Abstract:
A socket assembly configured to be reflow soldered to a circuit board comprising a perimeter frame having a central open area surrounded by perimeter walls. The socket assembly may be configured to be surface mounted on a circuit board, wherein at least one of the perimeter walls includes a post extending downward therefrom. The socket assembly also comprises a base fit into the open area of the perimeter frame. The base is separate and distinct from the socket frame. The base has a post hole therein positioned to mate with the post. Additionally, the socket assembly comprises contacts held by the base, and solder balls provided on a bottom of the base. The solder balls engage the contacts and, prior to, and after, soldering, extend beyond a bottom of the socket frame.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The present invention generally relates to a separable interface connector, and more particularly relates to a separable interface connector that joins a printed circuit board through reflow soldering to an electrical component, such as a motherboard.  
           [0002]    Various electronic systems, such as computers, comprise a wide array of components mounted on printed circuit boards, such as daughterboards and motherboards, which are interconnected to transfer signals and power throughout the system. The transfer of signals and power between the circuit boards requires electrical interconnection between the circuit boards.  
           [0003]    Certain interconnections include a socket assembly and a plug assembly, or integrated circuit (IC) chip. Some socket assemblies include spring contacts, which are configured to mate with conductive pads on the plug assembly. As the socket assembly and plug assembly mate, the spring contacts exert a normal force on the contact pads, thus ensuring proper electrical contact between the spring contacts and the conductive pads.  
           [0004]    In order to establish adequate contact, the spring contacts wipe across the conductive pads, cleaning both surfaces, as the plug assembly is mated into the socket assembly. Typically, during mating, the spring contacts are deflected. During deflection, the spring contacts exert a resistive force on the plug assembly. The resistive force typically has normal and tangential components. The normal force is usually referred to as the contact force and the tangential force is usually caused by the frictional behavior of the wiping motion.  
           [0005]    Typical socket assemblies, whether pin grid array (PGA), land grid array (LGA), or ball grid array (BGA) assemblies, are soldered to an electrical component, such as a motherboard. Typically, solder balls are attached to the bottom of the socket assembly. The socket assembly is positioned on a motherboard, and both components are passed through an oven, or other heating device, to begin the solder reflow process. During the solder reflow process, the solder balls melt and form a cohesive layer between the socket assembly and the motherboard. The solder layer cools after the heating and forms an electrically conductive bond between the socket assembly and the motherboard.  
           [0006]    Some socket assemblies are soldered to motherboards such that the solder layer is the only intervening material that supports and extends between the socket assembly and the motherboard. That is, the socket assembly does not contact the motherboard at any other point during or after the solder reflow process. When the plug assembly is mated into the socket assembly, however, the mating or clamping force exerted into the socket assembly is fully translated to, and absorbed by, the solder layer. The solder layer may be further collapsed, disrupted or otherwise compressed due to the forces absorbed. Consequently, the electrical connection between the socket assembly and the motherboard may be adversely affected.  
           [0007]    In order to counter the effects of mating or clamping forces being exerted into the solder layer, some socket assemblies include standoffs that support and stabilize the socket assembly onto the motherboard. Typically, the standoffs extend a distance that is less than that of the solder balls, but more than that of the natural reflow height of the solder balls. That is, before the solder reflow process, the standoffs do not touch the motherboard. When the socket assembly is soldered to the motherboard, the height of the socket assembly from the motherboard is dictated by the standoffs. U.S. Pat. No. 6,155,848, issued to Lin (“the &#39;848 patent”), describes an auxiliary device for a ZIF electrical connector that uses standoffs. The &#39;848 patent discloses that the height of the stand-off portion is less than the height of the solder balls before soldering, and equal to the height of the solder balls after soldering. Thus, after the solder reflow process, the resulting solder layer is dictated by the height of the standoffs. U.S. Pat. No. 6,220,884, issued to Lin (“the &#39;884 patent”) discloses a BGA socket that comprises an insulative cover supported by standoffs on a base. The standoffs of the cover extend beyond a bottom surface of the base. After the solder reflow process, the resulting solder layer is dictated by the height of the standoffs.  
           [0008]    Additionally, in both the &#39;848 and &#39;884 patents, the components (such as IC chips) that mate with each socket include pins. That is, the IC chips include pins that are mated into the socket. The existence of pins on the IC chips mandates that the height of the sockets is adequate to receive and retain the pins.  
           [0009]    However, conventional socket assemblies, including those of the &#39;848 and &#39;884 patents, do not allow the solder balls to reflow to the height they naturally would if there were no components that interfered. That is, the solder balls do not melt to a natural reflow height. Rather, the height of the resulting solder layer is dictated by the height of the standoffs. Because the solder layer is not necessarily at its natural height, electrical transmission through the solder layer may be adversely affected. For example, the solder layer may be too dense or too sparse due to the fact that the standoffs dictate the height of the solder layer.  
           [0010]    Thus, a need exists for a socket assembly that may be reflow soldered to an electrical component more efficiently, and in a manner that ensures a better conductive path through the resulting solder layer.  
         BRIEF SUMMARY OF THE INVENTION  
         [0011]    Certain embodiments of the present invention provide a socket assembly configured to be reflow soldered to a circuit board. The socket assembly comprises a socket frame, or perimeter frame, having a central open area surrounded by perimeter walls. The socket assembly may be configured to be surface mounted on a circuit board, wherein at least one of the perimeter walls includes a post extending downward therefrom. The socket assembly also comprises a socket board, or base, fit into the open area of the socket frame. The socket board is separate and distinct from the socket frame. Optionally, the socket frame may be integrally formed with the socket board as a single unit during manufacture. During assembly, the socket frame may then separate, or break away, from the socket board by way of a separation zone, such as a perforated area between the socket frame and the socket board.  
           [0012]    The socket board has a post hole therein positioned to mate with the post. Additionally, the socket assembly comprises contacts held by the socket board, and solder balls provided on a bottom surface of the socket board. The solder balls engage the contacts and, prior to, and after, soldering, extend beyond a bottom of the socket frame.  
           [0013]    The post is held partially seated in the post hole when the socket board and frame are positioned in a pre-soldered state. The post becomes fully seated in the post hole when the socket board and frame move during a plug assembly mating state, that is, when a plug assembly is mated into the socket assembly. The assembly process is controlled in that, after the reflow process, the post is able to move through the post hole in a mating direction defined by the direction of the plug assembly moving into the socket assembly. 
       
    
    
     BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS  
       [0014]    [0014]FIG. 1 is an isometric view of a socket assembly formed in accordance with an embodiment of the present invention.  
         [0015]    [0015]FIG. 2 is a top view of a socket frame of a socket assembly according to an embodiment of the present invention  
         [0016]    [0016]FIG. 3 is a bottom view of a socket frame according to an embodiment of the present invention.  
         [0017]    [0017]FIG. 4 is a cross-sectional view of a socket frame through line  4 - 4  of FIG. 2 according to an embodiment of the present invention.  
         [0018]    [0018]FIG. 5 is a bottom view of a post according to an embodiment of the present invention.  
         [0019]    [0019]FIG. 6 is a top view of a socket assembly according to an embodiment of the present invention.  
         [0020]    [0020]FIG. 7 is a bottom view of a socket assembly according to an embodiment of the present invention.  
         [0021]    [0021]FIG. 8 is a partial cross-sectional view of a socket assembly taken through line  8 - 8  shown in FIG. 6 according to an embodiment of the present invention.  
         [0022]    [0022]FIG. 9 is a side view of a socket assembly according to an embodiment of the present invention.  
         [0023]    [0023]FIG. 10 is a cross-sectional view of a socket assembly through line  10 - 10  of FIG. 6 according to an embodiment of the present invention.  
         [0024]    [0024]FIG. 11 is a side view of a socket assembly mounted on a motherboard before the reflow solder process, according to an embodiment of the present invention.  
         [0025]    [0025]FIG. 12 is a side view of a plug assembly mated into a socket assembly according to an embodiment of the present invention.  
         [0026]    [0026]FIG. 13 is a partial cross-sectional view of a socket assembly in a pre-soldered position according to an embodiment of the present invention.  
         [0027]    [0027]FIG. 14 is a partial cross sectional view of a socket assembly in a fully seated ion according to an embodiment of the present invention.  
         [0028]    [0028]FIG. 15 is an isometric view of a socket board according to an alternative embodiment of the present invention. 
     
    
       [0029]    The foregoing summary, as well as the following detailed description of certain embodiments of the present invention, will be better understood when read in conjunction with the appended drawings. For the purpose of illustrating the invention, there is shown in the drawings, certain embodiments. It should be understood, however, that the present invention is not limited to the arrangements and instrumentalities shown in the attached drawings.  
       DETAILED DESCRIPTION OF THE INVENTION  
       [0030]    [0030]FIG. 1 is an isometric view of a socket assembly  10  formed in accordance with an embodiment of the present invention. The socket assembly  10  is a two-piece, assembly that includes a socket board  12  and a socket frame  14 . The socket board  12  includes a plurality of spring contacts  16  mounted thereon. For the sake of simplicity, only one row of spring contacts  16  is shown in FIG. 1. The socket assembly  10  may be a Ball Grid Array (BGA) assembly.  
         [0031]    The socket board  12  and socket frame  14  are separate and distinct components. The socket board  12  connects to the perimeter frame by the mating, engagement or otherwise interaction of posts  26  (discussed below) of the socket frame  14  with post cavities  34  (discussed below) of the socket board  12 . The socket board  12  forms the base of the socket assembly  10 .  
         [0032]    [0032]FIG. 2 is a top view of the socket frame  14  of the socket assembly  10 . The socket frame  14  includes perimeter walls  18  having corners  20 , midsections  22  and an opening  24  defined between the perimeter walls  18 .  
         [0033]    [0033]FIG. 3 is a bottom view of the socket frame. The socket frame  14  also includes posts  26 , which extend downwardly from the bottom surface of the perimeter walls  18 . While five posts  26  are shown, more or less posts  26  may be formed on the perimeter walls  18 .  
         [0034]    [0034]FIG. 4 is a cross-sectional view of the socket frame  14  through line  4 - 4  of FIG. 2. The socket frame  14  also includes recesses  28  formed in the perimeter walls  18  between the corners  20  and the midsections  22 . The recesses  28  are formed so that the socket frame  14  may fit together with the socket board  12 . As shown in FIG. 4, the posts  26  extend downwardly from the bottom surface of the perimeter walls  18 . The posts  26  do not extend beyond the plane defined by the bottom surfaces of the corners  20  and midsections  22 . Alternatively, the posts  26  may extend beyond the plane defined by the bottom surfaces of the corners  20  and midsections  22 .  
         [0035]    [0035]FIG. 5 is a bottom view of a post  26 . The post  26  is hexagonal, but may be any shape that provides an adequate interference fit with a post hole or cavity formed in the socket board  12 . For example, the posts  26  may be formed as octagons, squares, triangles, circles, etc.  
         [0036]    [0036]FIG. 6 is a top view of the socket assembly  10 . FIG. 6 shows the socket frame  14  and the socket board  12  fitted together. The perimeter walls  18  of the socket frame  14  overlap outer edge  30  (as shown, for example, in FIG. 8) of the socket board  12  when the socket board  12  and the socket frame  14  are fit together. As shown in FIG. 6, a plurality of spring contacts  16  are mounted on the socket board  12 , which acts as the base of the socket assembly  10 . More or less spring contacts  16  than those shown may be positioned on the socket board  12 .  
         [0037]    [0037]FIG. 10 is a cross-sectional view of the socket assembly  10  taken through line  10 - 10  of FIG. 6. Each spring contact  16  includes a wiping tip  38  formed integrally with a deflectable extension portion  40 . The deflectable extension portion  40  is formed integrally with a curved transition portion  42 , which is in turn formed integrally with a retained portion  44 . The retained portion  44  is securely held by a contact cavity formed in the socket board  12  of the socket assembly  10 . A terminal end of the retained portion  44  contacts a solder ball  27 . As shown in FIG. 10, the socket board  12  is not formed integrally with the socket frame  14 . That is, the socket board  12  and the socket frame  14  abut against or are spaced apart from one another at interface  29 .  
         [0038]    [0038]FIG. 7 is a bottom view of the socket assembly  10  to better illustrate that the socket board  12  is generally formed as a square with chamfered corners  31 . Notches  32  are also cut in the sides of the socket board  12 . The corners  20  and the midsections  22  extending downward from the socket frame  14  are received by corresponding chamfered corners  31  and notches  32 , respectively, in the socket board  12 . That is, the socket board  12  and the socket frame  14  fit together through the interaction of corresponding corners and midsections  20  and  22  with chamfered corners and notches  31  and  32 , respectively. The socket board  12  also includes post cavities  34  arranged about the perimeter and an array of solder balls  27 , which may correspond to the number of spring contacts  16 .  
         [0039]    [0039]FIG. 8 is a partial cross-sectional view of the socket assembly  10  through line  88  shown in FIG. 6. The post cavities  34  are positioned on the outer edges of the socket board  12  and correspond to positions of the posts  26  located on the socket frame  14 . Upon initial mating of the post  26  and the post cavities  34 , a clearance area  36  is formed between the socket board  12  and the socket frame  14 . When a plug assembly (discussed below) is inserted into the socket assembly  10 , the clearance area  36  is either decreased or eliminated. That is, when the plug assembly is mated into the socket assembly  10 , the socket frame  14  is pressed toward the socket board  12  along with the plug assembly in the direction of line A until becoming fully seated. The posts  26  and post receptacles  34  are configured so that an interference fit exists between the two when mated. Further, the interference fit is such that additional force in the direction of line A moves the socket frame  14  into the socket board  12 . In other words, as shown, for example in FIGS. 8 and 13, the posts  26  of socket frame  14  are mated into the post cavities  34  of the socket board  12  to a pre-plug position (in which the socket assembly  10  does not touch a motherboard or other circuit board to which it is soldered). Also, after the socket assembly  10  is soldered to the board, but before the plug assembly is fully mated with the socket assembly  10 , the posts  26  may remain in the same position with respect to the post cavities  34 . After the plug assembly is fully mated into the socket assembly  10 , as shown for example in FIG. 14, the socket frame  14  is in its fully seated position with respect to the socket board  12  (in which the socket assembly  10  may abut the motherboard or other circuit board to which it is soldered).  
         [0040]    [0040]FIG. 9 is a side view of the socket assembly  10  before reflow soldering. As shown in FIG. 9, the solder balls  27  extend below the bottom surfaces of the corners  20  and the midsections  22  of the socket frame  14 . Because the solder balls  27  extend below the bottom surfaces of the corners  20  and the midsections  22 , the solder balls  27  are the only components of the socket assembly  10  that directly abut a motherboard  46  (as discussed below) when the socket assembly  10  is initially positioned on the motherboard  46 .  
         [0041]    [0041]FIG. 11 is a side view of the socket assembly  10  mounted on a motherboard  46  before the solder reflow process. Before the solder reflow process (i.e., heating of the solder balls  27 ), the only portion of the socket assembly  10  that touches the motherboard  46  is the solder balls  27 . The socket frame  14  does not touch, and is spaced a distance from, the motherboard  46 . As shown in more detail in FIG. 13, the clearance area  36  is formed between the socket board  12  and the socket frame  14 . Also, a clearance area  37  exists between the corners  20  (and midsections  22 , although not shown with respect to FIG. 13) and the motherboard  46 .  
         [0042]    As solder balls are heated, such as solder balls  27 , they melt to a natural height or level if there is no interfering or intervening components between the solder balls and the component to which they are being reflow soldered, such as the motherboard  46 . The natural height or level of solder reflow, that is, the natural height or level to which the solder balls melt, is determined by the physical properties of the solder balls. During the solder reflow process, the solder balls  27  are allowed to reflow naturally without any interfering structure, such as the corners  20  and midsections  22 , touching the motherboard  46 . Hence, the corners  20  and midsections  22  do not dictate the distance of the socket board  12  from the motherboard  46 . The distance between the socket board  12  and the motherboard after the reflow process is dictated by the natural height (HN) of the molten solder balls  27 .  
         [0043]    [0043]FIG. 12 is a side view of a plug assembly  47  mated into the socket assembly  10  after the solder reflow process is complete and the reflown solder balls  27  form solder connections  48  between the socket assembly  10  and the motherboard  46 . The height of the solder connections  48  is the natural height of the reflown solder balls (HN). The plug assembly  47 , or integrated circuit (IC) chip, mates with the socket assembly  10  in the direction of line A. The plug assembly  47  includes contacts, such as conductive pads (not shown), which mate with the spring contacts  16  positioned on the socket board  12 . The spring contacts  16  are deflected by the plug assembly  47  and wipe across the contacts of the plug assembly  47 . As the plug assembly  47  is mated into the socket assembly  10 , the mating force in the direction of line A causes the posts  26  to move further into the post cavities  34  (in the direction of line A), as discussed above with respect to FIG. 8. That is, the mating or clamping force of the plug assembly  47  into the socket assembly  10  causes the socket frame  14  to slide or otherwise move toward the socket board  12  by way of the posts  26  sliding through the post cavities  34 . The socket frame  14  is a moving frame in that it moves with respect to the socket board  12 .  
         [0044]    Upon full mating of the plug assembly  47  into the socket assembly  10 , the socket frame  14  may touch the motherboard  46 , as shown with respect to FIG. 14. That is, as the plug assembly  47  is mated into the socket assembly  10 , the movement of the plug assembly  47  in the direction of line A causes the socket frame  14  to move (by way of the interaction of the posts  26  through the post cavities  34 ) toward the motherboard  46 . Preferably, the socket frame  14  touches or abuts the motherboard  46  at the end of the mating process. In doing so, the excess clamping or mating force when joining the plug assembly  47  and the socket assembly  10  is translated into the socket frame  14 . Because the socket frame  14  touches the motherboard  46 , the excess mating or clamping force is translated directly to the motherboard  46 , but not through the solder connections  48 . Further, an accurate connection between the plug assembly  47  and the socket assembly  10  may be ensured if the socket frame  14  contacts the motherboard during the plug assembly/socket assembly mating process. That is, the corners  20  and midsections  22  may ensure that the mating surface of the plug assembly  47  is approximately parallel to the spring tips  38  of the socket assembly  10  (due to the bottom surfaces of the standoffs  20  and  22  being in parallel contact with the top surface of the motherboard  46 ). In any event, the natural reflow height of the solder balls  27  is not disturbed during the reflow process or the plug assembly  47 /socket assembly  10  mating process.  
         [0045]    [0045]FIG. 14 is a partial cross sectional view of a socket assembly  10  in a fully seated position. For the sake of clarity, the plug assembly  47  is not shown. However, the spring contact  16  is shown in a fully deflected position. In this view, the plug assembly  47 , while not shown, is in a fully mated position with respect to the socket assembly  10 . Further, the socket frame  14  is fully seated with respect to the socket board  12  and the motherboard  46 . It is to be noted that while the corners  20  (and midsections  22 , although not shown with respect to FIG. 14) abut the motherboard  46 , the corners  20  and midsections  22  do not abut the motherboard  46  during the reflow solder process. Only when the plug assembly  47  is fully seated into the socket assembly  10  does the socket frame  14  contact the motherboard  46 . That is, the mating force of the plug assembly  47  into the socket assembly  10  causes the posts  26  to slide through the post cavities  34 , and therefore the corners  20  and midsections  22  of the socket assembly  10  contact the motherboard  46 . Also, the clearance area  36  shown with respect to FIGS. 8 and 13 is eliminated or decreased when the socket assembly  10  is fully seated. Preferably, the socket frame  14  abuts the motherboard  46  before the plug assembly  47  is fully clamped into the socket assembly  10 , so that the motherboard  46  will absorb most, if not all, of the excess mating force.  
         [0046]    As mentioned above, more or less posts  26  and post cavities  34  may be used with the socket assembly  10 . Additionally, the shape of the socket frame  14  and socket board  12  may be different shapes, as long as both fit together. Additionally, the posts  26  may be any shape that interferingly fits into the post cavities  34 . Further, the post cavities  34  may be any shape that interferingly engages the posts  34 . Also, the posts may be positioned on, and extending upward from, the socket board  12 , while the cavities, or holes, are formed within the perimeter walls of the socket frame  14 .  
         [0047]    [0047]FIG. 15 is an isometric view of a socket board  60  according to an alternative embodiment of the present invention. The socket board  60  includes a base  62  having spring contacts  16  mounted thereon and a post  64  upwardly extending from the base  62 . The post  64  is configured to be slidably received by a corresponding hole in the plug assembly. Thus, instead of having a perimeter frame having posts, the socket board  60  includes the post  64 , over which the plug assembly may slide down into a fully seated position. Alternatively, the socket board  60  may include multiple posts  64  upwardly extending from various locations on the base  62 . For example, the posts  64  may be located in the corners.  
         [0048]    Thus, embodiments of the present invention provide a socket assembly that may be reflow soldered to a motherboard more efficiently. Because the resulting solder layer is reflown to its natural height, a more reliable electrical conductive path results. Also, when a plug assembly (such as an IC chip) is mated into the socket assembly, the excess clamping or mating force is translated into the motherboard. Thus, the solder layer is not excessively stressed during the mating process.  
         [0049]    While the invention has been described with reference to certain embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from its scope. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed, but that the invention will include all embodiments falling within the scope of the appended claims.