
     The insulating layers  10  function as insulating members between the conductive layers  11  apart from each other in the thickness direction or in a main surface direction (an X-Y plane direction) and insulating members between the via conductors  12  apart from each other in the main surface direction of. The insulating layers  10  include a first resin layer  13 , an inorganic insulating layer  14  disposed on the first resin layer  13 , and a second resin layer  15  disposed on the inorganic insulating layer  14 . 
     The first resin layer  13  functions as a bonding member between the insulating layers  10 . Moreover, part of the first resin layer  13  is disposed between the conductive layers  11  apart from each other in the main surface direction, and functions as an insulating member between the conductive layers  11 . 
     The thickness of the first resin layer  13  is, for example, not less than 3 μm and not more than 30 μm. The Young&#39;s modulus of the first resin layer  13  is, for example, not less than 0.2 GPa and not more than 20 GPa. The thermal expansion coefficient of the first resin layer  13  in each direction is, for example, not less than 20 ppm/° C. and not more than 50 ppm/° C. The Young&#39;s modulus of the first resin layer  13  is measured by a method pursuant to ISO14577-1:2002 by using Nano Indenter XP manufactured by MTS Systems Corporation. The thermal expansion coefficient of the first resin layer  13  is measured by a measurement method pursuant to JIS K7197-1991 by using a commercially available TMA (Thermo-Mechanical Analysis) apparatus. Hereinafter, the Young&#39;s modulus and thermal expansion coefficient of each member are measured similarly to those of the first resin layer  13 . 
     The first resin layer  13  contains, as shown in  FIG. 1( b ) , a first resin  22  and a plurality of first filler particles  23  dispersed in the first resin  22 . The content ratio of the first filler particles  23  in the first resin layer  13  is, for example, not less than 3% by volume and not more than 60% by volume. The content ratio of the first filler particles  23  in the first resin layer  13  may be measured by regarding as the content ratio (% by volume) the ratio of the area occupied by the first filler particles  23  in a given area of the first resin layer  13  on a cross section in the thickness direction of the wiring board  3 . Hereinafter, the content ratio of the particles in each member is measured similarly to that of the first filler particles  23 . 
     The first resin  22  is formed of a resin material such as epoxy resin, bismaleimide triazine resin, cyanate resin or polyimide resin, and is preferably formed of epoxy resin above all else. The Young&#39;s modulus of the first resin  22  is, for example, not less than 0.1 GPa and not more than 5 GPa. The thermal expansion coefficient of the first resin  22  in each direction is, for example, not less than 20 ppm/° C. and not more than 50 ppm/° C. 
     The first filler particles  23  are formed of an inorganic insulating material such as silicon oxide, aluminum oxide, aluminum nitride, aluminum hydroxide or calcium carbonate, and are preferably formed of silicon oxide above all else. The first filler particles  23  are, for example, spherical. The particle diameter of the first filler particles  23  is, for example, not less than 0.5 μm and not more than 5 μm. 
     The inorganic insulating layer  14 , which is formed of an inorganic insulating material high in rigidity and low in thermal expansion coefficient compared with resin materials, makes the wiring board  3  low in thermal expansion coefficient and high in rigidity. As a consequence, the rigidity of the wiring board  3  is enhanced while the difference in thermal expansion coefficient between the wiring board  3  and the electronic component  2  is reduced, whereby when heat is applied to the mounting structure  1  while the electronic component  2  is being mounted or operating, warpage of the wiring board  3  can be reduced. 
     The thickness of the inorganic insulating layer  14  is, for example, not less than 3 μm and not more than 30 μm. The Young&#39;s modulus of the inorganic insulating layer  14  is higher than the Young&#39;s moduli of the first resin layer  13  and the second resin layer  15 . The Young&#39;s modulus of the inorganic insulating layer  14  is, for example, not less than 10 GPa and not more than 50 GPa. The thermal expansion coefficient of the inorganic insulating layer  14  in each direction is lower than the thermal expansion coefficients of the first resin layer  13  and the second resin layer  15  in each direction. The thermal expansion coefficient of the inorganic insulating layer  14  in each direction is, for example, not less than 0 ppm/° C. and not more than 10 ppm/° C. 
     The inorganic insulating layer  14  contains, as shown in  FIG. 2  and  FIG. 3 , a plurality of inorganic insulating particles  16  partly connected to each other and a resin portion  18  disposed in part of a gap  17  among the inorganic insulating particles  16 . In the inorganic insulating layer  14 , the inorganic insulating particles  16  are connected to each other to thereby form a porous body which is a three-dimensional net-like structure. The connection portions between the inorganic insulating particles  16  are constricted and form a neck structure. 
     The inorganic insulating particles  16 , which are bound together and do not flow since they are partly connected to each other, enhance the Young&#39;s modulus of the inorganic insulating layer  14  and reduce the thermal expansion coefficient thereof in each direction. These inorganic insulating particles  16  contain a plurality of first inorganic insulating particles  19  partly connected to each other, a plurality of second inorganic insulating particles  20  larger in particle diameter than the first inorganic insulating particles  19  and apart from each other with the first inorganic insulating particles  19  in between, and a plurality of third inorganic insulating particles  21  larger in particle diameter than the first inorganic insulating particles  19  and the second inorganic insulating particles  20  and apart from each other with the first inorganic insulating particles  19  and the second inorganic insulating particles  20  in between. 
     The first inorganic insulating particles  19  function as connection members in the inorganic insulating layer  14 . Moreover, the first inorganic insulating particles  19 , which firmly connect as described later since they are small in particle diameter, can make the inorganic insulating layer  14  high in rigidity and low in thermal expansion coefficient. These first inorganic insulating particles  19  are formed of an inorganic insulating material such as silicon oxide, zirconium oxide, aluminum oxide, boron oxide, magnesium oxide or calcium oxide, and above all, silicon oxide is preferably used from the viewpoint of low thermal expansion coefficient and low dielectric tangent. 
     The first inorganic insulating particles  19  are, for example, spherical. The particle diameter of the first inorganic insulating particles  19  is not less than  3  nm and not more than  15  nm. Moreover, the Young&#39;s modulus of the first inorganic insulating particles  19  is, for example, not less than 40 GPa and not more than 90 GPa. Moreover, the thermal expansion coefficient of the first inorganic insulating particles  19  in each direction is, for example, not less than 0 ppm/° C. and not more than 15 ppm/° C. The particle diameter of the first inorganic insulating particles  19  is obtained by measuring the maximum diameter appearing on a cross section in the thickness direction of the wiring board  3 . Hereinafter, the particle diameter of each member is measured similarly to that of the first inorganic insulating particles  19 . 
     The second inorganic insulating particles  20  reduce crack extension in the region between the third inorganic insulating particles  21 . That is, when a crack extends to reach the second inorganic insulating particles  20  in the region between the third inorganic insulating particles  21 , it necessarily detours around the second inorganic insulating particles  20  which are large in average particle diameter, so that the extension of the crack can be reduced. Some of the second inorganic insulating particles  20  connect with the first inorganic insulating particles  19 , and the plurality of second inorganic insulating particles  20  bond together through the first inorganic insulating particles  19 . As the second inorganic insulating particles  20 , particles of a material and properties similar to those of the first inorganic insulating particles  19  may be used. The second inorganic insulating particles  20  are, for example, spherical. The particle diameter of the second inorganic insulating particles  20  is not less than 35 nm and not more than 110 nm. 
     The third inorganic insulating particles  21  further reduce crack extension in the inorganic insulating layer  14  than the second inorganic insulating particles  20 . That is, since the particle diameter of the third inorganic insulating particles  21  is larger than the particle diameter of the second inorganic insulating particles  20 , the energy necessary for detouring around the third inorganic insulating particles  21  is higher than the energy necessary for detouring the second inorganic insulating particles  20 , so that the third inorganic insulating particles  21  can further reduce crack extension than the second inorganic insulating particles  20 . Some of the third inorganic insulating particles  21  connect with the first inorganic insulating particles  19 , and the plurality of third inorganic insulating particles  21  bond together through the first inorganic insulating particles  19 . As the third inorganic insulating particles  21 , particles of a material and properties similar to those of the first inorganic insulating particles  19  may be used. The third inorganic insulating particles  21  are, for example, spherical. The particle diameter of the third inorganic insulating particles  21  is, for example, not less than 0.5 μm and not more than 5 μm. 
     The gap  17  is an open pore, and has openings on one main surface and the other main surface of the inorganic insulating layer  14 . Moreover, since the plurality of inorganic insulating particles  16  partly connected to each other form a porous body, at least part of the gap  17  is surrounded by the inorganic insulating particles  16  on a cross section in the thickness direction of the inorganic insulating layer  14 . 
     The resin portion  18 , which is formed of a resin material which more readily becomes elastically deformed than inorganic insulating materials, reduces the stress applied to the inorganic insulating layer  14  and reduces crack occurrence in the inorganic insulating layer  14 . 
     The second resin layer  15 , which is disposed between the inorganic insulating layer  14  and the conductive layer  11 , enhances the strength of bonding between the inorganic insulating layer  14  and the conductive layer  11 . Moreover, as described later, it reduces crack occurrence in the inorganic insulating layer  14 . The thickness of the second resin layer  15  is, for example, not less than 0.1 μm and not more than 5 μm. The Young&#39;s modulus of the second resin layer  15  is, for example, not less than 0.05 GPa and not more than 5 GPa. The thermal expansion coefficient of the second resin layer  15  in each direction is, for example, not less than 20 ppm/° C. and not more than 100 ppm/° C. 
     The second resin layer  15  contains, as shown in  FIG. 1( b ) , a second resin  24  and a plurality of second filler particles  25  dispersed in the second resin  24 . The content ratio of the second filler particles  25  in the second resin layer  15  is lower than the content ratio of the first filler particles  23  in the first resin layer  13 . As a consequence, the Young&#39;s modulus of the second resin layer  15  can be made lower than the Young&#39;s modulus of the first resin layer  13 . The content ratio of the second filler particles  25  in the second resin layer  15  is, for example, not less than 0.05% by volume and not more than 10% by volume. The second resin layer  15  does not necessarily contain the second filler particles  25 . 
     As the second resin  24 , for example, a resin of a material and properties similar to those of the first resin  22  may be used. As the second filler particles  25 , particles of a material and properties similar to those of the first filler particles  23  may be used. Moreover, the particle diameter of the second filler particles  25  is smaller than the particle diameter of the first filler particles  23 . As a consequence, the Young&#39;s modulus of the second resin layer  15  can be made lower than the Young&#39;s modulus of the first resin layer  13 . The particle diameter of the second filler particles  25  is, for example, not less than 0.05 μm and not more than 0.7 μm. 
     The conductive layers  11 , which are apart from each other in the thickness direction or in the main surface direction, function as wiring such as grounding wiring, power supply wiring or signal wiring. The conductive layers  11  are formed of a conductive material such as copper, silver, gold, aluminum, nickel or chromium, and above all, copper is preferably used. The thickness of the conductive layers  11  is, for example, not less than  3  pm and not more than  20  pm. The thermal expansion coefficient of the conductive layers  11  in each direction is, for example, not less than 14 ppm/° C. and not more than 18 ppm/° C. The Young&#39;s modulus of the conductive layers  11  is, for example, not less than 70 GPa and not more than 150 GPa. 
     The via conductors  12  electrically connect the conductive layers  11  apart from each other in the thickness direction, and function as wiring together with the conductive layers  11 . The via conductors  12  are filled in the via holes. The via conductors  12  are formed of a similar material to the conductive layers  11 , and have similar properties. 
     In the present embodiment, as shown in  FIG. 1 , the wiring board  3  includes the first resin layer  13 , the inorganic insulating layer  14  disposed on the first resin layer  13 , the second resin layer  15  disposed on the inorganic insulating layer  14  and having a lower Young&#39;s modulus than the first resin layer  13 , and the conductive layers  11  disposed on the second resin layer  15 . 
     As a consequence, the second resin layer  15  more readily becomes elastically deformed than the first resin layer  13  since it is lower in Young&#39;s modulus than the first resin layer  13 . For this reason, when stress is applied to the inside of the wiring board  3 , for example, due to warpage of the wiring board  3 , the second resin layer  15  disposed between the inorganic insulating layer  14  and the conductive layer  11  becomes elastically deformed, so that the stress applied to the inorganic insulating layer  14  can be reduced. Consequently, crack occurrence in the inorganic insulating layer  14  can be reduced. 
     Moreover, as shown in  FIG. 2 , the inorganic insulating layer  14  has a first region  26  located in the vicinity of the second resin layer  15  and a second region  27  located on a side opposite to a second resin layer  15  side of the first region  26 . The content ratio of the second inorganic insulating particles  20  in the first region  26  is lower than the content ratio of the second inorganic insulating particles  20  in the second region  27 . The vicinity of the second resin layer  15  is, for example, a region from the boundary between the second resin layer  15  and the inorganic insulating layer  14  to a thickness of 3 μm into the inorganic insulating layer  14 . 
     As a consequence, since the content ratio of the second inorganic insulating particles  20  in the first region  26  is lower than the content ratio of the second inorganic insulating particles  20  in the second region  27 , the content ratio of the resin portion  18  in the first region  26  can be made higher than the content ratio of the resin portion  18  in the second region  27 . For this reason, the first region  26  located in the vicinity of the second resin layer  15  readily becomes elastically deformed. Consequently, when stress is applied to the inside of the wiring board  3 , the stress caused between the second resin layer  15  which readily becomes elastically deformed and the inorganic insulating layer  14  which does not readily become elastically deformed can be reduced, so that crack occurrence in the inorganic insulating layer  14  can be reduced. Therefore, disconnection in the conductive layer  11  due to this crack is reduced, so that a wiring board  3  excellent in electrical reliability can be obtained. 
     Moreover, since the content ratio of the second inorganic insulating particles  20  in the second region  27  is higher than the content ratio of the second inorganic insulating particles  20  in the first region  26 , crack extension can be reduced by the second inorganic insulating particles  20  in the second region  27  located on the side opposite to the second resin layer  15  side of the first region  26 . Moreover, since the Young&#39;s modulus of the first resin layer  13  is higher than the Young&#39;s modulus of the second resin layer  15 , the rigidity of the wiring board  3  can be enhanced. The magnitude relation between the content ratio of the resin portion  18  in the first region  26  and the content ratio of the resin portion  18  in the second region  27  can be determined by performing EDS analysis using a transmission electron microscope on a cross section in the thickness direction of the inorganic insulating layer  14 . 
     In the present embodiment, the content ratio of the second inorganic insulating particles  20  in the first region  26  is not less than 0% by volume and not more than 10% by volume. The content ratio of the second inorganic insulating particles  20  in the second region  27  is more than 10% by volume and not more than 35% by volume. The content ratio of the first inorganic insulating particles  19  in the first region  26  and the second region  27  is not less than 15% by volume and not more than 45% by volume. The content ratio of the third inorganic insulating particles  21  in the first region  26  and the second region  27  is not less than 40% by volume and not more than 70% by volume. 
     Regarding the content ratios of the first, second and third inorganic insulating particles  19 ,  20  and  21  in the first and second regions  26  and  27 , like the content ratio of the first filler particles  23  of the first resin layer  13 , the ratios of the areas occupied by the first, second and third inorganic insulating particles  19 ,  20  and  21  in given areas of the first and second regions  26  and  27  on a cross section in the thickness direction of the wiring board  3  can be regarded as the content ratios (% by volume). 
     Here, the boundary between the first region  26  and the second region  27  defines a layered measurement region having a width of 2 μm at a pitch of 0.2 μm thickness from the boundary between the second resin layer  15  and the inorganic insulating layer  14  on a cross section in the thickness direction of the wiring board  3 , the ratio of the area of the second inorganic insulating particles  20  to the total area in the measurement region is the content ratio, measurement is successively performed from the boundary in the thickness direction, the region up to the measurement region of not more than 10% by volume is the first region  26 , and the region exceeding 10% by volume is the second region  27 . 
     The first region  26  preferably contains, of the first inorganic insulating particles  19  and the second inorganic insulating particles  20 , only the first inorganic insulating particles  19 . As a consequence, since the first region  26  does not contain the second inorganic insulating particles  20 , the first region  26  is made to more readily become elastically deformed, so that crack occurrence in the inorganic insulating layer  14  can be reduced. The fact that the first region  26  contains, of the first inorganic insulating particles  19  and the second inorganic insulating particles  20 , only the first inorganic insulating particles  19 , can be confirmed by observing five places of a cross section in the thickness direction of the inorganic insulating layer  14 . 
     Further, the first region  26  preferably contains the third inorganic insulating particles  21 . As a consequence, crack extension in the first region  26  can be reduced. 
     In the present embodiment, the thickness of the second resin layer  15  is smaller than the thickness of the first resin layer  13 . As a consequence, by making small the thickness of the second resin layer  15  having a low Young&#39;s modulus, the rigidity of the wiring board  3  can be enhanced. Moreover, by making large the thickness of the first resin layer  13  with a high Young&#39;s modulus, the rigidity of the wiring board  3  can be enhanced. Moreover, since the first resin layer  13  is easily filled in between the conductive layers  11  apart from each other in the main surface direction, the performance of insulation between the conductive layers  11  can be enhanced. The thickness of the second resin layer  15  of the present embodiment is smaller than the thicknesses of the inorganic insulating layer  14  and the conductive layers  11 . 
     In the present embodiment, the resin portion  18  has a first resin portion  28  disposed in the first region  26  and a second resin portion  29  disposed in the second region  27 . The first resin portion  28  is formed of the resin forming the second resin layer  15 , and this resin is part of the second resin  24 . As a consequence, since part of the second resin layer  15  enters the gap  17  in the first region  26 , the strength of bonding between the first region  26  and the second resin layer  15  can be enhanced by an anchor effect. 
     Moreover, the second resin portion  29  is formed of the resin forming the first resin layer  13 , and this resin is part of the first resin  22 . As a consequence, since part of the first resin layer  13  enters the gap  17  in the second region  27 , the strength of bonding between the second region  27  and the first resin layer  13  can be enhanced by an anchor effect. 
     In the present embodiment, the thickness of the first region  26  is smaller than the thickness of the second region  27 . As a consequence, the rigidity of the inorganic insulating layer  14  is enhanced, so that the rigidity of the wiring board  3  can be enhanced. The thickness of the first region  26  is, for example, not less than 0.2 μm and not more than 3 μm. The thickness of the second region  27  is, for example, not less than 3 μm and not more than 25 μm. 
     Next, a method of producing the mounting structure  1  described previously will be described with reference to  FIG. 4  to  FIG. 6 . 
     (1) As shown in  FIG. 4( a ) , the core substrate  5  is produced. Specifically, it is produced, for example, as follows: 
     The substrate  7  formed by curing a prepreg and a laminated plate formed of metallic foil such as copper foil disposed on both main surfaces of the substrate  7  are prepared. Then, a through hole is formed in the laminated plate by using laser processing, drilling or otherwise. Then, a conductive material is made to adhere to the inside of the through hole by using, for example, electroless plating, electrolytic plating, an evaporation method, sputtering or otherwise to form the tubular through hole conductor  8 . Then, uncured resin is filled into the through hole conductor  8  and cured to thereby form the insulator  9 . Then, after the conductive material is made to adhere onto the insulator  9  by using, for example, electroless plating, electrolytic plating or otherwise, patterning of the metal foil on the substrate  7  and the conductive material is performed to form the conductive layers  11 . The core substrate  5  can be produced in the way described above. 
     (2) As shown in  FIG. 4( b )  to  FIG. 6( a ) , a laminated sheet  33  is produced which includes a support sheet  30  formed of metal foil such as copper foil, a resin film such as a PET film or the like, a second uncured resin layer  31  disposed on the support sheet  30 , the inorganic insulating layer  14  disposed on the second uncured resin layer  31  and a first uncured resin layer  32  disposed on the inorganic insulating layer  14 . Specifically, it is produced, for example, as follows: 
     First, as shown in  FIG. 4( b ) , a support sheet  34  with resin is prepared which has the support sheet  30  and the second uncured resin layer  31  disposed on the support sheet  30 . The second uncured resin layer  31  contains an uncured resin which becomes the second resin  24  and the second filler particles  25 . 
     Then, as shown in  FIG. 4( c )  and  FIG. 4( d ) , slurry  36  is prepared which has the inorganic insulating particles  16  and a solvent  35  in which the inorganic insulating particles  16  are dispersed, and the slurry  36  is applied to one main surface of the second uncured resin layer  31 . Then, as shown in  FIG. 5( a )  and  FIG. 5( b ) , the solvent  35  is evaporated from the slurry  36  so that the inorganic insulating particles  16  remain on the support sheet  30 , thereby forming a powder layer  37  formed of the remaining inorganic insulating particles  16 . In this powder layer  37 , the first inorganic insulating particles  19  are in contact with each other at adjacent places. Then, as shown in  FIG. 5( c )  and  FIG. 5( d ) , the powder layer  37  is heated to connect the adjoining first inorganic insulating particles  19  at the adjacent places, thereby forming the inorganic insulating layer  14 . 
     Then, as shown in  FIG. 6( a ) , the first uncured resin layer  32  containing an uncured resin which becomes the first resin  22  and the first filler particles  23  is laminated onto the inorganic insulating layer  14 , and the laminated inorganic insulating layer  14  and first uncured resin layer  32  are heated and pressurized in the thickness direction, thereby filling part of the first uncured resin layer  32  into the gap  17 . The laminated sheet  33  can be produced in the way described above. 
     This laminated sheet  33  includes the support sheet  30 , the second uncured resin layer  31  disposed on the support sheet  30 , and the inorganic insulating layer  14  disposed on the second uncured resin layer  31 . The inorganic insulating layer  14  contains the plurality of first inorganic insulating particles  19  partly connected to each other and having a particle diameter of not less than  3  nm and not more than  15  nm, and the plurality of second inorganic insulating particles  20  disposed apart from each other with the first inorganic insulating particles  19  in between and having a particle diameter of not less than 35 nm and not more than 110 nm. 
     In the laminated sheet  33  of the present embodiment, the inorganic insulating layer  14  has the first region  26  located in the vicinity of the second uncured resin layer  31  and the second region  27  located on a side opposite to a second uncured resin layer  31  side of the first region  26 . The content ratio of the second inorganic insulating particles  20  in the first region  26  is lower than the content ratio of the second inorganic insulating particles  20  in the second region  27 . Part of the second resin  24  of the second uncured resin layer  31  is disposed in the gap  17  between the first inorganic insulating particles  19  in the first region  26 . 
     As a consequence, since the content ratio of the second inorganic insulating particles  20  in the first region  26  is lower than the content ratio of the second inorganic insulating particles  20  in the second region  27 , the volume of the gap  17  in the first region  26  can be increased. Consequently, since the content ratio of the second resin  24  of the second uncured resin layer  31  in the first region  26  can be increased, the strength of bonding between the second uncured resin layer  31  and the inorganic insulating layer  14  can be enhanced. Therefore, the separation between the second uncured resin layer  31  and the inorganic insulating layer  14  in the laminated sheet  33  is reduced, so that the production efficiency of the wiring board  3  using the laminated sheet  33  can be enhanced. 
     In the present embodiment, when the slurry  36  is applied to the second uncured resin layer  31 , part of the uncured resin of the second uncured resin layer  31  is dissolved or swelled by the solvent  35  in the slurry  36 . As a consequence, a space with a size of approximately 3 to 15 nm is caused in the uncured resin. And when the solvent  35  is dried, the first inorganic insulating particles  19  having a small particle diameter in the slurry  36  precipitate and readily enter the space in the uncured resin, whereas the second inorganic insulating particles  20  having a large particle diameter do not readily enter the space in the uncured resin. Consequently, when the first inorganic insulating particles  19  are connected to each other to form the inorganic insulating layer  14 , the content ratio of the second inorganic insulating particles  20  in the first region  26  can be made lower than the content ratio of the second inorganic insulating particles  20  in the second region  27 . 
     When the slurry  36  is applied to the second uncured resin layer  31 , by appropriately adjusting the degree of cure of the uncured resin, the size of the space in the uncured resin caused by the solvent  35  is adjusted, whereby the amount of entrance into the space by the second inorganic insulating particles  20  can be adjusted. Moreover, by appropriately adjusting the degree of cure of the uncured resin, the thickness of the first region  26  can be appropriately adjusted. 
     Moreover, since the third inorganic insulating particles  21  are present as the second filler in the second uncured resin layer  31  from the beginning, the first region  26  containing the third inorganic insulating particles  21  can be formed. 
     In the present embodiment, the slurry  36  containing the plurality of first inorganic insulating particles  19  whose particle diameter is not less than 3 nm and not more than 15 nm and the solvent  35  in which the first inorganic insulating particles  19  are dispersed is applied onto the support sheet  30 . As a consequence, since the particle diameter of the first inorganic insulating particles  19  is not less than 3 nm and not more than 15 nm, some of the plurality of first inorganic insulating particles  19  can be firmly connected to each other even under low temperature conditions. It is assumed that this happens because the atoms of the first inorganic insulating particles  19 , particularly, the atoms on the surface vigorously move since the first inorganic insulating particles  19  are minute and this lowers the temperature at which some of the first inorganic insulating particles  19  are firmly connected to each other. 
     Consequently, the plurality of first inorganic insulating particles  19  can be firmly connected to each other under low temperature conditions such as less than the crystallization start temperature of the first inorganic insulating particles  19 , and further, not more than 250° C. Moreover, by performing heating at a low temperature as mentioned above, the first inorganic insulating particles  19  can be connected to each other only in an adjacent region while the particle shape of the inorganic insulating particles  16  is maintained. As a consequence, a neck structure is formed at the connection portions, and the gap  17  which is an open pore can be easily formed. The temperature at which the first inorganic insulating particles  19  can be firmly connected to each other is, for example, approximately 150° C. when the average particle diameter of the first inorganic insulating particles  19  is set to 15 nm. 
     Moreover, in the present embodiment, the slurry  36  further containing the plurality of third inorganic insulating particles  21  whose particle diameter is not less than 0.5 μm and not more than 5 μm, is applied onto the support sheet  30 . As a consequence, since the space of the inorganic insulating particles  16  in the slurry  36  can be reduced by the third inorganic insulating particles  21  whose particle diameter is larger than those of the first inorganic insulating particles  19  and the second inorganic insulating particles  20 , the contraction of the powder layer  37  formed by evaporating the solvent  35  can be reduced. Consequently, by reducing the contraction of the powder layer  37  having a flat shape which is apt to largely contract in the main surface direction, crack occurrence in the thickness direction in the powder layer  37  can be reduced. 
     Moreover, in the present embodiment, the slurry  36  further containing the plurality of second inorganic insulating particles  20  whose particle diameter is not less than 35 μm and not more than 110 μm, is applied onto the support sheet  30 . As a consequence, the space of the inorganic insulating particles  16  in the regions between the third inorganic insulating particles  21  of the slurry  36  can be reduced by the second inorganic insulating particles  20  whose particle diameter is larger than that of the first inorganic insulating particles  19  and smaller than that of the second inorganic insulating particles  20 . Consequently, crack occurrence in the regions between the third inorganic insulating particles  21  of the powder layer  37  can be reduced. 
     The content ratio of the inorganic insulating particles  16  in the slurry  36  is, for example, not less than 10% by volume and not more than 50% by volume, and the content ratio of the solvent  35  in the slurry  36  is, for example, not less than 50% by volume and not more than 90% by volume. For the solvent  35 , for example, methanol, isopropanol, methyl ethyl ketone, methyl isobutyl ketone, xylene, or an organic solvent containing a mixture of two or more kinds selected therefrom can be used. Above all, methyl isobutyl ketone is preferably used as the solvent  35 . As a consequence, the second resin layer  15  can be appropriately dissolved or swelled, so that a desired first region  26  can be obtained. 
     The heating temperature when the powder layer  37  is heated is not less than the boiling point of the solvent  35  and less than the crystallization start temperature of the first inorganic insulating particles  19 , further, not less than 100° C. and not more than 250° C. Moreover, the heating time is, for example, not less than 0.5 hours and not less than 24 hours. 
     The applied pressure when the laminated inorganic insulating layer  14  and first uncured resin layer  32  are heated and pressurized is, for example, not less than 0.05 MPa and not more than 0.5 MPa, the pressurization time is, for example, not less than 20 seconds and not more than 5 minutes, and the heating temperature is, for example, not less than 50° C. and not more than 100° C. Since this heating temperature is less than the curing start temperature of the first uncured resin layer  32 , the first uncured resin layer  32  can be maintained in uncured state. 
     (3) As shown in  FIG. 6( b )  to  FIG. 6( c ) , the laminated sheet  33  is laminated on the core substrate  5  to form the insulating layer  10 , and the via conductor  12  passing through the conductive layer  11  disposed on the insulating layer  10  and the insulating layer  10  in the thickness direction thereof is formed. Specifically, this is performed, for example, as follows: 
     First, the laminated sheet  33  is laminated on the core substrate  5  while the first uncured resin layer  32  is disposed on the side of the core substrate  5 . Then, by heating and pressurizing in the thickness direction the core substrate  5  and the laminated sheet  33  which are laminated, the laminated sheet  33  is bonded to the core substrate  5 . Then, as shown in  FIG. 6( b ) , by heating the first uncured resin layer  32  and the second uncured resin layer  31 , the uncured resin is cured to make the first uncured resin layer  32  the first resin layer  13  and make the second uncured resin layer  31  the second resin layer  15 . As a consequence, the insulating layer  10  having the first resin layer  13 , the inorganic insulating layer  14  and the second resin layer  15  can be formed. In this case, part of the first uncured resin layer  32  having entered the gap  17  becomes the second resin portion  29 , and part of the second uncured resin layer  31  having entered the gap  17  becomes the first resin portion  28 . 
     Then, the support sheet  30  is mechanically or chemically removed from the insulating layer  10 . Then, using laser processing, a via hole passing through the insulating layer  10  in the thickness direction thereof is formed. When this is done, the conductive layer  11  is exposed at the bottom surface of the via hole. Then, as shown in  FIG. 6( c ) , using electroless plating or electrolytic plating, a conductive material is made to adhere to the inner wall of the via hole and the exposed one main surface of the insulating layer  10  to thereby form the conductive layer  11  and the via conductor  12 . 
     For the heating and pressurization when the core substrate  5  is bonded to the laminated sheet  33 , conditions similar to those of step (2) may be used. The heating temperature when the uncured resin is cured is, for example, not less than the curing start temperature of the uncured resin and less than the thermal decomposition temperature, and the heating time is, for example, not less than 10 minutes and not more than 120 minutes. 
     (4) As shown in  FIG. 6( d ) , by repeating steps (2) and (3), the buildup layers  6  are formed on the core substrate  5  to produce the wiring board  3 . By repeating these steps, the buildup layers  6  can be made more multi-layered. 
     (5) By flip-chip mounting the electronic component  2  on the wiring board  3  through the bump  4 , the mounting structure  1  shown in  FIG. 1( a )  is produced. The electronic component  2  may be electrically connected to the wiring board  3  by wire bonding or may be incorporated in the wiring board  3 . 
     The invention is not limited to the above-described embodiment and various modifications, improvements, combinations and the like are possible without departing from the scope of the invention. 
     For example, while in the above-described embodiment of the invention, by way of example, there is described a structure in which the buildup layers  6  have the first resin layer  13 , the inorganic insulating layer  14  and the second resin layer  15 , the core substrate  5  may have a structure corresponding to the first resin layer  13 , the inorganic insulating layer  14  and the second resin layer  15 . 
     Moreover, while in the above-described embodiment of the invention, there is described an example using as the wiring board  3  a buildup multi-layer board composed of the core substrate  5  and the buildup layers  6 , a different board may be used as the wiring board  3 ; for example, a single-layer board consisting only of the core substrate  5  or a coreless substrate consisting only of the buildup layers  6  may be used. 
     Moreover, while in the above-described embodiment of the invention, by way of example, there is described a structure in which the inorganic insulating particles  16  contain the third inorganic insulating particles  21 , the inorganic insulating particles  16  do not necessarily contain the third inorganic insulating particles  21 . 
     While in the above-described embodiment of the invention, an example structured so that the via conductors  12  adhere to the inner walls of the via holes is described, a structure in which the via conductors  12  are filled in the via holes may be used. 
     Moreover, while in the above-described embodiment of the invention, by way of example, there is described a structure in which the evaporation of the solvent  35  and the heating of the powder layer  37  are separately performed at step (2), these may be simultaneously performed. 
     REFERENCE SIGNS LIST 
     
         
           1 : Mounting structure 
           2 : Electronic component 
           3 : Wiring board 
           13 : First resin layer 
           14 : Inorganic insulating layer 
           15 : Second resin layer 
           16 : Inorganic insulating particle 
           17 : Gap 
           18 : Resin portion 
           19 : First inorganic insulating particle 
           20 : Second inorganic insulating particle 
           21 : Third inorganic insulating particle 
           22 : First resin 
           23 : First filler particle 
           24 : Second resin 
           25 : Second filler particle 
           26 : First region of inorganic insulating layer 
           27 : Second region of inorganic insulating layer 
           28 : First resin portion 
           29 : Second resin portion 
           30 : Support sheet 
           31 : second uncured resin layer 
           32 : First uncured resin layer 
           33 : Laminated sheet