Patent Publication Number: US-2021167555-A1

Title: Board-to-board connector and mounting structure therefor

Description:
TECHNICAL FIELD 
     This invention relates to a board-to-board connector and a mounting structure in which two connectors constituting the board-to-board connector are attached to two boards. 
     BACKGROUND ART 
     A board-to-board connector includes a first connector and a second connector. The first connector is attached to one surface of a first board while the second connector is attached to one surface of a second board. The first connector and the second connector fit together with the one surface of the first board and the one surface of the second board facing each other. The board-to-board connector electrically connects the first board and the second board. 
     As a conventional art, a board-to-board connector disclosed in Japanese Patent Application Laid Open No. 2017-33654 is shown in  FIGS. 1A, 1B, 2A, 2B, and 3 . The shown board-to-board connector includes a first connector  10  and a second connector  20 . 
     The first connector  10  includes an elongated insulating housing  11 , and the second connector  20  includes an elongated insulating housing  21 . Signal contacts  13  are arrayed at predetermined intervals along a length direction of the insulating housing  11 . Signal contacts  23  are arrayed at predetermined intervals along a length direction of the insulating housing  21 . 
     The insulating housing  11  includes proximal end portions lla at two ends in the length direction of the insulating housing  11  (that is, a length direction of the board-to-board connector), and the insulating housing  21  includes proximal end portions  21   a  at two ends in the length direction of the insulating housing  21  (that is, the length direction of the board-to-board connector). The insulating housing  11  includes an elongated central protruding portion  11   b  which is formed in a region between the two proximal end portions  11   a,  and the insulating housing  21  includes an elongated central recessed portion  21   b  which is formed in a region between the two proximal end portions  21   a.    
     A conductive shell  12  is placed around an outer peripheral portion of the insulating housing  11  and surrounds the outer peripheral portion of the insulating housing  11 . The conductive shell  12  functions as a shielding wall portion for the signal contacts  13 . A conductive shell  22  is placed around an outer peripheral portion of the insulating housing  21  and surrounds the outer peripheral portion of the insulating housing  21 . The conductive shell  22  functions as a shielding wall portion for the signal contacts  23 . Reference numerals  14  and  24  denote power contacts. 
     The conductive shell  12  is composed of two metal components. The two metal components each have an L-shape. The two components form a frame structure of the conductive shell  12 . One or more ground connecting portions  12   c  are formed at each of a lower end edge portion of a wall plate  12   a  along a length direction of the conductive shell  12  and a lower end edge portion of a wall plate  12   b  along a width direction. Each ground connecting portion  12   c  is joined with solder to a ground pad which is formed at a first wiring board  15 . 
     Ground connecting portions  12   c  which are formed at the wall plate  12   a  along the length direction of the conductive shell  12  are arranged at fixed intervals. An inspection window  12   d  is formed between each two adjacent ground connecting portions  12   c.  Through the inspection window  12   d,  a leg portion  13   a  of the signal contact  13  can be viewed. That is, a status of connection between the leg portions  13   a  and the first wiring board  15  or a connector assembly status can be confirmed. In this example, a length of the inspection window  12   d  in the length direction of the board-to-board connector corresponds to a length of a range including three leg portions  13   a  in a row. 
     A planar cover  12   e  links with an upper edge portion of the wall plate  12   a  along the length direction of the conductive shell  12 . Contact pieces  12   f  are formed at fixed intervals at sites astride a boundary between the wall plate  12   a  and the planar cover  12   e.  Each contact piece  12   f  is formed by cutting a part of a metal component and slightly raising a cut portion. The contact piece  12   f  is a metal piece in a plate spring shape which can make elastic contact with the conductive shell  22  of the second connector  20 . 
     The conductive shell  22  is composed of two metal components. The two metal components each have a staple shape. The two components form a frame structure of the conductive shell  22 . One or more ground connecting portions  22   c  are formed at each of a lower end edge portion of a wall plate  22   a  along a length direction of the conductive shell  22  and a lower end edge portion of a fixing and locking piece (that is, a wall plate along a width direction)  22   b.  Each ground connecting portion  22   c  is joined with solder to a ground pad which is formed at a second wiring board  25 . 
     Ground connecting portions  22   c  which are formed at the wall plate  22   a  along the length direction of the conductive shell  22  are arranged at fixed intervals. An inspection window  22   d  is formed between each two adjacent ground connecting portions  22   c.  Through the inspection window  22   d,  a leg portion  23   a  of the signal contact  23  can be viewed. That is, a status of connection between the leg portions  23   a  and the second wiring board  25  or a connector assembly status can be confirmed. In this example, a length of the inspection window  22   d  in the length direction of the board-to-board connector corresponds to a length of a range including three leg portions  23   a  in a row. 
     In a state in which the first connector  10  attached to the first wiring board  15  and the second connector  20  attached to the second wiring board  25  fit together, the signal contacts  23  and the power contacts  24  of the second connector  20  are in contact with the signal contacts  13  and the power contacts  14  of the first connector  10 . This results in mutual electrical connection between the first wiring board  15  and the second wiring board  25 . 
     In the state in which the first connector  10  and the second connector  20  fit together, the conductive shell  22  of the second connector  20  lies over the whole of the first connector  10  (see  FIG. 3 ). An inner wall surface of the conductive shell  22  of the second connector  20  is in elastic contact with the contact pieces  12   f  of the conductive shell  12  of the first connector  10 . Thus, the conductive shell  12  and the conductive shell  22  are electrically connected to each other via the contact pieces  12   f.    
     Along with miniaturization and densification of an electronic device using a board-to-board connector and use of a high-frequency electrical signal, measures against electromagnetic interference (EMI) for a board-to-board connector are required. 
     As for the above-described board-to-board connector according to the conventional art, a shielding effect is not necessarily good due to at least one of the three points below. 
     First, a void is present around the contact piece  12   f  for implementing electrical connection between the conductive shell  12  and the conductive shell  22 . Thus, the shielding effect is not necessarily good. 
     Second, in the state in which the first connector  10  and the second connector  20  fit together, there is a gap between the conductive shell  12  and the conductive shell  22  except at portions of contact between the contact pieces  12   f  and the conductive shell  22 . Thus, the shielding effect is not necessarily good. 
     Third, the conductive shell  12  and the conductive shell  22  have the inspection windows  12   d  and  22   d.  Thus, the shielding effect is not necessarily good. 
     BRIEF SUMMARY OF THE INVENTION 
     An object of this invention is to provide a board-to-board connector capable of obtaining a more potent shielding effect than in the conventional art and a mounting structure in which two connectors constituting the board-to-board connector are attached to two boards. 
     The following technical matters are described simply to facilitate the understanding of the main points of the present invention, not to limit the invention claimed in the claims explicitly or implicitly and not to express the possibility of accepting such a limitation that is imposed by a person other than those who will benefit from the present invention (for example, the applicant and the right holder). The general outline of the present invention described from other perspectives can be understood from, for example, the claims of this application as originally filed at the time of application. A board-to-board connector disclosed herein includes a first connector and a second connector. 
     The first connector includes a first shell which has four side wall portions arranged like a square frame. The second connector includes a second shell which has four side wall portions arranged like a square frame. In a state in which the first connector and the second connector are connected to each other, the second shell is fit inside the four side wall portions of the first shell, and the four side wall portions of the first shell and the four side wall portions of the second shell are in one-to-one contact with each other. 
     For any pair of two side wall portions, one of which is arbitrarily selected from among the four side wall portions of the first shell and the other of which is one of the four side wall portions of the second shell and which corresponds to the selected side wall portion, one of the two side wall portions has a protruding portion which extends linearly along the one side wall portion, and the other side wall portion does not have such a protruding portion. 
     A mounting structure for the board-to-board connector disclosed herein has a structure in which the first connector is attached to the first board and the second connector is attached to the second board. In this structure, a gap between the first board and the first connector is filled with solder, and a gap between the second board and the second connector is filled with solder. Specifically, at least one outer wall portion of the four outer wall portions of the above-described first shell has a first end edge and a first recessed portion formed at the first end edge, the first end edge except for the first recessed portion being in contact with the first board and the first recessed portion being for viewing an end portion of a contact which the first connector has. Additionally, at least one side wall portion of the four side wall portions of the above-described second shell has a second end edge and a second recessed portion formed at the second end edge, the second end edge except for the second recessed portion being in contact with the second board and the second recessed portion being for viewing an end portion of a contact which the second connector has. The first recessed portion and the second recessed portion are filled with solder. 
     These and other objects, features and advantages of the present invention will become apparent from the detailed description taken in conjunction with the accompanying drawings. 
     EFFECTS OF THE INVENTION 
     In the board-to-board connector according to this invention, a length, over which the four side wall portions of the first shell and the four side wall portions of the second shell are in contact with each other, is long due to the protruding portions extending along the square frame. A more potent shielding effect can thus be obtained than in the conventional art. 
     In the mounting structure for the board-to-board connector according to this invention, the recessed portions for viewing the end portions of the contacts are filled with solder. A more potent shielding effect can thus be obtained than in the conventional art. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The subject matter, which is regarded as the invention, is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The present invention itself, and manner in which it may be made or used, if any, may be better understood after a review of the following description in connection with the accompanying drawings in which: 
         FIG. 1A  is a perspective view of a first connector included in a board-to-board connector according to a conventional art; 
         FIG. 1B  is a front view of the first connector shown in  FIG. 1A ; 
         FIG. 2A  is a perspective view of a second connector included in the board-to-board connector according to the conventional art; 
         FIG. 2B  is a front view of the second connector shown in  FIG. 2A ; 
         FIG. 3  is an enlarged sectional view showing a state in which the first connector shown in  FIG. 1A  and the second connector shown in  FIG. 2A  fit together, together with wiring boards; 
         FIG. 4A  is a perspective view of a first connector included in a board-to-board connector according to an embodiment when viewed from above; 
         FIG. 4B  is a perspective view of the first connector shown in  FIG. 4A  when viewed from below; 
         FIG. 5A  is an enlarged front view of the first connector shown in FIG.  4 A; 
         FIG. 5B  is a sectional view taken along line C-C in  FIG. 5A ; 
         FIG. 6  is a front view showing a state in which the first connector shown in  FIG. 4A  is attached to a first board; 
         FIG. 7A  is a perspective view of a first insulator included in the first connector and first contacts held by the first insulator when viewed from above; 
         FIG. 7B  is a perspective view of the first insulator shown in  FIG. 7A  when viewed from below; 
         FIG. 8A  is a perspective view of a first shell included in the first connector when viewed from above; 
         FIG. 8B  is a perspective view of the first shell shown in  FIG. 8A  when viewed from below; 
         FIG. 9A  is a perspective view of a second connector included in the board-to-board connector according to the embodiment when viewed from above; 
         FIG. 9B  is a perspective view of the second connector shown in  FIG. 9A  when viewed from below; 
         FIG. 10A  is an enlarged front view of the second connector shown in  FIG. 9A ; 
         FIG. 10B  is a sectional view taken along line C-C in  FIG. 10A ; 
         FIG. 11  is a front view showing a state in which the second connector shown in  FIG. 9A  is mounted on a second board; 
         FIG. 12A  is a perspective view of a second insulator included in the second connector and second contacts held by the second insulator when viewed from above; 
         FIG. 12B  is a perspective view of the second insulator shown in  FIG. 12A  when viewed from below; 
         FIG. 13A  is a perspective view of a second shell included in the second connector when viewed from above; 
         FIG. 13B  is a perspective view of the second shell shown in  FIG. 13A  when viewed from below; 
         FIG. 14A  is a perspective view of a structure in which the first connector shown in  FIG. 4A  and the second connector shown in  FIG. 9A  are connected to each other when viewed from above; 
         FIG. 14B  is a perspective view of the structure of connection between the first connector and the second connector shown in  FIG. 14A  when viewed from below; 
         FIG. 15A  is an enlarged front view of the structure of connection between the first connector and the second connector shown in  FIG. 14A ; 
         FIG. 15B  is an enlarged side view of the structure of connection between the first connector and the second connector shown in  FIG. 14A ; 
         FIG. 15C  is a sectional view taken along line E-E in  FIG. 15B ; and  FIG. 15D  is a sectional view taken along line F-F in  FIG. 15A . 
     
    
    
     LIST OF REFERENCE NUMERALS 
     With regard to reference numerals used, the following numbering is used throughout the drawings. 
       10 : first connector 
       11 : insulating housing 
       11   a:  proximal end portion 
       11   b:  central protruding portion 
       12 : conductive shell 
       12   a:  wall plate 
       12   b:  wall plate 
       12   c:  ground connecting portion 
       12   d:  inspection window 
       12   e:  planar cover 
       12   f:  contact piece 
       13 : signal contact 
       13   a:  leg portion 
       14 : power contact 
       15 : first wiring board 
       20 : second connector 
       21 : insulating housing 
       21   a:  proximal end portion 
       21   b: central recessed portion 
       22 : conductive shell 
       22   a:  wall plate 
       22   b:  fixing and locking piece 
       22   c:  ground connecting portion 
       22   d:  inspection window 
       23 : signal contact 
       23   a:  leg portion 
       24 : power contact 
       25 : second wiring board 
       30 : first insulator 
       31 : bottom plate portion 
       31   a:  opening 
       31   b:  hole 
       31   c:  lower surface 
       31   d:  recessed portion 
       32 : side wall 
       32   a,    32   b:  recessed portion 
       33 : extension wall 
       34 : central protruding portion 
       34   a:  recessed portion 
       35 : protruding portion 
       35   a:  recessed portion 
       36 : step portion 
       36   a:  hole 
       40 : first contact 
       41 : end portion 
       42 : body portion 
       43 : U-shaped portion 
       44 : contact portion 
       45 : protuberant portion 
       50 : first shell 
       51 : outer wall portion 
       51   a:  recessed portion 
       51   b:  ground connecting portion 
       52 : outer wall portion 
       52   a:  recessed portion 
       52   b:  ground connecting portion 
       53 : side wall portion 
       53   a:  protruding portion 
       54 : side wall portion 
       54   a:  projection 
       55 : coupling portion 
       56 : arm portion 
       56   a:  distal end portion 
       60 : second insulator 
       61 : bottom plate portion 
       62 : side wall 
       62   a:  recessed portion 
       62   b:  step portion 
       63 : holding wall 
       64 : recessed portion 
       70 : second contact 
       71 : end portion 
       72 : U-shaped portion 
       80 : second shell 
       81 : side wall portion 
       81   a:  groove 
       81   b:  recessed portion 
       81   c:  ground connecting portion 
       82 : outer wall portion 
       82   a:  protruding portion 
       82   b:  recessed portion 
       82   c:  ground connecting portion 
       82   d:  projection 
       83 : coupling portion 
       83   a:  recessed portion 
       90 : solder region 
       91 : solder region 
       100 : first connector 
       200 : first board 
       300 : second connector 
       400 : second board 
       500 : board-to-board connector 
     DETAILED DESCRIPTION 
     Referring to the drawings, an embodiment of this invention will be described. A board-to-board connector  500  according to the embodiment includes a first connector  100  and a second connector  300 . 
     (First Connector) 
     The first connector  100  will be described with reference to  FIGS. 4A, 4B, 5A, 5B, 6, 7A, 7B, 8A, and 8B . The first connector  100  includes a first insulator  30 , first contacts  40  (in this example, the number of the first contacts  40  is ten) which are held by the first insulator  30 , and a first shell  50  which is attached to the first insulator  30 . 
     The first insulator  30  is made of resin. The first insulator  30  includes a rectangular bottom plate portion  31  and two side walls  32  in this example (see  FIGS. 7A and 7B ). The two side walls  32  are located at two ends of the bottom plate portion  31  in a length direction of the bottom plate portion  31 . 
     A recessed portion  32   a  is formed at an inner side surface of each of the two side walls  32 , and a recessed portion  32   b  is formed at an outer side surface of each of the two side walls  32 . Two extension walls  33  protrude inward from two sites located slightly inside two ends, in a length direction of the side wall  32 , of the inner side surface of the side wall  32 . A width of a portion of the bottom plate portion  31  which is located between the extension walls  33  located at one side wall  32  and the extension walls  33  located at the other side wall  32  is slightly smaller than a width of each of two end portions of the bottom plate portion  31 . 
     A central protruding portion  34  protrudes from an upper surface of a central portion of the portion having the slightly smaller width of the bottom plate portion  31 . The central protruding portion  34  extends in the length direction of the bottom plate portion  31 . A protruding portion  35  protrudes from each of two side sites of the bottom plate portion  31 . The two side sites of the bottom plate portion  31  are located at sides of the central protruding portion  34  in a width direction of the central protruding portion  34 . The protruding portions  35  extend in the length direction of the bottom plate portion  31 . 
     A step portion  36  slightly lower in height than the central protruding portion  34  is formed at each of two ends of the central protruding portion  34  in a length direction of the central protruding portion  34 . A hole  36   a  which opens upward is formed in each step portion  36 . An opening  31   a  is formed in a site of the bottom plate portion  31  which is located between the step portion  36  and the side wall  32 . 
     Recessed portions  35   a  (in this example, the number of the recessed portions  35   a  is five) are formed at predetermined intervals at a side surface, facing the central protruding portion  34 , of one protruding portion  35 . Recessed portions  35   a  (in this example, the number of the recessed portions  35   a  is five) are formed at the predetermined intervals at a side surface, facing the central protruding portion  34 , of the other protruding portion  35 . Recessed portions  34   a  (in this example, the number of the recessed portions  34   a  is five) are formed at predetermined intervals at a side surface, facing the one protruding portion  35 , of the central protruding portion  34 , and recessed portions  34   a  (in this example, the number of the recessed portions  34   a  is five) are formed at the predetermined intervals at a side surface, facing the other protruding portion  35 , of the central protruding portion  34 . That is, in this example, the first insulator  30  includes ten recessed portions  34   a  and ten recessed portions  35   a.  The ten recessed portions  34   a  correspond one-to-one to the ten recessed portions  35   a.  One recessed portion  34   a  and one recessed portion  35   a  corresponding to each other face each other. That is, assuming one recessed portion  34   a  and one recessed portion  35   a  corresponding to each other as one pair, the first insulator  30  includes ten pairs in this example. Ten holes  31   b  are formed in the bottom plate portion  31 , and the ten holes  31   b  correspond one-to-one to the ten pairs. One hole  31   b  arbitrarily selected from among the ten holes  31   b  communicates with one recessed portion  34   a  and one recessed portion  35   a  which constitute one pair corresponding to the one selected hole  31   b.  Each recessed portion  35   a  has an opening portion at an upper surface of the protruding portion  35 . An array of recessed portions  31   d  corresponding to an array of holes  31   b  is formed at each end of the lower surface  31   c  in a width direction of the bottom plate portion  31 . One hole  31   b  arbitrarily selected from among the ten holes  31   b  communicates with the recessed portion  31   d  corresponding to the selected hole  31   b.    
     Each first contact  40  is a thin metal piece which is bent at two or more sites and includes an end portion  41 , a body portion  42  which links perpendicularly with the end portion  41 , and a U-shaped portion  43  which links with the body portion  42  (see  FIG. 5B ). A contact portion  44  which is bent inward is formed at a distal end of the U-shaped portion  43 . A protuberant portion  45  is formed at a proximal end portion (that is, a portion close to a boundary between the body portion  42  and the U-shaped portion  43 ) of the U-shaped portion  43  which faces the contact portion  44 . The protuberant portion  45  faces the contact portion  44 . 
     The first contact  40  is attached to the first insulator  30  by inserting the first contact  40  into the hole  31   b.  Although not shown in detail, a projection is formed at the body portion  42 . The first contact  40  is fixed to the first insulator  30  by pushing the body portion  42  into the recessed portion  35   a.  A site close to the contact portion  44  of the U-shaped portion  43  is located in the recessed portion  34   a  of the central protruding portion  34 . 
     The first shell  50  in the shape of a square frame that has conductivity is formed by bending of a metal plate. As shown in  FIGS. 8A  and  8 B, the first shell  50  includes two outer wall portions  51  which are located at long sides of the square frame, two outer wall portions  52  which are located at short sides of the square frame, two side wall portions  53  which are located inside the two outer wall portions  51  and link with upper peripheral edges of the two outer wall portions  51 , and two side wall portions  54  which are located inside the two outer wall portions  52  and link with upper peripheral edges of the two outer wall portions  52 . The outer wall portion  51  and the outer wall portion  52  adjacent to each other are coupled at upper ends of them by a coupling portion  55 . 
     Five recessed portions  51   a  having small depths are formed at predetermined intervals in a length direction of the outer wall portion  51  at a lower end of the outer wall portion  51 . An array interval for the recessed portions  51   a  is equal to an array interval for five first contacts  40 . One recessed portion  52   a  having a small depth is formed at a lower end of the outer wall portion  52 . The lower end of the outer wall portion  51  except the recessed portions  51   a  serves as ground connecting portions  51   b.  The lower end of the outer wall portion  52  except the recessed portion  52   a  serves as ground connecting portions  52   b.    
     The two side wall portions  53  function as spring pieces supported by the outer wall portions  51  in this example. An elongated protruding portion  53   a  which extends linearly in a length direction of the side wall portion  53  is formed at an inner side surface of the side wall portion  53 . The protruding portion  53   a  extends from a site near one end of the side wall portion  53  to a site near the other end in this example. A shape of the protruding portion  53   a  in a cross-section orthogonal to the length direction of the side wall portion  53  is a semi-elliptical arc. 
     Two projections  54   a  are formed at two ends of one side wall portion  54  in a length direction of the one side wall portion  54 . Similarly, two projections  54   a  are formed at two ends of the other side wall portion  54  in the length direction of the other side wall portion  54 . An arm portion  56  extends from a central portion of a lower end of the side wall portion  54 . The arm portion  56  is a strip-shaped portion which is bent at two or more sites. The arm portion  56  extends from the lower end of the side wall portion  54  toward an inside of the first shell  50 , bends toward above the first shell  50 , extends toward the inside of the first shell  50 , and extends toward below the first shell  50 . 
     In the first shell  50 , a gap between the outer wall portion  51  and the outer wall portion  52  adjacent to each other is preferably made as narrow as possible. Similarly, a gap between the side wall portion  53  and the side wall portion  54  adjacent to each other is preferably made as narrow as possible. 
     The first shell  50  is attached to the first insulator  30  by pushing the first shell  50  against the first insulator  30  after putting the first shell  50  on the first insulator  30 . In this process, the side wall portion  54  having the projections  54   a  is pushed into the recessed portion  32   a  of the side wall  32  of the first insulator  30 , and a distal end portion  56   a  of the arm portion  56  is pushed into the hole  36   a  of the step portion  36  of the first insulator  30 . This results in completion of the first connector  100 . 
     The outer wall portions  51  are located outside the extension walls  33 , and the outer wall portions  52  are located in the recessed portions  32   b  of the side walls  32 . That is, the first shell  50  is an outer shell of the first connector  100 . The side wall portions  53  as spring pieces are located inside the extension walls  33  and are elastically pliable. 
     Although not shown in detail, the end portion  41  of the first contact  40  is soldered to a pad of a first board  200 , and the ground connecting portions  51   b  and  52   b  of the outer wall portions  51  and  52  are soldered to a ground pad of the first board  200  (see  FIG. 6 ). 
     As shown in  FIG. 5A , the end portion  41  of the first contact  40  can be viewed from outside the outer wall portion  51  through the recessed portion  51   a.  A position or a soldering condition of the end portion  41  can be confirmed, which ensures good mounting of the first connector  100 . After the first connector  100  is mounted on the first board  200 , all recessed portions  51   a  and  52   a  of the outer wall portions  51  and  52  are filled with solder. A region denoted by reference numeral  90  in  FIG. 6  is a solder region. 
     (Second Connector) 
     The second connector  300  will be described with reference to  FIGS. 9A, 9B, 10A, 10B, 11, 12A, 12B, 13A, and 13B . 
     The second connector  300  includes a second insulator  60 , second contacts  70  (in this example, the number of the second contacts  70  is ten) which are held by the second insulator  60 , and a second shell  80  which is attached to the second insulator  60 . 
     The second insulator  60  is made of resin. The second insulator  60  includes a rectangular bottom plate portion  61 , two side walls  62 , and two holding walls  63  in this example (see  FIGS. 12A and 12B ). The two side walls  62  are located at two ends, in a length direction of the bottom plate portion  61 , of the bottom plate portion  61 . The two holding walls  63  protrude upward from two ends, in a width direction of the bottom plate portion  61 , of the bottom plate portion  61 . The two holding walls  63  each extend from one side wall  62  to the other side wall  62 . 
     A length of each side wall  62  in the width direction of the bottom plate portion  61  is larger than a width of the bottom plate portion  61 . A recessed portion  62   a  is formed at an outer side surface of each of the two side walls  62 . At portions (that is, portions close to the bottom plate portion  61 ) inside two ends of the side wall  62  in a length direction of the side wall  62 , step portions  62   b  which are set back from the two ends are formed. 
     A height of each holding wall  63  is higher than a height of the side wall  62 . A shape of an upper end face of each holding wall  63  in a cross-section orthogonal to a length direction of the holding wall  63  is a semi-elliptical arc. A recessed portion  64  which extends in the length direction of the bottom plate portion  61  is formed between the two holding walls  63 . A bottom surface of the recessed portion  64  serves as the bottom plate portion  61 . One end of the recessed portion  64  extends into one side wall  62 , and the other end of the recessed portion  64  extends into the other side wall  62 . 
     The second contact  70  includes an end portion  71  and a U-shaped portion  72  (see  FIG. 10B ). One end of the U-shaped portion  72  links with the end portion  71 , and the other end of the U-shaped portion  72  faces the end portion  71 . The second contact  70  is fixed to the second insulator  60  through insert molding in this example. In the example, five second contacts  70  are arrayed at predetermined intervals in the length direction of one holding wall  63  at the one holding wall  63 . Similarly, five second contacts  70  are arrayed at the predetermined intervals in the length direction of the other holding wall  63  at the other holding wall  63 . A bent portion of the U-shaped portion  72  is located on a plane where the upper end face of the holding wall  63  is located. 
     The second shell  80  in the shape of a square frame that has conductivity is formed by bending of a metal plate. As shown in  FIGS. 13A and 13B , the second shell  80  includes two side wall portions  81  which are located at long sides of the square frame, two outer wall portions  82  which are located at short sides of the square frame, and two coupling portions  83  which are shaped like thin plates. One coupling portion  83  links with an upper end of one outer wall portion  82 , and two ends of the one coupling portion  83  link with two ends of upper ends of the two side wall portions  81 . The other coupling portion  83  links with an upper end of the other outer wall portion  82 , and two ends of the other coupling portion  83  link with two ends of the upper ends of the two side wall portions  81 . A recessed portion  83   a  is formed on a central inner side of each coupling portion  83 . 
     A groove  81   a  which extends from one end in a length direction of the side wall portion  81  to the other end is formed in an outer side surface of each side wall portion  81 . An elongated protruding portion  82   a  which extends linearly from a site near one end in a length direction of the outer wall portion  82  to a site near the other end is formed at an outer side surface of each outer wall portion  82 . A surface shape of the protruding portion  82   a  in a cross-section orthogonal to the length direction of the outer wall portion  82  is a semi-elliptical arc. 
     Five recessed portions  81   b  having small depths are formed at predetermined intervals in the length direction of the side wall portion  81  at a lower end of the side wall portion  81 . An array interval for the recessed portions  81   b  is equal to an array interval for five second contacts  70 . One recessed portion  82   b  having a small depth is formed at a lower end of the outer wall portion  82 . The lower end of the side wall portion  81  except the recessed portions  81   b  serves as ground connecting portions  81   c.  The lower end of the outer wall portion  82  except the recessed portion  82   b  serves as ground connecting portions  82   c.  Two projections  82   d  are formed at the two ends of one outer wall portion  82  in the length direction of the outer wall portion  82 . Two projections  82   d  are formed at the two ends of the other outer wall portion  82  in the length direction of the outer wall portion  82 . 
     In the second shell  80 , a gap between the side wall portion  81  and the outer wall portion  82  adjacent to each other is preferably made as narrow as possible. 
     The second shell  80  is attached to the second insulator  60  by pushing the second shell  80  against the second insulator  60  after putting the second shell  80  on the second insulator  60 . In this process, the outer wall portion  82  having the projections  82   d  is pushed into the recessed portion  62   a  of the side wall  62  of the second insulator  60 . This results in completion of the second connector  300 . The side wall portions  81  are located outside the step portions  62   b.  That is, the second shell  80  is an outer shell of the second connector  300 . 
     Although not shown in detail, the end portion  71  of the second contact  70  is soldered to a pad of a second board  400 , and the ground connecting portions  81   c  and  82   c  of the side wall portions  81  and the outer wall portions  82  are soldered to a ground pad of the second board  400  (see  FIG. 11 ). 
     As shown in  FIG. 10A , the end portion  71  of the second contact  70  can be viewed from outside the side wall portion  81  through the recessed portion  81   b.  A position or a soldering condition of the end portion  71  can be confirmed, which ensures good mounting of the second connector  300 . 
     After the second connector  300  is mounted on the second board  400 , all recessed portions  81   b  and  82   b  of the side wall portions  81  and the outer wall portions  82  are filled with solder. A region denoted by reference numeral  91  in  FIG. 11  is a solder region. 
     (Structure of Connection between First Connector and Second Connector) 
     A structure of connection between the first connector  100  and the second connector  300  will be described with reference to  FIGS. 14A, 14B, 15A, 15B, 15C, and 15D . The first board  200  and the second board  400  are not shown in these drawings. 
     The second shell  80  of the second connector  300  is fit inside the first shell  50 , that is, in a region surrounded by the side wall portions  53  and  54 . With this operation, the first connector  100  and the second connector  300  are connected to each other. The central protruding portion  34  of the first insulator  30  in the first connector  100  is fit in the recessed portion  64  of the second insulator  60  in the second connector  300 . 
     One holding wall  63  of the second insulator  60  is inserted between the central protruding portion  34  and one protruding portion  35  of the first insulator  30 , and the other holding wall  63  of the second insulator  60  is inserted between the central protruding portion  34  and the other protruding portion  35  of the first insulator  30 . As a result, the U-shaped portion  72  of the second contact  70  fits in the U-shaped portion  43  of the first contact  40  (see  FIG. 15D ). Since the U-shaped portion  72  is sandwiched between the contact portion  44  and the protuberant portion  45 , the second contact  70  and the first contact  40  come into contact with each other. 
     The two side wall portions  81  of the second shell  80  are held between the two side wall portions  53  of the first shell  50 . The protruding portion  53   a  of each side wall portion  53  fits in the groove  81   a  of the side wall portion  81  (see  FIG. 15D ). The side wall portion  53  of the first shell  50  and the side wall portion  81  of the second shell  80  are mechanically locked together. A worker or the like obtains a click feeling when the protruding portion  53   a  of the side wall portion  53  fits in the groove  81   a  of the side wall portion  81 . The side wall portion  54  of the first shell  50  and the outer wall portion  82  of the second shell  80  are locked together by contact resistance. That is, contact resistance is generated between the protruding portion  82   a  of the outer wall portion  82  and the side wall portion  54  (see  FIG. 15C ). The side wall portions  53  and  54  of the first shell  50  and the side wall portions  81  and the outer wall portions  82  of the second shell  80  are locked together, thereby electrically and mechanically connecting the first connector  100  and the second connector  300  to each other. 
     The above-described embodiment achieves the effects below. 
     (1) The first shell  50  of the first connector  100  and the second shell  80  of the second connector  300  have no contact pieces surrounded by a void. Thus, electromagnetic waves do not leak through a void, and a potent shielding effect is obtained. 
     (2) In a state in which the first connector  100  and the second connector  300  fit together, the two protruding portions  53   a  formed at the two side wall portions  53  of the first shell  50  are in contact with the two side wall portions  81  of the second shell  80 , and the two protruding portions  82   a  formed at the two outer wall portions  82  of the second shell  80  are in contact with the two side wall portions  54  of the first shell  50 . Since a length, over which the first shell  50  and the second shell  80  are in contact with each other, is long, a potent shielding effect is obtained. 
     (3) The recessed portions  51   a  are filled with solder after the first connector  100  is attached to the first board  200 , and the recessed portions  81   b  are filled with solder after the second connector  300  is attached to the second board  400 . Thus, electromagnetic waves do not leak through the recessed portions  51   a  and  81   b,  and a potent shielding effect is obtained. 
     As a modification of the embodiment, a protruding portion may be formed at the side wall portion  81 , and a groove, in which the protruding portion is to fit, may be formed in the side wall portion  53 , instead of forming the protruding portion  53   a  and the groove  81   a.    
     As another modification of the embodiment, a protruding portion may be formed at the side wall portion  54 , instead of forming the protruding portion  82   a.    
     In the embodiment, the end portion  41  of the first contact  40  corresponds one-to-one to the recessed portion  51   a,  and the end portion  71  of the second contact  70  corresponds one-to-one to the recessed portion  81   b.  However, only one recessed portion  51   a  corresponding to any one end portion  41  of the end portions  41  of the first contacts  40  may be formed. Alternatively, only one recessed portion  81   b  corresponding to any one end portion  71  of the end portions  71  of the second contacts  70  may be formed. 
     The recessed portions  52   a  and  82   b  are formed in consideration of respective patterns formed at the first board  200  and the second board  400  and are not essential features. 
     Addendum 
     While the invention has been described with reference to exemplary embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular system, device or component thereof to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiments disclosed for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims. 
     Moreover, the use of the terms “first”, “second”, “i-th”, etc., if any, do not denote any order or importance, but rather the terms “first”, “second”, “i-th”, etc. are used to distinguish one element from another. The term “first” does not necessarily mean “coming before all others in order”. The terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the invention in any way. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprise”, “comprises”, and/or “comprising,” when used in this specification and/or the appended claims, specify the presence of stated features, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, steps, operations, elements, components, and/or groups thereof. The same goes for “include”, “includes”, and/or “including”. The term “and/or”, if any, includes any and all combinations of one or more of the associated listed items. In the claims and the specification, unless otherwise noted, “connect”, “join”, “couple”, “interlock”, or synonyms therefor and all the word forms thereof, if any, do not necessarily deny the presence of one or more intermediate elements between two elements, for instance, two elements “connected” or “joined” to each other or “interlocked” with each other. Connection between elements, if required, may be physical connection, electrical connection, or a combination thereof. In the claims and the specification, unless otherwise noted, the term “arbitrary”, if any, should be understood as a term having the same meaning as the universal quantifier V. 
     For example, the expression “for arbitrary X” has the same meaning as “for every X” or “for each X”. Thus, for example, the expression “one arbitrary X of ten Xs” or “one arbitrarily selected X of ten Xs” or an expression similar thereto is not an expression describing one particular X but an expression describing each of ten Xs. For example, the expression “proposition P holds for a pair consisting of one arbitrarily selected X of three Xs and one Y, corresponding to the one selected X, of three Ys” or an expression similar thereto means that the proposition P holds for each of the three pairs (X 1 , Y 1 ), (X 2 , Y 2 ), and (X 3 , Y 3 ) which are generated by one-to-one correspondence between the set (X 1 , X 2 , X 3 ) and the set (Y 1 , Y 2 , Y 3 ). 
     Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by those skilled in the art to which the invention belongs. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and the present disclosure and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein. 
     In describing the invention, it will be understood that a number of techniques and steps are disclosed. Each of these has individual benefit and each can also be used in conjunction with one or more, or in some cases all, of the other disclosed techniques. Accordingly, for the sake of clarity, this description will refrain from repeating every possible combination of the individual techniques or steps in an unnecessary fashion. Nevertheless, the specification and claims should be read with the understanding that such combinations are entirely within the scope of the invention and the claims. 
     The corresponding structures, materials, acts, and equivalents of all means or step plus function elements in the claims below, if any, are intended to include any structure, material, or act for performing the function in combination with other claimed elements as specifically claimed. 
     The foregoing description of the embodiments of the invention has been presented for the purpose of illustration and description. It is not intended to be exhaustive and to limit the invention to the precise form disclosed. Modifications or variations are possible in light of the above teaching. The embodiments were chosen and described to provide the best illustration of the principles of the invention and its practical application, and to enable one of ordinary skill in the art to utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. All such modifications and variations are within the scope of the invention as determined by the appended claims when interpreted in accordance with the breadth to which they are fairly, legally, and equitably entitled.