Method for realizing reliable mounting of power chip through low-temperature rapid sintering

Abstract

The invention provides a method for realizing reliable surface mounting of a power chip through low-temperature rapid sintering. The method comprises the following steps: synthesizing antioxidant nickel-coated copper core-shell nanoparticles through spontaneous epitaxial growth; the nickel-coated copper nano particles and an organic solvent are centrifugally stirred to be uniform, and nickel-coated copper nano soldering paste is obtained; the nickel-coated copper nano soldering paste is uniformly coated on the surface of the heat dissipation substrate through steel mesh printing, and then the heat dissipation substrate is placed on a heating platform to be preheated; and a power chip is placed on the substrate with the nanometer soldering paste, then the substrate is transferred to an ultrasonic welding platform for ultrasonic-assisted sintering, and the sintered interconnection connector is obtained. According to the preparation method, the antioxidant nickel-coated copper core-shell nanoparticles with small and uniform size are prepared, and the preparation of the sintered interconnection joint with a good bonding interface and high shear strength is realized through an ultrasonic-assisted sintering process.

Description

Technical Field

[0001] The present invention relates to the technical field of micro-connection and electronic packaging, and in particular to a method for realizing reliable mounting of power chips by low-temperature rapid sintering. Background Art

[0002] As Moore's Law for semiconductors approaches its physical limits, especially with the third-generation semiconductor industry booming, the miniaturization of components, high current / voltage, and high-temperature service characteristics have become key factors restricting the widespread application of high-power devices in automotive electronics, 5G communication base stations, aerospace, and power electronics equipment. However, the interconnection materials and technologies for chip mounting of high-power devices face enormous challenges. Traditional tin-based solders generally have a low melting point (below 250°C), making them difficult to adapt to the high-temperature, high-power, and long-term service requirements of power chips. In contrast, metal nanoparticle materials, mainly silver and copper, have significant advantages in power device chip mounting due to their nanoscale effects, high surface activity, good electrical and thermal conductivity, and high-temperature service characteristics. Among them, copper nanoparticles have a lower price than silver and higher resistance to chemical electromigration, making them the most promising low-temperature sintering interconnection material. However, there are still certain problems with copper nanoparticles themselves: on the one hand, copper nanoparticles themselves have weak antioxidant ability, and the copper oxide generated after oxidation will seriously reduce the electrical and thermal conductivity and service reliability of the interconnection joints; on the other hand, compared with silver nanoparticles, the sintering conditions of copper nanoparticles are more stringent, and often require sintering at higher temperatures (above 250°C), for a long time, and in a high vacuum or inert atmosphere. Summary of the Invention

[0003] The main purpose of the present invention is to provide a method for achieving reliable power chip mounting by low-temperature rapid sintering, aiming to improve the traditional copper nanoparticle preparation method and sintering technology, generate small and uniformly sized oxidation-resistant nickel-coated copper core-shell nanoparticles by spontaneous growth, and further improve the stability of the copper nanoparticles by forming an amine organic coating layer on the surface of the nickel shell. Ultrasonic-assisted sintering is then performed using a self-made ultrasonic welding platform, combining the ultrasonic friction effect and the rapid diffusion of copper and nickel elements to improve the strength and reliability of the sintered interconnection joints, thereby solving the difficult problems of low-temperature rapid connection and high-temperature reliable service in high-power device chip mounting.

[0004] In order to achieve the above object, the present invention proposes a method for achieving reliable mounting of power chips by low-temperature rapid sintering, the method comprising the following steps:

[0005] Step S10, synthesis of antioxidant nickel-coated copper core-shell nanoparticles: copper salt and nickel salt are dispersed in oleylamine respectively, and heated and stirred to obtain a dark blue solution and a dark green solution; after mixing the two solutions, an excess of a high-carbon fatty alcohol is quickly added, and the mixture is heated to 150-170° C. with stirring and maintained for 5-10 minutes to obtain copper nanoclusters; heating is continued to 180-200° C., and after the solution turns into an olive brown, it is maintained for 10-40 minutes to allow a nickel shell to spontaneously grow on the surface of the copper nanoclusters, and the suspension is cooled to room temperature in an ice-water bath to obtain a black suspension; acetone and ethanol are added to the suspension for ultrasonic treatment, and then the suspension is centrifuged and ultrasonically dispersed 4-6 times, and then dried in a vacuum oven to obtain nickel-coated copper core-shell nanoparticle powder;

[0006] Step S20, preparing nickel-coated copper nano-solder paste: placing the nickel-coated copper core-shell nanoparticle powder in a centrifuge tube, and adding 12-15% by mass of an organic solvent thereto; placing the entire centrifuge tube in a planetary centrifugal agitator and mixing uniformly to obtain a viscous nickel-coated copper nano-solder paste;

[0007] Step S30, Printing and Preheating: Nickel-coated copper nano-solder paste is evenly coated on the surface of the heat sink substrate using stencil printing to a thickness of 80-120 μm. The heat sink substrate is then transferred to a heating platform and preheated at 80°C for 10-15 minutes to evaporate the organic solvent in the solder paste.

[0008] Step S40, ultrasonic assisted sintering: After preheating, a power chip is placed on top of a substrate with nano solder paste to form a three-layer mounting structure of "chip-nano solder paste-substrate", which is then transferred to an ultrasonic welding platform for ultrasonic assisted sintering; in order to apply the ultrasonic field, an N-type mold is placed on the mounting structure, the ultrasonic head is directly applied to the mold, and the ultrasonic energy is transmitted to the substrate through the N-type mold; the specific sintering process parameters are: heating temperature of 180-250°C, heating rate of 5°C / min, applied pressure of 2-4MPa, ultrasonic application time of 8-12s, ultrasonic power of 180-200W, and ultrasonic wave is shear wave; after each application of ultrasound, the ultrasonic head needs to be kept for 3s before being lifted; after the ultrasonic head is lifted, the mounting structure is transferred to the side of the fan for heat dissipation, and after removing the N-type mold, a sintered joint with a good mounting interface is obtained.

[0009] A further technical solution of the present invention is that in step S10, the mass ratio of the copper salt to the nickel salt is 1:1.8.

[0010] A further technical solution of the present invention is that the high-carbon fatty alcohol refers to a fatty alcohol containing more than 6 carbon atoms in the chemical formula.

[0011] A further technical solution of the present invention is that the high-carbon fatty alcohol includes one or a mixture of n-heptanol, n-octanol, n-nonanol, and n-decanol.

[0012] A further technical solution of the present invention is that, in step S10, nitrogen gas is introduced during the synthesis of the oxidation-resistant nickel-coated copper core-shell nanoparticles, wherein the gas flow rate is 20 to 40 ml / min.

[0013] A further technical solution of the present invention is that the average particle size of the nickel-coated copper core-shell nanoparticle powder in step S10 is 20-50 nm, and the nickel shell layer is coated with a layer of amine organic structure.

[0014] A further technical solution of the present invention is that the centrifugal stirring operation in step S20 is set to three gears, wherein the centrifugal speed of the first gear is 1200-1500 rpm, the centrifugal speed of the second gear is 800-1200 rpm, and the centrifugal speed of the third gear is 500-800 rpm. Each gear takes 2-3 minutes, and the viscosity of the obtained nickel-clad copper nano solder paste ranges from 1100 to 1400 mPa·s.

[0015] A further technical solution of the present invention is that a plurality of limiting holes are provided on the welding platform and the N-type mold in step S40 to prevent the mounting structure from shifting under the action of lateral ultrasonic waves, thereby reducing the alignment accuracy and sintering quality; the height of the N-type mold is controlled within 6 mm, and the area of ​​the upper surface is controlled within 20×20 mm. 2 To ensure that the ultrasonic energy can be transmitted to the substrate to achieve high-density sintering under the ultrasonic process.

[0016] A further technical solution of the present invention is that in step S40, a layer of graphite paper wrapped in aluminum foil is placed between the N-type mold and the power chip to isolate the damage effect of the ultrasonic wave propagating to the N-type mold on the chip and to evenly apply pressure on the chip.

[0017] A further technical solution of the present invention is that the thickness of the graphite paper is 2 to 4 mm, and the role of the aluminum foil is to prevent the graphite paper from directly contacting the chip, so that the bottom graphite sheet of the graphite paper adheres to the chip, causing chip contamination and requiring additional cleaning operations.

[0018] The beneficial effects of the method of the present invention for achieving reliable mounting of power chips by low-temperature rapid sintering are:

[0019] (1) The technical solution of the present invention improves the preparation method of traditional copper nanoparticles, and prepares small and uniform sized anti-oxidation nickel-coated copper core-shell nanoparticles in one step by spontaneous epitaxial growth, with an average particle size of less than 50 nm. With the help of the passivation effect of the dense nickel shell, the oxidation of copper nanoparticles is significantly inhibited. The oxidation tendency during transfer and storage in the two-step preparation of core-shell nanoparticles is avoided. At the same time, a layer of amine organic coating is generated on the surface of the nickel shell through the coordination reaction of fatty alcohol and solvent. With the help of the hydrophobic effect of the amino group, the agglomeration phenomenon between nanoparticles is effectively hindered, further improving the stability of the copper nanoparticles.

[0020] (2) Compared with the traditional vacuum hot pressing sintering process, the sintering conditions required in this technical solution are milder. Due to the presence of the nickel shell and the amine organic coating layer, the nickel-coated copper nano-solder paste can be sintered in an atmospheric environment without the need for vacuum conditions, significantly reducing production costs. At the same time, the friction effect caused by the ultrasonic energy field and the copper-nickel solid solution diffusion process enable this technical solution to achieve the preparation of sintered interconnects with high shear strength (46.6-60.5MPa) at temperatures as low as 180-250°C, pressures as low as 2-4MPa, and times as low as 8-12s. Its strength significantly exceeds the shear strength of traditional tin-lead solder. There are no defects such as pores and residual organic matter at the bonding interface, which improves the sintering quality.

[0021] (3) The ultrasonic welding device and ultrasonic assisted sintering technology proposed in the present invention are not limited to the size and number of chips, and will not cause damage to the chips. It has a large cross-sectional area (20×20mm 2 ) and tolerance height (2-4mm), providing a practical technical solution for large-volume, large-area, and high-flatness power chip mounting scenarios. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 This is a schematic flow chart of a method for implementing reliable mounting of power chips by low-temperature rapid sintering according to the present invention;

[0023] Figure 2 Schematic diagram of the synthesis process of the oxidation-resistant nickel-coated copper nanoparticles in Example 1 of the present invention;

[0024] Figure 3 The scanning electron microscope / secondary electron microscope (SEM / SE) image and its magnified image of the oxidation-resistant nickel-coated copper nanoparticles in Example 1 of the present invention are shown;

[0025] Figure 4Transmission electron microscopy (TEM) images of the antioxidant nickel-coated copper nanoparticles in Example 1 of the present invention and their surface scanning element distribution maps: (a) bright field image, (b) dark field image, (c) copper element distribution map, (d) nickel element distribution map, (e) copper and nickel element distribution overlay map, (f) carbon element distribution map, (g) oxygen element distribution map, (h) nitrogen element distribution map;

[0026] Figure 5 Schematic diagram of the ultrasonic assisted sintering process in Example 1 of the present invention;

[0027] Figure 6 Figures 1 and 2 show the cross-sectional microstructures of the sintered interconnects obtained under different ultrasonic-assisted sintering processes in Examples 1 and 2 of the present invention: (a) the heating temperature is 180°C, the applied pressure is 2 MPa, and the ultrasonic application time is 10 s; (b) the heating temperature is 250°C, the applied pressure is 2 MPa, and the ultrasonic application time is 10 s.

[0028] Figure 7 Surface scanning element distribution diagrams of the cross-sectional microstructure of the sintered interconnect obtained in Example 2 of the present invention: (a) copper element distribution diagram, (b) nickel element distribution diagram, and (c) carbon element distribution diagram;

[0029] Figure 8 This is a cross-sectional microstructure of the sintered interconnect obtained in Comparative Example 1;

[0030] Figure 9 This is a cross-sectional microstructure of the sintered interconnect obtained in Comparative Example 2;

[0031] Figure 10 This is a comparison chart of the strength of sintered interconnect joints obtained under different sintering processes in shear tests in Examples 1-2 of the present invention and Comparative Examples 1-2.

[0032] Figure 11 This is the energy dispersive spectrometer (EDS) result of the fracture surface of the sintered interconnection joint obtained in Example 2 of the present invention after shear testing.

[0033] Figure 5 Description of the figures in the attached figure:

[0034] Copper substrate 1; chip 2; welding platform 3; N-type mold 4; ultrasonic head 5; limiting hole 6; graphite paper 7; heating device 8; nickel-coated copper nano paste 9.

[0035] In order to make the objectives, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below with reference to the accompanying drawings and embodiments. DETAILED DESCRIPTION

[0036] It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention.

[0037] In order to solve the problems of easy oxidation and harsh sintering process of copper nano solder paste in the existing power chip mounting technology, the present invention proposes a solution.

[0038] Specifically, the present invention proposes a method for achieving reliable mounting of power chips by low-temperature rapid sintering, such as Figure 1 As shown, the method for achieving reliable mounting of power chips by low-temperature rapid sintering of the present invention comprises the following steps:

[0039] Step S10, synthesis of oxidation-resistant nickel-coated copper core-shell nanoparticles, synthesizing oxidation-resistant nickel-coated copper core-shell nanoparticles by spontaneous epitaxial growth: copper salt and nickel salt are dispersed in oleylamine respectively, and heated and stirred to obtain dark blue solution and dark green solution.

[0040] The copper salt can be copper acetate, and the nickel salt can be nickel acetate. The mass ratio of the copper salt to the nickel salt is 1:1.8.

[0041] After mixing the above two solutions, an excess of a high-carbon fatty alcohol is quickly added, and the mixture is stirred and heated to 150-170°C and maintained for 5-10 minutes to obtain copper nanoclusters. Among them, the mixture can be stirred and heated to 150°C, 160°C or 170°C and maintained for 5 minutes, 8 minutes, or 10 minutes to obtain copper nanoclusters. The high-carbon fatty alcohol refers to a fatty alcohol containing more than 6 carbon atoms in the chemical formula. Preferably, in this embodiment, the high-carbon fatty alcohol includes one or more of n-heptanol, n-octanol, n-nonanol, and n-decanol.

[0042] Continue heating to 180-200° C. and keep the solution for 10-40 minutes after it turns into olive brown, so that a nickel shell spontaneously grows on the surface of the copper nanoclusters. Cool to room temperature in an ice-water bath to obtain a black suspension.

[0043] The temperature may be heated to 180° C., 190° C. or 200° C. and maintained for 10 min, 15 min, 20 min or 40 min, so that a nickel shell spontaneously grows on the surface of the copper nanoclusters.

[0044] Acetone and ethanol are added to the suspension for ultrasonic treatment, followed by centrifugal washing and ultrasonic dispersion 4 to 6 times, and then dried in a vacuum oven to obtain nickel-coated copper core-shell nanoparticle powder. The nickel-coated copper core-shell nanoparticle powder has an average particle size of 20 to 50 nm, and the nickel shell is coated with an amine organic structure.

[0045] In step S10, nitrogen gas is introduced during the synthesis of the oxidation-resistant nickel-coated copper core-shell nanoparticles, wherein the gas flow rate is 20-40 ml / min.

[0046] Step S20, preparing nickel-coated copper nano-solder paste, centrifugally stirring the nickel-coated copper nanoparticles and the organic solvent to obtain nickel-coated copper nano-solder paste: placing the above-mentioned nickel-coated copper core-shell nanoparticle powder in a centrifuge tube, and adding 12-15% by mass of an organic solvent thereto; placing the entire centrifuge tube in a planetary centrifugal agitator and mixing evenly to obtain a viscous nickel-coated copper nano-solder paste.

[0047] Wherein, the organic solvent is terpineol.

[0048] The centrifugal stirring operation in step S20 is set to three gears, wherein the centrifugal speed of the first gear is 1200-1500 rpm, the centrifugal speed of the second gear is 800-1200 rpm, and the centrifugal speed of the third gear is 500-800 rpm. Each gear takes 2-3 minutes, and the viscosity of the obtained nickel-clad copper nano solder paste ranges from 1100 to 1400 mPa·s.

[0049] Step S30: Printing and preheating. Nickel-coated copper nanosolder paste is evenly coated on the surface of the heat sink substrate using stencil printing. The substrate is then placed on a heating platform for preheating. The nickel-coated copper nanosolder paste is evenly coated on the surface of the heat sink substrate using stencil printing to a thickness of 80 to 120 μm. The heat sink substrate is then transferred to the heating platform and preheated for 10 to 15 minutes to volatilize the organic solvent in the solder paste. The preheating temperature is 80°C.

[0050] Step S40, ultrasonic assisted sintering, placing a power chip on top of the substrate with nano solder paste, and then transferring it to the ultrasonic welding platform for ultrasonic assisted sintering to obtain a sintered interconnected joint: after preheating treatment, placing a power chip on top of the substrate with nano solder paste to form a three-layer mounting structure of "chip-nano solder paste-substrate", and then transferring it to the ultrasonic welding platform for ultrasonic assisted sintering; in order to apply the ultrasonic field, an N-type mold is placed on the mounting structure, the ultrasonic head is directly applied to the mold, and the ultrasonic energy is transmitted to the substrate through the N-type mold; the specific sintering process parameters are: heating temperature of 180-250°C, heating rate of 5°C / min, applied pressure of 2-4MPa, ultrasonic application time of 8-12s, ultrasonic power of 180-200W, and ultrasonic wave is shear wave; after each application of ultrasound, the ultrasonic head needs to be kept for 3s before lifting; after the ultrasonic head is lifted, the mounting structure is transferred to the side of the fan for heat dissipation, and after removing the N-type mold, a sintered joint with good mounting interface bonding is obtained.

[0051] The welding platform and the N-type mold in step S40 are both provided with multiple limiting holes to prevent the mounting structure from shifting under the action of lateral ultrasonic waves, thereby reducing the alignment accuracy and sintering quality; the height of the N-type mold is controlled within 6 mm, and the area of ​​the upper surface is controlled within 20×20 mm2 to ensure that the ultrasonic energy can be transmitted to the substrate under the ultrasonic process to achieve high-density sintering.

[0052] In step S40, a layer of graphite paper wrapped in aluminum foil is placed between the N-type mold and the power chip to isolate the chip from damage caused by ultrasonic waves propagating to the N-type mold and to apply pressure evenly to the chip.

[0053] The thickness of the graphite paper is 2 to 4 mm, and the aluminum foil is used to prevent the graphite paper from directly contacting the chip, so that the bottom graphite sheet of the graphite paper adheres to the chip, causing chip contamination and requiring additional cleaning operations.

[0054] The method for achieving reliable mounting of power chips by low-temperature rapid sintering of the present invention will be further described in detail below in conjunction with Example 1, Example 2 and Comparative Example 1 and Comparative Example 2.

[0055] Example 1

[0056] This embodiment provides a method for achieving reliable mounting of power chips by low-temperature rapid sintering, including the following steps:

[0057] Step S1, synthesis of antioxidant nickel-coated copper core-shell nanoparticles: 400 mg of copper acetate and 720 mg of nickel acetate were dispersed in 30 ml of oleylamine, respectively, and heated and stirred to obtain a dark blue solution and a dark green solution; after mixing the two solutions, 60 ml of n-octanol was quickly added, and the mixture was heated to 160° C. with stirring and maintained for 10 minutes to obtain copper nanoclusters; the mixture was further heated to 190° C. and maintained for 15 minutes after the solution turned into olive brown, so that a nickel shell spontaneously grew on the surface of the copper nanoclusters. The growth process is as follows: Figure 1 As shown, the suspension was cooled to room temperature in an ice-water bath to obtain a black suspension; 30 ml of acetone and 60 ml of ethanol were added to the suspension for ultrasonic treatment, and then centrifugal washing and ultrasonic dispersion were performed 4 times, and the suspension was placed in a vacuum oven for drying to obtain nickel-coated copper core-shell nanoparticle powder.

[0058] Step S2, Preparation of Nickel-Clad Copper Nanosolder Paste: The nickel-clad copper core-shell nanoparticle powder is placed in a centrifuge tube and 12-15% by mass of terpineol is added. The entire centrifuge tube is then placed in a planetary centrifugal agitator for uniform mixing. The centrifugal agitation is set to three different speeds: the first speed is 1200-1500 rpm; the second speed is 800-1200 rpm; and the third speed is 500-800 rpm. Each speed setting takes 2-3 minutes. A viscous nickel-clad copper nanosolder paste is obtained, having a viscosity of approximately 1200 mPa·s.

[0059] Step S3, Printing and Preheating: Nickel-coated copper nanosolder paste is evenly coated on the heat sink substrate surface via stencil printing to a thickness of 80 μm. The heat sink substrate is then transferred to a heating platform and preheated at 80°C for 10–15 minutes to evaporate the organic solvent in the solder paste.

[0060] Step S4, ultrasonic assisted sintering: After preheating, the power chip 2 is placed on the copper substrate 1 with nickel-coated copper nano-paste 9 to form a three-layer mounting structure of "chip-nano solder paste-substrate", and then transferred to the ultrasonic welding platform 3 for ultrasonic assisted sintering. The sintering process diagram is shown in FIG. Figure 5 As shown. In order to apply the ultrasonic field, an N-type mold 4 is placed on the mounting structure, the ultrasonic head 5 is directly applied to the mold, and the ultrasonic energy is transmitted to the substrate through the N-type mold 4. The specific sintering process parameters are as follows: the heating temperature of the heating device 8 is 180°C, the heating rate is 5°C / min, the applied pressure is 2MPa, the ultrasonic application time is 10s, the ultrasonic power is 200W, and the ultrasonic wave is a transverse wave. After each application of ultrasound, the ultrasonic head 5 needs to be kept for 3s before being lifted. After the ultrasonic head 5 is lifted, the mounting structure is transferred to the side of the fan for heat dissipation. After removing the N-type mold 4, a sintered joint with a good mounting interface is obtained.

[0061] Two limiting holes 6 are provided at both ends of the welding platform 3 and the N-type mold 4 to prevent the mounting structure from shifting under the action of lateral ultrasonic waves, thereby reducing the alignment accuracy and sintering quality.

[0062] The height of the N-type mold 4 is 4 mm, and the area of ​​the upper surface is 10×10 mm. 2 to ensure that the ultrasonic energy can be transmitted to the substrate during the ultrasonic process to achieve high-density sintering.

[0063] A layer of aluminum foil-wrapped graphite paper 7 is placed between the N-type mold 4 and the chip to isolate the chip from any damage caused by ultrasonic waves propagating through the mold 4 and ensure uniform pressure on the chip. The graphite paper 7 is 3 mm thick. The aluminum foil prevents direct contact between the graphite paper 7 and the chip, preventing the underlying graphite sheet from adhering to the chip, contaminating it and requiring additional cleaning.

[0064] This embodiment prepares small and uniform size oxidation-resistant nickel-coated copper core-shell nanoparticles by spontaneous epitaxial growth. Figures 3-4 As shown, the average particle size of nickel-coated copper core-shell nanoparticles is 45nm. From the element distribution diagram of copper and nickel, it can be seen that a dense nickel shell is wrapped around the copper core. From the element distribution diagram of carbon, oxygen and nitrogen, it can be seen that a layer of amine organic structure is also coated on the outside of the nickel shell. This embodiment significantly inhibits the oxidation of copper nanoparticles by virtue of the passivation effect of the dense nickel shell, and effectively avoids the oxidation tendency during transfer and storage in the process of preparing core-shell nanoparticles by the two-step method. At the same time, a layer of amine organic coating is generated on the surface of the nickel shell by the coordination reaction of n-octanol and oleylamine. By virtue of the hydrophobic effect of the amine group, the agglomeration phenomenon between the nanoparticles is effectively hindered, further improving the stability of the copper nanoparticles.

[0065] The cross-sectional structure of the sintered interconnect was observed. Figure 6 As shown, when the sintering temperature is 180℃ ( Figure 6 Under the conditions of a), only a small number of holes and cracks exist inside the interface of the sintered interconnection joint. At the same time, a serrated interface formed by the ultrasonic friction effect can be observed near the interface, further indicating the significant role of the ultrasonic energy field in the sintering process. Finally, the average shear strength of the sintered interconnection joint reached 46.6MPa ( Figure 10 ), which is significantly higher than the shear strength of traditional tin-lead solder (about 27MPa), and has great application prospects in the field of power chip mounting.

[0066] Example 2

[0067] This embodiment provides another method for implementing reliable mounting of power chips by low-temperature rapid sintering. The difference from the first embodiment is that the sintering temperature is 250° C., and the other processes are the same as those in the embodiment.

[0068] Further increase the sintering temperature to 250℃( Figure 6 b) There are no holes or cracks inside the interface, achieving the preparation of sintered interconnects with no interface cracks, low porosity, and high strength. At the same time, the increase in temperature further enhances the ultrasonic friction effect, and the sintered interface is completely composed of a serrated interface. Figure 7As shown in the figure, the distribution diagram of copper, nickel and oxygen shows that the copper and nickel elements are evenly distributed inside the sintering interface, forming a dispersed solid solution structure and does not contain any residual organic matter (no carbon element). This shows that the friction effect of the ultrasonic energy field also effectively removes the organic matter remaining inside the sintering interface and improves the sintering quality. The average shear strength of the final sintered interconnection joint reaches 60.5MPa ( Figure 10 ), compared with Example 1, the shear strength increased by about 30%. The EDS results of its fracture morphology ( Figure 11 ) It can be seen that the fracture position after shearing is inside the sintered layer, and there is no residual organic matter, which further confirms the application of oxidation-resistant nickel-coated copper core-shell nanoparticles and their ultrasonic-assisted sintering technology as microconnections and power chip packaging interconnections.

[0069] Comparative Example 1

[0070] On the basis of Examples 1 and 2, the sintering process of this comparative example is different, mainly in that no ultrasonic energy field is applied, and the sintering and interconnection of nickel-coated copper nanoparticles is achieved only by temperature and pressure.

[0071] The results are as follows Figure 8 and Figure 10 As shown in the figure, even without the application of an ultrasonic energy field, the majority of the nickel-coated copper nanoparticles still exhibited a dispersed spherical morphology. Furthermore, a crack extending through the sintered layer, exceeding 120 μm in length, was present near the sintering interface. This indicates that the sintering process did not achieve a true sintered interconnection. Testing revealed an average shear strength of only 3.2 MPa.

[0072] Comparative Example 2

[0073] On the basis of Examples 1 and 2, the pressure applied in the ultrasonic assisted sintering process of this comparative example is different, mainly in that the applied pressure is 0.2 MPa.

[0074] The results are as follows Figure 9 and Figure 10 As shown, even at an applied pressure of only 0.2 MPa, numerous pores and short-range cracks still exist within the sintered layer. Furthermore, less pronounced jagged cracks appear near the sintering interface, indicating that the frictional effect of the ultrasonic energy field is not significant under these low-temperature and low-pressure conditions. Testing revealed an average shear strength of 34.3 MPa for this sintered joint, significantly lower than that of Examples 1 and 2.

[0075] The oxidation-resistant nickel-coated copper core-shell nanoparticles and the ultrasonic-assisted sintering technology thereof in the present invention can be used as microconnections and power chip packaging interconnections.

[0076] The beneficial effects of the method of the present invention for achieving reliable mounting of power chips by low-temperature rapid sintering are:

[0077] (1) The technical solution of the present invention improves the preparation method of traditional copper nanoparticles, and prepares small and uniform sized anti-oxidation nickel-coated copper core-shell nanoparticles in one step by spontaneous epitaxial growth, with an average particle size of less than 50 nm. With the help of the passivation effect of the dense nickel shell, the oxidation of copper nanoparticles is significantly inhibited. The oxidation tendency during transfer and storage in the two-step preparation of core-shell nanoparticles is avoided. At the same time, a layer of amine organic coating is generated on the surface of the nickel shell through the coordination reaction of fatty alcohol and solvent. With the help of the hydrophobic effect of the amino group, the agglomeration phenomenon between nanoparticles is effectively hindered, further improving the stability of the copper nanoparticles.

[0078] (2) Compared with the traditional vacuum hot pressing sintering process, the sintering conditions required in this technical solution are milder. Due to the presence of the nickel shell and the amine organic coating layer, the nickel-coated copper nano-solder paste can be sintered in an atmospheric environment without the need for vacuum conditions, significantly reducing production costs. At the same time, the friction effect caused by the ultrasonic energy field and the copper-nickel solid solution diffusion process enable this technical solution to achieve the preparation of sintered interconnects with high shear strength (46.6-60.5MPa) at temperatures as low as 180-250°C, pressures as low as 2-4MPa, and times as low as 8-12s. Its strength significantly exceeds the shear strength of traditional tin-lead solder. There are no defects such as pores and residual organic matter at the bonding interface, which improves the sintering quality.

[0079] (3) The ultrasonic welding device and ultrasonic assisted sintering technology proposed in the present invention are not limited to the size and number of chips, and will not cause damage to the chips. It has a large cross-sectional area (20×20mm 2 ) and tolerance height (2-4mm), providing a practical technical solution for large-volume, large-area, and high-flatness power chip mounting scenarios.

[0080] The above description is only a preferred embodiment of the present invention and does not limit the scope of the invention. Any equivalent structure or process transformation made by using the contents of the present invention description and drawings, or directly or indirectly applied in other related technical fields, are also included in the scope of protection of the present invention.

Claims

1. A method for achieving reliable mounting of power chips by low-temperature rapid sintering, characterized in that: The method comprises the following steps: Step S10, synthesis of antioxidant nickel-coated copper core-shell nanoparticles: copper salt and nickel salt are dispersed in oleylamine respectively, and heated and stirred to obtain a dark blue solution and a dark green solution; after mixing the two solutions, an excess of a high-carbon fatty alcohol is quickly added, and the mixture is heated to 150-170° C. with stirring and maintained for 5-10 minutes to obtain copper nanoclusters; heating is continued to 180-200° C., and after the solution turns into an olive brown, it is maintained for 10-40 minutes to allow a nickel shell to spontaneously grow on the surface of the copper nanoclusters, and the suspension is cooled to room temperature in an ice-water bath to obtain a black suspension; acetone and ethanol are added to the suspension for ultrasonic treatment, and then the suspension is centrifuged and ultrasonically dispersed 4-6 times, and then dried in a vacuum oven to obtain nickel-coated copper core-shell nanoparticle powder; Step S20, preparing nickel-coated copper nano-solder paste: placing the nickel-coated copper core-shell nanoparticle powder in a centrifuge tube, and adding 12-15% by mass of an organic solvent thereto; placing the entire centrifuge tube in a planetary centrifugal agitator and mixing uniformly to obtain a viscous nickel-coated copper nano-solder paste; Step S30, Printing and Preheating: Nickel-coated copper nano-solder paste is evenly coated on the surface of the heat dissipation substrate via stencil printing, with a stencil printing thickness of 80-120 μm. The heat dissipation substrate is then transferred to a heating platform and preheated for 10-15 minutes to volatilize the organic solvent in the solder paste. The preheating temperature is 80°C. Step S40, ultrasonic-assisted sintering: After preheating, a power chip is placed on top of a substrate with nano-solder paste to form a three-layer mounting structure of "chip-nano-solder paste-substrate", which is then transferred to an ultrasonic welding platform for ultrasonic-assisted sintering. In order to apply the ultrasonic field, an N-type mold is placed on the mounting structure, and the ultrasonic head is directly applied to the mold, and the ultrasonic energy is transmitted to the substrate through the N-type mold. The specific sintering process parameters are: heating temperature of 180-250°C, heating rate of 5°C / min, applied pressure of 2-4MPa, ultrasonic application time of 8-12s, ultrasonic power of 180-200W, and ultrasonic wave is shear wave. After each application of ultrasound, the ultrasonic head needs to be kept for 3s before being lifted. After the ultrasonic head is lifted, the mounting structure is transferred to the side of the fan for heat dissipation. After removing the N-type mold, a sintered joint with a good mounting interface is obtained.

2. The method for achieving reliable mounting of power chips by low-temperature rapid sintering according to claim 1, characterized in that: In step S10, the mass ratio of the copper salt to the nickel salt is 1:1.

8.

3. The method for achieving reliable mounting of power chips by low-temperature rapid sintering according to claim 1, characterized in that: The high carbon number fatty alcohol refers to a fatty alcohol containing more than 6 carbon atoms in the chemical formula.

4. The method for achieving reliable mounting of power chips by low-temperature rapid sintering according to claim 3, characterized in that: The high carbon number fatty alcohol includes one or a mixture of n-heptanol, n-octanol, n-nonanol and n-decanol.

5. The method for achieving reliable mounting of power chips by low-temperature rapid sintering according to claim 1, characterized in that: In step S10, nitrogen gas is introduced during the synthesis of the oxidation-resistant nickel-coated copper core-shell nanoparticles, wherein the gas flow rate is 20-40 ml / min.

6. The method for achieving reliable mounting of power chips by low-temperature rapid sintering according to claim 1, characterized in that: In the step S10, the average particle size of the nickel-coated copper core-shell nanoparticle powder is 20-50 nm, and the nickel shell layer is coated with a layer of amine organic structure.

7. The method for achieving reliable mounting of power chips by low-temperature rapid sintering according to claim 1, characterized in that: The centrifugal stirring operation in step S20 is set to three gears, wherein the centrifugal speed of the first gear is 1200-1500 rpm, the centrifugal speed of the second gear is 800-1200 rpm, and the centrifugal speed of the third gear is 500-800 rpm. Each gear takes 2-3 minutes, and the viscosity of the obtained nickel-clad copper nano solder paste ranges from 1100 to 1400 mPa·s.

8. The method for achieving reliable mounting of power chips by low-temperature rapid sintering according to claim 1, characterized in that: The welding platform and the N-type mold in step S40 are both provided with a plurality of limiting holes to prevent the mounting structure from shifting under the action of lateral ultrasonic waves, thereby reducing the alignment accuracy and sintering quality; the height of the N-type mold is controlled within 6 mm, and the area of ​​the upper surface is controlled within 20×20 mm. 2 To ensure that the ultrasonic energy can be transmitted to the substrate to achieve high-density sintering under the ultrasonic process.

9. The method for achieving reliable mounting of power chips by low-temperature rapid sintering according to claim 1, characterized in that: In step S40, a layer of graphite paper wrapped in aluminum foil is placed between the N-type mold and the power chip to isolate the chip from damage caused by ultrasonic waves propagating to the N-type mold and to apply pressure evenly to the chip.

10. The method for achieving reliable mounting of power chips by low-temperature rapid sintering according to claim 9, characterized in that: The thickness of the graphite paper is 2-4 mm. The aluminum foil is used to prevent the graphite paper from directly contacting the chip, so that the bottom graphite sheet of the graphite paper adheres to the chip, causing chip contamination and requiring additional cleaning operations.

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