As integrated circuits have become smaller and more powerful, the thermal resistance between the integrated circuits and their heat sinks which remove their generated heat is becoming a limiting factor in cooling. Generally, because of considerations in regard to differential thermal expansion between the integrated circuit and the heat sink, some sort of deformable intermediate material is placed between the integrated circuit and the heat sink, but that results in two thermal interfaces, one from the integrated circuit to the intermediate material and another between the intermediate material and the heat sink.

Even when the intermediate material itself has good thermal conductivity, the contact surfaces on either side of it add considerable thermal resistance. High thermal conductivity fibers of carbon and other materials have been used successfully to construct such intermediate materials, but they have not solved the thermal interface problem. It would be a great advantage for cooling integrated circuits if at least one of the thermal interfaces between the integrated circuit and the heat sink could be eliminated.

SUMMARY OF THE INVENTION The present invention is a heat sink structure or heat pipe casing which minimizes the thermal resistance at the interface of the heat sink or heat pipe with the heat source. This is accomplished by constructing the heat sink, or the heat pipe casing, as a matrix with heat conductive fibers captured within a body. The fibers are oriented in the direction of heat flow, and the fibers protrude out of the matrix to form the intermediate deformable material which contacts the heat source. The structure of the preferred embodiment incorporates carbon fibers directly into the volume of and extending out of the surface of the heat sink.

The body is cast around the carbon fibers, and the material of the body can be copper, tungsten, kovar, or even ceramic or plastic. The fibers are continuous through the casting and protrude from the matrix structure at least from the surface of the matrix adjacent to the heat source, but can also protrude through the other side of the body to serve as part of the cooling system.

When the structure of the invention is used as the wall of a heat pipe casing, such fibers protrude into the interior of the heat pipe and form a capillary evaporator wick.'In a more conventional heat sink, the fibers can protrude out of a surface of the heat sink other than the surface at the heat source, and the fibers can serve as fluid cooled spines with air or liquid flowing through them.

The method of forming such a structure is relatively simple. All that is necessary is to orient the fibers in a parallel configuration within a casting crucible and to cast the body around the fibers. Typically this results in a relatively long structure which can then be cut into wafer-like structures with the thermally conductive fibers oriented across the thickness of the wafer. The ends of the fibers are then exposed on one or both of the large surfaces of the wafer by etching away the material surrounding the fibers.

Such a wafer can be used as one wall of a heat pipe when other walls are joined to the wall to form a full enclosure, and for additional vacuum integrity a layer of copper can be deposited on the inner surface of such a heat pipe casing.

By essentially integrating the intermediate stress relieving material directly into the heat sink, the invention provides a structure which completely eliminates the thermal interface between the intermediate material and the heat sink.

Thus, since conventionally the two thermal interfaces at the intermediate material account for most of the thermal resistance in the cooling system, the present invention almost halves the thermal resistance involved in cooling integrated circuits.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a side view of tne preferred embodiment of the heat sink of the invention showing the heat conductive fibers extending out of opposite vlrfaces of the heat sink.

FIG. 2 is a cutaway view of a portion of a heat pipe with one wall constructed according to the invention.

FIG. 3 is a cutaway view of a portion of a heat pipe using the invention with an additional sealing layer within the heat pipe.

DETAILED DESCRIPTION OF THE INVENTION FIG. 1 is a side view of the preferred embodiment of the invention showing heat sink 10 with heat conductive fibers 12 extending out of opposite surfaces 14 and 16 of body 18 of heat sink 10.

When heat sink 10 is in use, surface 16 is located adjacent to a heat source such as an integrated circuit package (not shown) from which heat sink 10 removes heat. Although fibers 12 can extend out of surfaces 14 and 16 by virtually any distance, typically the length of fibers 12 extending from surface 16 adjacent to the heat source are relatively short so that the length of the thermal path is minimized.

In the preferred embodiment, body 18 of heat sink 10 is constructed of a heat conductive metal such as copper, tungsten, or kovar, which is cast around the heat conductive fibers, but it would also be possible to use a heat insulator or a plastic. It should be appreciated that body 18 is typically a disk or rectangular plate and that FIG-. 1 is a view from one edge of such a plate. Fibers 12 thus are extending from the relatively large flat surfaces which would normally be considered the top and bottom of such a plate, and fibers 12 cover virtually all of those larger surfaces. It should be appreciated that the drawings have been made to more easily describe the invention and do not picture the invention in accurate scale. For instance, the density of fibers 12 is far greater than that suggested by the drawings. Typically heat sink 10 will have fibers packed together so that fibers 12 actually protrude from surfaces 14 and 16 in densities in the range of 100,000 to one million fibers per square centimeter.

Heat conductive fibers 12 are well known in the art of heat transfer. They are typically carbon or carbon composite material, 0.5 to 5 microns in diameter, and are available in continuous lengths. One such fiber is manufactured by BT-Amoco Chemicals and is identified as K1100 or T300.

Heat conductive fibers 12 serve not only to conduct the heat from the heat source but also facilitate disposal of the heat. The virtual surface formed by the ends of fibers 12 extending out of surface 14 directly radiates heat into the environment because the carbon fibers have very good emissivity. However, it is also practical to use a fluid to cool fibers 12 extending from surface 14. To accomplish that it is only necessary to direct a flow of air or liquid at and through fibers 12. The alternate directions of such a cooling air flow are indicated by arrows A and B on FIG. 1, and are easily generated by fans (not shown).

The benefit of this structure is the elimination of one of the two usual thermal interfaces between the heat source and the heat sink. A deformable intermediate material is conventionally used between the heat sink and the heat source, and this intermediate material must be held in thermal contact with both the heat sink and the heat source, with a thermally resistive interface at each contact. However, while the ends of the fibers extending from surface 16 must still contact the heat source to form one interface, the second interface, the one at the heat sink, no longer exists. Instead, the fibers which contact the heat source are integrated directly into the heat sink and directly conduct the heat either through it or into it.

FIG. 2 shows another way to transfer heat to or from a structure by use of the invention. FIG. 2 is a cutaway view of a portion of a heat pipe 20 with one wall 22 constructed according to the invention. As previously described, wall 22 of heat pipe 20 is constructed with heat conducting fibers 24 oriented across the thickness of wall 22 and extending from both inner surface 26 and outer surface 28 of wall 22. Fibers 24 extending from outer surface 28 act in the same manner as those discussed in regard to FIG. 1 in that they are in contact with a structure (not shown) and conduct heat away from or to the structure.

However, ends 30 of fibers 24 which extend from internal surface 26 of wall 22 act in a different manner. Ends 30 not only conduct heat through the wall of the heat pipe, but because they are closely packed fibers, they form a capillary surface at the inside surface of wall 22 of heat pipe 20.

Thus, liquid within heat pipe 20 is distributed throughout the interior surface of wall 22 by the capillary action of fiber ends 30.

When wall 22 acts as an evaporator, liquid is delivered to fiber ends 30 by means of conventional heat pipe wick 32 attached to walls 34 and distributed throughout all the fiber ends 30 by capillary action. The heat being conducted to fiber ends 30 from a heat source in contact with the exterior ends of fibers 24 then evaporates the liquid and the resulting vapor moves the heat to the heat pipe condenser. Fiber ends 30 thereby become the evaporator surfaces of heat pipe 20, which results in an extremely large effective evaporator surface area.

However, wall 22 can also act as the condenser of heat pipe 20. In that case, heat is removed from exterior surface 28 of wall 22 and the exterior ends of fibers 24 located at surface 28. This cools interior ends 30 of fibers 24 and causes condensation of vapor within the heat pipe. The condensed liquid is then transported to the heat pipe evaporator by the capillary action of fiber ends 30 and conventional capillary wick 32.

Fibers 24 within wall 22 which is formed around the fibers therefore act in the same manner as in the heat sink of FIG. 1, and the fibers eliminate one of the two thermal interfaces conventionally used when transferring heat between a heat pipe and either a heat source or a heat sink.

FIG. 3 is a cutaway view of a portion of heat pipe 21 which has mostly the same construction as heat pipe 20 shown in FIG. 2. However, the interior fiber ends have been omitted and additional sealing layer 36 is added on interior surface 26 of wall 22 of heat pipe 20. Sealing layer 36 is a metal plated layer added to wall 22, and is used when wall 22 is thin enough that there may be some possibility of long term deterioration of the vacuum within heat pipe 21 because of leakage of air along the heat conducting fiber boundaries within the matrix of wall 22. In such a circumstance the typical plated metal is a 0.001 to 0.020 inch thick layer of copper.

Since there are no interior heat conducting fiber ends within heat pipe 21, a different capillary structure is required. To furnish the capillary surface on wall 22, conventional heat pipe wick 32 is extended from walls 34 onto wall 22 and attached to and covering sealing layer 36 to form capillary wick 38 of heat pipe 21. As in heat pipe 20 of FIG. 2, wall 22 of heat pipe 21 can then function as either the evaporator or the condenser of the heat pipe.

It should be appreciated that, despite the absence of internal fiber ends in heat pipe 21, the essential benefit of the invention is still present because the structure of FIG. 3 still eliminates the conventional thermal interface between the outside of the heat pipe wall and the usual intermediate material. As with the other embodiments of the invention, heat is transferred directly from the exterior ends of the heat conducting fibers into the heat pipe wall into which the fibers are integrated.

The heat transfer interface between the fibers and the internal portions of the wall formed around the fibers is quite different from the conventional heat conducting interface between a structure and an adjacent heat sink. While with the conventional heat sink the surface area at the interface with any other structure is determined by the contact surface area between the heat sink and the structure, the surface area between the fibers and the body formed around them is virtually unlimited. The heat transfer surface area is the sum total of the surface areas of all the fibers encased in the body and is therefore very large. This results in an extremely low thermal resistance because the thermal resistance across a surface is inversely proportional to the area of the surface. Thus, even when the heat conducting fibers are not conducting heat all the way through the body which encases them, they act to reduce the thermal resistance of heat transfer with the body to an insignificant factor.

The method of constructing the heat sink or heat pipe wall of the invention is relatively simple. The first step is to place the heat conducting fibers in a parallel orientation within a casting mold and to cast a material around the fibers with the fibers extending from one end of the cast body to the other. For the production of multiple heat sinks, the fibers used can be relatively long, resulting in a long structure.

This structure is then cut into multiple wafer-like structures with the length of the thermally conductive fibers oriented across the thickness of the wafer. Segments of the ends of the fibers are then exposed on one or both of the large surfaces of the wafer by etching away the material surrounding'the fibers.

When the resulting wafers are to be used as walls for heat pipes, it is desirable to cast the body within a vacuum environment because this prevents subsequent degassing of the material and deterioration of the internal vacuum within the heat pipe.

The invention can thus be manufactured in large batches which provides low costs, and the resulting structure not only eliminates the cost of the conventional intermediate stress relieving parts, but also dramatically reduces the thermal resistance in the heat removal system for devices such as integrated circuits.

It is to be understood that the form of this invention as shown is merely a preferred embodiment. Various changes may be made in the function and arrangement of parts; equivalent means may be substituted for those illustrated and described; and certain features may be used independently from others without departing from the spirit and scope of the invention as defined in the following claims.

For example, various materials other than those described can be used for heat conducting fibers 12 and 24, bodies 18 and 22, or sealing layer 36.

What is claimed as new and for which Letters patent of the United States are desired to be secured is: