Abstract:
A flexible printed wiring board includes a first conductor layer in the element mounting part adjacent to the top surface of the wiring board; a second conductor layer in the element mounting part adjacent to the bottom surface of the wiring board; and a third conductor layer between the first conductor layer and the second conductor layer, wherein the first and third conductor layers extend through and beyond the bending part, and the second conductor layer is absent in the bending part.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation of U.S. application Ser. No. 11/229,672 filed on Sep. 20, 2005, and claims the benefit of Japanese Application No. 2004-273669, filed on Sep. 21, 2004 in Japan, the entire contents of each of which are hereby incorporated by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    This invention relates to a flexible printed wiring board, and more particularly, to a flexible printed wiring board having an element mounting part where circuit elements are mounted and a bending part to be bent around a bending axis. 
         [0004]    2. Discussion of the Related Art 
         [0005]    Recent progress in information society invites rapid increase in information quantity, and rapid exchange and transmission of a large capacity of information data are required. Consequently, integration in electronic circuit elements has been improved, and enhancement in performance, enhancement in function and enhancement in integration are in progress for electronic information apparatus. In such a trend, printed wiring boards used in electronic information apparatus are also undergoing enhancement in thinning, miniaturization, and intensified function, and various proposals have been made for flexible printed wiring boards as well. See Patent Document Nos. 1-5, listed below, for example. 
         [0006]    Patent Document No. 1: Japanese Patent Laid-Open No. 1993-243741 
         [0007]    Patent Document No. 2: Japanese Patent Laid-Open No. 1994-216537 
         [0008]    Patent Document No. 3: Japanese Patent Laid-Open No. 1996-130351 
         [0009]    Patent Document No. 4: Japanese Patent Laid-Open No. 1996-125342 
         [0010]    Patent Document No. 5: Japanese Patent Laid-Open No. 1995-202358 
         [0011]    Among such flexible printed wiring boards, with respect to those with memory elements mounted thereon, a new structure with an increased memory capacity and speed has been proposed and put into practical use. For example, flexible printed wiring boards in which a chip size package (CSP) is mountable to both surfaces have been developed. 
         [0012]    Such a flexible printed wiring board with CSPs mounted on both surfaces is arranged to have a layered structure throughout the substrate surface, as shown in  FIGS. 15A and 15B , typically consisting of three conductor layers (PT 1  to PT 3 ), two insulating layers (IN 1  and IN 2 ) isolating these conductor layers, and two coverlay layers (CL 1  and CL 2 ) (hereafter referred to as “related art example”). In addition, in a flexible printed wiring board of the related art example, memory elements, for example, are mounted on both surfaces of an element mounting part  60 ′ as shown in  FIG. 15A . A bending part  70 ′ is bent around the bending axis AX′, so that a motherboard connecting part  80 ′, which is formed at the top surface on the other end, is made in electric contact with the motherboard MB ( FIG. 15B ). Consequently, the efficiency in implementation of memory elements on a motherboard can be improved. 
         [0013]    When the above described related art example of a flexible printed wiring board is bent along the bending axis, tensile stress is applied to the outer side, while compressive stress is applied to the inner side. In addition, depending on the curvature at the time of bending, cracks may occur in the insulating layers. Therefore, it is preferable to decrease the curvature of bending; but the decrease in the curvature of bending would impede high-density mounting of memory elements. 
         [0014]    Therefore, currently, a flexible printed wiring board with improved bending-withstanding properties is desired. 
       SUMMARY OF THE INVENTION 
       [0015]    Accordingly, the present invention is directed to a flexible printed wiring board and a method of manufacturing the same that substantially obviate one or more of the problems due to limitations and disadvantages of the related art. 
         [0016]    An object of the present invention is to provide a flexible printed wiring board that has improved crack resistance. 
         [0017]    Another object of the present invention is to provide a flexible printed wiring board that has an improved electronic continuity property. 
         [0018]    Another object of the present invention is to provide a coreless thin-type flexible wiring board that has superior crack resistance and electronic continuity properties. 
         [0019]    Another object of the present invention is to provide a flexible printed wiring board that has improved characteristic impedance. 
         [0020]    Another object of the present invention is to provide an efficient and high-yield manufacturing method for the flexible printed wiring board of the present invention. 
         [0021]    Another object of the present invention is to provide a both-surface mountable flexible wiring board that enables high-density low-profile mounting of circuit elements on a host board. 
         [0022]    Additional features and advantages of the invention will be set forth in the description which follows and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings. 
         [0023]    To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, according to one aspect of the invention, there is provided a flexible printed wiring board having an element mounting part configured to mount circuit elements on both top and bottom surfaces thereof and a bending part that is to be bent around a bending axis that extends substantially in parallel with the wiring board, the top surface of the wiring board being defined as a surface that comes to the outermost side when the bending part is bent around the bending axis, the bottom surface of the wiring board being defined as a surface that comes to the innermost side when the bending part is bent around the bending axis, the flexible printed wiring board including a first conductor layer in the element mounting part adjacent to the top surface of the wiring board; a second conductor layer in the element mounting part adjacent to the bottom surface of the wiring board; and a third conductor layer between the first conductor layer and the second conductor layer, wherein the first and third conductor layers extend through and beyond the bending part, and the second conductor layer is absent in the bending part. 
         [0024]    In another aspect, the present invention provides a flexible printed wiring board having three parts of an element mounting part, a folding part, and a board mounting part, which are laterally disposed in that order, the element mounting part being configured to mount circuit elements on both top and bottom surfaces of the wiring board, the board mounting part being configured such that the top surface of the board mounting part can be mounted on an external circuit board, the flexible printed wiring board being configured to be folded over at the folding part around a virtual folding line that extends substantially in parallel with the wiring board such that the top surface of the wiring board faces an exterior when the flexible printed wiring board is bent, the flexible printed wiring board including: a bottom conductor pattern in the element mounting part, the bottom conductor pattern being configured to mount a first circuit element on the bottom surface of the wiring board through a first plurality of pads that are directly in contact with the bottom conductor pattern, the bottom conductor pattern being absent in the folding part; a lower insulating layer on the bottom conductor pattern in all of said three parts, the lower insulating layer having a first plurality of via holes in the element mounting part; a middle conductor pattern on the lower insulating layer in all of said three parts, portions of the middle conductor pattern being electrically in contact with portions of the bottom conductor pattern via said first plurality of via holes; an upper insulating layer on the middle conductor pattern in all of said three parts, the upper insulating layer having a second plurality of via holes in the element mounting part; and a top conductor pattern on the upper insulating layer in all of said three parts, the top conductor pattern in the element mounting part being configured to mount a second circuit element on the top surface of the wiring board through a second plurality of pads that are directly in contact with the top conductor pattern, the top conductor pattern in the board mounting part being configured to mount the flexible printed wiring board to an external circuit board through a plurality of pads that are directly in contact with the top conductor pattern, portions of the top conductor pattern being electrically in contact with portions of the middle conductor pattern via said second plurality of via holes. 
         [0025]    In another aspect, the present invention provides a flexible wiring board for mounting circuit elements on both surfaces on one end, the other end of the flexible wiring board being configured to be folded over against said one end and configured to be mounted on an external host board to enable high-density low profile mounting of the circuit elements on the external host board, the flexible wiring board having a three-layered conductor structure on said one end so that the circuit elements can be mounted on both surfaces and having a two-layered conductor structure in a fold-over portion at which the flexible wiring board is to be folded over, the two layered conductor structure being configured to provide for improved crack resistance at the fold-over portion. 
         [0026]    In another aspect, the present invention provides a method for manufacturing a flexible printed wiring board having an element mounting part configured to mount circuit elements on both top and bottom surfaces thereof and a bending part that is to be bent around a bending axis that extends substantially in parallel with the wiring board, the top surface of the wiring board being defined as a surface that comes to the outermost side when the bending part is bent around the bending axis, the bottom surface of the wiring board being defined as a surface that comes to the innermost side when the bending part is bent around the bending axis, the method including the steps of: forming a first conductor layer in the element mounting part adjacent to the top surface of the wiring board; forming a second conductor layer in the element mounting part adjacent to the bottom surface of the wiring board; and forming a third conductor layer between the first conductor layer and the second conductor layer, wherein the first and third conductor layers extend through and beyond the bending part, and the second conductor layer is absent in the bending part. 
         [0027]    It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory, and are intended to provide further explanation of the invention as claimed. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0028]    The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this application, illustrate embodiments of the invention and together with the description serve to explain the principle of the invention. In the drawings: 
           [0029]      FIGS. 1A and 1B  illustrate a construction of a flexible printed wiring board according to one embodiment of the present invention.  FIG. 1A  is a schematic perspective view.  FIG. 1B  is a side view. 
           [0030]      FIG. 2  is a cross-sectional view of a flexible printed wiring board according to an embodiment of present invention. 
           [0031]      FIG. 3  is a schematic view illustrating the wiring direction of the signal conductor pattern of a flexible printed wiring board according to an embodiment of the present invention. 
           [0032]      FIGS. 4A and 4B  are cross-sectional views illustrating manufacturing steps of a flexible printed wiring board according to an embodiment of the present invention. 
           [0033]      FIG. 5  is a schematic view for showing the directions of the warp and the weft of the fiber-reinforced fabric in an insulating layer of a flexible printed wiring board according to an embodiment of the present invention. 
           [0034]      FIGS. 6A and 6B  are cross-sectional views illustrating manufacturing steps of a flexible printed wiring board according to an embodiment of the present invention. 
           [0035]      FIGS. 7A and 7B  are cross-sectional views illustrating manufacturing steps of a flexible printed wiring board according to an embodiment of the present invention. 
           [0036]      FIGS. 8A and 8B  are cross-sectional views illustrating manufacturing steps of a flexible printed wiring board according to an embodiment of the present invention. 
           [0037]      FIG. 9  is a cross-sectional view illustrating the manufacture of a flexible printed wiring board according to an embodiment of the present invention. 
           [0038]      FIG. 10  is a cross-sectional view illustrating the manufacture of a flexible printed wiring board according to an embodiment of the present invention. 
           [0039]      FIGS. 11A and 11B  are cross-sectional views illustrating manufacturing steps of a flexible printed wiring board according to an embodiment of the present invention. 
           [0040]      FIG. 12  is a cross-sectional view illustrating a manufacturing step of a flexible printed wiring board according to an embodiment of the present invention. 
           [0041]      FIGS. 13A and 13B  are cross-sectional views illustrating manufacturing steps of a flexible printed wiring board according to an embodiment of the present invention. 
           [0042]      FIG. 14  is a sectional view showing a construction of a flexible printed wiring board according to an embodiment of the present invention. 
           [0043]      FIGS. 15A and 15B  illustrate a construction of a flexible printed wiring board according to the related art.  FIG. 15A  is a schematic perspective view.  FIG. 1B  is a side view. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0044]    A flexible printed wiring board according to a preferred embodiment of the present invention includes an element mounting part where circuit elements are mounted and a bending part to be bent around the bending axis. An inner conductor layer is formed on the inside of the board. In the element mounting part, element mounting part conductor layers are formed on both surfaces at outer sides of the wiring board, respectively, and in the bending part, a bending part conductor layer is formed only on the surface on the outer side that faces the exterior when the wiring board is bent. 
         [0045]    Here, in the above described element mounting part, the element mounting part conductor layers are formed on both surfaces at the outer sides of the wiring board, respectively. In contrast, in the bending part, the bending part conductor layer is formed only on the side that faces the exterior when it is bent. Therefore, as compared with the related art example described above, this configuration allows a reduction of the number of conductor layers in the bending part by one and the number of coverlay layer by one as well. Thus, by reducing the number of layers in the bending part, the total thickness can be made thin as compared with the related art flexible printed wiring boards. Consequently, when the flexible wiring board is bent at the same curvature, the stress is reduced, and therefore crack resistance is improved. 
         [0046]    In addition, in the above described inner layer conductor layer, a conductor pattern that imparts a ground potential in terms of alternating current may be formed. Thus, by forming a conductor pattern to impart a ground potential in terms of alternating current as an interior layer conductor layer, the characteristic impedance of a signal circuit pattern (also referred to as “outer layer conductor pattern”) formed at the side that faces the exterior can be stabilized even when the flexible printed wiring board is bent along the bending axis. 
         [0047]    In particular, in the inner conductor layer in the bending part, a conductor pattern to impart the ground potential in terms of alternating current may be formed in a plane pattern. In this case, signal lines of the outer layer conductor pattern in the bending part can be formed to constitute a microstrip configuration, thereby further stabilizing the characteristic impedance. 
         [0048]    The insulating layer formed between the above described conductor patterns is preferably made of fiber-reinforced plastic. Such fiber-reinforced plastic can include, for example, fiber-glass reinforced plastic (GFRP), carbon fiber reinforced plastic (CFRP) and the like. More specifically, glass-fiber-reinforced epoxy, glass-fiber-reinforced polyester resin and the like may be used. 
         [0049]    The thickness of the above described insulating layer is preferably about 25 μm to about 65 μm. Here, when the thickness of the above described insulating layer is less than about 25 μm, it is difficult to form an insulating layer with a uniform thickness, while when the thickness exceeds about 65 μm, preferable crack resistance may not be obtained. 
         [0050]    In addition, the warp and the weft contained in the above described fiber-reinforced plastic preferably extend in the directions intersecting the direction of the above described bending axis by an angle of about 30° or more and about 60° or less. With this configuration, when the wiring board is bent, the warp and the weft mechanically cooperate to improve crack resistance. 
         [0051]    The signal conductor pattern formed in the above described bending part conductor layer preferably extends in the direction that is obliquely disposed relative to the direction of the above described bending axis. This way, when the bending part is bent around the bending axis, the crack-prevention performance of the signal conductor pattern is further improved and the occurrence of cracks can be further reduced. 
         [0052]    Incidentally, the above described circuit element can be a memory element. In this case, high density implementation of memory elements, which has been desired more and more in recent years, becomes possible. 
         [0053]    One advantage of the present invention is that a flexible printed wiring board with improved crack resistance can be provided. 
         [0054]      FIGS. 1A and 1B  shows a configuration of a flexible printed wiring board  10  according to an embodiment of the present invention.  FIG. 1A  is a perspective view thereof  FIG. 1B  is an XZ side view of the flexible printed wiring board  10  which is bent at a bending part along the bending axis AX and is attached to a motherboard MB. 
         [0055]    In this embodiment, on the upper surface (i.e., the surface facing in the +Z direction) of the flexible printed wiring board  10  shown in  FIG. 1A , pads  44 U 1  to  44 U N  for mounting a circuit element  100 A and pads  47 L 1  to  47 L N  are provided, and pads  44 U 1  to  44 U N  and pads  47 L 1  to  47 L N  are electrically connected, respectively, for example. 
         [0056]    In addition, although not shown in  FIG. 1A , on the lower surface (i.e., the surface facing in the −Z direction) of the flexible printed wiring board  10 , pads  45 U 1  to  45 U N  for mounting a circuit element  100 B are formed at locations that correspond to the pads  44 U 1  to  44 U N . For example, these pads  45 U 1  to  45 U N  and pads  47 L 1  to  47 L N  are also electrically connected, respectively. 
         [0057]    Here, in the present embodiment, both circuit element  100 A and circuit element  100 B can be memory elements of the same type. 
         [0058]    The flexible printed wiring board  10  of the present embodiment is employed as follows. As shown in  FIG. 1 , the element  100 A is mounted on the upper surface and the element  100 B is mounted on the lower surface of the flexible printed wiring board  10 . In mounting the flexible printed wiring board  10  on a host board, such as a mother board, the flexible printed wiring board  10  is bent around the bending axis AX, and is mounted on the motherboard MB, as shown in  FIG. 1B . In this case, the circuit element  100 B of the flexible printed wiring board  10  is bonded via a bonding layer  90  to a surface of the folded-over portion of the flexible printed wiring board, which is opposite to the surface having the pads  47 L 1  to  47 L N . 
         [0059]      FIG. 2  shows an XZ cross-sectional view of the flexible printed wiring board  10  of the present embodiment. As shown in  FIG. 2 , the flexible printed wiring board  10  of the present embodiment includes an element mounting parts  60 A and  60 B (also referred to as “element mounting part  60 ” collectively) where the circuit elements  100 A and  100 B as described above are to be mounted, a bending part  70  where this flexible printed wiring board is bent around the bending axis AX, and a motherboard connecting part  80 , which is to be connected to a motherboard. 
         [0060]    In addition, the flexible printed wiring board  10  includes, in the element mounting part  60 , (a) an insulating layer  13 , (b) an insulating layer  17 U formed on a surface in the +Z direction side of the insulating layer  13 , (c) an insulating layer  20 U, which is the outmost layer formed on a surface in the +Z direction side of the insulating layer  17 U and (d) an insulating layer  22  formed on a surface in the −Z direction side of the insulating layer  13 . Here, the insulating layers  20 U and  22  respectively function as coverlay layers. Since the insulating layer  22  is not formed in the bending part  70 , the bending part  70  includes the layers (a) to (c), but does not include the insulating layer  22  ( d ). 
         [0061]    In addition, the flexible printed wiring board  10  includes (e) a conductor pattern  34 U′ formed on the surface in the +Z direction of the insulating layer  13 , (f) a first conductor pattern  36 U′, which is a signal line pattern, formed on the surface in the +Z direction of the insulating layer  17 U and (g) a second conductor pattern  33 L, which is a signal line pattern, formed on the surface in the −Z direction of the insulating layer  13  in the element mounting part  60 . 
         [0062]    Here, this conductor pattern  34 U′ includes a conductor pattern to impart a ground potential in terms of alternating current (hereinafter also referred to as “ground pattern” or “GNP”), and additionally includes circuit patterns formed at via holes. The GNP may be formed in a solid pattern to cover substantially the entire area at the bending part  70 . 
         [0063]    The first conductor pattern  36 U′ includes a power supply pattern as well. Here, in the bending part  70 , the first conductor pattern  36 U′ is arranged to extend in the direction that intersects the bending axis AX (the Y direction) at an angle θ. In this embodiment, as shown in  FIG. 3 , conductor patterns P 1  to P N  (which electrically connect the pads  44 U 1 - 44 U N  to  47 L 1 - 47 L N , respectively, for example) are not disposed parallel to the X direction, but intersect a virtual line extending in the Y direction at angle θ. 
         [0064]    With the above-described configuration, because the ground potential pattern of the conductor pattern  34 U′ in the bending part  70  is formed to cover the substantially entire area, signal lines of the signal line pattern  36 U′ in the bending part  70  can be made very thin, thereby forming a microstrip configuration. In this case, the characteristic impedance can be stabilized. 
         [0065]    Moreover, the flexible printed wiring board  10  includes (h) via holes in the insulating layers  13  and  17 U, respectively, for providing interconnections among conductor pattern  36 U′ (including GNP), the first conductor pattern  36 U′, and the second conductor pattern  33 L. 
         [0066]    Here, although not depicted in  FIG. 2 , via holes are also provided in the motherboard connecting part  80  to provide interconnections between the GNP (conductor pattern  34 U′) and the first conductor pattern  36 U′. 
         [0067]    Pads  44 U 1  to  44 U N  for mounting circuit element  100 A are formed to be in contact with the first conductor pattern  36 U′. In addition, pads  45 U 1  to  45 U N  for mounting another circuit element  100 B are formed to be in contact with the second conductive pattern  33 L. 
         [0068]    Here, the length of wiring pattern connecting the pads  44 U j  (j=1 to N) with the pads  47 L j  and the length of wiring pattern connecting the pads  45 U j  with the pads  47 L j  can be made respectively the same to implement equal length wiring. 
         [0069]    As the material of the insulating layers  13  and  17 U, epoxy resin, glass-fiber-reinforced epoxy resin (hereinafter also referred to as “glass epoxy” or “prepreg”) obtained by impregnating epoxy resin into glass fiber, glass-fiber-reinforced polyimide resin obtained by impregnating polyimide resin into glass fiber, and the like can be used. In manufacturing the flexible printed wiring board of the present embodiment, glass epoxy is preferably used in terms of dimensional stability, mass productivity and thermal stability. Here, the insulating layers  13  and  17 U may be formed of the same material selected from the above described materials, or may be formed with mutually different materials. 
         [0070]    In addition, as for the insulating layers  20 U and  22  forming coverlay layers, polyimide resin coated with epoxy-based adhesive and the like can be used. In consideration of flexibility, heat resistance, insulating properties, and corrosion resistance, polyimide resin is preferable. 
         [0071]    As the material for the conductor patterns  33 L,  34 U′, and  36 U′, conductive metal such as copper, aluminum, stainless steel and the like can be used. In particular, in consideration of workability, copper is preferably used. 
         [0072]    Next, manufacturing steps of the flexible printed wiring board  10  will be described. At first, a supporting member (hereinafter also referred to as “reinforcing layer”)  11  shown in  FIG. 4A  is prepared. Here, as the supporting member  11 , from the viewpoint of ease in handling during manufacturing steps, prepreg is preferably used. Specifically, GHPL830 (manufactured by Mitsubishi Gas Chemical Company, Inc.), E679 (manufactured by Hitachi Chemical Co., Ltd.), R1661 (manufactured by Matsushita Electric Works, Ltd.) and the like can be used. In terms of costs as well as dimensional stability, R1661 is preferable. 
         [0073]    Next, a conductor film with a carrier ( 31 L,  32 U), an insulating layer  12 , and a conductor foil  32 L are prepared. The conductor film with a carrier ( 31 L,  32 U) is to be laminated on the surface in the −Z direction of the supporting member  11 . The insulating layer  12  is to be laminated on the surface in the −Z direction of the conductor film with a carrier ( 31 L,  32 U). The conductor foil  32 L is to be laminated on the surface in the −Z direction of the insulating layer  12 . In addition, a conductor film with a carrier ( 31 U,  33 L), an insulating layer  13 , and a conductor foil  33 U are prepared. The conductor film with a carrier ( 31 U,  33 L) is to be laminated on the surface in the +Z direction of the supporting member  11 . The insulating layer  13  is to be laminated on the surface in the +Z direction of the conductor film with a carrier ( 31 U,  33 L). The conductor foil  33 U is to be laminated on the surface in the +Z direction of the insulating layer  13 . 
         [0074]    The above-mentioned conductor film with a carrier can be manufactured by pressing a conductor film ( 32 U,  33 L) to adhere onto the surface of a carrier member ( 31 L,  31 U). The conductor film ( 32 U,  33 L) is attached to the carrier member by an adhesive, such as an adhesive that contains benzotriazole or benzotriazole derivative. For example, VERZONE (SF-310, manufactured by DAIWA KASEI K. K.) and the like can be used so that the resulting film can be delaminated at a later time. In addition, commercially available products may be appropriately selected and used. 
         [0075]    Such commercially available products allow subsequent delamination of a carrier member from the conductor film. The examples include Micro-thin (manufactured by Mitsui Mining and Smelting Co., Ltd.), XTR (manufactured by Olin Brass), and UTC-Foil (manufactured by METFOILS AB). 
         [0076]    Prepreg is preferably used as the insulating layers  12  and  13 . As commercially available products, prepreg with a thickness of about 25 μm to about 100 μm, such as GHPL830 (manufactured by Mitsubishi Gas Chemical Company, Inc.), E679 (manufactured by Hitachi Chemical Co., Ltd.), and R1661 (manufactured by Matsushita Electric Works, Ltd.) and the like can preferably be used in terms of the required thickness of the final product. In light of the thinning trend and improvement in crack resistance of flexible printed wiring boards, those with a thickness of about 25 μm to about 65 μm are more preferable. 
         [0077]    Here, as shown in  FIG. 5 , in the prepreg used as the insulating layers  12  and  13 , the direction of the warp WA (and therefore the weft WE) of the prepreg is preferably arranged to obliquely intersects the direction of the bending axis AX (i.e., the Y direction). 
         [0078]    The intersection angle φ is not particularly limited. However, from the viewpoint of improvement in crack resistance at the time of bending, the angle φ is preferably about 30° to about 60°. When the angle φ is about 45°, it provides the greatest prevention effect on crack occurrence in insulating layers. 
         [0079]    Referring to  FIG. 4A , the conductor film with a carrier ( 31 L,  32 U) is laminated on the reinforcing layer  11  so that the surface in the −Z direction of the reinforcing layer  11  and the surface in the +Z direction of the conductor film with a carrier ( 31 L,  32 U) are brought into contact. The insulating layer  12  is formed on the conductor film with a carrier ( 31 L,  32 U) so that the surface in the −Z direction of the conductor film with a carrier ( 31 L,  32 U) and the surface in the +Z direction of the insulating layer  12  are brought into contact. 
         [0080]    The conductor film with a carrier ( 31 U,  33 L) is laminated on the reinforcing layer  11  so that the surface in the +Z direction of the reinforcing layer  11  and the surface in the −Z direction of the conductor film with a carrier ( 31 U,  33 L) are brought into contact. The insulating layer  13  is formed on the conductor film with a carrier ( 31 U,  33 L) so that the surface in the +Z direction of the conductor film with a carrier ( 31 U,  33 L) and the surface in the −Z direction of the insulating layer  13  are brought into contact. 
         [0081]    The reinforcing layer  11  and the two insulating layers laminated as shown in  FIG. 4A  are pressed under predetermined conditions, for example, at about 185° C. under a pressure of about 40 kg/m 2  for about an hour, to produce a laminated body ( FIG. 4A ). 
         [0082]    Subsequently, a CO 2  laser process is performed to form a via hole  41 U. The opening  41 U is formed so as to reach the surface in the +Z direction of the conductor layer  33 L from the surface in the +Z direction of the insulating layer  13  (see  FIG. 4B ). 
         [0083]    To form the opening  41 U, first, a conductor layer  33 U is formed on the insulating layer  13 , and a region of the conductor layer  33 U at which the via hole  41 U will be formed on the surface in the +Z direction of the conductor layer  33 U undergoes blackening. Subsequently, this region having undergone blackening is irradiated with a laser beam having a predetermined energy from the above to form the opening  41 U. 
         [0084]    In forming a via hole  41 L on the surface in the −Z direction of the insulating layer  12 , the similar process is implemented (see  FIG. 4B ). 
         [0085]    The conductor layers  33 U and  32 L are formed by pressing the conductor film  33 U and the conductor film  32 L to adhere onto the surface in the +Z direction of the insulating layer  13  and the surface in the −Z direction of the insulating layer  12 , respectively. 
         [0086]    Copper foil and the like may be used as the conductor films  33 U and  32 L. A conductor film with a carrier can be used to form a very thin layer of the conductor films  33 U and  32 L. In such a case, the conductor film with a carrier is first laminated on the corresponding insulating layer, and thereafter the carrier member is pealed off to leave the thin conductor film on the insulating layer. 
         [0087]    Here, it is preferable to use a conductor film with a carrier having a conductor film thickness of about 3 μm to about 9 μm, such as Micro-Thin (manufactured by Mitsui Mining and Smelting Co., Ltd.), XTR (manufactured by Olin Brass), UTC-Foil (manufactured by METFOILS AB) or the like. 
         [0088]    Referring to  FIG. 6A , the remaining upper surface in the +Z direction of the conductor pattern  33 U, the side surface of the opening  41 U and the bottom surface of the opening  41 U (that is, the exposed surface in the +Z direction of the conductor pattern  33 L inside the opening  41 U) undergo metal plating so that a plated opening is formed and a conductor film  34 U is formed. Similarly, the remaining conductor pattern  32 L, the side surface of the opening  41 L, and the bottom surface of the opening  41 L undergo metal plating so that a plated opening is formed and a conductor film  34 L is formed. 
         [0089]    The plating can be performed with a copper plating bath with a composition shown in Table 1 below. 
         [0000]    
       
         
               
             
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Copper sulfate plating bath composition 
               
               
                 Plating bath 
               
             
          
           
               
                   
                 Name of compound 
                 Quantity (g/L) 
               
               
                   
                   
               
               
                   
                 Copper sulfate 
                 125 to 250 
               
               
                   
                 Sulfuric acid 
                  30 to 100 
               
               
                   
                   
               
             
          
         
       
     
         [0090]    Subsequently, referring to  FIG. 6B , a resist layer is formed on the entire upper surface of the laminated body and is patterned by a known lithography process to form a resist pattern  16 U, which covers the plated via hole  41 U′. Similarly, a resist pattern  16 L is formed on the lower surface in the −Z direction of the laminated body to cover the plated via hole  41 L′. 
         [0091]    As the resist layer, an acrylic dry film resist, such as HW440 (manufactured by Hitachi Chemical Co., Ltd.), for example, can be used. Moreover, NIT1025 (manufactured by Nippon Synthetic Chemical Industry Co., Ltd.), SA-50 (manufactured by DuPont) and the like can also be used. 
         [0092]    Subsequently, by employing a tenting process using an etching solution including copper (II) chloride or the like, the solder delamination process using a metal resist, or the micro-etching process suitable for fine pattern forming or the like, etching is performed until the surface in the +Z direction of the insulating layer  13  and the surface in the −Z direction of the insulating layer  12  are exposed (see  FIG. 7A ). 
         [0093]    As a result, a conductor pattern  34 U′ is formed on the surface in the +Z direction of the insulating layer  13 . Also, a plated non-through via hole  41 U′ for electrically connecting the conductor pattern  34 U′ to the conductor layer  33 L is formed. Likewise, on the surface in the −Z direction of the insulating layer  12 , a conductor pattern  34 L′ is formed, and a plated non-through via hole  41 L′ for electrically connecting the conductor pattern  34 L′ to the conductor layer  32 U is formed. 
         [0094]    Next, an insulating layer  17 U is formed on the surface in the +Z direction of the insulating layer  13 , and an insulating layer  17 L is formed on the surface in the −Z direction of the insulating layer  12 . Here, the insulating layers  17 U and  17 L may be formed by lamination pressing through pin lamination. For these insulating layers  17 U and  17 L, a material similar to that used for the insulating layers  12  and  13  can be used. Subsequently, conductor layers  35 U and  35 L are formed on the surface in the +Z direction of the insulating layer  17 U and on the surface in the −Z direction of the insulating layer  17 L, respectively (see  FIG. 7B ). 
         [0095]    The conductor layers  35 U and  35 L are formed by pressing the conductor film  35 U and the conductor film  35 L to adhere onto the surface in the +Z direction of the insulating layer  17 U and the surface in the −Z direction of the insulating layer  17 L, respectively. 
         [0096]    Copper foil and the like can be used as the conductor films  35 U and  35 L. A conductor film with a carrier may be used to form a very thin layer of the conductor films  35 U and  35 L. In such a case, the conductor film with a carrier is laminated on the corresponding insulating layer, and thereafter the carrier member is pealed off to leave the thin conductor film on the insulating layer. 
         [0097]    Here, it is preferable to use a conductor film with a carrier having a conductor film thickness of about 3 μm to about 9 μm, such as Micro-Thin (manufactured by Mitsui Mining and Smelting Co., Ltd.), XTR (manufactured by Olin Brass), UTC-Foil (manufactured by METFOILS AB) or the like. 
         [0098]    Subsequently, using a process similar to the process for forming the above-described openings  41 U and  41 L, an opening  42 U is formed on the insulating layer  17 U and an opening  42 L is formed on the insulating layer  17 L (see  FIG. 8A ). Subsequently, using a plating process similar to the plating process described above, conductor films  36 U and  36 L are formed (see  FIG. 8B ). Thereafter in a manner similar to the manner described above, formation of a resist layer, and etching and removal of the resist layer are performed to form conductor pattern  36 U′ and  36 L′ (see  FIG. 9 ). 
         [0099]    Next, referring to  FIG. 10 , an ink is printed and hardened to form a coverlay layer  20 U having openings  43 U in a manner similar to the photolithography method. Likewise, the cover layer  20 L having openings  43 L is formed. Here, polyimide resin such as CKSE (manufactured by NIKKAN INDUSTRIES Co., Ltd.), for example, can be used to form the coverlay layers  20 U and  20 L. In the alternative, instead of an ink, a resist film may be laminated to form the coverlay layers. 
         [0100]    Consequently, laminated bodies  10 U and  10 L are formed on the respective surfaces of the reinforcing layer  11  (see  FIG. 10 ). Here, as shown in  FIG. 10 , the laminated body  10 U includes the conductor layer  33 L, the insulating layer  13 , the insulating layer  17 U and the coverlay layer  20 U. The insulating layers  13  and  17 U are respectively provided with via holes for inter-layer connection. 
         [0101]    As shown in  FIG. 10 , the laminated body  10 L includes the conductor layer  32 U, the insulating layer  12 , the insulating layer  17 L and the coverlay layer  20 L. The insulating layers  12  and  17 L are respectively provided with via holes for inter-layer connection. 
         [0102]    The following steps will be described with reference to the laminated body  10 U. The laminated body  10 L will be processed in the same or similar manner. The laminated body  10 U formed on the surface in the +Z direction of the reinforcing layer  11  is separated from the reinforcing layer  11  at the interface between the carrier member  31 U and the conductor layer  33 L (see  FIG. 11A ). 
         [0103]    Subsequently, using the conductor layer  33 L, which is formed on the surface in the −Z direction of the insulating layer  13 , as a plating lead, nickel plating is carried out on portions of the upper surface of the laminated body  10 U that are not covered by the coverlay layer  20 U ( FIG. 11B ). Here, the nickel plating can be conducted with a plating bath shown in Table 2 under the following conditions: pH 4 to 5, liquid temperature of 40 to 60° C. and current density of approximately 2 to 6 A/dm 2 . 
         [0000]    
       
         
               
             
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Nickel electrolytic plating bath composition 
               
               
                 Plating bath 
               
             
          
           
               
                   
                 Name of compound 
                 Quantity (g/L) 
               
               
                   
                   
               
               
                   
                 Nickel sulfate 
                  Approximately 300 
               
               
                   
                 Nickel chloride 
                 Approximately 50 
               
               
                   
                 Boric acid 
                 Approximately 40 
               
               
                   
                   
               
             
          
         
       
     
         [0104]    Subsequently, gold plating can be performed on the portion that has undergone nickel plating using a plating bath with a composition shown in Table 3 under the following conditions: liquid temperature of 20 to 25° C. and current density of 0.2 to 1.0 A/dm 2 . Here, in  FIG. 11B , the two plated layers are illustrated as one layer. 
         [0000]    
       
         
               
             
               
               
               
             
           
               
                 TABLE 3 
               
             
             
               
                   
               
               
                 Au electrolytic plating bath composition 
               
               
                 Plating bath 
               
             
          
           
               
                   
                 Name of compound 
                 Quantity (g/L) 
               
               
                   
                   
               
               
                   
                 Gold 
                 10 
               
               
                   
                 Sodium cyanide 
                 30 to 35 
               
               
                   
                 Ammonia 
                 50 to 60 
               
               
                   
                   
               
             
          
         
       
     
         [0105]    After completion of the above-described plating process, an ink is printed and hardened on the conductor layer  33 L provided on the surface in the −Z direction of the laminated body  10 U to form a resist layer  21 L in a matter similar to the photolithography method ( FIG. 12 ). Here, AUS series (manufactured by TAIYO INK MFG CO., LTD.) and DSR series (manufactured by TAMURA Corporation), for example, can be used to form the resist layer. 
         [0106]    Here, the resist layer  21 L can be formed only in the element mounting part where circuit elements will be mounted on the surface in the −Z direction of the conductor layer  33 L. Alternatively, it may be formed on the entire surface except the bending part. 
         [0107]    Subsequently, by disposing soldering paste onto the openings  43 U by screen printing and subsequently performing a solder reflow process, or by using the solder ball direct mounting method or the like, pads  44 U 1  to  44   N  are formed ( FIG. 13A ). 
         [0108]    Next, as shown in  FIG. 13A , the uncovered portion of conductor layer  33 L is etched to expose the surface in the −Z direction of the insulating layer  13 . Then, the resist layer  21 L is removed by making it come up using NaOH, thereby exposing the resulting conductor pattern  33 L. 
         [0109]    Subsequently, as shown in  FIG. 13B , a coverlay layer  22  is formed so as to cover the surface in the −Z direction of the exposed insulating layer  13  and the conductor layer  33 L, and openings  44 L are formed in a manner similar to that used for forming the openings  43 U. 
         [0110]    Subsequently, by a process similar to the process described above, pads  45 U 1  and  45 U N  are formed at the openings  44 L, thereby completing a coreless thin type flexible printed wiring board  10 . 
         [0111]    The manufacturing process of the flexible printed wiring board  10  described above provides an excellent yield. 
         [0112]    Moreover, in the above-described manufacturing method, the conductor layer  33 L is used as the plating lead for plating, and this conductor layer  33 L is processed after plating to form a conductor pattern. Therefore, the step of providing a plating lead and the step of pealing it off, which are required in the conventional art, is no longer required. This expedites the production of flexible printed wiring boards. 
         [0113]    The laminated body  10 L formed on the surface in the −Z direction of the reinforcing layer  11  undergoes the same process as the above-described process for the laminated body  10 U so that a flexible printed wiring board having the same structure as the laminated body  10 U is manufactured. 
         [0114]    In the above described embodiment, the ground pattern included in the conductor pattern  34 U′ in the bending part  70  is formed as a solid pattern that substantially covers the entire area over which signal lines are formed. Alternatively, a power source pattern that imparts a ground potential in terms of alternating current may be formed in a similar solid pattern. In this case, signal lines by the outer layer conductor pattern  36 U′ can be formed to constitute a microstrip configuration in the bending part  70 . Therefore, the characteristic impedance can be further stabilized. 
         [0115]    Moreover, as for the metal plating used for the above-described manufacture of the flexible printed wiring board, nickel plating and subsequent gold plating were employed. However, a different combination of the same or different metal materials may be used in the plating. 
         [0116]    Here, in the above described embodiment, the equal length wiring was realized by providing element mounting parts  60 A and  60 B on the left-hand side. Alternatively or in addition, as shown in  FIG. 14 , the other side (the right side) of the flexible printed wiring board  10  may be provided with the circuit pattern  33 L and the coverlay layer  22  to implement equal length wiring. 
         [0117]    The flexible printed wiring board of the present embodiment is useful as a thin type flexible printed wiring board. In particular, the flexible printed wiring board of the present embodiment has a stable in-line impedance when high-speed multi-pin logic LSIs and the like are mounted thereon and has excellent crack resistance. 
         [0118]    Moreover, the method of manufacturing a flexible printed wiring board of the present embodiment is suitable for manufacturing a thin type flexible printed wiring board with an excellent yield. 
         [0119]    The flexible printed wiring board  10  manufactured as described above is bent (folded over) along the bending axis AX after electronic circuit chips, such as the memory elements  100 A and  100 B, are mounted onto the element mounting parts  60 A and  60 B, respectively. Then, the surface in the —X direction of the memory element  100 B is affixed to the surface in the −Z direction of the flexible printed wiring board  10  with an adhesive. Subsequently, as shown in  FIG. 1B , the motherboard MB and the motherboard connecting part  80  are electrically connected so that the flexible printed wiring board with the circuit chips is mounted on electronic information apparatus. 
         [0120]    Working examples of the present invention will now be described in detail. However, the present invention will not be limited by these examples in any ways. Manufacture of Flexible Printed Circuit Boards in Working Examples 1 to 10. 
         [0121]    As the reinforcing layer  11 , R1661 (manufactured by Matsushita Electric Works, Ltd.) was used. In forming the conductor film with a carrier ( 31 L,  32 U) to be laminated on the lower surface of the reinforcing layer  11 , the conductor foils  32 L and  33 U to be laminated on the insulating layers  12  and  13 , respectively, and the conductor film with a carrier ( 31 U,  33 L) to be laminated on the upper surface of the reinforcing layer  11 , Micro-thin (manufactured by Mitsui Mining and Smelting Co., Ltd.) was used. For these conductor films with a carrier, XTR (manufactured by Olin Brass) or UTC-Foil (manufactured by METFOILS AB) may also be used in place of Micro-thin. 
         [0122]    As the insulating layers  12  and  13 , GHPL830 (manufactured by Mitsubishi Gas Chemical Company, Inc.) was used. Alternatively, E679 (manufactured by Hitachi Chemical Co., Ltd.) or R1661 (manufactured by Matsushita Electric Works, Ltd.) may also be used. The thickness of the prepreg ranged from about 25 μm to about 65 μM (as shown in Table 7 below). 
         [0123]    The prepregs used as the insulating layers  12  and  13  were arranged so that the direction of the fabric of the warp WA (and therefore the weft WE) of the prepreg obliquely intersects a line extending in the Y direction to form an angle ranging from 30° to 60°, depending on Working Examples (see Table 7 below). 
         [0124]    The reinforcing layer  11  and the insulating layers  12  and  13  were laminated as shown in  FIG. 4A  and were pressed under a pressure of about 40 kg/m 2  at about 185° C. for about one hour to form a laminated body. 
         [0125]    Then the conductor layers  33 U and  32 L were formed. Subsequently, portions on the conductor layers  33 U and  32 L over the insulating layers  12  and  13  at which the non-through via holes  41 L  41 U should be formed were blackened and irradiated with a CO 2  laser beam to form the openings  41 U and  41 L, respectively ( FIG. 4B ). 
         [0126]    Next, the remaining surface of the conductor pattern  33 U and the interior of the opening  41 U as well as the remaining surface of the conductor pattern  32 L and the interior of the opening  41 L underwent metal plating with a plating bath using the composition shown in Table 4 below to form the conductor films  34 U and  34 L ( FIG. 6A ). 
         [0000]    
       
         
               
             
               
               
               
             
           
               
                 TABLE 4 
               
             
             
               
                   
               
               
                 Copper Sulfate Plating Bath Composition 
               
               
                 Plating bath 
               
             
          
           
               
                   
                 Name of compound 
                 Quantity (g/L) 
               
               
                   
                   
               
               
                   
                 Copper sulfate 
                 125 to 250 
               
               
                   
                 Sulfuric acid 
                  30 to 100 
               
               
                   
                   
               
             
          
         
       
     
         [0127]    Subsequently, an acrylic dry film resist HW440 (manufactured by Hitachi Chemical Co., Ltd.) was laminated on the whole surface of conductor film  34 U, and the resist is patterned by a known lithography method to form a resist pattern  16 U defining regions where a conductor pattern should be formed ( FIG. 6B ). 
         [0128]    In a similar fashion, a resist pattern  16 L was formed on the surface of conductor film  34 L to define regions where a conductor pattern should be formed ( FIG. 6B ). 
         [0129]    Subsequently, by employing a tenting process using copper (II) chloride, the solder delamination process using a metal resist, or the micro-etching process suitable for fine pattern forming, etching was performed until the surface in the +Z direction of the insulating layer  13  and the surface in the −Z direction of the insulating layer  12  were exposed ( FIG. 7A ). 
         [0130]    Accordingly, a conductor pattern  34 U′ was formed on the surface in the +Z direction of the insulating layer  13 , and a plated non-through via hole  41 U′ for electrically connecting the conductor pattern  34 U′ to the conductor layer  33 L was formed. Likewise, on the surface in the −Z direction of the insulating layer  12 , a conductor pattern  34 L′ was formed, and a plated non-through via hole  41 L′ for electrically connecting the conductor pattern  34 L′ to the conductor layer  32 U was formed ( FIG. 7A ). 
         [0131]    Next, by lamination pressing through pin lamination, an insulating layer  17 U was formed on the surface in the +Z direction of the insulating layer  13 , and an insulating layer  17 L was formed on the surface in the −Z direction of the insulating layer  12  ( FIG. 7B ). Subsequently, by pressing Micro-Thin (with thickness of approximately 5 μm, manufactured by Mitsui Mining and Smelting Co., Ltd.) to adhere onto the surface in the +Z direction of the insulating layer  17 U and onto the surface in the −Z direction of the insulating layer  17 L, respectively, and by pealing off the carrier member, conductor layers  35 U and  35 L were formed ( FIG. 7B ). 
         [0132]    Subsequently, using a process similar to the process for forming the openings  41 U and  41 L, an opening  42 U was formed in the insulating layer  17 U and an opening  42 L was formed in the insulating layer  17 L ( FIG. 8A ). Subsequently, using a plating process that is the same as or similar to the above-described process for forming the conductor layers  34 U and  34 L, conductor films  36 U and  36 L were formed ( FIG. 8B ). Then, using a process that is the same as or similar to the above-described process for forming the conductor pattern  34 U′ and  34 L′, conductor patterns  36 U′ and  36 L′ were formed ( FIG. 9 ). 
         [0133]    Subsequently, an ink is printed and hardened to form a coverlay layer  20 U having openings  43 U in a manner similar to the photolithography method. Likewise, the cover layer  20 L having openings  43 L is formed on the opposite side. As a result, laminated bodies  10 U and  10 L were formed on the respective surfaces of the reinforcing layer  11  ( FIG. 10 ). 
         [0134]    As described above, the laminated body  10 U formed on the surface in the +Z direction of the reinforcing layer  11  was separated from the reinforcing layer  11  at the interface between the carrier member  31 U and the conductor layer  33 L ( FIG. 11A ). 
         [0135]    Subsequently, using the conductor layer  33 L, which has been formed on the surface in the −Z direction of the insulating layer  13 , as the plating lead, nickel plating was carried out on the whole surface of the portions that were not covered by the coverlay layer  20 U using a plating bath with the composition shown in Table 5 under the following conditions: pH 4 to 5, liquid temperature of 40 to 60° C. and current density of approximately 2 to 6 A/dm 2 . 
         [0000]    
       
         
               
             
               
               
               
             
           
               
                 TABLE 5 
               
             
             
               
                   
               
               
                 Nickel Electrolytic Plating Bath Composition 
               
               
                 Plating bath 
               
             
          
           
               
                   
                 Name of compound 
                 Quantity (g/L) 
               
               
                   
                   
               
               
                   
                 Nickel sulfate 
                  Approximately 300 
               
               
                   
                 Nickel chloride 
                 Approximately 50 
               
               
                   
                 Boric acid 
                 Approximately 40 
               
               
                   
                   
               
             
          
         
       
     
         [0136]    Subsequently, gold plating was performed on the portions that have undergone nickel plating using a plating bath with the composition shown in Table 6 under the following conditions: liquid temperature of 20 to 25° C. and current density of 0.2 to 1.0 A/dm 2  ( FIG. 11B ). 
         [0000]    
       
         
               
             
               
               
               
             
           
               
                 TABLE 6 
               
             
             
               
                   
               
               
                 Au Electrolytic Plating Bath Composition 
               
               
                 Plating bath 
               
             
          
           
               
                   
                 Name of compound 
                 Quantity (g/L) 
               
               
                   
                   
               
               
                   
                 Gold 
                 10 
               
               
                   
                 Sodium cyanide 
                 30 to 35 
               
               
                   
                 Ammonia 
                 50 to 60 
               
               
                   
                   
               
             
          
         
       
     
         [0137]    After the completion of the above-described plating process, a resist layer  21 L was formed on the conductor layer  33 L provided on the surface in the −Z direction of the laminated body  10 U using AUS series (manufactured by TAIYO INK MFG. CO., LTD.). In stead of AUS series, DSR series (manufactured by TAMURA Corporation) ( FIG. 12 ) may be used. 
         [0138]    Subsequently, by disposing a soldering paste onto the openings  43 U by screen printing and by solder reflowing, pads  44 U 1  to  44 U N  were formed. Instead of using screen printing, the solder ball direct formation method may be used to form the pads. 
         [0139]    Next, etching was performed to expose the surface in the −Z direction of the insulating layer  13  and the resist layer  21 L was removed by making it come up using NaOH of 20 to 40 g/L, thereby forming conductor pattern  33 L ( FIG. 13A ). 
         [0140]    Subsequently, a coverlay layer  22  was formed so as to cover the surface in the −Z direction of the exposed insulating layer  13  and the surface in the −Z direction of the conductor layer  33 L, and openings  44 L were formed in a manner similar to that used for forming the openings  43 U ( FIG. 13B ). 
         [0141]    Subsequently, in a manner similar to that used for forming pads  44 U 1  to  44 U N , pads  45 U 1  and  45 U N  were formed inside the openings  44 L, thereby completing working examples of a coreless thin type flexible printed wiring board  10  according to the present invention. Working Examples 1-10 differ among themselves in terms of the following various manufacturing and dimensional parameters: the thickness of insulating layers  13  and  17 L, the width of the conductor pattern  36 U′, the angle φ of the prepreg fiber of the insulating layers  13  and  17 L ( FIG. 5 ), and the angle φ of the conductor pattern  36 U′ in the bending part  70  ( FIG. 3 ). These parameters are listed in Table 7. 
         [0142]    Here, in Working Examples 1 to 10 and Reference Examples 1 to 4 (which will be described below), the coverlay layer  22  was formed only in the portion  60 B where electronic circuit chips are mounted on the surface in the −Z direction of the conductor layer  33 L (see  FIG. 2 ). 
       Bending Test and Continuity Test 
       [0143]    Bending tests and continuity tests were carried out for flexible printed wiring boards of Working Examples 1 to 10 with impedance of 50Ω (design value), which were manufactured as described above. The MIT (flexural fatigue resistance) test of JIS5016 was adopted as the bending test, and occurrence of cracks in insulating layers and conductor layers were examined. In addition, the continuity test was conducted with a TCT (thermo cycle test) tester, and the continuity was examined after predetermined numbers of repetition of the thermal cycle consisting of raising the temperature from −55° C. to 125° C. in 30 minutes and lowering the temperature in the reverse manner. The continuity was evaluated at 50 cycles and 100 cycles. The lost continuity results were indicated as NG. The test results for Working Examples 1-10 are shown in Table 7. Manufacture of Flexible Printed Circuit Boards of Comparative Examples 1 to 5. 
         [0144]    Comparative Examples 1 to 5 were manufactured and tested. In Comparative Examples 1 to 5, the conductor pattern  33 L was formed in the bending part  70  as well. Thus, the number of conductor layers in the bending part  70  was three (3). Comparative Examples 1-4 differ among themselves in terms of the fiber directional angle φ of the prepreg in the insulating layer  17 U and  13  and the bending angle φ of the conductor pattern  36 U′. Otherwise, Comparative Examples were manufactured in the same way as in the manufacture of the flexible printed wiring boards of Working Examples 1 to 10. The above described bending tests as well as the continuity tests were conducted with respect to Comparable Examples 1-4. The results are shown in Table 8. 
       Manufacture of Flexible Printed Circuit Boards of Reference Examples 1 to 4 
       [0145]    The flexible printed wiring boards of Reference Examples 1 and 2 were manufactured in the same way as in the manufacture of the flexible printed wiring boards of Working Examples 1 to 10 except that the angle φ of the warp and the weft of the glass fiber in the insulating layers  17 L and  13  relative to the bending axis was set to 25° and 65°, respectively, and that the bending angle θ of the conductor pattern  36 U′ was set to 0°. These parameters and the results of the bending and continuity tests are listed in Table 8. 
         [0146]    In addition, the flexible printed wiring boards of Reference Examples 3 and 4 were manufactured in the same way as in the manufacture of the flexible printed wiring boards of Working Examples 1 to 10 except that the thickness of the insulating layers  17 L and  13  was set to 100 μm, the angle φ of the warp and the weft of the glass fiber in the insulating layers  17 L and  13  relative to the bending axis was set to 45°, and that the bending angle θ of the conductor pattern  16 U′ was set to 0°. These parameters and the results of the bending and continuity tests are listed in Table 8. 
         [0000]    
       
         
               
               
               
               
               
             
               
               
               
               
               
               
               
               
             
               
               
               
               
             
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                   
                 TABLE 7 
               
               
                   
                   
               
             
             
               
                   
                 Conductor layer counts 
                 Conductor layer 
                 Insulating layer 
                   
               
             
          
           
               
                   
                 Implementation 
                 Bending 
                 Width 
                 Thickness 
                 Thickness 
                 Angle of 
                 Bending angle 
               
               
                 Classification 
                 surface 
                 Part 
                 (μm) 
                 (μm) 
                 (μm) 
                 glass fiber (φ) 
                 of pattern (θ) 
               
               
                   
               
               
                 Example 1 
                 3 
                 2 
                 50 
                 20 
                 40 
                 35 
                 90 
               
               
                 Example 2 
                 3 
                 2 
                 50 
                 20 
                 40 
                 45 
                 90 
               
               
                 Example 3 
                 3 
                 2 
                 50 
                 20 
                 40 
                 55 
                 90 
               
               
                 Example 4 
                 3 
                 2 
                 50 
                 20 
                 40 
                 60 
                 45 
               
               
                 Example 5 
                 3 
                 2 
                 50 
                 20 
                 40 
                 30 
                 30 
               
               
                 Example 6 
                 3 
                 2 
                 50 
                 20 
                 40 
                 45 
                 45 
               
               
                 Example 7 
                 3 
                 2 
                 30 
                 20 
                 25 
                 45 
                 45 
               
               
                 Example 8 
                 3 
                 2 
                 30 
                 20 
                 25 
                 45 
                 90 
               
               
                 Example 9 
                 3 
                 2 
                 80 
                 20 
                 65 
                 45 
                 45 
               
               
                 Example 10 
                 3 
                 2 
                 80 
                 20 
                 65 
                 45 
                 90 
               
               
                   
               
             
          
           
               
                   
                 Status of crack occurrence 
                   
                   
               
               
                   
                 at the time of bending 
                 Continuity test 
               
             
          
           
               
                   
                 Insulating 
                 Conductor 
                 Number of cycles 
               
             
          
           
               
                   
                 Classification 
                 Impedance* 
                 layer 
                 layer 
                 50 
                 100 
               
               
                   
                   
               
               
                   
                 Example 1 
                 50 
                 None 
                 None 
                 OK 
                 OK 
               
               
                   
                 Example 2 
                 50 
                 None 
                 None 
                 OK 
                 OK 
               
               
                   
                 Example 3 
                 50 
                 None 
                 None 
                 OK 
                 OK 
               
               
                   
                 Example 4 
                 50 
                 None 
                 None 
                 OK 
                 OK 
               
               
                   
                 Example 5 
                 50 
                 None 
                 None 
                 OK 
                 OK 
               
               
                   
                 Example 6 
                 50 
                 None 
                 None 
                 OK 
                 OK 
               
               
                   
                 Example 7 
                 50 
                 None 
                 None 
                 OK 
                 OK 
               
               
                   
                 Example 8 
                 50 
                 None 
                 None 
                 OK 
                 OK 
               
               
                   
                 Example 9 
                 50 
                 None 
                 None 
                 OK 
                 OK 
               
               
                   
                 Example 10 
                 50 
                 None 
                 None 
                 OK 
                 OK 
               
               
                   
                   
               
               
                   
                 *Design value (Ω) 
               
             
          
         
       
     
         [0000]    
       
         
               
               
               
               
               
             
               
               
               
               
               
               
               
               
             
               
               
               
               
             
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                   
                 TABLE 8 
               
               
                   
                   
               
             
             
               
                   
                 Conductor layer counts 
                 Conductor layer 
                 Insulating layer 
                   
               
             
          
           
               
                   
                 Implementation 
                 Bending 
                 Width 
                 Thickness 
                 Thickness 
                 Angle of 
                 Bending angle 
               
               
                 Classification 
                 surface 
                 part 
                 (μm) 
                 (μm) 
                 (μm) 
                 glass fiber(φ) 
                 of pattern (θ) 
               
               
                   
               
               
                 Comp. Ex. 1 
                 3 
                 3 
                 50 
                 20 
                 40 
                 0 
                 0 
               
               
                 Comp. Ex. 2 
                 3 
                 3 
                 50 
                 20 
                 40 
                 15 
                 0 
               
               
                 Comp. Ex. 3 
                 3 
                 3 
                 50 
                 20 
                 40 
                 45 
                 0 
               
               
                 Comp. Ex. 4 
                 3 
                 3 
                 50 
                 20 
                 40 
                 15 
                 45 
               
               
                 Comp. Ex. 5 
                 3 
                 3 
                 50 
                 20 
                 40 
                 25 
                 45 
               
               
                 Ref. Ex. 1 
                 3 
                 2 
                 50 
                 20 
                 40 
                 25 
                 0 
               
               
                 Ref. Ex. 2 
                 3 
                 2 
                 50 
                 20 
                 40 
                 65 
                 0 
               
               
                 Ref. Ex. 3 
                 3 
                 2 
                 120 
                 20 
                 100 
                 45 
                 0 
               
               
                 Ref. Ex. 4 
                 3 
                 2 
                 120 
                 20 
                 100 
                 45 
                 0 
               
               
                   
               
             
          
           
               
                   
                 Status of crack occurrence 
                   
                   
               
               
                   
                 at the time of bending 
                 Continuity test 
               
             
          
           
               
                   
                 Insulating 
                 Conductor 
                 Number of cycles 
               
             
          
           
               
                   
                 Classification 
                 Impedance* 
                 layer 
                 layer 
                 50 
                 100 
               
               
                   
                   
               
               
                   
                 Comp. Ex. 1 
                 50 
                 Occurred 
                 Occurred 
                 NG 
                 — 
               
               
                   
                 Comp. Ex. 2 
                 50 
                 Occurred 
                 Occurred 
                 NG 
                 — 
               
               
                   
                 Comp. Ex. 3 
                 50 
                 None 
                 Occurred 
                 NG 
                 — 
               
               
                   
                 Comp. Ex. 4 
                 50 
                 Occurred 
                 Occurred 
                 NG 
                 — 
               
               
                   
                 Comp. Ex. 5 
                 50 
                 Occurred 
                 None 
                 NG 
                 — 
               
               
                   
                 Ref. Ex. 1 
                 50 
                 None 
                 None 
                 NG 
                 — 
               
               
                   
                 Ref. Ex. 2 
                 50 
                 None 
                 None 
                 OK 
                 NG 
               
               
                   
                 Ref. Ex. 3 
                 50 
                 None 
                 None 
                 OK 
                 NG 
               
               
                   
                 Ref. Ex. 4 
                 50 
                 None 
                 None 
                 OK 
                 NG 
               
               
                   
                   
               
               
                   
                 *Design value (Ω) 
               
             
          
         
       
     
         [0147]    As shown in Table 8, as for the flexible printed wiring boards of Comparative Examples 1 to 5, occurrence of crack was observed in bending tests. Also in continuity tests, electrical continuity was already lost at 50 cycles. 
         [0148]    In any of flexible printed wiring boards of Reference Examples 1 to 4, occurrence of crack was not observed at the time of bending. However, in the continuity test, the flexible printed wiring board of Reference Example 1 already lost electrical continuity at 50 cycles. 
         [0149]    As described above, it was found that a decrease in the number of conductor layers in the bending part reduces occurrence of cracks at the time of bending. In addition, it was found that by angularly offsetting the direction of the glass fiber in insulating layers relative to the bending axis, it is possible to form a conductor pattern that can maintain continuity after 50 cycles. 
         [0150]    For each of the Working Examples 1 to 10, cracks did not occur in the insulating layers in bending tests, and electrical continuity was maintained after 100 cycles in the continuity test. Thus, it was found that a decrease in the number of conductor layers in the bending part coupled with an angular configuration of either or both of the glass fiber in the insulating layers and the signal line conductor pattern further improves crack resistance. 
         [0151]    As described above, thin-type flexible printed wiring boards of Working Examples 1 to 10 excelled in crack resistance. 
         [0152]    The flexible printed wiring board of the present invention is useful as a thin-type flexible printed wiring board and is particularly suitable for miniaturizing high-speed and large-capacity memories and the like. 
         [0153]    Moreover, the method of manufacturing the flexible printed wiring board of the present invention is suitable for manufacturing thin-type flexible printed wiring boards that have superior crack resistance and has an excellent yield. 
         [0154]    It will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the spirit or scope of the invention. Thus, it is intended that the present invention cover the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.