Abstract:
A heat dissipation device and method provides thermal spreading and cooling for a heat-producing body. A thin film evaporator in thermal communication with the heat-producing body removes heat from the heat-producing body using a working fluid. A heat pipe integrated with the thin film evaporator, and extending from the thin film evaporator, dissipates heat removed by the thin film evaporator to the external environment of the heat dissipation device. A pumping element at least one of: 1) pumps working fluid to the thin film evaporator; and 2) augments transfer of working fluid to the thin film evaporator.

Description:
BACKGROUND 
       [0001]    The present application relates generally to thermal management. It finds particular application in conjunction with cooling heat-producing bodies, such as integrated circuits, and will be described with particular reference thereto. However, it is to be appreciated that the present application is also amenable to other like applications. 
         [0002]    Thermal management for electronics packaging is an active area of research given the increasing demands for high power density and three-dimensional (3D) integrated circuit (IC) architectures. With reference to  FIG. 1 , a typical electronics package for a 3D IC is shown. As can be seen, the electronics package relies upon a passive heat sink for thermal management. 
         [0003]    Most approaches for electronics gaseous working fluid thermal management are focused on back-side thermal management. An example of such an approach is described in U.S. Patent Application Publication No. 2005/0280162 to Mok et al. which describes an integrated vapor chamber thermal interposer on the backside of an IC. However, increasingly, 3D architectures (e.g., with heat fluxes approaching 100 watts per square centimeter (W/cm2)) have made this approach insufficient. 
         [0004]    3D die stacks have severe problems with hotspots on each of the layers. Spreading this heat is a major challenge since the typical combination of solder balls, (relatively) low thermal conductivity filler and copper-filled through silicon vias (TSVs) does not provide a sufficiently conductive thermal path. Further, spreading the heat only partially solves the problem as the heat still needs to be dissipated to the environment. 
         [0005]    One proposed solution is described in U.S. Patent Application Publication No. 2010/0044856 to SRI-JAYANTHA et al. SRI-JAYANTHA et al. modifies the active side or lateral thermal path to improve heat transfer away from the 3D IC using an integrated thermal interposer that utilizes either an additional spreader layer on the backside or a very complex micro channel cooler. This former realization demonstrates fairly modest thermal improvement, while the latter entails considerable complexity in terms of fluidic sealing and pumping. 
         [0006]    Another proposed solution is to use thermoelectric thermal bump, examples of which are shown in  FIG. 1 . However, thermoelectric thermal bumps suffer from the typical shortcomings of thermoelectric materials: poor efficiency and high cost. 
         [0007]    Yet another proposed solution is thin film evaporation. It uses a phase change process such as boiling, but mitigates some of the well-known shortcomings of boiling. Specifically, thin film evaporation mitigates the amount of superheat required to initiate boiling, the unpredictability of boiling nucleation sites, handling of combined vapor-liquid flow after boiling (in a flow system) and the critical heat flux (CHF) in which the hot surface dries out if the heat flux is too high. 
         [0008]    The foregoing is described in Ohadi et al., “Ultra—Thin Film Evaporation (UTF)—Application to Emerging Technologies in Cooling of Microelectronics”, Microscale Heat Transfer Fundametals and Applications, pages 321-338 (2005):
       Ultra Thin Film (UTF) evaporation is perhaps one of the most effective methods of heat removal from a high heat flux surface for several reasons:   1. a small quantity of fluid is required to remove the heat by evaporation at the surface of the thin layer of fluid,   2. a very high heat transfer coefficient results from the minimized thermal resistance across the thin liquid layer,   3. the surface experiences a very small temperature rise above the saturation temperature of the working fluid, as long as a sufficient quantity of fluid is provided to wet the surface,   4. a minimum amount of energy is required to circulate the working fluid due to no pressure drop across the thin film evaporator,   5. unlike nucleation boiling where a complex set of parameters determine the stability of the system, UTF is virtually conduction across a very thin film,   6. the upper limit on cooling performance would be limited by the homogeneous nucleation temperature and/or kinetics of vapor formation at a free interface, rather than the relatively low CHF.       
 
         [0016]    The second reason can be clarified using the relationship between heat transferred, thermal resistance and temperature drop, and the definition of the heat transfer coefficient. The heat transfer coefficient R can be defined by Q*R=DT, where Q is heat transferred in watts (W), R is in degrees Celsius per watt (C/W) and DT is temperature drop in degrees Celsius (C). A high thermal resistance means less heat is transferred and there is a large temperature gradient across the material of interest. The thermal resistance and heat transfer coefficient R are related by the relationship R=L/(kA) for conduction, where L is the conduction path length, A is the heat transfer area, and k is the thermal conductivity. 
         [0017]    Additional heat dissipation due to evaporation is what makes thin film evaporation particularly compelling. The amount of heat Q that can be removed at the vapor-liquid interface is Q=m(ΔH v ), where m is the mass flow of evaporating liquid and (ΔH v ) is the latent heat of vaporization of the refrigerant. Taking a 2 centimeter (cm)×2 cm die, as typical of integrated circuit packages, and a thermal density of 25 W/cm2, the heat to transfer is 100 W. Using water as a heat transfer fluid, the latent heat is 2260 joules per gram (J/g), so the amount of mass flux is 25/2260=0.01 grams per second (g/s) or 10 microliters per second (μL/s) of fluid must be evaporated to dissipate this much heat. 
         [0018]    In view of this, it is clear that large heat transfer area A and small conduction path length L provide a small conduction resistance through the thin film, which increases the amount of heat transferred to the phase change interface. A challenge in thin film evaporator design is feeding the thin film with enough material to match the evaporation rate for high heat fluxes. One solution is to employ electrohydrodynamics (EHD) polarization pumping to draw a thin film of dielectric liquid along a hot surface. However, this solution suffers from orientation dependence and the resulting film is not especially thin. Even so, this solution has been shown to be able to transfer heat fluxes of up to 40 W/cm2. 
         [0019]    It is well-known in the heat pipe community that a significant fraction of heat transfer in the evaporator section of heat pipes occurs in the thin film region where conductive losses are low and evaporation rates are highest. As such, there has been significant work on using wicking structures to maximize the thin film region. A realization of such work combines the application of an actively pumped microchannel cooler with a porous membrane for evaporation. The evaporation rate is augmented with air jet impingement to further improve the heat transfer. This realization demonstrates the ability to dissipate a heat flux of 500 W/cm2. However, this realization requires external infrastructure for pumping and a relatively large thin film. Roughly 85% of the heat transfer is due to the forced convection in the microchannel. 
         [0020]    The present application provides new and improved methods and systems which overcome the above-referenced challenges. 
       BRIEF DESCRIPTION 
       [0021]    In accordance with one aspect of the present application, a heat dissipation device to provide thermal spreading and cooling for a heat-producing body is provided. A thin film evaporator in thermal communication with the heat-producing body removes heat from the heat-producing body using a working fluid. A heat pipe integrated with the thin film evaporator, and extending from the thin film evaporator, dissipates heat removed by the thin film evaporator to the external environment of the heat dissipation device. A pumping element at least one of: 1) pumps working fluid to the thin film evaporator; and 2) augments transfer of working fluid to the thin film evaporator. 
         [0022]    In accordance with another aspect of the present application, a heat dissipation method to provide thermal spreading and cooling for a heat-producing body is provided. By a thin film evaporator in thermal communication with the heat-producing body, heat from the heat-producing body is removed using a working fluid. By a heat pipe integrated with the thin film evaporator and extending from the thin film evaporator, heat removed by the thin film evaporator is dissipated to the external environment of the heat dissipation device. By a pumping element, at least one of: 1) working fluid is pumped to the thin film evaporator; and 2) transfer of working fluid to the thin film evaporator is augmented. 
         [0023]    In accordance with another aspect of the present application, a heat dissipation device to provide thermal spreading and cooling for a heat-producing body is provided. A sealed housing includes a fluid reservoir of working fluid in liquid phase and a vapor chamber, the heat-producing body thermally coupled to an external surface of the sealed housing. A thin film evaporator is within the sealed housing and in thermal communication with an internal surface of the sealed housing adjacent the external surface. The thin film evaporator receives working fluid in liquid phase from the fluid reservoir and vaporizes the received working fluid to working fluid in gaseous phase using heat from the heat-producing device. A heat pipe within the sealed housing transfers the working fluid in gaseous phase away from the thin film evaporator, condenses the working fluid in gaseous phase to liquid phase, and returning the condensed working fluid to the fluid reservoir. A pumping element at least one of: 1) pumps working fluid to the thin film evaporator; and 2) augments transfer of working fluid to the thin film evaporator. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0024]      FIG. 1  illustrates an electronics package relying on passive cooling; 
           [0025]      FIG. 2  illustrates an embodiment of a heat dissipation device according to aspects of the present application; 
           [0026]      FIG. 3A  illustrates a cross sectional view of the heat dissipation device of  FIG. 2 ; 
           [0027]      FIG. 3B  illustrates an alternative cross sectional view of the heat dissipation device of  FIG. 2 ; 
           [0028]      FIG. 4  illustrates a return wick according to aspects of the present application; and 
           [0029]      FIG. 5  illustrates another embodiment of a heat dissipation device according to aspects of the present application. 
           [0030]      FIG. 6  illustrates another embodiment of a heat dissipation device using pulsating heat pipe technology according to aspects of the present application. 
           [0031]      FIG. 7  illustrates another view of the heat dissipation device according to  FIG. 6 . 
       
    
    
     DETAILED DESCRIPTION 
       [0032]    This present application combines an actively driven thin film evaporator for spreading heat with an integrated planar heat pipe extended surface for heat sinking. The thin film evaporator allows for a high rate of heat removal to remove hot spots, and the integrated planar heat pipe transports heat from the thin film evaporator to an extended surface for dissipation to the environment or sinking to an interposer layer. 
         [0033]    With reference to  FIG. 2 , a perspective view of a heat dissipation device  10  of the present application is provided. The heat dissipation device  10  provides thermal spreading and cooling to an associated heat-producing body  12 , such as an integrated circuit (IC) package (illustrated). Typically, but not necessarily, the IC package is three-dimensional (3D). As will be seen, the heat dissipation device  10  is capable of managing heat fluxes of 10 to 1000 Watts per square centimeter (W/cm2) or more, such as 100 W/cm2. 
         [0034]    The heat dissipation device  10  includes a sealed housing  14 , which is constructed of a thermally conductive material. The thermally conductive material can, for example, include one or more of copper, copper foil, copper alloys, aluminum, aluminum alloys, polyimides, metals, and the like. The sealed housing  14  can be flexible and seals in a working fluid  16  ( FIGS. 3A  &amp; B) for transfer of heat away from the heat-producing device  12 , as discussed hereafter. Advantageously, when the sealed housing  14  is flexible, the sealed housing  14  can be shaped after manufacture when installing the heat dissipation device  10  for the heat-producing body  12 . 
         [0035]    An external surface  18  of an interface portion  20  of the sealed housing  14  thermally contacts the heat-producing body  12 . For example, the external surface  18  can directly contact the heat-producing body  12 . As another example, the external surface  18  can indirectly contact the heat-producing body  12  by way of a substrate upon which the heat-producing body  12  rests or a thermal interface material intermediate the heat-producing body  12  and the external surface  18 . Typically, the interface portion  20  is formed from copper, copper foil, copper alloys, aluminum, or aluminum alloys, but other materials are contemplated. 
         [0036]    Extending away from the interface portion  20 , the sealed housing  14  further includes one or more extended portions or fins  22  (two as illustrated). As discussed hereafter, the extended portions  22  are used to convey heat into the external environment, typically by convection, or to sink heat to an interposer layer. The extended portions  22  are typically formed from a flexible polyimide or metallic substrate to allow the extended portions  22  to be shaped into a desired form factor after manufacture, but other materials are contemplated. 
         [0037]    With reference to  FIGS. 3A &amp; 3B , within the sealed housing  14 , the heat dissipation device  10  includes a vapor chamber  24  and a fluid reservoir  26 . The vapor chamber  24  includes an internal surface  28  of the interface portion  20  adjacent the external portion  18  of the interface portion  20 . Further, the vapor chamber  24  extends into each of the extended portions  22  of the sealed housing  14 , typically to the distal ends  30  of the extended portions  22 . The fluid reservoir  26  is typically disposed in the interface portion  20  of the sealed housing  14  central to the extended portions  22 . 
         [0038]    The fluid reservoir  26  holds working fluid  16  in the liquid phase (i.e., liquid working fluid  32 ), and the vapor chamber  24  holds working fluid  16  in the gaseous phase (i.e., gaseous working fluid  34  shown by the arrows in the vapor chamber  24 ). The working fluid  16  can include, for example, water, Freon, acetone, alcohol, and the like. As described hereafter, the working fluid  16  is employed to transfer heat away from the heat-producing body  12  through the sealed housing  14  using thin film evaporation, where the extended portions  22  act as heat pipes. In this way, heat fluxes of 10 to 1000 Watts per square centimeter (W/cm2) or more can be managed. 
         [0039]    The heat dissipation device  10  includes a thin film evaporator  36  for evaporating liquid working fluid  32  from the fluid reservoir  26  into gaseous working fluid  34  with heat from the heat-producing body  12 . Typically, the thin film evaporator  36  is actively driven to ensure sufficient transfer of working fluid  16  to cool the heat-producing body  12 , as discussed above, but it can also be passive. For example, the thin film evaporator  36  can be actively driven when the heat-producing body  12  is producing heat exceeding a predetermined threshold and passively driven when the heat-producing body  12  is producing heat less than the predetermined threshold. 
         [0040]    The thin film evaporator  36  includes an evaporator wick  38  in thermal contact with the internal surface  28  of the interface portion  20  of the sealed housing  14  and within the vapor chamber  24 . Typically, the surface area of the evaporator wick  38  in contact with the internal surface  28  is approximately (i.e., +/−5%) equal to, or greater than, the surface area of the heat-producing device  12  in contact with the external surface  18 . The evaporator wick  38  receives liquid working fluid  32  from the fluid reservoir  26  and disperses the liquid working fluid  32  substantially uniformly on the internal surface  28  of the interface portion  20  of the sealed housing  14  to form a thin layer  40  of liquid working fluid  32 . Suitably, the evaporator wick  38  is engineered to maximize the extent of capillary wicking and the area of the thin layer  40 . 
         [0041]    The heat dissipating device  10 , particularly the thermal coupling with the heat-producing body  12 , is designed to allow sufficient transfer of heat to the thin layer  40  of liquid working fluid  32  to cool the heat-producing body  12 . The heat transferred from the heat-producing body  12  to the thin layer  40  of liquid working fluid  32  is dictated by the heat transfer coefficient R for conduction, where R=L/(kA), L is the conduction path length, A is the heat transfer area, and k is the thermal conductivity. The greater the heat transfer coefficient R, the less transfer of heat. Hence, the greater the area of the thin layer  40  of liquid working fluid  32  and the thermal conductivity of the material intermediate the heat-producing body  12  and the thin layer  40  of liquid working fluid  32 , the greater the heat transfer. Similarly, the less the conduction path length, the greater the heat transfer. 
         [0042]    A feed conduit  42  of the sealed housing  14  extends between the fluid reservoir  26  and the evaporator wick  38  to provide liquid working fluid  32  to the evaporator wick  38  from the fluid reservoir  26 . The evaporator wick  38  draws liquid working fluid  32  from the fluid reservoir  26  by way of the feed conduit  42  using capillary action. This capillary action also serves to disperse the liquid working fluid  32  on the internal surface  28  of the interface portion  20  of the sealed housing  14 . The greater the dispersion of liquid working fluid  32 , the greater the transfer of heat from the heat-producing body  12 . Additional feed channels are also contemplated to improve the transfer of liquid working fluid  32  to the evaporator wick  38 . 
         [0043]    One or more synthetic jets  44  (two as illustrated) within the sealed housing  14  can be employed to improve the transfer of liquid working fluid  32  to gaseous working fluid  34  by removing gaseous working fluid  34  from the vapor chamber  24  and cooling the evaporator wick  38  to allow greater dispersion of the liquid working fluid  32  before evaporation. Typically, the synthetic jets  44  include a plurality of synthetic jets arranged in a grid or other two-dimensional arrangement to cool, and/or remove gaseous working fluid  34  from, the whole of the evaporator wick  38 . Power is provided to the synthetic jets  44  by way of corresponding wires  46  and power sources  48 . 
         [0044]    The synthetic jets  44  create a series of vortex rings of gaseous working fluid  34  in the vapor chamber  24  using corresponding orifices  50  and corresponding oscillating actuators  52 . The axes of the vortex rings are suitably perpendicular to the internal surface  28 . Suitably, the oscillating actuators  52  are piezoelectric actuators (illustrated), but other oscillating actuators are contemplated. While any configuration of the synthetic jets  44  is contemplated, the oscillating actuators  52  of the synthetic jets  44  typically oscillate corresponding diaphragms along the axes of the vortex rings. The oscillating actuators  52  can be the diaphragms (e.g., piezoelectric diaphragms), as illustrated, or merely oscillate the corresponding diaphragms. 
         [0045]    The orifices  50  typically include corresponding open ends  54  through which the vortex rings enter the vapor chamber  24  from the orifices  50 . The oscillating actuators  52  can be, for example, positioned within the orifices  50  to push vapor within the orifices  50  out the open ends  54 . The orifices  50  can further include additional corresponding open ends  56  opposite the open ends  54  through which the vortex rings enter the vapor chamber  24  from the orifices  50 . The oscillating actuators  52  can then be, for example, positioned at the additional open ends  54  to create the vortex rings using, for example, diaphragms spanning the additional open ends  54 . 
         [0046]    The oscillating actuators  52  can also be employed to pump liquid working fluid  32  to the evaporator wick  38  by way of the feed conduit  42  to ensure that sufficient liquid working fluid  32  is provided to the evaporator wick  38  to prevent dry out of the thin layer  40  of liquid working fluid  32 . Typically, the oscillating actuators  52  are out of plane (i.e., oscillate perpendicular to the direction flow of the liquid working fluid  32 ). In such instances, it&#39;s important to ensure that the liquid working fluid  32  can only flow in the direction of the feed conduit  42 . Other approaches to pumping the liquid working fluid  32  can also be employed. 
         [0047]    One approach to pump liquid working fluid  32  using the oscillating actuators  52  is to employ corresponding diaphragms with the oscillating actuators  52 . As noted above, the oscillating actuators  52  can be the diaphragms (illustrated) or merely oscillate the corresponding diaphragms. In such an approach, the diaphragms partially define the wall of the fluid reservoir  26  and oscillate in and out of the fluid reservoir  26 . Typically, the oscillations are perpendicular to the thin film evaporator  36  and the flow of liquid working fluid  32 . As the diaphragms moves in to the fluid reservoir  26 , the diaphragms pump liquid working fluid  32 . As the diaphragms move out of the fluid reservoir  26 , the diaphragms creates the above described vortex rings. 
         [0048]    With specific reference to  FIG. 3B , in addition to, or as an alternative to, using the feed conduit  42  to provide liquid working fluid  32  to the evaporator wick  38 , the synthetic jets  44  can spray liquid working fluid  32  from the fluid reservoir  26 , or some other fluid, on to the evaporator wick  38  substantially uniformly. As illustrated in  FIG. 3B , the synthetic jets  44  receive liquid working fluid  32  from corresponding feed wicks or conduits  58 , which control the flow of liquid working fluid  32  from the fluid reservoir  26  to the orifices  50 . This can help to disperse the liquid working fluid  32  on the internal surface  28  of the interface portion  20  of the sealed housing  14  to form the thin layer  40  of liquid working fluid  32 . 
         [0049]    While the thin film evaporator  36  employs the evaporator wick  38  for receiving and dispersing the liquid working fluid  32 , other approaches for receiving and dispersing the liquid working fluid  32  can be employed. For example, a wickless approach or an electrohydrodynamics (EHD) polarization pumping in conjunction with an electrically conductive wick can be employed. As another example, the synthetic jets  44  can spray the liquid working fluid  32 , as described above, on to the internal surface  28  to create the thin layer  40  of liquid working fluid  32  without the evaporator wick  38 . As another example, the thin film evaporator  36  can work without the synthetic jets  44 , but optionally with the oscillating actuators  52  pumping liquid working fluid  32  as described above. 
         [0050]    The thin film evaporator  36  and/or the synthetic jets  44  need to be designed to transfer and disperse a sufficient amount of liquid working fluid  32  to remove the heat transferred by the heat-producing body  12  to the thin layer  40  of liquid working fluid  32 . The amount of heat Q that can be removed at the thin layer  40  of liquid working fluid  32  is Q=m(ΔH v ), where m is the flow of evaporating liquid work fluid  32  and (ΔH v ) is the latent heat of vaporization of the working fluid  16 . Hence, thin film evaporator  36  and the synthetic jets  44  are designed around this equation. 
         [0051]    As the thin layer  40  of liquid working fluid  32  evaporates, the gaseous working fluid  34  is transported to the extended portions  22 , typically to the distal ends  30  of the extended portions  22 , by way of the vapor chamber  24 . The synthetic jets  44  facilitate transport of the gaseous working  34  fluid to the extended portions  22  by pushing the gaseous working fluid  34  to the extended portions  22 . Within the extended portions  22 , the gaseous working fluid  34  dissipates and condenses back into liquid working fluid  32 . 
         [0052]    Adjacent the vapor chamber  24 , each extended portion  22  includes a return wick  60  at least extending from the corresponding distal ends  30  to the fluid reservoir  26  and typically lining the extended portion  22 . The return wicks  60  capture working fluid  16  as it condenses back to liquid and return it to the fluid reservoir  26 , typically using capillary action. In this way, the extended portions  22  can be viewed as planar heat pipes. 
         [0053]    The design of the return wicks  60  is important to the successful operation of the heat dissipation device  10 . The flow of working fluid  16  through the return wicks  60  must be sufficient to complete the working fluid recirculation loop (shown by the arrows). The return wicks  60  are multi-layer wicks with engineered hydrophobic condensation surfaces and a sub-layer of feed channels that return the liquid working fluid  32  to the fluid reservoir  26 . With reference to  FIG. 4 , gaseous working fluid  34  condenses into a ball  62  at the apex  64  of one of the feed channels  66  before flowing into the feed channel for transport back to the fluid reservoir  26 . Further, examples of the return wicks  60  at different magnifications (corresponding to images A through D) are provided. 
         [0054]    Different approaches to returning the liquid working fluid  32  to the fluid reservoir  26  can also be employed. For example, a wickless approach or an electrohydrodynamics (EHD) polarization pumping in conjunction with an electrically conductive wick can be employed. 
         [0055]    The coupling of a heat pipe to a thin film evaporator, as described above, is to be contrasted with conventional thermal packaging arrangement in which the heat sink is connected to a spreader by way of a thermal interface material (See  FIG. 1 ). Each of these additional layers introduces thermal resistance and increases the potential for hotspots. On the active side, it is even less straightforward to dissipate heat away. The present application rectifies this problem by coupling extended portions or fins into the thermal path. These extended portions can be viewed as planar heat pipes with a conductive wick lining. 
         [0056]    With reference to  FIG. 5 , another embodiment of the heat dissipation device  10  is illustrated.  FIG. 5  shows a cut away of the heat dissipation device  10  with an emphasis on the extended portions  22 . Also, the extended portions  22  are not bent upward as done in the  FIGS. 2 ,  3 A and  3 B. As noted above, the heat dissipation device  10  can be shaped as need be after manufacture. The heat dissipation device  10  of this embodiment works as described in connection with the embodiment of  FIG. 2 . Hence, elements paralleling those of the discussion of the embodiment of the heat dissipation device  10  of  FIG. 2  are labeled the same. 
         [0057]    As illustrated, the evaporator wick  38  receives liquid working fluid  32  using capillary action and/or the synthetic jets  44  from the fluid reservoir  26 . Using this liquid working fluid  32 , the evaporator wick  38  creates the thin layer  40  (not shown in  FIG. 5 ) of liquid working fluid  32  on the internal surface  28  (not shown in  FIG. 5 ) of the interface portion  20  of the sealed housing  14 . The gaseous working fluid  34  (not shown in  FIG. 5 ) generated by evaporation of the liquid working fluid  32  in the thin layer  40  then travels to the extended portions  22  of the sealed housing  14  by way of the vapor chamber  24 . The synthetic jets  44  can be employed to move the gaseous working fluid  34  to the extended portions  22 . 
         [0058]    Within the extended portions  22 , the gaseous working fluid  34  cools and condenses back to liquid working fluid  32 . This liquid working fluid  32  can collect at corresponding capture reservoirs  68  at the distal ends  30  of the extended portions  22  and/or be collected by the return wicks  60 . The return wicks  60  return liquid working fluid  32  collected thereby and/or from the capture reservoirs  68  to the fluid reservoir  26 , typically using capillary action. In this way, the working fluid  34  follows a fluid recirculation loop, which is shown by the arrows. 
         [0059]    In some embodiments, additional or alternative approaches for removing gaseous working fluid from the interface portion can be employed. For example impinging jets can be employed. Further, in some embodiments, additional or alternative approaches to spreading heat in the extended portions  22  can be employed. For example, although less efficient than the preferred realization, the extended portions  22  can include pulsating heat pipes (PHPs). Such embodiments employing pulsating heat pipes would be limited by the conduction contact area between the PHPs and the vapor chamber. 
         [0060]    With reference to  FIGS. 6 and 7 , another embodiment of the heat dissipation device  10  employing PHP technology, which is known in the art, is illustrated. For ease of illustration, only a single extended portion  22  or fin is described and it&#39;s is not bent upward as done in the  FIGS. 2 ,  3 A and  3 B. However, as noted above, the heat dissipation device  10  can be shaped as need be after manufacture. But for the use of PHP technology, the heat dissipation device  10  of this embodiment works as described in connection with the embodiment of  FIG. 2 . Hence, elements paralleling those of the discussion of the embodiment of the heat dissipation device  10  of  FIG. 2  are labeled the same. 
         [0061]    Referring to  FIG. 6 , a top view of the heat dissipation device  10  is illustrated. Of note, all the detail of the interface portion  20  is not shown. Each of the extended portions  22  includes one or more PHPs  80  and a heat exchanger  82 . Alternatively, the extended portions  22  can share a common PHP. The heat exchangers  82  are typically positioned proximate the interface portion  20  at the bases of the corresponding extended portions  22 . Further, the PHPs  80  typically extend from the distal ends  30  of the corresponding extended portions  22  in to the corresponding heat exchangers  82 . 
         [0062]    The heat exchangers  82  receive gaseous working fluid  34  from the vapor chamber  24 . Within the heat exchangers  82 , the heat from the gaseous working fluid  34  is absorbed by the PHPs  80 , which transfer the absorbed heat to the distal ends  30  of the extended portions  22  for dissipation to the external environment. As the heat is absorbed by the PHPs  80 , the gaseous working fluid  34  condenses back to liquid working fluid  32  and is returned to the fluid reservoir  26 . 
         [0063]    With reference to  FIG. 7 , a partial, perspective view of the heat exchangers  82  is illustrated. Each of the heat exchangers  82  includes a capture reservoir  84  in which condensed working fluid  32  collects. The capture reservoirs  84  are typically positioned under the portions of the PHPs  80  extending into the heat exchangers  82 . Further, notwithstanding the orientation of the PHPs  80 , those skilled in the art will appreciate that other orientations are amenable. 
         [0064]    “Drop-wise condensation” is generally desired since it gives higher heat fluxes. This is encouraged by coating and/or encapsulating the portions of the PHPs  80  extending into the heat exchangers  82  with hydrophobic material  86 . For example, a thin layer of Polytetrafluoroethylene (PTFE), such as that found on a nonstick cooking pan, can coat these portions of the PHPs  80 . As illustrated, the PHPs  80  are encapsulated in hydrophobic material  86  and cause droplets  88  of liquid working fluid  32  to form and fall into the capture reservoirs  82 . 
         [0065]    To return the condensed working fluid  32  to the fluid reservoir  26 , each of the extended portions  22  includes a return wick  90  extending from the corresponding capture reservoir  84  to the fluid reservoir  26 . The return wick  90  uses capillary action as described above to transfer the liquid working fluid  32  in the captured. 
         [0066]    It will be appreciated that variants of the above-disclosed and other features and functions, or alternatives thereof, may be combined into many other different systems or applications. Various presently unforeseen or unanticipated alternatives, modifications, variations or improvements therein may be subsequently made by those skilled in the art which are also intended to be encompassed by the following claims.