Dissolve polyvinyl pyrrolidone (PVP) in ethylene glycol (EG) or diethylene glycol (DEG) at a tempera-ture of 90-160° C. with mechanical agitation.

Add an optional reducing agent to the above PVP solution. The selection of a reducing agent and its concentration depends on the selected metal salt.

Dissolve a metal salt in DI water or EG solution.

Add the silver salt solution to the PVP solution with controlled speed.

Separate silver nanoparticles from solution by centrifu-gation after the reactions are completed.

Purify the nanoparticles by washing with acetone/alcohol solvents and separate by centrifugation. Alternatively, a silver nanoparticle mixture may be formed with a catalytic reduction modifier PVP. As a pro-tective agent, the dose and molecular weight of PVP on the morphology, dispersibility, and size of silver particles are well controlled. The molecular weight of PVP is selected to be from 10000 to 100000. When the Mw is below 10000, the low molecular weight (Mw) and low glass transition tem-perature (Tg), the metal salt cannot be easily controlled by the PVP as the size distribution of nano silver is wider within the process of reduction, and when the Mw larger than 100000, the molecular weight of PVP is likely to form a thread to make the nano silver tend to assemble into a wire structure. A molecular weight of 40000 is selected as the molecular weight of the PVP, the size of nano silver is well controlled as schematically indicated below: n O N Ag N O n The mass fraction between the silver salt and PVP is selected to be from 1:3 to 1:7; including 1:4 to 1:6, 1:5 to 1:7, and 1:4 to 1:7. Below 1:3, the average particle size of nano silver is too large and easily aggregates, while when more than 1:7, the modified layer of nano silver will be too thick to wash out. The as-formed silver nanoparticle 28, as shown in FIG. 5, has an average size of 10-100 nm. The as-formed silver nanoparticle 28 with PVP capping layer 29 is washed with acetone/alcohol solvents and separated by centrifugation several times until the PVP capping layer 29 thickness is no less than approximately 3 nm and no thicker than approxi-mately 7 nm as confirmed under TEM and SEM. The PVP capping layer synthesis with the above procedure contains the Ag ions which for the formation of the in-situ silver nanoparticles. Reductive Solvent An optional solvent may be used with the silver particle mixture. Any known solvent functioning as a reducing agent may be used as the solvent. This solvent is preferably a glycol-based solvent. Examples of glycol-based solvents include ethylene glycol, diethylene glycol, triethylene glycol and polyethylene glycol, propylene glycol, dipropylene gly-col, etc. The trimodal nano-silver paste composition option-ally includes at least one reductive solvent; the solvents may be either one of the above solvents or their combination. The amount of reductive solvent is preferably from approxi-mately 10 to 30 percent by mass, including 10 to 20 percent by mass and 20 to 30 percent by mass. The solvent functioning as the reducing agent not only converts silver ions to silver nanoparticles on the surface of the core silver nanoparticle, but also removes any oxidation layer on the substrate to be bonded and provides a strong connection between the bonding line layer and the substrate or the connection between the bonding line and the die. In particular, the reducing ability of the reductive solvent is further increased as the temperature increases during the curing process. Thus, the curing time may be extended during nucleation of the silver nanoparticles to obtain the maximum amount of in-situ-formed silver nanoparticles. Extending the curing time at an appropriate temperature further removes any oxidation layer from the substrate. Moreover, the reducing solvent also removes any surface coatings of the silver flake portion of the paste, for example, stearic acid or oxides. Such coatings on the silver flakes would otherwise lower the thermal conductivity of the bonding line layer. Consequently, the cured film is dense, has high electrical conductivity, and has high adhesion to the substrate and die. Another consideration for the selection of a reductive solvent or solvents is the boiling point. Since the silver paste is sintered at temperatures of up to 260° C., the boiling point of the solvent should not exceed 260° C. Formation of in-situ silver nanoparticles occurs at a temperature between approximately 80-120° C.; therefore, selection of a reduc-tive solvent with a boiling point over 120° C. may be desirable. Consequently, the boiling point of the solvent is selected to be approximately 120 to 260° C., and more preferably 150 to 260° C. To further extend the time for the solvent to remain in the paste to ensure sufficient time for in-situ particle formation and oxidation layer removal, the solvent may be a combination of two different solvents, each with different boiling points. Thus, the solvent will evapo-rate in a stepwise manner, which will also contribute to a denser bond structure. An amount of the solvent is preferably 10 to 20 parts by mass to the silver. If this amount is less than 10 parts by mass, the viscosity increases and workability may be lower, and if it is over 20 parts by mass, the viscosity is lowered and the paste may be too thin to be printed. Anti-Oxidation Reducing Agent An anti-oxidation reducing agent may be added to the composition of the trimodal nano-silver paste. In the present invention, the anti-oxidization reducing agents refer to those materials having flux activity for removing an oxide coating film of a substrate, die, stearic acid coating on the surface of micron-sized silver particles, and the residual PVP coating on the silver nanoparticles. It also prevents the substrate surface from being re-oxidized during elevated temperature curing. A clean substrate surface will contribute to the mechanical strength at the interface and avoid crack forma-tion. Further, a clean surface of the micron-sized silver particles will contribute to high thermal conductivity. Examples of anti-oxidization reducing agents include citric acid, caffeic acid, malic acid and dicarboxylic acids such as ethanedioic acid, propanedioic acid, butanedioic acid, pen-tanedioic acid, hexanedioic acid, heptanedioic acid, octane-dioic acid, nonanedioic acid, decanedioic acid, etc. The amount of anti-oxidization reducing agent is preferably from approximately 0.1 to 1 percent by mass. If the amount is less than 0.1%, there will be no reducing effect and if the amount of anti-oxidization reducing agents exceeds 1 percent by mass, the thermal conductivity of the bond will be reduced. The decomposition temperature of the anti-oxidization reducing agents should be no more than 200° C. such that the reducing agent is removed at 150-200° C. before the silver sintering process starts. Failure to remove the reduc-ing agent prior to sintering may negatively impact the bond, leading to poor shear strength. Optional Non-Conductive Filler The trimodal nano-silver paste composition of the present invention may optionally include a non-conductive filler. The non-conductive filler may be selected from one or more of SiC, SiO2, TiO2, Al₂O3, or other ceramic and polymer particles such as silicone resin microspheres that are stable at sintering temperature. The purpose of adding non-con-ductive filler to the paste is to densify the paste structure and avoid aggregation of the voids under the grain growth B₂ process, which will have a negative influence on both shear strength and thermal conductivity. The addition of nonconductive particles improves the high-temperature stability and high-hydrothermal stability of the sintered nano-silver joints. It may also limit thermal resistance increase during thermal cycling between low and high temperature. Shear strength increases with an increasing proportion of SiC in the trimodal nano-silver paste. The size of the non-conduc-tive filler particles is preferably between approximately 50-1000 nm. The amount of non-conductive filler particles, when present, is preferably 0.1 to 1 percent by mass to 100 parts by mass of the trimodal nanosilver paste components. The Step-Curing Densification Method In another aspect, the present invention provides a multistep curing process that self-densifies the metal paste and permits the formation of a strong bond at low temperature without the need for the application of pressure during bonding. The multi-step curing method includes (S₁) a step for in-situ silver nanoparticle formation, (S₂) a step for removing the oxidation layer of the micron-sized silver particles and bonding silver nanoparticles, (S₃) a step for removing the residual anti-oxidation reducing agents, and (S₄) a step for sintering of the silver nanoparticles. FIG. 6 schematically depicts the multi-step curing method of the present invention. Accordingly, the multi-step curing method of the trimodal nanosilver paste provides a dense bond structure between the die and substrate without the need for application of external pressure during bonding. The first step creates in-situ silver nanoparticles from metal salt in the polymer layer surrounding the core nan-oparticles; the formation temperature is between approxi-mately 80∼120° C. for a period of approximately 10-30 mins. The second step removes an oxidation layer from the substrate and the organic layer on the micron-sized silver flakes/spherical silver particles; the temperature is between approximately 120∼150° C. for a time period of approxi-mately 10-30 mins. The third step removes the residual anti-oxidation reducing agents; the temperature is between approximately 150∼200° C. for a time period of approxi-mately 10-30 mins. The fourth step sinters the silver nan-oparticles; the temperature is between approximately 200-260° C. for a time period of approximately 30-90 mins. FIG. 7 schematically depicts the particles in a solvent followed by the particles binding to the micron-sized silver particles as the solvent is removed. Using the inventive trimodal nanosilver paste composi-tion and the multi-step curing method, in-situ silver nan-oparticles form around the central bonding nanoparticle from the polymer/PVP coating containing silver salt. With the presence of glycol-based solvents which also act as reducing agents, there will be a certain amount of small-sized silver nanoparticles (3-9 nm) generated on the surface of the bonding nanoparticles, forming a shell of smaller nanoparticles on a core larger nanoparticle as seen in FIG. 5. Importantly, the small nanoparticles 28 are chemically bonded to the base silver nanoparticle 27 prior to sintering to the larger, micron-sized silver flakes/spherical silver par-ticles. The composition is optimized to ensure a high packing density. Due to the slow removal of the solvent materials during the multi-step curing and sintering process, voids created by vaporization/decomposition of solvents/additives are filled by silver nanoparticles 27. That is, the structure is self-densifying due to its high packing density and the multi-step curing process. When trimodal nano-silver paste is sintered in the final step of the curing/sintering process, voids rarely form due to the densified structure formed during removal of the solvents and additive in steps 2 and 3. The in-situ formed silver nanoparticles sinter at a low temperature (approximately 112° C.) and act as metal adhe-sives on the silver nanoparticles so that the silver nanopar-TABLE Examples of Specific Example 1 2 3 Silver flakes 60 60 Synthesised AgNP 40 40 Commercialized AgNP EG 10% 5% DEG 5% 5% TEG 5% DBE Citric Acid 0.7% 0.7% 0.7% Caffeic acid hydroxybutanedioic acid 0.3% 0.3% 0.3% SiC powder 0.1% Organosilicone resin powder 0.1% P₁ ✓ ✓ ✓ P₂ P₃ Porosity <25% <20% <20% Electric Resitivity [Ω· cm] 4.60E−06 5.50E−06 5.50E−06 Thermal conductivity [W/m · K] 200 170 150 Shear strength before thermal 37 33 aging [Mpa] Shear strength after 1000 h 31 32 34 thermal aging [Mpa] Themal resistance increase >25% <25% <25% after 1000 thermal cycles (−55 to 150° C.) *Step-curing profile: The step-curing densification method disclosed above. Control Curing profile: P2: Directly curing under 260° C. P3: Equilibrium at 30° C., raising temperature at a speed of 10° C./min to 130° C., Isothermal min. ticles form strong bonds with the silver flakes/spherical silver particles. The silver nanoparticles act as strong bond-ing particles among the larger micron-sized particles. The final bond structure of three kinds of silver particles is shown in FIG. 2A. EXAMPLE 1: Synthesis of PVP Capped Silver Nanoparticle 36.4 g of polyvinyl pyrrolidone (Mw: 50000) was dis-solved in 75 mL of ethylene glycol with magnetic stirring while heated to 120° C., 5.8 g of silver nitrate was dissolved in 25 mL of ethylene glycol. The silver nitrate solution was added to PVP solution and the reaction mixture was stirred for 1 hour before cooling down to room temperature. After completion of the reaction, the mixture was centrifuged out at 10000 rpm and washed with ethanol and acetone. The purified silver nanoparticles were not dried, but kept in an ethanol environment for subsequent ink/paste formulation. The invention claimed is:

Dissolve polyvinyl pyrrolidone (PVP) in ethylene glycol (EG) or diethylene glycol (DEG) at a tempera-ture of 90-160° C. with mechanical agitation.

Add an optional reducing agent to the above PVP solution. The selection of a reducing agent and its concentration depends on the selected metal salt.

Dissolve a metal salt in DI water or EG solution.

Add the silver salt solution to the PVP solution with controlled speed.

Separate silver nanoparticles from solution by centrifu-gation after the reactions are completed.

Purify the nanoparticles by washing with acetone/alcohol solvents and separate by centrifugation. Alternatively, a silver nanoparticle mixture may be formed with a catalytic reduction modifier PVP. As a pro-tective agent, the dose and molecular weight of PVP on the morphology, dispersibility, and size of silver particles are well controlled. The molecular weight of PVP is selected to be from 10000 to 100000. When the Mw is below 10000, the low molecular weight (Mw) and low glass transition tem-perature (Tg), the metal salt cannot be easily controlled by the PVP as the size distribution of nano silver is wider within the process of reduction, and when the Mw larger than 100000, the molecular weight of PVP is likely to form a thread to make the nano silver tend to assemble into a wire structure. A molecular weight of 40000 is selected as the molecular weight of the PVP, the size of nano silver is well controlled as schematically indicated below: n O N Ag N O n The mass fraction between the silver salt and PVP is selected to be from 1:3 to 1:7; including 1:4 to 1:6, 1:5 to 1:7, and 1:4 to 1:7. Below 1:3, the average particle size of nano silver is too large and easily aggregates, while when more than 1:7, the modified layer of nano silver will be too thick to wash out. The as-formed silver nanoparticle 28, as shown in FIG. 5, has an average size of 10-100 nm. The as-formed silver nanoparticle 28 with PVP capping layer 29 is washed with acetone/alcohol solvents and separated by centrifugation several times until the PVP capping layer 29 thickness is no less than approximately 3 nm and no thicker than approxi-mately 7 nm as confirmed under TEM and SEM. The PVP capping layer synthesis with the above procedure contains the Ag ions which for the formation of the in-situ silver nanoparticles. Reductive Solvent An optional solvent may be used with the silver particle mixture. Any known solvent functioning as a reducing agent may be used as the solvent. This solvent is preferably a glycol-based solvent. Examples of glycol-based solvents include ethylene glycol, diethylene glycol, triethylene glycol and polyethylene glycol, propylene glycol, dipropylene gly-col, etc. The trimodal nano-silver paste composition option-ally includes at least one reductive solvent; the solvents may be either one of the above solvents or their combination. The amount of reductive solvent is preferably from approxi-mately 10 to 30 percent by mass, including 10 to 20 percent by mass and 20 to 30 percent by mass. The solvent functioning as the reducing agent not only converts silver ions to silver nanoparticles on the surface of the core silver nanoparticle, but also removes any oxidation layer on the substrate to be bonded and provides a strong connection between the bonding line layer and the substrate or the connection between the bonding line and the die. In particular, the reducing ability of the reductive solvent is further increased as the temperature increases during the curing process. Thus, the curing time may be extended during nucleation of the silver nanoparticles to obtain the maximum amount of in-situ-formed silver nanoparticles. Extending the curing time at an appropriate temperature further removes any oxidation layer from the substrate. Moreover, the reducing solvent also removes any surface coatings of the silver flake portion of the paste, for example, stearic acid or oxides. Such coatings on the silver flakes would otherwise lower the thermal conductivity of the bonding line layer. Consequently, the cured film is dense, has high electrical conductivity, and has high adhesion to the substrate and die. Another consideration for the selection of a reductive solvent or solvents is the boiling point. Since the silver paste is sintered at temperatures of up to 260° C., the boiling point of the solvent should not exceed 260° C. Formation of in-situ silver nanoparticles occurs at a temperature between approximately 80-120° C.; therefore, selection of a reduc-tive solvent with a boiling point over 120° C. may be desirable. Consequently, the boiling point of the solvent is selected to be approximately 120 to 260° C., and more preferably 150 to 260° C. To further extend the time for the solvent to remain in the paste to ensure sufficient time for in-situ particle formation and oxidation layer removal, the solvent may be a combination of two different solvents, each with different boiling points. Thus, the solvent will evapo-rate in a stepwise manner, which will also contribute to a denser bond structure. An amount of the solvent is preferably 10 to 20 parts by mass to the silver. If this amount is less than 10 parts by mass, the viscosity increases and workability may be lower, and if it is over 20 parts by mass, the viscosity is lowered and the paste may be too thin to be printed. Anti-Oxidation Reducing Agent An anti-oxidation reducing agent may be added to the composition of the trimodal nano-silver paste. In the present invention, the anti-oxidization reducing agents refer to those materials having flux activity for removing an oxide coating film of a substrate, die, stearic acid coating on the surface of micron-sized silver particles, and the residual PVP coating on the silver nanoparticles. It also prevents the substrate surface from being re-oxidized during elevated temperature curing. A clean substrate surface will contribute to the mechanical strength at the interface and avoid crack forma-tion. Further, a clean surface of the micron-sized silver particles will contribute to high thermal conductivity. Examples of anti-oxidization reducing agents include citric acid, caffeic acid, malic acid and dicarboxylic acids such as ethanedioic acid, propanedioic acid, butanedioic acid, pen-tanedioic acid, hexanedioic acid, heptanedioic acid, octane-dioic acid, nonanedioic acid, decanedioic acid, etc. The amount of anti-oxidization reducing agent is preferably from approximately 0.1 to 1 percent by mass. If the amount is less than 0.1%, there will be no reducing effect and if the amount of anti-oxidization reducing agents exceeds 1 percent by mass, the thermal conductivity of the bond will be reduced. The decomposition temperature of the anti-oxidization reducing agents should be no more than 200° C. such that the reducing agent is removed at 150-200° C. before the silver sintering process starts. Failure to remove the reduc-ing agent prior to sintering may negatively impact the bond, leading to poor shear strength. Optional Non-Conductive Filler The trimodal nano-silver paste composition of the present invention may optionally include a non-conductive filler. The non-conductive filler may be selected from one or more of SiC, SiO2, TiO2, Al₂O3, or other ceramic and polymer particles such as silicone resin microspheres that are stable at sintering temperature. The purpose of adding non-con-ductive filler to the paste is to densify the paste structure and avoid aggregation of the voids under the grain growth B₂ process, which will have a negative influence on both shear strength and thermal conductivity. The addition of nonconductive particles improves the high-temperature stability and high-hydrothermal stability of the sintered nano-silver joints. It may also limit thermal resistance increase during thermal cycling between low and high temperature. Shear strength increases with an increasing proportion of SiC in the trimodal nano-silver paste. The size of the non-conduc-tive filler particles is preferably between approximately 50-1000 nm. The amount of non-conductive filler particles, when present, is preferably 0.1 to 1 percent by mass to 100 parts by mass of the trimodal nanosilver paste components. The Step-Curing Densification Method In another aspect, the present invention provides a multistep curing process that self-densifies the metal paste and permits the formation of a strong bond at low temperature without the need for the application of pressure during bonding. The multi-step curing method includes (S₁) a step for in-situ silver nanoparticle formation, (S₂) a step for removing the oxidation layer of the micron-sized silver particles and bonding silver nanoparticles, (S₃) a step for removing the residual anti-oxidation reducing agents, and (S₄) a step for sintering of the silver nanoparticles. FIG. 6 schematically depicts the multi-step curing method of the present invention. Accordingly, the multi-step curing method of the trimodal nanosilver paste provides a dense bond structure between the die and substrate without the need for application of external pressure during bonding. The first step creates in-situ silver nanoparticles from metal salt in the polymer layer surrounding the core nan-oparticles; the formation temperature is between approxi-mately 80∼120° C. for a period of approximately 10-30 mins. The second step removes an oxidation layer from the substrate and the organic layer on the micron-sized silver flakes/spherical silver particles; the temperature is between approximately 120∼150° C. for a time period of approxi-mately 10-30 mins. The third step removes the residual anti-oxidation reducing agents; the temperature is between approximately 150∼200° C. for a time period of approxi-mately 10-30 mins. The fourth step sinters the silver nan-oparticles; the temperature is between approximately 200-260° C. for a time period of approximately 30-90 mins. FIG. 7 schematically depicts the particles in a solvent followed by the particles binding to the micron-sized silver particles as the solvent is removed. Using the inventive trimodal nanosilver paste composi-tion and the multi-step curing method, in-situ silver nan-oparticles form around the central bonding nanoparticle from the polymer/PVP coating containing silver salt. With the presence of glycol-based solvents which also act as reducing agents, there will be a certain amount of small-sized silver nanoparticles (3-9 nm) generated on the surface of the bonding nanoparticles, forming a shell of smaller nanoparticles on a core larger nanoparticle as seen in FIG. 5. Importantly, the small nanoparticles 28 are chemically bonded to the base silver nanoparticle 27 prior to sintering to the larger, micron-sized silver flakes/spherical silver par-ticles. The composition is optimized to ensure a high packing density. Due to the slow removal of the solvent materials during the multi-step curing and sintering process, voids created by vaporization/decomposition of solvents/additives are filled by silver nanoparticles 27. That is, the structure is self-densifying due to its high packing density and the multi-step curing process. When trimodal nano-silver paste is sintered in the final step of the curing/sintering process, voids rarely form due to the densified structure formed during removal of the solvents and additive in steps 2 and 3. The in-situ formed silver nanoparticles sinter at a low temperature (approximately 112° C.) and act as metal adhe-sives on the silver nanoparticles so that the silver nanopar-TABLE Examples of Specific Example 1 2 3 Silver flakes 60 60 Synthesised AgNP 40 40 Commercialized AgNP EG 10% 5% DEG 5% 5% TEG 5% DBE Citric Acid 0.7% 0.7% 0.7% Caffeic acid hydroxybutanedioic acid 0.3% 0.3% 0.3% SiC powder 0.1% Organosilicone resin powder 0.1% P₁ ✓ ✓ ✓ P₂ P₃ Porosity <25% <20% <20% Electric Resitivity [Ω· cm] 4.60E−06 5.50E−06 5.50E−06 Thermal conductivity [W/m · K] 200 170 150 Shear strength before thermal 37 33 aging [Mpa] Shear strength after 1000 h 31 32 34 thermal aging [Mpa] Themal resistance increase >25% <25% <25% after 1000 thermal cycles (−55 to 150° C.) *Step-curing profile: The step-curing densification method disclosed above. Control Curing profile: P2: Directly curing under 260° C. P3: Equilibrium at 30° C., raising temperature at a speed of 10° C./min to 130° C., Isothermal min. ticles form strong bonds with the silver flakes/spherical silver particles. The silver nanoparticles act as strong bond-ing particles among the larger micron-sized particles. The final bond structure of three kinds of silver particles is shown in FIG. 2A. EXAMPLE 1: Synthesis of PVP Capped Silver Nanoparticle 36.4 g of polyvinyl pyrrolidone (Mw: 50000) was dis-solved in 75 mL of ethylene glycol with magnetic stirring while heated to 120° C., 5.8 g of silver nitrate was dissolved in 25 mL of ethylene glycol. The silver nitrate solution was added to PVP solution and the reaction mixture was stirred for 1 hour before cooling down to room temperature. After completion of the reaction, the mixture was centrifuged out at 10000 rpm and washed with ethanol and acetone. The purified silver nanoparticles were not dried, but kept in an ethanol environment for subsequent ink/paste formulation. The invention claimed is:

Dissolve polyvinyl pyrrolidone (PVP) in ethylene glycol (EG) or diethylene glycol (DEG) at a tempera-ture of 90-160° C. with mechanical agitation.

Add an optional reducing agent to the above PVP solution. The selection of a reducing agent and its concentration depends on the selected metal salt.

Dissolve a metal salt in DI water or EG solution.

Add the silver salt solution to the PVP solution with controlled speed.

Separate silver nanoparticles from solution by centrifu-gation after the reactions are completed.

Purify the nanoparticles by washing with acetone/alcohol solvents and separate by centrifugation. Alternatively, a silver nanoparticle mixture may be formed with a catalytic reduction modifier PVP. As a pro-tective agent, the dose and molecular weight of PVP on the morphology, dispersibility, and size of silver particles are well controlled. The molecular weight of PVP is selected to be from 10000 to 100000. When the Mw is below 10000, the low molecular weight (Mw) and low glass transition tem-perature (Tg), the metal salt cannot be easily controlled by the PVP as the size distribution of nano silver is wider within the process of reduction, and when the Mw larger than 100000, the molecular weight of PVP is likely to form a thread to make the nano silver tend to assemble into a wire structure. A molecular weight of 40000 is selected as the molecular weight of the PVP, the size of nano silver is well controlled as schematically indicated below: n O N Ag N O n The mass fraction between the silver salt and PVP is selected to be from 1:3 to 1:7; including 1:4 to 1:6, 1:5 to 1:7, and 1:4 to 1:7. Below 1:3, the average particle size of nano silver is too large and easily aggregates, while when more than 1:7, the modified layer of nano silver will be too thick to wash out. The as-formed silver nanoparticle 28, as shown in FIG. 5, has an average size of 10-100 nm. The as-formed silver nanoparticle 28 with PVP capping layer 29 is washed with acetone/alcohol solvents and separated by centrifugation several times until the PVP capping layer 29 thickness is no less than approximately 3 nm and no thicker than approxi-mately 7 nm as confirmed under TEM and SEM. The PVP capping layer synthesis with the above procedure contains the Ag ions which for the formation of the in-situ silver nanoparticles. Reductive Solvent An optional solvent may be used with the silver particle mixture. Any known solvent functioning as a reducing agent may be used as the solvent. This solvent is preferably a glycol-based solvent. Examples of glycol-based solvents include ethylene glycol, diethylene glycol, triethylene glycol and polyethylene glycol, propylene glycol, dipropylene gly-col, etc. The trimodal nano-silver paste composition option-ally includes at least one reductive solvent; the solvents may be either one of the above solvents or their combination. The amount of reductive solvent is preferably from approxi-mately 10 to 30 percent by mass, including 10 to 20 percent by mass and 20 to 30 percent by mass. The solvent functioning as the reducing agent not only converts silver ions to silver nanoparticles on the surface of the core silver nanoparticle, but also removes any oxidation layer on the substrate to be bonded and provides a strong connection between the bonding line layer and the substrate or the connection between the bonding line and the die. In particular, the reducing ability of the reductive solvent is further increased as the temperature increases during the curing process. Thus, the curing time may be extended during nucleation of the silver nanoparticles to obtain the maximum amount of in-situ-formed silver nanoparticles. Extending the curing time at an appropriate temperature further removes any oxidation layer from the substrate. Moreover, the reducing solvent also removes any surface coatings of the silver flake portion of the paste, for example, stearic acid or oxides. Such coatings on the silver flakes would otherwise lower the thermal conductivity of the bonding line layer. Consequently, the cured film is dense, has high electrical conductivity, and has high adhesion to the substrate and die. Another consideration for the selection of a reductive solvent or solvents is the boiling point. Since the silver paste is sintered at temperatures of up to 260° C., the boiling point of the solvent should not exceed 260° C. Formation of in-situ silver nanoparticles occurs at a temperature between approximately 80-120° C.; therefore, selection of a reduc-tive solvent with a boiling point over 120° C. may be desirable. Consequently, the boiling point of the solvent is selected to be approximately 120 to 260° C., and more preferably 150 to 260° C. To further extend the time for the solvent to remain in the paste to ensure sufficient time for in-situ particle formation and oxidation layer removal, the solvent may be a combination of two different solvents, each with different boiling points. Thus, the solvent will evapo-rate in a stepwise manner, which will also contribute to a denser bond structure. An amount of the solvent is preferably 10 to 20 parts by mass to the silver. If this amount is less than 10 parts by mass, the viscosity increases and workability may be lower, and if it is over 20 parts by mass, the viscosity is lowered and the paste may be too thin to be printed. Anti-Oxidation Reducing Agent An anti-oxidation reducing agent may be added to the composition of the trimodal nano-silver paste. In the present invention, the anti-oxidization reducing agents refer to those materials having flux activity for removing an oxide coating film of a substrate, die, stearic acid coating on the surface of micron-sized silver particles, and the residual PVP coating on the silver nanoparticles. It also prevents the substrate surface from being re-oxidized during elevated temperature curing. A clean substrate surface will contribute to the mechanical strength at the interface and avoid crack forma-tion. Further, a clean surface of the micron-sized silver particles will contribute to high thermal conductivity. Examples of anti-oxidization reducing agents include citric acid, caffeic acid, malic acid and dicarboxylic acids such as ethanedioic acid, propanedioic acid, butanedioic acid, pen-tanedioic acid, hexanedioic acid, heptanedioic acid, octane-dioic acid, nonanedioic acid, decanedioic acid, etc. The amount of anti-oxidization reducing agent is preferably from approximately 0.1 to 1 percent by mass. If the amount is less than 0.1%, there will be no reducing effect and if the amount of anti-oxidization reducing agents exceeds 1 percent by mass, the thermal conductivity of the bond will be reduced. The decomposition temperature of the anti-oxidization reducing agents should be no more than 200° C. such that the reducing agent is removed at 150-200° C. before the silver sintering process starts. Failure to remove the reduc-ing agent prior to sintering may negatively impact the bond, leading to poor shear strength. Optional Non-Conductive Filler The trimodal nano-silver paste composition of the present invention may optionally include a non-conductive filler. The non-conductive filler may be selected from one or more of SiC, SiO2, TiO2, Al₂O3, or other ceramic and polymer particles such as silicone resin microspheres that are stable at sintering temperature. The purpose of adding non-con-ductive filler to the paste is to densify the paste structure and avoid aggregation of the voids under the grain growth B₂ process, which will have a negative influence on both shear strength and thermal conductivity. The addition of nonconductive particles improves the high-temperature stability and high-hydrothermal stability of the sintered nano-silver joints. It may also limit thermal resistance increase during thermal cycling between low and high temperature. Shear strength increases with an increasing proportion of SiC in the trimodal nano-silver paste. The size of the non-conduc-tive filler particles is preferably between approximately 50-1000 nm. The amount of non-conductive filler particles, when present, is preferably 0.1 to 1 percent by mass to 100 parts by mass of the trimodal nanosilver paste components. The Step-Curing Densification Method In another aspect, the present invention provides a multistep curing process that self-densifies the metal paste and permits the formation of a strong bond at low temperature without the need for the application of pressure during bonding. The multi-step curing method includes (S₁) a step for in-situ silver nanoparticle formation, (S₂) a step for removing the oxidation layer of the micron-sized silver particles and bonding silver nanoparticles, (S₃) a step for removing the residual anti-oxidation reducing agents, and (S₄) a step for sintering of the silver nanoparticles. FIG. 6 schematically depicts the multi-step curing method of the present invention. Accordingly, the multi-step curing method of the trimodal nanosilver paste provides a dense bond structure between the die and substrate without the need for application of external pressure during bonding. The first step creates in-situ silver nanoparticles from metal salt in the polymer layer surrounding the core nan-oparticles; the formation temperature is between approxi-mately 80∼120° C. for a period of approximately 10-30 mins. The second step removes an oxidation layer from the substrate and the organic layer on the micron-sized silver flakes/spherical silver particles; the temperature is between approximately 120∼150° C. for a time period of approxi-mately 10-30 mins. The third step removes the residual anti-oxidation reducing agents; the temperature is between approximately 150∼200° C. for a time period of approxi-mately 10-30 mins. The fourth step sinters the silver nan-oparticles; the temperature is between approximately 200-260° C. for a time period of approximately 30-90 mins. FIG. 7 schematically depicts the particles in a solvent followed by the particles binding to the micron-sized silver particles as the solvent is removed. Using the inventive trimodal nanosilver paste composi-tion and the multi-step curing method, in-situ silver nan-oparticles form around the central bonding nanoparticle from the polymer/PVP coating containing silver salt. With the presence of glycol-based solvents which also act as reducing agents, there will be a certain amount of small-sized silver nanoparticles (3-9 nm) generated on the surface of the bonding nanoparticles, forming a shell of smaller nanoparticles on a core larger nanoparticle as seen in FIG. 5. Importantly, the small nanoparticles 28 are chemically bonded to the base silver nanoparticle 27 prior to sintering to the larger, micron-sized silver flakes/spherical silver par-ticles. The composition is optimized to ensure a high packing density. Due to the slow removal of the solvent materials during the multi-step curing and sintering process, voids created by vaporization/decomposition of solvents/additives are filled by silver nanoparticles 27. That is, the structure is self-densifying due to its high packing density and the multi-step curing process. When trimodal nano-silver paste is sintered in the final step of the curing/sintering process, voids rarely form due to the densified structure formed during removal of the solvents and additive in steps 2 and 3. The in-situ formed silver nanoparticles sinter at a low temperature (approximately 112° C.) and act as metal adhe-sives on the silver nanoparticles so that the silver nanopar-TABLE Examples of Specific Example 1 2 3 Silver flakes 60 60 Synthesised AgNP 40 40 Commercialized AgNP EG 10% 5% DEG 5% 5% TEG 5% DBE Citric Acid 0.7% 0.7% 0.7% Caffeic acid hydroxybutanedioic acid 0.3% 0.3% 0.3% SiC powder 0.1% Organosilicone resin powder 0.1% P₁ ✓ ✓ ✓ P₂ P₃ Porosity <25% <20% <20% Electric Resitivity [Ω· cm] 4.60E−06 5.50E−06 5.50E−06 Thermal conductivity [W/m · K] 200 170 150 Shear strength before thermal 37 33 aging [Mpa] Shear strength after 1000 h 31 32 34 thermal aging [Mpa] Themal resistance increase >25% <25% <25% after 1000 thermal cycles (−55 to 150° C.) *Step-curing profile: The step-curing densification method disclosed above. Control Curing profile: P2: Directly curing under 260° C. P3: Equilibrium at 30° C., raising temperature at a speed of 10° C./min to 130° C., Isothermal min. ticles form strong bonds with the silver flakes/spherical silver particles. The silver nanoparticles act as strong bond-ing particles among the larger micron-sized particles. The final bond structure of three kinds of silver particles is shown in FIG. 2A. EXAMPLE 1: Synthesis of PVP Capped Silver Nanoparticle 36.4 g of polyvinyl pyrrolidone (Mw: 50000) was dis-solved in 75 mL of ethylene glycol with magnetic stirring while heated to 120° C., 5.8 g of silver nitrate was dissolved in 25 mL of ethylene glycol. The silver nitrate solution was added to PVP solution and the reaction mixture was stirred for 1 hour before cooling down to room temperature. After completion of the reaction, the mixture was centrifuged out at 10000 rpm and washed with ethanol and acetone. The purified silver nanoparticles were not dried, but kept in an ethanol environment for subsequent ink/paste formulation. The invention claimed is:

Dissolve polyvinyl pyrrolidone (PVP) in ethylene glycol (EG) or diethylene glycol (DEG) at a tempera-ture of 90-160° C. with mechanical agitation.

Add an optional reducing agent to the above PVP solution. The selection of a reducing agent and its concentration depends on the selected metal salt.

Dissolve a metal salt in DI water or EG solution.

Add the silver salt solution to the PVP solution with controlled speed.

Separate silver nanoparticles from solution by centrifu-gation after the reactions are completed.

Purify the nanoparticles by washing with acetone/alcohol solvents and separate by centrifugation. Alternatively, a silver nanoparticle mixture may be formed with a catalytic reduction modifier PVP. As a pro-tective agent, the dose and molecular weight of PVP on the morphology, dispersibility, and size of silver particles are well controlled. The molecular weight of PVP is selected to be from 10000 to 100000. When the Mw is below 10000, the low molecular weight (Mw) and low glass transition tem-perature (Tg), the metal salt cannot be easily controlled by the PVP as the size distribution of nano silver is wider within the process of reduction, and when the Mw larger than 100000, the molecular weight of PVP is likely to form a thread to make the nano silver tend to assemble into a wire structure. A molecular weight of 40000 is selected as the molecular weight of the PVP, the size of nano silver is well controlled as schematically indicated below: n O N Ag N O n The mass fraction between the silver salt and PVP is selected to be from 1:3 to 1:7; including 1:4 to 1:6, 1:5 to 1:7, and 1:4 to 1:7. Below 1:3, the average particle size of nano silver is too large and easily aggregates, while when more than 1:7, the modified layer of nano silver will be too thick to wash out. The as-formed silver nanoparticle 28, as shown in FIG. 5, has an average size of 10-100 nm. The as-formed silver nanoparticle 28 with PVP capping layer 29 is washed with acetone/alcohol solvents and separated by centrifugation several times until the PVP capping layer 29 thickness is no less than approximately 3 nm and no thicker than approxi-mately 7 nm as confirmed under TEM and SEM. The PVP capping layer synthesis with the above procedure contains the Ag ions which for the formation of the in-situ silver nanoparticles. Reductive Solvent An optional solvent may be used with the silver particle mixture. Any known solvent functioning as a reducing agent may be used as the solvent. This solvent is preferably a glycol-based solvent. Examples of glycol-based solvents include ethylene glycol, diethylene glycol, triethylene glycol and polyethylene glycol, propylene glycol, dipropylene gly-col, etc. The trimodal nano-silver paste composition option-ally includes at least one reductive solvent; the solvents may be either one of the above solvents or their combination. The amount of reductive solvent is preferably from approxi-mately 10 to 30 percent by mass, including 10 to 20 percent by mass and 20 to 30 percent by mass. The solvent functioning as the reducing agent not only converts silver ions to silver nanoparticles on the surface of the core silver nanoparticle, but also removes any oxidation layer on the substrate to be bonded and provides a strong connection between the bonding line layer and the substrate or the connection between the bonding line and the die. In particular, the reducing ability of the reductive solvent is further increased as the temperature increases during the curing process. Thus, the curing time may be extended during nucleation of the silver nanoparticles to obtain the maximum amount of in-situ-formed silver nanoparticles. Extending the curing time at an appropriate temperature further removes any oxidation layer from the substrate. Moreover, the reducing solvent also removes any surface coatings of the silver flake portion of the paste, for example, stearic acid or oxides. Such coatings on the silver flakes would otherwise lower the thermal conductivity of the bonding line layer. Consequently, the cured film is dense, has high electrical conductivity, and has high adhesion to the substrate and die. Another consideration for the selection of a reductive solvent or solvents is the boiling point. Since the silver paste is sintered at temperatures of up to 260° C., the boiling point of the solvent should not exceed 260° C. Formation of in-situ silver nanoparticles occurs at a temperature between approximately 80-120° C.; therefore, selection of a reduc-tive solvent with a boiling point over 120° C. may be desirable. Consequently, the boiling point of the solvent is selected to be approximately 120 to 260° C., and more preferably 150 to 260° C. To further extend the time for the solvent to remain in the paste to ensure sufficient time for in-situ particle formation and oxidation layer removal, the solvent may be a combination of two different solvents, each with different boiling points. Thus, the solvent will evapo-rate in a stepwise manner, which will also contribute to a denser bond structure. An amount of the solvent is preferably 10 to 20 parts by mass to the silver. If this amount is less than 10 parts by mass, the viscosity increases and workability may be lower, and if it is over 20 parts by mass, the viscosity is lowered and the paste may be too thin to be printed. Anti-Oxidation Reducing Agent An anti-oxidation reducing agent may be added to the composition of the trimodal nano-silver paste. In the present invention, the anti-oxidization reducing agents refer to those materials having flux activity for removing an oxide coating film of a substrate, die, stearic acid coating on the surface of micron-sized silver particles, and the residual PVP coating on the silver nanoparticles. It also prevents the substrate surface from being re-oxidized during elevated temperature curing. A clean substrate surface will contribute to the mechanical strength at the interface and avoid crack forma-tion. Further, a clean surface of the micron-sized silver particles will contribute to high thermal conductivity. Examples of anti-oxidization reducing agents include citric acid, caffeic acid, malic acid and dicarboxylic acids such as ethanedioic acid, propanedioic acid, butanedioic acid, pen-tanedioic acid, hexanedioic acid, heptanedioic acid, octane-dioic acid, nonanedioic acid, decanedioic acid, etc. The amount of anti-oxidization reducing agent is preferably from approximately 0.1 to 1 percent by mass. If the amount is less than 0.1%, there will be no reducing effect and if the amount of anti-oxidization reducing agents exceeds 1 percent by mass, the thermal conductivity of the bond will be reduced. The decomposition temperature of the anti-oxidization reducing agents should be no more than 200° C. such that the reducing agent is removed at 150-200° C. before the silver sintering process starts. Failure to remove the reduc-ing agent prior to sintering may negatively impact the bond, leading to poor shear strength. Optional Non-Conductive Filler The trimodal nano-silver paste composition of the present invention may optionally include a non-conductive filler. The non-conductive filler may be selected from one or more of SiC, SiO2, TiO2, Al₂O3, or other ceramic and polymer particles such as silicone resin microspheres that are stable at sintering temperature. The purpose of adding non-con-ductive filler to the paste is to densify the paste structure and avoid aggregation of the voids under the grain growth B₂ process, which will have a negative influence on both shear strength and thermal conductivity. The addition of nonconductive particles improves the high-temperature stability and high-hydrothermal stability of the sintered nano-silver joints. It may also limit thermal resistance increase during thermal cycling between low and high temperature. Shear strength increases with an increasing proportion of SiC in the trimodal nano-silver paste. The size of the non-conduc-tive filler particles is preferably between approximately 50-1000 nm. The amount of non-conductive filler particles, when present, is preferably 0.1 to 1 percent by mass to 100 parts by mass of the trimodal nanosilver paste components. The Step-Curing Densification Method In another aspect, the present invention provides a multistep curing process that self-densifies the metal paste and permits the formation of a strong bond at low temperature without the need for the application of pressure during bonding. The multi-step curing method includes (S₁) a step for in-situ silver nanoparticle formation, (S₂) a step for removing the oxidation layer of the micron-sized silver particles and bonding silver nanoparticles, (S₃) a step for removing the residual anti-oxidation reducing agents, and (S₄) a step for sintering of the silver nanoparticles. FIG. 6 schematically depicts the multi-step curing method of the present invention. Accordingly, the multi-step curing method of the trimodal nanosilver paste provides a dense bond structure between the die and substrate without the need for application of external pressure during bonding. The first step creates in-situ silver nanoparticles from metal salt in the polymer layer surrounding the core nan-oparticles; the formation temperature is between approxi-mately 80∼120° C. for a period of approximately 10-30 mins. The second step removes an oxidation layer from the substrate and the organic layer on the micron-sized silver flakes/spherical silver particles; the temperature is between approximately 120∼150° C. for a time period of approxi-mately 10-30 mins. The third step removes the residual anti-oxidation reducing agents; the temperature is between approximately 150∼200° C. for a time period of approxi-mately 10-30 mins. The fourth step sinters the silver nan-oparticles; the temperature is between approximately 200-260° C. for a time period of approximately 30-90 mins. FIG. 7 schematically depicts the particles in a solvent followed by the particles binding to the micron-sized silver particles as the solvent is removed. Using the inventive trimodal nanosilver paste composi-tion and the multi-step curing method, in-situ silver nan-oparticles form around the central bonding nanoparticle from the polymer/PVP coating containing silver salt. With the presence of glycol-based solvents which also act as reducing agents, there will be a certain amount of small-sized silver nanoparticles (3-9 nm) generated on the surface of the bonding nanoparticles, forming a shell of smaller nanoparticles on a core larger nanoparticle as seen in FIG. 5. Importantly, the small nanoparticles 28 are chemically bonded to the base silver nanoparticle 27 prior to sintering to the larger, micron-sized silver flakes/spherical silver par-ticles. The composition is optimized to ensure a high packing density. Due to the slow removal of the solvent materials during the multi-step curing and sintering process, voids created by vaporization/decomposition of solvents/additives are filled by silver nanoparticles 27. That is, the structure is self-densifying due to its high packing density and the multi-step curing process. When trimodal nano-silver paste is sintered in the final step of the curing/sintering process, voids rarely form due to the densified structure formed during removal of the solvents and additive in steps 2 and 3. The in-situ formed silver nanoparticles sinter at a low temperature (approximately 112° C.) and act as metal adhe-sives on the silver nanoparticles so that the silver nanopar-TABLE Examples of Specific Example 1 2 3 Silver flakes 60 60 Synthesised AgNP 40 40 Commercialized AgNP EG 10% 5% DEG 5% 5% TEG 5% DBE Citric Acid 0.7% 0.7% 0.7% Caffeic acid hydroxybutanedioic acid 0.3% 0.3% 0.3% SiC powder 0.1% Organosilicone resin powder 0.1% P₁ ✓ ✓ ✓ P₂ P₃ Porosity <25% <20% <20% Electric Resitivity [Ω· cm] 4.60E−06 5.50E−06 5.50E−06 Thermal conductivity [W/m · K] 200 170 150 Shear strength before thermal 37 33 aging [Mpa] Shear strength after 1000 h 31 32 34 thermal aging [Mpa] Themal resistance increase >25% <25% <25% after 1000 thermal cycles (−55 to 150° C.) *Step-curing profile: The step-curing densification method disclosed above. Control Curing profile: P2: Directly curing under 260° C. P3: Equilibrium at 30° C., raising temperature at a speed of 10° C./min to 130° C., Isothermal min. ticles form strong bonds with the silver flakes/spherical silver particles. The silver nanoparticles act as strong bond-ing particles among the larger micron-sized particles. The final bond structure of three kinds of silver particles is shown in FIG. 2A. EXAMPLE 1: Synthesis of PVP Capped Silver Nanoparticle 36.4 g of polyvinyl pyrrolidone (Mw: 50000) was dis-solved in 75 mL of ethylene glycol with magnetic stirring while heated to 120° C., 5.8 g of silver nitrate was dissolved in 25 mL of ethylene glycol. The silver nitrate solution was added to PVP solution and the reaction mixture was stirred for 1 hour before cooling down to room temperature. After completion of the reaction, the mixture was centrifuged out at 10000 rpm and washed with ethanol and acetone. The purified silver nanoparticles were not dried, but kept in an ethanol environment for subsequent ink/paste formulation. The invention claimed is: