Abstract:
A heat sink is constructed including at least one thermally conductive pedestal, allowing configuration of the heat sink to make contact with a plurality of heat-generating electronic devices where the devices may not be co-planar due to tolerance stack-up. The pedestals may be raised and lowered and tilted as needed to match the heights and tilts of the electronic devices. Within the heat sink is a cavity above the pedestal that may be filled with a thermally conductive material, such as solder, or a thermally conductive liquid, during construction to create a low thermal resistance contact between the pedestal and the heat sink fins. Also, thermally conductive material, such as thermal paste or a thermal pad, may be used between the heat generating device and the pedestal to create a low thermal resistance contact.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates generally to the field of heat sinks and more specifically to the field of heat sinks configured to maximize thermal conduction with heat generating devices that may not be co-planar with the heat sink.  
         BACKGROUND OF THE INVENTION  
         [0002]    Modern electronics have benefited from the ability to fabricate devices on a smaller and smaller scale. As the ability to shrink devices has improved, so has their performance. Unfortunately, this improvement in performance is accompanied by an increase in power as well as power density in devices. In order to maintain the reliability of these devices, the industry must find new methods to remove this heat efficiently.  
           [0003]    By definition, heat sinking means that one attaches a cooling device to a heat-generating component and thereby removes the heat to some cooling medium, such as air or water. Unfortunately, one of the major problems in joining two devices to transfer heat through a common surface is that a thermal interface is created at the junction. This thermal interface is characterized by a thermal contact impedance. Thermal contact impedance is a function of contact pressure, surface finish, and gap size.  
           [0004]    As the power density of electronic devices increases, heat transfer from the heat generating devices to the surrounding environment becomes more and more critical to the proper operation of the devices. Many current electronic devices incorporate heat sink fins to dissipate heat to the surrounding air moving over the fins. These heat sinks are thermally connected to the electronic devices by a variety of techniques. Some devices use a thermally conductive paste in an attempt to lower the contact resistance. Others may use solder between the two elements both for mechanical strength and thermal conductance. However, these two solutions require additional cost and process steps that would not be necessary except for presence of the contact resistance.  
           [0005]    Many present electronic modules include a plurality of heat-generating electronic devices on a single substrate. Often these devices to not have a co-planer upper surface which would allow a single heat sink to be thermally coupled to the plurality of devices. Thermal paste and other thermally conductive materials may be used to fill any gaps between the heat-generating electronic devices and the single heat sink, however large gaps, caused by tolerance stack-up issues between the heat-generating devices, are often not capable of being filled by a paste. Thermal gap pads are capable of filling gaps on the order of 20 to 200 mils, however, they have relatively low thermal conductivity, and may not be usable with high performance devices that generate large amounts of heat. In such cases, multiple heat sinks may be used, however, this adds cost and reduces the efficiency of the heat dissipation.  
         SUMMARY OF THE INVENTION  
         [0006]    A heat sink is constructed including at least one thermally conductive pedestal, allowing configuration of the heat sink to make contact with a plurality of heat-generating electronic devices where the devices may not be co-planar due to tolerance stack-up. The pedestals may be raised and lowered and tilted as needed to match the heights and tilts of the electronic devices. Within the heat sink is a cavity above the pedestal that may be filled with a thermally conductive material, such as solder, or a thermally conductive liquid, during construction to create a low thermal resistance contact between the pedestal and the heat sink fins. Also, thermally conductive material, such as thermal paste or a thermal pad, may be used between the heat generating device and the pedestal to create a low thermal resistance contact.  
           [0007]    Other aspects and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]    [0008]FIG. 1 is a cross-sectional view of an example embodiment of a heat sink according to the present invention.  
         [0009]    [0009]FIG. 2 is a cross-sectional view of an example embodiment of a heat sink including three threaded pedestals according to the present invention.  
         [0010]    [0010]FIG. 3 is a cross-sectional view of an example embodiment of a heat sink according to the present invention.  
         [0011]    [0011]FIG. 4 is a top view of an example embodiment of a heat sink according to the present invention.  
         [0012]    [0012]FIG. 5 is a flow chart of an example method of constructing a heat sink according to the present invention.  
         [0013]    [0013]FIG. 6 is a cross-sectional view of an example embodiment of a heat sink according to the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0014]    [0014]FIG. 1 is a cross-sectional view of an example embodiment of a heat sink according to the present invention. A heat-generating electronic device  100  is attached to a substrate  102 . A thermally conductive threaded pedestal  104  is thermally coupled with the electronic device  100  on a side opposite to that of the substrate  102 . A heat sink including a heat sink base  108 , a plate  112 , fins  120  and a thin plate  110  is attached to the threaded pedestal  104 . Note that in some embodiments of the present invention, the heat sink base  108 , plate  112 , and fins  120  may all be constructed as integral parts of a heat sink, instead of being constructed separately and assembled into a heat sink. In some embodiments of the present invention, the thin plate  110  will be configured such that the threaded pedestal  104  may be threaded into it at a small angle to match a tilt in the upper surface of the heat-generating electronic device  100 . Note that the threads shown in FIG. 1 are exaggerated in size for purposes of illustration. Many embodiments of the present invention will use threads proportionally smaller than those shown in this figure. In some embodiments of the present invention the plate  112  may be formed as a contiguous portion of the heat sink base  108 , as shown in FIG. 2. Other embodiments of the present invention may construct the plate  112  separately from the heat sink base  108  and physically connect them together to form a surface for the attachment of the heat sink fins  120 , as shown in FIG. 1. The plate  112  includes a drive access hole  114  and a solder overflow vent  116 . The threaded pedestal  104  includes a drive socket  106 .  
         [0015]    [0015]FIG. 2 is a cross-sectional view of an example embodiment of a heat sink including three threaded pedestals according to the present invention. In an example embodiment of the present invention, a plurality of thermally conductive threaded pedestals may be used with a single heat sink, allowing heat dissipation from a plurality of heat-generating electronic devices with non-co-planar upper surfaces. In the example embodiment of the present invention shown in FIG. 2 three heat-generating electrical devices with different heights are thermally coupled with a single heat sink body  220  and a single set of heat sink fins  240 . A first heat-generating electrical device  202  having a first height is attached to a substrate  200 , along with a second heat-generating electrical device  208  having a second height and a third heat-generating electrical device  214  having a third height. The first, second, and third heights may all be different as shown in the example embodiment of the present invention of FIG. 2. A heat sink body  220  is constructed including a first solder cavity  222 , a second solder cavity  228 , and a third solder cavity  234 . A first thin plate  242  including an opening  248  sized to fit a first pedestal  204  is attached to the heat sink body  220  under the first solder cavity  222 . A second thin plate  244  including an opening  250  sized to fit a second pedestal  210  is attached to the heat sink body  220  under the second solder cavity  228 . A third thin plate  246  including an opening  252  sized to fit a third pedestal  216  is attached to the heat sink body  220  under the third solder cavity  234 . A first solder overflow vent  226  and a first drive access hole  224  are included in the portion of the heat sink body  220  above the first solder cavity  222 . A second solder overflow vent  232  and a second drive access hole  230  are included in the portion of the heat sink body  220  above the second solder cavity  228 . A third solder overflow vent  238  and a third drive access hole  236  are included in the portion of the heat sink body  220  above the third solder cavity  234 . A first threaded pedestal  204  including a first drive socket  206 , a second threaded pedestal  210  including a second drive socket  212 , and a third threaded pedestal  216  including a third drive socket  218  are provided. In use of the example embodiment of the present invention shown in FIG. 2, the three threaded pedestals  204 ,  210 , and  216  are adjusted by a drive tool through the three drive access holes  224 ,  230 , and  236  to match the differing heights of the three heat-generating electrical devices  202 ,  208 , and  214 . Other embodiments of the present invention may not require the use of a drive socket and drive access hole. The threaded pedestals may be threaded into the heat sink to a known depth before the assembled heat sink is placed over the substrate, eliminating the need for a drive socket and drive access hole. A thermally conductive material, such as a solder paste, thermal grease, or a thermal pad, may be applied between the three heat-generating electrical devices  202 ,  208 , and  214  and the three threaded pedestals  204 ,  210 , and  216 . The three solder cavities  222 ,  228 , and  236  may be filled with melted solder to create a low resistance thermal connection between the three threaded pedestals  204 ,  210 , and  216  and the heat sink body  220 . The three solder cavities  222 ,  228 , and  236  may be filled either before or after the heat sink is mechanically attached to the substrate  200 . Upon filling of the three solder cavities  222 ,  228 , and  236  excess solder may escape via the three solder overflow vents  226 ,  232 , and  238 . The presence of solder at the three solder overflow vents  226 ,  232 , and  238  may be used as a visual indication that the three solder cavities  222 ,  228 , and  236  are full.  
         [0016]    [0016]FIG. 3 is a cross-sectional view of an example embodiment of a heat sink according to the present invention. In some embodiments of the present invention it may be desirable to simplify the heat sink body  308  by attaching a plate  312  between the heat sink body  308  and the heat sink fins  320 . Note that in some embodiments of the present invention, the heat sink base  308 , plate  312 , and fins  320  may all be constructed as integral parts of a heat sink, instead of being constructed separately and assembled into a heat sink. In this embodiment, the solder overflow vent  316  and the drive access hole  314  may be created in the plate  312  instead of into the heat sink body  308 . A thin plate  310  is attached to the bottom of the heat sink body  308  below a solder cavity  318 . A heat-generating electrical device  300  is attached to a substrate  302  and a thermally conductive threaded pedestal  304  including a drive socket  306  is threaded into the thin plate  310 . Other than the addition of the plate  312  this example embodiment of the present invention is similar to that shown in FIG. 1.  
         [0017]    [0017]FIG. 4 is a top view of an example embodiment of a heat sink according to the present invention. Cross-section A is the cross-section used in FIG. 1 and FIG. 3. Heat sink fins  400  are shown attached to a heat sink body  408 , as in the embodiment of the present invention shown in FIG. 2. In the example embodiments of the present invention shown in FIG. 1 and FIG. 3, the heat sink body  408  of FIG. 4 would show instead a plate. A solder overflow vent  406  and a drive access hole  402  are shown in the heat sink body  408 . A drive socket  404  may be seen through the drive access hole  402 .  
         [0018]    [0018]FIG. 5 is a flow chart of an example method of constructing a heat sink according to the present invention. In a step  500  a heat sink body including a solder cavity is provided. In an optional step  502  heat sink fins are attached to the heat sink body. In other example embodiments of the present invention, heat sink fins may be formed as an integral part of the heat sink body, or may not be needed at all. In a step  504  a solder overflow vent into the solder cavity is formed in the heat sink body. In an optional step  506  a drive access hole into the solder cavity is formed in the heat sink body. In a step  508  a thin plate including an opening sized to fit a thermally conductive pedestal is mechanically attached to the heat sink body under the solder cavity. In a step  510  a thermally conductive pedestal is threaded into the opening in the thin plate. In a step  512  the pedestals are adjusted to correspond to the height of a heat-generating electrical device on a substrate. In a step  514  the solder cavity is filled with molten solder. In an optional step  516  a thermally conductive material, such as a thermal paste is placed between the thermal pedestal and the electrical device. In an optional step  518  the heat sink assembly is mechanically attached to a substrate. Some embodiments of the present invention may not require the heat sink assembly to be mechanically attached to a substrate. They may use other techniques to prevent the heat sink from shifting within the scope of the present invention. Still other embodiments of the present invention may attach the heat sink assembly to a substrate before filling the solder cavity with molten solder.  
         [0019]    [0019]FIG. 6 is a cross-sectional view of an example embodiment of a heat sink according to the present invention. A heat-generating electronic device  600  is attached to a substrate  602 . A thermally conductive toothed pedestal  604  is thermally coupled with the electronic device  600  on a side opposite to that of the substrate  602 . Instead of spiral threads, the toothed pedestal  604  has a series of circular saw-tooth cuts about the outside of the pedestal. A heat sink including a heat sink base  608 , a plate  612 , fins  620  and a thin plate  610  is attached to the push-in pedestal  604 . In assembly, the toothed pedestal  604  is simply forced into an appropriately sized opening in the thin plate  610  and the saw-teeth in the surface of the toothed pedestal  604  keep it from backing out of the heat sink. Note that the teeth shown in FIG. 6 are exaggerated in size for purposes of illustration. Many embodiments of the present invention will use teeth proportionally smaller than those shown in this figure. The thin plate  610  is configured to allow the toothed pedestal  604  to fit snugly, but also allow the toothed pedestal  604  to fit into the plate at an angle, allowing for use over heat-generating devices  600  that are not parallel to the thin plate  610 . In some embodiments of the present invention the plate  612  may be formed as a contiguous portion of the heat sink base  608 , as shown in FIG. 2. Other embodiments of the present invention may construct the plate  612  separately from the heat sink base  608  and physically connect them together to form a surface for the attachment of the heat sink fins  620 , as shown in FIG. 6. The plate  612  includes a drive access hole  614  and a solder overflow vent  616 . The push-in pedestal  604  includes a drive socket  606 .  
         [0020]    The foregoing description of the present invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and other modifications and variations may be possible in light of the above teachings. The embodiments were chosen and described in order to best explain the principles of the invention and its practical application to thereby enable others skilled in the art to best utilize the invention in various embodiments and various modifications as are suited to the particular use contemplated. It is intended that the appended claims be construed to include other alternative embodiments of the invention except insofar as limited by the prior art.