# Patent

## Title
Heat spreader and method of making the same

## Abstract

A heat spreader having at least two adjoining strips of pyrolytic graphite material is made by cutting a strip from a sheet of pyrolytic graphite in the z direction. Thermal conductivity in the xy plane of the graphite sheet is greater than in the z direction. The z direction cut provides strips which are then each individually oriented 90 degrees such that the thickness direction of the original pyrolytic graphite sheet becomes the width or length of the cut strip. A face on the side of a first strip adjoins a face on the side of a second strip. Due to the greater thermal conductivity in the xy plane of the strips as compared to in the z direction heat transfers more rapidly in the length and thickness direction of the strips than across adjoining sides of the oriented strips.

## Background
BACKGROUND
The present invention relates to a heat spreader for conducting heat from a device and a method of making the heat spreader. Electronic components are becoming smaller while heat dissipation requirements are becoming greater. In order to dissipate heat generated by these electronic components, heat spreaders are utilized between the electronic component and a heat sink. Heat spreaders can be made of a solid thermally conductive metal. The solid conductive metal has a limited ability to spread heat and has limited thermal conductivity characteristics.
SUMMARY
According to the present invention, a heat spreader and a method for making the heat spreader is provided, and a method of dissipating from a heat source are disclosed.
In some embodiments, a heat spreader is provided which has at least two adjoining planar elements or strips of pyrolytic graphite material. The strips are made by cutting strips from a sheet of pyrolytic graphite such that the sheet has a cut there through in the z direction. Thermal conductivity in the xy plane of the pyrolytic graphite sheet is greater than in the z direction. The z direction cut provides strips which are then each individually oriented about 90 degrees such that the thickness direction of the original pyrolytic graphite sheet becomes the width or length of the cut strip. A portion of a lateral side of a first strip which has been formed by cutting the sheet of graphite and oriented about 90 degrees adjoins a face on the side of a second strip. Due to the greater thermal conductivity in the xy plane of the strips as compared to in the z direction heat transfers more rapidly along the length of the strip and in the thickness direction of the oriented strips than across a side of a strip which adjoins an adjoining strip.
In some embodiments of the invention the side of a first strip which adjoins the side of a second strip is coextensive with the second side.
In some embodiments of the invention three or more strips of substantially equal length are placed side by side.
Another embodiment of the invention is a method of making a heat spreader by providing at least two pyrolytic graphite strips or planar elements from a sheet of pyrolytic graphite. A cut is made in the thickness direction of the sheet which is known as the z direction. The thermal conductivity of the sheet in the z direction or as is commonly referred to as the c direction is relatively low as compared to the thermal conductivity in the xy plane or as is commonly referred to as the a directions or axes. The side of a first strip is then placed such that the side adjoins the side of a second strip. In this configuration heat transfers more rapidly along the length of the strip and in the thickness direction of the oriented strips than across a side of the strip which adjoins an adjacent strip.
Another embodiment of the invention is a method of placing the heat spreader in a heat conducting relationship with a heat source by providing adjoining pyrolytic graphite strips. The side of a first strip is placed such that the side adjoins the side of a second strip. Heat transfers more rapidly along the length of the strip and in the thickness direction of the oriented strips than across a side of the strip which adjoins an adjacent strip. Heat is conducted from the heat source into the first strip and second strip. Heat is conducted through the heat spreader in the direction of the a directions or axes of the pyrolytic graphite strips.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a parallel perspective view of a sheet of pyrolytic graphite for use in the present invention showing the direction of the a and c axes of the layers of pyrolytic graphite of the sheet;
FIG. 2 is a parallel perspective view of the sheet of pyrolytic graphite of FIG. 1 showing a first planar element which has been diced from the sheet and separated therefrom;
FIG. 3a shows the first planar element of FIG. 2 after orientation of about 90 degrees;
FIG. 3b shows the first planar element and the second planar element prior to adjoining;
FIG. 4 shows an embodiment of the heat spreader of the present invention and the direction of the a and c axes of the pyrolytic graphite in the first planar element and second planar element;
FIG. 5 shows another embodiment of the heat spreader of the present invention and the direction of the a and c axes of the pyrolytic graphite in the first planar element and second planar element;
FIG. 6 shows another embodiment of the heat spreader of the present invention and the direction of the a and c axes of the pyrolytic graphite in the first planar element, second planar element and third planar element;
FIG. 6A shows a third planar element of the heat spreader of FIG. 6;
FIG. 7 shows the heat spreader of the present invention in combination with an electronic device and a heat sink; and
FIG. 8 shows another embodiment of the heat spreader of the present invention having a throughhole in the thickness direction of the planar elements of the heat spreader.



DETAILED DESCRIPTION OF THE INVENTION
The invention will now be described in detail by reference to the following specification and non-limiting examples.
Without further elaboration, it is believed that one skilled in the art can, using the preceding description, utilize the present invention to its fullest extent. The following embodiments are, therefore, to be construed as merely illustrative, and not limitative of the remainder of the disclosure in any way whatsoever.
Graphite is made up of layer planes of hexagonal arrays or networks of carbon atoms. These layer planes of hexagonal arranged carbon atoms are substantially flat and are oriented so as to be substantially parallel and equidistant to one another. The substantially flat parallel layers of carbon atoms are referred to as basal planes and are linked or bonded together in groups arranged in crystallites. Conventional or electrolytic graphite has a random order to the crystallites. Highly ordered graphite has a high degree of preferred crystallite orientation. Accordingly, graphite may be characterized as laminated structures of carbon having two principal axes, the “c” axis or direction which is generally identified as the axis or direction perpendicular to the carbon layers and the “a” axes or directions parallel to the carbon layers and transverse to the c axes.
Referring now to the drawings in detail, wherein like reference numerals indicate like elements through the several views, there is shown in FIG. 1 a sheet 10 for making the heat spreader of the present invention having axes a which are in the direction of the hexagonal array of carbon atoms. The c axis as shown is perpendicular to the carbon layers.
Graphite materials that exhibit a high degree of orientation include natural graphite and synthetic or pyrolytic graphite. Natural graphite is commercially available in the form of flakes (platelets) or as a powder. Pyrolytic graphite is produced by the pyrolysis of a carbonaceous gas on a suitable substrate at elevated temperature. Briefly; the pyrolytic deposition process may be carried out in a heated furnace and at a suitable pressure, wherein a hydrocarbon gas such as methane, natural gas, acetylene etc. is introduced into the heated furnace and is thermally decomposed at the surface of a substrate of suitable composition such as graphite having any desirable shape. The substrate may be removed or separated from the pyrolytic graphite. The pyrolytic graphite may then be further subjected to thermal annealing at high temperatures to form a highly oriented pyrolytic graphite commonly referred to as HOPG.
In FIG. 2 is shown a sheet 10 of pyrolytic graphite having the direction of the a axes and the c axis as shown. A first planar element 12 or strip is cut or diced from the sheet 10 of pyrolytic graphite and after the first planar element 12 is cut from the sheet the direction of the a axes and c axis within the first planar element 12 remain in the same direction as when the first planar element 12 formed part of the sheet 10.
Planar element 12 after being cut from sheet 10 is oriented about 90 degrees or about 270 degrees such that the direction of the c axis of the first planar element 12 changes from the direction shown in FIG. 2 to the direction shown in FIG. 3a. Accordingly, it can be seen that after orientation of the first planar element 12 the relative location of first side 14 of first planar element 12 has changed from that shown in FIG. 2 to that shown in FIG. 3a. A second planar element 16 as shown in FIG. 3b is cut from sheet 10 and orientated 90 or 270 degrees in a manner similar to that described above for the first planar element 12.
According to an embodiment of the present invention, a first side 14 of first planar element 12 which is out of the plane of the plane of the planar element 12 is adjoined with second side 18 of second planar element 16 which is out of the plane of the second planar element 16 such that at least a portion the first side 14 adjoins at least a portion of the second side 18 as seen in FIG. 5.
In another embodiment of the invention, the first side 14 of the first planar element 12 can extend substantially coextensively with the second side 18 of the second planar element 16.
As can be seen in FIGS. 4 and 5, the portion of the first side 14 of the first planar element 12 which adjoins the portion of the second planar element 16 extends substantially normal to the first plane of the first planar element 12. The first plane of the first planar element 12 is defined by the direction in which the major dimension h and minor dimension g extend as shown in FIG. 4. The major dimension h and minor dimension g can be of equal magnitude, however the major dimension h and minor dimension g are not the thickness dimension of the planar element.
The major dimension h and minor dimension g can be the first lateral dimension and second lateral dimensions of the first planar element 12.
The direction of the first lateral dimension or major direction h of the planar element 12 and the thickness direction i of the first planar element can be the direction of the a axes of the sheet 10 of pyrolytic graphite from which the first planar element 12 is formed. The direction of the second lateral dimension can be the direction of the c axis of the sheet 10 of pyrolytic graphite from which the first planar element 12 is formed as seen in FIG. 1. Therefore, as seen in FIG. 4, the first planar element has a relatively high thermal conductivity in the first lateral dimension, here, major dimension h of the planar element and in the thickness direction i of the first planar element but a relatively low thermal conductivity in the second lateral dimension or minor dimension g. Therefore, heat is conducted more readily along major dimension h and in the thickness direction i than in minor dimension g than across first side 14 of first planar element 12 to second side 18 of second planar element 16.
The heat spreader of the present invention can be made such that the planar elements each have three sets of parallel sides. Each side can be orthogonal to two other sides of the planar element. The two opposite sides of a planar element can be spaced apart at substantially the same distance along each side.
The sheets of pyrolytic graphite from which the planar elements are cut or diced by any means for cutting the sheets such as wirecutting machines, dicing machines, or slicing machines are available in sizes having a thickness in the f dimension shown in FIG. 1 of from 0.2 millimeters up to 5 centimeters. A typical thickness is 1.3 centimeters. Commercially available pyrolytic graphite sheets are available having a length or d dimension of about 3 meters and the width dimension e can be as large as 40 centimeters. Pyrolytic graphite sheets suitable for use in the present invention are available from the Pyrogenics Group of Minteq International Inc. of New York, N.Y. An example is PYROID® HT pyrolytic graphite.
In one embodiment the distance in which the first side and the second side of the first planar element are spaced apart is at least about 1.5 millimeters.
In another embodiment the distance at which the first side and the second side of the first planar element are spaced apart is from about 1.5 millimeters to about 1.3 centimeters.
In another embodiment the distance at which the first side and the second side of the first planar element are spaced apart is from about 1.3 centimeters to about 2.5 centimeters.
In another embodiment the distance at which the first side and the second side of the first planar element are spaced apart is at least about 1.3 centimeters.
In another embodiment the distance at which the first side and the second side of the first planar element are spaced apart is at least about 4.0 centimeters.
In another embodiment the distance at which the first side and the second side of the first planar element are spaced apart is from about 1.3 centimeters to about 5.0 centimeters.
In another embodiment the distance at which the third side and the fourth side of the first planar element are spaced apart is at least about 1.0 centimeter.
In another embodiment the distance at which the third side and the fourth side of the first planar element are spaced apart is from about 1.0 centimeters to about 40 centimeters.
The thermal conductivity of the sheets in the a axes of the sheets can be from about 450 to about 2000 Watts/m° K and the particular thermal conductivity for a particular application can be tailored. The thermal conductivity in the z direction or along the c axis can be as low as about 2.0 Watts/m° K or in the case of PYROID® HT pyrolytic graphite 7 Watts/m° K. By comparison the thermal conductivity of copper is 400 Watts/m° K. As copper has a density of 8.9 g/cc as compared to values for pyrolytic graphite of as high as 2.25 g/cc, greater efficiencies and weight savings can be achieved using the heat spreader of the present invention.
Thermal grease can be used at the interface between the first planar element 12 and second planar element 16. The heat spreader 22 of FIG. 4 can be adjoined to a substrate such as a heat sink, here a copper plate 20 as seen in FIG. 7 by any suitable means for adjoining the first planar element 12 and second planar element 16 to a substrate. In the event that the heat spreader 22 is adjoined to a heat sink the means for adjoining the heat spreader 22 to the substrate permits the transfer of heat from the heat spreader 22 to the substrate. A mechanical means such as a clamping means can be a means to adjoin the heat spreader to a substrate which in turn transfers heat from the heat spreader to a heat sink. Also, the heat spreader can be adjoined directly to a heat sink. Additional means for adjoining the heat spreader to a substrate or heat sink can be a bonding means. The bonding means can be a layer of metal or a layer which comprises metal on a planar element of the heat spreader which is bonded to the substrate such as by soldering at least a portion of the metal containing layer to the substrate or heat sink. The layer is applied to a planar element on at least a portion of the planar element which is to adjoin the substrate. After application of the metal containing layer on at least a portion of the planar element, the planar element can be adjoined to the substrate or heat sink by techniques used in the semiconductor industry such as soldering or even by a mechanical means such as a mechanical fastener.
Application of the metal containing layer on a portion of the planar element which adjoins the substrate can be achieved by metallization techniques, sputtering or by applying a layer of solder to the portion of the planar element which is to be joined to the substrate. The planar elements can be provided with a surface treatment prior to the application of the metal containing layer using techniques suitable for use on semiconductors.
Any means for joining the first planar element 12 and the second planar element 16 can be used. For example, a mechanical clamping means such as a mechanical fastener can be used to join the first planar element 12 and the second planar element 16 together or the first planar element 12 and the second planar element 16 can be soldered together using techniques which are capable of joining carbon-based surfaces together. Upon adjoining of the first planar element 12 and the second planar element 16 heat can transfer from the first planar element 12 and the second planar element 16 along the portion wherein the first planar element 12 and the second planar element 16 are adjoined.
In another embodiment of the present invention, a heat spreader has a first planar element 12, a second planar element 16 and a third planar element 24 as seen in FIGS. 6 and 6A. The third planar element 24 is cut or diced from the sheet 10 of pyrolytic graphite and oriented in a manner similar to that in which the first planar element 12 and the second planar element 16 are cut. A third side 26 of second planar element 16 is arranged such that the third side 26 adjoins a fourth side 28 of third planar element 24. In a similar way, heat spreaders of the present invention can be made with a fourth, fifth or sixth etc. planar element. Each additional planar element has a side which adjoins an adjacent side of a planar element of the heat spreader such that heat transfers more rapidly in the two dimensions of the heat spreader which do not have a side which adjoins an adjacent planar element.
Because the a and c axes of the pyrolytic graphite of all three of the strips which make up this embodiment of the invention are arranged in the direction shown in FIG. 6, heat is transferred more readily in the j and k dimensions as compared to the 1 dimension.
In FIG. 7 a heat spreader of the present invention is shown in combination with an electronic device 30 and a heat sink 20 which is a copper plate. Heat from the electronic device 30 is transferred to the heat spreader 22. From the heat spreader 22, heat is transferred most rapidly in the direction of the thickness dimension i and in the direction of lateral dimension h which are oriented in the a axes of the pyrolytic graphite from which the heat spreader 22 is made. Heat is transferred less rapidly across the interface between the first planar element 12 and the second planar element 16.
The electronic device can be a microprocessor, an integrated circuit, high power devices such as laser diodes, LEDs, wide band gap, RF and microwave devices, power amplifiers, insulated gate bipolar transistors (IGBTs), application specific integrated circuits (ASICs), liquid crystal display (LCDs) and other types of video displays.
In yet another embodiment of the present invention, at least one of the first planar element 12 and the second planar element 16 has a throughhole 32 at least partially therethrough. A core 34 of material which can be isotropic or anisotropic such as pyrolytic graphite can be inserted into the throughhole 32. The core can be or can comprise a metal having a relatively high thermal conductivity or even diamond. The core 34 in the throughhole permits the transfer of more heat in the thickness direction i of the first planar element 12 or the second planar element 16.
The invention also includes another embodiment disclosing a method of dissipating heat from a heat source comprising providing a heat spreader having a first planar element and second planar element arranged as described above. The heat spreader is placed in a heat conducting relationship with a heat source such that the heat spreader conducts heat from the heat source into the first strip and second strip. Heat is conducted through the heat spreader in the direction of relatively high thermal conductivity.
Accordingly, it is understood that the above description of the present invention is susceptible to considerable modifications, changes and adaptations by those skilled in the art, and that such modifications, changes and adaptations are intended to be considered within the scope of the present invention, which is set forth by the appended claims.

## Claims

1. A heat spreader comprising:
a) a first planar element of pyrolytic graphite having a relatively high thermal conductivity in a direction of a first lateral dimension of a first plane of the first planar element and in a thickness direction of the first planar element and having a relatively low thermal conductivity in a direction of second lateral dimension of the first planar element, and
b) a second planar element of pyrolytic graphite having a relatively high thermal conductivity in a direction of a first lateral dimension of a second plane of the second planar element and in a thickness direction of the second planar element and having a relatively low thermal conductivity in a direction of a second lateral dimension of the second planar element,
wherein at least a portion of a first side of the first planar element which extends in a direction out of the first plane of the first planar element adjoins at least a portion of a second side of the second planar element which extends in a direction out of the second plane of the second planar element.

2. The heat spreader according to claim 1 wherein the portion of the first side of the first planar element which adjoins the portion of the second side of the second planar element extends substantially normal to the first plane of the first planar element, and the portion of the second side of the second planar element which adjoins the portion of the first side of the first planar element extends substantially normal to the second plane of the second planar element.

3. A heat spreader comprising:
a) a first planar element of pyrolytic graphite having a relatively high thermal conductivity in a direction of a first lateral dimension of a first plane of the first planar element and in the thickness direction of the first planar element and having a relatively low thermal conductivity in a direction of second lateral dimension of the first planar element, wherein the direction of the first lateral dimension of the first plane of the first planar element and the thickness direction of the first planar element extend substantially in directions of orientation of a axes of the pyrolytic graphite of the first planar element and the direction of the second lateral dimension of the first planar element extends substantially in the direction of the c axis of the pyrolytic graphite of the first planar element,
b) a second planar element of pyrolytic graphite having a relatively high thermal conductivity in a direction of a first lateral dimension of a second plane of the second planar element and in the thickness direction of the second planar element and having a relatively low thermal conductivity in a direction of a second lateral dimension of the second planar element, wherein the direction of the first lateral dimension of the second plane of the second planar element and the thickness direction of the second planar element extend substantially in directions of orientation of a axes of the pyrolytic graphite of the second planar element and the direction of the second lateral dimension of the second planar element extends substantially in the direction of the c axis of the pyrolytic graphite of the second planar element,
wherein at least a portion of a first side of the first planar element which extends in a direction out of the first plane of the first planar element adjoins at least a portion of a second side of the second planar element which extends in a direction out of the second plane of the second planar element.

4. A heat spreader comprising:
a) a first planar element of pyrolytic graphite having a relatively high thermal conductivity in a direction of a first lateral dimension of a first plane of the first planar element and in the thickness direction of the first planar element and having a relatively low thermal conductivity in a direction of second lateral dimension of the first planar element, wherein the direction of the first lateral dimension of the first plane of the first planar element and the thickness direction of the first planar element extend substantially in directions of orientation of a axes of the pyrolytic graphite of the first planar element and the direction of the second lateral dimension of the first planar element extends substantially in the direction of the c axis of the pyrolytic graphite of the first planar element,
b) a second planar element of pyrolytic graphite having a relatively high thermal conductivity in a direction of a first lateral dimension of a second plane of the second planar element and in the thickness direction of the second planar element and having a relatively low thermal conductivity in a direction of a second lateral dimension of the second planar element, wherein the direction of the first lateral dimension of the second plane of the second planar element and the thickness direction of the second planar element extend substantially in directions of orientation of a axes of the pyrolytic graphite of the second planar element and the direction of the second lateral dimension of the second planar element extends substantially in the direction of the c axis of the pyrolytic graphite of the second planar element,
wherein each of the first planar element and second planar element have a first side and a second side, the first side and second side of each of the first planar element and second planar element being substantially parallel and being spaced apart in the direction of the second lateral dimension at a first distance, the first side and second side of each of the first planar element and second planar element extending substantially normal to the direction of the second lateral dimension of the first planar element and the second planar element respectively wherein at least a portion of the first side of the first planar element adjoins at least a portion of the second side of the second planar element.

5. The heat spreader according to claim 4 wherein each of the first planar element and second planar element have a third side and fourth side, the third and fourth side of each of the first planar element and second planar element being substantially parallel and being spaced apart in the direction of the first lateral dimension at a second distance, the third side and fourth side of each of the first planar element and second planar element extending substantially normal to the direction of the first lateral dimension of the first planar element and the second planar element respectively.

6. The heat spreader according to claim 4 wherein the first distance at which the first side and the second side of the first planar element are spaced apart is at least about 1.5 millimeters.

7. The heat spreader according to claim 4 wherein the first distance at which the first side and the second side of the first planar element are spaced apart is from about 1.5 millimeters to about 1.3 centimeters.

8. The heat spreader according to claim 4 wherein the first distance at which the first side and the second side of the first planar element are spaced apart is from about 1.3 centimeters to about 2.5 centimeters.

9. The heat spreader according to claim 4 wherein the first distance at which the first side and the second side of the first planar element are spaced apart is at least about 1.3 centimeters.

10. The heat spreader according to claim 4 wherein the first distance at which the first side and the second side of the first planar element are spaced apart is at least about 4.0 centimeters.

11. The heat spreader according to claim 4 wherein the first distance at which the first side and the second side of the first planar element are spaced apart is from about 1.3 centimeters to about meters to about 5.0 centimeters.

12. The heat spreader according to claim 5 wherein the second distance at which the third side and the fourth side of the first planar element are spaced apart is at least about 1.0 centimeter.

13. The heat spreader according to claim 5 wherein the second distance at which the third side and the fourth side of the first planar element are spaced apart is from about 1.0 centimeters to about 40 centimeters.

14. The heat spreader according to claim 3 wherein the thermal conductivity along the a axis in the first planar element and in the second planar element is from about 450 to about 2000 Watts/meter° K.

15. The heat spreader according to claim 3 wherein the thermal conductivity, along the a axis in the first planar element and in the second planar element is from about 1000 to about 2000 Watts/meter° K.

16. The heat spreader according to claim 3 wherein the thermal conductivity along the a axis in the first planar element and in the second planar element is from about 1200 to about 2000 Watts/meter° K.

17. A method of making a heat spreader comprising the steps of:
a) providing a first planar element of pyrolytic graphite having a relatively high thermal conductivity in a direction of a first lateral dimension of a first plane of the first planar element and in a thickness direction of the first planar element and having a relatively low thermal conductivity in a direction of second lateral dimension of the first planar element, and
b) providing a second planar element of pyrolytic graphite having a relatively high thermal conductivity in a direction of a first lateral dimension of a second plane of the second planar element and in a thickness direction of the second planar element and having a relatively low thermal conductivity in a direction of a second lateral dimension of the second planar element,
wherein at least a portion of a first side of the first planar element which extends in a direction out of the first plane of the first planar element adjoins at least a portion of a second side of the second planar element which extends in a direction out of the second plane of the second planar element.