Mixed soldering paste for power device chip bonding and preparation method and application thereof

Abstract

The invention discloses a mixed soldering paste for power device chip bonding and a preparation method and application of the mixed soldering paste, the mixed soldering paste comprises mixed particles and soldering flux, and the mixed particles are composed of micron Cu-coated Ag core-shell particles, nano Ag and micron spherical graphite. The used materials have excellent electric conduction and heat conduction performance, meanwhile, due to the fact that the nano Ag particles are large in specific surface area, high in surface activity and large in surface atomic diffusion coefficient and generate the size effect, connection can be achieved at the low temperature, the size effect disappears after sintering, the normal melting point is recovered, and high-temperature service can be achieved. Meanwhile, the nano Ag particles can effectively fill pores between the micron Cu-coated Ag and the micron carbon spheres in the sintering process, and by means of mixed use of micro-nano materials, the sintered welding seam can have good density. In addition, the micron carbon balls can prevent the nano-silver component from being excessively sintered, the thermal stress generated by the nano-silver component in the high-temperature service process is relieved, and the reliability of the mixed soldering paste is improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of electronic packaging micro-interconnection, and relates to a solder paste, in particular to a hybrid solder paste for power device chip bonding, its preparation method and application. Background Art

[0002] With the rapid development of the modern electronics industry, electronic components are gradually becoming smaller and more power-intensive. At the same time, with the rise of power semiconductors, especially the emergence of third-generation semiconductor materials, devices need to meet requirements such as high thermal stability, high breakdown voltage, and high-temperature service. Therefore, high-temperature solders first need to have a suitable melting temperature to meet the production and assembly of components; in addition, the formed solder joints should also have high creep resistance, good thermal fatigue resistance, and good electrical and thermal conductivity, while common tin-based solders do not meet the new requirements of high-temperature solders. Among new high-temperature solders, nanomaterials can be sintered at lower temperatures due to their size effect and have high reliability at high temperatures. Nano-Ag particles have excellent thermal, electrical, mechanical, corrosion-resistant, and oxidation-resistant properties and are widely used in the packaging interconnection of wide-bandgap semiconductor devices. However, there are electrochemical migration problems in sintered silver, which may cause short-circuit failures of electronic devices, seriously affecting the reliability of products. At the same time, during sintering and service, Ag nano-solders are prone to over-sintering, and their solder joints will further densify at high temperatures, generating high thermal stress in the solder joints and ultimately leading to interface cracking. Cu nano-particles have significant advantages in terms of cost and anti-electromigration, but their spontaneous oxidation will increase the sintering temperature and also affect the mechanical strength and electrical and thermal conductivity of the solder joints, having a significant impact on the sintering quality.

[0003] To solve the above problems, some researchers have prepared micron In-Cu@In core-shell structures to effectively overcome the easy oxidation property of Cu and effectively reduce the welding temperature. Regarding the electromigration problem of Ag, some researchers have proposed Cu@Ag core-shell structures, AgCu solid solution structures, AgIn mixed particles, etc., but none of them can fully overcome the electromigration short-circuit problem of Ag, and some methods will greatly reduce the properties such as silver conductivity. Regarding the problem that Ag nano-solders are prone to over-sintering, none of the above methods can effectively solve it, and there is also less research in this area currently. Summary of the Invention

[0004] To solve the problems existing in the background art, the present invention provides a hybrid solder paste for power device chip bonding, its preparation method and application.

[0005] The object of the present invention is achieved through the following technical solutions:

[0006] A hybrid solder paste for power device chip bonding, comprising hybrid particles and a flux, wherein:

[0007] The mixed particles are composed of 56.61 - 70.05 wt.% of micron Cu@Ag core-shell particles, 3.39 - 28.95 wt.% of nano-Ag, and 1 - 20 wt.% of micron spherical graphite;

[0008] The mass ratio of the mixed particles to the soldering flux is 1 - 3:1;

[0009] The soldering flux is one of polyethylene glycol - 400, UV223, MK504, RMA223, etc.;

[0010] The particle size of the micron Cu@Ag core-shell particles is 10 - 45 microns, and can be 10μm, 20μm, 30μm, 45μm;

[0011] The particle size of the nano-Ag is 10 - 20 nm, and can be 10 nm, 20 nm;

[0012] The particle size of the micron spherical graphite is 4 - 6 microns, and can be 4μm, 5μm, 6μm.

[0013] A preparation method of the above-mentioned mixed solder paste for power device chip bonding includes the following steps:

[0014] Step 1: Mix the micron Cu@Ag, nano-Ag, and micron C particles evenly to obtain a Cu@Ag-Ag-C mixed powder. The specific mixing method is as follows: Add the micron Cu@Ag core-shell particles, nano-Ag particles, and micron C particles into an anhydrous ethanol solution, and ultrasonically vibrate to mix them evenly. Then remove the supernatant and vacuum-dry the precipitate to obtain the mixed powder;

[0015] Step 2: Add the soldering flux to the mixed powder and mix it evenly to obtain a mixed solder paste containing micron Cu@Ag particles, nano-Ag particles, and micron C particles.

[0016] An application of the above-mentioned mixed solder paste in power device chip bonding specifically includes the following steps:

[0017] Step S1: Apply the mixed solder paste onto the substrate by screen printing, and place the chip above the mixed solder paste to form a sandwich structure, where:

[0018] Before using the substrate, remove the surface oxide with dilute hydrochloric acid, remove the oil stain on the substrate surface with anhydrous ethanol, then dry the substrate and polish it to further remove the surface oxide film. Place the treated substrate in a vacuum drying oven and store it at room temperature under a vacuum condition of -1 MPa for standby;

[0019] The coating thickness of the mixed solder paste is 50 - 150μm;

[0020] Step S2: Preheat the sandwich structure in a constant-temperature drying oven to remove the contaminants between the chip, solder paste and substrate, volatilize the organic matter part, avoid generating excessive pores during the subsequent welding process, and at the same time reduce the welding stress. Then, perform laser welding or thermocompression bonding on it, where:

[0021] The preheating temperature is 100 - 150 °C, and the preheating time is 10 - 30 min;

[0022] The specific method of thermocompression bonding is: put the assembled sandwich structure into a hot press for bonding. The heating method is bilateral heating. Control the temperature rise rate to be 5 °C / min, the pressure to be 2 - 8 MPa, the heating temperature to be 150 - 350 °C, and the heating time to be 20 - 60 min;

[0023] The specific method of laser welding is: apply pressure to the assembled sandwich structure sample and place it in a laser micro - solder - joint welding device for rapid bonding. Control the heating power to be 75 - 125 W, the heating time to be 15 - 30 s, the applied pressure to be 2 - 8 Mpa, and the defocus range to be 0 - 1.2 cm.

[0024] Compared with the prior art, the present invention has the following advantages:

[0025] (1) The materials used in the present invention all have excellent electrical and thermal conductivity. At the same time, due to the large specific surface area, high surface activity, and large surface atomic diffusion coefficient of nano - Ag particles, a size effect is generated. Therefore, it can be connected at a lower temperature, and after sintering, the size effect disappears and it returns to the normal melting point, thus being able to serve at high temperature. Therefore, it can achieve low - temperature connection and high - temperature service. At the same time, nano - Ag particles can effectively fill the pores between micron Cu@Ag and micron carbon spheres during the sintering process. The mixed use of micro - nano materials can make the sintered weld seam have good density.

[0026] (2) Electromigration short - circuit occurs because the migration rate of Ag is very high. When the device is working, the Ag material will continuously dissolve on one side of the solder joint and precipitate at the adjacent solder joint, resulting in a path being formed between two adjacent solder joints and causing short - circuit failure. And the migration rate of Ag is 1000 times that of Cu. Therefore, the Cu@Ag core - shell structure can effectively inhibit the occurrence of electromigration short - circuit failure, and at the same time overcome the oxidation problem of Cu and improve the packaging reliability.

[0027] (3) Nano - Ag paste has excellent sintering performance, but the solder joint will be further densified at high temperature, generating high thermal stress in the solder joint and ultimately leading to interface cracking. In the present invention, by doping carbon into the nano - Ag paste, the carbon spheres will prevent the excessive sintering of the silver component, relieve the thermal stress generated by the nano - silver component during high - temperature service, and improve the reliability of the mixed solder paste.

[0028] (4) The density differences between micron Cu@Ag particles, nano-Ag particles, and micron C particles are too large. Therefore, it is very difficult for carbon spheres to be evenly distributed during the mixing process and they are prone to agglomeration. In the present invention, the mixed particles are added to an absolute ethanol solution, and ultrasonic vibration stirring is carried out to make them evenly mixed. After drying, uniform mixed particles are obtained. Finally, after sintering, the carbon spheres in the weld are evenly distributed without agglomeration.

[0029] (5) The present invention can be preferably applied to fields such as the manufacture of high-power semiconductor devices, microelectronics packaging, and power electronics packaging. Description of the Drawings

[0030] Figure 1 It is the morphology diagram and EDS line scan diagram of the micron Cu@Ag core-shell structure prepared by electroless plating method in Example 1, (a) SEM morphology diagram, (b) EDS line scan diagram;

[0031] Figure 2 It is a schematic diagram of the mixed solder paste of Cu@Ag particles, nano-Ag particles, and micron C particles prepared in Example 1;

[0032] Figure 3 It is a schematic diagram of the thermocompression bonding using the mixed solder paste of Cu@Ag particles, nano-Ag particles, and micron C particles in Example 1;

[0033] Figure 4 It is the morphology diagram of the solder joint formed by the mixed solder paste of Cu@Ag particles, nano-Ag particles, and micron C particles and the copper substrate in Example 1;

[0034] Figure 5 It is the surface scan diagram of the solder joint formed by the mixed solder paste of micron Cu@Ag particles, nano-Ag particles, and micron C particles and the copper substrate in Example 1. (b), (c), and (d) are the surface scan diagrams of Cu, Ag, and C elements in sequence;

[0035] Figure 6 It is the comparison diagram of the shear strength of the mixed material solder paste of micron Cu@Ag particles, nano-Ag particles, and micron C particles with different ratios in Example 1, Example 3 - 8;

[0036] Figure 7 It is a schematic diagram of the laser rapid connection using the mixed material solder paste of micron Cu@Ag particles, nano-Ag particles, and micron C particles in Example 14. Detailed Embodiments

[0037] The technical solutions of the present invention will be further described below in conjunction with the embodiments, but it is not limited thereto. Any modification or equivalent replacement of the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention shall be covered within the protection scope of the present invention.

[0038] Example 1:

[0039] This example provides a micron Cu@Ag core-shell, micron C, and nano Ag material hybrid solder paste material interconnection process. The micron Cu@Ag core-shell material is prepared by electroless plating and mixed with micron C particles and nano Ag particles, and an appropriate amount of polyethylene glycol-400 is added to finally obtain a hybrid material solder paste of micron Cu@Ag core-shell, micron C, and nano Ag. The hybrid material solder paste of micron Cu@Ag core-shell, micron C, and nano Ag is placed on the substrate by screen printing, and the chip, hybrid solder paste, and substrate are assembled into a sandwich structure to obtain an overall device. The overall device is connected under a certain pressure to obtain an interconnected device. The specific implementation steps are as follows:

[0040] Step 1. Preparation of the micron Cu@Ag core-shell material: First, perform pre-plating treatment on the micron Cu particles to remove the surface oxide film and oil, and then obtain a micron Cu@Ag particle solution through electroless plating. The specific steps are as follows: (1) Disperse the micron Cu particles in dilute hydrochloric acid, absolute ethanol, and deionized water in sequence, and remove the organic matter and oxides on the surface of the micron Cu particles through ultrasonic treatment and high-speed centrifugal cleaning; (2) Weigh 0.06 mol of the cleaned micron Cu particles, disperse them into 100 ml of ethylene glycol solvent through ultrasonic waves, and make the copper evenly dispersed through magnetic stirring; (3) Under light-shielding conditions, dissolve 0.001 mol of AgNO3 in 60 ml of ethylene glycol and continuously apply magnetic stirring to accelerate dissolution; (4) Heat the two obtained mixed solutions to 40 °C on a magnetic stirrer, slowly add all the AgNO3 organic solution to the suspension of micron Cu particles, continuously apply magnetic stirring while maintaining 40 °C, and continuously react for 45 minutes to obtain a micron Cu@Ag particle solution. After ultrasonic treatment, centrifuge and separate, and vacuum-dry the precipitate to obtain micron Cu@Ag particles. The morphology diagram is as shown in Figure 1 (a). It can be seen that the surface of the core-shell is relatively uniform. Combining with Figure 1 (b) the line scan image, it can be clearly seen that the copper particles have been completely coated by the silver shell, and the thickness of the silver shell is relatively uniform.

[0041] Step 2. Preparation of the hybrid solder paste of micron Cu@Ag core-shell, micron C, and nano Ag materials: Weigh a certain amount of dry micron C particles, nano Ag particles, and micron Cu@Ag core-shell particles, add them to an absolute ethanol solution, and ultrasonically vibrate to make them uniformly mixed. Place them in a vacuum drying oven to dry to obtain a mixed powder. Then, uniformly mix the dried mixed powder with polyethylene glycol-400 according to a mass ratio of 2:1 to obtain a hybrid solder paste. The schematic diagram of the hybrid solder paste is as shown in Figure 2As shown, micron C is evenly distributed between micron Cu@Ag core-shell particles and nano-Ag particles. In the mixed powder, the mass percentages of micron Cu@Ag core-shell particles, nano-Ag, and micron C particles are 69.35%, 28.65%, and 2% respectively. The particle size of the nano-Ag particles is 20 nm, the particle size of the micron C particles is 6 μm, and the particle size of the micron Cu@Ag core-shell particles is 20 μm.

[0042] Step 3. Substrate treatment: First, use acetone to remove the oil stains on the substrate surface, and then use dilute hydrochloric acid to remove the surface oxides. After drying, the cleaned copper substrate of the substrate is polished successively with 1000# and 2000# sandpapers, which can further remove the surface oxide film and increase the surface roughness, helping the wetting of the solder paste during the soldering process. Place the treated substrate in a vacuum drying oven and store it at room temperature under a vacuum condition of -1 MPa.

[0043] Step 4. Addition of the mixed solder paste: Add the mixed solder paste onto the substrate treated in Step 3 by screen printing. The thickness of the solder paste is 50 μm. Assemble the chip, the solder paste, and the substrate into a sandwich structure and place it in a constant-temperature drying oven. Preheat it at 100 °C for 10 min to remove the contaminants between the chip, the solder paste, and the substrate, avoid the formation of excessive pores in the connecting tissue, and at the same time reduce the soldering stress.

[0044] Step 5. Place the assembled sandwich structure into a hot press for connection. The schematic diagram of the thermocompression bonding is as Figure 3 shown, where: The heating method adopts bilateral heating. Compared with unilateral heating, the specimen is heated more evenly, which can reduce the thermal stress. Control the temperature rise rate at 5 °C / min, the pressure at 8 MPa, the heating temperature at 300 °C, and the heating time at 30 min. The morphology diagram of the solder joint formed by the mixed solder paste and the copper substrate after thermocompression bonding is as Figure 4 shown, and the combined surface scan diagram is as Figure 5 shown. It can be seen that the black part in the morphology diagram is micron C. It can be seen that micron C is evenly distributed and there is no agglomeration. The connection between micron Cu@Ag particles, micron C particles, and nano-Ag is dense. Through Figure 6 it can be known that its shear strength is very high, reaching 85.49 MPa.

[0045] Example 2:

[0046] The difference between this example and Example 1 is that in Step 2, the dried mixed powder is put into a preform mold, gently oscillated and shaken to make the powder evenly spread at the bottom of the mold, and pressure is applied above the mold and held for a certain time to obtain a preform of the mixed material of Cu@Ag particles, nano-Ag particles, and micron C particles.

[0047] Example 3:

[0048] The difference between this embodiment and Embodiment 1 is that in Step 2, the mass percentage contents of Cu@Ag particles, nano-Ag particles, and micron C particles are 70.05%, 28.95%, and 1% respectively. The shear strength is as Figure 6 shown, being 69.48 MPa.

[0049] Embodiment 4:

[0050] The difference between this embodiment and Embodiment 1 is that in Step 2, the mass percentage contents of Cu@Ag particles, nano-Ag particles, and micron C particles are 68.64%, 28.36%, and 3% respectively. As Figure 6 shown, the shear strength is 80.41 MPa.

[0051] Embodiment 5:

[0052] The difference between this embodiment and Embodiment 1 is that in Step 2, the mass percentage contents of Cu@Ag particles, nano-Ag particles, and micron C particles are 67.93%, 28.07%, and 4% respectively. As Figure 6 shown, the shear strength is 73.58 MPa.

[0053] Embodiment 6:

[0054] The difference between this embodiment and Embodiment 1 is that in Step 2, the mass percentage contents of Cu@Ag particles, nano-Ag particles, and micron C particles are 67.22%, 27.78%, and 5% respectively. As Figure 6 shown, the shear strength is 63.43 MPa.

[0055] Embodiment 7:

[0056] The difference between this embodiment and Embodiment 1 is that in Step 2, the mass percentage contents of Cu@Ag particles, nano-Ag particles, and micron C particles are 63.68%, 26.32%, and 10% respectively. As Figure 6 shown, the shear strength is 37.39 MPa.

[0057] Embodiment 8:

[0058] The difference between this embodiment and Embodiment 1 is that in Step 2, the mass percentage contents of Cu@Ag particles, nano-Ag particles, and micron C particles are 56.61%, 23.39%, and 20% respectively. As Figure 6 shown, the shear strength is 18.09 MPa.

[0059] Embodiment 9:

[0060] The difference between this embodiment and Embodiments 1 - 8 is that in Step 5, the heating temperature is 300 °C; the heating time is 60 min.

[0061] Embodiment 10:

[0062] The difference between this embodiment and Embodiment 9 is that in Step 5, the heating temperature is 350 °C and the heating time is 60 min.

[0063] Embodiment 11:

[0064] The difference between this embodiment and Embodiment 9 is that in Step 5, the heating temperature is 350 °C and the heating time is 30 min.

[0065] Embodiment 12:

[0066] The difference between this embodiment and Embodiment 9 is that in Step 5, the heating temperature is 250 °C and the heating time is 60 min.

[0067] Embodiment 13:

[0068] The difference between this embodiment and Embodiment 9 is that in Step 5, the heating temperature is 250 °C and the heating time is 30 min.

[0069] Embodiment 14:

[0070] The difference between this embodiment and Embodiments 1 - 8 is that in Step 5, a laser micro - solder joint welding device is used for rapid connection, the heating power is controlled at 100 W, the sintering time is 15 s, the applied pressure is 2 MPa, and the defocus amount is 1.0 cm. The laser connection schematic diagram is as Figure 7 shown, where the load is realized through an iron ring, and the laser passes through the glass and is applied to the surface of the copper substrate, thereby realizing rapid heating and sintering.

[0071] Embodiment 15:

[0072] The difference between this embodiment and Embodiment 14 is that in Step 5, the heating power is controlled at 100 W, the sintering time is 20 s, the applied pressure is 2 MPa, and the defocus amount is 1.0 cm.

[0073] Embodiment 16:

[0074] The difference between this embodiment and Embodiment 14 is that in Step 5, the heating power is controlled at 125 W, the sintering time is 15 s, the applied pressure is 2 MPa, and the defocus amount is 1.0 cm.

[0075] Embodiment 17:

[0076] The difference between this embodiment and Embodiment 14 is that in Step 5, the heating power is controlled at 125 W, the sintering time is 20 s, the applied pressure is 2 MPa, and the defocus amount is 1.0 cm.

[0077] Embodiment 18:

[0078] The difference between this embodiment and Embodiments 1-17 is that: in Step 2, the soldering flux is UV223, MK504 or RMA223, and the mass ratio of the mixed particles to the soldering flux is 1:1.

[0079] Embodiment 19:

[0080] The difference between this embodiment and Embodiment 18 is that: the mass ratio of the mixed particles to the soldering flux is 3:1.

Claims

1. A hybrid solder paste for power device chip bonding, characterized in that The mixed solder paste includes mixed particles and a flux, wherein: The mixed particles are composed of 56.61 - 70.05 wt.% of micron Cu@Ag core-shell particles, 3.39 - 28.95 wt.% of nano-Ag, and 1 - 20 wt.% of micron spherical graphite; The mass ratio of the mixed particles to the flux is 1 - 3:

1.

2. The hybrid solder paste for power device chip bonding according to claim 1, characterized in that The flux is one of polyethylene glycol-400, UV223, MK504, and RMA223.

3. The hybrid solder paste for bonding power device chips according to claim 1, wherein The particle size of the micron Cu@Ag core-shell particles is 10 - 45 microns, the particle size of the nano-Ag is 10 - 20 nm, and the particle size of the micron spherical graphite is 4 - 6 μm.

4. A method for preparing a hybrid solder paste for power device chip bonding according to any one of claims 1-3, characterized in that The method includes the following steps: Step 1: Mix the micron Cu@Ag, nano-Ag, and micron C particles evenly to obtain a Cu@Ag-Ag-C mixed powder; Step 2: Add the flux to the mixed powder and mix evenly to obtain a mixed solder paste containing micron Cu@Ag particles, nano-Ag particles, and micron C particles.

5. The preparation method of the hybrid solder paste for power device chip bonding according to claim 4, wherein The specific mixing method in Step 1 is as follows: Add the micron Cu@Ag core-shell particles, nano-Ag particles, and micron C particles into an anhydrous ethanol solution, and ultrasonically vibrate to mix them evenly. Then remove the supernatant and vacuum-dry the precipitate to obtain the mixed powder.

6. Application of the mixed solder paste for power device chip bonding according to any one of claims 1 - 3 in power device chip bonding.

7. Use of the hybrid solder paste for bonding power device chips according to claim 6 in bonding power device chips, characterized in that The power device chip bonding includes the following steps: Step S1: Apply the mixed solder paste onto the substrate by screen printing, and place the chip above the mixed solder paste to form a sandwich structure; Step S2: Place the sandwich structure in a constant-temperature drying oven for preheating to remove contaminants between the chip, solder paste, and substrate, volatilize part of the organic matter, avoid generating excess pores during subsequent welding, and reduce welding stress at the same time. Then perform laser welding or thermocompression bonding on it.

8. Use of the hybrid solder paste for power device chip bonding according to claim 7 in power device chip bonding, characterized in that The coating thickness of the mixed solder paste is 50 - 150 μm; the preheating temperature is 100 - 150 °C, and the preheating time is 10 - 30 min.

9. Use of the hybrid solder paste for bonding power device chips according to claim 7 in bonding power device chips, characterized in that The specific method of thermocompression bonding is: Place the assembled sandwich structure in a hot press for connection. The heating method is bilateral heating, control the temperature rise rate to be 5 °C / min, the pressure is 2 - 8 MPa, the heating temperature is 150 - 350 °C, and the heating time is 20 - 60 min.

10. Use of the hybrid solder paste for power device chip bonding according to claim 7 in power device chip bonding, characterized in that The specific method of laser welding is: Apply pressure to the assembled sandwich structure sample and place it in a laser micro-solder joint welding device for rapid connection. Control the heating power to be 75 - 125 W, the heating time to be 15 - 30 s, the applied pressure to be 2 - 8 Mpa, and the defocus amount range to be 0 - 1.2 cm.

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