High temperature ni barrier sintering paste

Abstract

To provide a silver paste improved by using an alloying method in order to increase the melting point of paste particles.SOLUTION: The improved paste 200 has silver coated nickel particles, silver coated silica particles or glass frit added to the silver paste in various concentrations to form an improved paste that functions when sintered at a temperature above the melting point of the silver particles 204. The modified paste 200, when sintered, forms a layer of alloy 206 on the substrate 208.SELECTED DRAWING: Figure 2

Description

[Background technology]

[0001] [CROSS-REFERENCE TO RELATED APPLICATIONS] This application claims the benefit of priority under 35 U.S.C. § 119 to U.S. Provisional Patent Application No. 63 / 673,833, filed July 22, 2024, entitled "HIGH-TEMPERATURE NI-BARRIER SINTERING PASTE," the disclosure of which is incorporated herein by reference in its entirety.

[0002] Silver paste is known for its excellent electrical conductivity, thermal stability, and adhesive properties and is a material used in the electronics industry. Many manufacturers use silver paste as electrodes in electronic components such as fuses, varistors, etc. Due to its high adhesive strength and conductivity, silver paste can also be applied to PCBs to form conductive paths, connections, and contacts. A typical silver paste is composed of silver particles with sizes in the nanometer to micrometer range, but usually less than 10 micrometers in diameter. The silver particles are dispersed in a binder material (e.g., a polymer).

[0003] One important feature of silver paste compared to alternative pastes is its ability to function reliably over a wide range of temperatures and exhibit high compatibility for adhesion to substrates. However, the usefulness of silver paste is limited when processing tasks requiring temperatures above approximately 950°C due to the melting point of silver. For example, when processing materials that require sintering at temperatures above 950°C, the utility of silver paste is limited. In one particular example, a multilayer varistor may use zinc oxide as the substrate, which must be sintered at temperatures above 1000°C. The use of pure silver paste at these temperatures is not feasible. Figure 1 shows an example of the results of sintering pure silver paste above the melting point of silver.

[0004] Therefore, it would be desirable to be able to provide a modified silver paste that is sinterable at temperatures above the melting point of silver (ie, 968.1° C.). Summary of the Invention

[0005] This Summary is provided to introduce various concepts in a simplified form that are further described below in the Detailed Description. This Summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to serve as an aid in determining the scope of the claimed subject matter.

[0006] Disclosed herein is a novel silver paste that uses an interdiffusion method to increase the melting point of the paste particles. In one embodiment, a metal such as nickel, which has a high melting point, can be used to alloy with the silver particles to increase the melting point of the silver paste. In a second embodiment, an additive such as glass frit or silver-coated fused silica can be used to form a protective layer disposed between the substrate and the silver paste to prevent the diffusion of silver onto the substrate. [Brief explanation of the drawings]

[0007] Certain exemplary embodiments of the systems and methods of the present disclosure will now be described, by way of example, with reference to the accompanying drawings, in which:

[0008] [Figure 1] Prior art results are shown for sintering pure silver paste on a substrate at temperatures above 1000°C.

[0009] [Figure 2] 1 is a schematic diagram of a modified paste according to a first embodiment of the present invention, showing a mixture of silver and nickel particles dispersed in a binder.

[0010] [Figure 3A] 1 shows the modified paste of the first embodiment applied onto a substrate to form conductive paths and contacts. [Figure 3B] FIG. 11 is a view after sintering, showing the conductive paths and contacts formed.

[0011] [Figure 4A] 1 shows the improved paste of the first embodiment before sintering. [Figure 4B] 1 shows the improved paste of the first embodiment after sintering at 960° C. [Figure 4C] 1 shows the improved paste of the first embodiment after sintering at 1000° C.

[0012] [Figure 5] FIG. 2 is a schematic diagram of a modified paste according to a second embodiment of the present invention, showing a mixture of silver particles and additive particles dispersed in a binder.

[0013] [Figure 6A] 1 shows the modified paste of the second embodiment applied onto a substrate to form conductive paths and contacts. [Figure 6B] FIG. 11 is a view after sintering, showing the conductive paths and contacts formed.

[0014] [Figure 7A] Shows before and after images of printed lines of silver paste, no nickel particles. [Figure 7B] Shows before and after images of printed lines of silver paste, mixed with silver coated nickel particles.

[0015] [Figure 8] 1 is a graph showing a partial phase diagram between nickel and silver. DETAILED DESCRIPTION OF THE INVENTION

[0016] Disclosed herein are novel materials including silver pastes with additives that improve the material's resistance to the high temperatures typically used to sinter certain materials, such as zinc oxide, which require sintering at temperatures above 1000°C.

[0017] In a first embodiment of the present invention, nickel particles or silver-coated nickel particles are mixed into a silver paste binder to form an improved paste. At temperatures near 900°C, interdiffusion between the silver and nickel particles begins to form a mixed structure. The nickel is between the molten silver and the substrate (e.g., ZnO) and acts as a barrier to prevent the molten silver from directly contacting and potentially reacting with the substrate. Nickel's higher melting point and its solid state at this temperature make it a good candidate to act as a diffusion barrier. However, even with nickel acting as a barrier, some diffusion of silver through the nickel can occur, and depending on the thickness and integrity of the nickel layer, some silver may eventually reach the substrate. Nevertheless, the presence of nickel significantly delays or limits this interaction.

[0018] 2 is a schematic diagram illustrating a modified paste 200 formed in this embodiment of the present invention. Both silver particles 204 and nickel particles 202 are dispersed in a binder 201. The nickel particles 202 are preferably coated with silver to improve distribution and promote bonding with the silver particles 204. In a preferred embodiment, the nickel particles 202 comprise between 10% and 50% of the total particle population, with the remaining particles being silver particles. The nickel and silver particles are preferably approximately the same diameter, preferably less than about 10 micrometers in diameter.

[0019] In variations of the first embodiment, other metals with higher melting points than silver may be used in place of nickel. Furthermore, several different metals may be used in any proportion.

[0020] When modified paste 200 is sintered, it forms an alloy layer 206 on substrate 208. The diffusion seen in Figure 1 does not occur. Figure 3A shows modified paste 200 before sintering in layout 300 forming conductive pathways 301 and contacts 302 on the substrate, and Figure 3B shows a post-sintering view of layout 300a forming conductive pathways 301a and contacts 302a. The diffusion seen in Figure 1 does not occur.

[0021] In a second embodiment of the present invention, additive particles are dispersed in a silver paste to form an improved paste. In a variation of the second embodiment, the additive particles may include, for example, glass frit or silver-coated silica, or a combination thereof. The glass frit may include silicon oxide having a melting point above the melting point of silver, preferably above 1000°C. The silver-coated silica can absorb some of the heat present during high-temperature sintering to improve the overall resistance of the improved paste to high temperatures. In a variation of the second embodiment, both the glass frit and silver-coated silica particles may be used together in any ratio.

[0022] 4A-4C show a multi-layer modified silver paste 400, in which silver-coated nickel particles 401 are sandwiched between zinc oxide (ZnO) layers 402. The ZnO layers may be formed on a substrate. FIG. 4A shows the modified paste of the first embodiment after drying but before sintering. FIG. 4B shows the paste after sintering at 960°C, and FIG. 4C shows the paste after sintering at 1000°C. As can be seen, there is no delamination or discontinuities in the printed paste layers.

[0023] 5 is a schematic diagram illustrating a modified paste formed in accordance with a second embodiment of the present invention. Both silver particles 504 and additive particles 502 are dispersed in a binder 500. As with the first embodiment, the additive particles preferably comprise between 10% and 50% of the total particles present in the modified paste, with the remaining particles being silver particles. The additive particles and silver particles preferably have approximately the same diameter, preferably less than about 10 micrometers in diameter.

[0024] The modified paste, when sintered, forms a protective layer 506 of alloy on the substrate 508. No diffusion occurs, as seen in Figure 1. A layer 510 of silver particles forms on top of the protective layer 506.

[0025] Figure 6A shows the modified paste in a layout 600 forming conductive paths 601 and contacts 602 on a substrate before sintering, and Figure 6B shows the post-sintering view, with no diffusion occurring as shown in Figure 1.

[0026] FIG. 7A shows a layer of pure silver paste 701 sintered at 960°C. In this figure, the substrate is ZnO 702, formed both above and below the printed silver paste. As can be seen, there are discontinuities in the pure silver paste lines, and the silver paste lines are quite thin. FIG. 7B shows a layer of modified silver paste with silver-coated nickel particles 701a, also sintered at 960°C. As in FIG. 7A, the printed paste layer is sandwiched between formed layers of ZnO 702a. In contrast to FIG. 7A, FIG. 7B shows a continuous, thick layer of printed silver paste, even after sintering.

[0027] Figure 8 is a graph showing a partial phase diagram between nickel and silver, illustrating the intermixing of the atoms of these two elements above the solidus temperature of 960°C. The graph shows that nickel can be completely mixed with silver up to a weight percentage of 30%.

[0028] The advantages of both embodiments of the present invention are clearly demonstrated. The modified paste allows the use of silver paste for sintering at temperatures above the melting point of silver. A high level of compatibility with substrates that require high sintering temperatures is therefore established. Furthermore, the adhesion of the modified paste to the substrate is improved compared to silver paste.

[0029] As one skilled in the art will recognize, many variations of the materials described herein are within the intended scope of the invention. For example, a different metal in the first embodiment and a different material in the second embodiment having a melting point above that of silver may be used without departing from the intended scope of the invention. Accordingly, the exemplary embodiments disclosed herein should not be considered limiting of the invention, but rather illustrative of the invention. The scope of the invention is defined by the following claims.

Claims

1. Binder; silver particles dispersed in the binder; and metal particles dispersed in the binder, the metal particles having a melting point higher than the melting point of the silver particles; A conductive paste comprising:

2. The conductive paste of claim 1 , wherein the metal particles include nickel particles coated with silver.

3. The conductive paste of claim 2 , wherein the nickel particles comprise between 10% and 50% of the total number of particles in the binder.

4. The conductive paste of claim 1 , wherein the silver particles comprise between 50% and 90% of the total number of particles in the binder.

5. The conductive paste of claim 1 , wherein the silver particles and the metal particles have a diameter of less than 10 microns.

6. The conductive paste of claim 1 , wherein the binder is a polymer.

7. The conductive paste of claim 1 , further comprising additive particles dispersed in the conductive paste, the additive particles being selected from the group consisting of silver coated silica and glass frit.

8. The conductive paste according to claim 7 , wherein the glass frit contains silicon oxide having a melting point higher than that of the silver particles.

9. a substrate; and A sintered alloy paste including silver particles and silver-coated nickel particles, each of the silver particles and the silver-coated nickel particles disposed in a binder, and the alloy disposed on the substrate to form a conductive pathway. An electrode comprising:

10. 10. The electrode of claim 9, wherein the sintered alloy paste is disposed on the substrate and forms a conductive contact portion, the contact portion being connected to the conductive path.

11. 10. The electrode of claim 9, wherein the nickel particles comprise between 10% and 50% of the total number of particles in the binder.

12. 10. The electrode of claim 9, wherein the nickel particles and the silver particles are approximately 10 microns in diameter.

13. The electrode of claim 9 wherein the binder is a polymer.

14. 10. The electrode of claim 9, further comprising the sintered alloy paste disposed between zinc oxide layers.

15. 10. The electrode of claim 9, further comprising additive particles dispersed in the sintered alloy paste, the additive particles being selected from the group consisting of silver-coated silica and glass frit having a melting point above the melting point of the silver particles.

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