Abstract:
In a method of manufacturing a circuit forming board, a first sheet having a first direction is transferred in a second direction, so that the first direction of the first sheet is parallel to the second direction. Films are stuck onto both surfaces of the first sheet while transferring the first sheet in a third direction orthogonal to the first direction of the first sheet. This method allows connecting member, such as conductive paste, to electrically coupling between layers of the circuit forming board.

Description:
TECHNICAL FIELD  
       [0001]     The present invention relates to a method of manufacturing a circuit forming board used in electronic apparatuses.  
       BACKGROUND OF THE INVENTION  
       [0002]     As electronic apparatuses have had a reduced size and included dense arrangement in their packages, circuit-forming boards having electronic components mounted thereon employs double-sided and multi-layer boards which can have more circuits and more components mounted thereon.  
         [0003]     A conventional circuit forming board disclosed in Japanese Patent Laid-Open Publication No. 6-268345 will be explained.  FIG. 6  illustrates a method of manufacturing prepreg sheet  13 , a material of the board. A fiber sheet, such as glass cloth  11 , used as a reinforcing member is dipped into varnish  12 , impregnating impregnation material containing solvent having thermosetting resin diluted therein, thus impregnating glass cloth  11  with a predetermined amount of varnish  12 . In order to adjust the amount of varnish  12 , varnish  12  is squeezed by rollers, and then, is heated to change into a semi-cured state (stage B). The glass cloth in the stage B containing varnish is cut into pieces having a predetermined size, thus providing prepreg sheets  13  as material of circuit forming boards.  
         [0004]     As shown in  FIG. 6 , prepreg sheet  13  is cut into a rectangle having longitudinal direction  202 . The longitudinal direction  202  is oriented along a machine direction (MD), a direction in which glass cloth  11  is transferred. Film is stuck onto each surface of prepreg sheet  13  by a method shown in  FIGS. 7 and 8 .  
         [0005]      FIG. 7  is a perspective view illustrating a process of sticking the film on prepreg sheet  13 .  FIG. 8  illustrates prepreg sheet  13  and film  14  seen along direction  204  in the process. Prepreg sheet  13  is introduced between films  14  along direction  201  (MD), i.e. the longitudinal direction  202 , and then, films  14  are pressed onto prepreg sheet  13  by heated rollers  15 , thereby heating and pressing prepreg sheet  13 . Since the resin impregnated in prepreg sheet  13  is in the semi-cured state, the resin melts due to the heating, hence allowing film  14  to adheres temporarily onto prepreg sheet  13 . Then, films  14  are cut into pieces having predetermined sizes along line  203 , hence providing laminated prepreg sheet  16 .  
         [0006]     A method of manufacturing the circuit forming board will be described hereinafter.  FIGS. 9A-9G  show sectional views of the circuit forming board for illustrating the method of manufacturing the board.  FIG. 9A  shows laminated prepreg sheet  16  including prepreg sheet  13  and films  14  adhering onto both surfaces of sheet  13 . As shown in  FIG. 9B , prepreg sheet  16  is pierced by laser beam, thereby forming a via-hole  17 . The hole  17  is then filled with conductive paste  18  by printing, as shown in  FIG. 9C . Conductive paste  18  is formed by mixing metallic particles, such as copper, with thermosetting resin, such as epoxy resin. Then, as shown in  FIG. 9D , films  14  are peeled off. Since a small amount of resin component of films  14  melting on the surfaces of prepreg sheet  13  to allow films  14  to be temporarily stuck onto prepreg sheet  13 , films  14  can be peeled off easily. As shown in  FIG. 9D , paste  18  projects from sheet  13  by the thicknesses of films  14  after the peeling of the films. Then, as shown in  FIG. 9E , copper foils  19  are placed on upper and lower surfaces of sheet  13 , and then is hot-pressed by a hot presser, such as a vacuum hot presser. The resin component of sheet  13  melts and undergoes molding and curing steps, and then conductive paste  18  is compressed. As a result, copper foils  19  on both surfaces of sheet  13  are electrically connected to paste  18 , as shown in  FIG. 9F . Then, copper foils  19  are etched to have a predetermined pattern for forming circuit  20 , as shown in  FIG. 9G , thus providing a double-sided circuit forming board.  
         [0007]     In the foregoing method, sheet  13  shown in  FIG. 9D , upon having an uneven thickness, causes variation of a compressing rate of paste  18  at the hot-pressing, as shown in  FIGS. 9E and 9F . The variation causes variation of a resistance of paste  18  which connects circuits  20  on both surfaces of the circuit forming board, thereby adversely affecting quality and reliability of the board.  
         [0008]     That is, via-hole  17  formed in a thick portion of prepreg sheet  13 , as shown in  FIG. 9D , reduces a compression rate of paste  18 , hence increasing a resistance of paste  18 .  
         [0009]     On the contrary, via-hole  17  formed in a thin portion of sheet  13  increases the compression rate of paste  18 , hence reducing a resistance of paste  18 . As such, the compression rate actually varies from a standard rate estimated based on an amount of the metallic particles mixed with the thermosetting resin, hence reducing the reliability of the coupling.  
         [0010]     If the circuit forming board is required to have equal resistances, for instance, for handling high frequency signals, it is undesirable to allow electric resistance of paste  18  to vary according to a position of the board.  
         [0011]     Even after Films  14  is stuck onto prepreg sheet  13  having a large variation of the thickness, the variation of the thickness of sheet  13  still remains, hence allowing the variation of the thickness of the sheet  13  to remain even in laminated prepreg sheet  16 . This problem also occurs in cases other than the case that glass cloth  11 , woven fabric, is employed as material of prepreg sheet  13 , i.e., the fiber sheet as the reinforcing member. This problem may occur in the case that unwoven fabric is employed. Woven fabric is impregnated with amount of varnish  12  less than unwoven fabric. A larger amount of varnish  12  is accordingly attached at the surfaces of prepreg sheet  13 , hence allowing the variation of the thickness of the woven fabric to affect electrical properties of conductive paste  18 .  
         [0012]     As thinner circuit forming boards have been recently demanded, glass cloth  11  becomes thin. This increases the amount of varnish  12  attached at the surface of prepreg sheet  13 , hence making the above problem more serious.  
         [0013]     In order to prevent the problem, it is essential that prepreg sheet  13  shown in  FIG. 9A  has an even thickness.  
         [0014]     However, an accuracy limitation of manufacturing apparatuses limits flatness of the thickness of prepreg sheet  13  to a certain level when varnish  12  is squeezed with rollers, as shown in  FIG. 6 , and when films  14  are stuck onto both surfaces of sheet  13 .  
       SUMMARY OF THE INVENTION  
       [0015]     In a method of manufacturing a circuit forming board, a first sheet having a first direction is transferred in a second direction, so that the first direction of the first sheet is parallel to the second direction. Films are stuck onto both surfaces of the first sheet while transferring the first sheet in a third direction orthogonal to the first direction of the first sheet.  
         [0016]     This method allows connecting member, such as conductive paste, to electrically coupling between layers of the circuit forming board.  
     
    
     BRIEF DESCRIPTIONS OF THE DRAWINGS  
       [0017]      FIG. 1  is a perspective view illustrating a method of manufacturing a prepreg sheet in accordance with an exemplary embodiment of the present invention.  
         [0018]      FIGS. 2A-2C  are perspective and sectional views of the prepreg sheet in accordance with the embodiment.  
         [0019]      FIG. 3  is a perspective view illustrating the method of manufacturing the prepreg sheet in accordance with the embodiment.  
         [0020]      FIG. 4  is a sectional view illustrating the method of manufacturing the prepreg sheet in accordance with the embodiment.  
         [0021]      FIGS. 5A-5G  are sectional views of the circuit forming board for illustrating the method of manufacturing the board in accordance with the embodiment.  
         [0022]      FIG. 6  is a perspective view illustrating a conventional method of manufacturing a prepreg sheet.  
         [0023]      FIG. 7  is a perspective view illustrating the conventional method.  
         [0024]      FIG. 8  is a side sectional view illustrating the conventional method.  
         [0025]      FIGS. 9A-9C  are sectional views of the circuit forming board for illustrating the conventional method of manufacturing the board.  
         [0026]      FIGS. 10A-10C  are perspective and sectional views of the conventional prepreg sheet. 
     
    
     DESCRIPTION OF PREFERRED EMBODIMENT  
       [0027]     According to experiments, causes of a variation of a thickness of conventional prepreg sheet  13  manufactured by a method shown in  FIGS. 6-8  was confirmed.  
         [0028]     Conventional prepreg sheet  13  having an uneven thickness shown in  FIG. 9A  will be described.  FIG. 10A  is a perspective view of prepreg sheet  13 .  FIG. 10B  is a sectional view of prepreg sheet  13  taken along line  10 B- 10 B shown in  FIG. 10A .  FIG. 10C  is a sectional view of prepreg sheet  13  taken along line  10 C- 10 C shown in  FIG. 10A .  
         [0029]     Films  14  were peeled off from both surfaces of sheet  13  to observe the cross section of sheet  13 . A larger variation of the thickness was found in the section shown in  FIG. 10B  taken along line  10 B- 10 B shown in  FIG. 10A . A smaller variation of the thickness was found in the cross section shown in  FIG. 10C  taken along line  10 C- 10 C shown in  FIG. 10A .  
         [0030]     As shown in  FIG. 6 , glass cloth  11  impregnated with liquid varnish  12  is transferred in a machine direction (MD)  201  while varnish  12  is squeezed with rollers in order to adjust an amount of varnish  12  impregnated in the glass cloth. Along MD  201 , a small variation of the amount of varnish  12  was found, namely, a variation of the thickness of sheet  13  was small. However, a large variation of the thickness of sheet  13  was found along a direction orthogonal to MD  201 .  
         [0031]      FIG. 1  shows a perspective view illustrating a method of manufacturing prepreg sheet  3  in accordance with an exemplary embodiment of the present invention. Fiber sheet used as a reinforcing member, such as glass cloth  1 , is dipped into varnish  2 , i.e. impregnating impregnation material formed by diluting thermosetting resin with solvent, and glass cloth  1  is impregnated with a predetermined amount of varnish  2 . In order to adjust the amount of varnish  2  impregnated, varnish  2  is squeezed with rollers, and is then heated to be in a semi-cured state (stage B). The glass cloth in stage B containing varnish is cut into pieces having predetermined sizes, thus providing prepreg sheet  3 , a material of a circuit forming board.  
         [0032]     As shown in  FIG. 1 , prepreg sheet  3  is cut to shaped in a rectangle having a short-side direction  102  is oriented along the direction  201  in which glass cloth  1  is transferred, namely, a machine direction (MD).  
         [0033]      FIG. 1  shows a single prepreg sheet  3 ; however, plural prepreg sheets  3  may be actually arranged in a row with taking into consideration about a width of glass cloth  1  and intervals of cutting. For example, glass cloth  1  has a width of about 1 m provides two prepreg sheets  3  each having a long-side of 400 mm and a short-side of 300 mm wide arranged in a row by cutting glass cloth  1 .  
         [0034]      FIG. 2A  is a perspective view illustrating prepreg sheet  3  thus manufactured.  FIG. 2B  is a sectional view of prepreg sheet  3  taken along line  2 B- 2 B shown in  FIG. 2A .  FIG. 2C  is a sectional view of prepreg sheet  3  taken along line  2 C- 2 C shown in  FIG. 2A . A variation of the thickness along a short-side direction shown in  FIG. 2B  is larger than a variation of the thickness along the long-side direction shown in  FIG. 2C .  
         [0035]     Next, films  4  are pressed onto prepreg sheet  3  with heated rollers  5 A and  5 B, and are stuck onto sheet  3 , as shown in  FIGS. 3 and 4 . While films  4  are adhered, the long-side direction  103  of sheet  3  matches MD  104 , namely, the direction in which sheet  3  is transferred.  
         [0036]     Factors, such as variations of diameters of heated rollers  5 A and  5 B, a pressure applied to them, a heating temperature, and parallelism between rollers  5 A and  5 B tend to disperse along axial direction  105  of the heated rollers. As shown in  FIG. 3 , axial direction  105  of rollers  5 A and  5 B matches short-side direction  102  of prepreg sheet  3 , thereby reducing the width of sheet  3  with respect to MD  104  while films  4  are stuck onto sheet  3 . This arrangement reduces adverse effect due to the variations of rollers  5 A and  5 B, hence increasing adhesion quality of films  4  onto sheet  3 .  
         [0037]     According to the embodiment, MD  101  for manufacturing prepreg sheet  3  is orthogonal to MD  104  for sticking film  14  with respect to prepreg sheet  3 .  
         [0038]     This arrangement reduces the variation of the thickness of sheet  3 .  
         [0039]     Variations of the thickness, the heating temperature, and a pressure along MD  101  and MD  103  are small during the manufacturing of prepreg sheet  3  and the sticking of films  14 . However, the variations are large along a direction orthogonal to MD  101  and MD  103 . Therefore, if MD  101  matches MD  103  with respect to prepreg sheet  3  during the manufacturing of the prepreg sheet and the sticking of films  14 , the variation of the thickness of sheet  3  remains until the circuit forming board is manufactured.  
         [0040]     However, if MD  101  for manufacturing prepreg sheet  3  is orthogonal to MD  103  for sticking films  14  with respect to prepreg sheet  3 , the variations occurring in the both processes are cancelled, hence providing sheet  3  in laminated prepreg sheet  6  with a uniform thickness.  
         [0041]     In other words, the variation of the thickness due to the variation of the amount of varnish  12  attached to fiber sheet  3  during the manufacturing of sheet  3  is reduced by heating and pressing prepreg sheet  6  during the sticking of films  4 .  
         [0042]     Next, a method of manufacturing the circuit forming board in accordance with the embodiment will be described with reference to  FIGS. 5A-5G . Laminated prepreg sheet  6 , a material of the board, shown in  FIG. 5A  includes prepreg sheet  3  and films on both surfaces of sheet  3 . As shown in  FIG. 5B , sheet  3  is pierced by laser beam, thereby forming via-holes  7 , and then, via-holes  7  are filled with conductive paste  8  by printing. Conductive paste  8  is formed by mixing metallic particles, such as copper, with thermosetting resin, such as epoxy resin.  
         [0043]     Then, as shown in  FIG. 5D , films  4  are peeled off from sheet  6 . Since just a small amount of resin component of films  14  melting on the surfaces of sheet  3  to allow films  4  to be temporarily stuck onto sheet  3 , films  4  can be peeled off easily. As shown in  FIG. 5D , paste  8  projects from sheet  3  by thicknesses of films  4  after the peeling. Next, as shown in  FIG. 5E , copper foils  9  are placed on upper and lower surfaces of sheet  3 , and are then hot-pressed by a hot presser, such as a vacuum hot presser. The resin component of sheet  3  melts and undergoes molding and curing steps, and then, conductive paste  8  is compressed. As a result, copper foils  9  on both surfaces of sheet  3  are electrically connected to paste  8 , as shown in  FIG. 5F . Then, copper foils  9  are etched to have predetermined pattern for forming circuit  10 , as shown in  FIG. 5G , thus providing a double-sided circuit forming board.  
         [0044]     Films  4  were peeled off from laminated prepreg sheet  6  shown in  FIG. 5A , and then, the thickness of prepreg sheet  3  was measured. It was observed that the sheet has a uniform thickness also from the appearance of films  4  of laminated sheet  6 . Since having uniform surfaces, the sheet  3  stabilizes an electric resistance of conductive paste  8  becomes stable, and can be handled easily during the forming of the via-holes, thus having stable dimensions in each process.  
         [0045]     According to the embodiment, prepreg sheet  3 , a board material is made of woven fabric, i.e. a fiber sheet, such as regular glass cloth  1 , or non-woven fabric, impregnated with thermosetting resin in stage B. The fiber sheet can employ organic fabric, such as aramid, instead of the glass cloth.  
         [0046]     Varnish  2  can be replaced with inorganic material which is cured to provide a rigid board, other than the thermosetting resin. A film base or a film in stage B that does not include fiber sheet, the reinforcing member, may be used as prepreg sheet  3 .  
         [0047]     Fiber sheet  1 , the reinforcing member, may employ material containing woven fabric and unwoven fabric mixed with the woven fabric, for example, may employ material including two glass fiber sheets and non-woven glass fiber sheet between the two sheets.  
         [0048]     The thermosetting resin according to this embodiment may be one of epoxy resin, epoxy-melamine resin, unsaturated polyester resin, phenol resin, polyimide resin, cyanate-based resin, ester cyanate resin, naphtalene resin, urea resin, amino resin, alkyd resin, silicon resin, furan-based resin, polyurethane resin, aminoalkyd resin, acrylic resin, fluoro resin, polyphenylene ether resin, ester cyanate resin. The thermosetting resin may be thermosetting resin composition formed by mixing plural resins selected from the foregoing resins, or thermosetting resin composition modified by thermoplastic resin. Fire-retardant or inorganic filler may be added to the thermosetting resin if necessary.  
         [0049]     According to this embodiment, the method of manufacturing a double-sided circuit forming board is described. Processes of this method may be repeated for manufacturing a multi-layer circuit forming board, that is, circuit boards including prepreg sheets filled with conductive paste may be attached to each other, thereby providing a multi-layer circuit board.  
         [0050]     Conductive paste  8  for coupling copper foils  9  is formed by mixing conductive particles, such as copper powder, with thermosetting resin containing curing agent. Paste  8  may be formed by mixing the conductive particles with polymer material having an appropriate viscosity which is discharged into the board material at the heat-pressing. The paste may be formed by mixing the conductive particles with solvent.  
         [0051]     Instead of the conductive paste, post-shaped conductive projections formed by plating may be used. Conductive particles having large diameters and not formed into paste may be placed between copper foils  9  for coupling.  
         [0052]     Boards are pierced for plating after the heat-pressing similarly to an ordinary multi-layer printed circuit board, so that layers of the boards can be coupled. Such boards nay be manufactured by the method according to the embodiment.  
       INDUSTRIAL APPLICABILITY  
       [0053]     The circuit forming board according to the present invention employs coupling member, such as conductive paste, between respective layers, hence providing stable and quality electrical connection between copper foils on both surfaces of the board.