Abstract:
There is described a multi-layer printed circuit board and a method of installing it. The circuit board includes a first signal layer formed on its obverse surface; a ground layer arranged at a position next to the first signal layer; an electronic power source layer arranged at a position next to the ground layer; and a second signal layer formed on its reverse surface. The first and second patterns are formed around peripheral areas of the first and second signal layers, respectively. The first ground pattern and the second ground pattern are electrically coupled to each other by plural through holes, and the multi-layer printed circuit board is installed on an electro-conductive housing in such a manner that a substantially whole area of the second ground pattern electrically contacts a mounting area of the electro-conductive housing, the mounting area being an electro-conductive area continuously coupled to the electro-conductive housing.

Description:
BACKGROUND OF THE INVENTION 
   The present invention relates to a method of installing a printed circuit board onto an electro-conductive housing, so as to prevent the printed circuit board from emitting electromagnetic wave noise. 
     FIG. 1(   a ) and  FIG. 1(   b ) show a conventional method for installing a printed circuit board onto an electro-conductive housing. In order to reduce an electromagnetic wave noise (EMI) emitted from the printed circuit board, the ground pattern formed on a signal layer of the printed circuit board is electrically coupled to the electro-conductive housing of the electronic apparatus. 
   As shown in the plan view of  FIG. 1(   a ), the conventional printed circuit board is mounted onto the electro-conductive housing by fastening four metal screws through the installation holes located at the four corners of the printed circuit board. 
     FIG. 1(   b ) shows a cross sectional view of the installation hole and its peripheral. Since the inner surface of the installation hole is treated with a through-hole metal coating, the ground pattern of the signal layer on the obverse surface and the inner ground layer in the printed circuit board are electrically coupled to the metal post. 
     FIG. 2  shows a partial plain view of the printed circuit board, in which the inner surface of the installation hole is finished without a through-hole metal coating. In this configuration, the ground pattern of the signal layer on the obverse surface is coupled to the inner ground layer by forming a lot of through holes around the periphery of the installation hole. 
   As shown in a plain view of the printed circuit board shown in  FIG. 3 , it has been well known as a conventional technique to increase a number of connecting points between the printed circuit board and the electro-conductive housing, in order to improve an electro-magnetic noise reduction effect. 
   The electro-magnetic noise reduction effect caused by connecting the ground pattern of the signal layer on the printed circuit board to the electro-conductivity housing lays on the two reasons described as follow:
     (1) Since the area of the ground pattern in the signal layer equivalently increases, a capacitance between the ground layer and the earth would increase, resulting in a reduction of voltage fluctuations on the ground layer.   (2) In the printed circuit board having more than four layers, a power source layer, the ground layer and the electro-conductive housing are laminated in an order of the ground layer, the power source layer and the electro-conductive housing. Accordingly, since the power source layer is structurally shielded by the two other layers, the electro-magnetic noises emitted from the gap between the power source layer and the ground layer are reduced. The lager the number of installation points is, the grater the shielding effect is, resulting in a further noise reduction.   

   By connecting the ground layer of the printed circuit board to the electro-conductive housing, however, sometimes the electromagnetic noises adversely increase. The reasons of this will be detailed in the following. 
     FIG. 4  is a schematic view, which shows the state that a printed circuit board is installed on the electro-conductive housing. 
   As shown in  FIG. 4 , by connecting the ground layer of the printed circuit board to the electro-conductive housing, a resonance would occur in the loop between connecting points. Accordingly, for instance, when the printed circuit board is mounted on the electro-conductive housing by employing the screws arranged in intervals of every 15 cm, the loop distance becomes 30 cm, resulting in the resonance at 1 GHz. Therefore, the noise reduction effect at this frequency cannot be exhibited, but the electro-magnetic noise component residing at this frequency would be adversely emphasized, possibly resulting in an occurrence of great amount of electromagnetic noises. 
   Generally speaking, since harmonics of clock signals mainly comprise odd-number harmonic components and the energy included in up to ninth harmonics is great, it should be prevented that the resonance occurs at the tenth harmonic frequency component. 
   In late years, since the clock frequency becomes high and the clock frequency of 100 MHz is generally employed in computer apparatus, it is necessary to set the resonance frequency larger than 1 GHz. To attain this goal, it is necessary to connect the printed circuit board to the electro-conductive housing at an interval smaller than 15 cm. (references: “EMC Design of Print Circuit”, published by Ohmsha) 
   The technology for solving the abovementioned problem is also set forth in Tokkai 2000-183533. 
   According to the abovementioned technology, since the ground layers are disposed at upper and lower sides of the power supply layer of the multi-layer printed circuit board, and the upper and lower ground layer are coupled to each other through the through holes established at an outer circumferential area of the power supply layer, the electro-magnetic noises emitted from a gap between the power supply layer and the ground layer can be reduced. However, when this technology is applied to the normal 4-layer printed circuit board, additional layers for the ground layers are necessary and, as a result, a 6-layer printed circuit board would be required. This fact is disadvantageous for its cost reduction aspect. 
   Further, another technology for improvement is set forth in Tokkai 2001-210922. This technology employs the electro-conductive housing, instead of the second ground layer set forth in Tokkai 2000-210922. According to this technology, in order to connect the ground to the electro-conductive housing, the ground pattern is continuously formed along the outer circumferential area of the outer layer of the printed circuit board, and is coupled to the ground of the inner layer through a lot of through holes, so that the power supply layer is electronically shielded by continuously coupling the ground pattern located at the outer circumferential area to the electro-conductive housing with a metal post or a metal plate. This is a superior technology, which effectively utilizes the electro-conductive housing for the low-cost 4-layer printed circuit board to have the same effect as set forth in Tokkai 2000-210922. 
   Patent Document 1: Tokkai 2000-183533, Scope of the claims 
   Patent Document 2: Tokkai 2001-210922, Scope of the claims 
   Non-Patent Document 1: “EMC Design of Printed Circuit Board” published by ohm publishing Co., LTD. 
   The prior art of “PRINTED WIRING BOARD AND PRINTED WIRING BOARD IMPLEMENTATION STRUCTURE” set forth in Tokkai 2001-210922 would be an effective technology when the size of the housing is approximately equal to that of the printed wiring board, but is insufficient for the printed circuit board mounted on a big housing. 
     FIG. 5  shows a figure explaining an outflow of a common mode electric current caused by resonance.  FIG. 6  shows a characteristic curve of the electromagnetic noise level versus frequency. 
   Even if the whole outer circumferential area of printed circuit board is connected to the electro-conductive housing, when the size of the housing is larger than that of printed circuit board as shown in  FIG. 5 , a resonance phenomenon is generated at a specific frequency f 0  by the common mode electric current flowing on the outer surface of the electro-conductive housing as shown in  FIG. 6 , resulting in a cause of the electro-magnetic noises. In other words, although the electro-magnetic noise emission from the gap between the power supply layer and the ground layer can be eliminated, it is impossible to prevent the electromagnetic noise emission caused by the common mode electric current flowing in the electro-conductive housing itself. 
   SUMMARY OF THE INVENTION 
   To overcome the abovementioned drawbacks in conventional technologies, it is an object of the present invention to provide a technology, which make it possible to not only reduce electromagnetic noises emitted from the gap between the power supply layer and the signal layer, but also simultaneously prevent the common mode electric current flowing in the electro-conductive housing. 
   Accordingly, to overcome the cited shortcomings, the abovementioned object of the present invention can be attained by multi-layer printed circuit boards and installing methods described as follow.
     (1) A multi-layer printed circuit board for mounting electronic parts on it, comprising: a first signal layer formed on an obverse surface of the multi-layer printed circuit board; a ground layer arranged at a position next to the first signal layer; an electronic power source layer arranged at a position next to the ground layer; and a second signal layer formed on a reverse surface of the multi-layer printed circuit board; wherein a first ground pattern is formed around a peripheral area of the first signal layer, while a second ground pattern is formed around a peripheral area of the second signal layer; and wherein the first ground pattern and the second ground pattern are electrically coupled to each other by a plurality of through holes, and the multi-layer printed circuit board is installed on an electro-conductive housing in such a manner that a substantially whole area of the second ground pattern electrically contacts a mounting area of the electro-conductive housing, the mounting area being an electro-conductive area continuously coupled to the electro-conductive housing.   (2) The multi-layer printed circuit board of item  1 , wherein a snubber circuit, being a serial connection of a resistor and capacitor, is coupled between the ground layer and the electronic power source layer.   (3) A method for installing a multi-layer printed circuit board on an electro-conductive housing, the multi-layer printed circuit, on which electronic parts are mounted, comprising a first signal layer formed on an obverse surface of the multi-layer printed circuit board, a ground layer arranged at a position next to the first signal layer, an electronic power source layer arranged at a position next to the ground layer, and a second signal layer formed on a reverse surface of the multi-layer printed circuit board, the method comprising the steps of: forming a first ground pattern around a peripheral area of the first signal layer; forming a second ground pattern around a peripheral area of the second signal layer; coupling electrically the first ground pattern to the second ground pattern with a plurality of through holes; and mounting the multi-layer printed circuit board on a mounting area of the electro-conductive housing in such a manner that a substantially whole area of the second ground pattern electrically contacts the mounting area of the electro-conductive housing, the mounting area being an electro-conductive area continuously coupled to the electro-conductive housing.   (4) The method of item  3 , wherein the mounting area of the electro-conductive housing is formed in a protruded shape, so that the electronic parts are held by the multi-layer printed circuit board without physically touching the electro-conductive housing.   (5) The method of item  3 , wherein an inner area surrounded by the mounting area of the electro-conductive housing is formed in a concaved shape, so that the electronic parts are held by the multi-layer printed circuit board without physically touching the electro-conductive housing, and the inner area is opposed against the reverse surface of the multi-layer printed circuit board.   (6) The method of item  3 , wherein a part of an inner area surrounded by the mounting area of the electro-conductive housing is formed in a concaved shape, so that an electronic part, being one of the electronic parts, is held by the multi-layer printed circuit board without physically touching the electro-conductive housing, and the inner area is opposed against the reverse surface of the multi-layer printed circuit board.   (7) The method of item  3 , wherein an inner area surrounded by the mounting area of the electro-conductive housing is a flat plane continued from the mounting area, so that the multi-layer printed circuit board is mounted on the electro-conductive housing with a insulation layer between the inner area and the flat plane, and the inner area is opposed against the reverse surface of the multi-layer printed circuit board.   (8) A method for installing a multi-layer printed circuit board on an electro-conductive housing, the multi-layer printed circuit, on which electronic parts are mounted, comprising a first signal layer formed on an obverse surface of the multi-layer printed circuit board, a ground layer arranged at a position next to the first signal layer, an electronic power source layer arranged at a position next to the ground layer, and a second signal layer formed on a reverse surface of the multi-layer printed circuit board, the method comprising the steps of: forming a first ground pattern around a peripheral area of the first signal layer; forming a second ground pattern around a peripheral area of the second signal layer; coupling electrically the first ground pattern to the second ground pattern with a plurality of through holes; and mounting the multi-layer printed circuit board on a mounting area of the electro-conductive housing in such a manner that a substantially whole area of the first ground pattern is electrically coupled to the electro-conductive housing through an electro-conductive member.   (9) The method of item  8 , wherein the multi-layer printed circuit board is held between the electro-conductive member and the electro-conductive housing by fastening them with screws, so that the electro-conductive member press-contacts the first ground pattern while press-contacting the electro-conductive housing.   (10) The method of item  8 , wherein an inner area surrounded by the mounting area of the electro-conductive housing is formed in a concaved shape, so that the electronic parts are held by the multi-layer printed circuit board without physically touching the electro-conductive housing, and the inner area is opposed against the reverse surface of the multi-layer printed circuit board.   (11) The method of item  8 , wherein a part of an inner area surrounded by the mounting area of the electro-conductive housing is formed in a concaved shape, so that an electronic part, being one of the electronic parts, is held by the multi-layer printed circuit board without physically touching the electro-conductive housing, and the inner area is opposed against the reverse surface of the multi-layer printed circuit board.   (12) The method of item  8 , wherein an inner area surrounded by the mounting area of the electro-conductive housing is a flat plane continued from the mounting area, so that the multi-layer printed circuit board is mounted on the electro-conductive housing with a insulation layer between the inner area and the flat plane, and the inner area is opposed against the reverse surface of the multi-layer printed circuit board.   

   Further, to overcome the abovementioned problems, other printed circuit boards and installation methods, embodied in the present invention, will be described as follow:
     (13) A printed circuit board, characterized in that,   

   in the printed circuit board that has at least a ground layer and an electronic power source layer and mounts electronic parts, 
   (a) an order of layer structure of a multi-layer printed circuit board having equal to or more than four layers and signal layers on obverse and reverse surfaces is structured in an order of an outer signal layer, a ground layer, an electronic power source layer, and a signal layer at an electro-conductive housing side, 
   (b) a ground pattern is formed along a circumferential area of each of the signal layers, and the electro-conductive housing electrically and continuously coupled to the ground pattern with contacting surfaces to each other, 
   (c) the ground pattern is electrically coupled to the ground layer with a plurality of through holes, and 
   (c) the ground layer is coupled to the electronic power source layer through a snubber circuit, being a serial connection of a resister and capacitor.
     (14) An installation method for installing the printed circuit board, that has at least a ground layer and an electronic power source layer and mounts electronic parts, onto an electro-conductive housing, characterized in that,   

   (a) the structure of printed circuit board is the same as that described in item  13 , 
   (b) the ground pattern, formed along the circumferential area of the outer layer of the printed circuit board, is formed at the surface of the printed circuit board opposing to the electro-conductive housing, 
   (c) an electro-conductive convex-shaped member is integrally formed at a portion at which the electro-conductive housing corresponds to the ground pattern of the printed circuit board, and the electronic parts mounted on the printed circuit board are held without contacting the electro-conductive housing.
     (15) An installation method for installing the printed circuit board, that has at least a ground layer and an electronic power source layer and mounts electronic parts, onto an electro-conductive housing, characterized in that,   

   (a) the structure of printed circuit board is the same as that described in item  13 , 
   (b) the ground pattern, formed along the circumferential area of the outer layer of the printed circuit board, is formed at the surface of the printed circuit board opposing to the electro-conductive housing, 
   (c) the surface of the electro-conductive housing, opposing to the printed circuit board and corresponding to inner side of the ground pattern, is formed in a concave-shape, and the electronic parts mounted on the printed circuit board are held without contacting the electro-conductive housing.
     (16) An installation method for installing the printed circuit board, that has at least a ground layer and an electronic power source layer and mounts electronic parts, onto an electro-conductive housing, characterized in that,   

   (a) the structure of printed circuit board is the same as that described in item  13 , 
   (b) the ground pattern, formed along the circumferential area of the outer layer of the printed circuit board, is formed at the surface of the printed circuit board opposing to the electro-conductive housing, 
   (c) in the surface of the electro-conductive housing, opposing to the printed circuit board and corresponding to inner side of the ground pattern, at least an area, near a portion at which an electronic part of the printed circuit board protrudes toward the electro-conductive housing side, is formed in a concave-shape, and the electronic parts mounted on the printed circuit board are held without contacting the electro-conductive housing.
     (17) An installation method for installing the printed circuit board, that has at least a ground layer and an electronic power source layer and mounts electronic parts, onto an electro-conductive housing, characterized in that,   

   (a) the structure of printed circuit board is the same as that described in item  13 , 
   (b) the ground pattern, formed along the circumferential area of the outer layer of the printed circuit board, is formed at the surface of the printed circuit board opposing to the electro-conductive housing, 
   (c) the surface of the electro-conductive housing, opposing to the printed circuit board, is formed in a flat plain with a insulation layer between them, and the electronic parts mounted on the printed circuit board are held without contacting the electro-conductive housing.
     (18) An installation method for installing the printed circuit board, that has at least a ground layer and an electronic power source layer and mounts electronic parts, onto an electro-conductive housing, characterized in that,   

   (a) the structure of printed circuit board is the same as that described in item  13 , and 
   (b) the ground pattern is formed at an opposite side of a surface of the printed circuit board opposing to the electro-conductive housing, and the electro-conductive housing is coupled to the ground pattern through an electro-conductive member. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other objects and advantages of the present invention will become apparent upon reading the following detailed description and upon reference to the drawings in which: 
       FIG. 1(   a ) and  FIG. 1(   b ) show a conventional method for installing a printed circuit board onto an electro-conductive housing; 
       FIG. 2  shows a partial plain view of the printed circuit board, in which the inner surface of the installation hole is finished without a through-hole metal coating; 
       FIG. 3  shows a plain view of the printed circuit board; 
       FIG. 4  is a schematic view, which shows the state that a printed circuit board is installed on the electro-conductive housing; 
       FIG. 5  shows a figure explaining an outflow of a common mode electric current caused by resonance; 
       FIG. 6  shows a characteristic curve of the electro-magnetic noise level versus frequency; 
       FIG. 7(   a ) shows a cross-sectional view of a state that the printed circuit board is mounted on the electro-conductive housing, and  FIG. 7(   b ) shows a perspective view of the electro-conductive housing connected to the printed circuit board and the electro-conductive post; 
       FIG. 8  shows a cross-sectional view of the printed circuit board; 
       FIG. 9(   a ),  FIG. 9(   b ),  FIG. 9(   c ) and  FIG. 9(   d ) show an exploded plan view of the printed circuit board; 
       FIG. 10(   a ) and  FIG. 10(   b ) show schematic diagrams of the layer construction of a six-layer printed circuit board; 
       FIG. 11(   a ) shows a perspective view of the printed circuit board,  FIG. 11(   b ) shows a plain view and  FIG. 11(   c ) shows a schematic diagram of a layer structure; 
       FIG. 12(   a ),  FIG. 12(   b ),  FIG. 12(   c ) and  FIG. 12(   d ) show cross sectional views of the printed circuit board mounted on the electro-conductive housing corresponding to various kinds of installing methods; and 
       FIG. 13(   a ),  FIG. 13(   b ) and  FIG. 13(   c ) show cross sectional views of the printed circuit board mounted on the electro-conductive housing corresponding to other various kinds of installing methods; and 
       FIG. 14(   a ) and  FIG. 14(   b ) show a plain view and perspective view of an electro-conductive housing with electro-conductive post members, as another embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Referring to  FIGS. 7(   a )–( b ), the preferred embodiment of the present invention will be detailed in the following. 
     FIG. 7(   a ) shows a cross-sectional view of a state that printed circuit board  1  is mounted on electro-conductive housing  2  (hereinafter, also referred to as a housing),  FIG. 7(   b ) shows a perspective view of electro-conductive housing  2  connected to printed circuit board  1  and electro-conductive post  3 ,  FIG. 8  shows a cross-sectional view of printed circuit board  1 , and  FIG. 9(   a ),  FIG. 9(   b ),  FIG. 9(   c ) and  FIG. 9(   d ) show an exploded plan view of printed circuit board  1 . 
   The printed circuit board  1  shown in the above drawings is a four layer printed circuit board having signal layer  14 , ground layer  12 , power source layer  13  and signal layer  15 . 
   Signal layer  14  is fixed at upper side of ground layer  12  with insulation sheet  16 A between them in the direction shown in  FIG. 7(   a ). Signal layer  15  is formed at lower side of power source layer  13  with insulation sheet  16 B between them in the direction shown in  FIG. 7(   a ). 
   The conductive area of ground layer  12  is formed as larger than that of power source layer  13 . Further, signal lines from which the electro-magnetic noise emission is relatively weak, for instance, such as analogue lines, are printed on signal layer  14 , while signal lines from which the electro-magnetic noise emission is relatively strong, for instance, such as clock signal lines and a bus line, are preferentially printed on signal layer  15 . 
   Printed circuit board  1  is mounted on electro-conductive housing  2  in such a manner that the side of signal layer  15  is located directly opposite to electro-conductive housing  2 . 
   In  FIG. 7(   a ), ground pattern  17  is disposed at the circumferential area of signal layer  14  serving as a first layer, while ground pattern  18 , being different from ground pattern  17 , is disposed at the circumferential area of signal layer  15  serving as a fourth layer (refer to  FIG. 9(   a ) and  FIG. 9(   b )). 
   By mounting printed circuit board  1  onto electro-conductive housing  2 , ground layer  12  is electronically connected to electro-conductive housing  2  through electro-conductive post  3  (an electro-conductive protrusion-shaped member) and through holes  19  detailed later. 
   As shown in  FIG. 7(   b ), electro-conductive post  3  is formed in a frame shape having a surface substantially the same shape as that of ground pattern  17  formed on signal layer  14 . 
   Ground pattern  17  and ground pattern  18  are electronically connected to each other by a plurality of through holes  19 . The interval distance between through holes  19  should be established at a distance shorter than 5 cm, which is ⅙ of 30 cm equivalent to the wavelength at 1 GHz, being the upper limit value defined in the standard of electromagnetic noise emission. 
   According to the abovementioned configuration, power source layer  13  of printed circuit board  1  and signal layer  15 , on which the lines emitting relatively strong electro-magnetic noises are printed, are surrounded by ground layer  12 , through holes  19  and ground pattern  18  (refer to  FIGS. 9(   a )– 9 ( d )). 
   Accordingly, it becomes possible to suppress the electro-magnetic noise emissions from the gap between power source layer  13  and ground layer  12  and from the signal pattern of signal layer  15 . The shielding effect could be improved by reducing a mounting distance of electro-conductive post  3 . 
     FIG. 10(   a ) and  FIG. 10(   b ) show schematic diagram of the layer construction of a six-layer printed circuit board. 
   With respect to the six-layer printed circuit, the layer order of signal layer  14 , ground layer  12 , power source layer  13  and signal layer  15  is the same as that of the four-layer printed circuit board, while the layer order of second ground layer  12 A and second signal layer  14 A are added other than them. Since power source layer  13  is shielded by sandwiching it with signal layer  15  and electro-conductive housing  2 , it becomes possible to drastically reduce the electro-magnetic noises. 
   Incidentally, in order to electronically conduct to electro-conductive housing  2 , the solder-resist treatment is not applied to the signal pattern of signal layer  15 . 
     FIG. 11(   a ) shows a perspective view of printed circuit board  1 ,  FIG. 11(   b ) shows a plain view and  FIG. 11(   c ) shows a schematic diagram of a layer structure. 
   The electronic parts, such as integrated circuit IC, transistor TR, crystal oscillator Q for generating the clock signals, resister R, capacitor C, etc., are mounted on signal layer  14  and signal layer  15 . 
   As shown in  FIGS. 11(   a )– 11 ( c ), a plurality of snubber circuits, each of which is a serial connection of resistor R and capacitor C, are mounted between power source layer  13  and signal layers  14 ,  15  around the circumferential area of printed circuit board  1 . Generally speaking, the mounting distance between the snubber circuits is several centimeters. This snubber circuit is effective for preventing an abrupt change of the electronic current flowing into the electronic parts, resulting in a suppression of the resonance, a reduction of the common mode current flowing out from electro-conductive housing  2  and a reduction of the electro-magnetic noise emission. 
     FIG. 12(   a ),  FIG. 12(   b ),  FIG. 12(   c ) and  FIG. 12(   a ) show cross sectional views of the printed circuit board mounted on electro-conductive housing  2  in various kinds of mounting structures. The connecting structure of printed circuit board  1  with electro-conductive housing  2  is not only the structure shown in  FIG. 7(   a ), but also anyone of the structures shown in  FIGS. 12(   a )– 12 ( d ) selected depending on a presence or absence of the electronic parts mounted on the rear side of printed circuit board  1  and contents of the housing design. 
     FIG. 12(   a ) shows a structure in which printed circuit board  1  is mounted on electro-conductive post  3  fixed on electro-conductive housing  2 . According to the abovementioned structure and the mounting method, the surface disposed opposite to printed circuit board  1  is formed in a convex shape with electro-conductive post  3 , which corresponds to ground pattern  18  formed on the circumferential area of printed circuit board  1 . Accordingly, printed circuit board  1  can be fixed on electro-conductive housing  2  by employing screws, etc. without contacting electronic parts  4  to electro-conductive housing  2 . 
     FIG. 12(   b ) shows a structure in which concave section  2 A is formed at the surface disposed opposite to printed circuit board  1 , and located at an inner portion corresponding to ground pattern  18 . According to the abovementioned structure and the mounting method, printed circuit board  1  can be fixed on electro-conductive housing  2  by employing screws, etc. without contacting electronic parts  4  to electro-conductive housing  2 . 
     FIG. 12(   c ) shows a structure in which concave section  2 B is formed at the surface disposed opposite to printed circuit board  1 , and located at an inner portion surrounded by ground pattern  18 . According to the abovementioned structure and the mounting method, printed circuit board  1  can be fixed on electro-conductive housing  2  by employing screws, etc. without contacting electronic parts  4  to electro-conductive housing  2 . 
     FIG. 12(   d ) shows a structure in which a flat plane is formed at the surface disposed opposite to printed circuit board  1 , and located at an inner portion continued from ground pattern  18 . According to the abovementioned structure and the mounting method, printed circuit board  1  can be fixed on electro-conductive housing  2  by employing screws, etc. with insulating sheet  16 C between printed circuit board  1  and electro-conductive housing  2 . This structure is applicable only for a case that no electronic parts exist on signal layer  15 . 
   In the mounting structures shown in  FIGS. 12(   a )– 12 ( d ), since ground pattern  18  on signal layer  15  is coupled to electro-conductive housing  2 , ground pattern  17  at the circumferential area of signal layer  14  serving as a first layer is not necessary required. 
     FIG. 13(   a ),  FIG. 13(   b ) and  FIG. 13(   c ) show cross sectional views of printed circuit board  1  mounted on the electro-conductive housing  2 , indicating another embodiment of the present invention. 
   In the configuration shown in  FIG. 13(   a ), electro-conductive housing  2  has concave section  2 A in the same manner as shown in  FIG. 12(   b ), and press-contacting member  5 , being electro-conductive and disposed at the end portion of printed circuit board  1 , press-contacts ground pattern  17  at the circumferential area of printed circuit board  1 , while another end portion of press-contacting member  5  press-contacts electro-conductive housing  2 , so as to electrically couples ground pattern  17  to electro-conductive housing  2 . 
   In the configuration shown in  FIG. 13(   b ), electro-conductive housing  2  has concave section  2 B in the same manner as shown in  FIG. 12(   c ), and press-contacting member  5 , being electro-conductive and disposed at the end portion of printed circuit board  1 , press-contacts ground pattern  17  at the circumferential area of printed circuit board  1 , while another end portion of press-contacting member  5  press-contacts electro-conductive housing  2 , so as to electrically couples ground pattern  17  to electro-conductive housing  2 . 
   In the configuration shown in  FIG. 13(   c ), electro-conductive housing  2  is a flat plane in the same manner as shown in  FIG. 12(   d ), and press-contacting member  5 , being electro-conductive and disposed at the end portion of printed circuit board  1 , press-contacts ground pattern  17  at the circumferential area of printed circuit board  1 , while another end portion of press-contacting member  5  press-contacts electro-conductive housing  2 , so as to electrically couples ground pattern  17  to electro-conductive housing  2 . 
   In the mounting structures shown in  FIGS. 13(   a )– 13 ( c ), since ground pattern  17  on signal layer  14  is coupled to electro-conductive housing  2  through press-contacting member  5 , ground pattern  18  at the circumferential area of signal layer  15  serving as a fourth layer is not necessary required. 
     FIG. 14(   a ) and  FIG. 14(   b ) show a plain view and perspective view of an electro-conductive housing with electro-conductive post members, as another embodiment of the present invention. 
   In the abovementioned embodiment shown in  FIG. 7(   a ) and  FIG. 7(   b ), electro-conductive post  3  is coupled to electro-conductive housing  2  without any slit over the whole circumferential area of printed circuit board  1 . As shown in  FIG. 14(   b ), however, even if slits  3 A, which cut off a part of electro-conductive post  3  shaped in a frame, are established in it, such configuration would result in only a little deterioration of the efficiency. 
   According to the present invention, the following effects can be attained.
     (1) By changing a connecting configuration between printed circuit board  1  and electro-conductive housing  2  from a plural points contacting mode to a line-coupling mode, it becomes possible to prevent an electro-magnetic noise emission caused by resonance current in the loop formed between the installing points.   (2) When a multi-layer printed circuit board having equal to or more than two layers is employed, by arranging the layers in an order of the signal layer, the electronic power source layer and the electro-conductive housing, and by coupling the ground pattern at circumferential area of the signal layer to the signal layer at inner layer with a lot of through holes, the electronic power source layer can be shielded, resulting in a reduction of the electromagnetic noise emission emitted from a gap between the electronic power source layer and the ground layer.   (3) By coupling the electronic power source layer power source layer to the ground layer with a plurality of snubber circuits, each being a serial connection of a resistor and a capacitor, it becomes possible to prevent not only an occurrence of the resonance between the electronic power source layer and the ground layer, but also the common mode current flowing into the electro-conductive housing, resulting in a reduction of the electro-magnetic noise emission.   

   Disclosed embodiment can be varied by a skilled person without departing from the spirit and scope of the invention.