Abstract:
A sealed pouch constructed of thermally conductive flexible material containing a low melting point, thermally conductive material is placed between two components that require thermal continuity. This assembly is then loaded in compression and heated to the melting point of the low melting point, thermally conductive material, which then melts within the sealed pouch, and conforms to the shape of the two components. The sealed pouch also may contain a springy material made of a metal, or a solder compatible plastic or organic to help maintain shape of the pouch in some applications.

Description:
This is a divisional of application Ser. No. 10/104,730, filed Mar. 21, 2002 also entitled now abandoned, “Thermal Pouch Interface,” which is hereby incorporated by reference herein. 

   FIELD OF THE INVENTION 
   The present invention relates generally to the field of heat transfer and more specifically to the field of heat transfer between irregularly shaped objects. 
   BACKGROUND OF THE INVENTION 
   Modem 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. 
   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 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 and the absence or presence of material filling small gaps or surface variations in the interface. 
   The heat-sinking problem is particularly difficult in devices such as multi-chip modules (“MCMs”) where multiple components need to have topside cooling into a single cold plate or heat sink. The various components within the multi-chip module may not be of equal thickness, creating a non-coplanar surface that often must be contacted to a single planar surface of the cold plate or heat sink. Engineers have developed a variety of approaches to solving the non-coplanar surface problem, such as, gap fillers comprising thick thermal pads capable of absorbing 10 to 20 mils of stack up differences. However, the thickness and composition of these thermal pads often results in a relatively high thermal resistance making them suitable only for low power devices. Others have used pistons with springs attached to them attached to a plurality of small cold plates or heat sinks to account for the irregularity of the stack up. However, this can become an expensive solution to the problem. Still others have used an array of small cold plates connected together by flexible tubing allowing some flexibility between the plates to account for the variations in height of the components. However, once again, this solution may become too expensive for many products. 
   Other solutions include the use of thermal grease or phase change materials, such as paraffin, to fill in small gaps, such as the microscopic roughness between two surfaces. However, thermal grease and phase change materials are unable to fill larger gaps such as those present in multi-chip modules. 
   SUMMARY OF THE INVENTION 
   A sealed pouch constructed of thermally conductive flexible material containing a low melting point, thermally conductive material is placed between two components that require thermal continuity. This assembly is then loaded in compression and heated to the melting point of the low melting point, thermally conductive material, which then melts within the sealed pouch, and conforms to the shape of the two components. The sealed pouch also may contain a springy material made of a metal, or a solder compatible plastic or organic to help maintain shape of the pouch in some applications. 
   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 
       FIG. 1  is a cross-section of the interface between two surfaces. 
       FIG. 2  is a graph of temperature versus position through an interface between two thermal conductors. 
       FIG. 3A  is a cross-section of an example embodiment of a thermal pouch interface according to the present invention during construction. 
       FIG. 3B  is a cross-section of the example embodiment of a thermal pouch interface from  FIG. 3A  after construction. 
       FIG. 4  is a cross-section of an example embodiment of a thermal pouch interface according to the present invention prior to use between two components. 
       FIG. 5  is a cross-section of the example embodiment of a thermal pouch interface from  FIG. 4  during use between two components. 
       FIG. 6A  is a cross-section of an example embodiment of a thermal pouch interface according to the present invention including a single spring. 
       FIG. 6B  is a cross-section of an example embodiment of a thermal pouch interface according to the present invention including a plurality of springs. 
       FIG. 7  is a flow chart of an example method for the construction of a thermal pouch according to the present invention. 
   

   DETAILED DESCRIPTION 
     FIG. 1  is a cross-section of the interface between two surfaces. In this greatly magnified view of the interface between two surfaces, a first object  100  having a first surface  102  is brought into contact with a second object  104  having a second surface  106 . Neither surface is perfectly flat resulting in an imperfect mating of the two surfaces. This imperfect interface contributes to a thermal contact resistance at the interface between the two objects. 
     FIG. 2  is a graph of temperature versus position through an interface between two thermal conductors. In this view of two thermally conductive objects joined together, a graph of temperature versus position is shown below a cross-sectional view of the two objects including the thermal interface  210  between them. A first object  200  is joined with a second object  202  producing a thermal interface  210  at the point where the objects join. As shown in  FIG. 1 , this interface between the two objects is not a perfect joint and contributes to a thermal contact resistance at the thermal interface  210 . When thermal energy as heat  204  enters the first object  200 , passes through it to the second object  202 , before exiting the second object as heat  206 , the thermal energy must pass through the thermal interface  210  between the two objects. The thermal energy enters the first object  200  at a position  208  and a temperature T 1   214 , and decreases to a temperature T 2   216  as it passes through the first object  200 . At the thermal interface  210  between the two objects the thermal energy must overcome a thermal contact resistance and the temperature decreases to a temperature T 3   218  as it enters the second object  202 . The temperature decreases to a temperature T 4   220  as it passes through the second object  202  where it is radiated as heat  206  at a position  212 . 
     FIG. 3A  is a cross-section of an example embodiment of a thermal pouch interface according to the present invention during construction. A top sheet  300  and a bottom sheet  302  of a flexible, thermally conductive material are constructed to enclose a quantity of low melting point, thermally conductive material  306 , such as low melting point solder or liquid metal. As discussed in more detail below, the low melting point, thermally conductive material  306  will be in liquid form as the thermal pouch is compressed. Therefore, the melting point of the low melting point, thermally conductive material  306  must be lower than the melting point of the materials used in the devices that are to be thermally joined by the thermal pouch. Those skilled in the art will recognize that many different flexible, thermally conductive materials may be used to create the top sheet and bottom sheet within the scope of the present invention. Some example materials include, copper, aluminum, and mylar. Those skilled in the art will also recognize that there are many different methods of creating a top sheet and a bottom sheet from flexible, thermally conductive material within the scope of the present invention. Some example methods include, creating two separate sheets of material, folding a single sheet of material to form a top sheet and a bottom sheet and sealing three edges, or forming a cylinder of the material to create a top sheet and a bottom sheet. Other embodiments of the present invention may use a thermally conductive liquid, such as mercury, as the low melting point, thermally conductive material  306 . While mercury is toxic, if kept sealed within a thermal pouch, it may pose little risk. Optionally, a springy material  304  may be included in the construction if needed to help maintain contact pressure between the thermal pouch and the two components it will be sandwiched between. The springy material  304  may comprise a metal or solder compatible plastic or organic that has sufficient springy properties to resist deformation to some extent. In a preferred embodiment of the present invention, the low melting point, thermally conductive material  306 , once melted, will fill the interstices within the springy material  304  but not penetrate the individual wires or fibers of the springy material  304 . Other embodiments of the present invention may use one or more springs as the springy material  304  as shown in  FIGS. 6A and 6B . Still other embodiments may use metal wool as the springy material  304 . Steel wool and copper wool are two examples of metal wool. Other embodiments of the present invention may not require any springy material  304  and be constructed containing only a low melting point, thermally conductive material  306 . 
     FIG. 3B  is a cross-section of the example embodiment of a thermal pouch interface from  FIG. 3A  after construction. After the top sheet  300  and bottom sheet  302  have been sealed together, a thermal pouch filled with a low melting point, thermally conductive material  306 , and optionally a quantity of springy material  304  is created. In a preferred embodiment of the present invention, the final thermal pouch will be completely filled with the low melting point, thermally conductive material  306  and springy material  304  with all air (or other gases) expelled from the pouch during construction. This eliminates any air pockets within the thermal pouch that may cause a reduction in thermal conductivity of the thermal pouch. Thus, once the thermal pouch is sealed, and the temperature raised above the melting point of the low melting point, thermally conductive material  306 , the thermal pouch will be flexible enough to conform to non-planar surfaces of the devices it is used to thermally join. Other embodiments of the present invention may include a coating  308  such as thermal grease, phase change material, or solder on the outer surfaces of the thermal pouch to fill in the very small irregularities in the interface between the thermal pouch and any components it contacts. Note that some embodiments of the present invention may use a different coating  308  on the top surface of the thermal pouch, than the coating  308  on the bottom surface of the thermal pouch. Also, some embodiments of the present invention may use a coating  308  on only one surface of the thermal pouch, or not use any coating  308  at all. 
     FIG. 4  is a cross-section of an example embodiment of a thermal pouch interface according to the present invention prior to use between two components. A completed thermal pouch is placed between a top component  400  and a bottom component  402  having non-coplanar surfaces. The thermal pouch comprises a top sheet  300 , a bottom sheet  302 , a quantity of low melting point, thermally conductive material  306  and a quantity of springy material  304 , as shown in  FIGS. 3A and 3B . In an example use of the present invention, the bottom component  402  may be a multi-chip module and the top component  400  may be a heat sink. One embodiment of the present invention may use a single thermal pouch interface between the multi-chip module and the heat sink, while another embodiment may use a plurality of small thermal pouch interfaces between the individual components on the multi-chip module and the heat sink. 
     FIG. 5  is a cross-section of the example embodiment of a thermal pouch interface from  FIG. 4  during use between two components. The temperature of the thermal pouch is raised above the melting point of the low melting point, thermally conductive material  306 , yet below the melting point of materials within the two components  400  and  402 . The two components  400  and  402  from  FIG. 4  are now moved to their final positions, compressing the thermal pouch between them. Since the low melting point, thermally conductive material  306  is in a liquid state during this compression of the thermal pouch, the pouch flexes to conform to any non-planarity in the surfaces of the two components  400  and  402 . The thermal pouch comprises a top sheet  300 , a bottom sheet  302 , a quantity of low melting point, thermally conductive material  306  and a quantity of springy material  304 , as shown in  FIGS. 3A and 3B . Note that the thermal pouch has deformed to match the non-coplanar shapes of the two components  400  and  402  creating a low thermal resistance thermal contact between the two components  400  and  402 . 
     FIG. 6A  is a cross-section of an example embodiment of a thermal pouch interface according to the present invention including a single spring.  FIG. 6A  shows an example embodiment of the present invention similar to that of  FIG. 3B  where the springy material  304  comprises a single spring  600 . 
     FIG. 6B  is a cross-section of an example embodiment of a thermal pouch interface according to the present invention including a plurality of springs.  FIG. 6B  shows an example embodiment of the present invention similar to that of  FIG. 3B  where the springy material  304  comprises a plurality of springs  602 . 
     FIG. 7  is a flow chart of an example method for the construction of a thermal pouch according to the present invention. In a step  700  a top sheet  300  of a thermal pouch is created. In a step  702  a bottom sheet  302  of a thermal pouch is created. In an optional step  704  a quantity of springy material  304  is placed between the top sheet  300  and the bottom sheet  302 . In a step  706  a quantity of low melting point, thermally conductive material is placed surrounding the springy material  304 . In a step  708  the top sheet  300  and bottom sheet  302  are affixed to each other forming a thermal pouch. In an optional step  710  one or more surfaces of the thermal pouch are coated with thermal grease, phase change material, or solder  308 . 
   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.