Abstract:
A heat dissipating assembly including a layered stack of materials with a highly thermally conductive path for cooling a circuit, the stack including a structurally isolated material having a high coefficient of thermal expansion connected between materials having low coefficients of thermal expansion.

Description:
GOVERNMENT INTERESTS 
       [0001]    The present invention was developed with support from the U.S. government under a contract with the United States Department of Energy, Contract No. DE-NA0000622. Accordingly, the U.S. government has certain rights in the present invention. 
     
    
     BACKGROUND 
       [0002]    Embodiments of the present invention relate to assemblies for dissipating heat in electronic circuits. 
         [0003]    Heat dissipating assemblies used with electrical circuits often include multiple layers of materials having high thermal conductivity for quickly removing heat away from the circuit. Multiple layers are required due to geometric, electronic, and manufacturing limitations for any single material. For example, an electrical circuit may have a complex shape with a complex heat profile, so the layer of the heat dissipation assembly connected to the circuit must be formed into a complementary complex shape to closely bond to the circuit at high heat-generating areas. Other layers of the heat dissipation assembly do not need to have a complex shape but must be able to dissipate heat quickly. The coefficients of thermal expansion (CTE), the amount that the materials expand for an amount of heat applied to them, of these materials vary widely. The different expansion rates may result in the generation of high stresses, which may cause fractures in the materials or may cause the layers to separate. These fractures or separations introduce air into the heat dissipation path, causing heat to build up in the dissipation assembly, which may cause circuit overheating. 
         [0004]    Accordingly, there is a need for an improved heat dissipating assembly that overcomes the above-described limitations. 
       SUMMARY 
       [0005]    Embodiments of the present invention solve the above-mentioned problems and provide a distinct advance in the art of heat dissipating assemblies. More particularly, embodiments of the invention provide a heat dissipating assembly including a layered stack of materials with a highly thermally conductive path for cooling a circuit, the stack including a structurally isolated material (having a coefficient of thermal expansion (CTE) unmatched to the CTE of the circuit) connected between materials having CTEs matched to the CTE of the circuit. Having matched CTEs means essentially that over a normal manufacturing and operating temperature range, for specific sizes and geometries of the parts, and for specific construction of the assembly, failures due to thermal stresses are not normal. The layered stack is constructed in a way that the structurally isolated material, which is a relatively soft material (i.e., a material having a low modulus of elasticity), dissipates thermally induced stresses and strains by deforming within the layered stack. 
         [0006]    One embodiment of the heat dissipation assembly includes a heat spreader for connecting to an electrical circuit, a substrate connected to the heat spreader on a first side of the substrate, a heat slug connected to the heat spreader on a first side of the heat slug and adjacent to the substrate, and a backplate connected to the heat slug and the substrate on second sides opposite the first sides of the heat slug and the substrate. The heat spreader, the substrate, and the backplate form layers of materials with a highly thermally conductive path for cooling the circuit. The heat spreader and the circuit have matching CTEs, and the substrate and the backplate have matching CTEs. In this way, the heat spreader and the backplate provide structural support with minimal stress buildup while the heat slug is disposed between the heat spreader and the backplate for dissipating thermally induced stresses and strains and for dissipating heat away from the circuit to the backplate. 
         [0007]    This summary is provided to introduce a selection of concepts in a simplified form that are further described below in the detailed description. This summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used to limit the scope of the claimed subject matter. Other aspects and advantages of the current invention will be apparent from the following detailed description of the embodiments and the accompanying drawing figures. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING FIGURES 
         [0008]    Embodiments of the current invention are described in detail below with reference to the attached drawing figures, wherein: 
           [0009]      FIG. 1  is a top perspective view of a heat dissipation assembly constructed in accordance with an embodiment of the invention; 
           [0010]      FIG. 2  is a cross section view of the assembly of  FIG. 1 ; 
           [0011]      FIG. 3  is a cross section view of a heat dissipation assembly wherein the heat spreader overlaps a first portion of a surface-level RF ground plane of the substrate, in accordance with an embodiment of the invention; and 
           [0012]      FIG. 4  is a cross section view of a heat dissipation assembly wherein the heat spreader and the heat slug are disposed within a cavity of the substrate. 
       
    
    
       [0013]    The drawing figures do not limit the current invention to the specific embodiments disclosed and described herein. The drawings are not necessarily to scale, emphasis instead being placed upon clearly illustrating the principles of the invention. 
       DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0014]    The following detailed description of the invention references the accompanying drawings that illustrate specific embodiments in which the invention may be practiced. The embodiments are intended to describe aspects of the invention in sufficient detail to enable those skilled in the art to practice the invention. Other embodiments may be utilized and changes may be made without departing from the scope of the current invention. The following detailed description is, therefore, not to be taken in a limiting sense. The scope of the current invention is defined only by the appended claims, along with the full scope of equivalents to which such claims are entitled. 
         [0015]    In this description, references to “one embodiment”, “an embodiment”, or “embodiments” mean that the feature or features being referred to are included in at least one embodiment of the technology. Separate references to “one embodiment”, “an embodiment”, or “embodiments” in this description do not necessarily refer to the same embodiment and are also not mutually exclusive unless so stated and/or except as will be readily apparent to those skilled in the art from the description. For example, a feature, structure, act, etc. described in one embodiment may also be included in other embodiments, but is not necessarily included. Thus, the current technology may include a variety of combinations and/or integrations of the embodiments described herein. 
         [0016]    Turning to the figures, and initially  FIGS. 1 and 2 , a heat dissipation assembly  10  for dissipating heat in an electronic circuit  12  is shown. An embodiment of the assembly  10  has multiple layers of materials with high thermal capacities including a heat spreader  14  for connecting to the circuit  12 , a substrate  16  connected to the heat spreader  14 , a heat slug  18  connected to the heat spreader  14 , and a backplate  20  connected to the heat slug  18  and the substrate  16 . The heat slug  18  is structurally isolated between the heat spreader  14  and the backplate  20 , for reasons described below. In this description, “matching CTEs” means that over a normal manufacturing and operating temperature range, for specific sizes and geometries of the parts, and for specific construction of the assembly, failures due to thermal stresses are not normal. Factors that determine whether two CTEs are matched include temperature ranges to which the components are subjected, the modulus of elasticity of each component, the Poisson&#39;s ratio of each component, size and geometry of each component, and alignment between the components. For exemplary purposes only, “matching CTEs” may mean two CTEs being the same or having a difference of less than approximately 3 ppm/° C. It will be understood that for various assemblies, matching CTEs may be within larger or smaller ranges. The circuit  12  and the heat spreader  14  have matching CTEs. The substrate  16 , and the backplate  20  have matching CTEs. The CTEs of the circuit  12  and the heat spreader  14  may match the CTEs of the substrate  16  and the backplate  20 . The heat slug  18  may have a CTE unmatched with the aforementioned CTEs. Because the heat slug  18  is structurally isolated between the heat spreader  14  and the backplate  20 , the heat slug  18  may comformably or compliantly expand or shift relative to the heat spreader  14  or the backplate  20  without causing a structural failure therebetween as may happen if the components are conventionally constructed. 
         [0017]    In more detail, and as shown in  FIGS. 1-4 , the exemplary circuit  12  is an integrated circuit chip but may be any kind of heat generating electronic circuit or circuit component such as a resister, capacitor, amplifier, or computer component such as a hard drive, display, or processor. The circuit  12  may be formed of Gallium-arsenide, Gallium-nitride, Silicon, Silicon-carbide, Germanium, laminated FR4, duroid®, or other material. The circuit  12  may have a complex geometry such as one having a plurality of edges, contours, curves, angles, depressions, etched surfaces, and mounted elements. The complex geometry may also include bosses, protrusions, or mounts for connecting the circuit  12  to the heat spreader  14 . The circuit  12  may be manufactured using complex manufacturing processes such as printing, molding, or assembling. The circuit  12  may also have a complex heat profile, meaning the circuit  12  does not uniformly generate heat across its surface(s). That is, certain areas or parts generate more heat than others. 
         [0018]    The heat spreader  14  is a plate, bar, board, wafer, or similar layer that is connected to and draws heat away from the circuit  12 . The heat spreader  14  may be made of any material that has a high material strength, a high thermal conductance to efficiently remove heat from the circuit  12 , and a CTE that matches the CTE of the circuit  12  so that stresses are minimized when the heat spreader  14  increases in temperature and expands. For example, the heat spreader  14  may be formed of Aluminum, Aluminum-silicon-carbide, Copper-tungsten, Tungsten-nickel-copper alloys, Nickel-chromium alloys, Nickel-iron alloys, Diamond, Copper, Copper-molybdenum, Kovar, Alloy 42, MET Graph 350 composite, or other material. The heat spreader  14  may have a CTE slightly lower than the CTE of the circuit  12  (e.g., 0.001 to 3 ppm/° C. lower) which results in non-damaging compression on the circuit  12  when the assembly increases in temperature. The heat spreader  14  may also exert tension or sheer forces on the circuit  12 . 
         [0019]    The heat spreader  14  may have a complex geometry complementary or conformable to the complex shape and/or heat profile of the circuit  12  so that the heat spreader  14  is configured to be connected to the circuit  12  and to efficiently remove heat from higher heat-generating areas of the circuit  12 . The heat spreader  14  also may be shaped to reduce electrical or radio frequency (RF) interference such as having a “continuous” grounding plane (i.e., a plate or a large surface). The heat spreader  14  may also be shaped to minimize a wire bond length between the circuit  12  and the substrate  16 . The complex geometry may include bosses, protrusions, or mounts for connecting the heat spreader  14  to the circuit  12 . The heat spreader  14  may be connected to the circuit  12  via fasteners, clamps, bonding such as soldering, glue, welding, conductive epoxy, or other connection means (described below). The heat spreader  14  is also connected to the substrate  16  and heat slug  18  on a side opposite that of the circuit  12 . The heat spreader  14  may extend over the substrate  16  ( FIGS. 1-3 ) or may be disposed between portions of the substrate  16  ( FIG. 4 ). The heat spreader  14  and the circuit  12  may be removed from the substrate  16  and the heat slug  18  and replaced, and the substrate  16  and the heat slug  18  may be reused, if the circuit  12  is determined to be defective or broken. 
         [0020]    The substrate  16  may be formed of a low temperature co-fired ceramic (LTCC), a high temperature co-fired ceramic (HTCC), Aluminum-nitride, Aluminum-oxide, Beryllium-oxide, laminated FR4, duroid®, a printed wire assembly/printed wire board, a wafer, or other material having a CTE matching the CTE of the backplate  20 . Again, having matching CTEs reduces stresses between the materials. The substrate  16  does not necessarily need to have a high thermal conductivity. This is because heat from the circuit  12  primarily travels through the heat slug  18 , described below. The substrate  16  is connected to the heat spreader  14  on a first side of the substrate  16  via fasteners, clamps, bonding such as soldering, glue, or welding, or other connection means, as described below, and may extend above the heat spreader, as shown in  FIG. 2 . In some embodiments, the substrate  16  includes a cavity  26  or a space that extends to opposite sides of the substrate  16 , so as to form a through-hole or a through-space wherein the heat slug  18  is disposed. In this way, the substrate  16  acts as a spacer between the heat spreader  14  and the backplate  20 . The substrate  16  may also include a continuous ground plane  22  for reducing RF interference. The ground plane  22  may be embedded in ( FIGS. 1 and 2 ), printed on ( FIG. 3 ), or connected to the substrate  16 . The ground plane  22  is connected to the heat spreader  14  via conductive bonding material such as epoxy or solder. The ground plane  22  may instead be connected via a low inductance wire  24 , cable, or other electrical connector to the circuit  12  or the heat spreader  14  to form a grounding connection. Alternatively, the substrate  16  may have a plurality of wires (not shown) emanating therefrom for connecting to the circuit  12 . The geometry of the substrate  16  (and the heat spreader  14  as mentioned above) enables the length of the wire  24  to be minimized. The wire  24  may be connected to the substrate  16  and the circuit  12  via fasteners, clamps, bonding such as soldering, glue, welding, conductive epoxy, thermosonic or thermocompressive wire bonding, or other means, as described below. 
         [0021]    The heat slug  18  has a high thermal conductivity and a CTE that does not necessarily match the CTEs of the circuit  12 , the heat spreader  14 , the substrate  16 , and/or the backplate  20 . The heat slug  18  may have a low material strength for conformably or compliantly expanding, deforming, or shifting relative to the stronger heat spreader  14  and/or backplate  20  without inducing a structural failure. Thus, the heat slug  18  may be formed of a material such as high purity Copper, high purity Gold, high purity Platinum, Silver, Aluminum-silicon alloy, Copper-tungsten alloy, Kovar, Alloy 42, Diamond composites, or graphite materials such as thermally pyrolytic graphite or graphite-metal composites. These materials are difficult to form into precise or complex shapes, and so the heat slug  18  may be formed into a simple shape with very few features. In one embodiment, the heat slug  18  has a uniform cross sectional shape and is formed by a cost effective manufacturing process such as extrusion. The heat slug  18  is connected to the heat spreader  14  on the second side of the heat spreader  14  (opposite the circuit  12 ) and on a first side of the heat slug  18 . The heat slug  18  is also connected to the backplate  20  (described below) on a second side of the heat slug  18  opposite the first side, so that the heat slug  18  is situated in between the heat spreader  14  and the backplate  20 . The heat slug  18  is also adjacent to the substrate  16  and/or disposed in the cavity  26 , so that the heat slug  18  is attached to the heat spreader  14  near a first end of the cavity  26  and attached to the backplate  20  near a second end of the cavity  26 . The heat slug  18  may be connected to the heat spreader  14  and the backplate  20  via fasteners, clamps, bonding such as soldering, glue, welding, conductive epoxy, or other means, as described below. 
         [0022]    The backplate  20  has a high material strength, a high thermal conductivity, and a CTE that matches the CTE of the substrate  16  to prevent the buildup of stresses therebetween, and is formed of Aluminum, Aluminum-silicon-carbide, Copper-tungsten, Tungsten-nickel-copper alloys, Nickel-chromium alloys, Nickel-iron alloys, Diamond, 
         [0023]    Copper, Copper-molybdenum, Kovar, Alloy 42, MET Graph 350 composite, or other material. The backplate  20  sandwiches the heat slug  18  and the substrate  16  between the heat spreader  14  and itself to isolate the heat slug  18  therebetween. 
         [0024]    The above components are connected via connectors  28  such as Tin-lead, Indium-lead, or Gold-tin solders, Diemat, ABLEBOND®, or LORD® epoxy, brazes, welds, glue, adhesives, fasteners, clamps, or other means. The connectors  28  connecting the heat spreader  14  to the heat slug  18  and the heat slug  18  to the backplate  20  may be flexible, malleable, compressible, or expandable compared to the materials that form the heat spreader  14 , the heat slug  18 , the substrate  16 , and the backplate  20 . Therefore, when the heat slug  18  expands at a greater rate than the heat spreader  14 , the substrate  16 , and the backplate  20 , the solder, epoxy, etc. reversibly compresses, squeezes, or deforms without forming cracks or other failures. The solder, epoxy, etc. also may decompress as the heat slug  18  retracts, thereby forming a constant, thermally conductive connection between the heat slug  18  and the heat spreader  14  and the heat slug  18  and the backplate  20 . The connectors  28  may also be electrically conductive and may help to minimize electrical or RF interference. 
         [0025]    Although the invention has been described with reference to the embodiments illustrated in the attached drawing figures, it is noted that equivalents may be employed and substitutions made herein without departing from the scope of the invention as recited in the claims.