Abstract:
The present invention is directed to a method of constructing a semiconductor chip cooling device consists of multiple fans and heat sinks to provide redundant cooling capability. Heat coming from a semiconductor chip is first distributed to several heat sinks using multiple heat pipes. The heat sinks are placed around the fan outlet such that air is pulled in near the center of the fan and then pushed to across the heat sinks. Multiple fans and heat sinks are stacked up to form a complete cooling device. An external control circuitry is used to monitor and control the fans. In case of one fan fails, the other fans will be speeded up to make up the lost of air flow.

Description:
FIELD OF THE INVENTION  
       [0001]     The invention is directed to improve the capability and reliability of a cooling device for semiconductor chips. The invention is in the field of heat transfer and cooling of semiconductor chips used in the computer and telecommunication equipment.  
       BACKGROUND AND RELATION TO THE PRIOR ART  
       [0002]     The problem to be solved by this invention is to provide an improved cooling device to a semiconductor chip using multiple fans and heat sinks. This cooling device has multiple fans connected in a redundant mode such that if one fan malfunctions, the remaining fans will spin up to make up the lost of air flows to the heat sinks. The current fan-heatsink uses only one fan and has no redundant feature.  
         [0003]     (1) U.S. Pat. No. 5,309,983—“Low Profile Integrated Heat Sink and Fan Assembly”, describes a heat sink with straight fins and a fan integrated in a cavity in the fin area. The base of the heat sink is a flat, solid plate and the fins are straight fins. The fan has air inlets on the top surface of the fan. Air is moving through the fan and the spacing between fins at two opposite directions. The solid heat sink base does not provide a compliant interface to the heated surface underneath and the air flows are restricted to own two directions which are inherently inefficiency. Furthermore, the air inlets are on one surface only.  
         [0004]     (2) U.S. Pat. No. 5,502,619—“Heat Sink Assembly for Computer Chips”, teaches another type of fan-heatsink assembly which has the fins stacked up by a plurality of sheets with openings and the fan placed on the top of the stacked sheets. This stacking method of making the fins for the heat sink will impede the heat flow from the heated source underneath the heat sink and hence rendering it inefficient for heat transfer. This heat sink has a solid base and the fan has inlets at the top surface only.  
         [0005]     (3) U.S. Pat. No. 5,584,339—“Heat Sink Assembly for the Central Processor of Computer”, describes another type of fan-heatsink assembly which has a fan with cover placed in the middle of the heat sink that has post-like fins. The post-like fins will give air multiple air flow paths with the heat sink but the fan still has one inlet on one surface. The base of the heat sink is also a solid plate.  
         [0006]     (4) U.S. Pat. No. 5,785,116—“Fan Assisted Heat sink Device”, teaches a heat sink assembly having heat sink housing surrounding the fan. Cold air enters the heat sink house near the top of the house and hot air exits the house from the bottom end in all the directions. Since there is no provision to separate the cold and hot air, air mixing may happen and the efficiency of the cooling device will degrade. Furthermore, the base of the heat sink is a solid plate which cannot provide compliant interface to the hot surface underneath. The hot air coming out from the heat sink device will add heating effects to the surrounding chips. This is considered undesirable.  
       SUMMARY OF THE INVENTION  
       [0007]     The cooling device consists of multiple stacks of fan-heatsink sets, each of these sets has one centrifugal fan surrounded by heat sinking fins, and multiple heat pipes connecting the heatsinking fins to one heat distribution block. The heat distribution block is to be placed on top of an semiconductor chip. The heat distribution block has a few heat pipes inserted in for improving heat transfer within the block. Furthermore, the block can be made from a vapor chamber type in which liquid evaporation and condensation within the chamber is used to transfer heat from one surface of the block to the heat pipes inserted in the chamber.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]      FIG. 1  The 3-D view of the multiple fan heatsinks with heat pipes and fans.  
         [0009]      FIG. 2  The exploded view of the multiple fan heatsinks.  
         [0010]      FIG. 3  The structure of the fins for the multiple fan heatsinks.  
         [0011]      FIG. 4  Another embodiment of the fin structure.  
         [0012]      FIG. 5  The heat distribution block and heat pipe assemblies.  
         [0013]      FIG. 6  The vapor chamber with multiple heat pipes inserted therein.  
         [0014]      FIG. 7  The exemplary block diagram for controlling the multiple fans.  
     
    
     DESCRIPTION OF THE INVENTION  
       [0015]     The detailed structure of this cooling device using multiple fan and heat sinks is shown in the following figures.  FIG. 1  shows the fans  41  placed at the center surrounded by the heat sinking fins  31 . The fins  31  can be arranged in line or in staggered faction which is not shown in the picture. The fins  31  are thermally connected to four heat pipes  51 ,  52 ,  53 , and  54  respectively. All of the four heat pipes are then connected to a heat distribution block  21 .  FIG. 2  is the exploded view of the multiple fan heat sink. There are three sets of fan and fins in this figure. However, the number of the fan-fin sets can be any and should not be limited to just three. Each fan has its own motor and blades. Each fan can be run and controlled independently. The fans  41 ,  44 , and  47  in the figure have their respective blades  43 ,  46 , and  49 . Only fan  41  has motor  42  shown for the clarity of the drawing. The fans  41 ,  44 , and  47  are surrounded by the fin sets  31 ,  33 , and  35 , respectively. Four heat pipes  51 ,  52 ,  53 , and  54  are used to transfer heat from the heat distribution block  21  to their respective fin sets  31 ,  33 , and  35 . The heat distribution block is brought into a good thermal contact with a heat generating semiconductor chip. Heat from the semiconductor chip will, therefore, transfer to the heat distribution block, the multiple heat pips, the fin sets, and the moving air passing through the fins. Air will be pulled into the fan at the inlet near the fan motor, for instance, air comes in from the inlet  40  of the fan  41 . Similarly, air will come in from the inlet from the bottom fan  47 . Part of the air coming in from fans  41  and  47  will pass through the fans and go to the inlet of the fan  44 . All of the inlet air is then being pushed out to the fin sets by the centrifugal forces from the rotating blades  43 ,  46 , and  49 . The direction of the rotating blades of each fan can be the same or different.  
         [0016]      FIG. 3  shows the structure of one portion of the fin sets.  FIG. 3   a  shows one assembled fin set which consists of multiple fins  317  and  318  inserted and soldered into the fin bases  311  and  312 .  
         [0017]     The fins and fin bases are made of heat conductive materials such as aluminum or copper.  
         [0018]      FIG. 3   b  shows the details of the fin bases  311  and  312  before attaching the fins. Each fin base has slots  313  where the fins are to be inserted and soldered in. There are screw holes  314   a  and  314   b  on the fin bases. Two of the fin bases will be screwed together back to back with a heat pipe inside the hole  315 . The heat pipe in the hole  315  is slightly compressed to provide a good thermal contact between the heat pipe and the fin base. Alternatively, the fin bases can be soldered on the heat pipe directly.  FIG. 4  shows another embodiment of the fin sets. In this case, the fin base  321  looks like a half cylinder with horizontal slots  323  on the surface. The fin  327  has a notch  329  on the edge and is to be inserted in the slots  323 . Once all of the fins are inserted and soldered on the fin base  321 , a heat pipe is then placed in the recess  325  on back side of the fin base  321 . A cover which is not shown in the figure will be placed on the heat pipe to secure it on the fin base  321 . Alternatively, the cover and heat pipe can be soldered on the recess  325  directly.  
         [0019]      FIG. 5  shows the heat distribution block with multiple heat pipes inserted in. Referring to  FIG. 5   a , one end of the four heat pipes  51 ,  52 ,  53 , and  54  are inserted and soldered in the heat distribution block  21 . The four heat pipes  51 ,  52 ,  53 , and  54  can be bent up as shown in the figure or into different configurations to accommodate the fin sets which are brought into a good thermal contact with the other end of these four heat pipes.  FIG. 5   b  shows another heat distribution block arrangement, in which a couple of smaller heat pipes  22  and  23  are embedded in the block. These smaller heat pipes are bent into “U” shape such that both ends of the heat pipes are soldered into the block. The purpose of this arrangement is to reduce the temperature gradient within the heat distribution block  21 . As shown in the figure, one end of the “U” shape heat pipe  22  is placed near the bottom of the heat distribution block  21  and the other end near the top surface. This heat pipe will help to transfer part of the heat coming from the bottom to the top of the block and, therefore, reducing the vertical temperature gradient within the block.  FIG. 6  shows another structure of the heat distribution block  221  which is a vapor chamber with one end of the heat pipes inserted inside the vapor chamber. As shown in  FIG. 6 , The heat pipes  251 ,  252 , and  254  are inserted inside the vapor chamber  226 . For the clarity of the drawing, the forth heat pipe is not shown in the figure. As a matter of fact, the number of heat pipes can be any and should not be limited to four. There is a layer of wicks covering the inner surface of the vapor chamber  226  including the outer surface on the section of the heat pipes inside the chamber.  
         [0020]     The wicks can be made of copper foams, sintered copper, or fiber bundles from heat conducting materials such as copper and graphite. The vapor chamber  226  is a vacuum sealed environment which is partially filled with a working fluid such as water, ethanol, florinert, acetone, etc. When heat is supplied to the bottom of the block  221  from a semiconductor chip, the working fluid absorbs the heat and vaporize. The vapor is then flowing across the chamber and to be condensed on the surface of the wicks and heat pipes. Heat is released to the wicks and heat pipes when the vapor becomes fluids again. The capillary force of the fluid will pull the fluid back to the heating side of the chamber. This cycle continues as long as there is temperature difference between the hot side of the chamber and the surface of the heat pipes.  
         [0021]      FIG. 7  shows an exemplary block diagram for controlling the fans. The fan controller which has processing units and memory attached or built in, takes the control data and commends from a system host and drives the fans at a predetermined speed through the fan drivers. The rotating speed of each fan will be fed back to the fan controller. If the fan speed is deviated away from the predetermined value, the fan controller will try to adjust it through the pulse width modulation (PWM) which is common in the art of fan speed controlling. If the speed of one fan cannot be recovered by this adjustment, the controller will mark this fan and try to increase the speed of the other fans. The percentage of the fan speed increase is based on a pre-stored table of fan heatsink cooling performance in the fan controller by interpolation. The goal is to maintain the same cooling performance when all fans are running at the predetermined speed. For the purposes of clarity, only major components are drawn in these figures and they are not drawn to scale.