Abstract:
A housing includes a first member that has a first conductor, a second member that has a second conductor and cooperates with the first member to accommodate a circuit board on which an electronic element is mounted, inside a space surrounded by the first conductor and the second conductor, and a plurality of connecting elements that are arranged at a predetermined arrangement interval y to fix the second member to the first member.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2016-142988, filed on Jul. 21, 2016, the entire contents of which are incorporated herein by reference. 
       FIELD 
       [0002]    The embodiments discussed herein are related to a housing and an electronic device for which measures for Electro Magnetic Compatibility (EMC) are taken. 
       BACKGROUND 
       [0003]    With the development of electronic devices, problems have arisen, for example, in that electromagnetic noise emitted from an electronic device causes a failure in other electronic devices, or an electronic device malfunctions due to electromagnetic noise emitted from other electronic devices. Thus, there has been a demand for EMC measures in a process of designing and assembling an electronic device. 
         [0004]    Since the electromagnetic noise is blocked by a metal, it may be conceived to hermetically seal an electronic circuit which is a source of generating the electromagnetic noise, for example, within a housing made of a metal. As an example, it may be conceived to install flanges on two metal members (a housing body and a lid) forming a housing, respectively, and overlap and fix the flanges by screws, caulking, rivets, welding or the like (hereinafter, referred to as “connecting elements”). 
         [0005]    When the two metal members having the flanges are fixed by the connecting elements such as screws, an attenuation amount of the electromagnetic noise is relative to the widths of the flanges and the arrangement interval (pitch) of the connecting elements. For example, when a frequency is 1,000 MHz, and the arrangement interval of the connecting elements is 7.5 cm, the width of each flange which is required to reduce the electromagnetic noise by 15 dB is about 5 cm. 
         [0006]    However, in an electronic device such as a computer, when the flanges each having the width of 5 cm are installed, the size of the electronic device increases, and furthermore, a problem is also caused in view of design. 
         [0007]    The following is a reference document. 
       [Document 1] Japanese Laid-Open Patent Publication No. 2007-067195. 
     SUMMARY 
       [0008]    According to an aspect of the invention, a housing includes a first member that has a first conductor, a second member that has a second conductor and cooperates with the first member to accommodate a circuit board on which an electronic element is mounted, inside a space surrounded by the first conductor and the second conductor, and a plurality of connecting elements that are arranged at a predetermined arrangement interval y to fix the second member to the first member, wherein, when an overlapping width between the first conductor and the second conductor is x, a frequency of a clock signal flowing through the circuit board is f, a speed of light is c, and a wavelength of electromagnetic noise is λ=c/f, a value of the arrangement interval y is calculated by following equation (1) or (2), 
         [0000]      when  x≦ 2.2 mm,  y ≦(0.13 x+ 0.0015)λ  (1)
 
         [0000]      when  x&gt; 2.2 mm,  y =(10 −5   x   3 −0.0006 x   2 +0.012 x+ 0.25)λ  (2).
 
         [0009]    The object and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the claims. 
         [0010]    It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of the invention, as claimed. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0011]      FIG. 1  is a perspective view schematically illustrating an example of an electronic device; 
           [0012]      FIG. 2  is a perspective view schematically illustrating a housing in which a portion where close contact between a housing and a lid is insufficient occurs between screws; 
           [0013]      FIG. 3  is a view illustrating a result obtained from the inspection of the intensity of electromagnetic noise generated from a circuit board; 
           [0014]      FIG. 4  is a view schematically illustrating a part of the housing; 
           [0015]      FIG. 5  is a graph obtained by connecting points where an attenuation amount is 30 dB; 
           [0016]      FIG. 6  is a view divisionally illustrating a portion obtained by straight line approximation of the curve illustrated in  FIG. 5  and a portion obtained by curve approximation of the curve illustrated in  FIG. 5 ; 
           [0017]      FIG. 7A  is a cross-sectional view illustrating an electronic device according to an embodiment, and  FIG. 7B  is a perspective view illustrating a casing of the same electronic device; and 
           [0018]      FIGS. 8A to 8C  are schematic views illustrating examples of overlapping portions between the housing and the lid. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0019]    Hereinafter, embodiments will be described with reference to the accompanying drawings. 
         [0020]      FIG. 1  is a perspective view schematically illustrating an example of an electronic device. 
         [0021]    In the electronic device illustrated in  FIG. 1 , a housing  10  accommodates a circuit board  15  on which a central processing unit (CPU) and other electronic components are mounted. The housing  10  includes a housing body (a first member)  11  and a flat plate shaped lid (a second member)  12  disposed on the housing  11 . The housing body  11  is formed by a rectangular bottom plate and four side plates surrounding the bottom plate. The lid  12  is fixed to the housing body  11  by screws  13  (or other connecting elements (the same shall apply hereinafter)). 
         [0022]    The surfaces (the inner surfaces, the outer surfaces, and the edge surfaces) of the housing body  11  and the lid  12  are conductive, and the circuit board  15  is sealed in the space surrounded by the conductors. Accordingly, leakage of the electromagnetic noise from the circuit board  15  to the outside of the housing body  11  or entry of the electromagnetic noise from the outside into the housing  10  is suppressed. 
         [0023]    However, when the edge of the lid  12  and the hosing body  11  are thoroughly in close contact with each other as illustrated in  FIG. 1 , the electromagnetic noise may be blocked by the housing body  11 . However, actually, a portion where the close contact between the housing body  11  and the lid  12  is insufficient occurs between the adjacent screws  13  as illustrated in  FIG. 2 , and the electromagnetic noise leaks from the portion. 
         [0024]    In order to avoid the leakage of the electromagnetic noise between the housing body  11  and the lid  12 , a related technology has installed flanges each having a width determined depending on the arrangement interval of the screws  13  and the frequency of the electromagnetic noise, on the housing body  11  and the lid  12 , respectively. In that case, when the frequency of electromagnetic waves is 1,000 MHz, and the arrangement interval of the screws (the connecting elements) is 7.5 cm, the width of each of the flanges becomes 5 cm. 
         [0025]    The inventors of the present disclosure inspected the intensity of the electromagnetic noise generated from the circuit board on which electronic components such as the CPU are mounted. 
         [0026]      FIG. 3  is a view illustrating a result obtained from the inspection of the intensity of the electromagnetic noise generated from the circuit board, with the horizontal axis representing a frequency of a clock signal and the vertical axis representing a level of the electromagnetic noise. 
         [0027]    As may be seen from  FIG. 3 , the electromagnetic noise of about 80 dBμV is generated from the circuit board. Meanwhile, although the regulation level of the electromagnetic noise differs depending on countries, the regulation level is approximately 50 dBμV. Thus, in the housing, the electromagnetic noise generated from the circuit board needs to be suppressed by 30 dB. 
         [0028]    The inventors of the present disclosure variously changed the overlapping width x of the two members (the housing body and the lid) forming the housing and the arrangement interval y of the screws, and inspected the attenuation amount of the electromagnetic noise under each of the conditions. Here, the overlapping width x is the width of the portion where the conductors of the two respective members are in contact with each other. 
         [0029]      FIG. 4  is a view schematically illustrating a part of the housing  10 . Here, as illustrated in  FIG. 4 , it is assumed that the circuit board  15  which is a source of generating the electromagnetic noise is disposed inside the housing  10  formed by the housing body  11  and the lid  12  which are made of a metal. Further, it is assumed that the overlapping width between the side walls of the housing body  11  and the lid  12  is x, and the distance (the arrangement interval) between the adjacent screws  13  is y. 
         [0030]    In addition, while a method of a related technology inspects the attenuation amount of the electromagnetic noise by using a sinusoidal signal, the inventors of the present disclosure inspect the attenuation amount of the electromagnetic noise by using a clock signal (a pulse shaped rectangular wave signal) flowing through the circuit board  15 . This is because the electromagnetic noise generated from an electronic device such as a computer is mainly caused by a clock signal flowing through the circuit board  15 . For the analysis of the attenuation amount of the electromagnetic noise, the electromagnetic field analysis software (Poynting for Microwave™) of Fujitsu Limited was used. 
         [0031]    As a result, the relationship between the overlapping width x of the two members (the housing body and the lid) forming the housing and the arrangement interval y of the screws when the attenuation amount is set to 30 dB became as illustrated in  FIG. 5 . In addition, λ in  FIG. 5  is the wavelength (unit: mm) of the electromagnetic noise, and λ=c/f (c is the speed of light, and f is the frequency of the clock signal). 
         [0032]      FIG. 5  is a graph obtained by connecting points where the attenuation amount is 30 dB, with the horizontal axis representing the overlapping width x and the vertical axis representing the arrangement interval y of the screws. When the relationship between the overlapping width x of the housing body  11  and the lid  12  and the arrangement interval y of the screws  13  is lower than the curve illustrated in  FIG. 5 , the electromagnetic noise leaking to the outside of the housing  10  is reduced by 30 dB to become 50 dBμV or less. 
         [0033]    The inventors of the present disclosure divided the curve illustrated in  FIG. 5  into a straight line approximation portion (between A and B) and a curve approximation portion (between B and C) as illustrated in  FIG. 6 , and represented the portions with the following equations (1) and (2), respectively. Here, an X coordinate value (the overlapping width x) of the point B is 2.2 mm, and a Y coordinate value (the arrangement interval y of the connecting elements) thereof is λ/3.65. 
         [0000]        y =(0.13 x+ 0.0015)λ  (1)
 
         [0000]        y =(10 −5   x   3 −0.0006 x   2 +0.012 x+ 0.25)λ  (2)
 
         [0034]    According to  FIG. 6 , it may be understood that when the frequency of the clock signal is 1,000 MHz, and the arrangement interval y of the screws  13  is λ/4 (=7.5 cm), the overlapping width x between the housing body  11  and the lid  12  is 2 mm. In other words, when the overlapping width between the housing body  11  and the lid  12  is 2 mm or more, the arrangement interval of the screws  13  may be λ/4 or more. 
         [0035]    In addition, equations (1) and (2) are represented as the following equations (1)′ and (2)′ when the number of significant digits is expanded. 
         [0000]        y =(0.1285 x+ 0.0015)λ  (1)′
 
         [0000]        y =(10 −5   x   3 −0.0006 x   2 +0.0123 x+ 0.2498)λ  (2)′
 
         [0036]    Instead of equation (1) or (2) above, the equation (1)′ or (2)′ may be used. 
         [0037]      FIG. 7A  is a cross-sectional view illustrating an electronic device according to the present embodiment, and  FIG. 7B  is a perspective view illustrating a housing of the electronic device. 
         [0038]    The circuit board  15  is accommodated inside the housing  10 . The CPU and other electronic components  16  are mounted on the circuit board  15 , and a clock signal of 1,000 MHz flows through the circuit board  15 . 
         [0039]    The housing  10  includes the housing body  11  and the lid  12 . Here, it is assumed that the housing body  11  and the lid  12  are made of a resin, and the surfaces (the inner surfaces, the outer surfaces, and the edge surfaces) thereof are covered with a conductor such as a metal. However, both the housing body  11  and the lid  12  may be formed of a metal. 
         [0040]    The overlapping width x (see, e.g.,  FIG. 4 ) between the housing body  11  and the lid  12  is 2 mm. In addition, the lid  12  is fixed to the housing body  11  by the screws  13 . Further, the arrangement interval y of the screws  12  is set to 7.5 cm. 
         [0041]    In the electronic device according to the present embodiment, since the circuit board  15  which is a source of generating the electromagnetic noise is disposed inside the housing  10  as an EMC measure, the electromagnetic noise leaking to the outside of the housing  10  is reduced to 50 dBμV or less. Further, in the electronic device according to the present embodiment, since the overlapping width between the housing body  11  and the lid  12  is as small as 2 mm, and flanges protruding outwardly are unnecessary, the electronic device is small in size and excellent in design. 
         [0042]      FIGS. 8A to 8C  are schematic views illustrating an example of the overlapping portion between the housing body  11  and the lid  12 . 
         [0043]      FIG. 8A  illustrates an example in which a flange  11   a  is formed to extend toward the inner side of the housing body  11 , as a case where the overlapping width x between the housing body  11  and the lid  12  which is calculated from the formula (1) or (2) is larger than the thickness of the side wall. 
         [0044]      FIG. 8B  illustrates a case where the overlapping width x between the housing body  11  and the lid  12  which is calculated by the formula (1) or (2) is smaller than the thickness of the side wall. In this case, the thickness of the portion of the side wall of the housing body  11  where each of the screws  13  is disposed is set as the thickness required to fix the screws  13 , and the thickness of the other portion is set as the overlapping width x between the housing body  11  and the lid  12  which is calculated by the formula (1) or (2). 
         [0045]      FIG. 8C  illustrates an example in which a slit is formed on the flange  11   a  of the housing body  11 , and a crank shaped portion is formed in the lid  12  so as to be inserted into the slit as a connecting portion  12   a.    
         [0046]    All examples and conditional language recited herein are intended for pedagogical purposes to aid the reader in understanding the invention and the concepts contributed by the inventor to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions, nor does the organization of such examples in the specification relate to a showing of the superiority and inferiority of the invention. Although the embodiment of the present invention has been described in detail, it should be understood that the various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention.