Abstract:
A grounded conductive plate in a land grid array package assembly includes a plurality of openings. The openings allow contacts from the socket to pass through to contact a package. The diameter of each opening is customizable to produce desired impedance between the contacts and the conductive plate. Impedance discontinuity seen by signals passing through the socket may be reduced.

Description:
BACKGROUND  
       [0001]     This invention relates generally to sockets for electronic device packages.  
         [0002]     Electronic devices are operating at faster and faster speeds. With this increase in performance, a designer should take into consideration the possibility of increased noise, cross-talk, ringing, etc. that may occur on the signal lines of the electronic device.  
         [0003]     Electronic devices may reside in any of a number of package technologies, for examples, flat pack, dual in-line package (DIP), pin grid array (PGA), and land grid array (LGA). Electronic devices such as microprocessors generally reside on packages with multiple pins such as an LGA.  
         [0004]     Current LGA socket technology has inherent I/O performance limitations. Manufacturing capability limitations of LGA socket technology limit minimum socket height, socket self inductance, socket loop inductance, and socket capacitance. These aspects of the socket design impose impedance discontinuities that limit the performance (i.e., speed) of I/O signaling in electronic device products that use present LGA socket technology.  
         [0005]     Currently, these problems have been addressed by reducing socket height, controlling pitch, optimizing mold material, and optimizing the land configuration. However, these solutions have limitations. For example, regarding socket height, the height of the socket can only go so small to control inductance. Similarly, land pitch can only control inductance to a certain degree. Moreover, to reduce impedance discontinuities with land configuration, one may have to completely surround a signal land with ground lands. This requires too many lands to practically use a socket for a microprocessor application.  
         [0006]     At high frequencies, impedance (Zo) is equal to the square root of inductance divided by capacitance (Zo=(SQRT L)/C). Current solutions attempt to control the impedance by controlling the inductance (L). In current solutions however, the inductance is generally too high, or the inductance to capacitance ratio is not controlled to the degree desired. Therefore, when an electronic device in a LGA package, for example, is plugged into a socket, signals on the lands of the LGA package see impedance discontinuities causing signal integrity problems.  
         [0007]     Thus, there is a need for better LGA packages. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]      FIG. 1  is an enlarged, partial cross-sectional view of one embodiment of the present invention;  
         [0009]      FIG. 2  is an enlarged, cross-sectional view of a socket according to one embodiment of the present invention;  
         [0010]      FIG. 3  is an enlarged, cross-sectional view of another embodiment of the present invention;  
         [0011]      FIG. 4  is an enlarged, cross-sectional view of a socket according to another embodiment of the present invention; and  
         [0012]      FIG. 5  is a schematic depiction of one embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0013]     A grounded metal plate may be embedded within a land grid array (LGA) electronic socket. The plate may provide a balancing capacitance that compensates for the inductance of the socket, reducing the discontinuity presented by the socket interconnect elements in some embodiments.  
         [0014]     Referring to  FIG. 1 , a package assembly  10  includes an LGA package  12  coupled by contacts  22  to a socket  16 . The embedded conductive plate  18  is grounded. The socket contact  22  has a land  23  on one end, extends through the conductive plate  18 , and has a deformed end  26  over the plate  18  that spring contacts the package  12 . The grounded conductive plate  18  has openings  20  to allow contacts  22  to pass through. Capacitance C arises between a contact  22  and the grounded conductive plate  18  as indicated in  FIG. 1 .  
         [0015]     The conductive plate  18  provides capacitive coupling to each contact  22 , which may reduce the impedance discontinuity at the socket  16 . Morover, coupling between adjacent contact  22  pairs may be improved, enabling use of the socket  16  for differential signaling in some embodiments.  
         [0016]     Initially, the electrical properties of the electronic package  12  are identified. The electrical properties of the contacts  22  in the socket  16  are determined. An inductance is determined. A desired impedance between each contact  22  and the conductive plate  18  is determined. The inductance may be fixed for a particular socket. Therefore, by identifying a desired impedance, the capacitance C can be varied to get the desired performance.  
         [0017]     The diameter of each hole  20  in the conductive plate  18  is determined to achieve the desired impedance. The diameter of the hole  20  can be varied to vary the capacitance C between the conductive plate  18  and the contact  22 . Therefore, knowing a desired impedance, the hole  20  diameter may be set to achieve a particular capacitance C that produces the desired impedance.  
         [0018]     For a particular electronic package assembly  10 , the desired impedance may be the same for every contact  22  on the socket  16 . In this case, the diameter of each hole  20  in the grounded conductive plate  18  may be the same. However, it is possible that different impedances are desired for different contacts  22  on a socket  16  based, for instance, on the size of the contact  22  or the signal evolving from the contact. In this case, the grounded conductive plate  18  may have holes  20  of varying diameters.  
         [0019]     A land grid array package  12 , which may carry an integrated circuit, may be contacted from below by the deformed end  26 . The deformed end  26  may have a curved upper contact portion. The contact  22  may have a generally horizontally deformed portion  24 , and a bent section  21  that couples to a vertical section  23 . The vertical section  23  may be the portion of the contact  22  that extends through the embedded conductive plate  18 . In one embodiment, stamped metal contact land grid array technology may be utilized.  
         [0020]     Referring to  FIG. 2 , the package  12  may be clamped onto the socket  16  in accordance with one embodiment, depressing the contact  22  deformed ends  26 . Some of the contacts  22  may be coupled to solder balls  32 , which are electrically coupled to a grounded motherboard  28 . However, other contacts  30  are of a slightly different configuration. Those contacts  30  may have V-shaped contacting portions  31 , which have land surfaces  33 , which contact the embedded conductive plate  18  when the package  12  engages the socket  16 .  
         [0021]     As a result, when the package  12  is pressed onto the socket  16 , the deformed ends  26  of the contacts  22  are deformed to make tight spring biased electrical connections to the package  12 . However, the contacts  30  deform so that their lands  33  make electrical connection to the embedded conductive plate  18 . This connection grounds the embedded conductive plate  18  via solder balls  32  to the grounded motherboard  28 .  
         [0022]     Referring to  FIG. 3 , additional contacts  34  may be permanently electrically coupled to the embedded conductive plate  18  in one embodiment. The contacts  34  couple to ground through the motherboard  28  via solder balls  32 . Thus, in this embodiment, the metal conductive plate  18  is connected to ground through the motherboard  28 .  
         [0023]     In accordance with still another embodiment, shown in  FIG. 4 , the socket contacts  36  electrically contact the conductive plate  18  through land ends  38  when the package  12  is pressed onto the socket  16 . However, in this case, the socket contacts  36  are floating because they do not ground through the motherboard  28 .  
         [0024]     Sockets with conductive plates, according to some embodiments of the present invention, may reduce the impedance discontinuity of LGA contacts. Moreover, some embodiments allow extension of present LGA sockets to differential signaling applications. Further, electrical parasitics (inductance and capacitance) may be distributed to avoid potential resonance issues at high frequencies in some cases.  
         [0025]     Referring to  FIG. 5 , a processor-based system  46  may be a laptop computer, a desk top computer, an entertainment system, a personal digital assistant, a camera, a cellular telephone, to mention a few examples. The system  46  may include a package  12 , which includes a processor  40 . The processor  40  may be coupled over the motherboard  28  to a bus  48 . The bus  48  may in turn be coupled to input/output pads  42  and a storage  44 .  
         [0026]     While the present invention has been described with respect to a limited number of embodiments, those skilled in the art will appreciate numerous modifications and variations therefrom. It is intended that the appended claims cover all such modifications and variations as fall within the true spirit and scope of this present invention.