Abstract:
A printed wiring board includes a board made of insulator; a wiring pattern to transfer an electric signal which is made of patterned metallic conductor and formed on at least one of a main surface and a rear surface of the board; and an electric power layer formed on at least one of the main surface and the rear surface of the board; wherein the electric power layer includes a mechanism for controlling a characteristic impedance of the printed wiring board.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2007-194762, filed on Jul. 26, 2007; the entire contents of which are incorporated herein by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to a printed-wiring board particularly requiring high speed operation, large current rating and flexibility. 
         [0004]    2. Description of the Related Art 
         [0005]    A printed wiring board, on which (an) IC chip(s) with high speed operationality is (are) mounted and which requires large current rating, is available for various electronic devices such as a computer, a magneto-optical memory and a cellular phone. With a conventional printed-wiring board, a high frequency signal is transferred under a transfer mode using a micro strip line (MSL), a strip line (SL) or a coplanar waveguide (CPW). In the conventional printed-wiring board, the thickness of the board and the line width of the board are mainly designed so as to set the characteristic impedance of the board to a predetermined value. In this case, the characteristic impedance can be represented by the following equation (1): 
         [0000]    
       
         
           
             
               
                 
                   
                     Z 
                     0 
                   
                   = 
                   
                     
                       
                         R 
                         + 
                         
                           jω 
                            
                           
                               
                           
                            
                           L 
                         
                       
                       
                         G 
                         + 
                         
                           jω 
                            
                           
                               
                           
                            
                           C 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
         [0006]    However, a flexible wiring board with large degree of freedom in structure is being employed as the electronic device is downsized. Since the thickness of the flexible wiring board is small so as to realize the flexibility thereof, it is difficult to design the characteristic impedance of the flexible wiring board. 
         [0007]    On the other hand, such a design guide as controlling both of the current rating and the characteristic impedance, which are traded off with one another, is being expected as the current consumption is increased accompanied by the multifunction of the electronic device and the IC processing speed is increased. If the width of the wiring pattern of the flexible wiring board is decreased, the characteristic impedance of the flexible wiring board can be controlled. On the other hand, in this case, since the current rating of the flexible wiring board becomes small so that the wiring pattern of the flexible wiring board may be burned out. If the width of the wiring pattern of the flexible wiring board is increased, the characteristic impedance can not be appropriately controlled so that the effective electric power transfer efficiency may be decreased. 
         [0008]    In this point of view, conventionally, the width of the wiring pattern is decreased so that the characteristic impedance can be appropriately controlled through the reduction of the current rating of the flexible wiring board. Alternatively, the width of the wiring pattern is increased so that the current rating can be maintained as designed under the compromise of the impedance matching. In the latter case, the impedance matching can be realized by mounting resistances for impedance matching in series and/or parallel to the driving IC or the load element (passive element, active element, optical device, etc.) in the vicinity of and/or apart from the driving IC or the load element. In this case, however, since electric power consumption is increased as a whole and much current is consumed at the resistances, the electric power is wasted. 
         [0009]    In Reference 1, in the multilayered printed-wiring board which contains an MSL formed as the top layer thereof, the conductive pattern located below the MSL is partially removed so that the conductive pattern can not be superimposed with the MSL, and a ground pattern is formed as the bottom layer uniformly on the rear surface of the multilayered wiring board so that the ground pattern can be superimposed with the MSL, thereby controlling the characteristic impedance of the MSL, that is, the multilayered wiring board. 
         [0010]    The technique disclosed in Reference 1, however, is effective for the multilayered wiring board with three or more, preferably, four or more conductive layers containing the MSL, but not effective for a flexible wiring board such as a double-sided printed wiring board which is configured such that the conductive patterns are formed on the main surface and the rear surface of the core board, respectively. 
         [0011]    [Reference 1] JP-A 2006-74014 (KOKAI) 
       BRIEF SUMMARY OF THE INVENTION 
       [0012]    It is an object of the present invention, in view of the above-described problems, to provide a printed-wiring board which can conduct the control of the characteristic impedance independent of the number of conductive pattern composing the printed-wiring board. 
         [0013]    In order to achieve the above object, an aspect of the present invention relates to a printed wiring board, including: a board made of insulator; a wiring pattern to transfer an electric signal which is made of patterned metallic conductor and formed on at least one of a main surface and a rear surface of the board; and an electric power layer formed on at least one of the main surface and the rear surface of the board; wherein the electric power layer includes a mechanism for controlling a characteristic impedance of the printed wiring board. 
         [0014]    According to the aspect of the present invention, the electric power layer of the printed wiring board includes the mechanism for controlling the characteristic impedance of the printed wiring board. Therefore, the characteristic impedance of the printed wiring board can be easily controlled only by operating the mechanism even though the width of the wiring pattern is not controlled as in the past. As a result, the design of the wiring pattern in view of the current rating is not required so that the degree of freedom in design of the wiring pattern can be developed. For example, there is no advantage that the current rating of the wiring pattern, that is, the printed wiring board is increased even though the width of the wiring pattern is decreased in view of impedance matching. 
         [0015]    In an embodiment, the mechanism is a trench which is formed below and/or above the wiring pattern throughout the electric power layer over a long direction of the wiring pattern. In this case, the characteristic impedance of the printed wiring board can be controlled by adjusting the width of the trench. Concretely, the width of the trench is controlled in view of the shape and size of the wiring pattern and the intended characteristic impedance of the printed wiring board. For example, the width of the trench is set within a range of 0.1 W to 3 W when the width of the wiring pattern is defined as numeral character “W”. 
         [0016]    In the embodiment, it is desired that the center of the wiring pattern in the width direction thereof is matched with the center of the trench in the width direction thereof. In this case, the characteristic impedance of the printed wiring board can be controlled effectively and absolutely by adjusting the width of the trench. 
         [0017]    The aspects of the present invention as described above can be applied for any printed wiring board. However, the aspects can be applied more effectively and absolutely for a flexible printed wiring board by making the board of heat resistance resin base. 
         [0018]    The aspects of the present invention can be applied for a printed wiring board which is configured such that the wiring pattern is formed on the main surface of the board and the electric power layer is formed on the rear surface of the board, thereby including a double-sided wiring structure. Moreover, the aspects can be applied for a printed wiring board which is configured such that the electric power layer includes a first electric power layer and a second electric power layer so that the first electric power layer is formed on the main surface of the board and the second electric power layer is formed on the rear surface of the board, thereby including a double-sided wiring structure. 
         [0019]    The flexible printed wiring board and the double-sided printed wiring board include no multilayered conductive pattern. As disclosed in Reference 1, therefore, if the multilayered conductive pattern is formed so that the conductive pattern located below the MSL (wiring pattern) is partially removed and the conductive pattern can not be superimposed with the MSL, the manufacturing cost of the printed wiring board is increased and the flexibility of the printed wiring board is reduced because the printed wiring board includes the multilayered conductive pattern. On the other hand, according to the aspects of the present invention, the characteristic impedance of the printed wiring board can be effectively controlled. 
         [0020]    The characteristics of the flexible printed wiring board may be independent from or combined with the characteristics of the double-sided printed wiring board. For example, a double-sided flexible printed wiring board may be provided. 
         [0021]    The present invention is not limited to the flexible printed wiring board and the double-sided printed wiring board, but may be applied to any printed wiring board. For example, a rigid printed wiring board and a multilayered printed wiring board may be included within the scope of the present invention. 
         [0022]    As the double-sided printed wiring board may be exemplified a micro strip line (MSL) type printed wiring board, a strip line (SL) type printed wiring board and a coplanar waveguide type printed wiring board. 
         [0023]    According to the aspect of the present invention can be provided a printed-wiring board which can conduct the control of the characteristic impedance independent of the number of conductive pattern composing the printed-wiring board. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0024]      FIG. 1  is a perspective view illustrating an embodiment of the printed-wiring board according to the present invention. 
           [0025]      FIG. 2  is a side view of the printed-wiring board in  FIG. 1 , as viewed from the direction designated by the arrow “A”. 
           [0026]      FIG. 3  is a perspective view of another embodiment of the printed-wiring board according to the present invention. 
           [0027]      FIG. 4  is a perspective view illustrating still another embodiment of the printed-wiring board according to the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0028]    Hereinafter, the present invention will be described in detail with reference to the drawings.  FIG. 1  is a perspective view illustrating an embodiment of the printed-wiring board according to the present invention.  FIG. 2  is a side view of the printed-wiring board in  FIG. 1 , as viewed from the direction designated by the arrow “A”. 
         [0029]    The printed-wiring board  10  illustrated in  FIG. 1  includes aboard  11  made of insulator, a metallic conductor (wiring pattern)  13  to transfer (a) microwave electric signal(s) which is formed on the main surface of the board  11  and an electric power layer  15  formed on the rear surface of the board  11 , thereby constituting a double-sided printed wiring board with a double-sided wiring structure. The electric power layer  15  is maintained a standard electric potential or shifted slightly from the standard electric potential so that the potential shift from the standard electric potential can be small enough to set the characteristic impedance of the electric power layer  15  negligible within a signal frequency range of the wiring pattern  13  (e.g., to tithe or less of the transfer impedance). In other words, the electric power layer  15  may be shifted from the standard electric potential only if the potential shift from the standard electric potential can be small enough to set the characteristic impedance of the electric power layer  15  negligible within a signal frequency range of the wiring pattern  13  (e.g., to tithe or less of the transfer impedance). 
         [0030]    In this embodiment, the wiring pattern  13  is formed as a micro strip line (MSL). 
         [0031]    As shown in  FIGS. 1 and 2 , a trench  15 A is formed throughout the electric power layer  15  over the long direction of the wiring pattern  13  directly below the wiring pattern  13 . The trench  15 A functions as an impedance controller for the wiring pattern  13 , that is, the printed-wiring board  10 . Concretely, the characteristic impedance of the printed-wiring board  10  is entirely changed by controlling the width SW of the trench  15 A. Therefore, the desired characteristic impedance can be realized for the printed-wiring board  10  by appropriately controlling the width of the trench  15 A commensurate with the shape and size of the wiring pattern  13 . 
         [0032]    As shown in  FIG. 2 , it is desired that the center of the wiring pattern  13  in the width direction thereof is matched with the center of the trench  15 A in the width direction thereof. If not matched, the characteristic impedance of the printed-wiring board  10  may not be effectively and efficiently changed by appropriately controlling the width SW of the trench  15 A. 
         [0033]    Suppose that the width of the wiring pattern  13  is defined as numeral character “W”, the width SW of the trench  15 A is preferably set within a range of 0.1 W to 3 W, more preferably within a range of 0.1 W to 1 W. If the width SW of the trench  15 A is less than 0.1 W, the characteristic impedance of the printed-wiring board  10  may not be changed sufficiently in dependence with the size and shape of the wiring pattern  13 . Namely, if the width SW of the trench  15 A is less than 0.1 W, the characteristic impedance of the printed-wiring board  10  may not be changed sufficiently in comparison with a printed-wiring board  10  with no trench. Similarly, if the width SW of the trench  15 A is more than 3 W, the characteristic impedance of the printed-wiring board  10  may not be changed sufficiently in dependence with the size and shape of the wiring pattern  13 . The reason will be described below. 
         [0034]    In this embodiment, since the printed wiring board  10  is formed as the double-sided wiring structure, the wiring pattern  13  is disposed in the vicinity of the electric power layer  15  via the board  11  which is located at the center of the wiring board  10 . Therefore, since the wiring pattern  13  is electrically interfered with the electric power layer  15  strongly so that the electric interference between the wiring pattern  13  and the electric power layer  15  is changed remarkably when the configuration (shape and size) of the wiring pattern  13  and/or the electric power layer  15  is changed slightly. As a result, since the electric interference between the wiring pattern  13  and the electric power layer  15  is changed remarkably by forming the trench  15 A at the electric power layer  15  so that the capacitance formed between the wiring pattern  13  and the electric power layer  15  is also changed remarkably, the characteristic impedance of the printed wiring board  10  can be effectively and efficiently changed. 
         [0035]    In this point of view, if the width SW of the trench  15 A is set more than 3 W, since the electric interference between the wiring pattern  13  and the electric power layer  15  becomes extremely small or negligible, the capacitance formed between the wiring pattern  13  and the electric power layer  15  can not almost be changed so that the characteristic impedance of the printed wiring board  10  can be effectively and efficiently changed. 
         [0036]    Referring to the technical principle as described above, the thickness of the board  11  is set preferably to 200 μm or less, more preferably to 50 μm or less, particularly preferably to 12.5 μm or less. In this case, if the board  11  is made of a material as described below, the board  11  can be flexible so that the printed wiring board  10  can be a flexible printed wiring board. 
         [0037]    The lower limited value of the thickness of the board  11  may be set to several μm in view of the dielectric breakdown strength of the board  11  dependent on the constituent material. 
         [0038]    In the conventional technique as disclosed in Reference 1, a plurality of conductive patterns, located below the MSL, are partially removed so that the removal of the conductive patterns can be set larger than the width of the MSL and thus, the MSL can not be electrically interfered with the conductive patterns, thereby conducting the impedance control. In contrast, in the present embodiment (invention), the wiring pattern  13  is electrically interfered with the electric power layer  15  by intent, thereby conducting the control. As a result, the technical idea of Reference 1 is quite different from the technical idea of the present embodiment (invention). 
         [0039]    The board  11  may be made of a given insulator. As the insulator can be exemplified polyester, polyimide or glass epoxy based flexible material, polysulfone, polyetherimide or polyether thermoplastic resin and liquid crystal. 
         [0040]    In this embodiment (invention), a rigid printed wiring board is not excluded. In this case, paper (e.g., FR-1, FR-2, XXXpc, Xpc, FR-3), glass (e.g., FR-4, G-10, FR-5, G-11, GPY), epoxy or polyester based composite (CEM-1, CEM-3, FR-6), alumina, alumina nitride or silicon carbide low temperature sintered ceramic material can be exemplified. 
         [0041]    The wiring pattern  13  and the electric power layer  15  may be made of e.g., Cu, Ag, Au, aluminum or an alloy thereof. 
         [0042]    Not shown in  FIGS. 1 and 2 , adhesive layers may be formed between the board  11  and the wiring pattern  13  and/or between the board  11  and the electric power layer  15 . Then, cover layers (containing respective adhesive layers thereof) may be formed on the main surface and the rear surface of the board  11  over the wiring pattern  11 , the trench  15 A and the electric power layer  15 . Moreover, reinforcing boards may be formed on the respective cover layers. 
         [0043]      FIG. 3  is a perspective view of another embodiment of the printed-wiring board according to the present invention. The printed-wiring board  20  illustrated in  FIG. 3  includes a board  21  made of insulator, a metallic conductor (wiring pattern)  23  to transfer (a) microwave electric signal(s) which is formed in the board  21  in parallel with the main surface and the rear surface of the board  21 , a first electric power layer  25  and a second electric power layer  26  which are formed on the main surface and the rear surface of the board  21 , respectively. The wiring pattern  23  is disposed at the center area of the board  11  in the thickness direction of the board  21  and is elongated along the long direction of the board  21 . The first electric power layer  25  and the second electric power layer  26  are maintained a standard electric potential or shifted slightly from the standard electric potential so that the potential shift from the standard electric potential can be small enough to set the characteristic impedances of the electric power layers  25  and  26  negligible within a signal frequency range of the wiring pattern  23  (e.g., to tithe or less of the transfer impedance). In other words, the electric power layers  25  and  26  may be shifted from the standard electric potential only if the potential shift from the standard electric potential can be small enough to set the characteristic impedances of the electric power layers  25  and  26  negligible within a signal frequency range of the wiring pattern  23  (e.g., to tithe or less of the transfer impedance). 
         [0044]    In this embodiment, the wiring pattern  13  is formed as a strip line (SL). 
         [0045]    As shown in  FIG. 3 , in this embodiment, a trench  25 A is formed throughout the first electric power layer  25  over the long direction of the wiring pattern  23  directly above the wiring pattern  23  and a trench  26 A is formed throughout the second electric power layer  26  over the long direction of the wiring pattern  23  directly below the wiring pattern  23 . The trenches  25 A and  26 A function as impedance controllers for the wiring pattern  23 , that is, the printed-wiring board  20 . 
         [0046]    Concretely, the characteristic impedance of the printed-wiring board  20  is entirely changed by controlling the widths of the trenches  25 A and  26 A. Therefore, the desired characteristic impedance can be realized for the printed-wiring board  20  by appropriately controlling the widths of the trenches  25 A and  26 A commensurate with the shape and size of the wiring pattern  23 . 
         [0047]    In this case, it is also desired that the center of the wiring pattern  23  in the width direction thereof is matched with the centers of the trenches  25 A and  26 A in the width direction thereof. If not matched, the characteristic impedance of the printed-wiring board  20  may not be effectively and efficiently changed by appropriately controlling the widths of the trenches  25 A and  26 A. 
         [0048]    Suppose that the width of the wiring pattern  23  is defined as numeral character “W”, the widths of the trenches  25 A and  26 A are preferably set within a range of 0.1 W to 3 W, more preferably within a range of 0.1 W to 1 W. If the widths of the trenches  25 A and  26 A are less than 0.1 W, the characteristic impedance of the printed-wiring board  20  may not be changed sufficiently in dependence with the size and shape of the wiring pattern  23 . Similarly, if the widths of the trenches  25 A and  26 A are more than 3 W, the characteristic impedance of the printed-wiring board  20  may not be changed sufficiently in dependence with the size and shape of the wiring pattern  23 . 
         [0049]    In this embodiment, the trenches  25 A and  26 A are formed at the first electric power layer  25  and the second electric power layer  26 , respectively. However, if either of the trenches  25 A and  26 A is formed at the first electric power layer  25  or the second electric power layer  26 , the function/effect of the present invention, that is, the characteristic impedance of the printed wiring board  20  can be appropriately controlled. If both of the trenches  25 A and  26 A are formed at the first electric power layer  25  or the second electric power layer  26  as described in this embodiment, the characteristic impedance of the printed wiring board  20  can be controlled more effectively and efficiently. 
         [0050]    The thickness of the board  21  is set preferably to 200 μm or less, more preferably to 50 μm or less, particularly preferably to 12.5 μm or less. In this case, if the board  21  is made of a material as described below, the board  21  can be flexible so that the printed wiring board  20  can be a flexible printed wiring board. 
         [0051]    The lower limited value of the thickness of the board  21  may be set to several μm in view of the dielectric breakdown strength of the board  21  dependent on the constituent material. 
         [0052]    The board  21  may be made of a given insulator. As the insulator can be exemplified polyester, polyimide or glass epoxy based flexible material, polysulfone, polyetherimide or polyether thermoplastic resin and liquid crystal. 
         [0053]    In this embodiment (invention), a rigid printed wiring board is not excluded. In this case, paper (e.g., FR-1, FR-2, XXXpc, Xpc, FR-3), glass (e.g., FR-4, G-10, FR-5, G-11, GPY), epoxy or polyester based composite (CEM-1, CEM-3, FR-6), alumina, alumina nitride or silicon carbide low temperature sintered ceramic material can be exemplified. 
         [0054]    The wiring pattern  23 , the first electric power layer  25  and the second electric power layer  26  may be made of e.g., Cu, Ag, Au, aluminum or an alloy thereof. 
         [0055]    Not shown in  FIG. 3 , adhesive layers may be formed between the board  21  and the wiring pattern  23 , between the board  21  and the first electric power layer  25  and/or between the board  21  and the second electric power layer  26 . Then, cover layers (containing respective adhesive layers thereof) may be formed on the main surface and the rear surface of the board  11  over the wiring pattern  21 , the trenches  25 A,  26 A and the electric power layers  25 ,  26 . Moreover, reinforcing boards may be formed on the respective cover layers. 
         [0056]      FIG. 4  is a perspective view of still another embodiment of the printed-wiring board according to the present invention. The printed-wiring board  30  illustrated in  FIG. 4  includes a board  31  made of insulator, a metallic conductor  33  to transfer (a) microwave electric signal(s) which is formed on the board  31 , and an electric power layer  35 . The electric power layer  35  functions as a reference electrode for the metallic conductor  33  and is maintained constant electric potential for the metallic conductor  33  so that the microwave electric signal(s) can be transferred under good condition. The electric power layer  35  may be electrically grounded, but may be maintained a predetermined electric potential only if the microwave electric signal(s) can be transferred in the metallic conductor  33 . 
         [0057]    In this embodiment, a pair of grounded electrode layers  37  are formed on both sides of the metallic conductor  33  so as to sandwich the metallic conductor  33 . In this case, the metallic conductor  33  and the grounded electrode layers  37  constitute the wiring pattern as a coplaner waveguide (CPW). 
         [0058]    The grounded electrode layers  37  are maintained a standard electric potential or shifted slightly from the standard electric potential so that the potential shift from the standard electric potential can be small enough to set the characteristic impedance of the grounded electrode layers  37  negligible within a signal frequency range of the metallic conductor  33  (i.e., wiring pattern), for example, to tithe or less of the transfer impedance. In other words, the electric power layer  35  and the grounded electrode layers  37  may be shifted from the respective standard electric potential only if the potential shift from the standard electric potentials can be small enough to set the characteristic impedances of the electric power layer  35  and the grounded electrode layers  37  negligible within a signal frequency range of the wiring pattern (e.g., to tithe or less of the transfer impedance). 
         [0059]    As shown in  FIG. 4 , in this embodiment, a trench  35 A is formed throughout the electric power layer  35  over the long direction of the metallic conductor  33  directly below the metallic conductor  33  over the long direction of the metallic conductor  33 . The trench  35 A functions as an impedance controller for the metallic conductor (wiring pattern)  33 , that is, the printed-wiring board  30 . Concretely, the characteristic impedance of the printed-wiring board  30  is entirely changed by controlling the width of the trench  35 A. Therefore, the desired characteristic impedance can be realized for the printed-wiring board  30  by appropriately controlling the width of the trench  35 A commensurate with the shape and size of the metallic conductor (wiring pattern)  33 . 
         [0060]    In this case, it is also desired that the center of the metallic conductor  33  in the width direction thereof is matched with the centers of the trench  35 A in the width direction thereof. If not matched, the characteristic impedance of the printed-wiring board  30  may not be effectively and efficiently changed by appropriately controlling the width of the trench  35 A. 
         [0061]    Suppose that the width of the metallic conductor  33  is defined as numeral character “W”, the width of the trench  35 A is preferably set within a range of 0.1 W to 3 W, more preferably within a range of 0.1 W to 1 W. If the width of the trench  35 A is less than 0.1 W, the characteristic impedance of the printed-wiring board  30  may not be changed sufficiently in dependence with the size and shape of the wiring pattern containing the metallic conductor  33 . Similarly, if the width of the trench  35 A is more than 3 W, the characteristic impedance of the printed-wiring board  30  may not be changed sufficiently in dependence with the size and shape of the wiring pattern containing the metallic conductor  33 . 
         [0062]    The board  31  is preferably configured as the boards  11  and  21  as described above. Namely, the thickness of the board  31  is set preferably to 200 μm or less, more preferably to 50 μm or less, particularly preferably to 12.5 μm or less. The lower limited value of the thickness of the board  31  may be set to several μm in view of the dielectric breakdown strength of the board  31  dependent on the constituent material. 
         [0063]    The board  31  may be made of a given insulator. As the insulator can be exemplified flexible material such as polyester, polyimide or glass epoxy based flexible material, polysulfone, polyetherimide or polyether thermoplastic resin and liquid crystal or rigid material such as paper (e.g., FR-1, FR-2, XXXpc, Xpc, FR-3), glass (e.g., FR-4, G-10, FR-5, G-11, GPY), epoxy or polyester based composite (CEM-1, CEM-3, FR-6), alumina, alumina nitride and silicon carbide low temperature sintered ceramic material. 
         [0064]    The metallic conductor  33 , the electric power layer  35  and the grounded electrode layers  37  may be made of e.g., Cu, Ag, Au, aluminum or an alloy thereof. 
         [0065]    Not shown in  FIG. 4 , adhesive layers may be formed between the board  31  and the metallic conductor  33 , the grounded electrode layers  37  (that is, wiring pattern) and/or between the board  31  and the electric power layer  35 . Then, cover layers (containing respective adhesive layers thereof) may be formed on the main surface and the rear surface of the board  31  over the metallic conductor  33  and the grounded electrode layers  37  (i.e., wiring pattern), the trench  35 A and the electric power layer  35 . Moreover, (Example) 
         [0066]    In this Example, the printed wiring board  10  as shown in  FIGS. 1 and 2  was fabricated so that the change in characteristic impedance of the printed wiring board  10  was examined with the change in width SW of the trench  15 A. In this case, the width W of the wiring pattern (MSL)  13  was set to 100 μm and the thickness of the board  11  was set to 12.5 μm. As a result, it was confirmed the characteristic impedance of the printed wiring board  10  is changed within a range of 20 to 150Ω by changing the width SW of the trench  15 A within a range of 10 to 300 μm. Herein, the characteristic impedance of a printed wiring board with no trench was 18Ω. 
         [0067]    As a result, it was confirmed that if a trench is formed at an electric power layer of a printed wiring board throughout the electric power layer and then, the width of the trench is changed appropriately, the characteristic impedance of the printed wiring board can be controlled. 
         [0068]    Although the present invention was described in detail with reference to the above examples, this invention is not limited to the above disclosure and every kind of variation and modification may be made without departing from the scope of the present invention.