FIELD OF THE INVENTION 
     This invention relates to dissipating heat generated by electrical components, and more particularly, to an efficient, cost-effective, and easy-to-manufacture apparatus for transferring heat from a circuit board having a multilevel surface. 
     BACKGROUND OF THE INVENTION 
     As the desire for more intensive electronic applications increases, so does the demand for electrical systems that operate at faster speeds, occupy less space, and provide more functionality. To meet these demands, manufacturers design modules containing numerous components with different package types, such as integrated circuits (ICs), multi-chip modules (MCMs), hybrids, and the like, residing in relatively close proximity on a common substrate, for example, a circuit board. Certain components residing on the circuit board, such as a central processing unit (CPU) or processor, generate large amounts of heat which must be dissipated by some means. 
     Generally, heat is dissipated by transferring the heat to a heat-sinking medium such as air or water. Due to the expense and complexity associated with liquid media and, in many cases, the non-availability of such media, it is desirable to use air as a sinking medium. Heat-transfer from the heat source to the surrounding air is accomplished via direct contact between a component and the surrounding atmosphere, passive thermal transfer schemes (e.g., heat pipes), or active liquid cooling systems (e.g., a closed loop circulating cooling system) or a combination of these schemes. In the case of direct contact, heat transfer is generally enhanced by placing a thermally conductive heat sink with protruding fins in contact with an area of high heat flux, such as the upper surface of a component&#39;s package or the component&#39;s “face.” The heat sink fins greatly increase the heat transfer area to the surrounding atmosphere and reduce the thermal resistance between the heat source and heat sink. Typically, the surrounding air circulates over the heat sink fins by convection; however, in order to further enhance the heat transfer to the surrounding atmosphere, a fan may be used to mechanically move air over the heat sink fins. 
     In order to enhance the transfer of heat within the heat sink itself, some heatsinks enclose a heat pipe, while others attach to a separate housing which encloses a heat pipe. Such an enclosed heat pipe provides a thermally efficient conduit for transferring heat from small areas of high heat generation uniformly throughout the heat sink, creating a nearly isothermal surface on the heat sink. 
     In the prior art, an individual heat sink is typically adhesively bonded to (e.g., with a thermosetting, conducting epoxy) and/or mounted adjacent the face of a single heat-generating component with fastening devices (e.g., clips, retaining rings, press fits, and the like). For circuit boards having a reasonable number of components, with ample component-to-component spacing, the prior art use of individual heatsinks and fastening devices is usually effective for transferring heat away from the critical components of a circuit board. 
     As the complexity of a circuit board increases, however, the number and type of components are likely to increase, while the allotted space between components is likely to decrease. These two factors result in a densely populated, complex circuit board. These boards also have a multilevel surface due to the various heights of the numerous components, surface anomalies and fabrication tolerances such as inconsistencies resulting from solder ball attachments. Since many, and possibly even all, of the components on a circuit board require cooling, the high component density and multi-level surface coupled with the requirement that each heat sink be in intimate contact with its associated component results in a board with numerous individual closely spaced, multilevel heatsinks. 
     Furthermore, for dissipating heat generated by high power components, such as the next generation, SUN UltraSPARC® family of processors (in particular, those used in the next generation workgroup server), the size of a heat sink must be relatively large, often ten times the size and weight of the actual component to which it is attached. (UltraSPARC is a registered trademark of SPARC International, Inc. and is licensed by Sun Microsystems, Inc.) This size requirement may be difficult to meet in a densely populated board and the large and heavy heat sinks expose the attached component to shock and vibration problems during handling and shipping, especially with surface mount components (i.e., components electrically connected to a circuit board via solder balls, or the like). Consequently, the clutter of heatsinks, fastening mechanisms, and adhesives often results in a board with inadequate cooling means and unreliable electrical connections. Manufacturing, troubleshooting, and reworking such a board is difficult, and in some cases, practically even impossible. 
     Therefore, there is a need for an efficient and cost-effective heat sink apparatus that accommodates a circuit board having a multilevel surface with high power components in close proximity. It is also desirable that the apparatus ensure high structural integrity and reliable electrical connections for a heavy complex assembly. Further, the apparatus should simplify manufacturing, rework, and troubleshooting and use conventional cooling devices (e.g., tube axial fans and heatsinks). Finally, it is desirable that the apparatus allow for reuse of the heat sink, circuit board, and components thereon after rework and troubleshooting. 
     SUMMARY OF THE INVENTION 
     The present invention teaches an apparatus that is effective in transferring/dissipating heat from a substrate, particularly a heavy complex circuit board having a multilevel surface, which typically results from a plurality of electrical components thereon having different heights. Unlike the prior art, however, the invention offers an efficient solution that substantially reduces the number of individual heatsinks and fastening devices, yet still provides adequate heat spreading/dissipation by using a single heat sink. This results in an apparatus that is both readily attachable to and readily detachable from a circuit board, and thus, facilitates manufacturing, rework, and troubleshooting, and allows for component reuse after rework. 
     In accordance with the principles of the invention, heat is dissipated from a circuit board having a multilevel surface by transferring the heat to a single heat dissipating member (e.g., a heat sink device) via a phase change material and/or a resilient thermal-conductive filling material. A fastener (e.g., a combination of spring-loaded screws) secures the phase change material and/or the filling material between the bottom portion of the dissipating member and the multilevel surface, and compresses the phase change material and/or the filling material therein, creating a thermal path sufficient to transfer heat from the multilevel surface to the dissipating member so that the board operates within specified design parameters. 
     In another embodiment of the invention, a plurality of heat dissipating members, substantially less than the number of components on the circuit board, is used to dissipate heat from all of the components. Thus, heat generated by a portion of the board (i.e., a cluster of components) may be provided with a thermal path to a single heat-dissipating member. 
     In yet another embodiment of the invention, the heat-dissipating member encloses a heat pipe, or alternatively, is attached to a housing that encloses a heat pipe. The heat pipe aids in spreading/transferring heat from the smaller areas of high heat flux, typically a region on a circuit board where high power electrical components reside. 
     In yet another embodiment of the invention, the circuit board makes up a portion of a heavy/complex module (e.g., the next generation SUN UltraSPARC® workgroup server processor module). In order to provide support and structural integrity, a rigid stiffener plate is configured to receive the circuit board. The fastener secures the phase change material and/or the filling material and the circuit board between the top inlet side of the stiffener plate and the bottom portion of the single heat-dissipating member. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and further advantages of the invention may be better understood by referring to the following description in conjunction with the accompanying drawings in which: 
     FIG. 1 is a partially cut away side view of an embodiment of the present invention, using a single heat sink wherein gap filling material is utilized to provide a thermal path for dissipating heat. 
     FIG. 2 is a partially cut away side view of an embodiment of the present invention, wherein both gap filling material and phase change material are utilized to provide a thermal path for dissipating heat. 
     FIG. 3 is an exploded view of an embodiment the invention as implemented into a complex heavy module, for example, the next generation SUN UltraSPARC® workgroup server processor module. 
     FIG. 4 is an expanded view of one of the plurality of spring-loaded screws of FIGS.  1 - 3 . 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 illustrates a partially cut away side view of an embodiment of the present invention. In order to emphasize selected portions of the drawing, it is not drawn to scale. A circuit board  50  is populated with a plurality of components  10 - 1  to  10 - 7 , many of which dissipate high power and in close proximity to each other. The term “component” includes the various electronic devices that are familiar to those skilled in the art, such as integrated circuits (ICs), multi-chip modules (MCMs), hybrids, and the like. Each of the plurality of components  10 - 1  to  10 - 7  includes a package, such as a surface mount package, DIP package, or other package type that is familiar to those skilled in the art, which encloses, protects, and provides electrical connections to the components  10 - 1  to  10 - 7  (e.g., through pins or solder balls). Since each package is different, each component  10 - 1  to  10 - 7  varies in height, resulting in the circuit board  50  having a multilevel upper surface composed of the different planar levels of the plurality of component “faces”  20 - 1  to  20 - 7  (i.e., the upper surface of each component&#39;s package). It should be noted, however, that height variations also result due to fabrication tolerances. Thus, identical components, having identical packages, may in fact have slightly different vertical heights due to fabrication tolerances. 
     The plurality of components  10 - 1  to  10 - 7  generate excess heat requiring heat dissipation in order for the circuit board  50  to operate properly (i.e., within design parameters). Because of the number, close proximity, and different heights of the plurality of components  10 - 1  to  10 - 7 , prior art methods of attaching individual heatsinks to each of the faces  20 - 1  to  20 - 7  of the components  10 - 1  to  10 - 7  are impracticable. Therefore, a single heat sink  30  is used to dissipate heat generated by the plurality of components  10 - 1  to  10 - 7 , and a thermal path is formed to transfer the heat from the plurality of components  10 - 1  to  10 - 7  to the heat sink  30 . Heat sink  30  may be comprised of aluminum, copper, beryllium, white metal or any other suitable material with high heat conductivity. 
     In order to provide an adequate thermal path between the plurality of component faces  20 - 1  to  20 - 7  and the single heat sink  30 , a resilient thermal-conductive filling material  40 , having a predetermined compressibility rate, is sandwiched between the multilevel surface  5  and the single heat sink  30 . Thus, the faces  20 - 1  to  20 - 7  of the components  10 - 1  to  10 - 7  are covered with filling material  40 . Preferably, the shape and area of the filling material  40  substantially coincides with the shape and area of the plurality of component faces  20 - 1  to  20 - 7 . It will be appreciated that the choice of the resilient thermal-conductive filling material  40  enables an efficient thermal, as well as compliant, interface between the planar bottom portion  31  of the heat sink  30  and the plurality of faces  20 - 1  to  20 - 7 . 
     Screws  60 - 1  and  60 - 2  run through bores in the heat sink  30  and are threaded into threaded studs extending from bolster plates  65 - 1  and  65 - 2 . Those skilled in the art will appreciate the additional support provided by the bolster plates  65 - 1  and  65 - 2 , and the way in which they securely attach to the circuit board  50  so that the filling material  40  is secured between the substantially planar bottom portion  31  of the heat sink  30  and the multilevel surface of the circuit board  50 . Screws  60 - 1  and  60 - 2  may be loaded with springs  61 - 1  and  61 - 2  as shown or with elastomeric sleeves or other suitable loading devices. Springs  61 - 1  and  61 - 2  have a spring constant that is sufficient to compress the resilient filling material  40  so that it is in intimate contact with the plurality of component faces  20 - 1  to  20 - 7 . This provides a thermal path that is sufficient to transfer heat from the plurality of faces  20 - 1  to  20 - 7  to the heat sink  30  so that the circuit board  50  operates within predetermined design parameters. 
     Moreover, if necessary, the loading devices  61 - 1  and  61 - 2  may be configured to have a spring constant that is sufficient to secure one or more of the plurality of components  10 - 1  to  10 - 7  to electrical contacts on the circuit board  50  (e.g., a socket, a cluster of pads, or the like). This may be necessary to provide adequate electrical connectivity for one or more of the plurality of components  10 - 1  to  10 - 7  so that the circuit board  50  operates within predetermined design parameters. 
     FIG. 2 illustrates a side view, also not drawn to scale, of an embodiment of the present invention which is similar to the embodiment illustrated by FIG. 1 except that a thermal phase change material  100  is employed for transferring heat away from the high power central processing units (CPUs)  10 - 6  and  10 - 7 . Since the CPUs  10 - 6  and  10 - 7  generate the most heat during operation (i.e., among the plurality of components  10 - 1  to  10 - 7 ), the thermal phase change material  100 , as opposed to the filling material  40 , is inserted between the faces  20 - 6  and  20 - 7  of the CPUs  10 - 6  and  10 - 7  and heat sink  30 . The thermal phase change material  100  provides a better thermal path for the high power CPUs  10 - 6  and  10 - 7 , and thus, increases the flow of heat at this critical interface. 
     In a preferred embodiment, the phase change material  100  is FSF- 52  from ORCUS Inc., and it is sandwiched between the faces  20 - 6  and  20 - 7  of the CPUs  10 - 6  and  10 - 7  and the bottom of the heat sink  30 . The phase change material  100  is solid until it reaches a temperature of about 52° C., at which point, it melts, flows, and fills-in any microscopic voids on both the bottom portion  31  of the heat sink  30  and the faces  20 - 6  and  20 - 7  of the CPUs  10 - 6  and  10 - 7 , thereby creating an intimate thermal path for transferring heat away from the CPUs  10 - 6  and  10 - 7 . 
     FIG. 2 also shows a vapor chamber  70  (i.e., a type of heat pipe) that has been inserted into a machined recess  32  in heat sink  30 . The internal construction of the vapor chamber  70  is conventional and will not be described further. The vapor chamber  70  may also be affixed to the bottom of heat sink  30 . The vapor chamber  70  serves to spread the heat generated by components  10 - 1  to  10 - 7  evenly across the heat sink  30 . For heavy modules, a stiffener plate  80  is shown which reinforces the circuit board  50  over its entire area. Screws  60 - 1  and  60 - 2  now screw into bores  66 - 1  and  66 - 2  formed in the stiffener plate  80 . 
     Now referring to FIG. 3, the invention is depicted as part of a complex heavy module, for example, the next generation SUN UltraSPARC® workgroup server processor module  150  (not drawn to scale, nor in its entirety). This very large and complex module  150  illustratively measures 20″L by 9″W by 3″H, and weighs approximately 14 pounds when it is fully configured. To provide rigid support and flatness to the overall module  150 , a die-cast stiffener plate  80  is configured to receive the circuit board  50 . The stiffener plate  80 , which may be made of aluminum or other suitable stiff material, also houses the inject and eject levers  90 - 1  and  90 - 2 , allowing for the module  150  to be guided on rails within a system enclosure (not shown). 
     Adding to the complexity of the module  150  is the multitude of on-board components  10 - 1  to  10 -n, two of which are high power, next generation SUN UltraSPARC® processors  10 - 6  and  10 - 7 , which need to be sufficiently cooled. Because the components  10 - 1  to  10 -n are not of uniform height, the use of conventional heat sinks would result in attaching an individual heat sink to each component and/or attaching heat sinks to closely mounted chips of the same height. For high volume assembly, this would result in an undue burden during assembly and/or rework of the module. To overcome this problem, two heatsinks  30 - 1  and  30 - 2  are used to dissipate heat generated by the plurality of components  10 - 1  to  10 -n, and a thermal path is formed to transfer the heat from the plurality of component faces  20 - 1  to  20 -n to the heatsinks  30 - 1  and  30 - 2 . 
     The circuit board  50  sits on top of the die-cast aluminum stiffener plate  80 , and a filling material  40  and phase change material  100  are inserted between the circuit board  50  and the substantially planar bottom portions  31 - 1  and  31 - 2  of the heatsinks  30 - 1  and  30 - 2 . The filling material  40  covers the faces  20 - 1  to  20 -n of all of the components except for the processors  10 - 6  and  10 - 7 . The phase change material  100  covers the faces  20 - 6  and  20 - 7  of the processors  10 - 6  and  10 - 7  and creates an intimate thermal path from their faces  20 - 6  and  20 - 7  to the bottom portions  31 - 1  and  31 - 2  of the heatsinks  30 - 1  and  30 - 2 . 
     The presence of the filling material  40  and the phase change material  100  compensates for dimensional variations in the heights of all of the components  10 - 1  to  10 -n, and reduces mechanical vibrations that might otherwise be more directly transmitted to the plurality of component faces  20 - 1  to  20 -n. This vibration dampening reduces the magnitude of forces that might otherwise tend to break electrical bonds between the contacts (not shown) on the circuit board  50  and the contacts (not shown) on the plurality of components  10 - 1  to  10 -n. Contacts on the circuit board include solder pads, sockets, elastomeric frames, and the like known to those reasonably skilled in the art. Contacts on the plurality of components include pin leads, LGAs, BGAs, elastomeric columns, and the like which known to those reasonably skilled in the art. 
     It will be appreciated that the choice of the resilient thermal-conductive filling material  40  and the phase change material  100  enables an efficient thermal interface between the substantially planar bottom portions  31 - 1  and  31 - 2  of the heatsinks  30 - 1  and  30 - 2  and the plurality of faces  20 - 1  to  20 -n of the components  10 - 1  to  10 -n. Preferably, the filling material  40  is made from thermally conductive silicon based elastomer, such as Tflex material from Thermagon Inc. The thickness and compressibility of the filling material  40  is determined by the size of a particular module. For the module  150  illustrated in FIG. 3, the preferred thickness is between 0.090″ and 0.100″ and the preferred compressibility is 15%. Other thicknesses and compressibility ratios could also be used. 
     In the preferred embodiment, the phase change material  100  is FSF- 52  from ORCUS Inc., and it is sandwiched between the faces  20 - 6  and  20 - 7  of the processors  10 - 6  and  10 - 7  and the bottom portions  31 - 1  and  31 - 2  of the heatsinks  30 - 1  and  30 - 2 . The phase change material  100  is solid until it reaches a temperature of about 52° C., at which point, it melts, flows, and fills-in any microscopic voids on both the bottom portions  31 - 1  and  31 - 2  of the heatsinks  30 - 1  and  30 - 2  and the faces  20 - 6  and  20 - 7  of the processors  10 - 6  and  10 - 7 , thereby creating an intimate thermal path for transferring heat away from the processors  10 - 6  and  10 - 7 . 
     The filling material  40  and the phase change material  100  are subjected to an amount of pressure that is a function of the dimension and tolerances of the thickness of the filling material  40  and the phase change material  100 , the plurality of component faces  20 - 1  to  20 -n, the planar bottom portions  31 - 1  and  31 - 2  of the heatsinks  30 - 1  and  30 - 2 , the relative flexibility of the circuit board, and the amount of compressive force exerted by the spring-loaded screws  60 - 1  to  60 - 10 . 
     The spring-loaded screws  60 - 1  to  60 - 10 , as depicted in greater detail in FIG. 4 (not drawn to scale), include a head  410 , a cylindrical body  420 , a threaded tip  440 , and a spring  430  coiled around the cylindrical body  420 . An alternate embodiment includes a head  450 , a cylindrical body  470 , a threaded tip  480 , and an elastomeric sleeve  460  positioned around the cylindrical body  470 . In either embodiment, it is preferred that the head  410  or  450  has a socket, such as an allen socket, a hex socket, or another socket type that is familiar to those skilled in the art. 
     Referring back to FIG. 3, when the apparatus is fully configured, two of the spring-loaded screws  60 - 9  and  60 - 10  run through pass-through bores in the heat sinks  30 - 1  and  30 - 2  and pass-through bores in the circuit board  50 . Then, these spring-loaded screws  60 - 9  to  60 - 10  screw into threaded bores  66 - 9  to  66 - 10  in the stiffener plate  80 , while the remaining spring-loaded screws  60 - 1  to  60 - 8  engage threaded holes  66 - 1  to  66 - 8  in studs  67 - 1  to  67 - 8  which extend from the bolster plates  65 - 1  and  65 - 2  up through the stiffener plate  80 , the circuit board  50 , and into bores in the heat sinks  30 - 1  and  30 - 2 . When all of the spring-loaded screws  60 - 1  to  60 - 10  are tightened, the filling material  40  and the phase change material  100  are securely sandwiched between the bottom portions  31 - 1  and  31 - 2  of the heatsinks  30 - 1  and  30 - 2  and the circuit board  50 . 
     Moreover, the spring-loaded screws  60 - 1  to  60 - 10 , when tightened, secure the processors  10 - 6  and  10 - 7  to sockets (not shown) on the circuit board  50 . This is necessary to provide adequate electrical connectivity so that the circuit board  50  operates within predetermined design parameters. 
     In the preferred embodiment, the bolster plates  65 - 1  and  65 - 2  are made of stainless steel, aluminum or other suitable material, and each one has four studs  67 - 1  to  67 - 8  defining the threaded bores  66 - 1  to  66 - 8 . In addition to securing the filling material  40  and the phase change material  100  between the bottom portions  31 - 1  and  31 - 2  of the heatsinks  30 - 1  and  30 - 2  and the circuit board  50 , the bolster plates  65 - 1  and  65 - 2  aid in capturing and securing the processors  10 - 6  and  10 - 7  to the circuit board  50 . 
     The spring-loaded screws  60 - 1  to  60 - 10  are designed such that when they are threaded into the threaded holes  66 - 1  to  66 - 10 , they help supply enough downward force to sufficiently counteract the forces being applied between the circuit board  50  and the heat sink  30  by the filling material  40  and the elastomeric contacts of the processor sockets (not shown). The amount of compressive force is important because each contact, of which in this module  150  there are thousands, needs sufficient compression for electrical conduction and performance. Additionally, the filling material  40  and the phase change material  100  need to be sufficiently compressed to fill any surface anomalies at the substantially planar bottom portions  31 - 1  and  31 - 2  of the heatsinks  30 - 1  and  30 - 2  and at the faces  20 - 1  to  20 -n of the plurality of components  10 - 1  to  10 -n. Furthermore, the compressive force enhances the thermal performance of the materials. For this module  150 , each of the spring-loaded screws  60 - 1  to  60 - 10  has a compressive force, or spring constant, of about 376 pounds/in. 
     The heatsinks  30 - 1  and  30 - 2  have aluminum bodies  70 - 1  and  70 - 2  that can be affixed to flat copper enclosed vapor chambers  75 - 1  and  75 - 2 . For the module  150  illustrated in FIG. 3, the vapor chambers  75 - 1  and  75 - 2  are manufactured by the likes of Thermacore, Inc. The use of vapor chambers  75 - 1  and  75 - 2  to increase thermal conductivity and thermal spreading has been in practice for many years. When in contact with a heat source, the vapor chambers  75 - 1  and  75 - 2  help spread and transport smaller areas of high heat flux (i.e., from the faces  20 - 1  to  20 -n of the components  10 - 1  to  10 -n), resulting in a nearly isothermic surface. By uniformly spreading out the input heat sources over a larger area, the vapor chambers  75 - 1  and  75 - 2  allow the bodies  70 - 1  and  70 - 2  of the heatsinks  30 - 1  and  30 - 2  to more efficiently conduct the heat away from the critical chips that need cooling, particularly the processors  10 - 6  and  10 - 7 . 
     For additional cooling, an air ducting shroud  200  partially encloses the module  150  and directs airflow, which is generated by tube axial fans  201  and  203 , into the module, concentrating the air flow on the critical areas. The air ducting shroud  200  also provides protection for the module  150 , while allowing easy access to the components  10 - 1  to  10 -n therein. 
     For a circuit board having numerous components that are high power and in relatively close proximity, the present invention offers the following advantages: adequate thermal conduction for multiple components having different heights and surface anomalies; a significant reduction in the number of individual heatsinks and fastening mechanisms; high structural integrity to provide adequate protection from shock, vibration, and handling; reliable electrical connections; simplified and expedited manufacturing, rework, and troubleshooting; and reuse of the heat sink(s), the circuit board, and the components after rework and troubleshooting. 
     Although an exemplary embodiment of the invention has been disclosed, it will be apparent to those skilled in the art that various changes and modifications can be made which will achieve some of the advantages of the invention without departing from the spirit and scope of the invention. It will be obvious to those reasonably skilled in the art that other devices performing the same functions may be suitably substituted. Further, aspects such as the type of fastener and/or the specific materials of the heat sink and/or the specific materials of the vapor chambers and/or the specific materials of the filling material and/or the specific materials of the phase change material and/or the specific materials of the stiffener plate and/or the number of heatsinks and fastening components, as well as other modifications to the inventive concept, are intended to be covered by the appended claims.
For a circuit board populated with numerous components having different heights, the need for numerous individual heatsinks to accommodate the circuit board's multilevel surface is eliminated. Instead, heat is transferred to a single integral heatsink via a thermal-conductive material (i.e., a thermal phase change material and/or a resilient gap filling material). A fastener secures the thermal-conductive material between the bottom portion of the heatsink and the multilevel surface, and compresses the thermal-conductive material therein, creating a thermal path sufficient to transfer heat from the multilevel surface to the heatsink so that the circuit board operates within specified design parameters.