Abstract:
A heat transfer assembly includes a printed circuit board assembly supporting an electronic component assembly including one or more semiconductor chips. A heat sink assembly is adapted to be placed in thermal engagement with the one or more semiconductor chips. Included is a loading assembly for loading the one or more semiconductor chips toward engagement with the heat sink assembly. An encapsulating mechanism is provided that contains a sufficient amount of a thermally conductive medium to transfer heat between a surface of one or more of the semiconductor chips and the heat sink assembly, wherein the thermally conductive medium fills any gaps or space between the one or more semiconductor chips and the heat sink assembly.

Description:
BACKGROUND OF THE INVENTION 
   The present invention relates in general to the field of heat transfer, and, in particular, to a heat transfer assembly and method that efficiently removes heat from electronic components. 
   Electronic components, such as microprocessors and integrated circuits, must operate within certain specified temperature ranges to perform efficiently. Excessive heat degrades electronic component performance, reliability, life expectancy, and can even cause failure. Heat sinks are widely used for controlling excessive heat. Typically, heat sinks are formed with fins, pins, or other similar structures to increase the surface area of the heat sink and thereby enhance heat dissipation as air passes over the heat sink. In addition, it is not uncommon for heat sinks to contain high performance structures, such as vapor chambers and/or heat pipes, to enhance further heat transfer. Heat sinks are typically formed of metals, such as copper or aluminum. More recently, graphite-based materials have been used for heat sinks because such materials offer several advantages, such as improved thermal conductivity and reduced weight. 
   Electronic components are generally packaged using electronic packages (i.e., modules) that include a module substrate to which the electronic component is electronically connected. In some cases, the module includes a cap (i.e., a capped module) which seals the electronic component within the module. In other cases, the module does not include a cap (i.e., a bare-die module) wherein the electronic component directly engages the heat sink. 
   Bare-die modules are generally preferred over capped modules from a thermal performance perspective. In the case of a capped module, a heat sink is typically attached having a thermal interface gap material existing between a bottom surface of the heat sink and a top surface of the cap, and another thermal interface gap material existing between a bottom surface of the cap and a top surface of the electronic component. In the case of a bare-die module, a heat sink has a thermal interface gap material existing between a bottom surface of the heat sink and a top surface of the electronic component. Bare-die modules typically exhibit better thermal performance than capped modules because bare-die modules eliminate two sources of thermal resistance present in capped modules, i.e., the thermal resistance of the cap and the thermal resistance of the thermal interface gap material between the cap and the electronic component. Accordingly, bare-die modules are typically used to package electronic components, such as semiconductor chips, that require high total power dissipation. 
   Heat sinks are attached to modules using a variety of attachment mechanisms, such as clamps, bolts, and other hardware. The attachment mechanism typically applies a force that maintains a thermal interface gap, i.e., the thickness of the thermal interface gap material extending between the heat sink and the module. In the case of a capped module, the cap protects the electronic component from physical damage from the applied force. In the case of a bare-die module, however, the applied force is transferred directly through the electronic component itself onto the bare-die module. Consequently, when bare-die modules are used, the attachment mechanism typically applies a compliant force to decrease stresses on the electronic component. 
     FIG. 1  illustrates an example of a prior art attachment mechanism for attaching a heat sink to a bare-die module. There is illustrated a circuit board assembly  100  that includes a printed circuit board  105 , and a bare-die module  110 . Bare-die module  110  includes a module substrate  115 , an electronic component, such as a semiconductor chip  120 , and an electronic connection  125 . Semiconductor chip  120  is electrically connected to module substrate  115 . Electronic connection  125 , which electrically connects printed circuit board  105  to module substrate  115 , may be a pin grid array (PGA), a ceramic column grid array (CCGA), a land grid array (LGA), or the like. Semiconductor chip  120  is thermally connected to a heat sink  130  through a thermal interface gap material  135 . The thermal interface gap material maybe a layer of a thermally conductive medium, such as thermal paste, grease, oil, or other high thermal conductivity material. Typically, the thermal interface gap material  135  is relatively thin so that it may effectively transfer heat away from the bare-die module  110  and toward heat sink  130 . The thickness of thermal interface gap material  135  extending between heat sink  130  and semiconductor chip  120  is referred to as the thermal interface gap. 
   Heat sink  130  is attached to bare-die module  110  using bolts  140 . Bolts  140  pass through thru-holes  131  in heat sink  130  and thru-holes  106  in printed circuit board  105  and are threaded into threaded-holes  146  in a backside bolster  145 . Typically, bolts  140  are arranged one at each corner of the electronic component  120 , or one on each side of the electronic component  120 . Bolts  140  are tightened by threading a threaded portion of bolts  140  into threaded-holes  146  in backside bolster  145 . As bolts  140  are tightened, heat sink  130  engages semiconductor chip  120  through thermal interface gap material  135 . Additional tightening of bolts  140  causes deflection (bowing) of the printed circuit board  105 , which applies a compliant force to bare-die module  110 . More particularly, printed circuit board  105  is slightly flexed in a concave-arc fashion with respect to bare-die module  110 . 
   Presently, some computer systems use multiple chip assemblies that require high and stable loading. The multiple chip assemblies have very thin thermal gaps filled with a thermal interface layer to establish thermal engagement with the heat sink. However, due to physical chip height variations, the noted deflections can lead to non-planar thermal interfaces being formed, thereby resulting in possible thermal degradation of the CPU. Moreover, the detrimental effects due to differences in chip height get more pronounced with power cycling loading. During power cycling loading, the chip heat dissipation results in a temperature gradient across the chip, thermal interface layer, and heat sink base. This temperature gradient has thermal transients around the vicinity of the chip, thereby resulting in distortion or relative movement of the heat sink base to the chip. These effects are further compounded when a bare-die solution is used. Consequently, thermal efficiency is compromised. Accordingly, continuing efforts are being made to improve thermal efficiency in such situations. 
   SUMMARY OF THE INVENTION 
   An aspect of the present invention provides an enhanced mounting system and method for enhancing the efficient transfer of heat in a manner substantially without negative effect and that overcomes many of the disadvantages of prior art arrangements. 
   In accordance with one aspect of the present invention, provision is made for a heat transfer method comprising: providing a heat sink assembly; providing a source of heat having one or more heat source elements, wherein each of one of the one or more heat source elements has space between it and the heat sink assembly when joined to the source of heat under loading; and, encapsulating the one or more heat source elements in a thermally conductive medium so that the thermally conductive medium remains in any space between the one or more heat source elements and the heat sink assembly. 
   In accordance with another aspect of the present invention, provision is made for a heat transfer assembly comprising: a printed circuit board having an electronic component assembly including one or more semiconductor chips, a heat sink assembly adapted to be placed in thermal engagement with the electronic component assembly, and a loading assembly for urging the electronic component assembly toward engagement with at least one of the semiconductor chips; and a thermal conductive fluid medium assembly that contains a sufficient amount of a thermally conductive medium to transfer heat between a surface of alone or more of the semiconductor chips and the heat sink assembly. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The preferred exemplary embodiments of aspects of the present invention will hereinafter be described in conjunction with the appended drawings, wherein like designations denote like elements. 
       FIG. 1  illustrates an example of a prior art attachment mechanism for attachment of a heat sink to a bare-die module. 
       FIG. 2  is a cross-sectional longitudinal view of a heat transfer assembly. 
       FIG. 3  is a cross-sectional view of a heat transfer assembly shown in  FIG. 2  but with portions removed for clarity of illustrating the invention. 
       FIG. 4  is a flow diagram of a method for mounting a heat sink assembly in thermal transfer arrangement with an electronic component. 
   

   DETAILED DESCRIPTION 
   Reference is now made to  FIGS. 2-4  for illustrating a heat transfer assembly  200  in accordance with the preferred embodiments of the present invention, which implement an improved process for mounting a heat sink on a heat source, such as an electronic component.  FIGS. 2-4  are intended to depict the representative major components of heat transfer assembly  200  at a high level, it being understood that individual components may have lesser or greater complexity than represented in  FIG. 2-4 , and that the number, type and configuration of such components may vary. For example, heat transfer assembly  200  may contain a different number, type and configuration of heat sources (e.g., electrical assembly components) than shown. 
   As illustrated in  FIGS. 2-3 , there is provided a heat transfer assembly  200 . The heat transfer assembly  200  comprises a heat sink assembly  210 , an electronic component assembly  215  mounted on a printed circuit board assembly  220 , an encapsulating mechanism  230 , and a frame loading assembly  240 . The heat sink assembly  210  includes a base plate  242  and a plurality of fins  244  or other similar structures to increase the surface area of the heat sink assembly and thereby enhance heat dissipation as air passes over the heat sink assembly. It is also preferable for the heat sink assembly to contain high performance structures, such as vapor chambers and/or heat pipes (not shown) to further enhance heat transfer. The heat sink base plate  242  and fins  244  may be made of a variety of suitable materials, such as aluminum and graphite. In an illustrated embodiment, the electronic component assembly  215  includes a module substrate  250 , an electronic component  254 , and an electronic connection  256 . 
   In the illustrated embodiment, the electronic component  254  is a multi-chip module (MCM) that includes for example, one or more bare-die semiconductor chips  254   a ,  254   n  (collectively  254 ). The present invention envisions other embodiments, such as use of a single-chip module (SCM). Those skilled in the art will recognize that the invention can be practiced with more than one SCM or MCM and/or other electronic components. Each semiconductor chip will have when mounted a gap  260  in-between its upper surface as viewed in the drawings and a bottom surface of the heat sink. Because of the differences in gap dimensions, it is difficult to place the right amount of thermally conductive medium therein. This is so because of chip height differences. This is compounded with a known squeezing action exerted on the thermally conductive medium in the gaps when subjected to forces of the kind envisioned. 
   The electronic connection  256  maybe any one of several known connections. Exemplary connectors may include a pin grid array (PGA), a ceramic column grid array (CCGA), a land grid array (LGA), or the like. The printed circuit board assembly  220  includes a printed circuit board  268  that carries the electronic component assembly  215 . 
   One aspect of this invention is that it encapsulates the bare-die semiconductor chips  254  in an encapsulating mechanism  230  that insures an efficient transfer of heat without degradation of the heat efficiency. The encapsulating mechanism  230  defines an enclosed reservoir  272  to house a suitable thermally conductive medium  274 . The thermally conductive medium  274  is provided in an amount that is adequate to fill any gaps  260  between top surfaces of the chip(s) and the heat sink under loading conditions to be described. This compensates for chip height variations that provide different gaps. The thermally conductive medium  274 , preferably, may be a suitable thermally conductive fluid material, such as a thermal grease  274 , gels, and pastes. The thermally conductive medium will also remain in any other space between the chips and the heat sink assembly. Accordingly, sufficient thermal grease will remain in all the gaps or any other space for that matter between the several semiconductor chips  254  and the heat sink assembly regardless of chip height variations. Therefore, having a sufficient quantity of thermal grease  274  obviates the negative effects of chip height differences. It also minimizes the thermal grease being squeezed from the gaps during loading of the heat transfer assembly by the frame loading assembly  240 . 
   The encapsulating mechanism  230  includes, preferably, a continuous and compressible sealing element  276  or gasket element  276 . The sealing element  276  can be made from any suitable elastomeric material, such as a strip of resinous material, (e.g., silicone or epoxy resins) or natural rubber. The sealing element  276  may be treated, of course, with additive materials that can be added for effecting different functions, such as electromagnetic shielding materials being added for enhancing EMC. The sealing element  276  may be applied in any known manner to peripherally encompass the semiconductor chips  254 . The sealing element  276  may be adhesively bonded at its junctions or interfaces to the heat sink assembly and the module substrate. Alternatively and preferably, the sealing element  276  could be dispensed using a suitable gasket applicator device (not shown) to a controllable predetermined height in a suitable pattern, for example rectangular, to peripherally surround or encompass the semiconductor chips  254 . In this manner, the sealing element  276  can be applied without resorting to the stocking of gaskets having preformed predetermined sizes. Because the maximum chip height is known, it is a relatively straightforward procedure to adjust the height of the sealing element  276  to be adequately higher than the semiconductor chips  254  when compressed for ensuring the retention of the thermal grease  274 . Once the sealing element  276  has been applied, the heat sink assembly can be positioned over the chips and the normal load applied to the heat transfer assembly  200  by the loading mechanism  240 . Loading the sealing element  276  during loading of the heat sink assembly will affect a fluid-tight seal between the heat sink and the module substrate, thereby retaining the thermally conductive medium  274 . While the encapsulating mechanism  230  is self-contained, it will be appreciated that the thermal grease in the reservoir  272  can be externally supplied under pressure. Another aspect of the sealing element  276  is that it includes a thermal coefficient of expansion (CTE) value that falls within the different coefficient of expansion (CTE) values of the module substrate  250  and the heat sink assembly  210 . It has been determined that this relationship tends to minimize any stresses building up on the sealing element  276  relative to the heat sink assembly when the system is heated, thereby minimizing the likelihood of the sealing element becoming dislodged or distorted relative to the heat sink. 
   The loading mechanism  240  may be similar to that described above concerning  FIG. 1 . To supplement such understanding, however, the following brief description is set forth. The heat sink base  242  is attached to the electronic component assembly  215  using bolts  280 . Bolts  280  pass through thru-holes  281  in heat sink base  242  and thru-holes  282  in printed circuit board and are threaded into threaded-holes  288  in a backside bolster  286 . Typically, bolts  280  are arranged one at each corner of the electronic component assembly. The bolts  280  are tightened by threading a threaded portion of bolts into the threaded-holes  288  in the backside bolster  286 . As the bolts  280  are tightened, the heat sink base  242  is brought to a thermally juxtaposed relationship with the semiconductor chips  254 . Additional tightening of the bolts  280  causes deflection (bowing), not shown, of the printed circuit board  268  thereby applying a compliant force to the semiconductor chips  254 . More particularly, the printed circuit board is slightly flexed in a concave-arc (not shown) with respect to semiconductor chips when loaded so as to set the thermal gaps  260  in a thermal transfer relationship to the heat sink. 
     FIG. 4  is a flow diagram of a method  400  for mounting a heat sink in thermal contact with an electronic component according to a preferred embodiment of the present invention. Method  400  sets forth a preferred order of the steps. It must be understood, however, that the various steps may occur at any time relative to one another. The electronic component assembly  215  is soldered to the printed circuit board  268  (block  410 ). The frame loading assembly  240  is attached to the printed circuit board (block  430 ) and the heat sink assembly. The encapsulating mechanism  230  has its sealing element  276  applied to the module substrate  250  to a height sufficient to provide the thermal medium reservoir (block  440 ). The thermal grease  274  is loaded into the reservoir  272  (block  450 ). The sealing element  276  is cured (block  460 ) at appropriate temperatures and conditions, not forming a part of the invention, so that it will be appropriately bonded to and between the heat sink assembly and the module substrate  250  to form a fluid sealing relationship. The method  400  continues with the application of a preload force using the frame loading mechanism  240  to set the thermal interface gaps  260  (block  470 ). During loading (block  470 ), the actuation bolt  280  is turned an appropriate amount to apply a preload force (e.g.,  40  lbs) that provides the desired thermal interface gap (e.g.,  1 . 2  mils). In other words, the thermally conductive medium will remain in the gaps as the preload force is applied to set the gaps. Hence, the desired gap is filled with the thermally conductive medium. Once this point is reached, the thermally conductive medium may be thermally cured in any suitable manner. 
   The embodiments and examples set forth herein were presented in order to explain best the present invention and its practical application and to thereby enable those skilled in the art to make and use the invention. However, those skilled in the art will recognize that the foregoing description and examples have been presented for the purposes of illustration and example only. The description as set forth is not intended to be exhaustive or to limit the invention to the precise form disclosed. Many modifications and variations are possible in light of the above teaching without departing from the spirit and scope of the following claims.