Abstract:
An impedance matched circuit board utilizes a series of vias, one signal via that is surrounded by four ground vias in order to effect impedance matching with a coaxial signal transmission line. The vias are plated and extend through the thickness of the circuit board. Both opposing surfaces of the circuit board are provided with a conductive ground layer and each such ground layer has an opening formed there that encompasses one or more of the vias. On the top surface the opening surrounds the signal and ground via and on the bottom surface the opening only partially surrounds the signal via and the opening includes a convex portion formed therein.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention relates generally to circuit boards and more particularly to impedance matched circuit boards. 
         [0002]    Conventionally, in order to connect a high-frequency coaxial connector for transmitting signals at high frequencies such as a microwave to a microstrip line formed on a circuit board, a circuit structure arranging a ground layer around the microstrip line and at the rear side of the circuit board is used. This is shown in Japanese Utility Model Application Laid-Open (Kokai) No. 5-82111. 
         [0003]      FIG. 4  is a view showing a circuit structure, which is formed on a surface of a conventional circuit board, and  FIG. 5  is a view showing a circuit structure, which is formed on other surface of the conventional circuit board. 
         [0004]    In  FIGS. 4 and 5 ,  801  denotes a first ground layer formed by a metallic film and formed on one face of a circuit board made of a dielectric material, and  802  denotes a second ground layer formed by a metallic film formed on other face of the circuit board. In addition,  803  denotes a first through-hole, or via, penetrating through the circuit board; and circular first relief part  808  and circular second relief part  809 , from which the first ground layer  801  and the second ground layer  802  are removed at a predetermined radius, are formed around the first through-hole  803 .  804  denotes a second through-hole penetrating through the circuit board, and a plurality of such through-holes  804  are arranged so as to encircle the periphery of the first through-hole  803  in a horseshoe shape. Further, conductive metal plating is applied onto the inner circumferential faces of the first through-hole  803  and the second through-holes  804 . 
         [0005]    In addition, a soldering part  806  is formed on the first via  803  on one surface of the circuit board and the end of a microstrip line  807  made of a metallic film is connected to the first through-hole  803 . Then, a central conductor of the high-frequency coaxial connector (not illustrated) is connected to the first via  803  by the soldering part  806 . Further, the external conductor of the high-frequency coaxial connector is connected to the second ground layer  802 . 
         [0006]    Here, the first ground layer  801  is connected to the second ground layer  802  by the second vias  804 , so that the external conductor of the high-frequency coaxial connector, the vias  804 , the first ground layer  801 , and the second ground layer  802  have the same potentials. Therefore, the central conductor of the high-frequency coaxial connector and the first via  803  and the microstrip line  807 , which are connected to this central conductor, are encircled by the external conductor of the high-frequency coaxial connector, the second vias  804 , the first ground layer  801 , and the second ground layer  802  having the same potentials, so that it is possible to make an electric property of a connecting portion providing an electric connection between the high-frequency coaxial connector and the circuit board stable. 
         [0007]    Nevertheless, according to the conventional circuit structure, since both of the first ground layer  801  and the second ground layer  802  are formed around the first via  803  and the second via  804 , a third layer must be provided for forming the microstrip line  807 . 
         [0008]    In addition, a first opening  808  and a second opening  809  formed on the first ground layer  801  and the second ground layer  802 , respectively, are positioned inside the second vias  804 , and this results in that they have substantially the same shapes and sizes. Therefore, appropriate matching of electric impedance cannot be accomplished. 
       SUMMARY OF THE INVENTION 
       [0009]    Taking the foregoing problems encountered by the conventional art into consideration, the present invention has an object to provide a circuit board having a configuration such that a convex portion of a reference electric potential layer is formed in a region of a relief part around a through-hole, or via, which is formed on a face at the opposite side of a face on which a signal thin line functioning as a microstrip line is formed, so as to correspond with the signal thin line, thereby simplifying a structure of a connecting portion of a high-frequency coaxial connector while enabling it to easily and appropriately accomplish matching of electric impedance. 
         [0010]    Therefore, a circuit board in accordance with the present invention comprises a via for signal, to which a central conductor of a high-frequency coaxial connector is connected; vias for reference potential (i.e. ground vias), arranged around a periphery of the signal via and allowing a terminal for shield of the high-frequency coaxial connector to be connected thereto; a first reference potential (i.e. ground) layer formed by a conductive film disposed on one surface of the circuit board; a thin signal line, which is connected to the signal via on the one surface, the thin signal line being formed of a conductive strip, and functioning as a microstrip line; and a second reference potential(i.e. ground), layer formed by a conductive film arranged on other surface of the circuit board; wherein the first ground layer is provided with a first opening in which a portion of the conductive film is removed so as to encircle the periphery of the live signal via; and wherein the second ground layer is provided with a second opening part in which a portion of the conductive film is removed so as to encircle the periphery of the signal via and a convex portion configured to protrude toward the signal via at the region corresponding with the thin signal line in the second opening. 
         [0011]    In another circuit board according to the present invention, the concave portion further has a width thereof not less than seven times as large as the width of the signal thin line. 
         [0012]    In a further circuit board according to the present invention, further, a distance a front end of the concave portion is spaced from the via for signal is set depending on dielectric voltage to be specified. 
         [0013]    In a still further circuit board according to the present invention, the first ground further includes a first auxiliary opening, from which the conductive film is removed so as to have a width not less than three times as large as the width of the thin signal line at the opposite sides of the thin signal line, respectively. 
         [0014]    In a still another circuit board according to the present invention, the first opening accommodates within its perimeter, the signal via and all of the ground vias, the first opening being connected to the first auxiliary opening, and the second opening accommodates, within its perimeter, the signal via but none of the ground vias. 
         [0015]    In accordance with the present invention, the circuit board has the convex portion of the second ground layer, which is formed at the region corresponding to the thin signal line in the opening around the through-hole, which is formed on the face opposite to the circuit board tow surface on which the thin signal line functioning as the microstrip line is formed. Hence, the connecting portion of the high-frequency coaxial connector is simplified in its structure while enabling it to easily and appropriately accomplish matching of an electric impedance of the circuit board. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]    FIG  1 A is a top plan view of a circuit board constructed in accordance of the principle of the present invention; 
           [0017]      FIG. 1B  is a bottom plan view of the circuit board of  FIG. 1A ; according to an embodiment of the present invention; 
           [0018]      FIG. 2  is a cross-sectional view taken along a line A-A of  FIG. 1A , showing a distant relationship between the microstrip line and a first ground layer according to the embodiment of the present invention; 
           [0019]      FIG. 3  is a view showing a modified example of a convex portion of a second ground layer according to the embodiment of the present invention; 
           [0020]      FIG. 4  is a view showing a circuit structure, which is formed on one face of a conventional circuit board; and 
           [0021]      FIG. 5  is a view showing a circuit structure, which is formed on other face of the conventional circuit board. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0022]    Preferred embodiments of the present invention will be described below in detail with reference to the accompanying drawings.  FIG. 1  is a view showing a circuit board according to an embodiment of the present invention. Further,  FIG. 1A  shows a face on which a microstrip line is formed, and  FIG. 1B  shows a face of the opposite side thereof. 
         [0023]    In  FIGS. 1A  &amp; B, the circuit board  31  is used for an electronic device using a high frequency wave, for example, a telecommunication device, and a measurement device or the like. Then, for example, a high-frequency coaxial connector such as a SMA (Sub-Miniature Type A) connector (not illustrated) is connected to the circuit board  31 . In this embodiment, representations of directions such as up, down, left, right, front, rear, and the like, used for explaining the structure and movement of each part of the circuit board  31 , and the like, are not absolute, but relative. These representations are appropriate when each part of the circuit board  31 , and the like, is in the position shown in the figures. If the position of the circuit board  31 , and the like, changes, however, it is assumed that these representations are to be changed according to the change of the position of the circuit board  31 , and the like. 
         [0024]    The circuit board  31  is preferably a planar member, and is made of a dielectric material such as polyimide. As shown in  FIG. 1A , one surface of the circuit board  31  is covered with a first ground layer  56  as a first reference electric potential layer, and as shown in  FIG. 1B , the other surface of the circuit board  31  is also covered with a second ground layer  57  as a second reference electric potential layer. The first ground layer  56  and the second ground layer  57  are a conductive film, respectively, made of a metal with a low conductive resistance, for example, copper. These ground layers are laminated on the surfaces of the circuit board  31 . Further, the first ground layer  56  and the second ground layer  57  may be used as a power source layer for supplying a power source current to diverse kinds of electronic elements such as an IC (not illustrated) to be mounted on the circuit board  31 . However, in thin deception they will be explained as being only a ground layer used for grounding. 
         [0025]    On the circuit board  31 , penetrating through thickness of the circuit board  31  is a through-hole, or via,  61  for a signal is formed. Further, as encircling the periphery of this signal via  61 , a plurality of through-holes or vias  51  for ground, for example, four ground vias are formed as vias for a reference electric potential to travel through the circuit board  31  in the thickness direction. It is preferable that these ground vias  51  are generally formed on a circle encircling the periphery of the signal via  61  and at positions that preferably equally divide the circle. 
         [0026]    In addition, it is assumed that all of the ground vias  51  have the same sizes. For example, they are arranged so as to have diameters of approximately 1.6 mm and intervals between the adjacent holes, namely, pitches 5.0 mm. On the other hand, the signal via  61  may have a size different from that of the ground via  51 . The explanation herein assumes that the signal via  61  has the same size as that of the ground vias  51 . 
         [0027]    The inner peripheral faces of the ground vias  51  and the signal via  61  contain a coating film made of a metal with a low conductive resistance, for example, copper (formed by plating or the like). The circuit board  31  has a land formed thereon in the shape of an annularly extending guard so as to encircle the periphery of the opposite ends of the ground vias  51  and the signal vias  61 . 
         [0028]    This land is a coating or a film made of a metal with a low conductive resistance, which is integrally formed with the film covering the inner peripheral faces of the ground vias  51  and the signal via  61 . For example, the external diameter of each land is approximately 2.0 mm. 
         [0029]    Then, a central conductor set of a high-frequency coaxial connector (not illustrated) is connected to the signal via  61 . This central conductor is also made of a metal with a low conductor resistance such as copper and is connected to the central conductor of the high-frequency coaxial cable terminated by the high-frequency coaxial connector so as to transmit a high frequency signal. Therefore, a high frequency signal is transmitted to the signal via  61 . In addition, a terminal for the shield of the same high-frequency coaxial connector is connected to the ground vias  51 . This terminal is made of a conductive metal to be connected to a shield wire of the high-frequency coaxial cable, which is terminated to the high-frequency coaxial connector. 
         [0030]    As shown in  FIG. 1A , the top surface of the circuit board  31  has a first relief part, or opening  32   a , in which the conductive film of the first ground layer  56  is removed. This first opening  32   a  is defined by a circular first pattern line  33   a  indicating a boundary against the top surface ground layer  56 . This first pattern line  33   a  is a circle centering on the center of the for signal via  61 , and it is assumed that the first pattern line  33   a  has the size so as to be able to contain the signal via  61  and the ground vias  51 , for example, a diameter 112.0 mm. Reference numeral  91  denotes a dashed line that indicates the outline or edge of the coaxial connector mentioned above that is connected to the circuit board  31 . 
         [0031]    The first opening  32   a  is a part where the first ground layer  56  is removed and the dielectric material of the circuit board  31  is exposed outside to contain therein the signal via  61  and all the ground hole vias  51  through-holes. This first opening  32   a  is slightly larger than the outline  91  of the coaxial connector. The lands are formed at the ends of the ground via  51  and the signal via  61  as encircling the respective peripheries thereof. These lands are also contained in the first opening part  32   a . Therefore, the lands for all of the signal vias  61  are unconnected to the first ground layer  56 . The lands for respective signal vias  61  are mutually kept in an unconnected state, and the lands of the ground vias  51  and the land of the signal via  61  are also unconnected to each other. 
         [0032]    Further, on the top surface of the circuit board  31 , a thin signal line  66  is provided to function as a microstrip line. According to the illustrated example, this thin signal line  66  has one end thereof connected to the signal via  61 , and is extended in the right direction (as shown in the drawings) between of two ground vias  51 , which are arranged on the right side. In this case, in a range outside the first opening  32   a , a first auxiliary opening  32   b  is formed on each of the opposite sides of the thin signal line  66  as the opening from which the conductive film of the first ground layer  56  is removed. The first auxiliary opening  32   b  is a portion, which is defined by second pattern lines  33   b  parallel to a thin signal line  66  indicating a boundary to the first ground layer  56  and where the dielectric material of the circuit substrate  31  is exposed due to removal of the first ground layer  56 . Moreover, since the first auxiliary opening  32   b  is connected to the first opening  32   a , the first pattern line  33   a  is not defined as a complete circle but is formed in a shape having a part of the circle cut to be connected to the second pattern line  33   b . When combined the first opening  32   a  and  32   b  exhibit a spoon-like shape(FIG  1 A). 
         [0033]    The thin signal line  66  is a narrow line formed by a foil or a film made of a metal with a low conductive resistance such as copper. For example, the thin signal line  66  can be formed from the first ground layer  56  by a patterning method employing etching, laser-machining and so forth. However, the thin signal line  66  may be formed by means of any other methods. Further, the thin signal line  66  is connected to the land of the signal via  61 . The thin signal line  66  is not connected to the land of the ground via  51 . 
         [0034]    As shown in  FIG. 1B , on the other bottom surface of the circuit board  31 , a second opening  35   a  is formed where the conductor film of the second ground layer  57  has been removed. This second opening  35   a  is defined by a circular third pattern line  36   a  defining a boundary in the second ground layer  57 . This third pattern line  36   a  is a circle coaxial with the signal via  61  is sized to include therein the signal via  61 . Furthermore, the ground vias  51  ground are located outside the third pattern line  36   a , so that all of the ground vias  51  are connected to the second ground layer  57 . 
         [0035]    It is preferred that the second opening part  35   a  is large as possible within a range not including the ground via  51  for ground. It is preferred that the third pattern line  36   a  is defined to have a size thereof permitted to be in contact with each ground via  51 , for example, a diameter approximately 5.2 mm. 
         [0036]    It is desirable that the second ground layer  57  extends over the entire range in a longitudinal direction of the thin signal line  66  in view of appropriately achieving matching of impedance, namely, reducing mismatching of impedance. Therefore, on a part of the second ground layer  57  corresponding to an intermediate area extending between two ground vias  51  through which area the thin signal line  66  extended on the top surface of the circuit board  31 , a convex portion  57   a  is formed so as to protrude toward the signal via  61 . Then, the second relief part  35   a  opposing this convex portion  57   a  is formed in a second auxiliary opening  35   b  as an opening of a smaller diameter. 
         [0037]    As shown in  FIG. 1B , a fourth pattern line  36   b  defining a boundary between the convex portion  57   a  and the second auxiliary opening  35   b  is a circular arc. Then, in order to reduce the mismatching of impedance, it is preferred that the convex portion  57   a  comes close to the signal via  61  as much as possible. Therefore, a radius of the fourth pattern line  36   b  is approximately 1.3 mm, for example. The radius of the fourth pattern line  36   b  is determined to be slightly larger than the half of the outline (about 2.0 mm) of the afore-described land of the signal via  61  in order for the convex portion  57   a  not to contact the land but not to be separated apart from the land so much. Further, it is preferred that a value of “β” which indicates a distance from the land of the signal via  61  up to the front end of the convex portion  57   a  is as small as possible within the range that the insulation between the land and the convex portion  57   a  is not broken. In other words, it is preferred that the value of the distance “β” is preset to the shortest in a range that enables to endure a withstanding voltage according to an insulation resistance between the land and the convex portion  57   a , namely, a withstanding voltage property between the land and the convex portion  57   a . In addition, according to the example shown in  FIG. 1B , a value of “α”, which is a center angle of the fourth pattern line  36   b  of the circular arc, is  90 °, however, this angle may be appropriately changed. 
         [0038]    Then, the high-frequency coaxial connector is connected to one face of the circuit board  31  as shown in  FIG. 1A . Further, a terminal for shield of the high-frequency coaxial connector is connected to the ground via  51 , and the central conductor of the high-frequency coaxial connector for transmitting a high-frequency signal is connected to the signal via  61 . In this case, the first opening  32   a  and the first auxiliary opening  32   b  are formed large and the first ground layer  56  is spaced away from the thin signal line  66 , so that the first ground layer  56  has no influence on the electric impedance of the thin signal line  66 . Further, the ground via  51  is located to connect to the second ground layer  57  in the first opening part  32   a . However, the area of the land thereof is small, and the ground via  51  has no influence on the electric impedance of the thin signal line  66 . In addition, on the bottom surface of the circuit board  31  as shown in  FIG. 1B , the second relief part  35   a  is formed larger, so that it is also possible to diminish any adverse influence on the electric impedance by the second ground layer  57 . 
         [0039]    As a result, the impedance of the thin signal line  66  is determined depending on only coupling thereof with the second ground layer  57 . In addition, the impedance of the signal thin line  66  is not affected by the signal via  61 . Then, on the bottom face of the circuit board  31 , only by matching of the second ground layer  57  located at the position corresponding to the thin signal line  66 , matching of electric impedance of the thin signal line  66  can be achieved. In other words, impedance can be brought into being matched. Further, the second ground layer  57  comes close to the signal via  61  only at the concave portion  57   a , it is possible to reduce mismatching of impedance. 
         [0040]    Next, a modified example of the concave portion  57   a  of the second ground layer  57  will be described. 
         [0041]      FIG. 2  is a cross-sectional view, taken along the line A-A of  FIG. 1A  showing a distant relationship between a microstrip line and a first ground layer, according to the embodiment of the present invention, and  FIG. 3  is a view showing a modified example of a convex portion of a second ground layer according to the embodiment of the present invention. 
         [0042]    Generally, in the case where a width of a thin signal line made of a belt-like metallic material is defined as “W”, if the width of the part from which a metallic layer is removed at the opposite ends of the signal thin line is set in a range of 3W through 5W, even in the case of a coplanar structure provided with a ground layer, the signal thin line can be considered as a microstrip line. Therefore, according to the present embodiment, as shown in  FIG. 2 , if the width of the thin signal line  66  is defined to be “W”, the widths of the first auxiliary relief parts  32   b  on both sides of the thin signal line  66  are set to be 3W or more, respectively. 
         [0043]    Then, according to the example as shown in  FIG. 3 , the convex portion  57   a  of the second ground layer  57  is formed in approximately a rectangular shape. In this case, the width of this convex portion  57   a  is preset to be a value obtained from adding the widths of the first auxiliary openings  32   b  on both sides to the width W of the thin signal line  66 , namely, the value not less than 7W. Further, the distance from the land of the signal via  61  up to the front end of the convex portion  57   a  is preferably short as much as possible within a range such that an electric insulation between the land and the convex portion  57   a  is prevented from being broken, similar to the embodiment as illustrated in  FIG. 1B . 
         [0044]    In this case, as the convex portion  57   a  with the width not less than 7W is formed, the second ground layer  57  of sufficiently large width is arranged to come, on the signal thin line  66 , to a position where it closely opposes the signal via  61 , so that the impedance of the thin signal line  66  can be successfully matched on account of matching of the second ground layer  57  and thus, mismatching of impedance can be diminished. 
         [0045]    Hence, according to the present embodiment, the first ground layer  56  is provided with the first opening  32   a  where the conductor film is removed so as to encircle the periphery of the signal via  61  with the first opening  32   a , and the second ground layer  57  is provided with the second opening  35   a  where the conductor film is removed so as to encircle the periphery of the signal via  61  with the second opening  35   a  for signal, along with the concave portion  57   a  protruding toward the through-hole  61  for signal at the region corresponding to the thin signal line  66  in the second opening  35   a.    
         [0046]    Thus, the impedance of the thin signal line  66  can be matched only by matching of the second ground layer  57 . In addition, since the second ground layer  57  comes close to the signal via  61  only at the concave portion  57   a , it is possible to diminish any mismatching of the impedance. 
         [0047]    In addition, the concave portion  57   a  has the width not less than seven times as large as the width of the signal thin line  66 . In this case, since the second ground layer  57  with a sufficient large width corresponds up to the position adjacent to the signal via  61  in the signal thin line  66 , mismatching of the impedance can be diminished. 
         [0048]    The present invention is not limited to the above-described embodiments, and may be changed in various ways based on the gist of the present invention, and these changes are not eliminated from the scope of the present invention.