Abstract:
A microchannel cold plate for cooling semiconductor electronic devices which may include a semi-permeable wall in contact with liquid in the cooling passage. The cold plate, in turn, may be in contact with the integrated circuit. Cooling liquid passes through the cold plate where it starts boiling inside of the microchannels. The wall allows gas bubbles to pass through while preventing the passage of liquid. As a result, the gas bubbles may be removed from the liquid flow by upward buoyancy. The removal of the gas bubbles improves the operation of the cold plate in some embodiments.

Description:
BACKGROUND  
       [0001]     This invention relates generally to cooling integrated circuit chips.  
         [0002]     Integrated circuit chips may be cooled by passing a flow of cooling fluid through a cold plate in sufficiently close proximity to remove heat from a heated integrated circuit. Removing the heat is desirable to improve the performance of the circuit, and to prevent damage to the circuit and surrounding components.  
         [0003]     In some cases, a condition called dry out may occur in such devices. The dry out is a result of boiling of the cooling fluid and the formation of relatively large concentration of bubbles that interfere with heat transfer to the cooling fluid. This interference reduces the efficiency of heat transfer to the cooling fluid.  
         [0004]     Thus, there is a need for alternative ways for cooling integrated circuits, especially for early removal of the vapor bubbles from the flow. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0005]      FIG. 1  is a schematic depiction of one embodiment of the present invention;  
         [0006]      FIG. 2  is an enlarged, partial, cross-sectional view of one embodiment of the present invention;  
         [0007]      FIG. 3  is a greatly enlarged perspective view of the cooling plate shown in  FIG. 2  in accordance with one embodiment of the present invention; and  
         [0008]      FIG. 4  is an enlarged, cross-sectional view of one embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0009]     Referring to  FIG. 1 , a two-phase cooling fluid may be positioned below a semi-permeable membrane or wall  18 . Below the cooling liquid may be a heat source in the form of a packaged integrated circuit chip (not shown). As the chip is operated, it develops heat which causes the liquid molecules to transition to a gas phase. The gas molecules may move upwardly through the semi-permeable wall  18 . Because the wall is semi-permeable, the wall  18  allows the passage of gas molecules and prevents the passage of liquid. The gas molecules may then be exhausted for condensing while the liquid may continue for recycling.  
         [0010]     As a result, the gas bubbles are removed from the flow of cooling liquid beneath the semi-permeable wall  18 . Such gas bubbles can cause a partial dry out as a result of gas blockage of liquid flow.  
         [0011]     Referring to  FIG. 2 , a package  12 , containing a semiconductor integrated circuit, may be mounted on a cold plate  14 . In one embodiment, the cold plate may include microchannels  56  (shown in  FIG. 3 ), defined by upstanding plates  58 , on its upwardly facing surface  15 . A flow of liquid refrigerant, indicated by arrows A, is provided through a passage  16 . Along the passage  16  may be a housing  25 , including a semi-permeable wall  18 .  
         [0012]     Thus, a thin liquid flow passage  16  is provided between the housing  25  and the cold plate  14 . The liquid in that passage may be thin enough to produce thin film evaporation. In one embodiment, the passage  16  may be on the order of millimeters to less than a millimeter.  
         [0013]     The semi-permeable wall  18  may have a downwardly facing horizontal surface to receive bubbles from below. Effective cooling may occur because the gas bubbles may be exhausted into the housing  25  through the wall  18 . The flow passage  24  totally separates the vapor captured above the semi-permeable wall from the two-phase mixture below. The gas bubbles may be exhausted to a flow passage  24  and the liquid may continue in the flow passage  16 . In one embodiment, the gas molecules may be condensed to form liquid and re-circulate in a loop to continue to cool the package. In some embodiments, a pump may be provided in the flow passage  16  to pump the liquid.  
         [0014]     The cold plate  14  with microchannels removes heat more efficiently in some embodiments of the present invention due to the use of the semi-permeable wall  18 . This is because less vapor is in contact with the boiling surface which prevents partial dry out since the vapors can move upwardly and out of the way of the liquid flow.  
         [0015]     Without the semi-permeable wall  18 , nucleation may occur at the heat transfer surface. The nucleation or bubbles may reduce the amount of liquid contact with the heat transfer surface. The generation of bubbles on the surface may act as an insulation layer. In addition, the bubbles may tend to build up due to a choking effect. When the bubble concentration becomes high enough, the bubbles tend to remain on the liquid transfer surface. Of course, more powerful pumps may be used to pump the liquid to reduce the effect of choking, but larger pumps mean larger package volumes which may be unacceptable in some applications.  
         [0016]     By efficiently removing the bubbles using the semi-permeable wall  18 , the bubbles are less prone to choking and move upwardly through the liquid passage. This may translate into improved heat transfer.  
         [0017]     Due to the randomness in the bubble formation and size, the buoyancy force acting on each bubble may vary accordingly. Depending on the magnitude of this force and the design of the semi-permeable wall  18 , the bubbles may either penetrate through the wall  18  or may be entrained in the liquid flow. Due to the lower vapor quality, the two phase flow pressure drop decreases which, in turn, further decreases the thermal resistance.  
         [0018]     In some embodiments, thin film evaporation improves heat transfer coefficients. By positioning the membrane wall  18  close to the boiling surface, low liquid film thickness can be achieved in some embodiments. Boiling at low vapor quality may be a preferred heat transfer regime, in some cases, due to the fact that less vapor will be in contact with the boiling surface. Thus, a continuing wetting surface may be provided.  
         [0019]     A number of different semi-permeable membrane walls  18  may be utilized, including the GORE™ membrane vent material available from W.L. Gore &amp; Associates of Elkton, Maryland. This material is an expanded polytetrafluoroethylene (PTFE) membrane.  
         [0020]     In some cases, the vapor inside the housing  25  may be removed separately from the liquid flow, as illustrated in  FIG. 2 , and in other cases, the vapor may be intermixed with liquid. In some cases, a vacuum may be applied to the housing  25  and in other cases no such vacuum may be applied.  
         [0021]     Referring to  FIG. 3 , a heat exchanger or condenser  30  may be utilized in conjunction with the cold plate  14  to subsequently cool the two-phase mixture or 100% vapor flow which was heated by heat from the integrated circuit  12 . To this end, a forced airflow may be passed over the hotter two-phase fluid flow to extract the heat from or reduce the temperature of the two-phase fluid to enable the resulting liquid flow to be re-circulated. Heat radiating fins  32  may be utilized in an embodiment with forced air or unforced air cooling. While a forced airflow system is illustrated, other heat transfer techniques may also be utilized.  
         [0022]     In the initial region B of heat transfer between the airflow and the cooling liquid, the cooling liquid is hotter than the ambient air. As the two-phase fluid is cooled by the passing airflow, the heat transfer efficiency is reduced because of the decreased temperature difference between the airflow and the hotter cooling liquid. However, in the two-phase fluid flow, a change of phase occurs at constant temperature. Thus, the heat exchange efficiency may be the same in all areas depending on final arrangement and air flow direction.  
         [0023]     Thus, in one embodiment, the vapor drawn out of the housing  25  may be mixed with the cooling liquid before entering the condenser  30  (see  FIG. 4 ). This may be done in a mixing area before entering the condenser.  
         [0024]     In one embodiment, a semi-permeable membrane  34  is provided over openings in the cooling passage  16 . Thus, the hot gas from the housing  25  and/or passage  24  may be ducted via ducts  24   a  into the cooling liquid to effectively increase the temperature difference in the condenser  30  between the airflow and the liquid being cooled. The membrane  34  allows the gas to pass, but prevents liquid from the passage  16  from exiting. The membrane  34  may be the same type of material used in the housing  25  for the semi-permeable wall  18  in one embodiment of the present invention.  
         [0025]     The gas phase (indicated by dashed lines) from the passage  24  may be mixed with liquid phase from the passage  16  at  52 . The two phases may be separated prior to the mixing area  52 , by the wall  54 . A two phase mixture may result (indicated by dashed arrows) which passes through the condenser  30 . A pump  50  may be provided downstream of the condenser  50  to pump the liquid (solid arrows) exiting from the condenser  30 .  
         [0026]     While the present invention has been described with respect to a limited number of embodiments, those skilled in the art will appreciate numerous modifications and variations therefrom. It is intended that the appended claims cover all such modifications and variations as fall within the true spirit and scope of this present invention.