Method for preparing three-dimensional porous copper foam, packaging soldering sheet and semiconductor packaging method

Abstract

The application provides a preparation method of three-dimensional porous copper foam, a packaging solder piece and a semiconductor packaging method. The preparation method of the three-dimensional porous copper foam comprises the following steps: step S1, preparing the copper foam; step S2, heating the copper foam in an oxygen-containing atmosphere to generate an oxide layer on the pore wall surface of the copper foam, and obtaining the copper foam after oxidation; step S3, compressing the copper foam after oxidation to form a dense sheet-shaped copper foam; and step S4, removing the oxide layer and leaving the pore channel to obtain the copper foam with a three-dimensional porous structure. The pore wall surface of the copper foam obtained by the technical scheme of the application has a plurality of pore channels and a low Young's modulus, and can solve the problem of mismatch of the thermal expansion coefficient at the interface of the solder layer, greatly improve the reliability of the device during the thermal cycle in the service process, reduce the Young's modulus of the solder layer, relieve the brittle hardness of the solder layer, reduce the thermal fatigue, and resist frequent thermal shock during the operation of the device, and provide reliable connection strength to resist physical impact.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of electronic packaging, and particularly relates to a preparation method of three-dimensional porous copper foam, a packaging soldering sheet and a semiconductor packaging method. BACKGROUND

[0002] In order to fully tap the potential of the third generation semiconductor devices, a packaging integration process matched therewith needs to be provided. High power density packaging integration is a core guarantee for current and future SiC chips to exert their excellent performance. However, the current mature Si-based packaging integration technology is difficult to meet the service requirements of SiC devices under the action of extreme harsh conditions such as high field, high frequency and high temperature. The evolution of power electronic technology has always been towards the direction of high efficiency, high power density and integration, among which power semiconductor devices play a crucial role in this innovation. In applications below 600V, silicon metal oxide semiconductor field effect transistors (MOSFETs) dominate the market, while super junction metal oxide semiconductor field effect transistors (SJ-MOSFETs) and silicon insulated gate bipolar transistors (IGBTs) occupy the mainstream market from 600V to 6.5kV. Although Si power devices have made significant progress in the past few decades, in future development, as the performance of Si power devices is limited by physical limits such as carrier mobility, breakdown field, etc., further improvement becomes increasingly difficult. At the same time, due to the bipolar current conduction mechanism of IGBT, its switching speed is relatively slow, and therefore it is limited to low switching frequency applications.

[0003] On the other hand, the main chip mounting methods in current high service temperature power devices are nano-silver sintering, transient liquid phase bonding (TLP) and copper sintering. Nano-silver sintering is considered to be a potential high-temperature packaging material, which can realize low-temperature sintering, and the obtained solder joint can withstand higher service temperature, but the sintering seam contains a large number of pores, which not only affects the conductivity and heat dissipation, but also is extremely easy to become a source of micro-crack initiation and expansion, and there are also problems such as electromigration, etc. In addition, due to the large difference in thermal expansion coefficient between silver and copper, potential threats or fatal damage may be caused to subsequent service during thermal service. Transient liquid phase bonding can also achieve the purpose of low-temperature welding of solder joints resistant to high temperature, but due to the limitations of the process itself, in order to quickly obtain IMC products, the solder seam is very thin (≤30μm), and the thin full intermetallic compound seam has poor stress and strain absorption capacity, and the reliability is not high. Compared with silver paste, copper paste has another advantage because it can be better sintered on Cu or Ni surfaces, and these materials are often used as under bump metallization (UBM) layers of power device packaging modules.

[0004] However, the Young's modulus of sintered copper is too high to serve in the environment of thermal shock or thermal cycle. In the failure of package level, solder layer fatigue will lead to solder layer fracture, causing local thermal resistance of the chip, resulting in internal temperature difference of the chip, thus accelerating the overall failure of the device. That is, in the high and low temperature cycle of the device, the hysteresis phenomenon of stress and strain makes the solder layer prone to thermal fatigue, which easily leads to the failure of the power semiconductor device. SUMMARY

[0005] In view of the above technical problems, the present application discloses a preparation method of three-dimensional porous foam copper, a packaging solder sheet and a semiconductor packaging method. The foam copper has a three-dimensional porous structure, and the pore wall surface has a channel, so that it has a large surface area, a low Young's modulus, and in the welding process, the channel on the pore wall surface can provide more diffusion channels, thereby promoting sintering.

[0006] To this end, the technical solution adopted by the present application is as follows:

[0007] A preparation method of three-dimensional porous foam copper, comprising the following steps:

[0008] Step S1, preparing foam copper;

[0009] Step S2, heating the foam copper in an oxygen-containing atmosphere to form an oxide layer on the pore wall surface, to obtain oxidized foam copper;

[0010] Step S3, compressing the oxidized foam copper into a dense sheet of foam copper;

[0011] Step S4, removing the oxide layer to leave a channel, to obtain foam copper with a three-dimensional porous structure.

[0012] By using the oxide layer as a sacrificial layer, and forming a plurality of channels on the pore wall surface by removing the oxide layer, the foam copper has a three-dimensional porous structure and a large surface area, which allows more metal atoms to participate in the sintering reaction. In the welding process, the pore structure can provide more diffusion channels, thereby promoting sintering. In addition, the width of the channel can be controlled by adjusting the thickness of the oxide layer.

[0013] As a further improvement of the present application, step S1 includes: plating copper on the polyurethane foam in a copper plating solution, drying, and then annealing at 680-700℃ in a nitrogen-hydrogen mixed gas to obtain foam copper. By annealing at 680-700℃, the stress of the foam copper can be adjusted, and the polyurethane foam can be burned off to obtain foam copper.

[0014] By adopting the technical scheme, the foamed copper prepared from the polyurethane foam material has excellent oxygen transfer capacity when it is subjected to chemical reaction. The CuO crystal lattice contains oxygen vacancies, and the external oxygen participates in the reaction and enters the oxygen vacancies, which can promote the near-surface area strengthening and reduce the surface wrinkles caused in the thermal cycle process.

[0015] As a further improvement of the present application, the density of the foamed copper is 600 g / m 2 ~ 2400 g / m 2 . Preferably, the density of the foamed copper is 600 g / m 2 .

[0016] As a further improvement of the present application, the porosity of the polyurethane foam is ≥ 90%.

[0017] As a further improvement of the present application, before the copper plating, the polyurethane foam is subjected to oil removal and cleaning, and then is subjected to roughening treatment. Further, the polyurethane foam material is soaked in a commercially available oil removal liquid at a temperature of 40 ~ 60℃ for 15 min, and then is washed clean with distilled water.

[0018] Further, the roughening treatment is once or twice.

[0019] Further, the polyurethane foam material after oil removal is placed in a first roughening liquid for roughening once, the roughening time is 1 ~ 20 min, and the temperature is 35 ~ 40℃. After roughening once, the polyurethane foam material is washed clean with distilled water and is squeezed dry. The first roughening liquid is potassium permanganate and sulfuric acid solution, and further, the concentration of potassium permanganate is 8 ~ 10 g / L, and the concentration of sulfuric acid is 5 ~ 7 mL / L.

[0020] Further, the polyurethane foam material after roughening once is placed in a second roughening liquid for roughening twice, the time is 1 ~ 10 min, and then is placed in commercially available sensitizing liquid and activating liquid. In this technical scheme, the different surface densities of the foamed copper can be determined by adjusting the roughening time.

[0021] Further, the composition of the second roughening liquid is chromium trioxide 5 ~ 7 g / L and sulfuric acid 3 ~ 5 mL / L.

[0022] Further, the composition of the sensitizing liquid is tin dichloride 20 ~ 30 g / L and hydrochloric acid with a mass fraction of 36% 40 ~ 50 mL / L.

[0023] Further, the composition of the activating liquid is silver nitrate 3 ~ 8 g / L and ammonia water with a mass fraction of 25% 5 ~ 10 mL / L.

[0024] As a further improvement of the present application, the polyurethane foam after roughening treatment is placed in a copper plating liquid for pre-plating at a voltage of 4 ~ 6 V for 6 min, and then is subjected to copper plating at an apparent current density of 0.6 A / cm2 The electrodeposition is performed under the condition of temperature 25-35℃ for 20-30min.

[0025] As a further improvement of the present application, the volume percentage of hydrogen in the nitrogen-hydrogen mixed gas is 10%-30%. Further, the volume ratio of nitrogen to hydrogen is 1:3.

[0026] As a further improvement of the present application, in step S2, the oxygen-containing atmosphere includes any one of the following: a mixture of oxygen and hydrogen-nitrogen gas, a mixture of oxygen and hydrogen gas, and an air atmosphere.

[0027] With this technical solution, when the alloy is oxidized in hydrogen, the oxygen permeability at a higher hydrogen pressure increases the diffusion rate of oxygen atoms. Due to higher chemical driving force and larger lattice size, hydrogen in the metal promotes the diffusion rate of oxygen in the matrix, which can cause internal oxidation of Cu, steam formation in the oxide, which can lead to pore formation, thereby providing rapid transport of oxygen and accelerating corrosion. Secondly, the internal corrosion of oxygen makes the modified copper foam have a relatively small ligament spacing, good strain development and obtain stress relaxation, and improves the service reliability.

[0028] As a further improvement of the present application, in step S2, the heating temperature is 300-600℃. Further, the heating temperature is 300-400℃. Further, the heating time is 30min or more; further, the heating time is 60min or more; further preferably, the heating time is 60-120min. With this technical solution, the foam copper prepared by polyurethane foam is heated in an oxygen-containing atmosphere, and the heating temperature is less than the recrystallization temperature of copper, so that an oxide layer can be formed on the pore wall surface.

[0029] As a further improvement of the present application, in step S3, the oxidized foam copper is compressed by rolling or hot pressing.

[0030] As a further improvement of the present application, in step S4, the oxide layer is removed by using an etching liquid. Further, the etching liquid is an acidic etching liquid or an alkaline etching liquid. Further preferably, the acidic etching liquid is dilute hydrochloric acid or dilute sulfuric acid; and the alkaline etching liquid is a mixture of ammonia water and ammonium chloride.

[0031] As a further improvement of the present application, the alkaline etching liquid is obtained by adding ammonium chloride to concentrated ammonia water with a mass percentage of 15%-38%, and the mass percentage of ammonium chloride in the alkaline etching liquid is 10%-30%.

[0032] Further, the etching time is 30min or more; further, the etching time is 60min or more, and further, the etching time is 60-120min.

[0033] The application discloses a packaging soldering sheet, which comprises three-dimensional porous structure copper foam prepared by the method for preparing three-dimensional porous copper foam according to any one of the above.

[0034] As a further improvement of the application, the copper paste is prepared by the following method:

[0035] Step S21, preparing a soldering agent;

[0036] Step S22, taking the micron copper powder after pickling and drying;

[0037] Step S23, mixing the nano copper powder after washing PVP with the micron copper powder after pickling and drying, and mixing with the soldering agent to obtain the copper paste.

[0038] As a further improvement of the application, the components of the soldering agent are ethylene glycol, formic acid, 11-mercapto-1 alcohol, aniline, dimethylamine borane, isopropanolamine, ethanol and propylene glycol monofatty acid ester. Further, in the soldering agent, the concentration of the formic acid is 0.1-0.3 g / ml, the concentration of the 11-mercapto-1 alcohol is 0.001-0.002 g / ml, and the concentration of the aniline is 0.01-0.03 g / ml. By using the technical scheme, the formic acid reacts with copper particles on the surface to form copper formate, the polyether polyol can form a complex with the copper formate to protect the copper particles from being oxidized in the sintering process, and the aniline promotes the formation of the complex.

[0039] As a further improvement of the application, in step S23, the mass ratio of the copper powder to the soldering agent is (3.5-6.5):1, and the copper paste is obtained by mixing by using a paste mixer.

[0040] The application further discloses a semiconductor packaging method, which comprises the following steps: placing the packaging soldering sheet into a substrate and a semiconductor chip to be welded, and sintering under a reducing atmosphere, wherein the sintering temperature is 180-680 DEG C; and the reducing atmosphere comprises a formic acid atmosphere, a hydrogen atmosphere or a carbon monoxide atmosphere. The sintering can be pressurized or non-pressurized sintering.

[0041] Currently, power device packaging primarily relies on silver / copper paste sintering. However, the resulting sintered silver / copper often suffers from thermal expansion coefficient (CTE) mismatch at the solder interface due to its excessively high Young's modulus during device service, especially during thermal cycling, leading to failure. The present invention addresses this issue by introducing a low-Young's modulus foamed copper structure into the sintered copper structure. This solves the CTE mismatch problem at the solder interface, significantly improving the device's reliability during thermal cycling. Furthermore, the foamed copper's three-dimensional porous structure provides a large surface area, allowing more metal atoms to participate in the sintering reaction. During soldering, the porous structure provides more diffusion channels, promoting sintering and enabling interconnection at lower temperatures while withstanding higher service temperatures.

[0042] As a further improvement of the present invention, no pressure is applied during the sintering process, the sintering temperature is 180℃~280℃, the sintering time is 0.1-3 hours, and it can be reflowed multiple times.

[0043] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0044] First, the foamed copper obtained using the technical solution of this invention has several channels on its pore wall surface, resulting in a low Young's modulus. This solves the problem of thermal expansion coefficient (CTE) mismatch at the solder layer interface, greatly improving the reliability of the device during thermal cycling in service. Furthermore, the foamed copper structure of this invention allows for similar thermal expansion coefficients at the solder layer contact interface, reducing the Young's modulus of the solder layer, alleviating its brittleness and hardness, reducing thermal fatigue, and simultaneously resisting frequent thermal shocks during device operation, providing reliable connection strength to withstand physical impacts.

[0045] Secondly, the technical solution of this invention can greatly improve the reliability of the device during thermal cycling by mixing and sintering nano- and / or micro-sized copper with foamed copper. It can withstand 500 cycles at -45℃ to 150℃ without failure. The process is simple, low-cost, and highly practical, solving the problems of high chip bonding cost and poor service reliability of current power devices. Attached Figure Description

[0046] Figure 1 This is a schematic diagram of the preparation process of a three-dimensional porous structure according to the present invention.

[0047] Figure 2 These are magnified microstructure images of copper foam with different areal densities (600 g / m², 1200 g / m², 2400 g / m²) obtained in step S1 of the preparation of the three-dimensional porous copper foam of the present invention, and their local magnified microstructures; wherein, a) to c) are respectively images of copper foam with an areal density of 600 g / m². 2 1200g / m 2 2400g / m2 foamed copper; d) - f) are partial enlarged microstructure images of foamed copper with areal density of 600 g / m 2 , 1200 g / m 2 , 2400 g / m 2 .

[0048] Figure 3 are cross-sectional contrast images of foamed copper of the three-dimensional porous structure of the present application after oxidation for 30 min at different temperatures.

[0049] Figure 4 are cross-sectional images and partial enlarged images of foamed copper material with areal density of 600 g / m 2 after oxidation at 400℃ for different time, wherein a) - d) are oxidation time of 30 min, 60 min, 90 min and 120 min, respectively, and e) - h) are partial enlarged images of a) - d), respectively.

[0050] Figure 5 are strength comparisons of foamed copper material with areal density of 600 g / m 2 after oxidation at 400℃ for different time.

[0051] Figure 6 are surface morphology and partial enlarged images of foamed copper material with areal density of 600 g / m 2 after oxidation at 400℃ for 60 min and etching for different time, wherein a) - d) are etching time of 30 min, 60 min, 90 min and 120 min, respectively, and e) - h) are partial enlarged images of a) - d), respectively.

[0052] Figure 7 are welding effect images of foamed copper with areal density of 600 g / m 2 after oxidation-etching modification and double-sided brushing of copper paste.

[0053] Figure 8 are welding effect images of foamed copper with areal density of 600 g / m 2 after oxidation-etching modification and single-sided brushing of copper paste and partial enlarged images of A, B and C, wherein C is foamed copper.

[0054] Figure 9 is a welding effect image of foamed copper with areal density of 600 g / m 2 after oxidation-etching modification and single-sided brushing of commercially available copper paste.

[0055] Figure 10 are transmission electron microscope photos of oxygen transport channels in the oxidation process of foamed copper with three-dimensional porous structure obtained by the embodiment of the present application, wherein a) and b) are electron microscope photos with different magnifications.

[0056] Figure 11 Figure 1 is a low Young's modulus finite element calculation result of the three-dimensional porous structure of the foam copper obtained by the embodiment of the present application, wherein a) is a connection diagram, b) is a stress distribution diagram, and c) and d) are stress distribution diagrams of different positions. DETAILED DESCRIPTION

[0057] The preferred embodiment of the present application is further described in detail below.

[0058] In the first aspect, the present detailed description discloses a three-dimensional porous structure of foam copper, and a schematic diagram of the preparation process is shown in Figure 1, which comprises the following steps: Figure 1

[0059] Step S1, preparing foam copper, specifically comprising:

[0060] The polyurethane foam material with a porosity of ≥90% is soaked in a commercially available oil removal liquid at a temperature of 40-60°C for 15 min, and then washed clean with distilled water;

[0061] The oil-removed polyurethane foam material is placed in a potassium permanganate and sulfuric acid solution (potassium permanganate 8-10 g / L, sulfuric acid 5-7 mL / L) for primary roughening, the primary roughening time is 1-20 min, and the temperature is 35-40°C; after primary roughening, the polyurethane foam material is washed clean with distilled water and squeezed dry; the primary roughened polyurethane foam material is placed in a second roughening liquid (chromium trioxide 5-7 g / L, sulfuric acid 3-5 mL / L) for secondary roughening, the time is 1-10 min; and then placed in commercially available sensitization liquid and activation liquid (sensitization liquid: tin dichloride 20-30 g / L, 36% hydrochloric acid 40-50 mL / L, activation liquid: silver nitrate 3-8 g / L, 25% ammonia water 5-10 mL / L). By adjusting the roughening time, different face densities of the foam copper can be determined.

[0062] The polyurethane foam material obtained in the above step is placed in an electroplating liquid for pre-plating at a voltage of 4-6 V for 6 min, and then electrodeposited at an apparent current density of 0.6 A / cm 2 at a temperature of 30°C for 20-30 min, and dried. The dried foam copper is placed in a nitrogen-hydrogen mixed gas with a volume ratio of 1:3 for annealing treatment at 680-700°C to obtain the foam copper.

[0063] Step S2, heating the foam copper prepared by the polyurethane foam in an oxygen-containing atmosphere, the heating temperature is 300-600°C, so that an oxide layer is generated on the pore wall surface.

[0064] Step S3, compressing the foam copper to a dense state by rolling or hot pressing. ​

[0065] Step S4, the oxide layer is removed by solution etching to leave the channels, and then the final three-dimensional porous structure of the copper foam is obtained. The oxide layer acts as a sacrificial layer, and the thickness of the oxide layer is adjusted to control the channel width.

[0066] The oxygen-containing atmosphere includes a mixed atmosphere of oxygen and hydrogen, a mixed atmosphere of oxygen and hydrogen, and an air atmosphere. The heating temperature range is preferably 300-600°C.

[0067] The etching solution is acidic and alkaline, and one of them can be used. The acidic etching solution is dilute hydrochloric acid, dilute sulfuric acid, etc. The alkaline etching solution is a mixture of ammonia and ammonium chloride, and the preparation method is to add ammonium chloride to concentrated ammonia water with a content of 25%-28%, and the mass fraction of ammonium chloride is 18%.

[0068] The above technical solution has the following characteristics:

[0069] 1. The copper foam prepared by the polyurethane foam material has excellent oxygen transfer capacity when it undergoes chemical reaction. The CuO crystal lattice contains oxygen vacancies, and the external oxygen participates in the reaction and enters the oxygen vacancies, which can promote the strengthening of the near-surface region and reduce the surface wrinkles caused by thermal cycling.

[0070] 2. When the alloy is oxidized in hydrogen, the oxygen permeability increases at a higher hydrogen pressure, and the oxygen atom diffusion rate increases. Due to higher chemical driving force and larger lattice size, hydrogen in the metal promotes the diffusion rate of oxygen in the matrix, which can cause internal oxidation of Cu, and the formation of steam in the oxide can lead to pore formation, thereby providing rapid transport of oxygen and accelerating corrosion.

[0071] 3. Internal corrosion of oxygen makes the modified copper foam ligament spacing comparable, strain development good and stress relaxation obtained, improving service reliability.

[0072] In a second aspect, the specific embodiment discloses a packaging solder, which comprises three-dimensional porous structure of copper foam prepared by the preparation method of three-dimensional porous copper foam as described above; the surface of the three-dimensional porous structure of copper foam is coated with copper paste, which is micron copper paste, nano copper paste, or micro-nano mixed copper paste. Further, the modified copper foam is brushed with copper paste (commercially available copper paste can also be used) of the required thickness on both sides by means of steel screen printing process.

[0073] Specifically, the copper paste is prepared by the following steps:

[0074] First step: flux ingredients: 120g of ethylene glycol, 18ml of formic acid, 0.2g of 11-mercapto-1 alcohol, 2ml of aniline, 2g of dimethylamine borane, 2ml of isopropanol amine, use ethanol to constant volume 150ml, and supersaturated propylene glycol monofatty acid ester. Add 18ml of formic acid, 0.2g of 11-mercapto-1 alcohol, 2ml of aniline, 2g of dimethylamine borane, 2ml of isopropanol amine to ethylene glycol, use ethanol to constant volume 150ml. Put into a 50℃ water bath. Add propylene glycol monofatty acid ester, continuously stir in a 40℃ water bath until it can't be dissolved. Take out after 60 minutes of refrigeration at 4℃.

[0075] Second step: take 1.5g of 3.5 micron diameter copper powder after pickling and drying, the pickling solution is 10% hydrochloric acid solution.

[0076] Third step: take 3.5g of 50nm copper powder, wash off the PVP with sodium hydroxide solution, mix with the micron copper powder obtained in the second step, and drop the liquid obtained in the first step. The mass ratio of powder to liquid is (3.5-6.5):1, and the mixture is mixed using a paste mixer to obtain a copper paste.

[0077] In a third aspect, the specific embodiment discloses a packaging method of a semiconductor chip, which is packaged by using a hybrid sintering process.

[0078] Place the above-mentioned copper foam printed with copper paste on a copper flange substrate, and place a power semiconductor chip on the modified copper foam sheet.

[0079] Put the pre-connected sample into a reducing atmosphere furnace, sinter at 180-280℃ without pressure for 0.1-3 hours, and can be refluxed multiple times.

[0080] The following will be described in conjunction with specific examples.

[0081] Example 1

[0082] Place the polyurethane foam material with a void fraction of ≥90% in a commercially available oil removal liquid at a temperature of 60℃ for 15min, then rinse with distilled water until clean;

[0083] The polyurethane foam material after oil removal is placed in a potassium permanganate and sulfuric acid solution (potassium permanganate 8-10 g / L, sulfuric acid 5-7 mL / L) for primary roughening, the primary roughening time is 20 min, the temperature is 40°C, after primary roughening, the polyurethane foam material is rinsed with distilled water and squeezed dry; the polyurethane foam material after primary roughening is placed in a second roughening liquid (chromium trioxide 5-7 g / L, sulfuric acid 3-5 mL / L) for secondary roughening, roughening for 10 min; then it is placed in commercially available sensitizing liquid and activating liquid (sensitizing liquid: tin dichloride 20-30 g / L, 36% hydrochloric acid 40-50 mL / L, activating liquid: silver nitrate 3-8 g / L, 25% ammonia water 5-10 mL / L) for 5 min respectively. Foam copper with an areal density of 600 g / m 2 is obtained.

[0084] Example 2

[0085] The polyurethane foam material with a void fraction of ≥90% is immersed in commercially available oil removal liquid at a temperature of 50°C for 15 min, then rinsed with distilled water and dried;

[0086] The polyurethane foam material after oil removal is placed in a potassium permanganate and sulfuric acid solution (potassium permanganate 8-10 g / L, sulfuric acid 5-7 mL / L) for primary roughening, the primary roughening time is 10 min, the temperature is 40°C, after primary roughening, the polyurethane foam material is rinsed with distilled water and squeezed dry; the polyurethane foam material after primary roughening is placed in a second roughening liquid (chromium trioxide 5-7 g / L, sulfuric acid 3-5 mL / L) for secondary roughening, roughening for 10 min; then it is placed in commercially available sensitizing liquid and activating liquid (sensitizing liquid: tin dichloride 20-30 g / L, 36% hydrochloric acid 40-50 mL / L, activating liquid: silver nitrate 3-8 g / L, 25% ammonia water 5-10 mL / L) for 5 min respectively. Foam copper with an areal density of 1200 g / m 2 is obtained.

[0087] Example 3

[0088] The polyurethane foam material with a void fraction of ≥90% is immersed in commercially available oil removal liquid at a temperature of 50°C for 15 min, then rinsed with distilled water and dried;

[0089] The polyurethane foam material after oil removal is placed in a potassium permanganate and sulfuric acid solution (potassium permanganate 8-10 g / L, sulfuric acid 5-7 mL / L) for primary roughening, the primary roughening time is 1 min, and the temperature is 40°C. After primary roughening, the polyurethane foam material is rinsed clean with distilled water and squeezed dry. The polyurethane foam material after primary roughening is placed in a second roughening liquid (chromium trioxide 5-7 g / L, sulfuric acid 3-5 mL / L) for secondary roughening, roughening for 1 min. Then it is placed in commercially available sensitization liquid and activation liquid (sensitization liquid: tin dichloride 20-30 g / L, 36% hydrochloric acid 40-50 mL / L, activation liquid: silver nitrate 3-8 g / L, 25% ammonia water 5-10 mL / L) for 5 min respectively. The areal density of the obtained foam copper is 2400 g / m 2 .

[0090] Examples 4-6

[0091] The foam copper obtained in Examples 1-3 is placed in a tube furnace, nitrogen-hydrogen mixed gas (10% H2) is introduced, and then oxygen is introduced. The ratio of nitrogen-hydrogen mixed gas to oxygen is 1:1, the tube furnace is set to 400 degrees Celsius for oxidation for 30 min, and then it is taken out and rolled.

[0092] Examples 7-9

[0093] The porous copper raw material obtained in Examples 1-3 is placed in a tube furnace, pure oxygen is introduced. The tube furnace is set to 600 degrees Celsius for oxidation for 30 min, and then it is taken out and rolled.

[0094] Examples 10-12

[0095] The porous copper raw material obtained in Examples 1-3 is placed in a tube furnace, air (70% nitrogen, 30% oxygen) is introduced. The tube furnace is set to 500 degrees Celsius for oxidation for 30 min, and then it is taken out and rolled.

[0096] Examples 13-15

[0097] The porous copper raw material obtained in Examples 1-3 is placed in a tube furnace, nitrogen-hydrogen mixed gas (10% H2) is introduced, and then oxygen is introduced. The ratio of nitrogen-hydrogen mixed gas to oxygen is 1:1, the tube furnace is set to 400 degrees Celsius for oxidation for 60 min, and then it is taken out and rolled.

[0098] Examples 16-18

[0099] The porous copper raw material obtained in Examples 1-3 is placed in a tube furnace, pure oxygen is introduced. The tube furnace is set to 600 degrees Celsius for oxidation for 60 min, and then it is taken out and rolled.

[0100] Examples 19-21

[0101] The porous copper raw materials obtained in Examples 1 to 3 were placed in a tube furnace and air (70% nitrogen, 30% oxygen) was introduced. The tube furnace was oxidized at 500 degrees Celsius for 90 minutes, and then rolled after removal.

[0102] Examples 22-24

[0103] The porous copper raw material obtained in Example 1 was placed in a tube furnace, and a nitrogen-hydrogen mixture (10% H2) was introduced, followed by oxygen. The ratio of the nitrogen-hydrogen mixture to oxygen was 1:1. The tube furnace was oxidized at 400 degrees Celsius for 60 min, 90 min, and 120 min respectively, and then rolled after removal.

[0104] Examples 25-28

[0105] The sample obtained in Example 22 was placed in a solution of 25% concentrated ammonia water with added ammonium chloride and 18% ammonium chloride by mass for 30 min, 60 min, 90 min, and 120 min respectively, and then rolled after removal.

[0106] In the above embodiments, by changing the roughening time of the polyurethane foam, densities of 600 g / m³ were obtained. 2 1200g / m 2 2400g / m 2 Three types of copper foam were compared by examining their microstructures, such as... Figure 2 As shown, all three densities exhibit a three-dimensional porous structure with pores on the surface of the pore walls. The 600 g / m³ density... 2 The foamed copper has the fewest microscopic defects.

[0107] By comparing 600g / m 2 1200g / m 2 2400g / m 2 Cross-sections of porous copper raw materials after oxidation at different temperatures for 30 minutes, such as... Figure 3 As shown, copper foams of different densities can withstand oxidation at 400℃, with 600g / m³ being the most resistant. 2 The porous copper oxidation temperature is 400℃, resulting in a dense microstructure and more appropriate ligament spacing.

[0108] The following comparison uses a surface density of 600 g / m³. 2 Experiments were conducted using porous copper raw materials as an example.

[0109] Cross-sections of porous copper precursors prepared by oxidation at 400℃ for different times after 60 min of etching, as shown in the figure. Figure 4 As shown, 600g / m 2Porous copper can achieve a good oxidation interface after oxidation at 400℃ for more than 60 minutes, with shear strengths greater than 45 MPa. Copper foam after 60 minutes of oxidation exhibits the highest strength, reaching 53 MPa. Figure 5 As shown.

[0110] By comparing the surface density of 600 g / m² 2 Porous copper prepared by oxidizing porous copper raw material at 400℃ for 60 min was examined cross-sections after etching for different times. It was found that after etching for 30 min, the foamed copper exhibited a better surface microstructure, with the foamed copper etched for 60 min showing the densest surface microstructure and the most significant strengthening effect in the near-surface area. Figure 6 As shown.

[0111] Transmission electron microscopy (TEM) images of the oxygen transport channels during the oxidation process of copper foam obtained in Example 4 are shown below. Figure 10 As shown, the finite element calculation results of the low Young's modulus of the sample obtained in Example 26 are as follows: Figure 11 As shown, the calculated results are the stress distribution and Young's modulus after SiC and copper substrate are joined and subjected to thermal cycling. The average Young's modulus is approximately 220.0 MPa. It is evident that the three-dimensional porous structure of the foamed copper has a large surface area, allowing more metal atoms to participate in the sintering reaction. During the welding process, the porous structure provides more diffusion channels, thereby promoting sintering.

[0112] Example 29

[0113] Step 1: The flux composition is: 120g ethylene glycol, 18ml formic acid, 0.2g 11-mercapto-1 alcohol, 2ml aniline, 2g dimethylamine borane, 2ml isopropanolamine, and ethanol to a final volume of 150ml, along with supersaturated propylene glycol monofatty acid ester. Add 18ml formic acid, 0.2g 11-mercapto-1 alcohol, 2ml aniline, 2g dimethylamine borane, and 2ml isopropanolamine to the ethylene glycol, and then bring the volume to a final volume of 150ml with ethanol. Place in a 50°C water bath. Add the propylene glycol monofatty acid ester and stir constantly in a 40°C water bath until it cannot be dissolved. Freeze at 4°C for 60 minutes.

[0114] Step 2: Take 1.5g of copper powder with a diameter of 3.5 micrometers, pickle it, and then dry it. The pickling solution is a 10% hydrochloric acid solution.

[0115] Step 3: Take 3.5g of 50nm copper powder, wash away PVP with sodium hydroxide solution, mix with the micron-sized copper powder obtained in Step 2, and add the liquid obtained in Step 1 dropwise. The powder to liquid mass ratio is 4.5:1. Mix the mixture using a paste mixer to obtain copper paste.

[0116] Examples 30-31

[0117] The copper paste double-sided brush prepared from the copper paste prepared in Example 29 is applied to the modified foam copper obtained in Example 26, and placed on a copper substrate with a SiC chip having a copper electrode embedded thereon. The sample is placed in a formic acid furnace at 280°C, and refluxed for 1.5, 2.5 hours respectively in a formic acid atmosphere to obtain Examples 30-31.

[0118] Examples 32-34

[0119] A commercially available micro-nano mixed copper paste is prepared to obtain a copper paste double-sided brush, which is applied to the modified foam copper obtained in Example 26, and placed on a copper substrate with a SiC chip having a copper electrode embedded thereon. The sample is placed in a formic acid furnace at 280°C, and refluxed for 1, 1.5, 2 hours respectively in a formic acid atmosphere to obtain Examples 32-34.

[0120] In the above examples, the copper paste double-sided brush has a surface density of 600 g / m 2 The post-welding effect diagram of the copper paste double-sided brush of Example 29 after oxidation-corrosion modification of the foam copper (Example 30) is shown in Figure 7 The copper paste double-sided brush has a surface density of 600 g / m 2 The post-welding effect diagram of the copper paste single-sided brush of Example 29 after oxidation-corrosion modification of the foam copper (Example 30) is shown in Figure 8 The copper paste single-sided brush has a surface density of 600 g / m 2 The post-welding effect diagram of the commercially available copper paste single-sided brush of Example 29 after oxidation-corrosion modification of the foam copper (Example 33) is shown in Figure 9 It can be seen that the foam copper provided by the application can achieve good connection effect as the commercially available copper paste.

[0121] Examples 35-37

[0122] A commercially available nano mixed copper paste is prepared to obtain a copper paste double-sided brush, which is applied to the foam copper (unmodified) of the prior art, and placed on a copper substrate with a SiC chip having a copper electrode embedded thereon. The sample is placed in a reflux soldering furnace in a nitrogen-hydrogen mixed gas at 280°C, and refluxed for 0.1h / 0.2h / 0.3h respectively for three times to obtain Examples 35-37.

[0123] Example 38

[0124] The samples of Examples 30-37 are placed in a thermal cycle furnace at -65°C-150°C, and subjected to cold-heat cycles for 50 times, and the failure conditions are counted. See Table 1 below, and each example has 6 samples. As can be seen from the data in Table 1, the introduction of the foam copper structure with low Young's modulus in the sintered copper structure can solve the mismatch of the thermal expansion coefficient (CTE) at the solder layer interface, and greatly improve the reliability of the device during the thermal cycle in the service process.

[0125] Table 1

[0126]

[0127] The above description is further detailed in connection with specific preferred embodiments of the application, and it is not to be construed that the specific implementation of the application is limited to these descriptions. For those skilled in the art of the present application, without departing from the concept of the present application, a number of simple deductions or substitutions can be made, and all of them should be considered as falling within the protection scope of the present application.

Claims

1. A method for preparing three-dimensional porous copper foam, characterized in that: Includes the following steps: Step S1: Prepare copper foam; Step S2: Heat the copper foam in an oxygen-containing atmosphere to form an oxide layer on the surface of its pore walls, thus obtaining oxidized copper foam; the heating temperature is 300~600℃. Step S3: Compress the oxidized copper foam by rolling or hot pressing to form dense sheet-like copper foam. Step S4: Use an etching solution to remove the oxide layer, leaving pores, to obtain a three-dimensional porous copper foam; the etching solution is obtained by adding ammonium chloride to concentrated ammonia water with a mass percentage of 15%-38%, and the mass percentage of ammonium chloride is 10%-30%; the etching time is 60-120 min.

2. The method for preparing three-dimensional porous copper foam according to claim 1, characterized in that: Step S1 includes: placing polyurethane foam in a copper plating solution to plate copper, drying it, and then placing it in a nitrogen-hydrogen mixed gas for annealing at 680~700℃ to obtain foamed copper.

3. The method for preparing three-dimensional porous copper foam according to claim 2, characterized in that: The polyurethane foam has a porosity ≥90%. Before copper plating, the polyurethane foam is degreased and cleaned, then roughened. Next, it is pre-plated in a copper plating solution at 4-6V for 6 minutes, followed by plating at an apparent current density of 0.6 A / cm². 2 Electrodeposition is performed at a temperature of 25-35℃ for 20-30 minutes; the volume percentage of hydrogen in the nitrogen-hydrogen mixed gas is 10%-30%.

4. The method for preparing three-dimensional porous copper foam according to claim 1, characterized in that: In step S2, the oxygen-containing atmosphere includes any one of the following: a mixed atmosphere of oxygen and hydrogen-nitrogen, a mixed atmosphere of oxygen and hydrogen, or an air atmosphere.

5. The method for preparing three-dimensional porous copper foam according to claim 4, characterized in that: In step S2, the heating temperature is 300~400℃ and the heating time is 60min-120min.

6. A type of encapsulation pad, characterized in that: It includes a three-dimensional porous structure of foamed copper, which is prepared by the preparation method of three-dimensional porous foamed copper as described in any one of claims 1 to 5; the surface of the three-dimensional porous structure of foamed copper is coated with copper paste, which is micron copper paste, nano copper paste, or micro-nano mixed copper paste.

7. The encapsulation pad according to claim 6, characterized in that: The copper paste was prepared using the following method: Step S21, prepare the flux; Step S22: Take micron-sized copper powder, acid wash and then dry it; In step S23, after washing away PVP from the nano copper powder, it is mixed with the micron copper powder obtained after acid washing and drying in step S22, and then mixed with flux to obtain copper paste.

8. The encapsulation pad according to claim 7, characterized in that: The flux consists of ethylene glycol, formic acid, 11-mercapto-1 alcohol, aniline, dimethylamine borane, isopropanolamine, ethanol, and propylene glycol monofatty acid ester. In the solder, the concentration of formic acid is 0.1-0.3 g / ml, the concentration of 11-mercapto-1 alcohol is 0.001-0.002 g / ml, and the concentration of aniline is 0.01-0.03 g / ml; In step S23, the mass ratio of copper powder to brazing filler metal is (3.5-6.5):1, and copper paste is obtained by mixing using a paste mixer.

9. A semiconductor packaging method, characterized in that: The encapsulation pad described in claim 6 is placed between the substrate and the semiconductor chip to be soldered, and sintered in a reducing atmosphere at a temperature of 180°C-680°C; wherein the reducing atmosphere includes a formic acid atmosphere, a hydrogen atmosphere, or a carbon monoxide atmosphere.

10. The semiconductor packaging method according to claim 9, characterized in that: During the sintering process, no pressure is applied, the sintering temperature is 180℃~280℃, and the sintering time is 0.1-3 hours.

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