Heat spreader featuring matched thermal expansion coefficients and heat dissipation using same

Abstract

The heat spreader may be tailored to match the coefficient of thermal expansion (CTE) to an electronic component or other heat generating component in thermal communication with the electronic component. In some cases, the heat generating component may be bonded to the heat spreader while achieving a CTE match. The copper nanoparticles may be consolidated with a CTE adjuster under mild conditions to define a heat spreader configured to contact a heat source and a heat sink, at least a portion of the heat spreader including a copper composite including a CTE adjuster. The copper composite may be present in or in a coating on a thermal conductor that defines the heat spreader. The copper composite may contact the heat generating component to promote effective heat transfer and a strong bond between the copper composite and the heat generating component, such as in a printed circuit board (PCB), followed by dissipation of the heat to a heat sink or other heat receiving structure.

Description

[Technical Field]

[0001] Inefficient thermal communication between a heat source and a heat sink can hinder the dissipation of excess heat from a system, particularly within an electronic device. For example, heat-generating electronic components, such as high-power LEDs and high-power circuits, are continually becoming smaller and more powerful, thereby generating excessive heat loads that are increasingly concentrated in ever-decreasing spaces. Increasing levels of integration and complex electronic packaging in system-in-package (SIP) systems can also make heat dissipation difficult. The increased and concentrated excess heat generation can make effective heat removal particularly challenging, albeit important. Failure to remove excess heat from an electronic device can result in serious consequences, such as overheating, poor conduction, higher-than-normal power requirements, and / or the need for clock-down operation to avoid board burnout and device failure due to the presence of hot spots. Failure modes can include lateral displacement forces resulting from coefficient of thermal expansion (CTE) mismatch that can exceed the strength of bond lines, leading to circuit destruction and / or short circuits.

[0002] Ineffective heat conduction can be particularly ubiquitous in various types of circuit boards, especially printed circuit boards (PCBs), composite packaging, laminate boards, and system-in-package (SIP) components. PCBs and similar circuit boards are thermal insulators by the very nature of their construction. Specifically, PCBs may use thermally insulating substrates (e.g., glass fiber epoxy composites such as FR4, a common example with a thermal conductivity value of only 0.25 W / m·K) on which the appropriate electronic circuitry and various board components are placed. The low thermal conductivity value of PCB substrates can make removing excess heat from electronic devices quite difficult. Because leads or embedded metal traces are typically small in size, very little excess heat can be removed through them. Furthermore, traditional lead solder is not particularly thermally conductive (e.g., approximately one-tenth the thermal conductivity of more thermally conductive metals such as copper). th(See below.) For example, package substrates with heat slugs for GaN and SiC devices, monolithic microwave integrated circuits (MMICs), phased arrays, etc., as found in 5G base stations and power converters, can present similar problems. EMI shielding can present related issues, but without the short circuiting.

[0003] Thermal vias are one approach to removing excess heat generated by electronic components associated with printed circuit boards or similar structures. However, direct liquid casting of high melting point metals into vias is not compatible with currently used substrate materials (metal processing temperatures >1000°C compared to the much lower polymer melting points of materials typically used as PCB substrates). o C). Thus, vias are often filled with rosin or a similar filler and then either galvanically capped at both ends or left open with only a thick metal plating (e.g., copper) formed on the via walls (i.e., via barrel) to facilitate electrical communication through the PCB substrate. This approach is performed by slow electrodeposition and can result in suboptimal thermal communication by leaving gaps in the metal plug extending through the PCB. An alternative approach for filling vias using metal nanoparticles is described in U.S. Pat. No. 1,061,6994, incorporated herein by reference, which can promote more complete filling of the via hole and result in higher thermal conductivity. Large diameter vias can be compatible with such processes and provide more effective removal of excess heat. Even thermal vias may be insufficient to remove large amounts of excess heat.

[0004] Thermal markings are another approach to heat dissipation that can be used when more thermal conduction is needed than thermal vias can provide. Thermal markings are 3-4 mm diameter metal bodies pressed into the plane of a PCB or similar substrate. As a result of increased thermal conduction for thermal vias, size mismatches can be common, and the thickness of the PCB during fabrication and the thickness of the pre-fabricated thermal markings can vary, which can cause assembly problems when stacking multiple PCB layers together. Thermal markings are also typically fabricated in a limited range of shapes, which may not be applicable to certain PCB architectures.

[0005] Heat pipes are an alternative heat transfer medium that can facilitate the transfer of very large amounts of excess heat. While highly thermally conductive metals such as copper can only have thermal conductivity values ​​in the range of a few hundred W / m·K, heat pipes can provide much higher effective thermal conductivity values, ranging from several thousand W / m·K, such as approximately 10,000 W / m·K to approximately 100,000 W / m·K. Heat pipes function through direct heat transfer to a working fluid contained within an enclosed vessel; conduction is further supplemented by the liquid-vapor phase transition and subsequent condensation of the working fluid. Heat pipes have traditionally been utilized in applications where passive dissipation of heat in harsh operating environments is desirable. Examples include satellite and spacecraft applications. Miniaturized heat pipes, such as oscillating heat pipes, have recently been used to dissipate excess heat from printed circuit boards and similar small, heat-generating electronic components.

[0006] Similar to a heat pipe, a heat spreader may also facilitate the dissipation of excess heat from a heat source in contact with it. Heat spreaders, lacking the working fluid of a heat pipe, may facilitate the lateral diffusion of heat through a monolithic thermal conductor to facilitate more effective rejection of excess heat to a thermal reservoir. Heat spreaders may be tapered to facilitate the dissipation of excess heat from a heat source having a limited size, such as a point-like heat source, to a more extensive dissipation surface, typically at or adjacent to a heat sink.

[0007] A difficulty associated with both heat pipes and heat spreaders is that ineffective thermal communication can exist between the heat-generating component and the outer surface of the heat pipe or heat spreader due to coefficient of thermal expansion (CTE) mismatch. For example, copper is a highly thermally conductive metal often utilized to form the outer shell of a heat pipe or the monolithic metal body of a heat spreader, but this metal has a significantly different CTE than the ceramic materials typically found in heat-generating components on printed circuit boards or similar components that generate excess heat. CTE mismatch can lead to disengagement of the heat-generating component from the heat pipe or heat spreader when heat is generated, thereby significantly negating the heat pipe's or heat spreader's ability to dissipate excess heat from the heat-generating component. Disengagement of the heat spreader from a heat sink or other heat-receiving structure can also be problematic. Furthermore, the material used to bond the heat pipe or heat spreader to the heat-generating component can also contribute to a CTE mismatch. [Brief explanation of the drawings]

[0008] The following figures are included to illustrate certain aspects of the present disclosure and should not be viewed as exclusive embodiments. The disclosed subject matter is capable of significant modification, permutation, combination, and equivalents in form and function, as will occur to those skilled in the art and having the benefit of this disclosure. [Figure 1] 1 shows a diagram of the proposed structure of metal nanoparticles with a surfactant coating on their surface. [Figure 2] 1 shows a diagram of the proposed structure of metal nanoparticles with a surfactant coating on their surface. [Figure 3A] 1A-1C illustrate cross-sectional views of various configurations of the heat spreader of the present disclosure. [Figure 3B] 1A-1C illustrate cross-sectional views of various configurations of the heat spreader of the present disclosure. [Figure 3C] 1A-1C illustrate cross-sectional views of various configurations of the heat spreader of the present disclosure. [Figure 3D] 1A-1C illustrate cross-sectional views of various configurations of the heat spreader of the present disclosure. [Figure 4] 1 shows a diagram of an exemplary heat spreader in which multiple conductive fibers extend from a coating on one end of a thermal conductor. [Figure 5] 1 shows a diagram in which a heat spreader is bonded to the top surface of a heat generating component. [Figure 6] 1 shows a diagram in which a heat spreader is bonded to the bottom surface of a heat-generating component. [Figure 7] 1 shows a diagram in which a heat spreader is bonded to the top and bottom surfaces of a heat-generating component. [Figure 8] 1 shows a diagram in which multiple heat spreaders are bonded to the sides of a heat-generating component. DETAILED DESCRIPTION OF THE INVENTION

[0009] The present disclosure relates generally to thermal management, and more specifically to a heat spreader having at least an outer surface with an improved coefficient of thermal expansion (CTE) that matches heat-generating components used in printed circuit boards (PCBs) and related electronic devices, such as copper-clad substrates (CCBs) and substrates employing novel ceramics like AlN and SiN. The heat-generating components may use electrically and thermally insulating substrates, such as FR4 or other polymer substrates, or in some cases, electrically insulating but thermally conductive substrates, such as AlN. Advantageously, the present disclosure may facilitate tailoring the CTE of at least the outer surface of the heat spreader to match the CTE of a given heat-generating component. CTE matching to the CTE of a heat sink or similar heat-receiving structure may also be achieved. The heat spreader may be attached directly to the backside of a PCB, either as a monolithic thermal ground plane or as a finned heat sink. Direct metal bonding may be achieved in some system architectures using a bonding layer between the heat spreader and the heat-generating component, resulting in mechanical durability. Furthermore, the heat spreaders and related concepts of the present disclosure may avoid or minimize the use of thermal grease and gel pads that may be required to mitigate CTE mismatch, even at the cost of less effective thermal conductivity. CTE tuning may be achieved in a variety of ways, as described below.

[0010] As mentioned above, removing excess heat from heat-generating components in circuit boards and associated electronic assemblies can be challenging due to the ubiquity of insulating materials therein. While heat spreaders can be effective at dissipating large amounts of excess heat from such systems, the CTE mismatch between the metal components of the heat spreader and various portions of the electronic component or other heat-generating structure can be significant, especially in those that include non-metallic components such as various ceramics. CTE mismatch can be ubiquitous in heat-generating components that feature both thermally insulating and thermally conductive substrates such as AlN and SiN. If excessive CTE mismatch exists in either type of system, the heat spreader may separate from the heat-generating component when it heats up, thereby negating or severely limiting the heat spreader's ability to dissipate excess heat, resulting in overheating and burnout of the heat-generating component.

[0011] The present disclosure provides a heat spreader that can provide a more effective CTE match between at least the outer surface of the heat spreader and a heat-generating component. More specifically, the present disclosure provides a metal composite, such as a copper composite, including a CTE modifier, that can define at least the outer surface of the heat spreader and, optionally, a larger portion or even the entire heat spreader. The metal composite may be formed using metal nanoparticles, such as copper nanoparticles, consolidated together to form a bulk metal with a low degree of nanoporosity. Even before CTE tuning, the fused copper nanoparticles have a relatively low CTE of only about 7-11 ppm (varying based on the degree of nanoporosity present), which may be further tailored in the copper composite using at least one CTE modifier, as further described herein. The CTE of the metal composite can be easily modified by adjusting the loading of the CTE modifier in the continuous metal matrix to promote a more effective CTE match with the ceramic material in the heat-generating component, such as those containing SiC, GaN, AlN, etc. Metal composites can be readily formed from compositions containing metal nanoparticles, such as copper nanoparticles, such that the metal composite and heat spreader can be formed by solid-state sintering at low temperatures, well below the melting point of the molten metal. Heat sinks or similar heat-receiving structures may similarly be CTE-matched to heat spreaders by similarly utilizing metal nanoparticles and CTE adjusters in situ. Further details regarding metal nanoparticles, such as copper nanoparticles, and their properties that may facilitate low-temperature processing are described below.

[0012] Appropriate CTE modifiers can reduce the already low CTE (7-11 ppm) of bulk metals formed from copper nanoparticles to as little as 3 ppm at room temperature, in some cases, compared to the 17 ppm typically found in bulk copper. These features can significantly simplify PCB assembly and other heat transfer architectures, resulting in overall product cost savings, while significantly improving performance and reliability in areas where severe thermal shock and significant thermal cycling occur. Similarly, the CTE can be adjusted upward by including bulk copper powder in the copper nanoparticle consolidation, including CTE values ​​approaching 17 ppm. If necessary, even higher CTE values ​​may be achieved by including micron-sized particles of other metals, such as aluminum particles, flakes, or wire, in which case CTE values ​​approaching 23 ppm may be achieved. Thus, metal composites of the present disclosure may be formed from copper nanoparticles and at least one CTE modifier, optionally further combined with bulk copper powder, to provide a range of achievable CTE values.

[0013] In addition to facilitating improved CTE matching between the heat spreader and the substrate of an electronic component or similar heat-generating component, the metal nanoparticle composition may also facilitate direct bonding between the electronic component and the heat spreader through a bonding layer, similar to that produced by soldering or the use of a metal paste. For example, the metal nanoparticle composition may be applied as a bonding layer on the surface of the electronic component, and subsequent consolidation of the metal nanoparticles in the bonding layer may facilitate direct metallurgical bonding to at least a portion of the outer surface of the heat spreader, which is also formed from the consolidated metal nanoparticles. Direct metallurgical bonding significantly reduces the likelihood of disengagement between the heat-generating component and the heat spreader. Furthermore, because the heat spreader (or its outer coating) and the bonding layer may be formed from similar materials, CTE mismatch is less likely to occur, thereby limiting or even eliminating thermomechanical stresses. That is, the bonding layer may define a transition layer having a CTE intermediate between that of the electronic component and the outer surface of the heat spreader. With large electronic component sizes and high operating temperatures (e.g., below about 350°C), even small CTE differences can result in high thermomechanical stress values, leading to potential delamination and device failure. CTE matching of the outer surface of the heat spreader to a heat sink or similar heat-receiving structure may also be achieved by using metal nanoparticles and CTE modifiers to form at least a portion of the heat sink or similar heat-receiving structure, potentially resulting in a metallurgical bond.

[0014] The heat spreaders of the present disclosure may be used in conjunction with printed circuit boards and similar architectures where heat-generating electronic components are a concern. The heat spreaders may be connected to printed circuit boards and similar architectures in a variety of ways. Heat-generating components disposed on or embedded within the surface of a printed circuit board may be connected to the heat spreaders of the present disclosure on the front surface facing away from the non-conductive substrate of the PCB, on the side of the heat-generating electronic component, and / or on the underside of the heat-generating electronic component. In the latter configuration, the heat spreader may extend through the electrically insulating substrate of the PCB to contact the heat-generating component. Combinations of the aforementioned heat spreader configurations may be used to provide multiple heat transfer paths. The aforementioned heat spreader configurations for connecting the heat spreader to the heat-generating component may be utilized to facilitate stacking multiple printed circuit board layers on top of each other to create three-dimensional integration of devices and systems such as SIPs and memory devices on processors. The heat spreader may be in thermal communication with a structure for removing excess heat shunted through it, such as a liquid reservoir, radiator, or similar structure acting as a heat sink. The heat spreader may optionally be in further thermal communication or physical contact with a heat pipe, which may carry excess heat further away from the heat-generating electronic component, i.e., the heat pipe may be interposed between the heat spreader and the heat sink.

[0015] Similarly, for heat-generating components featuring an electrically insulating but highly thermally conductive substrate such as AlN or SiN, the heat spreader of the present disclosure may be disposed on either side of the substrate or at least partially within the substrate. When an electrically insulating but thermally conductive substrate is used, the heat spreader may be in thermal communication with the heat-generating component through the thermally conductive substrate rather than in direct contact with the heat-generating component. In some cases, AlN or SiN may be deposited as a thin film (e.g., about 300 microns to about 500 microns thick) on the surface of the electrically insulating substrate to impart thermal conductivity to the surface. The heat spreaders of the present disclosure may be used in these configurations as well.

[0016] Metal nanoparticles are uniquely suited for forming at least a surface coating on a heat spreader, or optionally the entire heat spreader. At the very least, the heat spreader may be better CTE-matched to a heat-generating component having a lower CTE, and if a bonding layer is further utilized, the bonding layer may provide a strong bonding interaction between the heat spreader and the heat-generating component, or the bonding layer may have a CTE intermediate between the CTE of the heat-generating component and the CTE of the heat spreader. Moderate processing conditions for consolidating the metal nanoparticles to form a bulk metal (e.g., bulk copper) in a metal composite with low nanoporosity (e.g., a copper composite with a CTE modifier) ​​may facilitate the formation of the bonding layer and other optional components of the heat spreader. As described in more detail below, the metal nanoparticles can be consolidated (fused) together into the corresponding bulk metal under a range of mild processing conditions well below the melting point of the metal itself. Due to copper's high thermal conductivity and relatively low cost, copper nanoparticles may be a particularly desirable type of metal nanoparticle for use in various embodiments of the present disclosure. Bulk copper formed from metal nanoparticles combined with a CTE modifier can effectively form a well-dispersed composite after metal nanoparticle consolidation. Suitable CTE modifiers may include, for example, carbon fibers, diamond particles, boron nitride particles or fibers, carbon nanotubes, graphene, W and / or Mo particles, silicon particles, graphite powder, silicon nitride particles or fibers, aluminum nitride particles or fibers, copper oxide nanoparticles, and any combination thereof. W and / or Mo particles may also impart oxidation resistance to copper. In addition to promoting CTE matching between the heat spreader surface and the heat-generating component, the CTE modifier and optional micron-sized metal particles may also limit shrinkage during consolidation of the metal nanoparticles, which can exceed 20% for other metal nanoparticle systems. Limited shrinkage may help alleviate thermomechanical stresses during operational high-temperature-low-temperature cycling that occurs during use of the heat spreader. Additionally, the nanoporosity obtained after metal nanoparticle consolidation may impart additional flexibility that may provide additional resistance to thermomechanical stresses.

[0017] In addition to the aforementioned advantages, metal nanoparticles can facilitate the fabrication of heat spreaders with improved structures for dissipating heat from the heat spreader. For example, some heat spreaders of the present disclosure may include a plurality of thermally conductive fibers extending from an end (cold end) of the heat spreader, which can facilitate rapid dissipation of excess heat to a heat sink, such as the ambient atmosphere, a marine environment (e.g., ocean, lake, or river water), or a radiator for space applications. The conductive fibers may be bonded to the heat spreader using a metal nanoparticle composition, which is also effective in promoting CTE matching, as briefly described above and described in more detail below. Bonding of the conductive fibers may be achieved during fabrication of the heat spreader without a separate bonding step by incorporating the ends of the conductive fibers into a suitable metal nanoparticle composition before consolidation of the metal nanoparticles occurs. Following consolidation of the metal nanoparticles, one set of conductive fiber ends remains firmly anchored within the resulting metal composite, while the other set of conductive fiber ends extends outward from the heat spreader to facilitate heat dissipation from the heat spreader.

[0018] As used herein, the term "metal nanoparticle" refers to a metal particle having a size of about 200 nm or less, without particular reference to the shape of the metal particle.

[0019] As used herein, the term "micron-scale metal particles" refers to metal particles that are greater than or equal to about 200 nm in size in at least one dimension.

[0020] The terms "consolidate," "consolidation," and other variations thereof are used interchangeably herein with the terms "fuse," "fusion," and other variations thereof.

[0021] As used herein, the terms "partially fused," "partial fusion," and other derivatives and grammatical equivalents refer to the partial coalescence of metal nanoparticles. While fully fused metal nanoparticles retain only minimal structural morphology of the original unfused metal nanoparticles (i.e., they resemble dense bulk metal but exhibit internal particle sizes in the 100-500 nm range with low nanoporosity), partially fused metal nanoparticles retain at least some of the structural morphology of the original unfused metal nanoparticles, e.g., higher levels of porosity, smaller average particle sizes, and more grain boundaries. The properties of partially fused metal nanoparticles can be intermediate between those of the corresponding bulk metal and the original unfused metal nanoparticles. In some embodiments, a fully dense (non-porous) bulk metal can be obtained after consolidation of the metal nanoparticles to obtain a metal composite. In other embodiments, the metal composite may have a porosity of less than about 10%, or less than about 20%, or less than about 30% above full densification (i.e., >0% porosity). Thus, in certain embodiments, a metal composite formed from metal nanoparticles and a CTE modifier may have a porosity (nanoporosity) in the range of about 2% to about 30%, or about 2% to about 5%, or about 5% to about 10%, or about 10% to about 15%, or about 15% to about 20%, or about 20% to about 25%, or about 25% to about 30%.

[0022] Before further describing more specific aspects of the present disclosure in more detail, we first provide a brief additional description of metal nanoparticles and their processing conditions, particularly copper nanoparticles. Metal nanoparticles exhibit several properties that may be significantly different from those of the corresponding bulk metal. One property of metal nanoparticles that may be particularly important for processing according to the present disclosure is nanoparticle fusion (consolidation), which occurs at the metal nanoparticles' melting temperature. As used herein, the term "fusion temperature" refers to the temperature at which metal nanoparticles liquefy, thereby giving the appearance of being molten. As used herein, the terms "fusion" and "consolidation" synonymously refer to the coalescence or partial coalescence of metal nanoparticles to form a bonded interface or a larger mass, such as a metal composite, on at least the outer surface of a heat spreader. The melting temperature may be as much as 80% below the melting point of the corresponding bulk metal. Thus, at least partial connectivity exists between the metal nanoparticles after heating above the melting temperature. After consolidation of the metal nanoparticles, the resulting nanoporosity can accommodate thermal stresses that arise during heating and cooling cycles. Without being bound by theory or mechanism, it is believed that the nanoporosity may absorb stresses resulting from the expansion or contraction of the heat spreader, rather than suffering from failure from the rapid release of thermomechanical stresses.

[0023] As size decreases, especially below a spherical equivalent diameter of about 20 nm, the temperature at which metal nanoparticles liquefy decreases dramatically from the liquidus temperature of the corresponding bulk metal. o For example, copper nanoparticles with a size of about 20 nm or less have a melting point of about 235 C, compared to the melting point of bulk copper. o C or less, or about 220 o C or less, or about 200 o The metal nanoparticles can have a melting temperature below 375°C. Thus, consolidation of the metal nanoparticles occurring at the melting temperature can enable the fabrication of structures containing the bulk metal at significantly lower processing temperatures than when the bulk metal itself is directly processed as the starting material. Processing conditions for consolidating the metal nanoparticles are typically around 375°C. oThis is within the normal PCB manufacturing parameters of 275-400 psi, but pressure is not necessary for metal nanoparticle fusion to occur. Applying pressure to promote consolidation of the metal nanoparticles may result in a denser bulk metal. Thus, in some cases, pressures as high as 1500 psi may be applied to the metal nanoparticles to promote consolidation. For example, for copper nanoparticles, the melting temperature (approximately 220°C or less) is below the temperature at which commonly used PCB substrates melt or deform. Therefore, although copper nanoparticle fusion can occur under temperature conditions associated with conventional PCB manufacturing processes, more vigorous consolidation conditions may optionally be used. Fusion may be performed under an inert atmosphere to prevent metal oxidation, or, in the case of larger surfaces or articles, there may be sufficient outgassing to limit oxidation even in the absence of an inert atmosphere. Thus, metal nanoparticles such as copper nanoparticles provide an easy to handle material for forming the bulk metal of a metal composite within at least a portion of a heat spreader or within at least a portion of a bonding layer on a heat spreader, particularly when incorporating the heat spreader within a PCB manufacturing process.

[0024] A number of scalable processes have been developed for producing large quantities of metal nanoparticles within a target size range. Most typically, such processes for producing metal nanoparticles involve the reduction of a metal precursor in the presence of one or more surfactants. The metal nanoparticles can then be isolated and purified from the reaction mixture by common isolation techniques and processed into formulations suitable for distribution.

[0025] Any suitable technique can be used to form the metal nanoparticles used in the present disclosure. Particularly easy metal nanoparticle production techniques are described in U.S. Patent Nos. 7,736,414, 8,105,414, 8,192,866, 8,486,305, 8,834,747, 9,005,483, 9,095,898, and 9,700,940, each of which is incorporated herein by reference in its entirety. As described therein, metal nanoparticles can be produced in a narrow size range by reducing a metal salt in a solvent in the presence of a suitable surfactant system, which can include one or more different surfactants. A target size distribution of metal nanoparticles, such as a bimodal size distribution, can be obtained by combining metal nanoparticles of different sizes together. Further description of suitable surfactant systems is provided below. Without being bound by any theory or mechanism, it is believed that the surfactant system can mediate the nucleation and growth of metal nanoparticles, limit surface oxidation of the metal nanoparticles, and / or inhibit extensive aggregation of the metal nanoparticles with one another before at least partially fusing them together. Suitable organic solvents for solubilizing metal salts and forming metal nanoparticles include, for example, formamide, N,N-dimethylformamide, dimethyl sulfoxide, dimethylpropylene urea, hexamethylphosphoramide, tetrahydrofuran, glyme, diglyme, triglyme, tetraglyme, proglyme, or polyglyme. Suitable reducing agents for reducing metal salts and promoting the formation of metal nanoparticles include, for example, alkali metals (e.g., lithium naphthalide, sodium naphthalide, or potassium naphthalide) or borohydride reducing agents (e.g., sodium borohydride, lithium borohydride, potassium borohydride, or tetraalkylammonium borohydride) in the presence of a suitable catalyst.

[0026] Figures 1 and 2 show diagrams of the possible structures of metal nanoparticles with surfactant coatings on their surfaces. As shown in Figure 1, metal nanoparticle 10 includes metal core 12 and surfactant layer 14 overcoating metal core 12. Surfactant layer 14 can contain any combination of surfactants, as described in more detail below. Metal nanoparticle 20 shown in Figure 2 is similar to that shown in Figure 1, except that metal core 12 is grown around nucleus 21, which can be the same metal as or a different metal from that of metal core 12. Nucleus 21 is deeply embedded within metal core 12 in metal nanoparticle 20 and is so small in size that it is not believed to significantly affect the overall nanoparticle properties. Nucleus 21 can include a salt or metal, which can be the same or different metal as metal core 12. In some embodiments, the nanoparticles can have an amorphous morphology.

[0027] As described above, metal nanoparticles have surfactant coatings on their surfaces that contain one or more surfactants. The surfactant coatings can be formed on the metal nanoparticles during their synthesis. The surfactant coatings are generally lost during consolidation of the metal nanoparticles when heated above their melting temperature, resulting in the formation of bulk metal with presumably uniform nanoporosity. Forming a surfactant coating on the metal nanoparticles during their synthesis can desirably limit the ability of the metal nanoparticles to prematurely fuse together, limit aggregation of the metal nanoparticles, and promote the formation of a population of metal nanoparticles with a narrow size distribution. Porosity values ​​after consolidation can be in the range of about 2-30% or about 2-15%, which can be adjusted based on several factors, including the type of surfactant(s) present and whether micron-scale metal particles come into contact with the metal nanoparticles during consolidation. With nanoporosity of about 2% to about 15%, the copper composite may contain about 85% to 98% densely fused copper nanoparticles, with closed pore nanoporosity having pore sizes in the range of about 50 nm to about 500 nm, or about 100 nm to about 300 nm, or about 150 nm to about 250 nm.

[0028] The types of metal nanoparticles suitable for use with various embodiments of the present disclosure are not particularly limited. Suitable metal nanoparticles include, but are not limited to, tin nanoparticles, copper nanoparticles, aluminum nanoparticles, palladium nanoparticles, silver nanoparticles, gold nanoparticles, iron nanoparticles, cobalt nanoparticles, nickel nanoparticles, titanium nanoparticles, zirconium nanoparticles, hafnium nanoparticles, tantalum nanoparticles, molybdenum nanoparticles, tungsten nanoparticles, and the like. Combinations of these metal nanoparticles may also be used. Micron-scale particles of these metals can be present in a metal nanoparticle paste composition that also contains metal nanoparticles. Copper can be a particularly desirable metal for use in embodiments of the present disclosure due to its low cost, strength, and excellent electrical and thermal conductivity values.

[0029] In various embodiments, the surfactant system present in the metal nanoparticles can include one or more surfactants. The different properties of various surfactants can be used to tailor the properties of the metal nanoparticles. Factors that can be taken into consideration when selecting a surfactant or combination of surfactants to include on the metal nanoparticles include, for example, the ease of surfactant dissipation from the metal nanoparticles during nanoparticle fusion, the nucleation and growth rates of the metal nanoparticles, and the metal content of the metal nanoparticles.

[0030] In some embodiments, an amine surfactant or a combination of amine surfactants, particularly aliphatic amines, can be present on the metal nanoparticles. Amine surfactants may be particularly desirable for use with copper nanoparticles. In some embodiments, two amine surfactants can be used in combination with each other. In other embodiments, three amine surfactants can be used in combination with each other. In more specific embodiments, primary amines, secondary amines, and diamine chelators can be used in combination with each other. In even more specific embodiments, the three amine surfactants can include a long-chain primary amine, a secondary amine, and a diamine with at least one tertiary alkyl group nitrogen substituent. Further disclosure regarding suitable amine surfactants follows below.

[0031] In some embodiments, the surfactant system can include a primary alkylamine. In some embodiments, the primary alkylamine can be a C2-C 18 In some embodiments, the primary alkylamine may be a C7-C 10 The alkylamine may be a C5-C6 alkylamine. In other embodiments, C5-C6 primary alkylamines may also be used. Without being bound by any theory or mechanism, it is believed that the exact size of the primary alkylamine can be balanced between being long enough to provide an effective reverse micelle structure during synthesis, being volatile, and / or being easy to handle during nanoparticle compaction. For example, primary alkylamines having more than 18 carbons may also be suitable for use in this embodiment, but may be more difficult to handle due to their waxy properties. In particular, C7-C6 alkylamines may be used. 10 Primary alkylamines can represent a good balance of desirable properties for ease of use.

[0032] In some embodiments, C2 to C 18The primary alkylamine can be, for example, n-hexylamine, n-heptylamine, n-octylamine, n-nonylamine, or n-decylamine. While these are all linear primary alkylamines, branched primary alkylamines can also be used in other embodiments. For example, branched primary alkylamines such as 7-methyloctylamine, 2-methyloctylamine, or 7-methylnonylamine can be used. In some embodiments, such branched primary alkylamines can be sterically hindered when they are attached to the nitrogen atom of the amine. Non-limiting examples of such sterically hindered primary alkylamines include, for example, t-octylamine, 2-methylpentan-2-amine, 2-methylhexane-2-amine, 2-methylheptan-2-amine, 3-ethyloctane-3-amine, 3-ethylheptan-3-amine, and 3-ethylhexane-3-amine. Additional branching can also be present. Without being bound by any theory or mechanism, it is believed that primary alkylamines can act as ligands in the metal coordination sphere, but can easily dissociate from the metal coordination sphere during compaction of the metal nanoparticles.

[0033] In some embodiments, the surfactant system can include a secondary amine. Secondary amines suitable for forming metal nanoparticles include linear, branched, or cyclic C4-C bonded to the nitrogen atom of the amine. 12 The alkyl group may be included. In some embodiments, branching can occur on the carbon atom attached to the nitrogen atom of the amine, thereby causing significant steric hindrance at the nitrogen atom. Suitable secondary amines include, but are not limited to, dihexylamine, diisobutylamine, di-t-butylamine, dineopentylamine, di-t-pentylamine, dicyclopentylamine, dicyclohexylamine, and the like. C4-C 12Secondary amines outside this range can also be used, but such secondary amines may have undesirable physical properties such as low boiling points or waxy consistency that can complicate their handling.

[0034] In some embodiments, the surfactant system can include a chelating agent, particularly a diamine chelating agent. In some embodiments, one or both nitrogen atoms of the diamine chelating agent can be substituted with one or two alkyl groups. When two alkyl groups are present on the same nitrogen atom, the two alkyl groups can be the same or different. Furthermore, when both nitrogen atoms are substituted, the alkyl groups can be the same or different. In some embodiments, the alkyl groups can be C1-C6 alkyl groups. In other embodiments, the alkyl groups can be C1-C4 alkyl groups or C3-C6 alkyl groups. In some embodiments, the C3 or higher alkyl groups can be linear or branched. In some embodiments, the C3 or higher alkyl groups can be cyclic. Without being bound by any theory or mechanism, it is believed that the diamine chelating agent can promote the formation of metal nanoparticles by promoting nanoparticle nucleation.

[0035] In some embodiments, suitable diamine chelating agents include N,N'-dialkylethylenediamines, particularly C1-C4 N,N'-dialkylethylenediamines. Corresponding methylenediamine, propylenediamine, butylenediamine, pentylenediamine, or hexylenediamine derivatives can also be used. The alkyl groups can be the same or different. C1-C4 alkyl groups that can be present include, for example, methyl, ethyl, propyl, and butyl groups, or branched alkyl groups such as isopropyl, isobutyl, s-butyl, and t-butyl groups. Exemplary N,N'-dialkylethylenediamines that may be suitable for inclusion on metal nanoparticles include, for example, N,N'-di-t-butylethylenediamine, N,N'-diisopropylethylenediamine, and the like.

[0036] In some embodiments, suitable diamine chelating agents include N,N,N',N'-tetraalkylethylenediamines, particularly C1-C4N,N,N',N'-tetraalkylethylenediamines. Corresponding methylenediamine, propylenediamine, butylenediamine, pentylenediamine, or hexylenediamine derivatives can also be used. The alkyl groups can similarly be the same or different and include those described above. Exemplary N,N,N',N'-tetraalkylethylenediamines that may be suitable for use in forming metal nanoparticles include, for example, N,N,N',N'-tetramethylethylenediamine, N,N,N',N'-tetraethylethylenediamine, and the like.

[0037] Surfactants other than aliphatic amines can also be present in the surfactant system.In this regard, suitable surfactants can include, for example, pyridine, aromatic amine, phosphine, thiol, or any combination thereof.These surfactants can be used in combination with aliphatic amines such as those mentioned above, or can be used in surfactant systems that do not contain aliphatic amines.Further disclosure of suitable pyridines, aromatic amines, phosphines, and thiols follows below.

[0038] Suitable aromatic amines include ArNR 1 R 2 wherein Ar is a substituted or unsubstituted aryl group and R 1 and R 2 are the same or different. R 1 and R 2 can be independently selected from H or an alkyl or aryl group containing 1 to about 16 carbon atoms. Exemplary aromatic amines that may be suitable for use in forming metal nanoparticles include, for example, aniline, toluidine, anisidine, N,N-dimethylaniline, N,N-diethylaniline, and the like. Other aromatic amines that can be used in conjunction with metal nanoparticles will occur to those skilled in the art.

[0039] Suitable pyridines include both pyridine and its derivatives. Exemplary pyridines suitable for inclusion on metal nanoparticles include, for example, pyridine, 2-methylpyridine, 2,6-dimethylpyridine, collidine, pyridazine, and the like. Chelating pyridines, such as bipyridyl chelators, may also be used. Other pyridines that can be used in conjunction with metal nanoparticles can be envisioned by those skilled in the art.

[0040] Suitable phosphines can have the formula PR3, where R is an alkyl or aryl group containing 1 to about 16 carbon atoms. The alkyl or aryl groups attached to the phosphorus center can be the same or different. Exemplary phosphines that can be present on the metal nanoparticles include, for example, trimethylphosphine, triethylphosphine, tributylphosphine, tri-t-butylphosphine, trioctylphosphine, triphenylphosphine, and the like. Phosphine oxides can also be used. In some embodiments, surfactants containing two or more phosphine groups configured to form a chelating ring can also be used. Exemplary chelating phosphines can include, for example, bisphosphines such as 1,2-bisphosphine, 1,3-bisphosphine, and BINAP. Other phosphines that can be used in conjunction with the metal nanoparticles will be apparent to those skilled in the art.

[0041] Suitable thiols can have the formula RSH, where R is an alkyl or aryl group having about 4 to about 16 carbon atoms. Exemplary thiols that can be present on the metal nanoparticles include, for example, butanethiol, 2-methyl-2-propanethiol, hexanethiol, octanethiol, benzenethiol, and the like. In some embodiments, surfactants containing two or more thiol groups configured to form a chelating ring can also be used. Exemplary chelating thiols can include, for example, 1,2-dithiols (e.g., 1,2-ethanethiol) and 1,3-dithiols (e.g., 1,3-propanethiol). Other thiols that can be used in conjunction with the metal nanoparticles can be envisioned by those skilled in the art.

[0042] As mentioned above, a distinctive feature of metal nanoparticles is their low melting temperature, which may facilitate consolidation to form bulk metal within metal composites according to the present disclosure. To facilitate distribution of the metal nanoparticles, the metal nanoparticles may be incorporated into pastes or similar formulations. Additional disclosure regarding metal nanoparticle paste compositions and similar formulations follows below.

[0043] Metal nanoparticle paste compositions or similar formulations can be prepared by dispersing as-produced or isolated metal nanoparticles in an organic matrix containing one or more organic solvents and various other optional ingredients. As used herein, the terms "nanoparticle paste formulation," "nanoparticle paste composition," "nanoparticle paste," and their grammatical equivalents are used interchangeably and refer synonymously to a fluid composition containing dispersed metal nanoparticles suitable for dispensing using a desired technique. The use of the term "paste" does not necessarily imply adhesive functionality of the paste alone. Judicious selection of the organic solvent(s) and other additives allows for the addition of metal nanoparticles, etc., and the facile dispensing of the metal nanoparticles and the formation of bulk metal.

[0044] Cracks can sometimes occur during metal nanoparticle consolidation. One way that the nanoparticle pastes of the present disclosure can facilitate a reduction in the degree of cracking and void formation after metal nanoparticle consolidation is by maintaining a high solids content. More specifically, in some embodiments, the paste composition can contain at least about 30 wt.% metal nanoparticles, particularly about 30 wt.% to about 98 wt.% metal nanoparticles of the paste composition, or about 50 wt.% to about 95 wt.% metal nanoparticles of the paste composition, or about 70 wt.% to about 98 wt.% metal nanoparticles of the paste composition, or about 85 wt.% to about 98 wt.% metal nanoparticles of the paste composition, or about 88 wt.% to about 99 wt.% metal nanoparticles of the paste composition. Furthermore, in some embodiments, in addition to the metal nanoparticles, small amounts of micron-scale particles, particularly micron-scale metal particles (e.g., about 0.01 wt.% to about 15 wt.%, or about 35 wt.%, or about 60 wt.% of the paste composition) can be present. The micron-scale metal particles may include any of particulate materials, fibers, and / or flakes. Such micron-scale metal particles can desirably facilitate the fusion of the metal nanoparticles into a continuous mass of bulk metal, further reducing the incidence of cracking. Rather than being liquefied and directly consolidated as with metal nanoparticles, micron-scale metal particles can simply bond together when they come into contact with liquefied metal nanoparticles that have been elevated above their melting temperature. These factors can reduce porosity after fusing the metal nanoparticles together. The micron-scale metal particles can contain the same or a different metal as the metal nanoparticles. Suitable metals for micron-scale metal particles include, for example, copper, silver, gold, aluminum, tin, molybdenum, and tungsten. In some embodiments, micron-scale graphite particles may also be included. In some embodiments, carbon nanotubes, boron nitride, diamond particles, and / or graphene may be included. According to some embodiments, a carbonaceous additive may increase the thermal conductivity resulting from the consolidation of the metal nanoparticles. Micron-scale metal particles or similarly sized micron-scale additives may also function in the capacity of a CTE adjuster.The CTE may be further adjusted to facilitate CTE matching according to the present disclosure through the addition of various CTE adjusters. Any of the aforementioned micron-scale particles may further function as crack deflectors to limit crack propagation during use, thereby increasing mechanical strength.

[0045] The reduction of cracking and void formation during metal nanoparticle consolidation can also be promoted by judicious selection of the solvent(s) forming the organic matrix. A tailored combination of organic solvents can desirably reduce the rate of cracking and void formation. More specifically, organic matrices containing one or more hydrocarbons (saturated, monounsaturated, polyunsaturated (two or more double bonds), or aromatic), one or more alcohols, one or more amines, and one or more organic acids can be particularly effective for this purpose. In some embodiments, one or more esters and / or one or more anhydrides may be included. In some cases, an alkanolamine, such as ethanolamine, may be present. Without being bound by any theory or mechanism, it is believed that this combination of organic solvents can promote the removal and sequestering of surfactant molecules surrounding the metal nanoparticles during consolidation, so that the metal nanoparticles can more easily fuse together. More specifically, it is believed that the hydrocarbon and alcohol solvents can passively solubilize surfactant molecules liberated from the metal nanoparticles by Brownian motion, reducing the ability of the surfactant molecules to reattach to the metal nanoparticles. In cooperation with the passive solubilization of surfactant molecules, amine solvents and organic acid solvents can actively sequester the surfactant molecules through chemical interactions so that they are no longer available to recombine with the metal nanoparticles.

[0046] Further adjustments to the solvent composition can be made to reduce the rapid volume shrinkage that occurs during surfactant removal and metal nanoparticle compaction. Specifically, two or more members of each class of organic solvent (i.e., hydrocarbons, alcohols, amines, and organic acids), optionally combined with one or more alkanolamines, esters, or anhydrides, can be present in the organic matrix, with the members of each class having boiling points separated from one another by a set temperature. For example, in some embodiments, the various members of each class are separated from one another by about 20°C. o C~about 50 o By using such a solvent mixture, the various components of the solvent mixture can have boiling points separated by a wide range of boiling points (e.g., about 50 o C ~ about 200 o C), the solvent can be removed gradually over time, minimizing sudden volume changes due to rapid loss of solvent during compaction of the metal nanoparticles.

[0047] In various embodiments, at least a portion of the one or more organic solvents is about 100 o In various other embodiments, at least a portion of the one or more organic solvents can have a boiling point of about 200 C or higher. o In some or other embodiments, the one or more organic solvents may have a boiling point of about 50 C or higher. o C ~ about 200 o C, or about 50 o C ~ approx. 250 o C, or about 50 o C ~ approx. 300 o C, or about 50 o C ~ approx. 350 o C, or about 50 o C ~ approx. 365 oThe organic solvents can have boiling points ranging from 0.1 to 1.5 C. The use of high-boiling organic solvents can desirably increase the pot life of the metal nanoparticle paste composition and limit rapid solvent loss that could otherwise result in cracking and void formation during nanoparticle consolidation. In some embodiments, at least one of the organic solvents can have a boiling point higher than the boiling point(s) of the surfactant(s) associated with the metal nanoparticles. Thus, the surfactant(s) can be removed from the metal nanoparticles by evaporation before removal of the organic solvent(s) occurs.

[0048] In some embodiments, the organic matrix can contain one or more alcohols, and in more particular embodiments, the one or more alcohols are C2 to C6 12 , C4~C 12 or C7~C 12In various embodiments, the alcohol may include a monohydric alcohol, a diol, or a triol. In certain embodiments, one or more glycol ethers (e.g., diethylene glycol and triethylene glycol), alkanolamines (e.g., ethanolamine, triethanolamine, etc.), or any combination thereof may be present, either alone or in combination with other alcohols. In some embodiments, various glymes may be present with one or more alcohols. In some embodiments, one or more hydrocarbons may be present in combination with one or more alcohols. As discussed above, it is believed that alcohols (and optionally glymes and alkanolamines) and hydrocarbon solvents can passively promote surfactant solubilization as the surfactant is removed from the metal nanoparticles by Brownian motion, limiting reassociation of the surfactant with the metal nanoparticles. Furthermore, hydrocarbon and alcohol solvents only weakly coordinate to the metal nanoparticles and therefore do not simply replace the substituted surfactant in the nanoparticle coordination sphere. Illustrative, but non-limiting examples of alcoholic and hydrocarbon solvents that may be present include, for example, light aromatic petroleum distillates (CAS 64742-95-6), hydrotreated light petroleum distillates (CAS 64742-47-8), tripropylene glycol methyl ether, ligroin (CAS 68551-17-7, C 10 ~C 13 Alkane mixtures), diisopropylene glycol monomethyl ether, diethylene glycol diethyl ether, 2-propanol, 2-butanol, t-butanol, 1-hexanol, 2-(2-butoxyethoxy)ethanol, and terpineol. In some embodiments, polyketone solvents can be used as well.

[0049] In some embodiments, the organic matrix can contain one or more amines and one or more organic acids. In some embodiments, the one or more amines and one or more organic acids can be present in an organic matrix that also includes one or more hydrocarbons and one or more alcohols. As discussed above, it is believed that the amines and organic acids can actively sequester surfactants passively solubilized by hydrocarbon and alcohol solvents, thereby making the surfactants unavailable for reassociation with the metal nanoparticles. Thus, organic solvents containing a combination of one or more hydrocarbons, one or more alcohols, one or more amines, and one or more organic acids can provide synergistic benefits for promoting the compaction of metal nanoparticles. Illustrative, but non-limiting, examples of amine solvents that can be present include, for example, tallow amine (CAS 61790-33-8), alkyl (C8-C9) amines, and alkyl (C8-C9) amines. 18 ) Unsaturated amine (CAS 68037-94-5), di(hydrogenated tallow)amine (CAS 61789-79-5), dialkyl (C8-C 20 ) amine (CAS 68526-63-6), alkyl (C 10 ~C 16 ) Dimethylamine (CAS 67700-98-5), Alkyl (C 14 ~C 18 ) dimethylamine (CAS 68037-93-4), dihydrogenated tallow methylamine (CAS 61788-63-4), and trialkyl (C6-C 12 ) amine (CAS 68038-01-7). Illustrative, but non-limiting examples of organic acid solvents that can be present in the nanoparticle paste composition include, for example, octanoic acid, nonanoic acid, decanoic acid, caprylic acid, pelargonic acid, undecylic acid, lauric acid, tridecylic acid, myristic acid, pentadecanoic acid, palmitic acid, margaric acid, stearic acid, nonadecylic acid, α-linolenic acid, stearidonic acid, oleic acid, and linoleic acid.

[0050] In some embodiments, the organic matrix can include two or more hydrocarbons, two or more alcohols, optionally two or more glymes (glycol ethers), two or more amines, and two or more organic acids. For example, in some embodiments, each class of organic solvent can have two or more members, or three or more members, or four or more members, or five or more members, or six or more members, or seven or more members, or eight or more members, or nine or more members, or ten or more members. Furthermore, the number of members in each class of organic solvent can be the same or different. Specific advantages of using multiple members of each class of organic solvent are described below. Higher boiling point organic solvents may provide safety advantages.

[0051] One particular advantage of using multiple members of each class of organic solvents is the ability to provide a wide range of boiling points in the metal nanoparticle paste composition. Providing a wide range of boiling points allows for gradual removal of the organic solvent as the temperature increases, while affecting the compaction of the metal nanoparticles, thereby limiting volumetric shrinkage and adversely affecting cracking. By gradually removing the organic solvent in this manner, less temperature control may be required to affect slow solvent removal than if a single solvent with a narrow boiling point range were used. In some embodiments, the members of each class of organic solvent are present in a range of about 50 o C ~ about 200 o C, or about 50 o C ~ approx. 250 o C, or about 100 o C ~ about 200 o C, or about 100 o C ~ approx. 250 o C, or about 150 o C ~ approx. 300 o C, or about 150 o C ~ approx. 350 o C, or about 150 o C ~ approx. 365 oIn more particular embodiments, the various members of each class of organic solvents can each have a boiling point window spanning a range of at least about 20 o C, specifically about 20 o C~about 50 o C. More specifically, in some embodiments, each hydrocarbon may have a boiling point that is separated from the other hydrocarbons in the organic matrix by about 20. o C~about 50 o C, each alcohol may have a boiling point that differs by approximately 20°C from the other alcohols in the organic matrix. o C~about 50 o C, each amine may have a boiling point that differs by approximately 20 C from the other amines in the organic matrix. o C~about 50 o C, each organic acid may have a boiling point that differs by approximately 20°C from the other organic acids in the organic matrix. o C~about 50 o The boiling points of the organic solvents may differ by a factor of 0. The more members of each class of organic solvents present, the smaller the difference between the boiling points. By reducing the difference between the boiling points, solvent removal can be more continuous, thereby limiting the amount of volumetric shrinkage that occurs at each stage. When four or five or more members of each class of organic solvents are present, each with boiling points that are separated from one another within the above range (e.g., four or more hydrocarbons, four or more alcohols, four or more amines, and four or more organic acids; or five or more hydrocarbons, five or more alcohols, five or more amines, and five or more organic acids), a reduced degree of cracking may occur.

[0052] In various embodiments, the metal nanoparticles used in the metal nanoparticle paste composition can be about 20 nm or less in size. In various other embodiments, the metal nanoparticles may be up to about 75 nm in size, or even up to about 200 nm in size, in at least one dimension. As noted above, metal nanoparticles in the size range below about 20 nm can have melting temperatures significantly lower than the melting point of the corresponding bulk metal, resulting in easier compaction together. In some embodiments, metal nanoparticles about 20 nm or less in size can be up to about 220 nm in size. o C or lower melting point (e.g., about 140 o C~ approx. 220 o (melting temperature within the range of 200°C) oC or less, thereby providing the above-mentioned advantages. In some embodiments, at least a portion of the metal nanoparticles can be about 10 nm or less in size, or about 5 nm or less in size. In more specific embodiments, at least a portion of the metal nanoparticles can be in a range of about 1 nm to about 20 nm in size, or about 1 nm to about 10 nm in size, or about 1 nm to about 5 nm in size, or about 3 nm to about 7 nm in size, or about 5 nm to about 20 nm in size. In some embodiments, substantially all of the metal nanoparticles can be present within these size ranges. In some embodiments, larger metal nanoparticles can be combined with metal nanoparticles of about 20 nm or less in size in the metal nanoparticle paste composition. For example, in some embodiments, metal nanoparticles ranging in size from about 1 nm to about 10 nm can be combined with metal nanoparticles ranging in size from about 25 nm to about 50 nm, or with metal nanoparticles ranging in size from about 25 nm to about 100 nm, or with metal nanoparticles ranging in size from about 25 nm to about 150 nm, or with metal nanoparticles ranging in size from about 25 nm to about 200 nm. As discussed further below, in some embodiments, micron-scale metal particles, other micron-scale particles, and / or nanoscale particles can also be included in the metal nanoparticle paste composition. While larger and micron-scale metal particles may not be liquefiable at the low temperatures of their smaller counterparts, they can still be consolidated upon contact with smaller metal nanoparticles liquefied above their melting temperatures, as outlined above.

[0053] In addition to the metal nanoparticles and organic solvent, other additives may also be present in the metal nanoparticle paste composition. Such additional additives may include, for example, rheology control aids, thickeners, micron-scale conductive additives, nanoscale conductive additives, and any combination thereof. Chemical additives may also be present. As described below, the inclusion of micron-scale conductive additives, such as micron-scale metal particles, may be particularly advantageous. In some cases, it may be desirable to include nano- or micron-scale diamond or other thermally conductive additives to promote more efficient heat transfer and adjust the CTE. Suitable CTE adjusters, unless otherwise specified, may be in the form of particles or fibers, and include, but are not limited to, carbon fibers, W particles, Mo particles, diamond particles, boron nitride, aluminum nitride, silicon nitride, copper oxide nanoparticles (e.g., containing CuO and / or CuO, sized from about 2 nm to about 200 nm), carbon nanotubes, graphene, graphite, and the like. Any of the aforementioned CTE adjusters may be micron-sized in at least one dimension.

[0054] In some embodiments, the paste composition can contain about 0.01 wt% to about 15 wt% micron-scale metal particles, or about 1 wt% to about 10 wt% micron-scale metal particles, or about 1 wt% to about 5 wt% micron-scale metal particles, or about 0.1 wt% to about 35 wt% micron-scale metal particles, or about 10 wt% to about 60 wt% micron-scale metal particles, or about 25 wt% to about 55 wt% micron-scale metal particles. The inclusion of micron-scale metal particles in the metal nanoparticle paste composition can desirably reduce the incidence of cracking during consolidation of the metal nanoparticles to form the bulk metal. Without being bound by any theory or mechanism, it is believed that the metal nanoparticles are liquefied and form a temporary liquid coating on the surfaces of the micron-scale metal particles, allowing the micron-scale metal particles to consolidate together. In some embodiments, the micron-scale metal particles can range in size from about 500 nm to about 100 microns in at least one dimension, or from about 500 nm to about 10 microns in at least one dimension, or from about 100 nm to about 5 microns in at least one dimension, or from about 100 nm to about 10 microns in at least one dimension, or from about 100 nm to about 10 microns in at least one dimension, or from about 100 nm to about 1 micron in at least one dimension, or from about 1 micron to about 10 microns in at least one dimension, or from about 5 microns to about 10 microns in at least one dimension, or from about 1 micron to about 100 microns in at least one dimension, or from about 1 micron to about 25 microns in at least one dimension, or from about 1 micron to about 5 microns in at least one dimension, or from about 5 microns to about 15 microns in at least one dimension. The micron-scale metal particles can contain the same metal as the metal nanoparticles, or can contain a different metal. Thus, by including micron-scale metal particles in a paste composition having a metal different from that of the metal nanoparticles, a metal alloy can be produced, i.e., a metal composite may optionally include a metal alloy.Metal alloys may also be formed by combining different types of metal nanoparticles. Suitable micron-scale metal particles include, for example, Cu, Ni, Al, Fe, Co, Mo, W, Ag, Zn, Sn, Au, Pd, Pt, Ru, Mn, Cr, Ti, V, Mg, or Ca particles. Non-metallic particles, such as Si micron-scale particles and B micron-scale particles, can also be used. In some embodiments, the micron-scale metal particles can be in the form of metal flakes, such as high-aspect-ratio copper flakes. Thus, in some embodiments, the metal nanoparticle paste compositions described herein can contain a mixture of copper nanoparticles and high-aspect-ratio copper flakes or other types of micron-scale copper particles. Specifically, in some embodiments, the metal nanoparticle paste compositions can contain about 30 wt % to about 90 wt % copper nanoparticles and about 0.01 wt % to about 15 wt % or 1 wt % to 35 wt % high-aspect-ratio copper flakes. A CTE adjuster may also be present in the metal nanoparticle paste composition.

[0055] Other micron-scale metal particles that can be used equivalently to high-aspect ratio metal flakes include metal nanowires and other high-aspect ratio particles, which can be, for example, about 300 microns or less in length. The ratio of metal nanoparticles to metal nanowires can be in the range of about 10:1 to about 40:1, according to various embodiments. Suitable nanowires can have lengths of, for example, about 5 microns to about 50 microns and diameters of about 100 nm to about 200 nm or about 100 nm to about 250 nm.

[0056] Additional materials that may optionally be present in the metal nanoparticle paste composition include, for example, flame retardants, UV protectants, antioxidants, carbon black, graphite, fibrous materials (e.g., chopped carbon fiber materials), diamond, and the like.

[0057] In some more specific embodiments, suitable nanoparticle paste compositions may include diamond particles or nanodiamond particles, which may be as large as possible in size to limit the grain boundaries that must be crossed by phonons during heat transfer, while remaining small enough so as not to impair the dispersibility of the metal nanoparticle paste composition.

[0058] In even more specific embodiments, diamond particles suitable for use in the metal nanoparticle paste composition may have a size within a range of about 1 micron to about 1000 microns, or about 0.5 microns to about 500 microns, or about 1 micron to about 10 microns, or about 2 microns to about 50 microns, or 50 microns to about 150 microns, which can provide good particle dispersion and acceptable paste dispersibility. Diamond particles having a size within a range of about 200 microns to about 250 microns or within a range of about 1 micron to about 10 microns can represent a good compromise between providing effective dispersion and minimizing grain boundaries to suppress phonon scattering. Other suitable size ranges for diamond particles can be about 25 microns to about 150 microns, or about 50 microns to about 250 microns, or about 100 microns to about 250 microns, or about 100 microns to about 200 microns, or about 150 microns to about 250 microns, or about 1 micron to about 100 microns, or about 10 microns to about 50 microns, or about 5 microns to about 25 microns.

[0059] In exemplary embodiments, the metal composite may contain about 10% to about 75% by volume of diamond particles after metal nanoparticle consolidation to form a monolithic metal body. Other conductive particles or CTE modifiers may be present in the same composition range. In some embodiments, one or more CTE modifiers may be present at 35% or about 60% by weight of the metal composite of the present disclosure.

[0060] The blending of copper nanoparticles and diamond particles may be desirable for several reasons. Copper is low cost compared to most other metals, has a relatively good impedance match with diamond, and possesses high thermal conductivity by itself. In some embodiments, the impedance match can be further improved by including a carbide-forming additive to form a thin layer of metal carbide (a monoatomic layer of about 10 nm or less, or a monoatomic layer of about 50 nm or less) on the diamond particles. Suitable carbide-forming metals may include, for example, Ti, Zr, Hf, Cr, Mo, W, V, Mn, Fe, and any combination thereof. Thus, the combination of copper nanoparticles and diamond particles can provide highly effective heat transfer in various embodiments of the present disclosure. Copper also provides high electrical conductivity to establish electronic communication between various substrate layers. Depending on the particular composition utilized, for example due to the amount of non-conductive additive, the electrical conductivity may be about 30-50% IACS, or about 35-60% IACS, or about 50-75% IACS, or about 55-90% IACS, or about 60-98% IACS (International Annealed Copper Standard).

[0061] Nanoparticle paste compositions suitable for use in accordance with the present disclosure can be formulated using any of the metal nanoparticle paste compositions described above. Furthermore, according to some embodiments, multiple metals may be present in the metal nanoparticle paste composition. In some or other embodiments, suitable metal nanoparticle paste compositions may include a mixture of metal nanoparticles, other nano-sized particles (i.e., particles having dimensions of about 200 nm or less), and / or micron-scale particles, such as micron-scale metal particles. According to more specific embodiments, the metal nanoparticle paste composition may include copper nanoparticles. In some embodiments, copper nanoparticles may be the major component (>50 wt%) of the metal nanoparticle paste composition.

[0062] Various heat spreaders and printed circuit boards utilizing heat spreaders may be formed, at least in part, from copper nanoparticles and copper nanoparticle paste compositions. In particular, copper nanoparticles or copper nanoparticle paste compositions may be utilized to fabricate at least a surface portion of a heat spreader. For example, a copper nanoparticle paste composition including a CTE modifier may be consolidated to form at least a partial coating on a thermal conductor, which coating may perform various functions described herein, or the copper nanoparticle paste composition may be consolidated to form the thermal conductor itself. It should be understood that alternative metal nanoparticles may be utilized to form the conductors or coatings on the conductors defining the heat spreader, as needed to facilitate CTE matching to certain ceramic materials within heat-generating components. Thus, it should be understood that any embodiment utilizing copper or copper nanoparticles in the following disclosure may utilize alternative metals or metal nanoparticles (optionally in combination with copper) depending on application-specific needs, unless otherwise specified.

[0063] Similarly, the copper nanoparticle paste composition may be applied as a bonding layer between a heat-generating component and a heat spreader containing a copper composite on at least a surface thereof. Following consolidation of the metal nanoparticles, the resulting bulk metal may provide a CTE-matched bonding interface between the heat-generating component and the surface of the heat spreader (also formed from the consolidated metal nanoparticles). The bonding layer may, in some embodiments, form a full or partial coating on at least one side of the heat spreader. The bonding layer may, in some embodiments, be multi-layered, with each layer having a different CTE. The bonding layer may be metallurgically bonded to the heat spreader, but not necessarily to the heat-generating component. In either case, the bonding layer may have a CTE intermediate between the CTE of the heat-generating component and the CTE of the heat spreader. A similar bonding layer may be interposed between the heat spreader and a heat sink or similar heat-receiving structure, and the heat spreader may optionally be metallurgically bonded to the heat sink or similar heat-receiving structure, and the bonding layer may have a CTE that is intermediate between the CTE of the heat spreader and the CTE of the heat sink or similar heat-receiving structure, such as a heat pipe.

[0064] The heat spreader of the present disclosure may include a thermal conductor configured to contact a heat source and a heat sink, with at least a portion of the thermal conductor being CTE-matched to the heat-generating component. In particular, at least a portion of the thermal conductor, a full or partial coating on the thermal conductor, or an interface material (bonding layer) between the thermal conductor and the heat-generating component may comprise a copper composite with a CTE adjuster, as further described herein. The thermal conductor may be a monolithic block that may comprise a metal, a metal alloy, or a metal composite. The thermal conductor may define a tapered heat spreader. The tapering of the heat spreader may distribute heat at its cold end and reduce the thermal load per unit area at this location. Various heat spreader configurations are defined below with reference to the drawings.

[0065] In various embodiments, a copper composite may be formed by consolidating copper nanoparticles with micron-sized copper particles and a CTE modifier, or by consolidating copper nanoparticles with a CTE modifier without micron-sized copper particles. The copper nanoparticles, micron-sized copper particles (if present), and CTE modifier may define a copper nanoparticle paste composition, as defined in more detail above. The copper nanoparticle paste composition may be used to form a thermal conductor of a heat spreader, a coating on a thermal conductor of a heat spreader, or as a bonding interface material between a heat spreader and a heat-generating component. A suitable copper nanoparticle paste composition may include about 30% to about 60% by weight of copper nanoparticles, about 5% to about 50% by weight of micron-sized copper particles, and an amount of a CTE modifier effective to target a specific CTE. The CTE modifier may be present in an amount ranging from about 1 wt.% to about 35 wt.%, or from about 4 wt.% to about 8 wt.%, or from about 5 wt.% to about 15 wt.%, or from about 10 wt.% to about 20 wt.%, or even about 35 wt.% or about 60 wt.%. One or more CTE modifiers may be present, such as two, three, four, or five or more different CTE modifiers. Micron-sized copper particles may be excluded in some embodiments.

[0066] Suitable CTE adjusters may include, but are not limited to, graphite / pitch-based carbon fibers (e.g., those having a diameter of 10 microns), W particles, Mo particles, diamond particles, boron nitride particles or fibers, aluminum nitride particles or fibers, silicon nitride particles or fibers, carbon nanotubes, graphene, graphite powder, etc., and any combination thereof. Unless otherwise specified, the CTE adjuster may be in one or more forms, such as powder, particles, fibers, flakes, etc. The amount of CTE adjuster may be selected to provide the desired degree of CTE matching. For example, a carbon-based additive can achieve a thermal expansion of about 2-3 ppm when added at about 16 vol%, or about 7 ppm when added at about 9 vol%, or about 6 ppm when added at about 11 vol%. A diamond addition of about 45 vol% can achieve a thermal expansion of about 5-6 ppm depending on the density (82%). At a loading of about 37% by volume and a density of 93%, the thermal expansion provided by diamond can be about 6 ppm. At diamond loadings greater than about 50% by volume, the thermal expansion drops to less than about 5 ppm.

[0067] CTE modifiers can also significantly increase thermal conductivity in some cases. For example, carbon nanotubes can increase the thermal conductivity of copper from a low of 400 W / m·K for bulk copper alone to approximately 600 W / m·K. The degree of thermal conductivity modification achievable with carbon nanotubes can depend on the length of the carbon nanotubes, with longer carbon nanotubes exceeding thermal conductivity values ​​of approximately 600 W / m·K.

[0068] The consolidated copper nanoparticles themselves exhibit a thermal expansion of approximately 7-12 ppm, depending on process conditions and density. As density increases, the thermal expansion approaches that of bulk copper (17 ppm). At approximately 91% density, the thermal expansion is approximately 7-8 ppm, and at approximately 93% density, the thermal expansion increases to approximately 10-11 ppm. At approximately 98% density, the thermal expansion reaches approximately 12 ppm. Even at such high density values, the thermal expansion is still below that of bulk copper, which is presumably due to the nanoporosity present after copper nanoparticle consolidation.

[0069] The addition of micron-scale metal particles to metal nanoparticles (e.g., copper nanoparticles) can increase thermal expansion, reaching 17 ppm or more depending on the particular metal. For example, the addition of Al particles, which have a bulk CTE of approximately 23-24 ppm, can increase the CTE of the resulting composite to values ​​exceeding that of bulk copper. The addition of approximately 55% micron-scale copper powder results in a thermal expansion of approximately 14 ppm at 96% density.

[0070] The thermal conductor of the heat spreader disclosed herein may comprise a monolithic block of metal, metal alloy, or metal composite. When the thermal conductor comprises a metal composite, additional components within the metal composite, such as CTE modifiers, diamond particles, or other types of particles with high thermal conductivity, may be distributed throughout the thermal conductor such that no portion of the thermal conductor is devoid of the additional component. That is, the thermal conductor does not include a compositional discontinuity where the additional component, such as a CTE modifier, is absent within a portion of the thermal conductor. Thus, the additional component, such as a CTE modifier, may be uniformly distributed in concentration within the thermal conductor, or may vary in concentration in a continuous or step-gradient manner. The thermal conductor may increase in size from a "hot" end that contacts the heat-generating component to a "cold" end configured to dissipate heat to a heat sink or other thermal reservoir, such as a radiator to the ambient atmosphere, a marine environment, or external space. The amount of additional component added may increase or decrease in concentration from the hot end to the cold end in the heat spreader configurations disclosed herein.

[0071] In some embodiments, the heat spreader may include a multilayer interface that promotes a gradual CTE change in a graded or stepped manner. The amount of CTE modifier and / or the composition of the CTE modifier in each layer of the multilayer interface may be adjusted to provide a desired degree of CTE difference between each layer. A stepped or graded change in CTE may provide less thermal stress between the heat-generating component and the heat spreader than if there were a more abrupt CTE change at the interface between the two. Any number of layers may be present in the multilayer structure, such as two, three, four, five, six, seven, eight, nine, or ten layers. Each layer may be between about 1 micron and about 25 microns thick, or between about 5 microns and about 10 microns thick. The CTE may vary between each layer by an amount ranging from about 1 ppm to about 2.5 ppm, or from about 0.8 ppm to about 1.5 ppm, or from about 1 ppm to about 2 ppm, or from about 1.5 ppm to about 2.2 ppm. For example, the CTE may be graded from 4 ppm in the heat generating component (e.g., containing SiC) to 17 ppm in the heat spreader through five layers having sequential CTEs of about 6.0 ppm, 8.2 ppm, 10.5 ppm, 12.8 ppm, and 15.0 ppm, or through ten layers having sequential CTEs of about 5 ppm, 6.1 ppm, 7.3 ppm, 8.7 ppm, 10.1 ppm, 11.5 ppm, 12 ppm, 13.7 ppm, 14.9 ppm, and 16 ppm. The multilayer interface may optionally be present as a bonding layer on the surface of the heat spreader.

[0072] The heat spreaders disclosed herein may be of any specified shape. Without limitation, the heat spreaders may be circular, prismatic, oval, triangular, flat, oblate, etc. The heat spreaders may be tapered or non-tapered. If tapered, the heat spreader may increase in size from the hot end to the cold end. The tapering may be continuous or discontinuous.

[0073] 3A-3D show cross-sectional views of various configurations of heat spreaders of the present disclosure. In FIG. 3A, heat spreader 300 includes a thermal conductor 310 and a coating 312 continuously disposed on thermal conductor 310. At least a portion of coating 312 may include a metal composite suitable for CTE matching according to the present disclosure. Alternatively, the thermal conductor may include a metal composite suitable for CTE matching according to the present disclosure, but a separate metal composite coating may not be present. If present, coating 312 does not necessarily need to be a continuous coating as shown in FIG. 3A. FIG. 3B shows a diagram of heat spreader 301 in which coating 312 is discontinuous on thermal conductor 310. The discontinuous coating shown in FIG. 3B may be utilized to form a thermal connection and / or bond to a heat-generating component (not shown). When disposed as a discontinuous coating, the discontinuous coating may interpose at least a portion of the space between the heat-generating component and thermal conductor 302. The metal nanoparticle paste composition may be disposed in the space between the heat generating component and the thermal conductor 302 to provide a bonding layer therebetween.

[0074] The heat spreaders of the present disclosure may be tapered to provide more effective heat dissipation. In FIG. 3C, heat spreader 302 includes a continuously tapered thermal conductor 310 having a coating 312 disposed on at least a portion of the thermal conductor 310, while in FIG. 3D, heat spreader 303 includes a discontinuously tapered thermal conductor 310 having a coating 312 disposed on at least a portion of the thermal conductor 310. Again, the coating 312 may be formed by consolidating a metal nanoparticle paste composition including a CTE adjuster between the thermal conductor 310 and the heat-generating component, thereby establishing a bonding layer between the two. In both heat spreaders 302 and 303, surface 314 may contact a heat source (heat-generating component), and surface 316 may contact or be in thermal communication with a heat sink or similar heat-receiving structure, such as a heat pipe (the heat source and heat sink are not shown in FIGS. 3C and 3D). For example, surface 314 may contact an electronic component that generates excess heat.

[0075] When a metal nanoparticle paste composition containing a CTE adjuster is used to form the thermal conductor of a heat spreader, the metal nanoparticle paste composition may be added to a suitable mold or die, and then the copper nanoparticles may be consolidated to form a monolithic metal block containing the CTE adjuster. The metal nanoparticle paste composition may be applied to a thermally conductive metal body to form a coating on the thermal conductor or to form a bonding layer between the thermally conductive metal body and a heat-generating component, or the metal nanoparticle paste composition may be sandwiched between the thermally conductive metal body and the heat-generating component, at which stage the metal nanoparticles in the metal nanoparticle paste composition may be consolidated to form a metal composite, such as a copper composite containing a CTE adjuster. Suitable conditions for processing the metal nanoparticle paste composition may include, for example, injection molding, hot pressing, or similar deposition and consolidation techniques. Localized heating of the metal nanoparticles may be performed during the deposition and consolidation process. Rapid localized heating to form the metal composite may be performed, for example, using a laser or Xe lamp, in non-limiting embodiments. Desirably, the fusion of metal nanoparticles to form a metal composite can be effected in the absence of an inert or reducing atmosphere, especially when rapid heating is performed.

[0076] In some configurations, multiple conductive fibers may extend from one end of the heat spreader. FIG. 4 shows a diagram of an exemplary heat spreader 400 having multiple conductive fibers 402 extending from one end of the thermal conductor 310. When configured in this manner, the conductive fibers 402 can provide a large surface area for dissipating conducted heat to a heat sink or similar thermal reservoir. In the configuration shown in FIG. 4, the thermal conductor 310 is formed from a metal composite containing a CTE modifier, and no separate coating (e.g., coating 312) is present thereon. The metal composite is manufactured by consolidating metal nanoparticles, as described in more detail above. In this manner, the conductive fibers 402 may be incorporated into the thermal conductor 310 during its formation, specifically by inserting the conductive fibers 402 into a metal nanoparticle paste composition formed into the shape of the conductor 310 and then consolidating the metal nanoparticles. It should be understood that, alternatively, the conductive fibers 402 may be bonded to the heat spreader 400 by the compaction of metal nanoparticles within a separate coating (if present). Other than the conductive fibers 402 extending from the thermal conductor 310 (or a coating thereon), the heat spreader 400 is similar to the heat spreaders 300-303 (FIGS. 3A-3D) and may be better understood by reference thereto. Common reference numerals are used to indicate elements of similar structure and function. Accordingly, any of the heat spreaders 300-303 may similarly incorporate the conductive fibers 402 in a manner similar to that described with respect to the heat spreader 400.

[0077] To introduce the conductive fibers 402 into the heat spreader 400, an unconsolidated metal nanoparticle paste composition may first be applied to (or used to form) the thermal conductor 310, and then the conductive fibers 402 may be disposed within the unconsolidated copper nanoparticle paste composition. Following consolidation of the copper nanoparticles, the conductive fibers 402 may be firmly secured to the heat spreader 400 in a matrix of bulk copper and dispersed CTE modifier formed from the copper nanoparticle paste composition.

[0078] Suitable conductive fibers may include, but are not limited to, graphite fiber bundles, which may exhibit thermal conductivity values ​​more than twice that of bulk copper (e.g., 800-1100 W / m·K or 550-1200 W / m·K). Other suitable conductive fibers may include, but are not limited to, metal fibers (e.g., Al fibers or Cu fibers), diamond fibers, carbon nanotube or carbon nanotube fibers, or any combination thereof. Suitable fiber lengths may be about 2-8 inches, depending on the flexibility of the fibers. Suitable fiber diameters may be about 5-50 microns, or about 5-10 microns, or about 5-20 microns, or about 30-50 microns. The fibers may also be in the form of a porous foam extending from the thermal conductor (or coating thereon), in which case a cooling fluid, such as air or liquid, may pass through the pores of the foam and / or over or through the fibers to carry away excess heat.

[0079] The heat spreaders disclosed herein may be utilized to dissipate heat from heat-generating components associated with a printed circuit board. The heat-generating components may comprise ceramics such as, for example, Si (CTE=2.6 ppm), SiC (CTE=4.2 ppm), GaN (CTE=5.6 ppm), or AlN (CTE=4.5 ppm). In a non-limiting example, the CTE of the heat spreader may be approximately 1 / 2 the CTE of the heat-generating component. + Within 50% or approximately + Within 25% or approximately + Within 20% or approximately + Within 15% or approximately + Within 10% or approximately + Within 5% or approximately + Within 4% or approximately + Within 3% or approximately + Within 2% or approximately + The thermal conductor of the heat spreader or a coating thereon may exhibit a CTE value at room temperature in the range of about 2 ppm to about 6 ppm, or about 3 ppm to about 7 ppm, or about 5 ppm to about 10 ppm, or about 10 ppm to about 15 ppm, or about 15 ppm to about 25 ppm.

[0080] Thus, in some embodiments, the present disclosure provides a heat dissipation system employing a heat spreader that can be CTE matched to a heat generating component and a heat sink or similar heat receiving structure. + 20%. A heat dissipation system may include a heat spreader having a thermal conductor, wherein at least a portion of the thermal conductor or a coating thereon comprises a copper composite including a coefficient of thermal expansion (CTE) modifier such that the thermal conductor has a coefficient of thermal expansion (CTE) of about 3 ppm to about 7 ppm; a heat-generating component in contact with a first surface of the thermal conductor, the heat-generating component having a CTE of about 3 ppm to about 7 ppm; and a heat sink or heat pipe in contact with a second surface of the thermal conductor, the heat sink or heat pipe also formed from the copper composite including the coefficient of thermal expansion modifier and having a CTE of about 3 ppm to about 7 ppm. Optionally, the thermal conductor of the heat spreader may be metallurgically bonded to the heat sink or heat pipe while achieving the aforementioned CTE match. Heat dissipation systems having the above-described features may be used in conjunction with printed circuit boards and other electronic components that generate excess heat, in non-limiting examples.

[0081] The printed circuit board of the present disclosure may include a heat-generating component disposed on or embedded within an electrically insulating substrate and at least one heat spreader in thermal communication with the heat-generating component, the at least one heat spreader being CTE-matched to at least the heat-generating component. The at least one heat spreader may be in thermal communication with the heat-generating component through direct or indirect physical contact and bonding with the heat-generating component. The electrically insulating substrate may also be thermally insulating, such as FR4, or thermally conductive, such as AlN or SiN. In various examples, the at least one heat spreader is a heat spreader of the present disclosure, at least a portion of which comprises a metal composite formed from metal nanoparticles and a CTE adjuster (e.g., a copper composite with a CTE adjuster). The at least one heat spreader may be bonded to the heat-generating component directly through a thermal conductor, indirectly through a coating on the thermal conductor, or indirectly through a bonding layer comprising a copper composite CTE-matched to the heat-generating component and at least a portion of the heat spreader.

[0082] It should be understood that printed circuit boards, as used herein, also refer equally to alternative structures that have similar heat dissipation issues, such as SIPs and packaged electronics.

[0083] The heat-generating component may be disposed on the top surface of the electrically insulating substrate or embedded within a recess in the electrically insulating substrate. At least one heat spreader may be bonded to the top surface of the heat-generating component or the bottom surface of the heat-generating component, one or more heat spreaders may be bonded to the side surfaces of the heat-generating component, or any combination thereof. Specific configurations are provided below.

[0084] FIG. 5 illustrates a diagram of a heat spreader bonded to the top surface of a heat-generating component. As shown, PCB 600 includes an electrically insulating substrate 602 and a heat-generating component 604 thereon. Heat spreader 606 is bonded to the top surface of heat-generating component 604. Heat spreader 606 may, in some cases, be integrated into and / or part of an integrated circuit housing. Although not shown in FIG. 5, heat-generating component 604 may be embedded within electrically insulating substrate 602, and heat spreader 606 may be placed on the surface of electrically insulating substrate 602.

[0085] FIG. 6 illustrates a diagram in which a heat spreader is bonded to the bottom surface of a heat-generating component. In this configuration, the heat spreader 606 extends through a via defined in the electrically insulating substrate 602 of the PCB 700 and contacts the backside of the heat-generating component 604. The via may be sized appropriately to allow the heat spreader 606 to extend through the via. The heat spreader 606 may increase in lateral size (increase in taper) after passing through the via. Although not shown in FIG. 6, the heat-generating component 604 may be embedded within the electrically insulating substrate 602.

[0086] Heat spreaders may also be bonded to the top and bottom surfaces of the heat-generating component, as shown in Figure 7. In PCB 800, heat spreader 606a is bonded to the top surface of heat-generating component 604, and heat spreader 606b extends through electrically insulating substrate 602 and is bonded to the bottom surface of heat-generating component 604. Although not shown in Figure 7, heat-generating component 604 may be embedded within electrically insulating substrate 602.

[0087] FIG. 8 illustrates multiple heat spreaders bonded to the sides of a heat-generating component. FIG. 8 shows a top view of a PCB 900, looking at the heat-generating component 604 and heat spreader 606 on the top surface of an electrically insulating substrate 602. The heat spreader 606 is bonded to both sides of the heat-generating component 604. While two heat spreaders 606 are shown with side bonding in the PCB 900, it is understood that one or more heat spreaders 606 may be similarly bonded. Side bonding of the heat spreader 606 may facilitate stacking on multiple PCB layers. While not shown, it is understood that upper and lower heat spreaders (see FIGS. 5-7) may also be present.

[0088] Thus, PCBs incorporating the heat spreaders of the present disclosure may be single layer or multilayer, according to various embodiments. Multilayer PCBs may include individual layers that are stacked together to define vias and other board features.

[0089] In various embodiments, the bonding layer between the heat spreader and the heat-generating component may include copper or may be formed from copper nanoparticles, or more specifically, may be formed from a copper nanoparticle paste composition containing other additives suitable for adjusting the CTE to match that of the heat-generating component. CTE adjusters may include carbon fibers, diamond particles, boron nitride, aluminum nitride, silicon nitride, carbon nanotubes, graphene, graphite, copper oxide nanoparticles, and the like. Other additives may be present in the copper nanoparticle paste composition for ease of dispensing and handling. The bonding layer may also include a second copper composite, which may be similarly CTE-matched to the copper composite of the heat spreader.

[0090] Embodiments disclosed herein include the following.

[0091] A. Heat Spreader. The heat spreader includes a thermal conductor configured to contact a heat source and a heat sink, wherein at least a portion of the thermal conductor or a coating thereon includes a copper composite including a coefficient of thermal expansion (CTE) modifier.

[0092] B. a printed circuit board having a heat generating component disposed on or embedded within an electrically insulating substrate; at least one heat spreader in thermal communication with the heat-generating component, the at least one heat spreader comprising: A printed circuit board comprising a thermal conductor, wherein at least a portion of the thermal conductor or a coating thereon comprises a copper composite including a coefficient of thermal expansion (CTE) modifier.

[0093] C. Heat Dissipation System. A heat dissipation system may include a heat spreader having a thermal conductor, wherein at least a portion of the thermal conductor or a coating thereon comprises a copper composite including a coefficient of thermal expansion (CTE) modifier such that the thermal conductor has a coefficient of thermal expansion (CTE) of about 3 ppm to about 7 ppm; a heat generating component in contact with a first surface of the thermal conductor, the heat generating component having a CTE of about 3 ppm to about 7 ppm; and a heat sink or heat pipe in contact with a second surface of the thermal conductor, the heat sink or heat pipe also formed from the copper composite including a coefficient of thermal expansion modifier and having a CTE of about 3 ppm to about 7 ppm. Optionally, the thermal conductor or a coating thereon has a CTE of about 3 ppm to about 7 ppm. + Optionally, the thermal conductor or coating thereon may have a CTE within about 20% of the CTE of the heat sink or heat pipe. + It may have a CTE within 20. Optionally, the thermal conductor or a coating thereon may be metallurgically bonded to the heat generating component and / or heat sink or heat pipe via a bonding layer.

[0094] Each of embodiments A to C may have one or more of the following additional elements in any combination.

[0095] Element 1: Copper composite is formed by consolidation of copper nanoparticles with micron-sized copper particles and a CTE modifier.

[0096] Element 1A: The copper composite is formed by consolidation of copper nanoparticles and a CTE modifier.

[0097] Element 2: The copper composite has uniform nanoporosity between about 2% and about 30%.

[0098] Element 3: The CTE adjuster comprises particles or fibers selected from the group consisting of carbon, W, Mo, diamond, boron nitride, aluminum nitride, silicon nitride, carbon nanotubes, graphene, graphite, copper oxide nanoparticles, and any combination thereof.

[0099] Element 4: The heat spreader further comprises a plurality of thermally conductive fibers extending from at least a portion of the thermal conductor or coating thereon, if present.

[0100] Element 5: The thermal conductor is formed entirely from a copper composite containing a CTE modifier.

[0101] Element 6: The CTE modifier is distributed throughout the thermal conductor.

[0102] Element 7: The heat conductor is tapered.

[0103] Element 8: The CTE modifier is present in the copper composite in a stepped or gradient concentration distribution.

[0104] Element 9: At least one heat spreader is bonded to the heat generating component via a bonding layer comprising a copper composite CTE matched to the heat generating component. Optionally, the CTE of the heat generating component and the CTE of the thermal conductor and / or coating thereon are less than about + 20% or less, or approximately + 10% or less, or + differ by less than 5%.

[0105] Element 10: The copper composite of the bonding layer has a uniform nanoporosity of about 2% to about 30%.

[0106] Element 11: A heat-generating component is disposed on or embedded within a surface of an electrically insulating substrate, and at least one heat spreader is bonded to a top surface of the heat-generating component, one or more heat spreaders are bonded to a side surface of the heat-generating component, at least one heat spreader is bonded to a bottom surface of the heat-generating component, at least one heat spreader extends through the electrically insulating substrate, or any combination thereof.

[0107] As a non-limiting example, exemplary combinations applicable to A through C include, but are not limited to, 1 or 1A and 2; 1 or 1A and 3; 1 or 1A and 4; 1 or 1A and 5; 1 or 1A and 6; 1 or 1A and 7; 1 or 1A and 8; 2 and 3; 2 and 4; 2 and 5; 2 and 6; 2 and 7; 2 and 8; 3 and 4; 3 and 5; 3 and 6; 3 and 7; 3 and 8; 4 and 5; 4 and 6; 4 and 7; 4 and 8; 5 and 6; 5 and 7; 5 and 8; 6 and 7; 6 and 8; and 7 and 8. With respect to B and C, any of the foregoing may be further combined with 9, 10, or 11. Further exemplary combinations applicable to B and C include, but are not limited to, 9 and 10; 9 and 11; 10 and 11; and 9-11.

[0108] To facilitate a better understanding of the embodiments of the present disclosure, the following examples of preferred or representative embodiments are given. In no way should the following examples be read to limit or define the scope of the invention.

[0109] Example Example 1. To fabricate a solid cylinder 11.7 mm high and 0.5 inches wide, 2.7 mL of 5.1 g / ccm high-density paste containing 43% (v / v) commercially available dried graphite powder, 24% (v / v) commercially available dried copper powder, and 33% (v / v) copper nanoparticles was placed into a 3-inch high by 2-inch wide graphite cell with a 0.5-inch hole closed at both ends with tightly fitting graphite rods. The cell was then placed in a hydraulic press. An initial pressure of 250 psi was applied while the cell was heated to 250°C. The pressure was increased to 500 psi at 100°C, 1000 psi at 200°C, 1750 psi at 250°C, and then maintained throughout the process. The target peak temperature was reached after approximately 15 minutes and maintained for an additional 75 minutes. The heat was then shut off, the cell was cooled to room temperature, and a part was extruded. The cylinder weighed 12.2 g and had a density of 92%.

[0110] Example 2. To fabricate a 6.8 mm high, 0.5 inch wide solid cylinder, 1.6 mL of 4.9 g / ccm high-density paste containing 33% (v / v) commercially available dry BN powder, 33% (v / v) commercially available dry copper powder, and 34% (v / v) copper nanoparticles was placed into a 3 inch high x 2 inch wide graphite cell with a 0.5 inch hole closed at both ends with tightly fitting graphite rods. The cell was then placed in a hydraulic press. An initial pressure of 250 psi was applied while the cell was heated to 250°C. The pressure was increased to 500 psi at 100°C, 1000 psi at 200°C, 1750 psi at 235°C, and then maintained throughout the process. The target peak temperature was reached after approximately 10 minutes and maintained for an additional 45 minutes. The heat was then shut off, the cell was cooled to room temperature, and the part was extruded. The cylinder weighed 6.9 g and had a density of 90%.

[0111] Example 3. To fabricate a 15.6 mm high, 0.5 inch wide solid cylinder, 2.9 mL of a 4.3 g / ccm high-density paste containing 7% (v / v) commercially available dry diamond powder, 23% (v / v) commercially available dry copper powder, and 70% (v / v) copper nanoparticles was placed into a 3 inch high x 2 inch wide graphite cell with a 0.5 inch hole closed at both ends with tightly fitting graphite rods. The cell was then placed in a hydraulic press. An initial pressure of 250 psi was applied while the cell was heated to 250°C. The pressure was increased to 500 psi at 100°C, 1400 psi at 200°C, and 1850 psi at 235°C, and then maintained throughout the process. The target peak temperature was reached after approximately 15 minutes and maintained for an additional 85 minutes. The heat was then shut off, the cell was cooled to room temperature, and the part was extruded. The cylinder weighed 11.7 g and had a density of 92%.

[0112] Additional specimens were prepared in a manner similar to that described above for Examples 1 to 3. The metal composite compositions and the CTE values ​​obtained from the metal composite compositions at various temperatures are shown in Table 1.

[0113] [Table 1]

[0114] Unless otherwise indicated, all numbers expressing quantities of ingredients, properties such as molecular weight, reaction conditions, and the like used in the specification and the related claims are to be understood as being modified in all instances by the term "about." Accordingly, unless otherwise indicated, the numerical parameters set forth in the following specification and appended claims are approximations that may vary depending upon the desired properties sought to be obtained by embodiments of the present invention. At the very least, and not as an attempt to limit the application of the doctrine of equivalents to the scope of the claims, each numerical parameter should at least be construed in light of the number of reported significant digits and by applying ordinary rounding techniques.

[0115] One or more exemplary embodiments incorporating features of the present disclosure are presented herein. For clarity, not all features of a physical implementation are described or illustrated in this application. It should be understood that in developing a physical embodiment incorporating the present disclosure, numerous implementation-specific decisions must be made to achieve the developer's goals, including compliance with system-related, business-related, government-related, and other constraints, which vary from implementation to implementation and from time to time. While the developer's efforts may require significant time, such efforts would nevertheless be routine for those skilled in the art and who have the benefit of this disclosure.

[0116] Thus, the present disclosure is well adapted to achieve the ends and advantages mentioned, as well as those inherent therein. The specific embodiments disclosed above are illustrative only, as the disclosure may be modified and practiced in different but equivalent manners apparent to those skilled in the art having the benefit of the teachings herein. Furthermore, no limitations are intended to the details of construction or design herein shown, other than as described in the following claims. It is therefore evident that the specific exemplary embodiments disclosed above may be altered, combined, or modified, and all such variations are considered to be within the scope and spirit of the present invention. The disclosure herein may suitably be practiced in the absence of any element not specifically disclosed herein and / or in the absence of any optional element disclosed herein. While compositions and methods are described in terms "comprising," "containing," or "including" various components or steps, the compositions and methods may also "consist essentially of" or "consist of" the various components and steps. All numbers and ranges disclosed above may be varied by some amount. Whenever a numerical range is disclosed with a lower and upper limit, any number falling within that range and any range encompassed therein is specifically disclosed. Specifically, all ranges of values ​​disclosed herein (in the form "from about a to about b," or, equivalently, "from approximately a to b," or, equivalently, "from approximately a to b") are to be understood to describe all numbers and ranges encompassed within the broader range of values. Furthermore, terms in the claims have their plain and ordinary meaning unless expressly and unambiguously defined otherwise by the patent owner. Furthermore, the indefinite articles "a" or "an," as used in the claims, are defined herein to mean one or more of the element they introduce.

Claims

1. A heat spreader, 1. A heat spreader comprising: a thermal conductor configured to contact a heat source and a heat sink, at least a portion of the thermal conductor comprising a copper composite including copper nanoparticles and a coefficient of thermal expansion (CTE) modifier.

2. 2. The heat spreader of claim 1, wherein the copper composite further comprises micron-sized copper particles and is formed by consolidation of the copper nanoparticles with the micron-sized copper particles and the CTE modifier.

3. The heat spreader of claim 1 , wherein the copper composite has a uniform nanoporosity of about 2% to about 30%.

4. 10. The heat spreader of claim 1, wherein the CTE modifier comprises particles or fibers selected from the group consisting of carbon, W, Mo, diamond, boron nitride, aluminum nitride, silicon nitride, carbon nanotubes, graphene, graphite, copper oxide nanoparticles, and any combination thereof.

5. The heat spreader of claim 1 , further comprising a plurality of thermally conductive fibers extending from at least a portion of the thermal conductor.

6. The heat spreader of claim 1 , wherein the thermal conductor is formed entirely from the copper composite including the CTE modifier.

7. The heat spreader of claim 6 , wherein the CTE modifier is distributed throughout the thermal conductor.

8. The heat spreader of claim 1 , wherein the thermal conductor is tapered.

9. The heat spreader of claim 1 , wherein the CTE modifier is present in the copper composite in a graded or gradient concentration distribution.

10. The heat spreader of claim 1 , wherein the copper composite has a CTE value at room temperature of about 3 ppm / ° C. to about 7 ppm / ° C.

11. 1. A printed circuit board (PCB), comprising: a heat-generating component disposed on or embedded in an electrically insulating substrate; and at least one heat spreader in thermal communication with the heat generating component, the at least one heat spreader comprising: a thermal conductor, at least a portion of which comprises a copper composite comprising copper nanoparticles and a coefficient of thermal expansion (CTE) modifier; A printed circuit board (PCB) wherein the copper composite is formed by consolidation of the copper nanoparticles and the CTE modifier.

12. 12. The PCB of claim 11, wherein the copper composite further comprises micron-sized copper particles and is formed by consolidation of the copper nanoparticles with the micron-sized copper particles and the CTE modifier.

13. The PCB of claim 11, wherein the copper composite has a uniform nanoporosity of about 2% to about 30%.

14. 12. The PCB of claim 11, wherein the at least one heat spreader is bonded to the heat generating component via a bonding layer comprising a second copper composite CTE matched to the heat generating component.

15. The PCB of claim 14, wherein the second copper composite of the bonding layer has a uniform nanoporosity of about 2% to about 30%.

16. 12. The PCB of claim 11, wherein the CTE adjuster comprises particles or fibers selected from the group consisting of carbon, W, Mo, diamond, boron nitride, aluminum nitride, silicon nitride, carbon nanotubes, graphene, graphite, copper oxide nanoparticles, and any combination thereof.

17. The PCB of claim 11 , wherein the thermal conductor is formed entirely from the copper composite including the CTE modifier.

18. 18. The PCB of claim 17, wherein the CTE modifier is distributed throughout the thermal conductor.

19. The PCB of claim 11 , wherein the thermal conductor is tapered.

20. 12. The PCB of claim 11, wherein the heat generating component is disposed on or embedded within a surface of the electrically insulating substrate, and: the at least one heat spreader is bonded to an upper surface of the heat generating component; the at least one heat spreader is bonded to a side surface of the heat-generating component; the at least one heat spreader is bonded to a bottom surface of the heat generating component, and the at least one heat spreader extends through the electrically insulating substrate; or any combination thereof.

21. The PCB of claim 11 , wherein the CTE modifier is present in the copper composite in a graded or gradient concentration distribution.

22. 12. The PCB of claim 11, wherein the copper composite has a CTE value at room temperature of about 3 ppm / °C to about 7 ppm / °C.

23. 1. A heat dissipation system comprising: a heat spreader having a thermal conductor, at least a portion of the thermal conductor comprising a copper composite including copper nanoparticles and a coefficient of thermal expansion (CTE) modifier such that the thermal conductor has a coefficient of thermal expansion (CTE) value at room temperature of about 3 ppm / °C to about 7 ppm / °C; a heat generating component in contact with a first surface of the thermal conductor, the heat generating component having a CTE value at room temperature of about 3 ppm / °C to about 7 ppm / °C; a heat sink or heat pipe in contact with the second surface of the thermal conductor, the heat sink or the heat pipe also being formed from the copper composite including the CTE modifier and having a CTE value at room temperature of about 3 ppm / °C to about 7 ppm / °C; The heat dissipation system, wherein the copper composite is formed by consolidation of the copper nanoparticles and the CTE modifier.

24. The heat dissipation system of claim 23 , wherein the thermal conductor is metallurgically bonded to the heat sink or heat pipe via a bonding layer.

25. 24. The heat dissipation system of claim 23, wherein the thermal conductor has a CTE within about ±20% of the heat-generating component.

26. A heat spreader as described in claim 9, wherein the heat spreader includes a multilayer interface and the CTE adjuster in the copper composite is adjusted between each layer to have a stepped or gradient concentration distribution.

27. ​​A heat spreader as described in claim 26, wherein the change in CTE value between each layer at room temperature is about 1 ppm / °C to about 2.5 ppm / °C.

28. A heat spreader as described in claim 1, wherein the copper composite further comprises diamond particles, the diamond particles having a size of about 0.5 to about 1000 microns.

29. A PCB as described in claim 21, wherein the heat spreader includes a multi-layer interface and the CTE adjuster in the copper composite is adjusted between each layer to have a stepped or gradient concentration distribution.

30. A PCB as described in claim 29, wherein the change in CTE value between each layer at room temperature is from about 1 ppm / °C to about 2.5 ppm / °C.

31. The PCB of claim 11, wherein the copper composite further comprises diamond particles, the diamond particles having a size of about 0.5 to about 1000 microns.

32. The heat dissipation system of claim 23, wherein the copper composite further comprises diamond particles, the diamond particles having a size of about 0.5 to about 1000 microns.

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