Abstract:
An apparatus and method are disclosed to provide heat pipe cooling for semiconductor chips mounted on a module. Each chip is provided its own heat pipe for cooling. A piston on the end of each heat pipe is loaded against each chip with a predetermined force without the need for counting turns of a screw or use of torque measurement tools. Each heat pipe draws heat away from the chip it is loaded against, carrying the heat to a heat sink for dissipation into the ambient. A large multichip module can be loaded against a land grid array interposer with uniform loading of the interposer. This interposer loading is accomplished independently of the loading of the heat pipes against the chips.

Description:
FIELD OF THE INVENTION 
     This invention relates to large land grid array integrated circuit packages, and in particular to removal of heat from integrated circuit chips mounted on such packages through the use of heat pipes. 
     BACKGROUND OF THE INVENTION 
     Packaging of semiconductor chips is a key contributor to the cost, performance, and reliability of any electronic system, especially electronic digital computer systems. Early transistorized computer systems, such as the IBM 1401, had individual transistors mounted in metal or plastic units with the transistor&#39;s three wires extending from the package. These wires would typically be inserted through holes in a printed wiring board (PWB) that had printed wiring on a top or bottom surface, or in some instances, on one or more internal planes in the card. A soldering operation would then electrically connect the transistor&#39;s three wires to the printed wiring on the PWB. Resistors, capacitors, diodes, and other electronic components would also be connected in the same way on the PWB to complete the desired circuitry, e.g., logic NANDs, logic NORs, latches, and so forth. The wiring length between these components reduced performance due to time of flight delays. Lengthy wires between the transistors and the PWB also introduced inductive and capacitive parasitics that degraded signal integrity. 
     By the mid 1960s, multiple transistors and resistors were placed in a single electronic package, or module. Modules were then connected physically and electrically to the PWB with conductors called pins that carried signals from the circuitry on the module to the PWB. These pins would go through holes in the PWB and solder would then make electrical and mechanical connections. These pins could be arranged along the edges of the module or could be arranged as an array of pins protruding from the bottom of the module. These pins brought power and ground connections to the circuitry on the module from the PWB and also provided paths for logic signals between the circuitry and the PWB. These pins were still very limiting in terms of providing low inductance power and ground to the circuitry and in the number of signals that could be received or driven between the circuitry on the module and the PWB. 
     Still later, direct solder attach between the module and the PWB was accomplished through “ball grid array”, “solder bump”, or “solder column” packages. Although these packages were electrically superior to the wired or pinned packages they had some limitations. One limitation was the difficulty of reworking the PWB to remove and replace a faulty module. Another limitation was the allowable width and length of the module. This limitation resulted from coefficient of thermal expansion (CTE) mismatches between the materials in the module and the materials in the PWB. The solder connections mechanically fatigued when flexed repeatedly. During a temperature change the solder connections flexed in proportion to how far they were from a central, or neutral, point on the module. Therefore, solder connections on larger modules became fatigued and electrically unreliable from normal thermal variations when the computer was turned on and off repeatedly, or even during normal periods of greater or lesser logic switching activity during normal use. Limitations of this kind of module to PWB connection restricted such packages to moderate width and length dimensions and number of rework cycles that could be performed. 
     A recent addition to packaging technology is the Land Grid Array (LGA) module. These LGA modules are socketed directly to the PWB with socket connectors, in which the electrical connection is achieved by aligning a contact array on the bottom of the module, a contact array on the top surface of the PWB, and an LGA interposer. The LGA interposer is then compressed between the module and the PWB. Upon sufficient compression, the LGA interposer provides electrical connection between the respective module and PWB array points. The advantage of LGA socket connectors include: field upgradeability; flexibility in system bring-up and diagnosis; reduced PWB assembly rework cost; reduced effects of CTE mismatch between PWB and module; improved electrical performance; and compact designs. There are various types of LGA interposers, including a version based on finely twisted wire, a technology marketed by Cinch Connectors Company. In this approach, palladium or gold plated molybdenum wire is randomly wound to form individual contacts, somewhat similar to a miniature wound-wire scouring pad. These contacts are housed in plastic molded frames. A similar “scouring pad” connector is made by Tecknit Corporation. Another type of LGA interposer is a Metallized Particle Interconnect (MPI™) technology such as is marketed by Tyco International, Ltd. MPI™ has silver-filled elastomeric contacts which are molded onto and through a carrier such as Kapton®, manufactured by the DuPont Corporation, which is staked onto a plastic frame. Yet another type of LGA interposer is the cLGA™ product from InterCon Corporation in which C-shaped clips are individually inserted into cavities molded in a plastic form. Further examples of interposer technologies can be found in the literature and US patents. 
     LGA socket assemblies are common today in the electronics industry but are typically used to attach single chip modules (SCMs) to PWBs. Relatively small multichip modules (MCM) also exist in the art. The demand for system performance is driving a requirement to place a number of chips, amounting to a large silicon area, on an MCM, while at the same time requiring a very large number of signal, ground, and power supply connections. Each electrical connection on an LGA interposer requires a small amount of force. However in today&#39;s large and very high speed systems there are enough connections that well over a half ton of force is required to compress the interposer enough to make reliable connections. Torqueing screws at the corners of the modules has accomplished conventional compression of the LGA interposer. With the increasing size of the sockets and modules, and the large forces required to compress the interposer, it has become problematic to keep the module and the card from unacceptably deforming when tightened from the corners in the conventional manner. Conventional compression pinches the edges of the module against the PWB underneath the edges with no concentrated force under the module itself, which means that the individual LGA connections towards the edge are compressed or deformed with more force than those LGA connections near the middle. Such differences in deformations can cause unreliable connections on the LGA interposer. 
     Another vital concern in the design of packaging of Very Large Scale Integrated (VLSI) circuit chips is the removal of heat that is produced from the circuitry on the chips. Chip temperature must not be allowed to exceed some maximum temperature because of reliability and performance considerations. Many designs in the past have provided paths for thermal conduction to fins for air cooling or to portions of the module that are cooled by liquids, such as water. A cooling mechanism that has recently been introduced to the field of cooling chips is the heat pipe. Heat pipes have superior heat transfer characteristics but tend to be physically large and must be pressed against the chip with a predefined force. Current use of heat pipes to cool chips involve torqueing tools or methods of counting turns of a screw to ensure proper loading of the heat pipe upon the chip. These methods of loading are error prone, time consuming, and expensive. Furthermore, prior technologies did not support large, LGA MCMs with heat pipe cooling of the chips. 
     Therefore, there is a need for an LGA socket assembly capable of holding a large MCM and compressing one of a variety of commercially available LGA interposers between the MCM bottom and the PWB top in a manner that maintains substantially uniform force across the LGA interposer. Such an LGA socket assembly further must provide for excellent heat transport away from the chips through the use of heat pipes and the heat pipes should be loaded against the chips without the requirement of counting of screw turns or torque measurement. 
     SUMMARY OF THE INVENTION 
     This invention provides an apparatus that provides for substantially uniform loading of an LGA interposer between an MCM and a PWB. The apparatus further provides a mechanism for loading of heat pipe cooling devices against semiconductor chips on the MCM without the requirement of torqueing tools or counting of turns of a screw. 
     The substantially uniform loading of the LGA interposer is accomplished by turning an actuation screw, which forces down upon the top of a module cap on the MCM. The screw is threaded through an X-shaped loading mechanism. The loading mechanism is mechanically coupled to load posts, which extend through or past the MCM cap; through or past an LGA interposer; through the PWB; through or past a backside stiffener; and are mechanically joined to a load spring. As the actuation screw is actuated, the loading mechanism is forced away from the module cap, putting tension on the load posts and transferring the force to the load spring. A load bushing situated substantially at the center of the load spring pushes against a backside stiffener at or near the center of the backside stiffener, which in turn, pushes against the PWB. This produces a slight convex deformation of the backside stiffener and the portion of the PWB the backside stiffener pushes against. The PWB in turn pushes upward, compressing the LGA interposer against the MCM. Although the actuation screw pushes down at the center of the MCM cap, the module cap supports the MCM at the edges. As the LGA interposer is pressed against the MCM, the center of the MCM is also slightly deformed in a convex manner in the same direction as the backside stiffener, the PWB, and the LGA interposer. In this manner, force can be distributed substantially evenly across the many contacts between the bottom of the MCM and the top of the LGA interposer and also between the bottom of the LGA interposer and the top of the PWB. 
     Power dissipated on the semiconductor chips must be removed from the chips. Very high-speed processor chips currently dissipate well over 100 watts. The chips must be kept below some maximum temperature for performance and reliability considerations. Current semiconductor chips used in commercial computers typically have a maximum temperature of about 100 degrees centigrade specified. Reliability of the semiconductor chips degrades rapidly beyond this temperature, forcing designers to reduce current densities in wiring on the chip. Other reliability considerations force other design limitations as temperatures rise. Complementary Metal Oxide Semiconductor (CMOS) circuits also degrade in performance as temperature rises, providing even further motivation to effectively remove heat from the chips. 
     Heat pipes are an excellent means to remove heat. This invention provides a mechanism to load a heat pipe against a chip with a predetermined amount of force without requiring use of torqueing tools or counting turns of a screw. A loading force of the heat pipe against the chip in this invention is determined by the geometries of a load collar and a piston, together with the geometry and spring constant of a spring. The load collar and the spring are coaxially and slideably placed on the heat pipe. The load collar is simply forced downwards, through a hole on the MCM cap, until a flange on the load collar comes into contact with the top of the MCM cap, which stops further travel of the load collar. The spring is compressed between a bottom of the load collar and a top of a piston affixed to a bottom end of the heat pipe, forcing a bottom of the piston against a top of the chip with a force determined by the geometries and the spring as stated above. O-rings on the inner and outer peripheries of the load collar prevent contamination from entering the volume between the top of the MCM and the MCM cap. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows an isometric view of the heat pipe/heat sink and module assembly. 
     FIG. 2 shows an exploded view of the heat pipe/heat sink and module assembly. 
     FIG. 3 shows a cutaway view showing the heat pipe/heat sink and module assembly. 
     FIG. 4 shows an isometric view showing details of the loading mechanism and actuation screw. 
     FIG. 5 shows an exploded partial cutaway view showing the module, chip, elastomeric window frame, module cap, piston, heat pipe, spring, and load collar. 
     FIG. 6 shows the elements of FIG. 5, but with the spring under compression and the piston in contact with the chip. 
     FIG. 7 shows a cross section of the load collar, showing the groove that retains the inner O-ring. 
     FIG. 8 shows a cross section of the module cap showing the load plate for heat pipe in a relaxed position and the load collar not fully engaged in its hole in the module cap. 
     FIG. 9 shows a cross section of the module cap showing the load plate for heat pipe in its engaged position with the flange of the load collar pressed against the top of the module cap. 
     FIG. 10 shows a top view showing the load plate for heat pipe in a position to push down on two load collars. 
     FIG. 11 shows a VLSI chip with an elastomeric window frame surrounding it. A circle shows the footprint of the piston. 
     FIG. 12 shows a cross section of the module, the LGA interposer, and the card to illustrate where forces occur as the actuation screw is turned. 
     FIG. 13 shows two embodiments of the load posts. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Having reference now to the figures, the invention will be described in detail. FIG. 1 illustrates an isometric view  100  of a heat pipe/heat sink LGA assembly. This shows the general grouping of major module components in their finally assembled positions, but without the PWB and LGA interposer, which will be shown later. A heat pipe  101  is a hollow pipe, with sealed ends, and is made of copper, aluminum, or other suitable material with the mechanical strength and heat conductivity for the application. Heat pipe  101  is partially filled with a liquid such as deionized water. A partial vacuum lowers the boiling point of the liquid. 
     A heat sink  102  is attached to each heat pipe  101  by adhesive attach, brazing, press fitting, shrink fitting, or equivalent means. Heat sink  102  is made of aluminum, copper, or other appropriately thermally conductive material and usually has fins (not shown) to enhance the transfer of heat from the fin to the surrounding air. Alternatively, heat sink  102  could be a liquid cooled heat sink wherein heat is removed as liquid is pumped or convectively flows through pipes in the heat sink. Water is often the liquid used in liquid cooled heat sinks. 
     FIG. 1 shows four heat pipes  101 , each with a heat sink  102  attached. A module cap  105  provides mechanical support for the module which is shown in a later figure. Module cap  105  provides for distribution of forces, also discussed later, and cylindrical holes through which heat pipes  101  enter. Load spring  104  distributes force to the center of a backside stiffener  103  as will be described later. 
     FIG. 2 shows an exploded view of the heat pipe/heat sink LGA assembly, generally shown as  200 . 
     A heat pipe heat sink assembly showing four heat pipes and their attached heat sinks is denoted as  201 . A loading mechanism  202  is an X-shaped element, which will transfer force to a set of load posts  205 . Four load posts  205  are normally used; two are shown for clarity in FIG.  2 . Interposer retention clips  203  loosely hold an LGA interposer  206  against the bottom of the module. Load bushing  204  transfers force from load spring  104  to backside stiffener  103 . A heat pipe load plate  207  presses heat pipe  101  against a semiconductor chip (not shown in FIG.  2 ). 
     FIG. 3 shows a cutaway side view of the heat pipe/heat sink LGA assembly. Heat pipes  101  and Heat sinks  102  are shown, with a load plate for heat pipe  207  pressed against a load collar  302 , thereby compressing springs  303  against pistons  301  in a manner that will be shown in more detail in FIGS. 5,  6 ,  7 ,  8 , and  9 . 
     FIG. 4 shows an isometric view of the assembly without heat pipes  101  and heat sinks  102 . FIG. 4 shows an actuation screw  401 , which is turned to actuate compressive force on the LGA as will be explained later. 
     FIG. 5 shows an exploded view of the heat transfer elements, generally denoted  500 , detailing how piston  301  is loaded against a semiconductor chip  503 . Piston  301  must be forced downward against chip  503 , providing a thermal path to heat pipe  101 . The amount of force must be large enough to ensure a good thermal path, but not so large that either chip  503  is damaged, or module  504  is separated from module cap  105 . A bottom surface of piston  301  preferably has a spherical crown of a suitably large radius. This slight curvature allows for slight tilting of heat pipe  101  or mechanical tolerances in piston  301  or tolerances of the attachment of chip  503  to a module  504 . Without the slight curvature, such tilting or tolerances could cause contact between piston  301  and chip  503  to occur at an edge or corner of the chip, leaving much of chip  503  an unacceptable distance away from the center of piston  301 . A thin layer of indium a few microns thick can be used to spread the contact area and provide a good heat transfer path. Various types of thermal “grease” or “paste” can be used to enhance the thermal interface between the bottom of piston  301  and a top of chip  503 . The piston/chip interface could also be thermally enhanced with a thermal pad or solder alloy. Any thermal enhancement that can be used between the top of chip  503  and the bottom of piston  301  is contemplated by this invention, and the examples given are illustrative only, and not limiting. 
     During assembly, piston  301  and the lower part of heat pipe  101  are inserted through a hole in module cap  105 . It is possible during this operation that heat pipe  101  can be dropped. An elastomeric window frame  505  is placed around chip  503  to prevent damage of chip  503  if heat pipe  101  is dropped. The height of elastomeric window frame  505  is slightly higher than the height of chip  503  as mounted on module  504 . Elastomeric window frame  505  serves as a cushion, to prevent damage to chip  503 , if heat pipe  101  is dropped. Any suitable elastomeric or cushioning material is contemplated by this invention, but an exemplary material would include polyurethane foam. One commercially available polyurethane foam is Poron® from Rogers Corporation. FIG. 11 shows the top view of chip  503 , elastomeric window frame  505  around it and the footprint of piston  301 . The relative dimensions are not important as long as the elastomeric window frame  505  can protect chip  503  in the event that heat pipe  101  is dropped. Elastomeric window frame  505  can be eliminated if the assembler chooses to accept the risk of damaging chip  503  or has effective means to prevent dropping heat pipe  101 . 
     Spring  303  is any type of spring that can be made stiff enough to load appropriate force downward on the piston  301  against elastomeric window frame  505  and chip  503  for the amount of downward travel load collar  302  is designed to move. Spring  303  is advantageously a wave spring but could be a coil spring, an elastometer, or other suitable spring material. 
     Load collar  302  is designed so that it is free to slide on the lower portion of heat pipe  101 . A load collar flange  501  is larger than the hole in module cap  105  that piston  301  and the lower portion of heat pipe  101  are inserted through. Load collar  302 , when pushed downward as shown in FIG. 6, is stopped from further travel when load collar flange  501  contacts the top of module cap  105 . The height of piston  301  and the distance from the bottom of load collar  302  to the bottom of load collar flange  501  and the height and stiffness of spring  303  are designed to exert a predetermined downward force from piston  301  to chip  503 . The amount of force required depends on the size of chip  503 ; the manner by which chip  503  is joined to module  504 ; and the nature of the mechanical interface between chip  503  and piston  301 . 
     Contaminants must be kept from entering the volume between module cap  105  and module  504 . Conventional means are used to join module  504  to module cap  105 . One such joining product is Sylgard® from Dow Corning Corporation. This joining process also seals that path from contamination. Contaminants must also be prevented from entering the volume between module  504  and module cap  105  through the hole in module cap  105  that piston  301  is inserted through. This is accomplished by putting a groove  701  as shown in FIG.  7  and an O-ring  502  as shown in FIG. 5 around an outer periphery of load collar  302  below load collar flange  501 , thus sealing the gap between the load collar  302  and the hole in module cap  105 . There is another gap between heat pipe  101  and load collar  302 . This gap is sealed as shown in FIG. 7 by putting groove  702  around an inner periphery of load collar  302 , which retains inner O-ring  703 . 
     FIG. 6 shows the components described in FIG. 5, but with load collar flange  501  in contact with the top of the module cap. Spring  303  is compressed and piston  301  has compressed the elastomeric window frame  502  and bears against chip  503 . FIG. 6 shows a gap between piston  301  and chip  503 , but this is only to distinctly show chip  503  and piston  301 , which physically bears against chip  503  when load collar flange  501  is in contact with the top of the module cap. In this position, the thermal path from chip  503  to piston  301  to heat pipe  101  is complete. Heat is then transferred from the heat pipe  101  to heat sink  102  (shown in FIG.  1 ), where the heat is removed by convection, blowing air over the heat sink, or by running water or other liquid through the heat sink to carry away the heat. 
     FIGS. 8,  9 , and  10  show one exemplary means for pushing down on load collar  302  until load collar flange  501  contacts the top of module cap  105 . As load collar  302  is pushed downward it transfers force through spring  303  to piston  301 , and loads piston  301  against chip  503 . 
     FIG. 10 shows a top view of the preferred embodiment wherein a load plate for heat pipe  207  is used to push down on the tops of two load collars  302 , thereby pushing two pistons against their respective chips as discussed above. Although FIG. 10 shows a single load plate  207  loading two load collars  302 , individual load plates for each load collar could be used. FIG. 8 shows load plate for heat pipe  207  in an unloaded position. As described above, until load collar flange  501  is pushed down against the top of module cap  105 , piston  301  in FIG. 5 is not pressed against chip  503 . Load plate for heat pipe  207  is forced downwards against the top of load collar flange  501  in the exemplary FIG. 8 by turning a screw  801 , pivoting load plate for heat pipe  207  around the point that load plate for heat pipe  207  contacts module cap  105 . Note that since load collar flange  501  is part of load collar  302 , a downward force can be exerted upon either flange  501  or load collar  302 . FIG. 9 shows load collar flange  501  fully pushed down against the top of the module cap  105 . Upon contact of load collar flange  501  with the top of module cap  105  screw  801  will stop turning. Since the predetermined force between chip  503  in FIG.  5  and piston  301  is determined by the geometries explained above and the spring constant of spring  303 , there is no need to count the turns on screw  801  or to require torqueing tools to properly load piston  301  against chip  503 . Any other suitable means to force load collar  302  downwards until load collar flange  501  contacts the top of module cap  105  can be used and are within the spirit and scope of the invention described and disclosed herein. 
     The above discussion has focused upon the invention&#39;s construction of the thermal path required to remove heat from chip  503 . This invention also teaches the construction of this thermal path in concert with a design that allows compression of LGA interposer  206  in a uniform fashion, putting substantially equal force on all contact areas of LGA interposer  206 , without interfering with the thermal path discussed above. The following discussion discloses how the independent loading of the LGA interposer is accomplished. 
     FIGS. 1,  2 ,  3 , and  4  seen previously show the components of the hardware in isometric views. FIG. 12 will be used together with reference to the previous figures to describe the invention&#39;s provision for proper loading of the LGA interposer while allowing the independent loading of piston  301  against chip  503  as described above. 
     FIG. 12 shows a cutaway view of the assembly without the thermal components described above which load piston  301  against chip  503 . Actuation screw  401  is turned, forcing loading mechanism  202  away from module cap  105  by pushing against the center of the top of module cap  105 . As loading mechanism  202  is forced upward, the load posts  205  are also forced upwards because load post flanges  1201  are mechanically engaged with loading mechanism  202 . Load posts  205  slideably go through the module cap; through or past LGA Interposer  206 ; slideably through a PWB  1202 ; and slideably through backside stiffener  103 . Load posts  205  are then mechanically engaged with load spring  104 . Load posts  205  thus pull upwards on load spring  104  at the points of mechanical engagement. Load bushing  204  transfers the force from load posts  205  to the center area of the backside stiffener  103 . Backside stiffener  103  pushes upwards against a bottom of PWB  1202 . Note that in some applications wherein wiring is done on the bottom of the PWB  1202  or for added insulation for the PWB, a nonconductive layer of plastic or other suitable material is placed between backside stiffener  103  and PWB  1202 . This nonconductive layer is not shown in FIG.  12 . It is important in large MCMs to bend module  504  and PWB  1202  with similar convexity, in order to uniformly load LGA interposer  206 , as described earlier. In the current invention, this is accomplished by pushing upwards with load bushing  204  through backside stiffener  103 . This will produce an upwardly convex deflection on PWB  1202 . Module  504  is attached to module cap  105  on its top edges as also shown in FIG.  12 . Since the module  504  is being pushed upward near its center through the force paths described above, deflection of the module  504  will also be convex upward, satisfying the need for similar convexity. 
     Although load posts  205  are illustrated as simple cylinders with flanges at the ends, the invention is not so limited. A preferred embodiment has the entire length of load post  205  at the same width as load post flange  1201  except for the small parts of the length that engage loading mechanism  202  and load spring  104 . FIG. 13 shows such a load post  205 ′ as a cylindrical rod with a narrowed section  1203  which will mechanically engage loading mechanism  202 , and another narrowed section  1204  which will mechanically engage load spring  104 . An advantage of such a design is that there is less elongation of the load posts as the actuation screw is turned. Other variants of the load posts  205 , such as making them tapered, octagonal, and so forth are within the spirit and scope of the invention described and disclosed herein. 
     In a preferred embodiment, loading mechanism  202  is X-shaped as shown in FIGS. 2 and 4 with four arms extending from the center. Each arm has a C-shaped form at or near the end of the arm. All four C-shaped forms are facing the same rotational direction, so that, for example, if said loading mechanism is rotated clockwise, the open side of each C-shaped form faces the direction of rotation. The C-shaped forms have essentially the same inner diameter as the outer diameter of load posts  205  directly below the load post flanges. In the load post embodiment  205 ′, the C-shaped forms have essentially the same inner diameter as the outer diameter of narrowed section  1203 . Actuation screw  401  is threaded through X-shaped loading mechanism  202 . Until actuation screw  401  is tightened, X-shaped loading mechanism  202  turns freely around actuation screw  401 , allowing easy mechanical engagement of the load posts  205  by the C-shaped forms by rotating X-shaped loading mechanism  202  slightly. Once this engagement has been done, load posts  205  will be pulled as actuation screw  401  is turned. Only a small degree of rotation of X-shaped loading mechanism  202  is required to engage load posts  205 . In the large MCM, as shown in FIG. 1, X-shaped loading mechanism  202  is X-shaped to allow engaging load posts  205  without interfering with heat pipes  101 . Other means of engaging load posts  205  with loading mechanism  202  could include threading the tops of the load posts and screwing them into threaded holes on the X-shaped arms. Other modified techniques to connect the load posts  205  to the loading mechanism  202  are considered to be within the spirit and scope of the current invention. 
     The present invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects as illustrative and not restrictive. The scope of the invention is, therefore, indicated by the appended claims rather than by the foregoing description. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope.