Abstract:
A substrate of the present invention includes pads which are provided on the surface of said substrate; and surface layers which are kept to the ground potential and cover the surface of said substrate except said pads and their peripheral. Another substrate of the present invention includes a part of circuit which is provided on the surface of said substrate; and a surface layers which are kept to the ground potential and cover the surface of said substrate except said part of circuit and its peripheral.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a circuit board, particularly relates to a circuit board wherein electronic parts and electrical components are mounted on the surface of a substrate such as a semiconductor chip. 
     2. Description of the Related Art 
     Currently, a circuit board is used for various electric equipment and information equipment. In a conventional circuit board, electronic parts are mounted on the top surface or the bottom surface of a substrate. As recent electric equipment and information equipment are miniaturized and are provided with high functions, technique for further integrating mounted electronic parts, forming a pattern of a few layers inside a circuit board and forming a circuit wherein these patterns are mutually connected has been also enhanced. 
     Such technique has been remarkably developed for a circuit board particularly used for a computer, however, as a digital signal is conducted in a circuit board used for this technical field, an electromagnetic wave is emitted from the circuit board as EMI noise. As EMI noise has an effect upon another circuit provided to the same electric equipment and information equipment or another electric equipment and information equipment, it is required to be reduced. 
     To prevent EMI noise from leaking outside the chassis of electric equipment and information equipment, a conductive sheet was arranged inside the chassis of electric equipment and the information equipment or the chassis was made of conductive material as a major conventional technique. Also, for another technique to reduce EMI noise in the electric equipment and the information equipment on which many circuit boards are mounted, a shield plate for shielding an electromagnetic wave was provided between circuit boards, or the surface of a circuit board is shielded in a state that electronic parts are mounted. 
     However, it has been difficult to reduce EMI emitted noise as the performance of electric equipment and information equipment is enhanced. Emitted EMI noise cannot be reduced by only applying the shield to the chassis of electric equipment and information equipment. Therefore, a method of directly reducing emitted EMI noise caused from a circuit board is required. 
     SUMMARY OF THE INVENTION 
     The object of the invention is to provide a circuit board wherein EMI noise emitted from the circuit board is reduced. 
     Another object of the invention is to provide a circuit board that can reduce noise mixed from an external device. 
     According to one aspect of the present invention, a substrate is provided which includes: pads which are provided on the surface of said substrate; and surface layers which are kept to the ground potential and cover the surface of said substrate except said pads and their peripheral. 
     According to another aspect of the present invention, a substrate is provided which includes: a part of circuit which is provided on the surface of said substrate; and a surface layers which are kept to the ground potential and cover the surface of said substrate except said part of circuit and its peripheral. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other features and advantages of the invention will be made more apparent by the following detailed description and the accompanying drawings, wherein: 
     FIG. 1 is a perspective drawing showing a part of a circuit board equivalent to a first embodiment of the invention; 
     FIG. 2 is a sectional view viewed along a line A—A of the circuit board according to the invention shown in FIG. 1; 
     FIG. 3 is a perspective drawing showing a part of a circuit board equivalent to a second embodiment of the invention; 
     FIG. 4 is a sectional view viewed along a line B—B of the circuit board according to the invention shown in FIG. 3; 
     FIG. 5 is a perspective drawing showing a part of a circuit board equivalent to a third embodiment of the invention; and 
     FIG. 6 is a sectional view viewed along a line C—C of the circuit bird according to the invention shown in FIG.  5 . 
    
    
     In the drawings, the same reference numerals represent the same structural elements. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A first embodiment of the present invention will be described in detail below. 
     Referring to FIG. 1, a circuit board  10  includes ground layers  12 ,  14  and  16 . The ground layer  12  is provided on the upper surface of the circuit board  10 . The ground layer  14  is provided on the lower surface of the circuit board  10 . The ground layers  12  and  14  form plural surface layers of the circuit board  10 . The ground layer  16  is provided inside the circuit board  10 . Specifically, the ground layer  16  is one of plural internal layers included in the circuit board  10 . The ground layer  16  is put between the ground layers  12  and  14 . The ground layers  12 ,  14  and  16  are electrically connected to a stable ground such as the body of chassis. For example, the ground layers  12 ,  14  and  16  are made of copper. The ground layers  12 ,  14  and  16  are formed so that they have the thickness of, for example, approximately 0.035 millimeter. 
     The circuit board  10  includes signal layers  18  and  22  and a power supply layer  20 . The signal layer  18  is put between the ground layers  12  and  16 . The signal layer  22  is put between the ground layers  14  and  16 . The power supply layer  20  is put between the ground layers  14  and  16 . Insulating material is filled between these layers. For example, glass epoxy resin is used For the insulating material. Pads  24   a ,  24   b  and  24   c  are provided on the upper surface of the circuit board  10 . The pads  24   a ,  24   b  and  24   c  are used to electrically connect electronic parts mounted on the surface of the circuit board  10 . The ground layer  12  is removed in parts in which the pads  24   a ,  24   b  and  24   c  are respectively provided. That is, the ground layer  12  is formed in the whole area except the pads  24   a ,  24   b  and  24   c  and their peripheral areas on the upper surface of the circuit board  10 . It is desirable that an interval between each pad  24   a ,  24   b  ,  24   c  and the ground layer  12  is as narrow as possible in a range where no short circuit is caused. The ground layer  14  is uniformly formed in the whole area of the lower surface of the circuit board  10 . 
     In this embodiment, plural pads are provided on the upper surface of the circuit board  10 , however, plural pads may be also provided on both the upper and lower surfaces of the circuit board  10 . In this case, the ground layer  14  is formed in the whole area except the plural pads and their peripheral areas on the lower surface of the circuit board  10 . 
     Pads are provided by making a hole in the ground layer formed on the surface of the circuit board to supply power and a signal to an electronic circuit mounted on the surface of the circuit board. The pads and the power supply layer or the signal layer are electrically connected via wiring. 
     Referring to FIG. 2, via holes  26 ,  28  and  30  are formed inside the circuit board  10 . The pad  24   a  is connected to the power supply layer  20  via the via hole  26  and power is supplied to an electronic part mounted on the upper surface of the circuit board  10 . The pads  24   b  and  24   c  are connected to the signal layers  18  and  22  via the via holes  28  and  30 , respectively. The pads  24   b  and  24   c  receive and transmit various electric information from/to an electronic part mounted on the upper surface of the circuit board  10 . 
     Conductive elements  32  and  36  are provided between the round layers  12  and  16 . Conductive elements  34  and  38  are provided between the ground layers  14  and  16 . The conductive elements  32 ,  36 ,  34  and  38  are conductive and electrically connect the ground layers  12  and  14 . Concretely, the conductive element  32  electrically connects the ground layers  12  and  16 . The conductive element  34  electrically connects the ground layers  16  and  14 . The conductive element  36  electrically connects the ground layers  12  and  16 . The conductive element  38  electrically connects the ground layers  16  and  14 . The conductive elements  32 ,  36 ,  34  and  38  may be made of the same material as that of a signal conductor. 
     An electronic part mounted on the circuit board is provided with a ground pin that defines ground potential. The ground pin can be connected to any of the ground layers  12 ,  14  and  16 , however, it is desirable that it is electrically connected to the ground layer  12  for convenience of wiring. 
     In the circuit board  10  in this embodiment, power is supplied from the power supply layer  20  provided inside the circuit board  10  to an electronic part mounted on the surface of the board and ground potential is defined by connecting the ground pin of the electronic part to the ground layer  12 . Also, a signal is received or transmitted from/to an electronic circuit via signal conductors  18  and  22 . Therefore, in the circuit board  10  in this embodiment, a digital signal is conducted in the signal conductors  18  and  22  arranged in the circuit board  10  and the circuit board  10  is formed so that the ground layers  12 ,  14  and  16  cover the signal conductors  18  and  22 . Therefore, less EMI noise is emitted from the circuit board  10 . 
     The circuit board according to the invention is not limited to that in the embodiment described above and may be freely varied in a range of the invention. For example, the number of ground layers, power supply layers and signal layers respectively provided between the ground layers  12  and  14  may be arbitrary. Also, respective wiring for supplying power, ground and a signal can be also provided in the same layer. 
     Also, in the embodiment described above, the surface layer of the circuit board  10  is kept to ground potential, covers an area except the pad and the periphery and is composed of two sides of the ground layers  12  and  14  or four sides of the ground layers  12  and  14  and both sides of the board  10 , however, the surface layer may be also composed of six sides of the ground layers  12  and  14 , both sides of the board  10  and the front and back of the board  10 . 
     In this embodiment, the pads  24  are formed on the upper surface of the circuit board  10 , however, a part of a circuit, for example, a pad, a pattern and a via hole, may be also formed on the upper surface of the circuit board  10  in stead of the pads. In this case, the pad and the via hole are connected via the pattern, and the pad and a signal conductor in an internal layer transmit a signal via the via hole and the pattern. 
     Next, a second embodiment of the present invention will be described in detail. The second embodiment is characterized in that a ground layer on the upper surface and a ground layer on the lower surface are electrically connected via a via hole. The other configuration is similar to that in the first embodiment. 
     Referring to FIGS. 3 and 4, a circuit board  10   a  includes plural via holes  21  and  23 . The via hole  21  electrically connects ground layers  12 ,  16  and  14 . Conductive material is filled inside the via hole  21 . The via hole  23  electrically connects the ground layers  12 ,  16  and  14 . Conductive material is filled inside the via hole  23 . 
     In this embodiment, the number of via holes is not particularly limited. Only one via hole may be provided to each side of the circuit board  10   a . Plural via holes may otherwise be also provided to each side. 
     Next, a third embodiment of the present invention will be described in detail. The third embodiment is characterized in that a ground layer on the upper surface and a ground layer on the lower surface are electrically connected via a conductive layer formed on the side. The other configuration is similar to that in the first embodiment. 
     Referring to FIGS. 5 and 6, a circuit board  10   b  is provided with conductive layers  40  and  42 . The conductive layer  40  is provided to one side of the circuit board  10   b  and the conductive layer  42  is provided to the other side. Each of the conductive layers  40  and  42  electrically connect the ground layers  12  and  14 . The conductive layers  40  and  42  may be made of the same material as that of the ground layers  12  and  14 . 
     As described above, according to the invention, as the ground layers are formed on the top surface and the bottom surface of the circuit board, EMI noise can be reduced. Also, as the ground layers are formed on the top surface and the bottom surface of the circuit board, noise mixed from an external device, which deteriorates a signal conducted in the signal layer, can be reduced. Also, as wiring for transmitting a signal is not formed on the top surface and the bottom surface of the circuit board, there is also effect that a value of characteristic impedance is readily standardized. 
     While this invention has been described in conjunction with the preferred embodiments described above, it will now be possible for those skilled in the art to put this invention into practice in various other manners.