Abstract:
A method and apparatus for an electronic package includes a substrate; a heat source component operably coupled to the substrate, and in direct contact with and electrically connected to a top surface of the substrate; a heat sink assembly in thermal communication with the substrate. The heat sink assembly includes a plurality of distinct vapor chambers, each containing a heat transfer fluid configured to evaporate on a wall in thermal contact with a back surface of the heat source component and condense on an opposing wall defining an exterior wall defining the vapor chambers. Each of the plurality of distinct vapor chambers are serially aligned having facing sidewalls defining each relative to contiguous vapor chambers and at least one of the plurality of distinct vapor chambers includes a lower sidewall defining one distinct vapor chamber substantially aligned with a bottom defining the heat source component such that a bottom portion defining the one distinct vapor chamber is substantially aligned with a bottom portion of the heat source component.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The present invention relates to dissipating heat generated by integrated circuit (IC) modules, and a method of constructing such devices. In particular, the present disclosure relates to a method and apparatus for eliminating a dry out condition of a heat transfer or cooling fluid in a vertical heat sink assembly configured to dissipate heat generated by integrated circuit modules.  
         [0002]     As is known, operating electronic devices produce heat. This heat should be removed from the devices in order to maintain device junction temperatures within desirable limits: failure to remove the heat thus produced results in increased device temperatures, potentially leading to thermal runaway conditions. Several trends in the electronics industry have combined to increase the importance of thermal management, including heat removal for electronic devices, including technologies where thermal management has traditionally been less of a concern, such as CMOS. In particular, the need for faster and more densely packed circuits has had a direct impact on the importance of thermal management. First, power dissipation, and therefore heat production, increases as the device operating frequencies increase. Second, increased operating frequencies may be possible at lower device junction temperatures. Finally, as more and more devices are packed onto a single chip, power density (Watts/cm 2 ) increases, resulting in the need to remove more power from a given size chip or module. These trends have combined to create applications where it is no longer desirable to remove the heat from modern devices solely by traditional air cooling methods, such as by using traditional air cooled heat sinks.  
         [0003]     For example, with the advent of multichip modules (MCMs), containing multiple integrated circuit (IC) chips each having many thousands of circuit elements, it has become possible to pack great numbers of electronic components together within a very small volume. As is well known, ICs generate significant amounts of heat during the course of their normal operation. Since most semiconductor or other solid state devices are sensitive to excessive temperatures, a solution to the problem of the generation of heat by IC chips in close proximity to one another in MCMs is of continuing concern to the industry.  
         [0004]     A conventional approach to cooling components in electronic systems in which devices contained in MCMs are placed on printed circuit/wire boards or cards is to direct a stream of cooling air across the modules with the addition of heat sinks attached to the module to enhance the effectiveness of the airflow.  
         [0005]     Limitation in the cooling capacity of the simple airflow/heat sink approach to cooling has led to the use of another technique, which is a more advanced approach to cooling of card-mounted MCMs. This technique utilizes heat pipe technology. Heat pipes per se are of course, well known and heat pipes in the form of vapor chambers are becoming common. In the related art, there are also teachings of heat pipes/vapor chambers for dissipating the heat generated by electronic components mounted on cards.  
         [0006]     One approach includes using a cooling fluid or heat transfer fluid in a vapor chamber heat sink. A vapor chamber base enables heat sinks to perform better as the thermal resistance to spreading the heat in the base is reduced. The heat is removed from one side of the base in thermal communication with a heat source by evaporation of the heat transfer fluid and travels rapidly in a gaseous state until it condenses on a fin side of the base. In this manner, the heat is transferred from the base to the fins for subsequent conduction to convectively cooled fins extending from the base.  
         [0007]     However, vapor chamber technology has several limitations when applied to MCMs. One limitation is that the above described heat transfer mechanism can fail if inadequate heat transfer fluid or cooling fluid is present on the evaporator surface of the base near the heat source. This is often the case when the vapor chamber is positioned vertically such that gravity causes the returning or condensed cooling fluid to accumulate at a lower area of the vertically oriented vapor chamber. For applications where the heat source is centrally located with respect to the vertically oriented heat sink, dry out conditions are often created near the heat source.  
         [0008]     For the foregoing reasons, therefore, for an efficiently cooled electronic module or MCM that employs vapor chamber cooling. In particular, there is a need in the art for a method and apparatus of providing a vertically oriented vapor chamber and corresponding heat source to be cooled with a fluid coolant, while simultaneously eliminating dry out conditions near the heat source.  
       SUMMARY OF THE INVENTION  
       [0009]     One embodiment is an electronic package includes a substrate; a heat source component operably coupled to the substrate, and in direct contact with and electrically connected to a top surface of the substrate; a heat sink assembly in thermal communication with the substrate. The heat sink assembly includes a plurality of distinct vapor chambers, each containing a heat transfer fluid configured to evaporate on a wall in thermal contact with a back surface of the heat source component and condense on an opposing wall defining an exterior wall defining the vapor chambers. Each of the plurality of distinct vapor chambers are serially aligned having facing sidewalls defining each relative to contiguous vapor chambers and at least one of the plurality of distinct vapor chambers includes a lower sidewall defining one distinct vapor chamber substantially aligned with a bottom defining the heat source component such that a bottom portion defining the one distinct vapor chamber is substantially aligned with a bottom portion of the heat source component.  
         [0010]     Another embodiment is a method for lowering a thermal resistance of a vertically oriented heat sink assembly to dissipate heat from a heat source component. The method includes configuring a heat sink assembly with a plurality of distinct vapor chambers, each of the distinct vapor chambers containing a heat transfer fluid configured to evaporate on a wall in thermal contact with a back surface of the heat source component and condense on an opposing wall defining an exterior wall of the heat sink assembly; and configuring each of the plurality of distinct vapor chambers to be serially aligned having facing sidewalls defining each relative to contiguous vapor chambers and at least one of the plurality of distinct vapor chambers includes a lower sidewall defining one distinct vapor chamber substantially aligned with a bottom defining the heat source component such that a bottom portion defining the one distinct vapor chamber is substantially aligned with a bottom portion of the heat source component. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]     Referring to the exemplary drawings wherein like elements are numbered alike in the accompanying Figures:  
         [0012]      FIG. 1  depicts a perspective view of a partially populated central electronics complex (CEC) illustrating an exposed vertically oriented MCM and a vertically oriented heat sink assembly disposed over another MCM;  
         [0013]      FIG. 2  depicts a partial exploded cross section view of the vertically oriented heat sink assembly disposed over a MCM of  FIG. 1 ;  
         [0014]      FIG. 3  depicts a partial exploded cross section view of an exemplary embodiment of a vertically oriented heat sink assembly having vapor chambers integrated into a base of a heat sink disposed over a MCM of  FIG. 1 ;  
         [0015]      FIG. 4  depicts a schematic side view of a prior art vertically oriented heat sink assembly having a single vapor chamber in thermal communication with a centrally located heat source;  
         [0016]      FIG. 5  depicts a schematic side view of a vertically oriented heat sink assembly having two distinct vapor chambers in thermal communication with the heat source of  FIG. 4  in accordance with an exemplary embodiment of the present disclosure; and  
         [0017]      FIG. 6  depicts a schematic side view of a vertically oriented heat sink assembly having two distinct vapor chambers in thermal communication with the heat source of  FIG. 5  in accordance with an alternative exemplary embodiment of the present disclosure. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0018]     The present disclosure will now be described in more detail by way of example with reference to the embodiments shown in the accompanying figures. It should be kept in mind that the following described embodiments are only presented by way of example and should not be construed as limiting the inventive concept to any particular physical configuration.  
         [0019]     Further, if used and unless otherwise stated, the terms “upper”, “lower”, “front”, “back”, “over”, “under”, and similar such terms are not to be construed as limiting the invention to a particular orientation. Instead, these terms are used only on a relative basis.  
         [0020]     For the purposes of the present disclosure, the terms printed circuit board (PCB) and printed wire board (PWB) are equivalent terms. The terms “in contact” and “contacting” indicate mechanical and thermal contact  
         [0021]      FIG. 1  illustrates a so-called central electronics complex  10  (CEC) of a computer system. The CEC  10  is comprised of an enclosure (such as a cage  12 ), a backplane or midplane  14  as illustrated, and a circuit board or daughter card, such as a blade or node  16  having two processor multi-chip modules (MCM)  17 , and a corresponding vapor chamber heat sink assembly  20  disposed over each MCM  17  (only one shown of each for sake of clarity), 256 GB memory on 16 cards (not shown), an input/output (I/O) card  18 , and a control multiplexer card (not shown), for example, attachable to the backplane  14 . Air inlets  22  are shown at a back of CEC  10  and air flow across fins of heat sink assembly  20  and out air exhausts  24  is generally shown with arrows  26 .  
         [0022]      FIG. 2  is a partial cross-sectional view of an exemplary embodiment of a multichip module (MCM) mounted on a PCB having a lid in accordance with the present disclosure. It will be recognized by one skilled in the pertinent art that a dual chip module (DCM) is also contemplated for use in the present disclosure. In  FIG. 2 , MCM  100  includes a substrate  102  having a multiplicity of components  105  mounted thereto, each component having a front surface  110  and a back surface  115 . MCM  100  is mounted to a PCB  120  by a multiplicity of solder balls  125 . Substrate  102  may be a single or multi-level substrate and may be ceramic, fiberglass or polymer based. MCM  100  also includes a lid  130 . Lid  130  is mounted to substrate  102  by lid support  132  connecting the periphery of lid  130  to the periphery of substrate  102 . Lid support  132  may be fabricated from the same material as lid  130  and may be integral with the lid. Alternatively, lid support  132  may be fabricated from a material different from that of lid  130 . Lid support  132  may provide a hermetic seal between lid  130  and substrate  102 .  
         [0023]     Lid  130  includes a lower wall  135  having an outer surface  140 , an upper wall  145  having an outer surface  150  and sidewalls  155  defining a vapor chamber  160 . It will be noted that opposing sidewalls  155  defining vapor chamber  160  are shown closer together than with respect to  FIG. 5  for sake of clarity in describing chamber  160 . Moreover,  FIG. 2  is shown with a single vapor chamber  160 , wherein two or more vapor chambers  160  are included in an exemplary embodiment depicted in  FIG. 5  in accordance with the present disclosure.  
         [0024]     Vapor chamber  160  contains a heat transfer fluid such as, inter alia, water, freon or glycol. Front sides  110  of components  105  are electrically connected to a top surface  165  of substrate  102 . Components  105  may be flip chip, wire-bonded or soldered to substrate  102 . A thermal transfer medium  170  is in contact with back surfaces  115  of components  105  and outer surface  140  of lower wall  135  of lid  130  to enable thermal contact, mechanical restraint and pressure support over the contacting region. Thermal transfer medium  170  enables heat generated by the operation of components  105  to be efficiently transferred to lid  130 .  
         [0025]     Because of the excellent heat transfer capability afforded to lid  130  by vapor chamber  160 , the lid may be fabricated from many different materials including but not limited to metals such as aluminum, copper, nickel, gold or Invar and other materials such as plastics, ceramics and composites. Because of the wide range of materials available, lid  130  may fabricated from a material having a CTE matched to (between about 25% to 700% of the coefficient of thermal expansion) substrate  102  or from the same material as the substrate. For example, if MCM  100  is a HyperBGA® International Business Machine Corp., Armonk, N.Y., in which substrate  102  is a polytetraflouroethylene (PTFE) based material having a CTE of about 10-12 ppm/° C., then lid  130  may be fabricated from an aluminum-silicon carbide composite having a CTE of about 10 ppm/° C.  
         [0026]     Thermal transfer medium  170  may include a thermal adhesive, thermal grease, thermal-conductive pads, phase change or other materials known in the art.  
         [0027]     A heat sink  180  having a plurality of horizontal fins  182  (see also  FIG. 1 ) is in thermal communication with outer surface  150  of lid  130 . Heat sink  180  is shown remove from outer surface  150  for sake of clarity. Heat sink  180  may be formed from aluminum, copper, beryllium, white metal or any other suitable material with high heat conductivity. Furthermore, it will be recognized by one skilled in the pertinent art that heat sink  180  may be fabricated from a material having a CTE matched to (between about 25% to 700%) the CTE of lid  130 . Moreover, it will be recognized that although heat sink  180  and lid  130  are shown as separable parts, heat sink  180  may be integrated with lid  130  in a single integral part.  
         [0028]     In an exemplary embodiment referring to  FIG. 3 , vapor chamber  160  is integrated in a base defining heat sink  180  while lid  130  is a solid substrate. It will be recognized by one skilled in the pertinent art that having vapor chamber  160  integrated in a base of the heat sink versus lid  130 , enables the vapor chambers to spread the heat well beyond the confines of the cap of a module (e.g., lid  130 ). In an exemplary embodiment depicted in  FIG. 3 , a width of the heat sink vapor chambers are about twice the width of lid  130 . In either case, it is noted that the vapor chamber heat sink assembly  20  of  FIG. 2  includes a combination of heat sink  180  and lid  130  or a base of heat sink  180  having vapor chamber  160  integrated therewith, as in  FIG. 3 .  
         [0029]     While MCM  100  has been illustrated in  FIGS. 1 and 2  and described above as a ball grid array (BGA) module, MCM  100  may be pin grid array (PGA) module. Instead of solder balls  125  (see  FIG. 2 ), Land Grid Array (LGA) connections between substrate  102  and PCB  120  are also contemplated. Since LGA connections are asperity contact connections, generally some degree of pressure must be maintained on the connection to ensure good electrical conductivity. Therefore, flanges  184  defining ends of heat sink  180  may be used accept a mechanical fastener and engage substrate  120  to provide the necessary pressure to ensure suitable electrical conductivity.  
         [0030]     Referring now to  FIG. 4 , a vertically oriented heat sink  180  is in thermal communication with a prior art evaporator  200  having a corresponding vertically oriented single vapor chamber  260 , which is in turn in thermal communication with a heat source  210 . Heat source  210  is centrally located with respect to a bottom surface  240  defining a length of evaporator  200 . It will be recognized that heat source  210  may be MCM  100  as described above. In many such evaporators, a heat transfer fluid generally indicated at  220  such as, inter alia, water, has a tendency to accumulate at a bottom of vapor chamber  260  generally indicated at  224  because of gravity acting on transfer fluid  220 . The result is inadequate liquid to evaporate heat load proximate heat source  210  and evaporator  200  proximate the heat source  240  dries out as the heat therefrom is conducted further towards the water accumulation at  224 , reducing thermal performance of the assembly. The thermal performance is reduced by the increase in thermal resistance due to an increase in path length for the heat to travel to heat sink  180  because of the dried out section local to the centrally located heat source  210 .  
         [0031]     Referring now to  FIG. 5 , vertically oriented heat sink  180  is illustrated in thermal communication with an exemplary embodiment of an evaporator  300  defining at least two distinct vapor chambers  360 , which is in turn in thermal communication with heat source  210 . Heat source  210  is substantially centrally located with respect to a bottom surface  340  defining a length of evaporator  300  as described with respect to heat source  210  in  FIG. 3 , however, heat source  210  may be aligned anywhere along a length defining bottom surface  340 . It will be recognized that heat source  210  may be MCM  100  as described above. Since heat transfer fluid  220  has the tendency to accumulate at a bottom of vertically oriented vapor chamber  260  in  FIG. 4 , vapor chamber  360  in  FIG. 5  includes an upper vapor chamber  361  and a lower vapor chamber  362  separated from upper chamber  361  via a horizontal barrier  370  therebetween. Barrier  370  is configured to prevent condensed heat transfer fluid  220  from being pulled by gravity past heat source  210  into vapor chamber  362 . Barrier  370  extends from bottom surface  340  to upper surface  350 , substantially normal to both.  
         [0032]     In an exemplary embodiment as illustrated, barrier  370  is a horizontal solid section separating vapor chamber  360  into two distinct chambers,  361 ,  362 . The larger upper vapor chamber  361  will have an ample supply of heat transfer fluid available near heat source  210  in spite of gravity acting thereon in vertical mount applications. Even though the lower vapor chamber  362  still works against gravity, lower vapor chamber  362  still provides some added cooling. Vapor chambers  361  and  362  together outperform a single vapor chamber in most vertical applications because adequate liquid is available and local to heat source  210  to evaporate heat load proximate heat source  210 . Heat from heat source  210  is less prone to dry out vapor chamber  361  since a bottom of vapor chamber  361  is substantially aligned with heat source  210 . In particular, a bottom of heat source  210  substantially coincides with a bottom of vapor chamber  361  where heat transfer fluid  220  would tend to accumulate due to gravity. Thus, the heat transfer path is less likely to increase because vapor chamber  361  insures that the local evaporator area is wet, thus lowering thermal resistance of heat transfer to heat sink  180 .  
         [0033]     The thermal performance is increased by lowering thermal resistance due to a decreased path length for the heat to travel to heat sink  180  because of eliminating a dried out section local to the centrally located heat source  210 .  
         [0034]     In an exemplary embodiment, upper vapor chamber  361  is configured having a bottom portion thereof substantially aligned or alternatively, not extending much past heat source  210  insuring that the evaporator area local to the heat source  210  is wet with condensed heat transfer fluid  220 . The lower vapor chamber  362  works against gravity over a region similar to that described with respect to the single vapor chamber  260  in  FIG. 3 , but still provides some added cooling. The mean path for the heat to travel from heat source  210  to find condensed heat transfer fluid is reduced by two or more separate vapor chambers defining evaporator  300 , thus lowering the thermal resistance to heat sink  180 . Although, upper vapor chamber  361  has been described as being larger, e.g., longer relative to vertical, than lower vapor chamber  362 , vapor chambers may be configured having substantially the same length or lower chamber  362  may be longer than upper vapor chamber  361 .  
         [0035]     For example,  FIG. 6  depicts lower vapor chamber  362  configured longer than upper chamber  361 . In this case, it will be noted that heat source  210  is then more efficiently disposed towards an upper portion defining a length of bottom surface  340 .  
         [0036]     Thus, an efficiently cooled IC, such as a MCM, that employs vapor chamber cooling with a plurality of separate vapor chambers in thermal communication with a vertically oriented heat sink assembly while minimizing dry out conditions and reducing thermal resistance has been described.  
         [0037]     While the invention has been described with reference to exemplary embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention is not to be limited to the particular embodiment disclosed as the best or only mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims. Moreover, the use of the terms first, second, etc. do not denote any order or importance, but rather the terms first, second, etc. are used to distinguish one element from another. Furthermore, the use of the terms a, an, etc. do not denote a limitation of quantity, but rather denote the presence of at least one of the referenced item.