Patent Publication Number: US-2022232709-A1

Title: Printed wiring board and method for manufacturing printed wiring board

Description:
TECHNICAL FIELD 
     The present disclosure concerns a printed wiring board having vias and a method for manufacturing a printed wiring board. 
     BACKGROUND ART 
     Conventionally, in printed wiring boards, an interlayer connection structure in which an interlayer connection conductor is installed in a hole provided in a resin layer, i.e., a so-called via, is often configured. The interlayer connection conductor electrically connects two conductor layers of different layers across the resin layer (JP 2003-209411A and JP 2014-187369A). 
     Vias are generally classified into: 
     through-holes through all layers of a printed wiring board; and 
     interstitial (inner) vias through some layers of a printed wiring board. 
     The interstitial (inner) vias are generally classified into: 
     blind vias that through surfaces and inner layers of a printed wiring board; and 
     buried vias through inner layers and inner layers of a printed wiring board. 
     In the case of through holes, two terminations of an interlayer connection conductor installed in a hole reach one surface and the other surface of a printed wiring board, respectively. 
     In the case of blind vias, one termination of an interlayer connection conductor installed in a hole reaches either one or the other surface of a printed wiring board. The other termination stays in an inner layer. 
     In the case of buried vias, two terminations of an interlayer connection conductor installed in a hole both stay in inner layers. 
     SUMMARY OF INVENTION 
     According to an aspect of the present disclosure, a printed wiring board includes:
         a core substrate; and   one or more build-up layers,   wherein   the core substrate corresponds to a layer range where an interlayer connection conductor that constitutes an inner via is continuous,   each of the one or more build-up layers comprises a resin layer layered on the core substrate and a conductor layer on the resin layer,   a via inner space inside the interlayer connection conductor that constitutes the inner via is hollow, and   the via inner space is connected to an outside through one or more holes provided in the one or more build-up layers.       

     According to another aspect of the present disclosure, a method for manufacturing a printed wiring board includes:
         preparing, as a core substrate, a member which corresponds to a layer range where an interlayer connection conductor constituting an inner via is continuous and which includes a via inner space which is hollow and which is inside the interlayer connection conductor constituting the inner via;   preparing, as one or more build-up layers, one or more members which include one or more holes formed in advance; and   layering the core substrate and the one or more build-up layers such that the one or more holes of the one or more build-up layers are continuous with the via inner space.       

    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic cross-sectional view of a printed wiring board according to one embodiment. 
         FIG. 2  is a schematic cross-sectional view of a printed wiring board according to one embodiment. 
         FIG. 3  is a schematic cross-sectional view of a printed wiring board according to one embodiment. 
         FIG. 4  is a schematic cross-sectional view of a printed wiring board according to one embodiment. 
         FIG. 5  is a schematic cross-sectional view illustrating a method for manufacturing a printed wiring board according to one embodiment. 
         FIG. 6  is a schematic cross-sectional view illustrating a method for manufacturing a printed wiring board according to one embodiment. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Holes through all layers for forming a through-hole are left hollow. Then, a hollow through-hole connecting one surface of a printed wiring board to the other surface is obtained. For example, the hollow through-hole can be used as a waveguide space that transmits a transmission signal from an antenna to a receiver circuit. The antenna is laid on one surface of a multilayer substrate. The receiver circuit is constituted by an MMIC or similar device mounted on the other surface. The waveguide space is filled with air. 
     However, a hole in a core substrate for forming an interstitial (inner) via is closed by build-up layers which are layered outside of it. 
     In another case, a hole through only a portion between inner layers to which a buried via connects is made hollow. A sealed hollow structure is made by confining the hole with build-up layers on both sides. The inside of the sealed hollow structure can be a waveguide space where air is trapped. 
     However, resin of the build-up layers flows into the hole. Alternatively, resin with low flowability is selected so that the resin does not flow into the hole. Even in that case, heat during a build-up process causes air inside to expand. It may destroy the build-up layer. It is difficult to obtain a sealed hollow structure. 
     First, a printed wiring board of an embodiment of the present disclosure will be described with reference to  FIGS. 1 and 2 . 
     As shown in  FIG. 1 , the printed wiring board  100  includes a core substrate  1 , build-up layers  2 ,  3 ,  4 ,  5  on the core substrate  1 , and a solder resist  6 . In  FIG. 1 , an example in which the printed wiring board  100  includes an IC chip  7  is shown. 
     The core substrate  1  includes circuit patterns formed on both sides of an insulating substrate  1   a.  The core substrate  1  includes resin layers  1   b,    1   d  and conductor layers  1   c,    1   e.    
     An inner via  10  is configured in the core substrate  1 . An interlayer connection conductor  11  is adhered to and formed on an inner surface of a hole through the core substrate  1 . It connects a circuit pattern of the conductor layer  1   c  to a circuit pattern of the conductor layer  1   e.  The core substrate  1  is a part corresponding to a layer range where the interlayer connection conductor  11  is continuous. The number of layers of the core substrate  1  can be any. 
     The build-up layer  2  includes a resin layer  2   a  and a conductor layer  2   b  on the resin layer  2   a.  The build-up layer  3  includes a resin layer  3   a  and a conductor layer  3   b  on the resin layer  3   a.  The build-up layer  4  includes a resin layer  4   a  and a conductor layer  4   b  on the resin layer  4   a.  The build-up layer  5  includes a resin layer  5   a  and a conductor layer  5   b  on the resin layer  5   a.    
     The build-up layer  2  is layered on an upper surface of the core substrate  1  in  FIG. 1 . Further, a build-up layer  4  is layered on an upper surface of the build-up layer  2 . Thus, build-up layers ( 2 ,  4 ) that continue in a layering direction are provided. 
     The build-up layer  3  is layered on a lower surface of the core substrate  1  in  FIG. 1 . Further, the build-up layer  5  is layered on a lower surface of the build-up layer  3 . Thus, build-up layers ( 3 ,  5 ) that continue in a layering direction are provided. 
     The build-up layers  2 ,  3  are first build-up layers on the core substrate  1 . The build-up layers  4 ,  5  are second build-up layers over the core substrate  1 . 
     As described above, the build-up layers are provided on both sides of the core substrate  1 . 
     A via inner space  12  inside the interlayer connection conductor  11  constituting the inner via  10  is hollow. 
     On an upper side of the core substrate  1  in  FIG. 1 , the via inner space  12  is connected to the outside through holes  2   c,    4   c  provided in the build-up layers  2 ,  4 , respectively. That is, the via inner space  12  is connected to the outside through the holes  2   c,    4   c  formed in the build-up layers  2 ,  4  so as to be continuous. 
     On a lower side of the core substrate  1  in  FIG. 1 , the via inner space  12  is connected to a hole  3   c  in the build-up layer  3  and is closed by a resin layer  5   a  of the build-up layer  5 . That is, in the printed wiring board  100  shown in  FIG. 1 , the via inner space  12  has a single-opening structure. In the single-opening structure, the via inner space  12  is connected to the outside through the holes  2   c,    4   c  formed in the build-up layers  2 ,  4  only on one side (upper side in  FIG. 1 ) of the printed wiring board  100 . 
     In  FIG. 1 , a mounting pad constituted by a conductor layer  5   b  of the build-up layer  5  is formed also at a portion right below the via inner space  12 . An IC chip  7  is mounted there. 
     On the other hand, as in a printed wiring board  200  shown in  FIG. 2 , a hole  5   c  may be provided also in the build-up layer  5 . In that case, the via inner space  12  is connected to the outside through the holes  3   c,    5   c  formed in the build-up layers  3 ,  5  respectively, even on the lower side of the core substrate  1  in  FIG. 2 . That is, the printed wiring board  200  shown in  FIG. 2  has a double-opening structure. In the double-opening structure, the via inner space  12  is connected to the outside through the holes ( 2   c,    3   c,    4   c,    5   c ) formed in the build-up layers ( 2 ,  3 ,  4 ,  5 ) on both sides (upper and lower sides in  FIG. 2 ) of the printed wiring board  200 . A mounting pad for the IC chip  7  is provided around the hole  5   c.  The IC chip  7  is mounted across the hole  5   c.    
     In the printed wiring boards  100 ,  200  of the embodiments, the topmost conductor layer  4   b  in the figure constitutes an antenna. As the IC chip  7  mounted on the lower side in the figure, an MMIC including a receiver circuit is applied. As the resin layer  4   a  of the build-up layer  4  constituting a surface antenna layer, a resin material different from the resin layers  2   a,    3   a,    5   a  of the build-up layers  2 ,  3 ,  5  constituting other control circuits is applied. To increase efficiency of signal transfer between the IC chip  7  and the antenna, resin materials of the resin layer  4   a  and the resin layer  5   a  are made the same. 
     A transmission signal from the antenna of the conductor layer  4   b  propagates in air with a relative dielectric constant of 1.0 in the holes  4   c,    2   c,  the via inner space  12 , and the hole  3   c  (and the hole  5   c ) to reach the IC chip  7 . 
     In the case of the printed wiring board  200  shown in  FIG. 2 , the antenna and the IC chip  7  face each other across a hollow space of the holes  4   c,    2   c,  the via inner space  12 , the hole  3   c,  and the hole  5   c.    
     Circuit components (through holes, inner vias, and circuit patterns of layers) other than those described above are shown in  FIGS. 1 and 2 . They may or may not be included, and may have any specific configuration. 
     Next, the following two manufacturing methods  1 ,  2  for printed wiring boards will be described. 
     Manufacturing Method  1 —FIG.  5   
     The manufacturing method  1  is an example in which a single-opening structure is made in an inner via of a printed wiring board. 
     Steps carried out in the method are: 
     a step of preparing the core substrate  1  including a hollow via inner space; 
     a step of preparing the build-up layers  2 ,  3  in which holes are formed in advance; and 
     a step of layering the core substrate  1  and the build-up layers  2 ,  3  such that the holes of the build-up layers  2 ,  3  is continuous with the via inner space  12 . 
     As shown in  FIG. 5 , a double-sided plate  20  is layered on the core substrate  1  via the resin layer  2   a.  The double-sided plate  20  is a composite of three parts, i.e., the conductor layer  2   b,  the resin layer  4   a,  and the conductor layer  4   b.  A double-sided plate  21  is layered via the resin layer  3   a.  The double-sided plate  21  is a composite of three parts, i.e., the conductor layer  3   b,  the resin layer  5   a,  and the conductor layer  5   b.  The resin layer  2   a  side of the build-up layer  2  is aligned with one surface of the core substrate  1 . The resin layer  3   a  side of the build-up layer  3  is aligned with the other surface of the core substrate  1 . They are joined by heat pressing. 
     The holes  2   c  are formed in the resin layer  2   a  and the conductor layer  2   b  in advance. The holes  3   c  are formed in the resin layer  3   a  and the conductor layer  3   b  in advance. The holes  4   c  are formed in the resin layer  4   a  and the conductor layer  4   b  in advance. The holes  2   c,    3   c,    4   c  are layered such that they continue to the via inner space  12 . 
     Further, for the double-sided plate  20 , a circuit formation process is carried out on the conductor layers  2   b,    4   b.    
     Prepreg is applied to the resin layer  2   a  and resin layer  3   a.  In the prepreg, the holes  2   c,    3   c  are formed by laser processing or the like before layering. Punching with a die, drilling, and the like can also be applied. 
     Materials with properties that prevent resin from flowing due to heat during heat pressing should be selected for the prepreg. GIA-671N (Hitachi Chemical) is suitably used. 
     This layering process is performed on the core substrate  1  while the inner via  10  is already configured and the via inner space  12  remains hollow. Common inner vias are filled with resin. Screen-printing is generally applied as the method. In a case where there is a via that needs to be filled with resin other than the inner via  10  which is to be left hollow, a portion of a screen printing plate corresponding to the inner via  10  is not opened (it is blocked). It allows the inner via  10  to remain hollow. 
     Except for the IC  7 , a multilayer structure of the printed wiring board  100  shown in  FIG. 1  is obtained by the layering process. 
     The build-up layer  4  forms an antenna and has the hole  4   c.  Therefore, as the double-sided plate  20 , a double-sided plate of low dielectric constant and low dielectric loss tangent material, such as R-F705T, R-5775, R-5885, R-5515 (Panasonic), or RO3003 (Rogers Corporation), is used. To increase efficiency of signal transfer between the IC chip  7  and the antenna, it is suitable to use the same resin as the resin layer  4   a  for the resin layer  5   a  of the build-up layer  5 . 
     After the layering process, a bottom through-hole is drilled. Electrolytic plating is applied to side walls of the bottom through-hole. Then, the circuit formation process is carried out. 
       FIG. 5  shows an example in which an eight-layer printed wiring board is manufactured by layering the double-sided boards  20 ,  21  on top and bottom of the core substrate  1 , respectively. In a case where wiring scale below the core substrate  1  is small, the double-sided plate  21  layered on the lower side of the core substrate  1  may be replaced with copper foil. The result is a seven-layer printed wiring board as shown in  FIG. 3 . In that case, as in the case of the eight-layer printed wiring board shown in  FIG. 5 , a bottom through-hole is drilled after the layering process. Electrolytic plating is applied to an inner wall of the bottom through-hole, and then the circuit formation process is carried out. When the prepreg is opened to form the hole  3   c,  an area of only copper foil appears in the hollow. Wrinkles tend to have a negative effect on circuit formation. Therefore, careful consideration should be given to provision of the hole  3   c.    
     Manufacturing Method  2 — FIG. 6   
     The manufacturing method  2  is an example where a double-opening structure is made in an inner via of a printed wiring board. 
     The method differs from the manufacturing method  1  in that holes for forming the holes  3   c,    5   c  are also pre-formed in a double-sided plate  22  constituted by the conductor layer  3   b,  the resin layer  5   a,  and the conductor layer  5   b.  The holes  2   c,    3   c,    4   c,    5   c  are layered such that they are continuous with the via inner space  12 . Except for the above, the manufacturing method  2  is the same as the first embodiment. 
     Similarly, as shown in  FIG. 4 , a seven-layer printed wiring board can be made in the double-opening structure as well. 
     The figure shows a case where an antenna pattern is placed right above the via inner space  12  as well. 
     According to the printed wiring board of the embodiment of the present disclosure described above, the interstitial (inner) via is hollow and open to the outside. 
     According to the method for manufacturing a printed wiring board of the embodiment of the present disclosure, a structure in which an interstitial (inner) via is hollow and open to the outside is obtained. 
     The embodiments of the present disclosure are described above, but these embodiments are shown as an example, and various other embodiments are possible. The components may be omitted, replaced or modified within the scope of the claims of the invention. 
     In the above embodiments, two build-up layers are provided on each side the core substrate  1 . Alternatively, one build-up layer may be provided on each side. Also, three or more build-up layers may be provided on each side. 
     In the above embodiments, build-up layers are provided on both sides of the core substrate  1 . The inner via  10  is a buried via. Alternatively, the build-up layer may be provided only on one side of the core substrate  1 . In that case, the inner via is a blind via. 
     In the examples, the present invention is applied to a communication circuit board. However, the printed wiring board of the present disclosure is not limited to those examples. 
     An interstitial (inner) via in the present disclosure is a via in the broad sense. It includes not only vias which have both ends kept in inner layers, but also vias which have just one end kept in inner layers. 
     INDUSTRIAL APPLICABILITY 
     The present disclosure may be used for a printed wiring board and a method for manufacturing a printed wiring board. 
     REFERENCE SIGNS LIST 
     
         
           1  core substrate 
           2 ,  3 ,  4 ,  5  build-up layer 
           2   a  resin layer 
           2   b  conductor layer 
           2   c  hole 
           3   a  resin layer 
           3   b  conductor layer 
           3   c  hole 
           4   a  resin layer 
           4   b  conductor layer 
           4   c  hole 
           5   a  resin layer 
           5   b  conductor layer 
           5   c  hole 
           10  inner via 
           11  interlayer connection conductor 
           12  via inner space 
           100 ,  200  printed wiring board