Patent Application
20090165302
This disclosure relates generally to methods of bonding thermo pyrolytic graphite (TPG) to metal materials to serve as heatsinks for various uses and, more particularly, to bonding TPG elements to at least one metal material for forming a metal heat-conductive structure for use as a heatsink.
Modem embedded computer systems contain very high thermal power electrical components in a volumetrically constrained environment. The volumes typically do not change as the power dissipation of the components increase, presenting significant challenges in the management of component temperatures. In the past, a variety of direct cooling techniques, such as active or passive heatsinks including high thermally conductive materials such as aluminum and/or copper have been used to manage rising temperatures. These materials, however, are only sufficient if a relatively large amount of surface area is presented to the airstream, necessitating a physically larger heatsink structure that occupies a large amount of the total available volume. As the physical size of the heatsink increases, the ability of the material to rapidly carry heat to the extremities of the heatsink, thereby exposing the heat to the airstream, is diminished.
Thermo Pyrolytic Graphite (TPG) materials have been found to have the ability to provide better heat conduction in a single (X-Y) plane as compared to conventional metal materials. Furthermore, TPG has been found to have an improved overall conductivity as compared to copper. Recently, a method has been developed to embed a TPG material into an aluminum structure using a diffusion bonding process. The diffusion bonding process, while resulting in a suitable thermal contact between the TPG material and the aluminum structure, has limitations in that specialized equipment is needed to create the TPG-embedded structures in a time-consuming process, resulting in an expensive product.
As such, there is a need for a method to create a cost-effective product having TPG bonded to one or more metal materials, such as an aluminum structure, to form a metal heat-conducting structure (i.e., heatsink) to provide effective thermal conductivity in the X-Y plane. Additionally, there is a need for such a method that is easily reproducible and performed in various facilities using various types of equipment.
In one aspect, a method for bonding thermo pyrolytic graphite (TPG) to a first metal material and a second metal material to form a heatsink is provided. The method includes forming at least one hole through a TPG element; forming at least one via in the first metal material, wherein the via is configured to be complementary to the hole through the TPG element; providing a thermal spacer made from the second metal material, wherein the thermal spacer is configured to be complementary to a heat source element; applying a metal-based coating to an outer surface of the TPG element; and bonding the via in the first metal material and the thermal spacer of the second metal material to the coated surface of the TPG element. The via, thermal spacer, and hole are bonded to form the heatsink configured to allow heat from the heat source element to be conducted through the thermal spacer to the via through the hole in the TPG element.
In another aspect, a method for bonding thermo pyrolytic graphite (TPG) to a first metal material and a second metal material to form a heatsink is provided. The method includes forming at least one hole through a TPG element; forming at least one via in the first metal material, wherein the via is configured to be complementary to the hole through the TPG element; providing a thermal spacer made from the second metal material, wherein the thermal spacer is configured to be complementary to a heat source element; and bonding the via in the first metal material and the thermal spacer of the second metal material to the TPG element using an electroplating process. The via, thermal spacer, and hole are bonded to form the heatsink configured to allow heat from the heat source element to be conducted through the thermal spacer to the via, and through the hole in the TPG element.
In another aspect, a method for bonding thermo pyrolytic graphite (TPG) to a first metal material to form a heatsink is provided. The method includes forming at least one hole through a TPG element; applying a metal-based coating to an outer surface of the TPG element; depositing at least one soldering ball to an outer surface of the first metal material, wherein the soldering ball is configured to fill the hole through the TPG element; pressing the first metal material to the TPG element such that the soldering ball fills the hole; and heating the first metal material to solder the first metal material to the TPG element.
The present disclosure is related to bonding thermo pyrolytic graphite (TPG) to at least one metal material for forming a heatsink. As used herein, “TPG” refers to any graphite-based material in which the graphite is aligned in one direction for optimal heat transfer. The materials are typically referred to as “aligned graphite”, “TPG”, and/or “Highly Oriented Pyrolytic Graphite (HOPG)”. The TPG elements provide improved thermal conductivity in the X-Y plane of the metal heat-conducting structure (i.e., heatsink). More specifically, it has been found that by using the methods of bonding TPG elements to at least one metal material as provided in the present disclosure, temperatures created by the use of electrical systems, such as computer systems, can be lowered by about 12° C. or more as compared to conventional thermal solutions. This improved temperature release allows for almost a doubling of the electrical systems' power capacity in the same volume environment. Furthermore, the increase in power may result in systems being supported that could not have otherwise been so, or may allow existing systems to be used in environments having higher ambient temperatures.
As noted above, the heatsink is formed by bonding a TPG element to at least one material. In one embodiment, as shown in
TPG element 100 can be obtained using any method and/or equipment known in the art for fabricating TPG elements. TPG elements 100 can further be obtained commercially from a supplier, such as Momentive Performance Material located in Wilton, Conn.
In one embodiment, as shown in
At least one hole 10 is formed through TPG element 100. Holes 10 can be formed using any method known in the art. In a particular embodiment, as shown in
Holes 10 can have any suitable shape known to one skilled in the art. Without limiting the scope of the present disclosure, each hole 10 may have a suitable shape including, for example, a circular, an oval, a square, a rectangular, or a triangular shape. In one embodiment, each hole 10 has a circular shape as circular holes are easier to manufacture. In a particular embodiment, each circular hole has a diameter of approximately 0.5 inches.
Additionally, at least one via 12 is formed in a first metal material 200. In one embodiment, the via 12 is configured to be positioned within a complementary or corresponding hole 10 formed through TPG element 100. As such, dimensions of vias 12, number of vias 12, and/or spacing between vias 12 formed in first metal material 200 depend upon the corresponding dimensions and/or number of holes 10 formed through TPG element 100. In one embodiment, a plurality of vias 12 are formed through first metal material 200, as shown in
In a particular embodiment, as shown in
In a further embodiment, via 12 is strategically configured into one or more individual mushroom-cap shaped button (not shown). By using a mushroom-cap shape, vias 12 are free to float apart from each other to allow for better bonding with TPG element 100 and, thus, with the heat source element (not shown). In one embodiment, when vias 12 are mushroom cap shaped, vias 12 further include stems. The stems extend through holes 10; that is, the stems extend through the entire thickness of TPG element 100. Other suitable shapes for vias 12 can include stem-only mushroom vias; that is mushroom-shaped vias having the stems only.
In an alternative embodiment, a hole is defined through a center of each vias 12. The hole can be sized and configured to allow for a separate mechanical coupling component to be inserted, thereby strengthening the connection between first metal material 200 and TPG element 100. For example, in one embodiment, the hole can be sized and configured to accept a screw or rivet to facilitate coupling the metal fin or conduction-cooled heatframe, as described herein, of a first metal material 200 to via 12 of a first metal material 200. The mechanical coupling component can be provided prior to, subsequent to, or simultaneously with, the bonding.
First metal material 200 is made from a metal material having a suitable thermal conductivity. For example, first metal material 200 may include aluminum, copper, indium, and combinations thereof. In one embodiment, first metal material 200 is aluminum. Both aluminum and copper have been shown to provide high conductivity when used in heatsinks. More specifically, aluminum provides good thermal conductivity in a “Z” plane when used in heatsinks. However, as noted above, aluminum and copper alone fail to provide sufficient heat transfer in an X-Y plane and, as such, the present disclosure has combined TPG with aluminum, copper, or combinations thereof.
In one embodiment, as shown in
When first metal material 200 includes metal fin assembly 400, it should be recognized that vias 12 formed in first metal material 200 may be formed as separate components from fins 2, 4, 6 of metal fin assembly 400.
In an alternative embodiment, first metal material 200 is a conduction-cooled heatframe intended to transfer heat to an edge of a heatframe. Conduction-cooled heatframes are known in the art and are commercially supplied, such as from the commercial supplier, Simon Industries, located in Morrisville, N.C.
As shown in
Thermal spacers 300 can have any suitable dimensions known to one skilled in the art. In one embodiment, the dimensions of thermal spacer 300 are approximately 1.4 inches×1.4 inches×0.25 inches.
As noted above, thermal spacer 300 is configured to be complementary to a heat source element. Generally, the heat source element is an electrical heat source element. For example, the heat source element is an integrated semiconductor circuit. As noted above, during use of the heat source element, such as an integrated circuit, a large amount of heat is generated that must be released to the outside environment to prevent overheating and/or malfunctioning of the heat source element. For example, in one embodiment, an integrated circuit dissipates approximately 30 Watts or greater of thermal power, with die temperatures reaching an excess of about 100° C. This heat must be released to prevent overheating of the integrated circuit.
In addition to TPG element 100, first metal material 200, and thermal spacer 300, in one embodiment, a third metal material (not shown) may be used to provide independent vias from vias 12. The vias formed in the third metal material are configured to be complementary to holes 10 in TPG element 100. The vias couple TPG element 100 to the heat dissipating structure of the heatsink, typically fins 2, 4, 6 of metal fin assembly 400 (shown in
As with vias 12 formed within first metal material 100, the vias of the third metal material can be any suitable dimensions known to one skilled in the art. In one embodiment, the dimensions of the vias within the third metal material are approximately 0.5 inches in diameter and approximately 0.25 inches in thickness.
In one embodiment, the method of the present disclosure includes applying a metal-based coating material to an outer surface 102 of TPG element 100. More specifically, when used, the metal-based coating material is applied to outer surface 102 facing towards first metal material 200. A layer of metal material such as aluminum, copper, iron, silver, gold, nickel, zinc, tin, or a combination thereof, is applied to outer surface 102of the TPG element 100. In one embodiment, the metal-based coating material is a copper coating material with a nickel overcoat. In an alternative embodiment, an indium metal-based coating material is used.
The metal-based coating material suitably provides mechanical strength and a point of contact for the solder material or adhesive (if used) during heating and attachment. The metal-based coating material may also provide a compliant surface that conforms to the surface to which it is coupled (e.g., vias 12). The metal-based coating material is typically at least about 0.001 inches thick. More suitably, the copper/nickel based coating material is applied to TPG element 100 having a thickness of from about 0.0005 inches to about 0.002 inches.
The metal-based coating material can be applied to outer surface 102 of TPG element 100 in any suitable pattern known in the art. In one embodiment, the metal-based coating material is applied in a cross-hatched pattern. In an alternative embodiment, the metal-based coating material is applied in a striped pattern.
In one embodiment, a thermal interface material 14 is applied to the surface of via 12, a part of first metal material 200 and, and the metal fin or conduction-cooled heatframe part of the first metal material 200. When more than one metal material is used, for example, when thermal spacer 300 and the third metal material are used, the thermal interface material 14 is applied between a surface of first metal material 200 and a via of the third metal material.
The thermal interface material fills imperfections in the surface finish of first metal material 200 and thermal spacer 300 to create a thermal interface with a lower thermal impedance. In one embodiment, as shown in
Now referring to
In one embodiment, the components are bonded using a suitable electroplating process. Any suitable electroplating process known in the art can be used in the methods of the present disclosure. Generally, an electrolytic apparatus containing an anode end, an opposing cathode end, and a non-conductive housing between the anode and cathode ends as known in the art is used for the electroplating process. The housing of the electrolytic apparatus includes an electrolytic solution. In one embodiment, the process includes contacting TPG element 100, first metal material 200, thermal spacer 300 (when used), and the third metal material (when used) simultaneously with an electrolytic solution. The plating is typically deposited in multiple iterations to build up layers to fill any voids that may be present. More specifically, once TPG element 100, first metal material 200, thermal spacer 300, and the third metal material are contacted with the electrolytic solution, electroplating is carried out by passing an electric current between the anode and cathode ends of the electrolytic apparatus.
In an alternative embodiment, TPG element 100, first metal material 200, thermal spacer 300 (when used), and the third metal material (when used) are bonded together using a soldering process (See
Suitable solder can be made from materials including, without limitation, lead/tin alloys, lead-free tin alloys, tin/silver alloys, tin/silver/copper alloys, and tin/silver/copper/antimony alloys. In one embodiment, solder paste is introduced at holes 10 and gaps of TPG element 100. The solder paste contains particles of lead/tin alloy suspended in a gel, which is applied in a wet state to first metal material 200 (and thermal spacer 300 and the third metal material, when used). Heat is applied to melt the non-conductive gel away and the solder 600 melts and bonds TPG element 100 to first metal material 200.
In a further embodiment, the method of the present disclosure includes bonding TPG element 100, first metal material 200, and thermal spacer 300 using a thermally conductive adhesive. Typically, the adhesive is applied to at least one of TPG element 100, first metal material 200, thermal spacer 300, and the third metal material. More specifically, the adhesive may generally be applied in a semi-solid state, such as in a paste, or gel-like form using any method known in the art.
In one embodiment, the thermally conductive adhesive is Arctic Silver Epoxy, commercially available from Arctic Silver, Inc., located in Visalia, Calif. Amounts of adhesive used will typically depend upon the specific heatsink configuration. In one embodiment, approximately 1.5 mL of adhesive is applied using a syringe and a spatula to spread the adhesive into a thin layer over TPG element 100, first metal material 200, and thermal spacer 300.
In one embodiment, the heatsink is applied to the heat source element using a TIC400 thermal grease available from Bergquist, located in Chanhassen, Minn.
As noted above, while the above-described methods for bonding (e.g., electroplating process, soldering process, and adhesive) are described singularly, it should be understood that any combination of the three bonding methods can be used in combination to form a heatsink without departing from the scope of the present disclosure.
While the invention has been described in terms of various specific embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the claims.
