Abstract:
Disclosed is land grid array (LGA) assembly using a compressive load. An LGA assembly includes a first component located on the top of the LGA assembly; a center load screw coupled to the first component; and a second component, wherein the center load screw is received on the second component upon turning the center load screw in a first direction. Further turning of the center load screw in the first direction after the center load screw is received on the second component, operates the first component to apply a compressive load within the LGA assembly.

Description:
TECHNICAL FIELD 
   The technical field is integrated circuit packaging, specifically land grid array assembly using a compressive load. 
   BACKGROUND 
   The use of increasingly high speed integrated circuits (ICs) in computer systems has given rise to new assembly challenges related to the attachment and support of the ICs. Due to the large, thermally induced stresses that impact the long term reliability of solder joints, high speed, high density IC module assemblies can not employ standard solder techniques for connecting modules to a circuit board. Therefore, interposer socket assembly techniques, specifically land grid array (LGA) sockets, have emerged as a substitute for solder joints. 
   An LGA socket is placed on the circuit board and makes electrical contact with the circuit board through a plurality of input/output (IO) interconnects. An IC module is placed on the LGA socket in electrical contact so that the LGA socket enables electrical connection between the IC module and the circuit board. Assemblies using LGA sockets require mechanisms for applying compressive load on the assembly such that the LGA socket establishes and maintains reliable electrical connection with the circuit board, thereby ensuring reliable electrical connection between the IC module and the circuit board. 
   Coil spring assemblies are widely used in the art for applying a compressive load on LGA assemblies. Typically, a coil spring assembly includes an anchor screw, a spring, one or more washers to eliminate metal debris and a clip to capture the screw to a heat sink. The spring may be compressed as the anchor screw is tightened. The coil spring assemblies may be located along the perimeter of the LGA assembly. Compressive load is derived from the coil spring assemblies and spread across a heat sin and applied to the LGA assembly. 
   Prior art systems utilizing coil spring assemblies are limited. Large loads are difficult to achieve with load systems utilizing coil spring assemblies. Systems using coil spring assemblies are also difficult to manufacture. The anchor screw of each coil spring assembly must be tightened to apply a compressive load. Each anchor screw must be tightened individually before moving on to another anchor screw located in a corresponding opposite direction. In order to apply an even compressive load on the LGA assembly, the anchor screws must be sequentially tightened in a series of repetitive steps performed in a cross pattern configuration. Typically, three or more repetitions of each cross pattern are required. For example, if four coil spring assemblies are used in a system, twelve or more repeated operations may be required. Additionally, systems using coil spring assemblies are susceptible to unevenly applied loads on the LGA socket. Transverse loads may be created that interfere with the alignment of the processor to the LGA socket. Further, conventional coil spring assemblies typically involve a high part count. For example, a coil spring assembly may comprise a screw, a spring, one or more washers and a clip to capture the screw to a heat sink. The increased part Count may result in increased assembly time and costs. Further, systems using coil spring assemblies suffer from impeded thermal performance. Typically, a heat sink is used to dissipate heat generated by the IC module. The heat sink typically comprises a plurality of fins exposed to the ambient air. In order to make space for the coil spring assemblies (i.e. footprint), a significant amount of fin area must be removed from the heat sink, thereby reducing the performance of the heat sink. The coil spring assemblies may also create an obstruction to airflow in the heat sink. 
   SUMMARY 
   Disclosed is a land grid array (LGA) assembly using a compressive load comprising a first component located on the top of the LGA assembly; a center load screw coupled to the first component; and a second component, wherein the center load screw is received on the second component upon turning the center load screw in a first direction. Further turning of the center load screw in the first direction after the center load screw is received on the second component, operates the first component to apply a compressive load on one or more of a plurality of components of the LGA assembly. 
   Also disclosed is a method for LGA assembly using a compressive load, comprising the steps of setting a first component on the top of the LGA assembly; coupling a center load screw to the first component; setting a second component, wherein the center load screw is received on the second component upon turning the center load screw in a first direction; and turning the center load screw in the first direction, wherein the center load screw is received on the second component and the first component is operated to apply a compressive load on one or more of a plurality of components of the LGA assembly. 
   Other aspects and advantages will become apparent from the following detailed description, taken in conjunction with the accompanying figures. 

   
     DESCRIPTION OF THE DRAWINGS 
     The detailed description will refer to the following drawings, wherein like numerals refer to like elements, and wherein: 
       FIG. 1  is an exploded diagram of a modular integrated apparatus; 
       FIG. 2  is a diagram of the modular integrated apparatus assembled; 
       FIG. 3  is a diagram showing a sectional view of an opening in a heat sink of the modular integrated apparatus; and 
       FIG. 4  shows a flowchart illustrating a method for land grid array (LGA) assembly using a compressive load. 
   

   DETAILED DESCRIPTION 
     FIG. 1  is an exploded diagram of a modular integrated apparatus  10  for a computer system.  FIG. 2  is a diagram of the modular integrated apparatus  10  assembled. The modular integrated apparatus  10  comprises a plurality of components described below and forms an infrastructure for a clamped attachment of a processor  20  to a circuit board  30  through compressive load applied to the modular integrated apparatus  10 . The processor  20  may be an integrated circuit module such as, for example, a multi-chip module or a single VLSI package. The VLSI package may be a flip chip assembly. The processor  20  may also be an application specific integrated circuit (ASIC). The circuit board  30  may be, for example, a printed circuit board such as a mother board. Only a section of the circuit board  30  is shown in  FIGS. 1 and 2 . 
   A receiving apparatus  40  comprises an insulator  42  and a bolster plate assembly  44 . The receiving apparatus  40  is set on a bottom surface of the circuit board  30 . The bolster plate assembly  44  may be attached to the circuit board  30 . The bolster plate assembly  44  comprises a plurality of threaded sockets  46  located along a perimeter of the bolster plate assembly  44 . For example, if the bolster plate assembly  44  is rectangular in shape, the threaded sockets  46  may be located at the four corners of the rectangular shape. The circuit board  30  comprises a plurality of holes  32  corresponding to the location of the threaded sockets  46  in the bolster plate assembly  44 . The insulator  42  is set over the bolster plate assembly  44  in between the bolster plate assembly  44  and the circuit board  30 . The insulator  42  creates insulation between a plurality of input/output (IO) pads (not shown) that are located on the bottom surface of the circuit board  30 . The insulator  42  prevents short circuiting between the IO pads. 
   A modular processor apparatus  50  comprises a processor stack  55  and a frame assembly  54 . The modular processor apparatus  50  is set on a top surface of the circuit board  30  over the receiving apparatus  40 . The processor stack  55  comprises a land grid array (LGA) socket  52 , the processor  20  and a thermal interface material (TIM)  53 . The TIM  53  comprises a thermal interface enhancement material for enhancing thermal conduction from the processor  20  to a heat sink. The TIM  53  is set on a top face of the processor  20 . The processor  20  is located on a top surface of the LGA socket  52 . The LGA socket  52  enables electrical connection between the processor  20  and the circuit board  30 . The LGA socket  52  comprises a plurality of IO interconnect elements (not shown) that make contact with the circuit board  30 . In order to establish and maintain a reliable electrical connection between the processor  20  and the circuit board  30 , the LGA socket  52  must be sufficiently compressed between the circuit board  30  and the processor  20 . The receiving apparatus  40  may drive the orientation and alignment for the modular processor apparatus  50  in relation to the circuit board  30 . Additionally, separate and distinct features may be used for the orientation and alignment of the LGA socket  52  to the circuit board  30  and the processor  20  to the LGA socket  52 . 
   The processor stack  55  fits into an opening of the frame assembly  54 , which may be, for example, rectangular in shape. The frame assembly  54  may be designed, for example, to attenuate electromagnetic interference (EMI) generated by the processor  20 . The frame assembly  54  comprises a plurality of holes  56  corresponding to the location of the threaded sockets  46  in the bolster plate assembly  44 . Optionally, the frame assembly  54  may align the processor stack  55  in relation to the circuit board  30  and the bolster plate assembly  44 . The frame assembly  54  and the LGA socket  52  lie on the top surface of the circuit board  30 . 
   A plurality of load studs  60  are inserted through the holes  56  of the frame assembly  54 , the holes  32  in the circuit board  30 , and are threaded into the sockets  46  of the bolster plate assembly  44 . Each load stud  60  comprises a head  61 , a top shoulder  62  and a bottom shoulder  63 . Each load stud  60  bottoms out on the top surface of the frame assembly  54  at the bottom shoulder  63 . The load studs  60  are threaded and may be, for example, long screws. 
   A heat sink  70  is set over the load studs  60  through a plurality of holes  72  located along a perimeter of the heat sink  70 . The holes  72  correspond to the location of the threaded sockets  46  in the bolster plate assembly  44 . The heat sink  70  is located on top of and in contact with the modular processor apparatus  50 . The heat sink  70  comprises an opening  73 . The heat sink  70  may, for example, comprise a plurality of fins  76  that provide a surface area for distributing heat generated from the processor  20 . Heat that is generated in processor  20  may be thermally conducted into the heat sink  70  and further conducted into the ambient airflow by the fins  76  to dissipate the heat. The heat sink  70  may, for example, be comprised of aluminum. Optionally, an EMI gasket  78  may be placed between the heat sink  70  and the frame assembly  54  to further attenuate EMI generated by the processor  20 . Further, additional EMI gaskets may be placed between the circuit board  30  and the frame assembly  54  and between the circuit board  30  and the bolster plate assembly  44  for further EMI attenuation. The load of the heat sink  70  is primarily born by the processor stack  55 . 
     FIG. 3  is a diagram showing a sectional view of the opening  73  in the heat sink  70  in more detail. A counterbore  74  is located at a base of the opening  73  and comprises a hardened seat  75  at a base of the counterbore  74 . 
   A load plate assembly  80  is set on top of the heat sink  70  and shuttled onto the load studs  60  through a plurality of holes  82  in the load plate assembly  80 . The location of the holes  82  correspond to the location of the threaded sockets  46  in the bolster plate assembly  44 . The holes  82  may be, for example, in the shape of key holes comprising a wide opening  84  and a narrow slot  85 . The load plate assembly  80  may comprise, for example, one or more compression plates  86 . The compression plates  86  are convex in shape and are oriented so that the outwardly curved surface of the compression plates  86  are directed toward the top of the heat sink  70 . The load plate assembly  80  may be lowered onto the load studs  60  through the wide opening  84  of each hole  82  and then shuttled laterally so that the narrow slot  85  of each hole  82  is placed between the head  61  and the top shoulder  62  of each load stud  60 . The load plate assembly also comprises a center hole  88  located directly above the seat  75 . The center hole  88  may be, for example, a threaded rivet for receiving a threaded center load screw  90 . 
   The center load screw  90  is inserted through the center hole  88  in the load plate assembly  80 . The center load screw  90  comprises a head  92 . The center load screw  90  is threaded into the center hole  88  by turning the center load screw  90  in a clock-wise direction for a right-hand screw, or in a counter-clock-wise direction for a left-hand screw. Upon turning the head  92  of the center load screw  90 , the center load screw  90  moves downward toward the base of the opening  73  in the heat sink  70 . The center load screw is prevented from moving further downward after a tip  91  of the center load screw  90  bears down on the seat  75 . As the center load screw  90  is turned further after the tip  91  impacts the seat  75 , a center of the convex compression plates  86  of the load plate assembly  80  is forced upward by the threaded interface of the center load screw  90  and the center hole  88 . As the center of the compression plates  86  is forced upward upon continued turning of the center load screw  90 , the outer edges of the compression plates  86  are also forced upward under the head  61  of each load stud  60 . A tensile load is therefore applied to the load studs  60  because the load studs  60  are threaded into the sockets  46  of the bolster plate assembly  44 . The tensile load is translated to the bolster plate assembly  44 , pulling the bolster plate assembly  44  upward opposing the force of the center load screw  90  on the seat  75  on the heat sink  70 . The result is that the processor stack  55  is compressed between the bolster plate assembly  44  and the heat sink  70 . The compressive load ensures reliable electrical contact of the LGA socket  52  between the processor  20  and the circuit board  30 . 
   As shown in  FIG. 3 , the tip  91  of the center load screw  90  may be, for example, spherical in shape. This design minimizes any torque imparted on the heat sink  70  by turning of the center load screw  90  after the tip  91  has impacted the seat  75 . Excessive torque on the heat sink  70  may cause misalignment of the processor stack  55 . The spherical shape of the tip  91  also minimizes metal debris that may result from the shear forces created as the tip  91  is turned on the surface of the seat  75 . Metal debris may cause short circuiting problems within the modular integrated apparatus  10  as well as other portions of the computer system. Additionally, the center load screw  90  may be treated with a dry film lubricant to minimize torque and friction at the interface of the center load screw  90  and the center hole  88 . Additionally, the center load screw  90  may be treated with a liquid lubricant, preferably having high dielectric properties, in addition to or instead of a dry film lubricant to further minimize torque and friction and reduce metal debris. Further, the seat  75  is hardened in order to prevent torque, friction and metal debris as the center load screw  90  bears down on the seat  75 . The seat  75  may, for example, be comprised of stainless steel. The diameters of the counterbore  74  and the center load screw  90  may be sized to capture within the counterbore  74  any metal debris generated as the center load screw  90  bears down on the seat  75 . 
   The center load screw  90  may be designed so that a predetermined number of turns of the head  92  of the center load screw  90  results in a desired compressive load. An operator may count the number of turns performed on the center load screw  90 . The operator may count the number of turns manually or use a tool that assists in counting the number of turns. Alternately, a tool for turning the center load screw  90  may be programmed to automatically turn the center load screw  90  the predetermined number of turns to achieve the desired compressive load. This feature enables greater control over the load range to be applied to the processor stack  55  by eliminating stack tolerances of the components of the modular integrated apparatus  10 . Load range reduction is important because excessive loading may result in deformation of the processor stack  55 , the circuit board  30  and the bolster plate assembly  44 . The opposing faces of the processor  20  and the circuit board  30  may not be parallel as a result of this deformation, thereby impairing socket and electrical function. Inadequate loading may result in an unreliable electrical connection between the LGA socket  52  and the circuit board  30 . 
     FIG. 4  shows a flowchart  200  illustrating a method for applying compressive load in land grid array applications. In step  210 , the receiving apparatus  40  is set on the bottom surface of the circuit board  30 . In step  220 , the modular processor apparatus  50  is set on the top surface of the circuit board  30  over the receiving apparatus  40 . In step  230 , the load studs  60  are inserted through the frame assembly  54  and the circuit board  30  and into the sockets  46  of the bolster plate assembly  44 . In step  240 , the heat sink  70  is set over the load studs  60 . In step  250 , the load plate assembly  80  is set on top of the heat sink  70 , wherein the load plate assembly  80  is shuttled onto the load studs  60  through the holes  82  in the load plate assembly  80 . In step  260 , the center load screw  90  is inserted through the center hole  88  in the load plate assembly  80 . In step  270 , the center load screw  90  is turned, wherein the tip  91  of the center load screw  90  impacts the seat  75 . As the center load screw  90  is turned further, the center of the convex compression plates  86  of the load plate assembly  80  is forced upward by the threaded interface of the center load screw  90  and the center hole  88 . The outer edges of the compression plates  86  are also forced upward under the head  61  of each load stud  60 , resulting in a tensile load applied to the load studs  60 , which are threaded into the sockets  46  of the bolster plate assembly  44 . The bolster plate assembly  44  is pulled upward opposing the force of the center load screw  90  on the seat  75  on the heat sink  70 . The result is that the processor stack  55  is compressed between the bolster plate assembly  44  and the heat sink  70 . The compressive load ensures reliable electrical contact of the LGA socket  52  between the processor  20  and the circuit board  30 . 
   The use of the load plate assembly  80  in conjunction with the center load screw  90  to apply compressive load to the modular processor apparatus  50  enables an easy to use, high load implementation made up of fewer parts and resulting in lower operating costs than prior art devices. Compressive loads of more than  800  pounds may be achieved using a plurality of compression plates  86  in the load plate assembly  80 . Additionally, a more compact footprint is created without significant intrusion into the thermal capacity of the heat sink  70  due to loss of fin area and air flow blockage. Further, the compressive load is applied in a single step without creating transverse loads that may interfere with the proper alignment or loading of the processor  20  to the LGA socket  52 . Further, an evenly applied compressive load prevents damage to the LGA socket  52  and the processor  20 , which may occur in LGA assemblies comprising coil spring assemblies. In such assemblies, damage to socket contacts or the processor may occur when the manufacturing process involved with the LGA assembly is not rigorously controlled. For example, one corner of a heat sink in the LGA assembly may be deflected excessively when one of the coil springs is tightened with too much force. High loads may be created when the opposite corner of the heat sink is deflected downward by tightening a corresponding coil spring in the LGA assembly. The high loads may cause the heat sink to pivot and damage the socket and/or the processor. Some sockets and processors that are made of a ceramic material, or are otherwise brittle, are especially vulnerable to this problem.