阅读说明：本技术 高温焊剂及由其形成的连接部 (High-temperature solder and connection formed by the same ) 是由 雅佛地·A·辛 R·斯托尔滕贝里 于 2019-08-08 设计创作，主要内容包括：铜纳米颗粒糊剂组合物可以通过包括晶粒生长抑制剂与适量的铜纳米颗粒而配制,用于形成能够在高温下工作的连接部。这样的纳米颗粒糊剂组合物可以包含铜纳米颗粒以及与铜纳米颗粒掺和的0.01wt.％至15wt.％的晶粒生长抑制剂或晶粒生长抑制剂的前体,其中,晶粒生长抑制剂包含金属。晶粒生长抑制剂不溶于块体铜基体,并且能够存在于块体铜基体中的一个或多个晶粒边界处。一个或多个晶粒边界可以在铜纳米颗粒经历固结以形成块体铜之后形成。晶粒生长抑制剂可以包含不溶于块体铜的各种金属。(The copper nanoparticle paste composition may be formulated by including a grain growth inhibitor with an appropriate amount of copper nanoparticles for forming a connection capable of operating at high temperature. Such nanoparticle paste compositions may comprise copper nanoparticles and 0.01 to 15 wt.% of a grain growth inhibitor or a precursor of a grain growth inhibitor admixed with the copper nanoparticles, wherein the grain growth inhibitor comprises a metal. The grain growth inhibitor is insoluble in the bulk copper matrix and can be present at one or more grain boundaries in the bulk copper matrix. One or more grain boundaries may be formed after the copper nanoparticles undergo consolidation to form bulk copper. The grain growth inhibitor may comprise various metals that are insoluble in bulk copper.)

1. A nanoparticle paste composition comprising:

copper nanoparticles; and

0.01 to 15 wt.% of a grain growth inhibitor or a precursor of a grain growth inhibitor admixed with the copper nanoparticles, the grain growth inhibitor comprising a metal;

wherein the grain growth inhibitor is insoluble in the bulk copper matrix and can be present at one or more grain boundaries in the bulk copper matrix.

2. The nanoparticle paste composition of claim 1, wherein the grain growth inhibitor comprises a metallic species, a metal carbide, a metal nitride, a metal boride, a metal silicide, a metal phosphide, or any combination thereof.

3. The nanoparticle paste composition of claim 1, wherein the grain growth inhibitor comprises a metallic species comprising a metal selected from the group consisting of: fe. Mn, Cr, Ru, Si, V, W, Nb, Ta, Y, Zr, Hf, Be, Tl, Ir, Ti, Mo, Re, Al, any alloy thereof, and any combination thereof.

4. The nanoparticle paste composition of claim 3 wherein the grain growth inhibitor comprises one or more metal nanoparticles.

5. The nanoparticle paste composition of claim 4 wherein the metal nanoparticles are about 10nm or less in size.

6. The nanoparticle paste composition of claim 1 wherein the copper nanoparticles are about 20nm or less in size.

7. The nanoparticle paste composition of claim 1, wherein the grain growth inhibitor is present within the copper nanoparticles as a seed.

8. The nanoparticle paste composition of claim 1 wherein the copper nanoparticles are coated with at least one amine surfactant.

9. A connection portion, comprising:

a bulk copper matrix formed by the fusion of copper nanoparticles, the bulk copper matrix comprising a plurality of grain boundaries; and

a grain growth inhibitor that is insoluble in the bulk copper matrix and disposed within at least a portion of the plurality of grain boundaries within the bulk copper matrix, the grain growth inhibitor comprising a metal in an amount in a range of about 0.01 wt.% to 15 wt.% in the connection.

10. The joint of claim 9, wherein the joint is operationally stable at temperatures up to about 90% of the melting point of bulk copper.

11. The joint according to claim 9, wherein the grain growth inhibitor comprises a metallic substance, a metal carbide, a metal nitride, a metal boride, a metal silicide, a metal phosphide, or any combination thereof.

12. The connection portion according to claim 9, wherein the grain growth inhibitor comprises a metallic substance comprising a metal selected from the group consisting of: fe. Mn, Cr, Ru, Si, V, W, Nb, Ta, Y, Zr, Hf, Be, Tl, Ir, Ti, Mo, Re, Al, any alloy thereof, and any combination thereof.

13. A method, comprising:

depositing the nanoparticle paste composition on a substrate;

wherein the nanoparticle paste composition comprises copper nanoparticles and from 0.01 wt.% to 15 wt.% of a grain growth inhibitor or a precursor of a grain growth inhibitor admixed with the copper nanoparticles, the grain growth inhibitor comprising a metal; and

consolidating the copper nanoparticles to form a bulk copper matrix comprising a plurality of grain boundaries, wherein the grain growth inhibitor is insoluble in the bulk copper matrix and is present within the plurality of grain boundaries in the bulk copper matrix.

14. The method of claim 13, wherein the grain growth inhibitor comprises a metallic species, a metal carbide, a metal nitride, a metal boride, a metal silicide, a metal phosphide, or any combination thereof.

15. The method of claim 13, wherein the grain growth inhibitor comprises a metallic species comprising a metal selected from the group consisting of: fe. Mn, Cr, Ru, Si, V, W, Nb, Ta, Y, Zr, Hf, Be, Tl, Ir, Ti, Mo, Re, Al, any alloy thereof, and any combination thereof.

16. The method of claim 15, wherein the grain growth inhibitor comprises one or more metal nanoparticles.

17. The method of claim 16, wherein the metal nanoparticles are about 10nm or less in size.

18. The method of claim 13, wherein the grain growth inhibitor is present as a seed within the copper nanoparticles.

19. The method of claim 13, wherein the copper nanoparticles are coated with at least one amine surfactant.

20. The method of claim 13, further comprising:

exposing the joint to a temperature of about 150 ℃ or above to a temperature of at most about 90% of the melting point of bulk copper.

Background

Some of the most challenging tasks in space, especially those facing extremely high temperatures, such as the Venus lander and solar observation station, require more powerful electronic product systems than are currently available. Modern commercial electronic products are designed to operate over a relatively narrow temperature range (typically in the range of about 0 ℃ to 100 ℃). Electronic products may remain effective in some harsher environments by improving packaging and thermal management, but these techniques often result in greater design complexity and do not always address specific task requirements. For example, one approach proposed for the next generation of Venus lander (surface temperature 462℃.) that remains effective is to build large refrigerators for electronic product systems. However, this approach is not easily scalable, as the refrigerator may quickly consume all available system quality, leaving little space for scientific payloads. Similar challenges in high temperature operating environments are similarly found in industries such as oil and gas exploration and production, operation of high power electric vehicles, mining, data storage, and energy production. All of these industries may require the use of electronic products that remain effective at high temperatures, e.g., temperatures in excess of 150 c, and sometimes in excess of 300 c. As another non-limiting example, data centers are becoming larger due to the popularity of information technology and IoT (internet of things), and the limiting factor in the use of data centers is efficient heat dissipation and cooling, which may cost more than the available energy budget of the data center, not to mention the harsh thermal environment associated with the data center. Being able to operate a blade-type server efficiently at higher operating temperatures will reduce cooling requirements and save a lot of energy, thereby allowing more energy budget to be used for data processing rather than cooling. However, current materials are largely unable to meet these requirements.

New types of integrated circuit devices made of materials such as silicon carbide, gallium nitride and diamond have emerged in recent years. More experimental systems based on carbon nanotubes and graphene are under investigation, but these materials face large-scale manufacturing problems and high integration challenges. The above materials are capable of operating at temperatures ranging from 300 ℃ to 700 ℃, opening the door not only for Venus landers and close sun missions, but also for large commercial markets where harsh thermal conditions must be addressed. Complementary passive devices and powerful substrates have also been developed to better handle the high temperatures of these and other operating environments.

However, the higher operating temperatures of electronic devices indicate that the encapsulation materials are a new limiting factor. The encapsulating material includes those components used to manufacture the device, such as solder (solder) used to connect electronic or structural components to each other. Typical fluxes will melt and reflow around 200 c, while high performance fluxes may melt and reflow around 300 c. Even with high performance solder, there is still a large gap between the maximum operating temperature required for the integrated circuit and the temperature at which the solder reflows. Even below reflow temperatures, various solders may exhibit behavior that weakens or destroys the solder joint through creep, intermetallic formation, and/or dendritic growth, each of which may accelerate at elevated temperatures, even well below reflow temperatures or melting points. Many conventional fluxes also have poor thermal conductivity, which becomes a greater problem at elevated operating temperatures.

Sintered silver and silver epoxy are currently used in some higher temperature applications to address the aforementioned problems associated with poor soldering performance. In the case of epoxy resins, the thermal conductivity is very poor (less than 1W/m · K) and the effectiveness of the extreme temperature retention is limited, both of which may be attributed to the high polymer content. Furthermore, outgassing of the epoxy resin at high temperatures may lead to mechanical failure due to delamination of the outgassing material and contamination of other surfaces. Silver also tends to exhibit poor electromigration, creep and dendritic growth during high temperature operation, all of which are typically observed in sintered silver products.

Gold-tin and gold-germanium eutectic alloys have been achieved by high temperature brazing (reflow temperatures of 280 c and 356 c, respectively), but the high cost of gold-tin and gold-germanium eutectic alloys makes widespread adoption cost prohibitive. Even so, the reflow temperatures of gold-tin and gold-germanium eutectic alloys are still too low to be effective for many high temperature applications. At the other end of the thermal spectrum, silver-copper alloys with a reflow temperature in the range of 700 ℃ to 800 ℃ are available. However, these reflow temperatures are still at the upper end of the thermal stability range even for high temperature electronic devices, making device manufacturing problematic and narrowing the applicability of these encapsulation materials.

Drawings

The following drawings are included to illustrate certain aspects of the present disclosure and should not be taken as exclusive embodiments. The disclosed subject matter is capable of considerable modification, alteration, combination, and equivalents in form and function, as will occur to those ordinarily skilled in the pertinent art and having the benefit of this disclosure.

Fig. 1 and 2 show diagrams of the putative structures of metal nanoparticles having a surfactant coating thereon.

Fig. 3A and 3B show X-ray images, which indicate that there are no large voids in the consolidated copper nanoparticles (fig. 3B) and the void area is reduced as a whole, compared to the AuSn flux (fig. 3A).

Fig. 4 shows a graph of the stability of a nanostructured material calculated for a fixed dimensionless temperature.

Detailed Description

The present disclosure relates generally to high temperature fluxes and, more particularly, to copper-based fluxes for high temperature electronics and methods of making and using the same.

As mentioned above, most modern electronic products are designed to operate within a relatively narrow temperature range of about 0 ℃ to 100 ℃. With improved packaging and thermal management, it may remain effective in some of the more severe operating environments where thermal demands increase, but there are still limitations in the fabrication and performance of the device. Beyond about 150 ℃, common silicon-based electronics begin to fail. SiC-based integrated circuits have been developed that can operate near 600 ℃, but these operating temperatures present packaging challenges for current solder materials. Even high performance solder fluxes reflow or begin to fail at around 300 c, which leaves a large gap between the maximum operating temperature of the high temperature integrated circuit and the temperature at which the solder can still operate reliably. In addition, high performance solders are generally expensive, and some high performance solders may not be compatible with other materials used within integrated circuits.

The present disclosure provides an unconventional flux material (metal binder) in the form of a nanoparticle paste composition and an electronic assembly including one or more connections formed from the nanoparticle paste composition, wherein each connection has good resistance to high temperature operating conditions. Advantageously, the nanoparticle paste composition can be processed at relatively low temperatures (about 200 ℃ to below 240 ℃) to form connections and maintain excellent connectivity and conductivity at high operating temperatures above 300 ℃ or even higher, up to about 900 ℃ or up to about 940 ℃ (including temperatures between 900 ℃ and 940 ℃). Instead of using conventional reflowable solder to form the connection, the present disclosure employs a copper nanoparticle (nanocopper) based material in the form of a nanoparticle paste composition that can be processed at relatively low temperatures (about 200 ℃, about 220 ℃, or about 240 ℃) to form a connection that includes a bulk copper matrix. That is, the nanoparticle paste compositions disclosed herein do not require processing (reflow) at temperatures near the melting point of bulk copper to form connections. Once the joint comprising the bulk copper matrix is formed at low processing temperatures, the joint can reliably operate at high temperatures in the range of about 150 ℃ to 900 ℃, particularly 300 ℃ to 900 ℃. As explained below, the low temperature treatment of copper nanoparticles to form a bulk copper matrix is achieved due to the enhanced activity of the nanoparticles relative to the corresponding bulk metal, allowing treatment at temperatures well below the high melting point of bulk copper (1084 ℃). In particular, copper nanoparticles fuse together at temperatures much lower than the melting point of bulk copper, facilitating low temperature manufacturing conditions compatible with a range of electronic materials suitable for forming integrated circuits. Once the fusion to form the bulk copper matrix is performed, the resulting connector provides high temperature compatibility up to a significant fraction of the melting point of bulk copper metal. Thus, the use of copper nanoparticles to form connections provides the dual advantage of compatibility of low temperature manufacturing conditions with high temperature operating conditions. With conventional flux materials, a combination of low temperature processing and high temperature compatibility is not possible.

Since copper has the highest melting point (1084 ℃) among the three most conductive metals (imprint metals) Cu, Ag, Au and the second highest conductivity among these metals (5.96 × 10 ℃)⁷S/m relative to 6.3X 10⁷S/m Ag) copper may be an ideal material for forming metal connections in electronic devices under high temperature operating conditions. Copper is also the hardest of these three metals, does not creep, and its conductivity is that of conventional solder materials (typically below 1 x 10)⁷S/m) is 7 times higher. These properties exceed the performance characteristics of even high temperature AuSn alloy solder (283 ℃ m.p.) and various high temperature, high lead alloys used for die bonding and similar packaging applications. Due to the RoHS regulation, lead-based fluxes are being phased out worldwide, so their use is limited to very special applications where no other flux with suitable characteristics is present. In addition, few high temperature fluxes are available in paste form, which limits the handleability and the available application space of the high temperature flux. Thus, copper nanoparticle-based fluxes in the form of nanoparticle paste compositions provide many of the permits that conventional flux materials cannot meetMulti-processing and operational advantages.

Even below the melting point of bulk copper, the bulk copper matrix may experience grain growth, particularly under repeated thermal cycling. Excessive grain growth may lead to tissue weakening and joint failure. At low operating temperatures, grain growth is not generally an urgent handling problem for copper nanoparticle-based fluxes. Advantageously, the present disclosure describes how a grain growth inhibitor may be combined with the copper nanoparticles in the nanoparticle paste composition to reduce the occurrence of grain growth after the bulk copper matrix has been formed, while maintaining the effective operating temperature of the bulk copper matrix as close to the melting point of the bulk copper as possible. As such, the inclusion of the grain growth inhibitor may improve the high-temperature reliability of the connection made of the copper nanoparticles.

In accordance with the present disclosure, a grain growth inhibitor, such as a metal (e.g., Al, Ti, Ta, Zr, or Hf) or other material that is insoluble in the bulk copper matrix, may be included with the copper nanoparticles in a suitable form to inhibit grain growth at elevated operating temperatures. In particular, grain growth can be inhibited by: the grain growth inhibitor migrates and is present at the grain boundaries present within the bulk copper matrix, effectively locking the grain boundaries in place. This process may be referred to as grain boundary pinning or Zener pinning (Zener pinning). The processing conditions may determine whether incorporation of insoluble material occurs at the grain growth boundaries. For example, some metals may be soluble or insoluble in copper depending on the processing conditions and the amount used and the form in which the metal is associated with the copper nanoparticles.

Before discussing embodiments of the present disclosure in further detail, a brief description of metal nanoparticles and metal nanoparticle paste compositions suitable for use in the present disclosure will first be provided, wherein copper nanoparticles are representative examples of metal nanoparticles that may be present as the predominant metal nanoparticles in the metal nanoparticle paste compositions. Metal nanoparticles exhibit a number of characteristics that may be significantly different from those of the corresponding bulk metal. One property of metal nanoparticles that is of particular importance is the fusing or consolidation of the nanoparticles that occurs at the fusing temperature of the metal nanoparticles. As used herein, the term "fusion temperature" refers to the temperature at which the metal nanoparticles liquefy to give a molten appearance. As used herein, the terms "fusion" and "consolidation" refer synonymously to coalescence or partial coalescence of metal nanoparticles with one another to form a bulk metal of greater mass (sintered mass), thereby defining a bulk metal matrix, such as a bulk copper matrix.

As the size decreases, particularly as the equivalent spherical diameter decreases below about 20nm, the temperature at which the metal nanoparticles liquefy drops sharply from that of the corresponding bulk metal. For example, copper nanoparticles having a size of about 20nm or less or about 70nm or less may have a fusion temperature of about 240 ℃ or less, or about 220 ℃ or less, or about 200 ℃ or less, as compared to the melting point of 1084 ℃ for bulk copper. Thus, the consolidation of the metal nanoparticles at the fusion temperature may allow the fabrication of objects comprising a bulk metal matrix at much lower processing temperatures than when working directly using the bulk metal itself as a starting material. Once formed, the bulk metal matrix has a melting point similar to that of the bulk metal and includes a plurality of grain boundaries.

As used herein, the term "metal nanoparticles" refers to metal particles having a size of about 100nm or less, without specific reference to the shape of the metal particles. As used herein, the term "copper nanoparticles" refers to metal nanoparticles made of copper or made primarily of copper.

As used herein, the term "micron-sized metal particles" refers to metal particles having a size of about 100nm or more in at least one dimension.

The terms "consolidation", "consolidation" and other variations thereof are used interchangeably herein with the terms "fusion", "fusion" and other variations thereof.

As used herein, the terms "partially fused", and other derivatives and grammatical equivalents thereof refer to the partial coalescence of metal nanoparticles with one another. Fully fused metal nanoparticles retain substantially no structural morphology of the original unfused metal nanoparticles (i.e., they resemble bulk metal with minimal grain boundaries), while partially fused metal nanoparticles retain at least some structural morphology of the original unfused metal nanoparticles. The properties of the partially fused metal nanoparticles may be intermediate between those of the corresponding bulk metal and the original unfused metal nanoparticles.

Many scalable processes have been developed to produce large quantities of metal nanoparticles in the targeted size range. Most typically, such processes for producing metal nanoparticles are carried out by reducing 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 conventional separation techniques and processed into paste compositions if desired.

Any suitable technique may be employed to form the metal nanoparticles used in the nanoparticle paste compositions and processes described herein. Particularly easy metal nanoparticle fabrication techniques and their uses 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, 9,700,940, 9,797,032, 9,881,895, and 9,976,042, each of which is incorporated herein by reference in its entirety. As described therein, metal nanoparticles can be prepared in a narrow size range by reducing a metal salt in a solvent in the presence of a suitable surfactant system, which may include one or more different surfactants. Further description of suitable surfactant systems follows. Without being bound by any theory or mechanism, it is believed that the surfactant system may mediate nucleation and growth of the metal nanoparticles, limit surface oxidation of the metal nanoparticles, and/or inhibit substantial aggregation of the metal nanoparticles with one another prior to at least partial fusion together. Suitable organic solvents for dissolving the metal salt and forming the metal nanoparticles may include, for example, formamide, N-dimethylformamide, dimethyl sulfoxide, dimethyl propylene urea, hexamethylphosphoramide, tetrahydrofuran and glyme, diglyme, triglyme and tetraglyme. Reducing agents suitable for reducing the metal salt and promoting the formation of metal nanoparticles may include, for example, an alkali metal in the presence of a suitable catalyst (e.g., lithium naphthanate, sodium naphthanate, or potassium naphthanate), or a borohydride reducing agent (e.g., sodium borohydride, lithium borohydride, potassium borohydride, or tetraalkylammonium borohydride).

Fig. 1 and 2 show diagrams of the putative structures of metal nanoparticles having a surfactant coating thereon. As shown in fig. 1, the metal nanoparticle 10 includes a metal core 12 and a surfactant layer 14 covering the metal core 12. The surfactant layer 14 may comprise any combination of surfactants as described in more detail below. The metal nanoparticles 20 shown in fig. 2 are similar to the metal nanoparticles depicted in fig. 1, except that: metal core 12 is grown around core 21, and core 21 may be the same or different metal as that of metal core 12. Because the core 21 is deeply embedded within the metal core 12 in the metal nanoparticle 20, it is not believed to significantly affect the overall nanoparticle performance. In some embodiments, the core 21 may include a substance that acts as a grain growth inhibitor that may be released into the grain boundaries when the metal nanoparticles are consolidated with one another. In some embodiments, the nanoparticles may have an amorphous morphology.

As discussed above, the metal nanoparticles have a surfactant coating comprising one or more surfactants on their surface. The surfactant coating may be formed on the metal nanoparticles during synthesis of the metal nanoparticles. When heated above the fusion temperature, the surfactant coating is typically lost during consolidation of the metal nanoparticles. In the synthesis of metal nanoparticles, forming a surfactant coating on the metal nanoparticles can desirably limit the ability of the metal nanoparticles to fuse to one another before heating above the fusion temperature, limit agglomeration of the metal nanoparticles, and promote formation of metal nanoparticle populations having a narrow size distribution.

In embodiments of the present disclosure, copper may be a particularly desirable metal due to its low cost, strength, and excellent electrical and thermal conductivity values, as well as other advantages further addressed herein. Although copper nanoparticles may be advantageous for use in embodiments herein, it should be understood that in alternative embodiments, other types of metal nanoparticles may be used in the presence of a suitable grain growth inhibitor to form the connection. Other metal nanoparticles that may be useful in high temperature packaging applications include, for example, aluminum nanoparticles, palladium nanoparticles, silver nanoparticles, gold nanoparticles, iron nanoparticles, cobalt nanoparticles, nickel nanoparticles, titanium nanoparticles, zirconium nanoparticles, hafnium nanoparticles, tantalum nanoparticles, and the like. Micron-sized particles of these metals may also be present in paste compositions containing metal nanoparticles, which may provide processing advantages in certain circumstances.

In various embodiments, the surfactant system present within the metal nanoparticles may include one or more surfactants. Different properties of various surfactants can be used to tune the properties of the metal nanoparticles. In selecting a surfactant or combination of surfactants for inclusion on the metal nanoparticles, factors that may be considered may include, for example, mitigating dissipation of the surfactant from the metal nanoparticles during nanoparticle fusion, nucleation and growth rates of the metal nanoparticles, metal components of the metal nanoparticles, and the like.

In some embodiments, an amine surfactant or combination of amine surfactants, particularly fatty amines, may be present on the metal nanoparticles. Amine surfactants may be particularly desirable for use in combination with copper nanoparticles. In some embodiments, two amine surfactants may be used in combination with each other. In other embodiments, three amine surfactants may be used in combination with each other. In more particular embodiments, primary, secondary, and diamine chelating agents may be used in combination with one another. In a more specific embodiment, the three amine surfactants can include long chain primary amines, secondary amines, and diamines having at least one tertiary alkyl nitrogen substituent. Further disclosure regarding suitable amine surfactants follows.

In some embodiments, the surfactant system may comprise a primary alkylamine. In some embodiments, the primary alkylamine may be C₂-C₁₈An alkyl amine. In some embodiments, the primary alkylamine may be C₇-C₁₀An alkyl amine. In other embodiments, C may also be used₅-C₆A primary alkylamine. Without being bound by any theory or mechanism, the exact size of the primary alkylamine can be balanced between being long enough to provide an effective reverse micelle structure during synthesis and being volatile and/or easy to handle during consolidation of the nanoparticle. For example, primary alkylamines having more than 18 carbons may also be suitable for this embodiment, but primary alkylamines having more than 18 carbons may be more difficult to handle due to their waxy nature. C₇-C₁₀Primary alkylamines in particular may represent a good balance of desirable properties for ease of use.

In some embodiments, for example, C₂-C₁₈The primary alkylamine can be n-hexylamine, n-heptylamine, n-octylamine, n-nonylamine, or n-decylamine. Although these are linear primary alkylamines, branched primary alkylamines may also be used in other embodiments. For example, branched primary alkylamines such as 7-methyloctylamine, 2-methyloctylamine or 7-methylnonylamine may be used. In some embodiments, these branched primary alkylamines may be sterically hindered when attached to the amine nitrogen atom. Non-limiting examples of such sterically hindered primary alkylamines may include, for example, tert-octylamine, 2-methylpentan-2-amine, 2-methylhexan-2-amine, 2-methylheptan-2-amine, 3-ethyloctylamine-3-amine, 3-ethylhept-3-amine, 3-ethylhex-3-amine, and the like. Other branches may 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 readily dissociate therefrom during consolidation of the metal nanoparticles.

In some embodiments, the surfactant system may comprise a secondary amine. Secondary amines suitable for use in forming metal nanoparticles can include a normal, branched, or cyclic C bonded to the amine nitrogen atom₄-C₁₂An alkyl group. In some embodiments, branching may occur at the carbon bound to the amine nitrogen atomAtomically, thereby creating a significant steric hindrance on the nitrogen atom. Suitable secondary amines can include, but are not limited to, dihexylamine, diisobutylamine, di-tert-butylamine, dipentylamine (dinonylamine), di-tert-pentylamine, dicyclopentylamine, dicyclohexylamine, and the like. C may also be used₄-C₁₂Secondary amines outside the range, but these may have undesirable physical properties, such as low boiling point or waxy consistency, which may complicate their handling.

In some embodiments, the surfactant system may comprise a chelating agent, particularly a diamine chelating agent. In some embodiments, one or both nitrogen atoms of the diamine chelant may be substituted with one or two alkyl groups. When two alkyl groups are present on the same nitrogen atom, the two alkyl groups may be the same or different. Furthermore, when both nitrogen atoms are substituted, the same or different alkyl groups may be present. In some embodiments, the alkyl group may be C₁-C₆An alkyl group. In other embodiments, the alkyl group may be C₁-C₄Alkyl or C₃-C₆An alkyl group. In some embodiments, C₃Or higher alkyl groups may be straight chain or branched. In some embodiments, the C3 or higher alkyl group can be cyclic. Without being bound by any theory or mechanism, it is believed that the diamine chelating agent may promote the formation of metal nanoparticles by promoting nanoparticle nucleation.

In some embodiments, suitable diamine chelating agents may include N, N' -dialkylethylenediamine, particularly C₁-C₄N, N' -dialkylethylenediamine. The corresponding methylenediamine, propylenediamine, butylenediamine, pentylenediamine or hexylenediamine derivatives may also be used. The alkyl groups may be the same or different. C that may be present₁-C₄Alkyl groups include, for example, methyl, ethyl, propyl, and butyl, or branched alkyl groups such as isopropyl, isobutyl, sec-butyl, and tert-butyl. Exemplary N, N ' -dialkylethylenediamine that may be suitably included on the metal nanoparticles include, for example, N ' -di-tert-butylethylenediamine, N ' -diisopropylethylenediamine, and the like.

In some embodiments, suitable diamine chelatesThe composition may comprise N, N, N ', N' -tetraalkylethylenediamine, in particular C₁-C₄N, N, N ', N' -tetraalkylethylenediamine. The corresponding methylenediamine, propylenediamine, butylenediamine, pentylenediamine or hexylenediamine derivatives may also be used. The alkyl groups may again be the same or different and include the alkyl groups described above. Exemplary N, N ' -tetraalkylethylenediamine that can be suitable for forming metal nanoparticles include, for example, N ' -tetramethylethylenediamine, N ' -tetraethylethylenediamine, and the like.

Surfactants other than fatty amines may also be present in the surfactant system. In this regard, suitable surfactants may include, for example, pyridine, aromatic amines, phosphines, thiols, or any combination thereof. These surfactants may be used in combination with the fatty amines including the above-mentioned fatty amines, or may be used in surfactant systems in which no fatty amine is present. The following is a further disclosure of suitable pyridines, aromatic amines, phosphines, and thiols.

Suitable aromatic amines may have the formula ArNR¹R²Wherein Ar is a substituted or unsubstituted aryl group, and R¹And R²Are the same or different. R¹And R²May be independently selected from H or alkyl or aryl groups containing from 1 to about 16 carbon atoms. Exemplary aromatic amines that may be suitable for use in forming the metal nanoparticles include, for example, aniline, toluidine, anisidine, N-dimethylaniline, N-diethylaniline, and the like. One of ordinary skill in the art can envision other aromatic amines that can be used in conjunction with the metal nanoparticles.

Suitable pyridines may include pyridine and its derivatives. Exemplary pyridines that may be suitable for inclusion on the metal nanoparticles include, for example, pyridine, 2-methylpyridine, 2, 6-dimethylpyridine, collidine, pyridazine, and the like. Chelating pyridines, such as bipyridine chelators, may also be used. One of ordinary skill in the art can envision other pyridines that can be used in conjunction with the metal nanoparticles.

Suitable phosphines may have the formula PR₃Wherein R is an alkyl or aryl group containing from 1 to about 16 carbon atoms. The alkyl or aryl groups attached to the phosphorus center may be the same or different. May consist inExemplary phosphines 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 a similar manner. In some embodiments, surfactants comprising two or more phosphine groups configured to form a chelating ring may also be used. Exemplary chelating phosphines may include, for example, 1, 2-bisphosphines, 1, 3-bisphosphines, and bisphosphines such as BINAP. One of ordinary skill in the art can envision other phosphines that may be used in conjunction with the metal nanoparticles.

Suitable thiols can have the formula RSH, where R is an alkyl or aryl group having from about 4 to about 16 carbon atoms. Exemplary thiols that may be present on the metal nanoparticles include, for example, butanethiol, 2-methyl-2-propanethiol, hexanethiol, octanethiol, benzenethiol, and the like. In some embodiments, surfactants comprising two or more thiol groups configured to form a chelating ring may also be used. Illustrative chelating thiols can include, for example, 1, 2-dithiol (e.g., 1, 2-ethanethiol) and 1, 3-dithiol (e.g., 1, 3-propanethiol). One of ordinary skill in the art can envision other thiols that can be used in conjunction with the metal nanoparticles.

The metal nanoparticles described above can be incorporated into various nanoparticle paste compositions, which can facilitate dispensing to form connections. The following is an illustrative disclosure of these nanoparticle paste compositions. As described herein, for joints that may be exposed to high operating temperatures, particularly under repeated thermal cycling, the copper nanoparticle paste composition is particularly useful for forming joints, particularly in the presence of grain growth inhibitors.

The nanoparticle paste composition may be prepared by dispersing the produced or isolated metal nanoparticles in an organic matrix containing one or more organic solvents and various other optional components. As used herein, the terms "nanoparticle paste formulation" and "nanoparticle paste composition" are used interchangeably and refer synonymously to a fluid composition comprising dispersed metal nanoparticles suitable for dispensing using a desired technique. The use of the term "paste" does not necessarily imply a separate adhesive function of the paste. Distribution of the metal nanoparticles in the desired locations can be facilitated by judicious selection of organic solvents and other additives, loading of the metal nanoparticles, and the like. Additionally, nanoparticle paste formulations, particularly those containing copper nanoparticles, may contain grain growth inhibitors adapted to prevent grain growth once the bulk metal matrix has been formed and exposed to high operating temperatures.

Cracking and shrinkage sometimes occur during consolidation of the metal nanoparticles. One way in which the nanoparticle paste composition can promote fracture and a reduction in the degree of void formation after consolidation of the metal nanoparticles is by maintaining a high solids content. More particularly, in some embodiments, the paste composition may comprise at least about 30% by weight of metal nanoparticles, particularly from about 30% to about 97% by weight of the nanoparticle paste composition, or from about 50% to about 97% by weight of the nanoparticle paste composition, or from about 70% to about 97% by weight of the nanoparticle paste composition. Further, in some embodiments, in addition to the metal nanoparticles, a small amount (e.g., from about 0.01% to about 15%, or from about 35% to about 60%, or from about 10% to about 35%, by weight of the paste composition) of micron-sized metal particles may be present. Such micron-sized metal particles can desirably facilitate fusing of the metal nanoparticles into a continuous mass and further reduce the occurrence of cracking and shrinkage. For example, shrinkage in the presence of the micron-sized particles may be reduced to about 5 vol.% or less when forming fused copper nanoparticles, as compared to shrinkage of about 20 vol.% to 30 vol.% in the absence of the micron-sized particles. Instead of being liquefied and undergoing direct consolidation, the micron-sized metal particles may simply be bonded together upon contact with the liquefied metal nanoparticles that have been raised above their fusion temperature. These factors can reduce porosity after fusing the metal nanoparticles together. The micron-sized metal particles may comprise the same or different metal as the metal nanoparticles. Suitable metals for the micro-scale particles may include, for example, copper, silver, gold, aluminum, tin, and the like. In some embodiments, micron-sized graphite particles may also be included. In some embodiments, carbon nanotubes and/or graphene may be included. Carbon black and/or nanocarbon may be included in other embodiments. Other additives such as diamond particles and cubic BN (boron nitride) may also be included.

In the disclosure herein, micron-sized metal particles can be distinguished from grain growth inhibitors because micron-sized metal particles are less easily incorporated into the grain boundaries between consolidated metal nanoparticles due to their relatively large size. Specific examples of grain growth inhibitors or precursors thereof suitable for use in the present disclosure are discussed in further detail below.

The reduction of cracking and void formation during consolidation of the metal nanoparticles may also be facilitated by judicious selection of the solvent or solvents that form the organic matrix. The tailored combination of organic solvents can desirably reduce the occurrence of cracking and void formation. More particularly, organic matrices comprising one or more hydrocarbons (saturated, monounsaturated, polyunsaturated (2 or more double bonds) or aromatic), one or more alcohols, one or more amines, and one or more organic acids are particularly effective for this purpose. In some embodiments, one or more esters and/or one or more anhydrides may be included. Without being bound by any theory or mechanism, it is believed that this combination of organic solvents may facilitate the removal and isolation of surfactant molecules around the metal nanoparticles during consolidation so that the metal nanoparticles may more easily fuse to each other. More particularly, it is believed that the hydrocarbon and alcohol solvents can passively solubilize the surfactant molecules released from the metal nanoparticles by brownian motion and reduce the ability of the surfactant molecules to re-attach to the metal nanoparticles. In conjunction with passive solubilization of the surfactant molecules, the amine and organic acid solvents can actively sequester the surfactant molecules through chemical interactions, such that the surfactant molecules are no longer available for recombination with the metal nanoparticles.

The solvent composition may be further adjusted to reduce the abruptness of the volume shrinkage that occurs during surfactant removal and consolidation of the metal nanoparticles. Specifically, more than one member of each class of organic solvents (e.g., hydrocarbons, alcohols, amines, and organic acids) may be present in the organic matrix, wherein the boiling points of the members of each class of organic solvents are separated from each other to some extent. For example, in some embodiments, the various members of each class of organic solvents may have boiling points that are separated from each other by about 20 ℃ to about 50 ℃. By using such a solvent mixture, sudden volume changes due to rapid loss of solvent during consolidation of the metal nanoparticles may be minimized, since various components of the solvent mixture may be gradually removed over a wide boiling point range (e.g., about 50 ℃ to about 250 ℃).

In some embodiments, at least some of the one or more organic solvents may have a boiling point of about 100 ℃ or above. In some embodiments, at least some of the one or more organic solvents may have a boiling point of about 200 ℃ or greater or about 300 ℃ or greater. In some embodiments, the boiling point of the one or more organic solvents may be in a range between about 50 ℃ to about 200 ℃. The use of high boiling point organic solvents can desirably increase the pot life of the nanoparticle paste composition and limit the rapid loss of solvent, which can lead to cracking and void formation during nanoparticle consolidation. In some embodiments, at least one of the organic solvents may have a boiling point higher than a boiling point of the one or more surfactants associated with the metal nanoparticles. Thus, the one or more surfactants may be removed from the metal nanoparticles by evaporation prior to removal of the one or more organic solvents.

In some embodiments, the organic matrix may comprise one or more alcohols. In various embodiments, the alcohol may include a monohydric alcohol, a glycol, a triol, a glycol ether (e.g., diethylene glycol and triethylene glycol), an alkanolamine (e.g., ethanolamine, triethanolamine, etc.), or any combination thereof. In some embodiments, one or more hydrocarbons may be present in combination with one or more alcohols. As discussed above, it is believed that when surface activity is removed from metal nanoparticles by Brownian motionAs an agent, the alcohol and hydrocarbon solvent may passively promote dissolution of the surfactant and limit recombination of the surfactant with the metal nanoparticles. In addition, hydrocarbon and alcohol solvents can only form weak coordination with metal nanoparticles, and therefore they cannot simply replace the substituted surfactant in the nanoparticle coordination sphere. Illustrative, but non-limiting, examples of alcohols 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-C₁₃A mixture of alkanes), diisopropylglycol monomethyl ether, diethylene glycol diethyl ether, 2-propanol, 2-butanol, tert-butanol, 1-hexanol, 2- (2-butoxyethoxy) ethanol and terpineol. In some embodiments, polyketone solvents may be used in a similar manner.

In some embodiments, the organic matrix may comprise one or more amines and one or more organic acids. In some embodiments, one or more amines and one or more organic acids may 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 amine and organic acid can effectively sequester surfactants that have been passively dissolved by hydrocarbon and alcohol solvents, thereby making the surfactants unavailable for recombination with the metal nanoparticles. Thus, an organic solvent comprising a combination of one or more hydrocarbons, one or more alcohols, one or more amines, and one or more organic acids may provide synergistic benefits for promoting consolidation of metal nanoparticles. Illustrative, but non-limiting, examples of amine solvents that may be present include, for example, tallow amine (CAS 61790-33-8), alkyl (C)₈-C₁₈) Unsaturated amine (CAS 68037-94-5), di-hydrogenated tallow amine (CAS 61789-79-5), dialkyl (C)₈-C₂₀) Amine (CAS 68526-63-6), alkyl (C)₁₀-C₁₆) Dimethylamine (CAS 67700-98-5), alkyl (C)₁₄-C₁₈) Dimethylamine (CAS 68037-93-4), dihydrotallow methylamine (CAS 61788-63-4) and trialkyl (C)₆-C₁₂) Amine (CAS 68038-01-7). Organic acids that may be present in nanoparticle paste compositionsIllustrative, but non-limiting examples of solvents include, for example, caprylic acid, pelargonic acid, capric acid, caprylic acid, pelargonic acid, undecanoic acid, lauric acid, tridecanoic acid, myristic acid, pentadecanoic acid, palmitic acid, heptadecanoic acid, octadecanoic acid, azelaic acid, alpha-linolenic acid, stearic acid, oleic acid, and linoleic acid.

In some embodiments, the organic matrix may include one or more hydrocarbons, one or more alcohols, one or more amines, and one or more organic acids. For example, in some embodiments, each class of organic solvent may 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. In addition, the number of members in each type of organic solvent may be the same or different. Particular benefits of using multiple members of each class of organic solvents are described below.

One particular advantage of using multiple members in each class of organic solvents may include the ability to provide a broad distribution of boiling points in the nanoparticle paste composition. By providing a broad distribution of boiling points, the organic solvent can be gradually removed as the temperature increases, while affecting the consolidation of the metal nanoparticles, thereby limiting volume shrinkage and not favoring fracture. As a result, greater structural integrity of the connection may be achieved. By gradually removing the organic solvent in this manner, less temperature control may be required to promote slow solvent removal than if a single solvent with a narrow boiling point range were used. In some embodiments, members within each class of organic solvents may have a boiling point range between about 50 ℃ to about 200 ℃, or about 50 ℃ to about 250 ℃, or about 100 ℃ to about 200 ℃, or about 100 ℃ to about 250 ℃. In some cases, a boiling point of up to about 350 ℃ is suitable. In a more specific embodiment, the individual members of each class of organic solvents can each have a boiling point that is separated from each other by at least about 20 ℃, specifically from about 20 ℃ to about 50 ℃. More specifically, in some embodiments, each hydrocarbon may have a boiling point that is about 20 ℃ to about 50 ℃ different from the other hydrocarbons in the organic matrix, each alcohol may have a boiling point that is about 20 ℃ to about 50 ℃ different from the other alcohols in the organic matrix, each amine may have a boiling point that is about 20 ℃ to about 50 ℃ different from the other amines in the organic matrix, and each organic acid may have a boiling point that is about 20 ℃ to about 50 ℃ different from the other organic acids in the organic matrix. The more members of each class of organic solvent are present, the smaller the difference between boiling points. By having a smaller difference between boiling points, the removal of solvent can be made more continuous, thereby limiting the degree of volumetric shrinkage that occurs at each stage. When four to five or more members (for example, 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) are present in each type of organic solvent while their boiling points are spaced from each other within the above-mentioned range, the degree of cracking is reduced.

In various embodiments, the size of the metal nanoparticles used in the nanoparticle paste composition may be about 20nm or less. In some embodiments, the metal nanoparticles can be up to about 70nm or 75nm in size. As discussed above, the fusion temperature of metal nanoparticles within this size range is significantly lower than that of the corresponding bulk metal, and thus tend to consolidate with one another. In some embodiments, metal nanoparticles having a size of about 20nm or less or about 70nm or less may have a fusion temperature of about 240 ℃ or less (e.g., a fusion temperature in the range of about 140 ℃ to about 240 ℃), or about 220 ℃ or less, or about 200 ℃ or less, which may provide the advantages described above. In some embodiments, at least a portion of the metal nanoparticles can be about 10nm in size or less, or about 5nm in size or less. In some embodiments, at least a portion of the metal nanoparticles may range in size from between about 1nm to about 20nm, or from between about 1nm to about 10nm, or from between about 1nm to about 5nm, or from between about 3nm to about 7nm, or from between about 5nm to about 20 nm. In some embodiments, substantially all of the metal nanoparticles may fall within these size ranges. In some embodiments, larger metal nanoparticles may be combined with metal nanoparticles having a size of about 20nm or less in the nanoparticle paste composition. For example, in some embodiments, metal nanoparticles from about 1nm to about 10nm in size can be combined with metal nanoparticles from about 25nm to about 50nm in size, or with metal nanoparticles from about 25nm to about 100 nm. As discussed further below, in some embodiments, nanoscale metal particles or nanoscale particles may also be included in the nanoparticle paste composition. Although the larger metal nanoparticles and micron-sized metal particles may not liquefy at the low temperatures of their smaller counterparts, they may still consolidate when contacting the smaller metal nanoparticles that have liquefied at or above their fusion temperature, as generally discussed above. The larger metal nanoparticles and micron-sized metal particles do not necessarily need to be uniformly distributed to accomplish this function.

In addition to the metal nanoparticles and organic solvent, other additives may also be present in the nanoparticle paste composition. Such additional additives may include, for example, rheology control aids, thickeners, micron-scale conductive additives, nano-scale conductive additives, and any combination thereof. Chemical additives may also be present. As discussed below, it may be particularly advantageous to include micron-sized conductive additives, such as micron-sized metal particles. In some cases, nano-or micro-sized diamond or other thermally conductive additives may be required.

In some embodiments, the nanoparticle paste composition may comprise from about 0.01% to about 15% by weight of micron-sized metal particles, or from about 1% to about 10% by weight of micron-sized metal particles, or from about 1% to about 5% by weight of micron-sized metal particles, or from about 0.1% to about 35% by weight of micron-sized particles, or from about 10% to about 35% by weight of micron-sized particles. The inclusion of micron-sized metal particles in the nanoparticle paste composition can desirably reduce the incidence of cracking that occurs during consolidation of the metal nanoparticles when forming a continuous metal trace. Without being bound by any theory or mechanism, it is believed that the micron-sized metal particles may partially consolidate with each other as the metal nanoparticles are liquefied and form a transient liquid coating on the micron-sized metal particles and fill the voids therebetween. In essence, the metal nanoparticles act as a "glue" that binds the micron-sized particles together. In some embodiments, the micron-sized metal particles may range in size in at least one dimension from between about 500nm to about 100 microns, or may range in size in at least one dimension from about 500nm to about 10 microns, or may range in size in at least one dimension from about 100nm to about 5 microns, or may range in size in at least one dimension from about 100nm to about 10 microns, or may range in size in at least one dimension from about 100nm to about 1 micron, or may range in size in at least one dimension from about 1 micron to about 10 microns, or may range in size in at least one dimension from about 5 microns to about 10 microns, or may range in size in at least one dimension from about 1 micron to about 100 microns. The micron-sized metal particles may comprise the same metal as the metal nanoparticles or comprise a different metal. Thus, a metal alloy can be made by including micron-sized metal particles and a metal different from the metal nanoparticles in a nanoparticle paste composition. Suitable micron-sized metal particles may include, for example, Cu, Ni, Al, Fe, Co, Mo, Ag, Zn, Sn, Au, Pd, Pt, Ru, Mn, Cr, Ti, V, Mg, or Ca particles. Borides, carbides, phosphides, nitrides and silicides of these metals and combinations thereof may also be used. Non-metallic particles, such as Si and B micron-sized particles, may be used in a similar manner. In some embodiments, the micron-sized metal particles may be in the form of metal flakes, such as high aspect ratio copper flakes. That is, in some embodiments, the nanoparticle paste compositions described herein may comprise a mixture of copper nanoparticles and high aspect ratio copper flakes. Specifically, in some embodiments, the paste composition may comprise from about 30% to about 97% by weight copper nanoparticles and from about 0.01% to about 15% by weight high aspect ratio copper flakes, or from about 0.1% to about 35% by weight high aspect ratio copper flakes, or from about 1% to about 35% by weight high aspect ratio copper flakes.

Other micron-sized metal particles that may be used equivalently with high aspect ratio metal flakes include, for example, metal nanowires and other high aspect ratio particles, which may have a length of up to about 300 microns or about 500 microns. According to various embodiments, the ratio of metal nanoparticles to metal nanowires may range between about 10:1 to about 40: 1. For example, suitable nanowires may be between about 5 microns to about 50 microns or about 100 microns in length and between about 100nm to about 200nm in diameter.

In some embodiments, a nanoscale conductive additive may also be present in the nanoparticle paste composition. These additives may desirably provide further structural reinforcement and reduce shrinkage during consolidation of the metal nanoparticles. Furthermore, the inclusion of the nanoscale conductive additive may increase the electrical and thermal conductivity values that may approach or even exceed those of the corresponding bulk metal after consolidation of the nanoparticles. In some embodiments, the nanoscale conductive additive may have a size in at least one dimension in a range between about 1 micron to about 100 microns, or in a range between about 1 micron to about 300 microns. Suitable nanoscale conductive additives may include, for example, carbon nanotubes, graphene, and the like. When present, the nanoparticle paste composition can comprise from about 1% to about 15% by weight of the nanoscale conductive additive, or from about 1% to about 10% by weight of the nanoscale conductive additive, or from about 1% to about 5% by weight of the nanoscale conductive additive.

Other substances that may optionally be present include, for example, flame retardants, UV protectors, antioxidants, carbon black, graphite, fibrous materials (e.g., chopped carbon fiber materials), diamond, and the like.

Nanoparticle paste compositions suitable for use in packaging applications according to the present disclosure may be formulated using any of the nanoparticle paste compositions described above, further including a grain growth inhibitor, particularly a metal-containing grain growth inhibitor. The grain growth inhibitor may be included in a suitable form such that the grain growth inhibitor is able to enter the grain boundaries after consolidation of the nanoparticles. If not included in a suitable form, ineffective grain growth inhibition may occur even if the grain growth inhibitor otherwise comprises a substance capable of providing grain growth inhibition.

In particular embodiments, a nanoparticle paste composition suitable for use in the disclosures herein may comprise copper nanoparticles and a suitable amount of a grain growth inhibitor to prevent substantial grain growth upon heating of a bulk copper matrix formed from the copper nanoparticles. According to various embodiments, a suitable amount of grain growth inhibitor may be in a range between 0.01 wt.% to about 15 wt.% of the nanoparticle paste composition. The effective temperature range in which the grain growth inhibitor can inhibit grain growth is considered below.

Thus, the nanoparticle paste compositions of the present disclosure may comprise copper nanoparticles and 0.01 to 15 wt.% of a grain growth inhibitor or a precursor of a grain growth inhibitor admixed with the copper nanoparticles, wherein the grain growth inhibitor comprises a metal. Suitable grain growth inhibitors are insoluble in the bulk copper matrix and can be present at one or more grain boundaries in the bulk copper matrix. Suitable grain growth inhibitors comprising metals are provided below.

The copper nanoparticle paste compositions disclosed herein are superior to conventional flux materials as well as those copper nanoparticle-based flux materials that lack grain growth inhibitors. In particular, the copper nanoparticle paste compositions described herein provide low processing temperatures of about 200 ℃, which are mild to Printed Circuit Boards (PCBs) and their components, reduce thermal stress during heating and cooling, prevent via explosions (blob ups) and delamination, reduce warpage of larger components and packaging systems, and allow high operating temperatures after initial nanoparticle fusion due to conversion of the copper nanoparticles to a bulk copper matrix. Such nanoparticle paste compositions allow for easy stacking of components and systems and multiple processing steps without lowering the flux melting point and compromising overall reliability. As discussed above, when the copper nanoparticles are heated above their fusion temperature, the copper nanoparticles transform into a bulk copper matrix and can be reheated to a much higher temperature than the original processing temperature without adverse effects, unlike conventional solders. For example, the shear strength of a bulk copper matrix prepared from copper nanoparticles according to the present disclosure may increase by 100% in air at 150 ℃ over 1000 hours, whereas SAC fluxes may deteriorate significantly under the same conditions, losing more than 50% of the green strength. The copper nanoparticle paste compositions described herein are readily dispensable, including dispensed amounts of less than 0.1mg per step. Since the small amounts of additives and surfactants in the paste will evaporate completely during fusing ("reflow"), no post-treatment cleaning is required. One does not have to worry about flux residue, out-gassing that can occur after cleaning and afterwards, and even possible damage to sensitive optical systems in various spatial environments. In addition, the copper nanoparticles lack a liquid transition state, which eliminates wicking during processing, thereby allowing very close spacing between components and leads/contacts. The joints formed by copper nanoparticles also have good wet strength (green strength) and a high adhesion coefficient, making it easy to handle the assembled parts in a "wet" state. Furthermore, the copper nanoparticles retain their macroscopic shape during fusing; they simply harden to form a consolidated mass having a shape similar to the shape of the unconsolidated metal nanoparticles after deposition. During consolidation, the metal matrix within the consolidated mass is characterized by very fine, uniformly distributed nanopores (typically 4% to 15% of the pore size is in the range of about 100nm to about 300nm, and mostly closed pores) that limit hot spots (hot spots) by ensuring a uniform heat distribution across the interface. However, the nanopores may be in the range of about 2% to about 15% (i.e., 85% to 98% densely fused copper nanoparticles with closed nanopores, pore sizes in the range of about 50nm to about 500nm, or in the range of about 100nm to about 300nm, or in the range of about 150nm to about 250 nm). Fig. 3A and 3B show X-ray images, which indicate that there are no large voids in the consolidated copper nanoparticles (fig. 3B) and the void area is reduced as a whole, compared to the AuSn flux (fig. 3A). It can also relieve thermal and mechanical stresses (plastic deformation).

Copper is further advantageous because it does not grow whiskers or dendrites, and therefore can handle high electrical loads at high temperatures without causing short circuits, making it very robust. The toughness of copper prevents creep in this case, making this metal a very reliable and reliable high temperature material. In addition, copper is widely used and less toxic (an element essential to humans, animals and many plants) than most fluxes containing lead, cadmium and other toxic components.

Copper also has good oxidation resistance, at least at ambient temperature, due to its dense and well-adhering native Cu₂And O surface protection layer. For example, a further improvement in oxidation resistance at elevated temperatures can be achieved by alloying copper with Ni, Zn, Sn, P, Si or Al. As discussed further below, Al and Si may not be alloyed under alternative conditions and serve as grain growth inhibitors. By including additional oxidation resistance, copper may provide high temperature stability at temperatures up to about 90% of its melting point. Additional stabilizers to convey oxidation resistance may be added to the copper nanoparticles in the form of metal nanoparticles or micron-sized particles, in which case these additional stabilizers may be incorporated into the bulk copper matrix after the copper nanoparticles are consolidated. Additional additives may also be added as an initial atomic level additive (e.g., as a coating in a core-shell nanoparticle or as a nanoparticle core).

A suitable alloy may be formed in situ with the copper by co-reduction/precipitation during the initial nanocopper formation process. In one configuration, the stabilizing atoms may be combined in the form of a metal salt and co-reduced with copper. In another configuration, the stabilized atoms may be present as metal organic compounds that decompose during reduction to form copper nanoparticles. In yet another configuration, the stabilized atoms may be introduced as a compound after the reduction step to form copper nanoparticles, where the compound is decomposed or reduced in the same reaction vessel in a subsequent reduction step. Finally, in another configuration, the compound containing the stabilized atoms may be first reduced or decomposed prior to forming the copper nanoparticles in the same reaction vessel. In this configuration, the stabilized atoms may serve as nanoparticle nucleation seeds to promote their growth.

Other oxidation protection for the copper nanoparticles may be provided using a variety of existing conformal coatings or solder mask materials, such as polyurethane, epoxy, or parylene. These materials may be introduced onto a bulk metal matrix formed after consolidating copper nanoparticles to each other.

Improving the nanostructure of a metal or alloy can improve its toughness and strength while still allowing significant deformation and elongation. The large number of grain boundaries in the nanostructured material formed from the metal nanoparticles is associated with the large amount of energy present in these regions, which supports rapid grain growth even at relatively low temperatures, thereby driving the system to thermodynamic equilibrium and stability. To ensure long-term high temperature stability and varying operating environments (e.g., thermal shock and cycling), it is desirable to preserve the fine grain structure. In embodiments of the present disclosure, retention of the grain structure may be achieved by adding one or more grain growth inhibitors, which are materials that do not dissolve in the copper matrix but accumulate at the grain boundaries, resulting in domain pinning (domain pinning). According to some embodiments, a suitable grain growth inhibitor is a substance that is insoluble in bulk copper and is itself a foreign nanoparticle in the size range of 10nm and below. Metal-containing grain growth inhibitors, particularly metal nanoparticles having a size of about 10nm or less, may be particularly desirable for inclusion in the bulk copper matrix. The small nanoparticle size allows the grain growth inhibitor to easily enter the grain boundaries. The inclusion of a grain growth inhibitor may limit grain growth by interfacial or zener pinning and ensure that the nanocrystalline grain structure remains even after prolonged exposure to high temperatures, frequent temperature cycling, and thermal shock. These effects prevent further diffusion and recombination of atoms.

Without being bound by theory or mechanism, it is believed that grain growth occurs and is inhibited according to the following mechanism. In order for the grains to grow, the grain boundaries must cross any incoherent (nano) particles in the boundary region, while the part of the boundary that will be inside the particle is substantially absent. In order to cross over the (nano) particles some new boundaries have to be created, which is energetically unfavorable. Essentially, the bond must be broken and reformed. When the region of the boundary near the particle is pinned, the rest of the boundary will continue to try to move forward under its own driving force. This causes the boundary to bend between those points anchored (bonded) to the particle. Thereby creating an energetic or activating barrier that slows grain growth in the presence of a grain growth inhibitor. When a sufficient number of such pinning events occur, grain growth will stop completely.

If the material is too soluble in the copper matrix, the material may not be an effective grain growth inhibitor. That is, the material is incorporated throughout the matrix and does not pin the grain boundaries. When the mixing energy (solubility) is greater than the potential separating at the grain boundaries, the nanostructure is unstable and grain growth (Δ H) may occur_o>ΔH^mix). Fig. 4 shows a phase diagram of a typical nanostructured material and its stability regions.

The grain pinning pressure (Pz) at the grain boundaries can be estimated from equation 1:

wherein, γ₀Is the surface energy, and f and r are the volume fraction and radius of the precipitated phase, respectively. This equation shows that the smaller size and larger volume fraction of precipitated phases are more effective in pinning grain boundaries than the smaller volume fraction of larger particles. In nanostructured copper alloys, enhancement of Hall-Petch grain size may dominate up to high homolog temperatures of-0.87 Tm (Tm ═ the melting point of bulk copper). Thus, the nanocopper may be stable up to an operating temperature of about 940 ℃.

Similarly, grain growth inhibition may be limited if the potential grain growth inhibitor is not able to fit effectively within the grain boundaries. Thus, in some cases, particularly suitable grain growth inhibitors may be nanoparticles, particularly metal nanoparticles having a size of about 10nm or less.

Suitable grain growth inhibitors may include metallic species, metal carbides, metal nitrides, metal borides, metal phosphides, or any combination thereof. Metal nanoparticles may be particularly suitable grain growth inhibitors. Particularly suitable grain growth inhibitors may include metal nanoparticles that do not undergo consolidation upon exposure to high temperature operating conditions.

According to particular embodiments of the present disclosure, suitable grain growth inhibitors may be metal nanoparticles that are insoluble in the bulk copper matrix. Suitable metals may include, for example, Fe, Mn, Cr, Co, Ru, Si, V, W, Nb, Ta, Y, Zr, Hf, Be, Tl, Ir, Ti, Mo, Re, Al, or any combination thereof, including alloys thereof. For the purposes of this disclosure, Si is considered to be a metal. Other suitable grain growth inhibitors may include, for example, carbides, nitrides, borides, silicides, or phosphides of the foregoing metals. Suitable borides may include, for example, Zr/Hf, V, or Nb/Ta. Similar metals may be suitable for carbides, nitrides, silicides, and phosphides, but any of the above may be suitable. Other suitable phosphides may include, for example, BP and SiP₂Such as covalent phosphides, e.g. Fe₃P、Fe₂P、Ni₂Transition metal phosphides of P, CrP, MnP, MoP, etc. For example, such metal-rich phosphides may be desirable due to their water insolubility, electrical conductivity, high melting point, thermal stability, hardness, and similar properties. Other suitable carbides may include, for example, BC (including B)_xC_yNon-stoichiometric carbides) and covalent carbides such as SiC, as well as transition metal carbides that similarly exhibit high melting points, hardness, electrical conductivity, and similar properties. In some cases, depending on the processing conditions, graphene and other nanocarbon materials may also be effective grain growth inhibitors.

Suitable grain growth inhibitors may be included in the copper nanoparticle paste composition in an amount in the range of about 0.01 wt.% to about 15 wt.% relative to the nanoparticle paste composition or relative to the joint created after the copper nanoparticles are fused to form a bulk copper matrix. In more specific embodiments, the grain growth inhibitor may be present in an amount in a range between about 0.01 wt.% and about 5 wt.%, or in a range between about 0.1 wt.% and about 0.5 wt.%. Particular copper nanoparticle paste compositions may include up to about 12 wt.% Al, or about 0.01 wt.% to 5 wt.% Zr/Hf. These particular grain growth inhibitors in the reference amounts may provide a temperature stability of up to about 940 ℃, 500 ℃ or 600 ℃, respectively.

Al: al can be incorporated into the copper nanoparticles in several ways (as described above) and both acts as a grain growth inhibitor and provides oxidation protection. A particular copper nanoparticle paste composition may contain up to about 12 wt.% Al, under appropriate processing conditions, forming the following stable insoluble polycrystalline phases in the grain boundary regions: AlCu₃、Al₄Cu₉AlCu and/or Al₂And (3) Cu. The area nanoparticle coverage with aluminum may be such that about 25% to 75% of the copper atoms are exposed for surfactant evaporation. The insoluble Al/Cu compound promotes grain boundary pinning. About 5 atomic percent of copper atoms are covered by aluminum on the surface of the copper nanoparticles. At higher amounts, Al alloys may be formed. Al can be incorporated in the form of nanoparticles, in micron-sized particles or flakes, or in the form of compounds that release aluminum metal atoms before, during, or after the primary copper nanoparticle formation event. There may also be a partial shell of aluminum atoms on the copper nanoparticles, as long as an insoluble aluminum phase is formed that is insoluble in bulk copper. The different phases precipitate in the grain boundary regions, preventing further grain growth until the temperature exceeds about 940 ℃. As a much lighter element (density: 2.7 g/cm)³) Al has a high density in more dense nanoparticles such as Cu (density: 8.92g/cm³) A tendency to accumulate/settle on the surface.

Zr and Hf: the copper nanoparticle paste composition may beUp to or about 0.01 to 5 wt.% Zr or 0.01 to 5 wt.% Zr/Hf. Zr and Cu₅Zr and CuZr₂The composition forms a stable polymorph. Cu₉Zr₂And Cu with a small amount of Zr impurities present. Hf may form similar compounds or different compounds. These particular grain growth inhibitors in the reference amounts may provide temperature stability of up to about 500 ℃ and 600 ℃, respectively.

Iron: cu and Fe are not mixed. It is known that single phase alloys do not form at low temperatures. The solid mixture of these two elements contains separate FCC Cu and BCC Fe phases, satisfying the requirement of an insoluble phase that accumulates in the grain boundary regions.

Y and other metals: y forms three stable polymorphs in the presence of copper under appropriate conditions: cu₅Y，Cu₂Y and CuY. These binary compounds may act as grain growth inhibitors. Tl, Mo, W, V, Nb, Co and Ir likewise do not form stable Cu alloys and form completely separate phase forms. Ti forms the following stable binary compounds: TiCu₃、Ti₃Cu₄TiCu and Ti₂And (3) Cu. These insoluble titanium compounds can also inhibit grain growth.

Si: si formation of stable polymorphs Cu₁₅Si₄Which inhibits grain growth.

And others: mn forms metastable Cu₅A Mn phase which inhibits grain growth. Cr and Be are slightly soluble in copper at 1% to 2%, but their insoluble fraction still inhibits grain growth at higher concentrations.

The grain growth inhibitor may be in various forms when incorporated/combined with the copper nanoparticles. In some embodiments, the grain growth inhibitor may be the nanoparticles themselves, particularly having a size of about 10nm or less. In other embodiments, the grain growth inhibitor may have a size in a range between 10nm and 100 nm.

When incorporated as nanoparticles, the agent for forming the grain growth inhibitor may be mixed with the agent for forming the copper nanoparticles, and then they are co-reduced to simultaneously form the copper nanoparticles and the grain growth inhibitor. Suitable agents for forming grain growth inhibitors may include, for example, metal nitrates, chlorides, bromides, or iodides. The grain growth inhibitor may also constitute a nanoparticle seed for the copper nanoparticles and then be incorporated into the copper matrix at the grain boundaries formed after the copper nanoparticles are fused. Nanoparticle seeds suitable for use as grain growth inhibitors may be prepared separately and combined with the reagents used to form the copper nanoparticles, or such nanoparticle seeds may be formed simultaneously with the formation of the copper nanoparticles. The carrier solvent may be used to disperse the agent for forming the nanoparticle seed/grain growth inhibitor prior to the dispersion of the copper nanoparticles or the precursor of the copper nanoparticles.

Alternatively, a preformed grain growth inhibitor may be mixed with the preformed copper nanoparticles either before or after the copper nanoparticles are formulated into a paste formulation.

In other alternative embodiments, a trialkylaluminum compound (e.g., trimethylaluminum) may be incorporated into a copper nanoparticle paste formulation. The trialkylaluminum may react during the consolidation of the copper nanoparticles to release aluminum or aluminum compounds into the grain boundaries. Additional general details of this process are provided above. The compound red al (redal) can be used in a similar manner to the trialkylaluminum compound.

Still further alternatively, a salt that forms a grain growth inhibitor after reduction may be mixed in the copper nanoparticle paste formulation and then reduced during consolidation of the copper nanoparticles to form the grain growth inhibitor. A carrier solvent may be used to facilitate mixing with the copper nanoparticle paste formulation.

In other embodiments, NaReO may be added₄And (4) preparing a grain growth inhibitor. The salts are compatible with aqueous and non-aqueous solvent conditions (including glyme solvent mixtures) and the same amines that can be used to form copper nanoparticles. Reducing agents such as NaBH₄、CaH₂Hydrazine, organomagnesium or organosodium compounds or redAI can be used to affect the reduction. In particular, NaReO₄With CuCl₂Dissolve togetherAnd co-precipitated/reduced during the copper reduction step or may be pre-dissolved and added as a solution after the initial copper reduction step.

Accordingly, the present disclosure also provides a connection formed from the copper nanoparticle paste composition, and a method of forming such a connection. Suitable connections may include a bulk copper matrix formed by the fusion of copper nanoparticles, wherein the bulk copper matrix includes a plurality of grain boundaries; and a grain growth inhibitor that is insoluble in the bulk copper matrix and disposed within at least a portion of the plurality of grain boundaries within the bulk copper matrix, and wherein the grain growth inhibitor comprises a metal. The grain growth inhibitor is present in an amount of about 0.01 wt.% to 15 wt.% relative to the connection or the copper nanoparticle paste composition forming the connection. As discussed above, the connection is operationally stable at temperatures up to about 90% of the melting point of bulk copper.

Methods for forming the connections described herein may include: depositing a nanoparticle paste composition comprising copper nanoparticles on a substrate; and consolidating the copper nanoparticles to form a bulk copper matrix comprising a plurality of grain boundaries, wherein the grain growth inhibitor is insoluble in the bulk copper matrix and is present within the plurality of grain boundaries in the bulk copper matrix. Consolidating the metal nanoparticles to one another may include applying pressure to the copper nanoparticles and/or heating the copper nanoparticles above their fusion temperature.

Embodiments disclosed herein include:

A. a nanoparticle paste composition suitable for high temperature applications. The nanoparticle paste composition comprises: copper nanoparticles; and 0.01 to 15 wt.% of a grain growth inhibitor or a precursor of a grain growth inhibitor admixed with the copper nanoparticles, the grain growth inhibitor comprising a metal; wherein the grain growth inhibitor is insoluble in the bulk copper matrix and can be present at one or more grain boundaries in the bulk copper matrix.

A1. Nanoparticle paste compositions suitable for high temperature use. The nanoparticle paste composition comprises: copper nanoparticles; and a grain growth inhibitor or a precursor of a grain growth inhibitor blended with the copper nanoparticles; wherein the grain growth inhibitor is insoluble in the bulk copper matrix and at least a portion of the grain growth inhibitor is present at one or more grain boundaries in the bulk copper matrix.

B. A connection suitable for high temperature use. The connecting portion includes: a bulk copper matrix formed by fusing copper nanoparticles, the bulk copper matrix comprising a plurality of grain boundaries; and a grain growth inhibitor that is insoluble in the bulk copper matrix and disposed within at least a portion of the plurality of grain boundaries within the bulk copper matrix, the grain growth inhibitor comprising a metal in an amount in a range of about 0.01 wt.% to 15 wt.% in the connection.

B1. A linker suitable for high temperature use and formed from the nanoparticle paste composition of a1.

C. A method for forming a connection. The method comprises the following steps: depositing the nanoparticle paste composition on a substrate; wherein the nanoparticle paste composition comprises copper nanoparticles and from 0.01 wt.% to 15 wt.% of a grain growth inhibitor or a precursor of a grain growth inhibitor admixed with the copper nanoparticles, the grain growth inhibitor comprising a metal; and consolidating the copper nanoparticles to form a bulk copper matrix comprising a plurality of grain boundaries, wherein the grain growth inhibitor is insoluble in the bulk copper matrix and is present within the plurality of grain boundaries in the bulk copper matrix.

Embodiments a through C may have one or more of the following additional elements in any combination:

element 1: wherein the grain growth inhibitor comprises a metallic species, a metal carbide, a metal nitride, a metal boride, a metal silicide, a metal phosphide, or any combination thereof.

Element 2: wherein the grain growth inhibitor comprises a metallic species comprising a metal selected from the group consisting of: fe. Mn, Cr, Ru, Si, V, W, Nb, Ta, Y, Zr, Hf, Be, Tl, Ir, Ti, Mo, Re, Al, and any combination thereof.

Element 3: wherein the grain growth inhibitor comprises one or more metal nanoparticles.

Element 4: wherein the metal nanoparticles have a size of about 10nm or less.

Element 5: wherein the copper nanoparticles are about 20nm or less in size.

Element 6: wherein the grain growth inhibitor is present as a seed within the copper nanoparticles.

Element 7: wherein the copper nanoparticles are coated with at least one amine surfactant.

Element 8: wherein the joint is stable in operation at temperatures up to about 90% of the melting point of the bulk copper.

Element 9: wherein the method comprises exposing the joint to a temperature of about 150 ℃ or above to a temperature of at most about 90% of the melting point of the bulk copper.

As non-limiting examples, exemplary combinations suitable for a through C include: 1 and 3; 1. 3 and 4; 1 and 5; 1 and 6; 1 and 7; 2 and 3; 2 to 4; 2 and 5; 3 and 5; 2 and 6; 3 and 6; 5 and 6; 5 and 7; and 6 and 7, any of these combinations may be further combined with 8 or 9 of B and C, or any of 1 to 7 may be further combined with 8 or 9 of B and C alone.

Unless otherwise indicated, all numbers expressing quantities of ingredients, properties (e.g., molecular weights), reaction conditions, and so forth, used in the specification and claims are to be understood as being modified in all instances by the term "about". Accordingly, unless indicated to the contrary, the numerical parameters set forth in the following specification and attached claims are approximations that may vary depending upon the desired properties sought to be obtained by the 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.

One or more illustrative embodiments incorporating features of the present disclosure are presented herein. In the interest of clarity, not all features of a physical embodiment are described or shown in this application. It will be appreciated that in the development of any such actual embodiment, as in the case of the actual implementation in combination with the present disclosure, numerous implementation-specific decisions must be made to achieve the developers' specific goals, such as compliance with system-related, business-related, government-related and other constraints, which will vary from one implementation to another and from one implementation to another. While a developer's efforts might be time consuming, such efforts would still be a routine undertaking for those of ordinary skill in the art having benefit of this disclosure.

The disclosure is therefore well adapted to attain the ends and advantages mentioned as well as those that are inherent therein. The particular 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 claims below. It is therefore evident that the particular illustrative embodiments disclosed above may be altered, combined, or modified and all such variations are considered within the scope and spirit of the invention. The disclosure herein may be suitably practiced in the absence of any element not specifically disclosed herein and/or any optional element disclosed herein. Although compositions and methods are described in terms of "comprising," "containing," or "including" various components or steps, the compositions and methods can also "consist essentially of" or "consist of" the various components and steps. All numbers and ranges disclosed above may vary by some amount. Whenever a numerical range with a lower limit and an upper limit is disclosed, any number and any included range falling within the range is specifically disclosed. In particular, each range of values (of the form "from about a to about b," or, equivalently, "from about a to b") disclosed herein is to be understood as listing each number and range encompassed within the broader range of values. Also, the terms in the claims have their ordinary meaning unless otherwise explicitly and clearly defined by the patentee. Furthermore, the indefinite articles "a" or "an" used in the claims are defined herein to mean one element or more than one of the elements it introduces.