Abstract:
For the purpose of achieving enhanced reliability with respect to interlayer connections of printed wiring boards, a manufacturing method of printed wiring boards of the present invention includes any one of the steps of A) restricting the resin flowing in hot press processing, B) joining fiber reinforcements together by fusion or adhesion, C) having the thickness of a board material reduced after a filling process and D) forming a low fluidity layer via a filler mixed in a board material. Such properties as allowing the resin flowing in hot press processing to be controlled are provided to a material for manufacturing printed wiring boards of the present invention or to a volatile ingredient contained therein to allow the thickness of a board material to be reduced efficiently after a filling process.

Description:
TECHNICAL FIELD 
   The present invention relates to methods and materials for manufacturing printed wiring boards used in diverse electronic apparatuses. 
   BACKGROUND ART 
   The recent trend towards smaller and higher-density electronic apparatuses is increasing the use of double-sided multi-layer boards compared with conventional one-side printed wiring boards on which electronic components are placed. Consequently, boards that can hold a higher density of circuits and components are being developed (e.g. “Marked development trend towards build-up multi-layer PWBs,” Kiyoshi Takagi, January 1997, “Surface Mounting Technology,” Nikkan Kogyo Shimbun). 
   Prior art is described next with reference to  FIGS. 6A to 6G . Board material  61  in  FIG. 6A  is a prepreg in B-Stage which is made by impregnating a thermosetting material such as epoxy resin into woven glass fabric for printed circuit boards, and then drying it. Film  62  is pasted on both faces of board material  61  by lamination, typically by hot rolling. 
   Next, as shown in  FIG. 6B , via hole  63  is created in board material  61  using processing methods such as a laser beam. Then, as shown in  FIG. 6C , conductive paste  64 , made typically by mixing together conductive particles such as copper powder, thermosetting resin, curing agent, and solvent, is injected into via hole  63 . Film  62  is then peeled off revealing conductive paste  64  protruding as shown in  FIG. 6D . Copper foil  65  is disposed on both faces over this conductive paste  64 , and heated and pressed using hot pressing equipment (not illustrated). This thermally cures board material  61  and compresses conductive paste  64  such that copper foils  65  on top and rear faces are electrically connected. Here, epoxy resin impregnated in board material  61  flows outward to form leaked portion  66 . An unneeded portion at the edge is then cut to make a shape as shown in  FIG. 6F , and then copper foil  65  is processed into a predetermined pattern, typically by etching, to create circuit  67 . The double-sided printed wiring board as shown in  FIG. 6G  is thus completed. 
   In the above manufacturing method, however, electrical connection between the top and rear faces of the printed wiring board is unsatisfactory in some cases. In addition, a similar failure occurs, in some cases, between the surface layer and inner layer circuits when a multi-layer printed wiring board is formed using the above manufacturing method. 
   A major cause of this failure is leaking particle  610 , originally a conductive particle in conductive paste  64 , flowing out of via hole  63  as shown in  FIG. 6E . To realize an ideal electrical connection, conductive paste  64  needs to be compressed vertically in  FIG. 6E  so that conductive particles in the conductive paste make firm and effective contact, and also firmly contact copper foil  65 . However, as is apparent from the formation of leaked portion  66  during the steps shown in  FIGS. 6D and 6E , the thermosetting resin in board material  61  flows outward. In this state, conductive particles in conductive paste  64  are pressed and flow horizontally as in  FIG. 6E , resulting in conductive paste  64  being insufficiently compressed. Accordingly, the electrical connection through conductive paste  64  is unstable. The above description refers to a board material using woven glass fabric and thermosetting resin. The same phenomenon occurs with the use of inorganic fibers other than glass fiber, organic fibers such as aramid fiber, or nonwoven fabric other than woven fabric as reinforcement. 
   If a woven fabric is used, the flow of thermosetting resin as described above is noticeable, since the flow resistance in woven fabric is particularly low. This makes it difficult to establish electrical connection using conductive paste. In addition, deviation of the fibers comprising woven fabric has a detrimental effect. This phenomenon is described next with reference to  FIGS. 7A to 7C . As shown in  FIG. 7A , via hole  63  is created by a laser beam on board material  61  containing woven glass fabric  68 . Looking at this area from the top, via hole  63  is made by cutting woven glass fabric  68  as shown in  FIG. 7B . The processes described using  FIGS. 6C to 6E  are then applied. As shown in  FIG. 7C , when via hole  63  on the printed wiring board is observed after these processes, conductive paste  64  is seen to have spread around via hole  63  and woven glass fabric  68  is also moved outward from via hole  63 , compared to the initial regular arrangement, due to the pressing force applied during hot pressing and the flow of impregnated resin. The occurrence of the above phenomenon impedes efficient compression of conductive paste  64 . This phenomenon is thus a disadvantage, manifested as variations in electrical connection resistance and less reliability, in the manufacture of printed wiring boards. 
   Since thinner printed wiring boards are in increasing demand, thin woven glass fabrics are often used. Such material contains a lower density of glass fiber, which means relatively larger spaces are present between fibers, aggravating the above disadvantage. In particular, the above phenomenon becomes serious when woven glass fabrics less than 100 μm thick are used. 
   The major factors determining the compression rate of conductive paste  64  are the degree of compression in the thickness direction of board material  61  in the hot press processing in  FIGS. 6D and 6E  and the protruding distance of conductive paste  64  from board material  61  in  FIG. 6D . Since there are numerous interlayer connecting points through via holes  63  on a high-density printed wiring board, another element for effecting compression of conductive paste  64  is required in addition to controlling the above two major factors. 
   SUMMARY OF THE INVENTION 
   A manufacturing method of printed wiring boards of the present invention restricts resin flow during hot press processing. This achieves efficient electrical connection by an interlayer connector such as conductive paste. 
   In a material for manufacturing printed wiring boards of the present invention, resin of controlled fluidity in hot press processing is used. This achieves efficient electrical connection by an interlayer connector such as conductive paste. 
   Consequently, the reliability of the interlayer electrical connection, typically using conductive paste, significantly increased. Thus, the present invention offers high-quality, high-density printed wiring boards. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIGS. 1A to 1G  are sectional views illustrating each step of a manufacturing method of printed wiring boards in accordance with a first exemplary embodiment of the present invention. 
       FIGS. 2A to 2E  are sectional views illustrating each step of a manufacturing method of printed wiring boards in accordance with a second exemplary embodiment of the present invention. 
       FIG. 3A  is a sectional schematic view illustrating a via formation step in a manufacturing method of printed wiring boards in accordance with a third exemplary embodiment of the present invention. 
       FIG. 3B  is a top view of the via before filling conductive paste in the manufacturing method of printed wiring boards in accordance with the third exemplary embodiment of the present invention. 
       FIG. 3C  is a top view of the via after filling conductive paste in the manufacturing method of printed wiring boards in accordance with the third exemplary embodiment of the present invention. 
       FIGS. 4A to 4H  are sectional views illustrating each step of a manufacturing method of printed wiring boards in accordance with a fourth exemplary embodiment of the present invention. 
       FIG. 5A  is a sectional schematic view illustrating a via formation step in a manufacturing method of printed wiring boards in accordance with a fifth exemplary embodiment of the present invention. 
       FIG. 5B  is a top view of the via before filling conductive paste in the manufacturing method of printed wiring boards in accordance with the fifth exemplary embodiment of the present invention. 
       FIG. 5C  is a top view of the via after filling conductive paste in a manufacturing method of printed wiring boards in accordance with the fifth exemplary embodiment of the present invention. 
       FIGS. 6A to 6G  are sectional views illustrating each step of a conventional manufacturing method of printed wiring boards. 
       FIG. 7A  is a sectional schematic view illustrating a via formation step in the conventional manufacturing method of printed wiring boards. 
       FIG. 7B  is a top view of the via before filling conductive paste in the conventional manufacturing method of printed wiring boards. 
       FIG. 7C  is a top view of the via after filling conductive paste in the conventional manufacturing method of printed wiring boards. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Exemplary embodiments of the present invention are described below with reference to drawings. Those with the same configurations are given the same reference marks for reasons of simplicity. 
   First Exemplary Embodiment 
     FIGS. 1A to 1G  are sectional views illustrating each step of a manufacturing method and manufacturing material of printed wiring boards in a first exemplary embodiment of the present invention. 
   As shown in  FIG. 1A , film  12  with a thickness of 20 μm is pasted on both faces of board material  11 . Board material  11  is made of a prepreg 100 μm thick using woven glass fabric as reinforcement, impregnated with thermosetting epoxy resin. Film  12  is made of polyethylene terephthalate (PET). Film  12  can be coated with thermosetting resin such as epoxy resin as required. 
   Then, as shown in  FIG. 1B , via holes  13  with a diameter of about 200 μm are created using a CO 2  gas laser beam. 
   Then, as shown in  FIG. 1C , conductive paste  14  is injected into via holes  13 , typically by screen printing. Conductive paste  14  is made by mixing copper powder particles about 5 μm in diameter, thermosetting resin, and curing agent. Solvent can also be added to the conductive paste  14  to adjust its viscosity. 
   Next, as shown in  FIG. 1D , conductive paste  14  protrudes from board material  11  for about the thickness of film  12  after film  12  on both faces of board material  11  is peeled off. Copper foil  15  is then placed on both of these faces. 
   Next, hot press processing is applied to heat and press board material  11  vertically into the shape shown in  FIG. 1E . In this process, thermosetting resin in board material  11  flows to create leaked portion  16 . 
   Next, as shown in  FIG. 1F , a peripheral area of board material  11  is cut to a predetermined size. Copper foil  15  is etched to form a pattern of circuit  17  to complete a double-sided printed wiring board (C-stage) as shown in  FIG. 1G . 
   In the above processes, a weight of resin which becomes fluid in the hot press process and flows out to the peripheral area of board material  11  in proportion to the weight of board material  11  before hot-pressing (i.e., the percentage of weight of leaked portion  16  in  FIG. 1E ) is called the resin flow rate. The resin flow rate needs to be 20% or less to solve the disadvantage of insufficient electrical connection by conductive paste  14 , as described in the Background Art. 
   Table 1 shows some of the results of an experiment on the resin flow rate examined by the inventor. 
   Table 1 summarizes the following measurement results. 
   1) Thickness of the board material before and after the hot press process; 2) The resin flow rate calculated based on the weight of the leaked portion generated in the peripheral area in the hot press process and the weight of the board material before the hot press process; and 
   3) Via connection resistance (average) per point calculated based on the electrical resistance of a test pattern circuit in which 500 via holes create series circuits using the copper foil on the top and rear faces of the board. 
   Sample 1 has a resin flow rate of 22.8%, and via connection resistance varies between several ohms and several hundred ohms. Worse, vias without electrical connection are present. Observation of the section of the via in Sample 1 reveals that the conductive particles in conductive paste  14  have flowed out. 
   However, if the hot press conditions are adjusted to reduce the resin flow rate, as in Samples 2 to 7, the via connection resistance falls. By maintaining the resin flow rate at 20% or less, a practical via connection resistance is achievable. Moreover, samples show preferable characteristics in the reliability tests when the resin flow rate is 20% or less, including measurements of secular changes in via connection resistance after being shelved for a long period at high temperature and high humidity. Furthermore, as apparent from the results shown in Table 1, good initial via connection resistance is achieved when the resin flow rate is 10% or less. In this case, even better results are also obtained with respect to reliability. 
   A lower resin flow rate is effective for achieving better electrical connections. On the other hand, as shown by the results of Sample 7, a resin flow rate of 1% or more is needed for good moldability of board material  11  during hot pressing. The “filling defect” indicated in Table 1 refers to the following phenomenon. After the hot press process, a whitish area is visible on the board of Sample 7. When this portion is magnified, bubbles and a rough surface of the board material are observed. This happens when the resin flow is insufficient for smoothening the rough surface of an inner-layer circuit, causing bubbles and unevenness. This is a defect, called measling, that is known to occur during the manufacture of printed wiring boards. When this measling occurs, characteristics such as the strength against peeling the copper foil and solder heat resistance are adversely affected. 
   
     
       
             
             
             
             
             
             
           
             
             
             
             
             
             
           
         
             
               TABLE 1 
             
             
                 
             
             
                 
               Board material 
               Board material 
                 
               Via 
                 
             
             
               Sam- 
               thickness 
               thickness 
               Resin 
               connection 
                 
             
             
               ple 
               before hot 
               after hot 
               flow 
               resistance 
                 
             
             
               No. 
               pressing (μm) 
               pressing (μm) 
               rate (%) 
               (mÙ) 
               Remarks 
             
             
                 
             
           
           
             
                 
             
           
        
         
             
               1 
               130 
               85 
               22.8 
               Large 
                 
             
             
                 
                 
                 
                 
               variation 
             
             
               2 
               130 
               90 
               20.3 
               18 
             
             
               3 
               130 
               100 
               15.2 
               10 
             
             
               4 
               130 
               110 
               10.1 
               5 
             
             
               5 
               130 
               120 
               5.1 
               4 
             
             
               6 
               130 
               125 
               2.5 
               4 
             
             
               7 
               130 
               129 
               0.5 
               3 
               Filling 
             
             
                 
                 
                 
                 
                 
               defect 
             
             
                 
             
           
        
       
     
   
   One effective means of achieving a resin flow rate of 20% or less, as mentioned above, is to control the rate of temperature rise to 3° C./min or slower during the hot pressing process. However, taking account of the possibility for an extremely long hot press process or the detrimental effect on resin moldability due to too slow of a temperature rise, the temperature rise speed is preferably set to 0.5° C./min or faster. 
   Also with consideration of the change in viscosity caused by increasing the temperature of resin contained in board material  11 , the temperature rise speed can be limited to 3° C./min or less only during the period when the flow rate increases in the hot press process, and the temperature is increased at a higher speed at other times in the process. 
   Alternatively, the characteristics of board material  11  can be controlled to achieve the above effect. Uncured epoxy resin is heated and dried to control its volatile ingredient, residual solvent, and progress in thermosetting by heating temperature and time to allow adjustment of its curing time. Based on this method, the curing time, which represents the characteristic of fusion and curing of the board material during hot pressing, is set to 110 seconds or less. The use of the board material containing such resin material allows the resin flow rate to be limited to 20% or less, assuring good electrical connection between layers by conductive paste. 
   To avoid the occurrence of the aforementioned filling defect, the curing time is preferably set to 10 seconds or longer. Furthermore, with consideration of bondability between copper foil  15  and board material  11  in  FIG. 1E  or balancing of variations in the resin amount, the curing time is preferably set to 50 seconds or longer. 
   Second Exemplary Embodiment 
   The first exemplary embodiment refers to double-sided printed wiring boards. As shown in  FIGS. 2A to 2E , the present invention is also applicable to the manufacture of multi-layer printed wiring boards. 
   First, the double-sided printed wiring board (C-stage) as shown in  FIG. 2A  is prepared. Then, as shown in  FIG. 2B , board material  11  filled with conductive paste  14  and copper foil  15  are positioned on the top and rear faces of the double-sided printed wiring board, and heat and pressure are applied, typically by hot pressing equipment. This results in molding and curing of board material  11 , as shown in  FIG. 2C . At this point, the ingredients of board material  11  flowing out forms leaked portion  26 . 
   In the above processes, the weight of leaked portion  26  in proportion to the weight of two board materials  11  (resin flow rate) is set to 20% or less using the methods described in the first exemplary embodiment. 
   Next, after cutting off the unneeded peripheral area to achieve the shape shown in  FIG. 2D , copper foil  15  is patterned, typically by etching, to create circuit  27 , and to obtain a four-layer printed wiring board as shown in  FIG. 2E . Also, in the manufacture of the above multi-layer printed circuit board, a good electrical connection between layers is established by applying the manufacturing method and material of printed wiring boards of the present invention. 
   To fill the rough surface of the circuit on the inner-layer printed wiring board, a resin flow rate of 1% or more is required in the manufacture of multi-layer printed wiring boards. In addition, if the resin flow rate is limited to 20% or less by controlling the curing time of the resin contained in board material  11 , the curing time is preferably at least 50 seconds and at most 110 seconds, taking into consideration the need to fill the inner-layer circuits. 
   The double-sided printed wiring board used in the second exemplary embodiment can be either the one described in the first exemplary embodiment or a board in which layers are connected using commonly employed methods such as plating. In addition, a structure for temporarily press-bonding board material  11  onto the double-sided printed wiring board is applicable in the step shown in  FIG. 2B . 
   Third Exemplary Embodiment 
   A third exemplary embodiment of the present invention is described next with reference to  FIGS. 3A to 3C . 
   As shown in  FIG. 3A , via hole  13  is created using a laser beam on prepreg board material  11  using woven glass fabric  38 . When seen from the top, via hole  13  is created by cutting through woven glass fabric  38  as shown in  FIG. 3B . Here, fused portion  39  is formed, as shown in  FIG. 3B , by applying a predetermined processing method. 
   After forming fused portion  39 , conductive paste  14  is filled into via hole  13  and hot pressed. In this case, the spreading of conductive paste  14  to around via hole  13  is prevented, as shown in  FIG. 3C . Accordingly, conductive paste  14  establishes a good electric connection between layers. 
   If via hole  13  is created by drilling, fused portion  39  can also be created by denaturing and solidifying the resin ingredient in board material  11 , typically using the frictional heat generated by drilling to fix woven glass fabric  38  around via hole  13 . Accordingly, fused portion  39  does not need to be composed only of reinforcement such as glass fiber. The same effect is achievable if the fluidity of the resin ingredient in board material  11  in the subsequent hot pressing is canceled by curing or denaturing the resin ingredient by processing heat. It is preferable, however, to form fused portion  39  mainly made of woven glass fabric  38  by fusing or denaturing woven glass fabric  38  when forming via hole  13  using a laser beam. 
   Table 2 shows some of the inventor&#39;s experimental results. The double-sided printed wiring board is manufactured by forming via holes  13  under a range of conditions using three types of laser oscillator. As described in the first exemplary embodiment, comparisons of via connection resistance are indicated in Table 2. 
   It is apparent from these results that via connection resistance is affected by the laser wavelength. The formation of the fused portion is confirmed at an oscillation wavelength of 10.6 μm when the board material with via hole  13  is precisely observed. However, no fused portion is confirmed when the oscillation wavelength is 9.4 μm. 
   
     
       
             
             
             
             
             
           
             
             
             
             
             
           
         
             
               TABLE 2 
             
             
                 
             
             
                 
                 
               Oscillation 
               Hole 
               Via connection 
             
             
               Sample 
               Laser 
               wavelength 
               diameter 
               resistance 
             
             
               No. 
               oscillator 
               (ìm) 
               (ìm) 
               (mÙ) 
             
             
                 
             
           
           
             
                 
             
           
        
         
             
               8 
               A 
               10.6 
               200 
               3 
             
             
               9 
               A 
               10.6 
               150 
               5 
             
             
               10 
               B 
               9.4 
               200 
               10 
             
             
               11 
               B 
               9.4 
               150 
               25 
             
             
               12 
               C 
               10.6 
               200 
               2.5 
             
             
                 
             
           
        
       
     
   
   When a range of laser beams including an excimer laser, YAG harmonic wave, and carbon dioxide gas laser are used, the fused portion can be formed under selected processing conditions. However, it is preferable to form the fused portion by heating using a carbon dioxide gas laser rather than abulation, which does not use heat, typically using an excimer laser. 
   Furthermore, as described above, a laser with a wavelength of 10 μm or longer is efficient for forming fused portion  39 . The use of a carbon dioxide gas laser is also practically advantageous with respect to processing speed and cost. Viewed from A) Processing efficiency and oscillation efficiency, B) Presence of multiple oscillation wavelengths in the generated laser beam, and C) Fine processing (i.e., light-focusing by an optical system), a processing method mainly using a laser beam with a wavelength of 10 to 11 μm is preferable. 
   Fourth Exemplary Embodiment 
   A fourth exemplary embodiment of the present invention is described with reference to  FIGS. 4A to 4H . 
   As shown in  FIG. 4A , board material  41  is a prepreg 100 μm thick, reinforced by woven glass fabric and sandwiched by film  12  of 20 μm thick made of polyethylene terephthalate (PET) on both faces. Film  12  can be coated with thermosetting resin such as epoxy resin as required. Different from board material  11  used in the first exemplary embodiment, this board material  41  contains a substantial amount of a volatile ingredient after the prepreg manufacturing. Based on the weight change before and after drying at 160° C. for one hour, the amount of the volatile ingredient is 3%. 
   Subsequent steps shown in  FIGS. 4B to 4D  are the same as those in the first exemplary embodiment. 
   Next, board material  41  is loaded to a vacuum drier (not illustrated), and dried for one hour in a vacuum at about 133 Pa. While drying, a process to introduce warm air at 50° C. and reduce pressure is repeated three times for preventing a temperature drop of board material  41 . In this process, the thickness of board material  41  slightly reduces, as shown in  FIG. 4E . The reduced thickness is about 2 μm. Accordingly, the height of conductive paste  14  protruding from board material  41  increases about 1 μm respectively on the top and rear faces of the board material and the protruding height becomes about 21 μm, which was originally 20 μm before drying. 
   Next, as shown in  FIG. 4F , copper foil  15  is disposed on both faces of board material  41 , and hot pressing is applied to heat and press board material  41  in the vertical direction in  FIG. 4F . Then, a peripheral area is cut off to complete the shape shown in  FIG. 4G . 
   Copper foil  15  is then patterned, typically by etching, to form circuit  17  and complete a double-sided printed wiring board (C-stage) shown in  FIG. 1H . 
   When printed wiring boards are manufactured using the above process, the protruding height of conductive paste  14  can be slightly increased for 2 μm. This achieves better interlayer electrical connection through conductive paste. 
   Since vacuum pressure is applied in a general hot press process, most of the volatile ingredient in the board material is assumed to be removed during hot pressing. In such a manufacturing method, ingredients in the board material flow relatively greatly during hot pressing. This impedes conductive paste from being compressed in the board thickness direction to establish an electrical connection. 
   In this exemplary embodiment, the volatile ingredient in board material  41  is removed by vacuum drying before hot pressing to control the fluidity during hot pressing. In addition, the height of conductive paste  14  protruding from board material  41  is increased by removing the volatile ingredient, contributing to an increasingly effective compression rate. Compression of conductive paste  14  in hot pressing thus becomes extremely efficient, establishing a sufficient electrical connection of circuit  17  on the top and rear faces of the printed wiring board. 
   The above method is also preferably applicable to board material  21  for manufacturing multi-layer printed wiring boards as described in the second exemplary embodiment. 
   Experimental results of the fourth exemplary embodiment reveal that an amount of volatile ingredient of 0.5% or more in board material  41  shows a significant effect. Since too much volatile ingredient may degrade the shelf life of board material  41 , the amount of the volatile ingredient is preferably kept at 5% or less in board material  41 . 
   As for the volatile ingredient, it is preferable to impregnate solvent with a high boiling point such as BCA (butyl carbitol acetate) during the manufacture of board material  41 . 
   The fourth exemplary embodiment refers to vacuum drying for reducing the thickness of board material  41 . However, general drying methods using heat can be used under conditions that do not affect the properties of board material  41 . 
   Moreover, protrusion of the interlayer connector from the board material can also be secured by selectively etching the board material using dry or wet etching employing plasma or excimer laser in the step of reducing the thickness of the board material. If this method is applied, a degree of reduction of the board thickness can also be stably controlled. 
   Fifth Exemplary Embodiment 
   A fifth exemplary embodiment of the present invention is described with reference to  FIGS. 5A to 5C . 
   As shown in  FIG. 5A , via hole  13  is formed by a laser beam on board material  51  which is a prepreg using woven glass fabric. Board material  51  contains filler  510  as solids content. 
   A general board material is manufactured by impregnating woven glass fabric  38  with liquid called varnish, which is thermosetting resin typically diluted by solvent, and then volatilizing a volatile ingredient such as solvent and adjusting a curing degree of thermosetting resin in the drying process. Board material  51  used in the fifth exemplary embodiment is manufactured by dispersing filler in this varnish. In this exemplary embodiment, silica filler containing silica (SiO 2 ) with about 1 to 2 μm diameter is used. 
   As shown in  FIG. 5A , a low fluidity layer  511  is formed around via hole  13 . Low fluidity layer  511  is formed mainly through the next phenomenon, including denature of thermosetting resin around via hole  13  by heat. This heat is generated by converting processing energy that filler  510  absorbs during laser processing. In addition, a layer of denatured thermosetting resin is formed using filler  510 , which is solids content, as a core. This layer is formed very efficiently when filler  510  exists, compared to the case without filler  510 . Low fluidity layer  511  naturally has the possibility of containing woven glass fabric  38 . 
   After low fluidity layer  511  is formed, conductive paste  14  is filled in via hole  13 . When hot pressing is applied, spreading of conductive paste  14  around via hole  13  is prevented, as shown in  FIG. 5C . Accordingly, a good interlayer electrical connection is achieved by conductive paste  14 . 
   Low fluidity layer  511  can also be formed by denaturing the resin ingredient in board material  51  by friction heat generated while drilling via hole  13 , and solidifying this denatured resin with filler  510 . However, it is preferable to make filler  510  absorb energy while forming via hole  13  by a laser beam, and convert it to heat for forming low fluidity layer  511 . 
   With respect to the laser wavelength in the above process, low fluidity layer  511  is formed efficiently when an oscillation wavelength of 9 μm or longer is used for the CO 2  gas laser. In addition, materials other than silica can be used for filler  510 . The same effect is achievable with talc, gypsum powder, metal hydroxides (e.g. aluminum hydroxide), etc. 
   In all of the above exemplary embodiments, the board material refers to B-stage material in which thermosetting resin is impregnated in woven glass fabric. However, it is apparent that nonwoven fabric may be used instead of woven glass fabric. Furthermore, organic fiber such as alamid fiber may be used instead of glass fiber. 
   In the first, second and fourth exemplary embodiments, a B-stage film can also be used instead of a prepreg for the board material. 
   In addition, a mixed material of woven and nonwoven fabric, such as nonwoven glass fabric sandwiched by two sheets of glass fiber, may be used as reinforcement. 
   In all of the above exemplary embodiments, thermosetting resin refers to epoxy resin. However, one of the following substances, thermosetting resin compositions made of a mixture of two or more of the following substances, or these thermosetting resin compositions denatured by thermoplastic resin can be used: epoxy melamine resin, unsaturated polyester resin, phenol resin, polyimide resin, cyanate resin, cyanic acid ester resin, naphthalene resin, urea resin, amino resin, alkyd resin, silicic resin, furan resin, polyurethane resin, amino alkyd resin, acrylic resin, fluorocarbon resin, polyphenylene ether resin, and cyanate ester resin. Flame retardant or inorganic filler can also be added as required. 
   Moreover, a circuit typically made of metal foil which is tentatively secured on a support can be used instead of the copper foil. 
   In the exemplary embodiments, the interlayer connector refers to conductive paste made by mixing conductive particles such as copper powder, curing agent, and thermosetting resin. Instead, diverse compositions such as those mixed with polymer material and conductive particles in an appropriate viscosity, which allows them to be discharged to the board material during hot pressing, or those with solvent can also be used. Furthermore, a conductive protruding post formed typically by plating, other than conductive paste, or an independent conductive particle, not in the form of paste, having a relatively large particle size can also be used as the interlayer connector. 
   INDUSTRIAL APPLICABILITY 
   The manufacturing method of printed wiring boards of the present invention has one of the following steps of: 
   A) restricting the resin flow during hot pressing; 
   B) joining fiber reinforcements together by fusion or adhesion; 
   C) thinning a board material after the filling process; and 
   d) forming a low fluidity layer with a filler mixed in the board material. 
   The material for manufacturing printed wiring boards of the present invention is given a property to allow the resin flow to be controlled during hot pressing, or contains a volatile ingredient to allow the thickness of the board material to be efficiently reduced after the filling process. Accordingly, the present invention efficiently establishes electrical connection by the interlayer connector such as conductive paste. 
   In particular, the use of a woven fabric as reinforcement for the board material demonstrates an exceptional effect for stabilizing interlayer connection while keeping the advantage of dimensional stability of the woven fabric. This is achieved by controlling the fluidity or applying a treatment to partially prevent the movement of fiber in an area of interlayer connection at the same time as when holes are created, or by reducing the thickness of the board material. 
   Consequently, the reliability of interlayer electrical connection by the interlayer connector such as conductive paste improves, providing high-quality, high-density printed wiring boards.