Alloy bonded graphene sheets for enhanced thermal spreaders

Abstract

A heat spreader for printed wiring boards and a method of manufacture are disclosed. The heat spreader is made from a plurality of graphene sheets that are thermo-mechanically bonded using an alloy bonding process that forms a metal alloy layer using a low temperature and pressure that does not damage the graphene sheets. The resulting heat spreader has a higher thermal conductivity than graphene sheets alone.

Claims

1. A printed wiring board (PWB), comprising: a first PWB layer having an upper surface for attaching electrical components and a lower surface; a second PWB layer having upper and lower surfaces; and a thermal core material inserted between the lower surface of the first PWB and the upper surface of the second PWB, said thermal core material further comprising first and second planar graphene sheets thermo-mechanically bonded by a metal alloy layer formed from a plurality of planar layers.

2. The PWB of claim 1, wherein metal alloy is further comprised by a plurality of metal layers that have been thermo-mechanically bonded.

3. The PWB of claim 2, wherein the plurality of metal layers further comprises a layer of gold (Au) and a layer of indium (In).

4. The PWB of claim 2, wherein the plurality of metal layers further comprises a layer of gold (Au) and a layer of tin (Sn).

5. The PWB of claim 2, wherein the plurality of metal layers further comprises a layer of indium (In) and a layer of tin (Sn).

6. The PWB of claim 2, wherein the plurality of metal layers further comprises a layer of copper (Cu) and a layer of indium (In).

7. The PWB of claim 2, wherein the thermal core material further comprises a first layer of titanium between the plurality of metal layers and the first graphene sheet and a second layer of titanium and a layer of molybdenum between the plurality of metal layers and the second graphene sheet.

8. The PWB of claim 7 further comprising a layer of molybdenum (Mo) between the plurality of metal layers and one of the titanium layers.

9. A heat spreader for a printed wiring board comprising: a first planar graphene sheet; a second planar graphene sheet; and a plurality of planar layers between said first and second planar graphene sheets, said plurality of planar layers thermo-mechanically bonded to form a metal alloy layer.

10. The heat spreader of claim 9, wherein the plurality of planar layers further comprises a layer of gold (Au) and a layer of indium (In) which are thermo-mechanically bonded to form the metal alloy layer.

11. The heat spreader of claim 10, wherein the plurality of planar layers further comprises a first layer of titanium between the metal alloy layer and the first graphene sheet and a second layer of titanium and a layer of molybdenum between the metal alloy layer and the second graphene sheet.

12. The heat spreader of claim 9, wherein the plurality of planar layers further comprises a layer of gold (Au) and a layer of tin (Sn).

13. The heat spreader of claim 9, wherein the plurality of planar layers further comprises a layer of indium (In) and a layer of tin (Sn).

14. The heat spreader of claim 9, wherein the plurality of planar layers further comprises a layer of copper (Cu) and a layer of indium (In).

15. The heat spreader of claim 11 further comprising a layer of molybdenum (Mo) between the plurality of metal alloy layer and one of the titanium layers.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Features of example implementations of the invention will become apparent from the description, the claims, and the accompanying drawings in which:

(2) FIG. 1 is a representation of a printed wiring board (PWB) according to the present invention.

(3) FIG. 2 is a representation of the process of manufacturing a heat spreader for a PWB according to the present invention.

(4) FIG. 3 is a graph of the thermal conductivity of various thicknesses of graphene sheets in comparison with the heat spreader of the present invention.

(5) FIG. 4 is a graph of model predicted thermal performance for two bonded graphene heat spreaders of this invention.

DETAILED DESCRIPTION

(6) Reference will now be made in detail to one or more embodiments of the invention. While the invention will be described with respect to these embodiments, it should be understood that the invention is not limited to any particular embodiment. On the contrary, the invention includes alternatives, modifications, and equivalents as may come within the spirit and scope of the appended claims. Furthermore, in the following description, numerous specific details are set forth to provide a thorough understanding of the invention. The invention may be practiced without some or all of these specific details. In other instances, well-known structures and principles of operation have not been described in detail to avoid obscuring the invention.

(7) This disclosure describes a thermal core material and process for joining a variable number of thin, high thermal conductivity graphene sheets to form a suitable thickness thermal core material that can be used as a heat spreader in printed wiring boards (PWB). The purpose of such thermal core/heat spreaders is to assist in heat removal from heat generating devices located on the PWB.

(8) An embodiment of the present invention using two PWBs and a thermal core/heat spreader is illustrated in FIG. 1. PWB 10 is formed of PWBs 12 and 16 which are, for example, a fiberglass and resin composite such as G10 or FR-4. A variety of components are mounted on a surface of PWB 12 as would be understood by one of ordinary skill in the art. The components are diverse and generate differing amounts of heat. Therefore, the use of a heat spreading thermal core 14 helps disperse the heat horizontally and transfer it from the components on PWB 12 to PWB 16 and out to the edges of PWB 10.

(9) Graphene-based thermal spreaders offer significant weight advantage over copper core heat spreaders. Graphene also is a better match with the thermal expansion properties of the materials most commonly used to make PWBs. Since the expansion and contraction of the layers of PWB 10 will be more closely matched, PWB 10 will experience lower thermo-mechanical stresses as well as less board warpage and distortion at elevated operating temperatures.

(10) The process of manufacturing the graphene sheet heat spreader 14 of FIG. 1 is shown schematically in FIG. 2. The various film and sheet thicknesses depicted in FIG. 2 are not drawn to scale.

(11) In a first series of steps shown at 40, two graphene sheets 20 and 22 are provided. Although true graphene is generally understood to be a single layer of carbon atoms, in reality, a graphene sheet typically has between 1 and 20 layers of carbon atoms in a lattice structure. A thin layer 24, 26 of titanium is deposited on each of graphene sheets 20 and 22. This layer is approximately 200 to 600 Å (20-60 nm) thick and is used to cover any rough surfaces of the graphene sheet and improve adhesion of the graphene sheet to subsequent layers.

(12) A layer 28 of molybdenum (Mo) is deposited on top of Ti layer 24. In an embodiment, Mo layer 28 is approximately 500 to 2000 Å (50-200 nm) thick. This layer further enhances adhesion, and also forms a barrier between Ti layer 24 and indium (In) layer 30. A layer 32 of gold (Au) is deposited on Ti layer 26.

(13) The thicknesses of In layer 30 and Au layer 32 are flexible and should be chosen at the proper proportions to form an alloy of a desired thickness and composition. In an embodiment, In layer 30 is approximately 30,000 Å (3000 nm) thick and Au layer 32 is approximately 20,000 Å (2000 nm) thick. In an embodiment, bonding layers 30 and 32 should be thin as possible to reduce thermal impedance, but thick enough to cover any surface defects/asperities. Layers 30 and 32 should also have a unform thickness for even heat distribution and transfer across the layer.

(14) In a next step, represented at 42 in FIG. 2, the assembled layers 22, 26 and 32 are flipped and stacked on top of the assembled layers 20, 24, 28 and 30, then subjected to thermo-mechanical alloy brazing process. In an embodiment, layers 20, 24, 28, 30, 32, 26 and 22 are subjected to a bonding process during step 42 that creates an AuIn alloy layer 46 as shown at step 44. In an embodiment, the thermo-mechanical bonding process involves compressing the layers at approximately 200° C. and 7 kPa. In an alternative embodiment, the bonding process is performed at a range of temperatures from 175-250° C. and pressures from 5-20 kPa. The resulting AuIn layer 46 is approximately 5,000 Å (50 nm) thick. Although an alloy layer 46 of Au and In has been discussed above, alternative embodiments may use alloys of gold and tin (AuSn), indium and tin (InSn) and copper and indium (CuIn), for example.

(15) The bonded sheet structure has significantly improved through-the-thickness (K.sub.z) thermal conductivity. In an embodiment, an AuIn composition alloy according to the present invention demonstrated greater than 40% enhancement in K.sub.z conductivity over a single sheet of graphene. The bonding process is able to improve K.sub.z without damaging and diminishing the sheet material's in-plane (K.sub.xy) conductivity.

(16) Maintenance of the conductivity of bonded graphene sheet using the alloy process can be seen from the barchart of FIG. 3. The barchart compares the thermal conductivity in W/mK to various thicknesses in micrometers of graphene sheets. The bars in section 50 of the chart depict K.sub.xy and K.sub.z of single unbonded graphene sheets. The bars in section 52 of the chart depict the thermal conductivity of bonded graphene sheets using the inventive method. Bars 54 and 56 depict a K.sub.xy of 500 W/mK and a K.sub.z of 7 W/mk for a 70 micrometer thick unbonded graphene sheet respectively. Bars 58 and 60 depict a K.sub.xy of 600 W/mK and a K.sub.z of 10 W/mK respectively for the same graphene sheets after the inventive bonding process, resulting in a total thickness of 150 micrometer. As shown, the thermal conductivity K.sub.z is improved significantly.

(17) Alloy bonded graphene sheets can be advantageously exploited for fabricating lighter weight conductive core PWB needed for weight restricted air and spacecraft electronic payloads.

(18) The inventive alloy brazing method allows for bonding difficult-to-join graphene sheets into thicker laminate structures which may be used as lightweight, low thermal expansion, high conductivity thermal heat spreaders. The alloy bonding process joins graphene sheets together with minimal surface preparation and is accomplished at low temperatures and pressures. The process does not physically damage the sheet material or cause it to distort and maintains or enhances its thermal conductivity.

(19) As seen in FIG. 4, bonded graphene thermal core/heat spreaders incorporated in PWB offer the thermal performance of copper for safely managing junction temperatures of next generation devices at a fraction of the metal weight. FIG. 4 depicts the junction temperature comparisons for devices on a PWB with a variable thickness thermal core. In particular, FIG. 4 depicts predicted thermal performance for two alloy bonded graphene heat spreaders according to the present invention. For reference, line 80 depicts the performance of a copper heat spreader. Lines 82 and 84 depict the temperature across the various thicknesses of alloy bonded graphene sheet materials from two different suppliers.

(20) If used and unless otherwise stated, the terms “upper,” “lower,” “front,” “back,” “over,” “under,” and similar such terms are not to be construed as limiting the invention to a particular orientation. Instead, these terms are used only on a relative basis.

(21) The steps or operations described herein are just for example. There may be many variations to these steps or operations without departing from the spirit of the invention.

(22) Although example implementations of the invention have been depicted and described in detail herein, it will be apparent to those skilled in the relevant art that various modifications, additions, substitutions, and the like can be made without departing from the spirit of the invention and these are therefore considered to be within the scope of the invention as defined in the following claims.