Abstract:
Solder balls are offset in the horizontal direction from fixing portions while their positional accuracy is ensured, and signal paths are shortened in a ball grid array IC socket. Contacts fixed to contact hosing apertures of a housing include base portions, contact arms, and terminal portions that extend from the lower ends of the base portions toward a circuit board. The contact arms protrude so as to extend upward and then are bent unidirectionally. The terminal portions are constituted by transition portions that link solder ball pads to the base portions, while offsetting the solder ball pads in the same direction as the contact arms. The transition portions include vertical portions which are substantially perpendicular to the solder ball pads or inclined portions angled upwardly from the solder ball pads. The vertical or inclined portions prevent movement of the solder balls, thereby preventing positional misalignment and deformation thereof.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to a ball grid array type IC socket and more particularly to a socket having an insulating housing with electrical contacts provided in a matrix on the insulating housing for electrically connecting to a LGA (land grid array) or a BGA (ball grid array) of an IC package, while also electrically connecting the electrical contacts to a printed circuit board via solder balls.  
         BACKGROUND OF THE INVENTION  
         [0002]    A ball grid array IC socket (hereinafter, simply referred to as “IC socket”) is disclosed in U.S. Pat. No. 6,132,222 (FIG. 3). This IC socket comprises contacts for contacting pin contacts of an IC package. The contacts comprise contact arms for contacting the pin contacts, fixing portions for fixing the contacts to the insulative housing of the IC socket, and solder feet to be connected to the circuit board. Generally, the IC sockets are provided to consumers with solder balls soldered on to the solder feet.  
           [0003]    In the known IC socket described above, the IC package has pin contacts. In the case that the IC package is a ball grid array (BGA) or a land grid array (LGA), the contacts of known IC sockets are modified to connect with the BGA or LGA contacts. The contact arms, which contact electrodes of the IC package, are curved after extending through an IC package mounting surface from the fixing portions, such that the contact points of the contact arms (i.e., the points on the contact arms that contact the contacts of the BGA or LGA) are horizontally offset. This horizontal offset reduces the height of the IC socket assembly having the IC package mounted thereon. The contacts of a ball grid array or a land grid array IC package are connected to the contact arms by application of a mechanical force. This construction is adopted in response to the miniaturization of products to which IC sockets are mounted.  
           [0004]    For IC sockets with contact arms that are offset in the manner described above, it is preferable that the positions of the solder balls on the opposite side of the housing from the contacts, approach the positions of the contact points in the horizontal direction. That is, it is preferable that the solder feet are similarly offset from the fixing portions in the same direction as are the contact points of the contact arms. This offset is to balance the arrangement of the LGA or BGA of the IC package and the arrangement of the BGA of the IC socket.  
           [0005]    Due to the recent and ongoing increases in the speed of transmitted signals, however, it is preferable that the signal paths of the contacts are as short as possible.  
           [0006]    In view of the above, it is desirable that the solder feet are offset from the fixing portions, while providing the shortest possible signal paths from the IC package to the circuit board.  
           [0007]    In the IC socket disclosed in U.S. Pat. No. 6,132,222, the solder feet are formed by bending the lower ends of the contacts, which extend downward from the fixing portions, at substantially a right angle. Therefore, in the case that the solder feet are to be offset, a problem arises in that the signal paths become elongated. In addition, solder fillets are formed between solder balls and the solder feet during soldering of the solder balls onto the solder feet. There is a possibility that the solder balls are pulled toward the right, that is, the side of the fixing portions, due to the surface tension of the molten solder fillets. This leads to the problem that the solder balls are formed on the solder ball pad at positions that deviate from their predetermined positions, where they are to be soldered to the circuit board. As a result, the positional accuracy of the solder balls will be deteriorated, thereby reducing the reliability of electrical connections. In addition, there is a risk that the spherical shapes of the solder balls will be altered due to the horizontal displacement thereof.  
         SUMMARY OF THE INVENTION  
         [0008]    The present invention a ball grid array IC socket having an insulative housing and a plurality of contacts. The insulative housing has an IC package mounting surface on one face thereof and a circuit board mounting surface on a face opposite the IC package mounting surface and a plurality of contact housing apertures extending from the IC package mounting surface to the circuit board mounting surface configured to receive a plurality of contacts. The plurality of contacts include contact arms that protrude from the first surface in a unidirectionally bent manner for contacting contact portions of an IC package mounted on the IC package mounting surface, fixing portions for engaging the interiors of the plurality of contact housing apertures, solder ball pads that protrude from the circuit board mounting surface for soldering solder balls thereto for connecting to a circuit board, and transition portions provided between the fixing portions and the solder ball pads for displacing the solder ball pads in substantially the same direction as the direction in which the contact arms are bent. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]    The present invention will be described with reference to the accompanying drawings, of which:  
         [0010]    [0010]FIG. 1 is a cross sectional view of a ball grid array IC socket according to an exemplary embodiment of the present invention;  
         [0011]    [0011]FIG. 2 is a detailed view of area  42  indicated in FIG. 1, showing only a housing and contacts;  
         [0012]    [0012]FIGS. 3A-4B show a contact utilized in the ball grid array IC socket according to an exemplary embodiment of the present invention with FIG. 3A being a left side view, FIG. 3B being a front view, and FIG. 3C being a right side view, FIG. 4A being a top plan view, and FIG. 4B being a bottom view of the contact:  
         [0013]    [0013]FIG. 5 is a partial detailed view of a terminal portion of the contact shown in FIGS. 3A-4B.  
         [0014]    [0014]FIG. 6 is a partial detailed view of a modified contact having an inclined transition portion according to an alternate exemplary embodiment of the present invention;  
         [0015]    [0015]FIG. 7 is a partial detailed view of the contact shown in FIGS. 3A-4B with a housing according to an alternate exemplary embodiment of the present invention; and  
         [0016]    [0016]FIG. 8 is a partial detailed view of the contact shown in FIGS. 3A-4B with a modified housing according to an alternate exemplary embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0017]    [0017]FIG. 1 shows a sectional view of an IC socket  1  according to an exemplary embodiment of the present invention. The IC socket  1  comprises an insulative housing  2 , a metal plate  20 , and a loading plate  19 . The metal plate  20  supports the housing  2  from the side of a circuit board mounting surface  10  on the housing  2 . The loading plate presses an IC package  30  onto the housing  2 . The metal plate  20  and the loading plate  19  may be formed, for example, by stamping and forming.  
         [0018]    The housing  2  of the IC socket  1  is rectangular. An IC package mounting surface  6  is provided on a first side of the housing  2 , and the circuit board mounting surface  10  is provided on the other side of the housing  2 . The IC package mounting surface  6  is surrounded by walls  4 . The circuit board mounting surface  10  is configured to be mounted on a circuit board  8 . Contact housing apertures  12 , which will be described later (refer to FIG. 2), are formed through the housing  2  from the first side, which is the IC package mounting surface  6  (or upper surface as shown in FIG. 2), to the other side or board mounting surface  10 , on which a circuit board  8  is mounted. The contact housing apertures  12  are arranged in a matrix. Contacts  14  are press fit and fixed within each of the contact housing apertures  12 .  
         [0019]    A step  16  is formed along the entire periphery of the lower surface of the housing  2 . An opening  18  is formed in the metal plate  20  for receiving the lower portion of the housing  2 , formed by the step  16 . When the metal plate  20  and the housing  2  are assembled together, the edge of the plate  20  adjacent to the opening  18  abuts the step  16  in the housing  2 . A support portion  28  is formed at one end of the metal plate  20 , for example by bending. The support portion holds a rotating axis  26  of a lever  22  that operates the loading plate  19 . A crank shaped operating portion  24  is formed on the rotating axis  26  to urge the loading plate  19  downward when it is rotated.  
         [0020]    A bearing  32  is formed at the end of the loading plate  19  opposite from the end of the lever  22 . The metal plate  20  is provided with claws  34  for rotatably engaging an aperture  32   a  formed through the bearing  32 . This structure enables the loading plate  19  to rotate in the direction indicated by arrow  36  of FIG. 1. A tongue piece  38 , which is to be pressed by the operating portion  24 , is formed at the end of the loading plate  19  opposite the end of the bearing  32 . In addition, a curved portion  40 , which curves downward in FIG. 1, is formed at the central portion of the loading plate  19 . When the loading plate  19  is closed by rotating the lever  22  and is in the position shown in FIG. 1, the curved portion  40  presses the IC package  30  (shown by broken lines in FIG. 1) toward the housing  2 . Thus, electrodes  31  (contacts) of the IC package  30 , that is, the LGA or the BGA, electrically connect with contact arms  46  of the contacts  14 .  
         [0021]    Next, the shape and the mounting structure of the contacts  14  will be described with reference to FIGS.  2  through FIG. 4B. FIG. 2 is a detailed view of the area  42  indicated in FIG. 1, showing only the housing  2  and the contacts  14 . FIGS. 3A, 3B, and  3 C show the contact  14 , which is utilized in the IC socket of the present invention. FIG. 3A is a left side view, FIG. 3B is a front view, and FIG. 3C is a right side view of the contact  14  of FIG. 2. FIG. 4A is a plan view, and FIG. 4B is a bottom view of the contact  14 .  
         [0022]    First, with reference to FIG. 2, it is clearly illustrated that the contacts  14  are engaged within the contact housing apertures  12  from the IC package mounting surface  6  to the circuit board mounting surface  10  of the housing  2 . Each of the contacts  14  in the illustrated exemplary embodiment, as more clearly shown in FIGS. 3A, 3B,  3 C,  4 A, and  4 B, is constructed by punching and bending a single metal plate. Each of the contacts  14  comprises a base portion  44  (also referred to as a fixing portion) that extends in the vertical direction of FIGS. 3A, 3B, and  3 C; a contact arm  46  that extends from the base portion  44  upwardly; and a terminal portion  48  that extends form the lower end of the base portion  44  downwardly toward the circuit board  8 . The contact arm  46 , as best shown in FIGS. 3A and 3B extends from the side of the base portion  44 , and is bent along a vertical line to overlap the base portion  44 , and extending upwardly beyond the base portion  44 . Note that the expressions up, down, left, and right will be employed to indicate those directions in each figure, to facilitate the description.  
         [0023]    The shapes of each portion of the contact  14  will be described in further detail. As most clearly shown in FIG. 3C, engagement protrusions  56  ( 56   a ,  56   b ,  56   c , and  56   d ), for frictionally engaging inner walls  54  of the contact housing apertures  12 . The engagement protrusions  56  are formed at the top and bottom of the base portion  44  on both side edges  50  and  52  thereof. The contact arm  46  is bent from the side edge  52  of the base portion  44  at a bend  58 . The contact arm  46  extends further upward from the bend  58 , and is bent toward the left in FIG. 3B. A contact point  60 , which has an arcuate upper surface for connecting with the contacts of the IC package  30 , are provided at the distal ends of the contact arms  46 .  
         [0024]    The terminal portion  48  comprises: a solder ball pad  62 , to which a solder ball  64  is soldered; and a transition portion  66 , for linking the base portion  44  with the solder ball pad  62 . The solder ball pad  62  is of a discoid shape having a diameter slightly smaller than that of the solder ball  64 , and extends substantially parallel to the circuit board mounting surface  10 . The transition portion  66  offsets the solder ball pad in substantially the same direction as that in which the contact point  60  is offset. The transition portion  66  will be described with reference to FIG. 5.  
         [0025]    [0025]FIG. 5 is a partial detailed view that shows the terminal portion  48  of the contact  14  of FIGS. 3A-4B. The transition portion  66  comprises a horizontal portion  66   a  that extends substantially parallel to the circuit board mounting surface  10  and a vertical portion  66   b  that is continuous with the horizontal portion  66   a  and substantially perpendicular to the solder ball pad  62 .  
         [0026]    Next, the operation of the transition portion  66  will be described in further detail. during soldering of the solder ball  64  onto the solder ball pad  62  a solder fillet  64   a  is formed, by partially molten solder, between the solder ball pad  62  and the solder ball  64  around the entire periphery thereof. Because the vertical portion  66   b , which is continuous with the solder ball pad  62 , is formed perpendicular thereto, the solder fillet  64   a  does not flow toward the vertical portion  66   b . Accordingly, the vertical portion  66   b  functions to prevent solder fillet formation thereon.  
         [0027]    If the transition portion  66  extends rightward from the solder ball pad  62  then upward, as shown by the broken lines of FIG. 5, then the solder fillet  64   a  would flow toward the right from the solder ball pad  62 . Then, the surface tension of the molten solder would cause the solder ball  64  to move to the right, and cause it to be fixed in a positionally misaligned state. As a result, the solder balls  64  and conductive pads of the circuit board (not shown) become misaligned, reducing the reliability of electrical connections therebetween.  
         [0028]    In sharp contrast, the IC socket of the present invention allows the solder balls  64  to be consistently formed at their predetermined positions. Therefore, there is a reduced risk that positional misalignment will occur. In addition to the transition portion  66 , the size of the solder ball pad  62  (slightly smaller than the solder ball  64 ) also works to achieve this characteristic. That is, the size of the solder ball pad  62  reduces the risk of horizontal movement of the solder ball  64 , thereby contributing to accurate positioning thereof.  
         [0029]    The transition portion  66  is not limited to being of the shape shown in FIG. 5. Various shapes may be considered, as long as they prevent the flow of the solder fillet  64   a . For example, a modified contact  14 , having a differently shaped transition portion, is shown in FIG. 6.  
         [0030]    [0030]FIG. 6 is a partial detailed view of a modified contact  14   a  having a transition portion  68  with a single inclined portion  68   a . Note that of the parts illustrated in FIG. 6, those in common with the parts illustrated in FIGS. 3A through 5 will be denoted with the same reference numerals in the following description. The transition portion  68  is inclined. Therefore, it is difficult for the solder fillet  64   a  to flow upward along the inclined portion  68   a . That is, it is difficult for the solder ball  64  to be pulled upward toward the transition portion  68 . In addition, the incline portion  68   a  links the base portion  44  and the solder ball pad  62  with a shorter distance, thereby shortening the electrical path.  
         [0031]    The transition portion may be of a variety of shapes that discourage movement of the solder fillet  64   a  toward the transition portion. For example, the shape of the transition portion may be a combination of the aforementioned vertical portion  66   b  and the inclined portion  68   a . Alternatively, the transition portion may be formed as an arcuate shape that curves diagonally upward.  
         [0032]    Next, an alternative exemplary embodiment of the present invention will be described with reference to FIG. 7. FIG. 7 is a partial detailed view showing the terminal portion  48  of the contact  14  of FIGS. 3A-4B, with an alternate housing  2   a . In this embodiment, a protrusion  70  having a triangular cross section is provided on the circuit board mounting surface  10  of the housing  2   a . The protrusion  70  is provided to discourage movement of the solder fillet  64   a  toward the transition portion  66  of the contact  14 . The protrusion  70  extends from the fixing portion of the circuit board mounting surface  10 , that is, the base portion  44  of the contact  14 , to the solder ball pad  62 . Therefore, the distal end  70   a  of the protrusion  70  prevents upward movement of the solder fillet  64   a  when it attempts to flow along the transition portion  66 . Accordingly, movement and deformation of the solder ball  64  is further prevented.  
         [0033]    Next, a modification of the protrusion will be described with reference to FIG. 8. FIG. 8 is a partial detailed view showing the terminal portion  48  of the contact  14  with an alternative modified housing  2   b . FIG. 8 shows a state in which the shape of a protrusion  72  (protrusive portion) copies that of the transition portion  66 . That is, the protrusion  72  has a shape that is complementary to the right side of the transition portion  6 - 6  of :the contact  14 . In this case as well, the distal end  72   a  of the protrusion  72  prevents movement of the solder fillet  64   a  toward the transition portion  66 .  
         [0034]    In addition, a protrusion may be formed in the housing  2 , in combination with the contact  14   a  having the inclined transition portion  68  (shown in FIG. 6). Again, upward movement of the solder fillet  64   a  along the transition portion  68  can be prevented by such a protrusion.  
         [0035]    In this manner, the transition portions  66  and  68  of the contacts  14  and  14   a  may act as solder fillet stops by themselves, without depending on the shape of the housing  2 . However, by additionally providing the aforementioned protrusions  70  and  72  to the housings  2   a  and  2   b , the solder balls  64  are enabled to be offset while more effectively preventing positional misalignment. In this manner, the protrusions  70  and  72  of the housings  2   a  and  2   b  also function as solder fillet stops.  
         [0036]    While the invention is illustrated and described with reference to particular exemplary embodiments, it should be understood that alternative equivalent structures are contemplated within the scope of the invention. For example, metal plate  20  does not have to be formed from metal, but could be formed from a non-metallic material.