Copper paste for low-temperature air heterogeneous bonding and preparation method and application thereof

Abstract

The invention provides copper paste for low-temperature air heterogeneous bonding as well as a preparation method and application of the copper paste. The copper paste comprises the following components in percentage by mass: 58-66wt% of surface-modified copper nanoparticles, 12-14wt% of soldering flux and 20-30wt% of an organic solvent, the surface modified copper nanoparticles are obtained by mixing Cu (CH3COO) 2, a stabilizer and a reducing agent and then stirring and reacting under a water bath heating condition; the stabilizer is prepared from at least one of Arabic gum, pyridine, 2-pyridinemethanol, isopropanolamine and 2-hydroxyethyl pyridine; the reducing agent comprises at least one of L-ascorbic acid, salicylic acid and serine; the organic solvent comprises at least one of propyl alcohol, glycerol, methanol and ethylene glycol. A connector obtained by sintering the copper paste is a compact welding connector, the welding quality is high, the mechanical property is good, and the connection strength is far higher than that of current mainstream Sn-based brazing filler metal.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of brazing materials, and particularly relates to a copper paste for low-temperature air hetero-bonding, a preparation method and application thereof. BACKGROUND

[0002] In the field of microelectronic packaging and high-power device interconnection, nano-metallic paste as a kind of key connecting material, plays multiple functions such as connection, conduction and heat dissipation. In recent years, with the development trend of device miniaturization, high integration and high working temperature, traditional solder such as Sn-Pb, Sn-Ag-Cu gradually cannot meet the demand of high reliability and high thermal stability. Therefore, developing a kind of sintering material with excellent conduction performance, high melting point and good thermal stability gradually becomes a research hotspot. Among them, nano-copper paste is widely considered as a solution to replace silver paste and traditional solder due to its low-temperature sintering and high-temperature service characteristics.

[0003] The application of nano-copper paste in microelectronic packaging is based on its unique size effect. Copper nanoparticles have high activity due to their high specific surface area, which can realize solid-state sintering at a lower temperature, and then form intermetallic connection without the need of external flux. However, copper has a strong tendency to oxidize, and it is easy to form a surface oxide film during storage, printing and sintering process, which seriously hinders the diffusion sintering between particles, and then affects the connection strength and reliability. Therefore, most of the current nano-copper paste still needs to be sintered in a reducing atmosphere (such as formic acid) or in a vacuum condition to prevent oxidation and hinder sintering.

[0004] The easy-oxidation characteristic of nano-copper paste makes it difficult to be used in large-scale industrial production, and the use of reducing atmosphere furnace has certain risks, which not only increases the production cost but also is accompanied by the risk of spillover of reducing gas. The formic acid furnace, as a typical reducing atmosphere furnace, is currently used in the bonding process of nano-copper paste. The formic acid steam in the formic acid furnace is extremely easy to corrode the equipment, affecting the safety of production, and also seriously endangering the health of the body. The use of vacuum sintering furnace can improve this problem, but the existing vacuum furnace is difficult to maintain high vacuum degree, and there is still a small amount of oxygen in the bonding process, which reduces the reliability of the joint. After the surface of copper nanoparticles is oxidized, a layer of cuprous oxide is first formed, which will hinder the surface diffusion sintering behavior of the nanoparticles, inhibit the formation of sintering neck, and reduce the sintering driving force, making it difficult to form a reliable sintering structure after bonding. At the same time, most of the current research work focuses on Cu-Cu bonding, and there is little report on heterogeneous substrate bonding. At present, Au and Ag are the two materials commonly used for the back metal of SiC chips, and the development of air low-temperature heterogeneous bonding of high-strength copper paste is particularly important, which can solve the pain points in the current industrial production and greatly improve the reliability of chip interconnection. Therefore, it has great practical significance and application potential to develop a kind of high-strength nano-copper paste which can be directly low-temperature heterogeneous bonded in air.

[0005] In order to solve the problem of easy oxidation of nano-copper, some researches have tried to use organic coating (such as polyvinyl alcohol, PVP, oleylamine, etc.), carboxylic acid surface treatment or mixing with reducing organic carrier (such as terpineol, polyethylene glycol) to realize self-reducing sintering connection. However, these methods still face the following problems in practical application: first, the process window is narrow, and the control requirements of sintering temperature, time and atmosphere are high; second, the residual pores in the joint caused by the residual of some organic matters will affect the bonding strength and reliability; third, the dependence on vacuum or reducing gas environment in batch production or large-area packaging increases the packaging cost and equipment complexity.

[0006] Therefore, it has become a key issue to be urgently broken through in the current advanced packaging technology to develop a copper paste system which can be directly sintered in air environment, has high bonding strength, and can meet the conditions of homogeneous or heterogeneous bonding. SUMMARY

[0007] In view of the above technical problems, the present application discloses a kind of low-temperature air heterogeneous bonding copper paste and its preparation method and application, and the Cu-Cu, Cu-Ag or Cu-Au joint obtained by sintering using the copper paste is dense, non-oxidized and high-strength welding joint, with high welding quality and connection strength far exceeding the current mainstream Sn-based solder.The low-temperature air heterogeneous bonding refers to sintering below 280 DEG C, and the high strength refers to the shear strength higher than that of commonly used SAC305 solder (i.e., higher than 30-40 MPa).

[0008] To this end, the technical scheme adopted by the present application is:

[0009] A copper paste for low-temperature air heterogeneous bonding, the components and their mass percentages are: surface-modified copper nanoparticles 58-66wt%, flux 12-14wt%, organic solvent 20-30wt%;

[0010] The surface-modified copper nanoparticles are obtained by mixing Cu(CH3COO)2, stabilizer and reducing agent, and then stirring and reacting under water bath heating conditions;

[0011] The stabilizer includes at least one of gum arabic, pyridine, 2-pyridine methanol, isopropanolamine, and 2-hydroxyethyl pyridine;

[0012] The reducing agent includes at least one of L-ascorbic acid, salicylic acid, and serine; and the organic solvent includes at least one of propanol, glycerol, methanol, and ethylene glycol.

[0013] By this technical scheme, the surface of the copper nanoparticles is coated with stabilizer and reducing agent through surface modification, which significantly improves the oxidation resistance of the copper nanoparticles, further enhances the surface protection effect, prolongs the storage stability of the paste, has good dispersibility, printability and sintering performance, and effectively inhibits the oxidation of copper particles by oxygen in the air. Moreover, the reducing agent can in-situ remove the cuprous oxide layer during the bonding process of the copper paste, maintain the surface activity of the copper nanoparticles, promote the diffusion of surface atoms, and help the formation of sintering necks, thereby improving the strength of the connection joint.

[0014] As a further improvement of the present application, the stabilizer is gum arabic, and the reducing agent is L-ascorbic acid.

[0015] As a further improvement of the present application, the mass ratio of Cu(CH3COO)2, gum arabic and L-ascorbic acid is 2:1:7.

[0016] As a further improvement of the present application, the particle size of the coated and modified copper nanoparticles is 220-280nm, and the thickness of the surface coating layer is 1-2nm.

[0017] As a further improvement of the present application, the mass ratio of the surface-coated and modified copper nanoparticles, flux and organic solvent is 66:12:22.

[0018] As a further improvement of the present application, the flux is MK504L, and the organic solvent is glycerol.

[0019] The present application discloses a preparation method of the copper paste for low-temperature air heterogeneous bonding as described above, which comprises:

[0020] Step S1, preparing surface-modified copper nanoparticles;

[0021] Step S2, stirring and mixing the surface-modified copper nanoparticles with the flux and the organic solvent to obtain copper paste for low-temperature air heterojunction bonding.

[0022] As a further improvement of the present application, step S1 comprises:

[0023] Step S11, adding copper acetate and gum arabic into anhydrous ethanol, heating and stirring in a water bath at 45-55°C to obtain solution A;

[0024] Step S12, stirring L-ascorbic acid in anhydrous ethanol to obtain solution B; drop solution B into solution A, and continue heating and stirring in a water bath at 45-55°C to obtain copper nanoparticle suspension;

[0025] Step S13, centrifugal washing of the obtained copper nanoparticle suspension to obtain copper nanoparticles;

[0026] Step S14, secondary washing of the centrifuged copper nanoparticles with deionized water and freeze-drying to obtain coated modified copper nanoparticles.

[0027] As a further improvement of the present application, in step S11, the mass ratio of anhydrous ethanol, Cu(CH3COO)2 and gum arabic is 25:2:1;

[0028] In step S12, the mass ratio of anhydrous ethanol and L-ascorbic acid is 20:7;

[0029] In steps S11 and S12, magnetic stirring is adopted, the stirring speed is 300-500 rpm / min, and the stirring time is set to 10-15 min;

[0030] In step S12, solution B is added dropwise into solution A at a dropwise speed of 4-6 mL / min;

[0031] In step S14, the ultrasonic power for ultrasonic treatment is 250-350 W, the ultrasonic frequency is 35-45 KHz, and the ultrasonic time is set to 4-6 min;

[0032] In step S13, the centrifugal washing adopts anhydrous ethanol at a rotation speed of 10000 rpm / min for at least 5 min to remove unreacted copper acetate particles, and the supernatant is discarded to obtain copper nanoparticles;

[0033] In step S14, the centrifugal washing comprises washing with deionized water at a rotation speed of 10000 rpm / min for at least 5 min, and then repeating the washing once more;

[0034] The freeze-drying comprises pre-freezing the cleaned copper nanoparticles in a cold trap at -55℃-65℃ for 15-20 min, and then vacuum drying for 3-4 h.

[0035] As a further improvement of the application, in step S2, a planetary paste mixer is used for paste preparation, and the rotation parameters of the planetary paste mixer are set as 350 rpm / min, 550 rpm / min, 750 rpm / min, 950 rpm / min and 1250 rpm / min for 1 min respectively.

[0036] The application discloses the application of the copper paste for low-temperature air heterogeneous bonding as described above to homogenous or heterogeneous interconnection between a copper-plated substrate and a different back-gold chip.

[0037] As a further improvement of the application, the copper paste for low-temperature air heterogeneous bonding is printed on the surface of the copper-plated substrate through screen printing, the different back-gold chip is attached to the printed pattern, extrusion is performed to make the combination firm, and hot-press bonding is performed in a hot-pressing furnace; before the bonding, preheating is performed at 260℃ without pressure for 2 min, the flux is fully volatilized, and a dense nano-copper sintered body is formed, and then pressure connection is performed.

[0038] Compared with the prior art, the application has the following beneficial effects:

[0039] Firstly, the technical scheme of the application significantly improves the oxidation resistance of the nano-copper particles by introducing a stabilizer and a reducing agent for synergistic coating during the preparation of the copper nanoparticles. In particular, pyridine, 2-pyridine methanol, gum arabic and 2-hydroxyethyl pyridine are used as stabilizers, and L-ascorbic acid, salicylic acid and serine are used as reducing agents. The stabilizers such as gum arabic are adsorbed on the surface of the copper nanoparticles through steric hindrance effect and van der Waals force, thereby realizing the size control of the copper nanoparticles. The reducing agents such as L-ascorbic acid exist in an excess form and are synergistically coated on the surface of the copper nanoparticles together with the stabilizers, thereby further enhancing the surface protection effect, prolonging the storage stability of the paste, and effectively inhibiting the oxidation of the copper particles by oxygen in the air.

[0040] Secondly, the application selects organic solvents with reducing property such as glycerol to cooperate with specific fluxes (such as MK504L), which can decompose to produce reducing gas and remove surface oxide layer to restore surface activity during sintering, so as to realize in-situ reduction of copper oxide layer. Reducing agents such as L-ascorbic acid can remove cuprous oxide layer in-situ during bonding, maintain the surface activity of copper nanoparticles, promote the diffusion of surface atoms, and help the formation of sintering neck. At the same time, the decomposition temperature of ascorbic acid is relatively low, which can be completely removed during bonding and will not cause organic residues. Organic solvents such as glycerol can control the printability of copper paste and also have a certain reducing effect. Moreover, the organic carrier has a suitable boiling point (210℃), which can fully volatilize at the sintering temperature to avoid defects such as pores caused by organic residues, thereby significantly improving the density and mechanical properties of the joint.

[0041] Thirdly, the technical scheme of the application can be sintered in an air environment without inert atmosphere or vacuum condition, a pressureless preheating link is introduced at the initial stage of sintering, which greatly promotes the volatilization of the flux, reduces the residues of organic and oxide phases in the solder joint, and the nano-copper sintered body formed after preheating can realize high-quality sintering connection with wide process window and strong adaptability. The copper paste of the technical scheme of the application can form a dense and pore-free metal connection structure under the condition of 260℃ and 8MPa for 30 minutes, the shear strength of Cu-Cu joint is not less than 90MPa, the shear strength of Cu-Ag solder joint is not less than 70MPa, and the shear strength of Cu-Au solder joint is not less than 40MPa. Compared with other existing nano-copper paste, under the same temperature, pressure and time conditions, the joint obtained by the application has higher strength, better density, more stable oxidation control effect and simpler process, which is not only more suitable for low-cost and high-efficiency packaging applications in actual production lines, but also suitable for the connection of heterogeneous interfaces, which greatly widens the application scenarios of nano-copper paste. BRIEF DESCRIPTION OF DRAWINGS

[0042] Figure 1 is a schematic diagram of the sintering and bonding process of the low-temperature air heterogeneous bonding copper paste of the application and the existing ordinary copper paste, wherein (a) is the low-temperature air heterogeneous bonding copper paste of the application, and (b) is the ordinary copper paste.

[0043] Figure 2 is the scanning and transmission electron microscope pictures of the copper nanoparticles obtained in Example 2 of the application, wherein (a) and (b) are the pictures of scanning electron microscope, (c) is the statistical distribution of nanoparticle particle size, (d) is the picture of transmission electron microscope, and (e) and (f) are the local enlarged views of (d).

[0044] Figure 3 is the infrared absorption spectrum diagram of the nano-copper particles obtained in Example 2 of the application.

[0045] Figure 4 is a cross-sectional morphology of a Cu-Cu solder joint obtained by the embodiment 9 of the present application.

[0046] Figure 5 is a cross-sectional morphology of a Cu-Cu solder joint obtained by the embodiment 10 of the present application.

[0047] Figure 6 is a cross-sectional morphology of a Cu-Cu solder joint obtained by the embodiment 11 of the present application.

[0048] Figure 7 is a cross-sectional morphology of a Cu-Ag solder joint obtained by the embodiment 12 of the present application.

[0049] Figure 8 is a cross-sectional morphology of a Cu-Ag solder joint obtained by the embodiment 13 of the present application.

[0050] Figure 9 is a cross-sectional morphology of a Cu-Au solder joint obtained by the embodiment 14 of the present application.

[0051] Figure 10 is a cross-sectional morphology of a Cu-Au solder joint obtained by the embodiment 15 of the present application. DETAILED DESCRIPTION

[0052] The preferred embodiments of the present application are further described in detail below.

[0053] A preparation method of a low-temperature air hetero-bonding copper paste, the specific steps comprising:

[0054] Firstly, Cu(CH3COO)2 and gum arabic stabilizer are added into anhydrous ethanol, heated and stirred in a water bath, and the temperature is set to 50℃ to obtain solution A; wherein the mass ratio of anhydrous ethanol, Cu(CH3COO)2 and gum arabic is 25:2:1.

[0055] Solution A is subjected to magnetic stirring at a stirring speed of 500 rpm / min for 12 min.

[0056] Secondly, L-ascorbic acid is added into anhydrous ethanol and stirred, and the stirring table temperature is set to 50℃ to obtain solution B; wherein the mass ratio of anhydrous ethanol and L-ascorbic acid is 20:7.

[0057] Solution B is subjected to magnetic stirring at a stirring speed of 500 rpm / min for 12 min. After being fully stirred and uniformly mixed, solution B is added dropwise into solution A at a dropping speed of 4-6 mL / min, and the water bath heating and stirring are continued to obtain a copper nanoparticle suspension.

[0058] Thirdly, the obtained copper nanoparticle suspension is centrifuged at a speed of 10000 rpm / min for 5 min, and then cleaned with anhydrous ethanol to obtain copper nanoparticles.

[0059] Fourthly, the centrifuged copper nanoparticles are cleaned with deionized water again. After adding deionized water, in order to more fully wash away impurities, an ultrasonic cleaner is used for ultrasonic cleaning for 5 min. The ultrasonic power is 250-350 W, the ultrasonic frequency is 35-45 KHz, and the ultrasonic time is set to 4-6 min.

[0060] Fifthly, the cleaned copper nanoparticles are freeze-dried to obtain copper nanoparticles with high oxidation resistance. The freeze-drying includes pre-freezing the cleaned copper nanoparticles in a cold trap at -55-65℃ for 15-20 min, and then vacuum drying for 3-4 h.

[0061] Sixthly, the coated and modified copper nanoparticles are mixed with flux and organic solvent by stirring to obtain copper paste for low-temperature air heterojunction bonding. The mass ratio of the coated and modified copper nanoparticles to the flux and the organic solvent is 10:2:3. The rotation of the planetary paste mixer is 350 rpm / min, 550 rpm / min, 750 rpm / min, 950 rpm / min, and 1250 rpm / min for 1 min, respectively.

[0062] The obtained copper paste for low-temperature air heterojunction bonding is sintered in an air environment. In the initial stage of sintering, a pressureless preheating link is carried out, and then hot pressing is carried out after preheating. The bonding process of the copper paste of the present application and the ordinary copper paste of the prior art is compared as shown in FIG. 1, wherein the ordinary copper paste is obtained by mixing copper nanoparticles without surface modification with flux and organic solvent. As can be seen from the comparison, the copper paste of the present application can form a dense sintered body after bonding, while the ordinary copper paste has been oxidized during the bonding process, which hinders the sintering process and cannot form a reliable sintered body. Figure 1

[0063] The following will be described in detail in conjunction with specific examples.

[0064] Example 1

[0065] Firstly, 2 g of Cu(CH3COO)2 and 1 g of pyridine are added to 25 mL of anhydrous ethanol and stirred to obtain solution A.

[0066] Secondly, 7 g of salicylic acid is added to 20 mL of anhydrous ethanol and stirred to obtain solution B. After being fully stirred and uniform, solution B is added dropwise to solution A at a dropwise speed of 4-6 mL / min, and water bath heating and stirring are continued to obtain a copper nanoparticle suspension.

[0067] ​Third step, centrifugal separation of the obtained suspension, pour the supernatant to get coated modified copper nanoparticles. After separation, using anhydrous ethanol for cleaning, fully shaking and then centrifugal separation again.

[0068] Fourth step, the centrifuged copper nanoparticles were washed with deionized water again, after adding deionized water, in order to more fully wash away impurities, using ultrasonic cleaner for 5 minutes. After ultrasonic shaking, put into the centrifuge.

[0069] Fifth step, the cleaned copper nanoparticles were freeze-dried, and then ground into fine particles.

[0070] Sixth step, 0.1g coated modified copper nanoparticles, 0.02g MK504L flux and 0.05g ethylene glycol were mixed to obtain air low-temperature sintering copper paste.

[0071] Example 2

[0072] First step, 2g Cu(CH3COO)2 and 1g 2-pyridine methanol were added to 25mL anhydrous ethanol and stirred to obtain solution A.

[0073] Second step, 7g L-ascorbic acid was added to 20mL anhydrous ethanol and stirred to obtain solution B. After fully stirring, solution B was added to solution A dropwise at a speed of 4-6mL / min, and water bath heating and stirring were continued to obtain copper nanoparticle suspension.

[0074] Third step, centrifugal separation of the obtained suspension, pour the supernatant to get coated modified copper nanoparticles. After separation, using anhydrous ethanol for cleaning, fully shaking and then centrifugal separation again.

[0075] Fourth step, the centrifuged copper nanoparticles were washed with deionized water again, after adding deionized water, in order to more fully wash away impurities, using ultrasonic cleaner for 5 minutes. After ultrasonic shaking, put into the centrifuge.

[0076] Fifth step, the cleaned copper nanoparticles were freeze-dried, and then ground into fine particles.

[0077] Sixth step, 0.1g coated modified copper nanoparticles, 0.02g MK504L flux and 0.03g propylene glycol were mixed to obtain air low-temperature sintering copper paste.

[0078] In this example, the scanning electron microscope and transmission electron microscope pictures of the copper nanoparticles obtained in the fifth step are shown in Figure 2 , the infrared absorption spectrum of the copper nanoparticles is shown in Figure 3 , Figure 2 (a) and Figure 2(b) It can be seen that the nanoparticles prepared in this example have small size and uniform particle size distribution, which is beneficial for sintering. Figure 2 (c) is a particle size distribution histogram showing that most of the particle sizes are distributed in the range of 220-280 nm. Figure 2 (e) is Figure 2 (d) is a partial enlargement, from which it can be seen that the surface has an amorphous coating layer about 1.7 nm thick, which, combined with Figure 3 The infrared spectra of (e) also confirm that ascorbic acid not only participates in the redox reaction as a reducing agent, but also successfully coats the copper nanoparticles on the surface, Figure 2 (f) only found the lattice fringes of pure copper element, indicating that the copper nanoparticles obtained have good oxidation resistance and do not contain other oxides.

[0079] Example 3

[0080] In the first step, 2 g of Cu(CH3COO)2 and 1 g of 2-hydroxyethylpyridine were added to 25 mL of anhydrous ethanol and stirred to obtain solution A.

[0081] In the second step, 7 g of serine was added to 20 mL of anhydrous ethanol and stirred to obtain solution B. After being fully stirred and uniform, solution B was added dropwise to solution A at a rate of 4-6 mL / min, and water bath heating and stirring were continued to obtain a copper nanoparticle suspension.

[0082] In the third step, the obtained suspension was centrifuged, and the supernatant was poured to obtain coated and modified copper nanoparticles. After separation, anhydrous ethanol was used for cleaning, and after sufficient shaking, centrifugation was performed again.

[0083] In the fourth step, the centrifuged copper nanoparticles were washed twice with deionized water. After adding deionized water, in order to more thoroughly wash away impurities, an ultrasonic cleaner was used for ultrasonic cleaning for 5 min. After ultrasonic cleaning, the mixture was shaken uniformly and then centrifuged.

[0084] In the fifth step, the cleaned copper nanoparticles were freeze-dried, and after being taken out, they were ground into fine particles.

[0085] In the sixth step, 0.1 g of coated and modified copper nanoparticles, 0.02 g of MK504L flux, and 0.03 g of methanol were stirred and mixed to obtain air low-temperature sintering copper paste.

[0086] Example 4

[0087] In the first step, 2 g of Cu(CH3COO)2 and 1 g of gum arabic were added to 25 mL of anhydrous ethanol and stirred to obtain solution A.

[0088] Second step, 7g L-ascorbic acid is added to 20mL anhydrous ethanol to stir to obtain solution B. After fully stirring to be uniform, solution B is added dropwise to solution A, the dropwise speed is 4-6mL / min, and water bath heating and stirring are continued to obtain copper nanoparticle suspension.

[0089] Third step, the obtained suspension is centrifuged and the supernatant is poured to obtain modified copper nanoparticles. After separation, anhydrous ethanol is used for cleaning, and after fully shaking, centrifugation is performed again.

[0090] Fourth step, the centrifuged copper nanoparticles are cleaned with deionized water again. After adding deionized water, in order to more fully wash away impurities, an ultrasonic cleaner is used for ultrasonic cleaning for 5min. After ultrasonic cleaning, shaking is uniform, and the centrifuge is used for centrifugation.

[0091] Fifth step, the cleaned copper nanoparticles are freeze-dried, and after taking out, grinding is performed to be fine particulate.

[0092] Sixth step, 0.1g of the modified copper nanoparticles, 0.02g of MK504L flux and 0.03g of propanol are stirred and mixed to obtain air low-temperature sintering copper paste.

[0093] Example 5

[0094] First step, 2g of Cu(CH3COO)2 and 1g of pyridine are added to 25mL of anhydrous ethanol to stir to obtain solution A.

[0095] Second step, 7g of serine is added to 20mL of anhydrous ethanol to stir to obtain solution B. After fully stirring to be uniform, solution B is added dropwise to solution A, the dropwise speed is 4-6mL / min, and water bath heating and stirring are continued to obtain copper nanoparticle suspension.

[0096] Third step, the obtained suspension is centrifuged and the supernatant is poured to obtain modified copper nanoparticles. After separation, anhydrous ethanol is used for cleaning, and after fully shaking, centrifugation is performed again.

[0097] Fourth step, the centrifuged copper nanoparticles are cleaned with deionized water again. After adding deionized water, in order to more fully wash away impurities, an ultrasonic cleaner is used for ultrasonic cleaning for 5min. After ultrasonic cleaning, shaking is uniform, and the centrifuge is used for centrifugation.

[0098] Fifth step, the cleaned copper nanoparticles are freeze-dried, and after taking out, grinding is performed to be fine particulate.

[0099] Sixth step, 0.1g of the modified copper nanoparticles, 0.02g of MK504L flux and 0.05g of methanol are stirred and mixed to obtain air low-temperature sintering copper paste.

[0100] Example 6

[0101] First step, 2g Cu(CH3COO)2 and 1g 2-pyridinemethanol were stirred in 25mL anhydrous ethanol to obtain solution A.

[0102] Second step, 7g salicylic acid was stirred in 20mL anhydrous ethanol to obtain solution B. After being stirred evenly, solution B was added dropwise into solution A at a speed of 4-6mL / min, and the stirring was continued under water bath heating to obtain copper nanoparticle suspension.

[0103] Third step, the obtained suspension was centrifuged, and the supernatant was discarded to obtain coated modified copper nanoparticles. After separation, the copper nanoparticles were washed with anhydrous ethanol, and then were centrifuged again after being shaken evenly.

[0104] Fourth step, the centrifuged copper nanoparticles were washed with deionized water again. After adding deionized water, in order to wash away the impurities more thoroughly, the copper nanoparticles were ultrasonically cleaned for 5min. After ultrasonic cleaning, the copper nanoparticles were shaken evenly and then were centrifuged.

[0105] Fifth step, the washed copper nanoparticles were freeze-dried, and then were ground into fine particles.

[0106] Sixth step, 0.1g coated modified copper nanoparticles, 0.02g MK504L flux and 0.05g propanol were stirred and mixed to obtain air low-temperature sintering copper paste.

[0107] Example 7

[0108] First step, 2g Cu(CH3COO)2 and 1g gum arabic were stirred in 25mL anhydrous ethanol to obtain solution A.

[0109] Second step, 7g salicylic acid was stirred in 20mL anhydrous ethanol to obtain solution B. After being stirred evenly, solution B was added dropwise into solution A at a speed of 4-6mL / min, and the stirring was continued under water bath heating to obtain copper nanoparticle suspension.

[0110] Third step, the obtained suspension was centrifuged, and the supernatant was discarded to obtain coated modified copper nanoparticles. After separation, the copper nanoparticles were washed with anhydrous ethanol, and then were centrifuged again after being shaken evenly.

[0111] Fourth step, the centrifuged copper nanoparticles were washed with deionized water again. After adding deionized water, in order to wash away the impurities more thoroughly, the copper nanoparticles were ultrasonically cleaned for 5min. After ultrasonic cleaning, the copper nanoparticles were shaken evenly and then were centrifuged.

[0112] Fifth step, the washed copper nanoparticles were freeze-dried, and then were ground into fine particles.

[0113] Sixth step, 0.1 g of coated modified copper nanoparticles were mixed with 0.02 g of MK504L flux and 0.03 g of glycerol by stirring to obtain air low-temperature sintering copper paste.

[0114] Example 8

[0115] The copper paste obtained in the above examples can be used to realize Cu-Cu, Cu-Ag and Cu-Au interconnection between the copper-plated substrate and the chip, including the following steps:

[0116] First step, the surface of the copper-plated substrate and the chip was polished for 30 s with sandpaper of 2500 or 4000 mesh to remove the surface organic residues and copper oxide, improve the surface activity and facilitate subsequent soldering.

[0117] Second step, the copper paste was printed on the surface of the copper-plated substrate by steel screen printing process using a doctor blade, and then different SiC gold-backed chips were attached to the substrate surface according to the printed pattern.

[0118] Third step, a layer of graphite paper and a layer of silicon oil paper were covered on the area above the chip to ensure uniform pressure on the chip and facilitate the graphite paper demolding after bonding. The chip and substrate treated in the second step were placed on a clean aluminum plate and put into a hot press furnace. The soldering parameters were set as follows: soldering temperature was 260℃, first preheating without pressure for 2 min, then immediately press bonding for 28 min, the pressure applied during bonding was 8 MPa, and the bonded sample was taken out immediately after bonding and air-cooled to room temperature.

[0119] Example 9

[0120] On the basis of Example 8, the copper paste obtained in Example 1 was used for air hot-press welding, and the cross-sectional morphology of the obtained Cu-Cu weld was as shown in Figure 4 .

[0121] Example 10

[0122] On the basis of Example 8, the copper paste obtained in Example 2 was used for air hot-press welding, and the cross-sectional morphology of the obtained Cu-Cu weld was as shown in Figure 5 .

[0123] Example 11

[0124] On the basis of Example 8, the copper paste obtained in Example 3 was used for air hot-press welding, and the cross-sectional morphology of the obtained Cu-Cu weld was as shown in Figure 6 .

[0125] Example 12

[0126] On the basis of Example 8, the copper paste obtained in Example 4 was used for air hot-press welding, and the cross-sectional morphology of the obtained Cu-Ag weld was as shown in Figure 7 .

[0127] Example 13

[0128] On the basis of Example 8, the copper paste obtained in Example 5 was used for air hot pressure welding, and the cross-sectional morphology of the obtained Cu-Ag weld seam is shown in Figure 8. Figure 8

[0129] Example 14

[0130] On the basis of Example 8, the copper paste obtained in Example 6 was used for air hot pressure welding, and the cross-sectional morphology of the obtained Cu-Au weld seam is shown in Figure 9. Figure 9

[0131] Example 15

[0132] On the basis of Example 8, the copper paste obtained in Example 7 was used for air hot pressure welding, and the cross-sectional morphology of the obtained Cu-Au weld seam is shown in Figure 10. Figure 10

[0133] Example 16

[0134] The copper paste of Examples 1-7 was subjected to air hot pressure welding, and the shear strength test was performed on the obtained weld joints, i.e., Examples 9-15. Ten samples were taken for each group for testing, and the average value was taken after removing the highest value and the lowest value. The results are shown in Table 1.

[0135] Table 1

[0136]

[0137] By comparing the cross-sectional morphologies of the welds obtained in Examples 9-15, it can be seen that the cross-sectional morphology of the welds obtained in Examples 9 and 10 can correspond to the relatively high shear strength, and the cross-section of the high-strength joint of Examples 9 and 10 is also very dense, and the porosity is very low. Figures 4-10

[0138] The sintering conditions and shear strength of the nano-copper paste of different particle sizes disclosed in the current mainstream literature are shown in Table 2. By comparing the data in Table 1, it can be seen that the Cu-Cu joint strength of Examples 9, 10 and 11 is greater than 90 MPa, and the highest can reach 131.13 MPa. The Cu-Ag joint strength of Examples 12 and 13 is greater than 70 MPa, and the highest can reach 82.53 MPa. The Cu-Au joint strength of Examples 14 and 15 is greater than 40 MPa, and the highest can reach 53.83 MPa. The joint strength of the nano-copper paste of the prior art is lower than that of the present application.

[0139] Table 2

[0140] Particle size Sintering process Shear strength (MPa) 57.5 nm Air, 250°C, 5 MPa, 20 min (Cu-Cu) 68.5 6.5 nm Air, 250°C, 2 MPa, 10 min (Cu-Cu) 28.3 50 nm Air, 250°C, 5 MPa, 5 min (Cu-Cu) 60.7 40 nm Air, 225°C, 10 MPa, 60 min (Cu-Cu) 50.4 3800 nm Air, 300°C, 10 MPa, 1 min (Cu-Cu) 46.2 100 nm Air, 225°C, 8 MPa, 10 min (Cu-Cu) 38.7 2890 nm Air, 300°C, 5 MPa, 1 min (Cu-Ag) 39.3 6500-7200 nm mixed particle size Air, 350°C, 5 MPa, 3 min (Cu-Au) 26.3 200 nm Air, 300°C, 2 MPa, 1 min (Cu-Au) 21.8 6.5 nm Air, 275°C, 2 MPa, 10 min (Cu-Ag) 35.1 20-100 nm mixed particle size Air, 250°C, 4 MPa, 5 min (Cu-Au) 20.2 50 nm Air, 300°C, 2 MPa, 10 min (Cu-Ag) 26.7

[0141] ​​​​The above is further detailed description of the present application in combination with specific preferred embodiments, and cannot be deemed as limitation of the specific implementation of the present application to these descriptions. For those skilled in the art to which the present application belongs, without departing from the concept of the present application, a number of simple deductions or substitutions can be made, and all should be deemed as falling within the protection scope of the present application.

Claims

1. A copper paste for low-temperature air heterogeneous bonding, characterized in that, Its composition and mass percentage are as follows: 58-66 wt% surface-modified copper nanoparticles, 12-14 wt% flux, and 20-30 wt% organic solvent; The surface-modified copper nanoparticles were obtained by mixing Cu(CH3COO)2, stabilizer, and reducing agent and then stirring the mixture under water bath heating conditions. The stabilizer includes at least one of gum arabic, pyridine, 2-pyridinemethanol, isopropanolamine, and 2-hydroxyethylpyridine; The reducing agent includes at least one of L-ascorbic acid, salicylic acid, and serine; The organic solvent includes at least one of propanol, glycerol, methanol, and ethylene glycol.

2. The copper paste for low-temperature air heterogeneous bonding according to claim 1, characterized in that: The stabilizer is gum arabic, and the reducing agent is L-ascorbic acid; the mass ratio of Cu(CH3COO)2, gum arabic, and L-ascorbic acid is 2:1:

7.

3. The copper paste for low-temperature air heterogeneous bonding according to claim 2, characterized in that: The coated and modified copper nanoparticles have a particle size of 220-280 nm and a surface coating thickness of 1-2 nm.

4. The copper paste for low-temperature air heterogeneous bonding according to claim 1, characterized in that: The mass ratio of the surface-coated modified copper nanoparticles to the flux and organic solvent is 66:12:22; The flux is MK504L, and the organic solvent is glycerol.

5. The method for preparing copper paste for low-temperature air heterogeneous bonding as described in any one of claims 1 to 4, characterized in that, include: Step S1: Prepare surface-modified copper nanoparticles; Step S2: The surface-modified copper nanoparticles are stirred and mixed with flux and organic solvent to obtain copper paste for low-temperature air heterobonding.

6. The method for preparing copper paste for low-temperature air heterogeneous bonding according to claim 5, characterized in that: Step S1 includes: Step S11: Add copper acetate and gum arabic to anhydrous ethanol, heat and stir in a water bath at 45-55℃ to obtain solution A; Step S12: Add L-ascorbic acid to anhydrous ethanol and stir to obtain solution B; add solution B dropwise to solution A and continue heating and stirring in a water bath at 45-55℃ to obtain a copper nanoparticle suspension. Step S13: The obtained copper nanoparticle suspension is centrifuged and washed to obtain copper nanoparticles. Step S14: The centrifuged copper nanoparticles are washed a second time with deionized water and then freeze-dried to obtain coated and modified copper nanoparticles.

7. The copper paste for low-temperature air heterogeneous bonding according to claim 6, characterized in that: In step S11, the mass ratio of anhydrous ethanol, Cu(CH3COO)2 and gum arabic is 25:2:1; In step S12, the mass ratio of anhydrous ethanol to L-ascorbic acid is 20:7; In steps S11 and S12, magnetic stirring is used at a speed of 300-500 rpm / min and a stirring time of 10-15 min. In step S12, solution B is added dropwise to solution A at a rate of 4-6 mL / min; In step S14, the ultrasonic power of the ultrasonic treatment is 250-350W, the ultrasonic frequency is 35-45KHz, and the ultrasonic time is set to 4-6min. In step S13, the centrifugal washing is performed using anhydrous ethanol at a speed of 10000 rpm / min for at least 5 minutes to remove unreacted copper acetate particles, and the supernatant is discarded to obtain copper nanoparticles. In step S14, the centrifugal cleaning includes washing with deionized water at a speed of 10,000 rpm / min for at least 5 minutes, and then repeating the cleaning once more. The freeze-drying process involves pre-freezing the cleaned copper nanoparticles in a cold trap at -55℃ to 65℃ for 15-20 minutes, followed by vacuum drying for 3-4 hours.

8. The copper paste for low-temperature air heterogeneous bonding according to claim 5, characterized in that: In step S2, a planetary mixer is used to prepare the paste. The rotation parameters of the planetary mixer are set sequentially to 350 rpm / min, 550 rpm / min, 750 rpm / min, 950 rpm / min, and 1250 rpm / min for 1 min each.

9. The application of the copper paste for low-temperature air heterogeneous bonding as described in any one of claims 1 to 4, characterized in that: Used for homogeneous or heterogeneous interconnection between copper-plated substrates and different gold-backed chips.

10. The application of the copper paste for low-temperature air heterogeneous bonding according to claim 9, characterized in that: The copper paste for low-temperature air heterogeneous bonding is printed onto the surface of a copper-plated substrate using a stencil. Different back-gold chips are mounted onto the printed pattern and pressed to ensure a strong bond. Hot-press bonding is then performed in a hot press furnace. Before bonding, the substrate is preheated without pressure for 2 minutes to allow the flux to fully evaporate and form a dense nano-copper sintered body. Then, pressure is applied for bonding.

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