Flaky copper powder for conductive paste and preparation method of flaky copper powder

Abstract

The invention relates to the technical field of copper powder processing, in particular to flaky copper powder for conductive paste and a preparation method of the flaky copper powder. The method comprises the steps that copper powder which is prepared through PVD and is high in sphericity degree is subjected to ball milling through three kinds of grinding balls which are gradually decreased from high to low, and flaky copper powder is prepared; and then an acidic chemical nickel plating solution is adopted, the pH value of the plating solution is dynamically regulated and controlled, the pH value of the initial stage ranges from 2.0 to 4.0, the pH value of the middle stage ranges from 5.0 to 5.5, the pH value of the later stage ranges from 4.5 to 5.0, and the plating of the flaky copper powder is achieved through ultrasonic assistance. The flaky copper powder prepared through the method is complete and uniform in structure and compact in plating layer, the problems that according to an existing method, the long-term oxidation resistance is poor, the conductivity of the flaky copper powder is affected by residual coatings, and the flaky copper powder is not suitable for being used in special environments such as high temperature and high humidity are solved, and the flaky copper powder obtained through the method is low in resistance and good in oxidation resistance and has good application prospects. The end electrode can be applied to a chip multilayer ceramic capacitor used in a severe environment.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of copper powder processing, in particular to a flaky copper powder for conductive paste and a preparation method thereof. BACKGROUND

[0002] Copper powder is an ideal substitute material for silver due to its excellent electrical conductivity and low price, and is prepared into conductive paste and applied in the fields of electronics and photovoltaics. However, copper powder is easy to oxidize, especially in high temperature or humid environment, and the oxidation layer of copper may affect the welding quality, which requires more stringent environmental control or the use of flux. In particular, flaky copper powder has a larger specific surface area and is more prone to oxidation due to a larger contact area with oxygen.

[0003] In the prior art, traditional ball milling uses polyacrylate dispersants, and the residues cause the volume resistivity to increase by ≥30%; in Chinese patent CN103273056B, an improved carbon nanotube assisted ball milling process is used, but the resistivity of the conductive adhesive is still 0.8x10-3Ω·cm, and the resistivity still needs to be further reduced. In view of the disadvantage that copper powder is easy to oxidize, the flaky copper powder subjected to single anti-oxidation treatment in the prior art has an oxidation weight gain of 2.7mg / cm 2 after 240 hours in an 85℃ / 85%RH environment, which is difficult to meet the requirements of IEC 62373 accelerated aging test; although the chemical nickel plating process can improve corrosion resistance, there are problems such as plating layer porosity >1.5% and grain boundary diffusion channels, and the anti-oxidation performance still needs to be further improved.

[0004] Therefore, it is necessary to develop a preparation method of flaky copper powder with better anti-oxidation performance and lower resistivity. SUMMARY

[0005] In order to solve the problems of the prior art, the present application provides a preparation method of flaky copper powder for conductive paste, comprising the following steps:

[0006] S1: mixing spherical copper powder and process control agent according to a mass ratio of (80-100):1; the process control agent is one or both of oleic acid and stearic acid;

[0007] S2: adding three specifications of zirconium oxide grinding balls into the mixture obtained in step S1 at the same time, and performing ball milling treatment, the diameters of the three specifications of grinding balls are successively reduced, the ratio of the diameter of the former grinding ball to the diameter of the latter grinding ball is ≥3, the mass ratio of the total amount of grinding balls to the material is 10-30:1, inert gas is introduced during the ball milling process, and flaky copper powder is obtained after ball milling;

[0008] S3: After the flaky copper powder obtained in step S2 is classified by screening, a nickel plating solution and a reducing agent are added, and the copper powder is coated on the surface by using an ultrasonic-assisted chemical nickel plating process. The nickel plating solution is an acidic nickel-containing solution, the nickel concentration of the nickel plating solution is 0.08-0.15 mol / L, the temperature is 70-90℃ (temperature control accuracy ±2℃), the initial stage pH is 2.0-4.0, the middle stage pH is adjusted to 5.0-5.5, the later stage pH is adjusted to 4.5-5.0, the ultrasonic frequency is 30-60 kHz (to prevent copper powder from settling), and flaky nickel-plated copper powder is obtained. Meanwhile, the nickel ion concentration is monitored in real time by an ion selective electrode, and the error is not more than 0.1 g / L by replenishment.

[0009] S4: The flaky nickel-plated copper powder obtained in step S3 is washed and dried to obtain flaky copper powder for conductive paste.

[0010] Further, the content of copper element in the spherical copper powder in step S1 is ≥99%, preferably the copper powder is a spherical copper powder prepared by PVD process (physical vapor deposition) with good sphericity (>0.95) and narrow particle size distribution, to ensure uniform thickness and particle size of the flaky copper powder after ball milling.

[0011] Further, the three kinds of grinding balls in step S2 are: the first kind Φ1=15-25 mm, the second kind Φ2=5-10 mm, and the third kind Φ3=0.5-1 mm. Among them, the volume of the second kind of grinding ball is the largest, the volume of the first kind of grinding ball is the second, and the volume of the third kind of grinding ball is the smallest. Preferably, the volume ratio of the three kinds of grinding balls is 3:5:2.

[0012] Further, in step S2, the ball milling treatment adopts a vertical stirring ball mill, a horizontal stirring ball mill or a planetary stirring ball mill, the ball mill speed is 100-600 rmp, and the ball milling time is 2-15 h.

[0013] Further, in step S3, the nickel plating solution is nickel sulfate, the reducing agent is sodium hypophosphite, the concentration of sodium hypophosphite is 0.3-0.4 mol / L, the nickel plating time is 15-60 min (plating layer thickness 2-5 nm), and the volume ratio of the flaky copper powder to the nickel plating solution is 1:(10-30) to avoid local overreaction.

[0014] Further, in step S4, the washing process is first performed by alcohol washing 3-6 times, and then deionized water is used for washing 3-5 times. The drying temperature is 60-80℃, the drying time is 2-6 hours, and the gas atmosphere during the drying process is any one of vacuum, nitrogen, argon or helium.

[0015] A flaky copper powder for conductive paste is prepared according to the above preparation method, and the volume resistivity is less than 4×10 -6Ω*cm. No surface oxidation spots after IPC-TM-650 2.6.25 damp heat test for 500 hours.

[0016] The beneficial effects of the present application are: the present application improves the ball milling efficiency by three specifications of ball milling, and the shear efficiency is improved by 40% through DEM simulation verification, and the edge burr of the flake layer is eliminated; then the pH value is controlled during the chemical nickel plating process: the nucleation is accelerated in the initial stage, the grain epitaxy is promoted in the middle stage, and the dense filling is completed in the later stage, so that the porosity of the plated layer is <0.3%. The present application realizes the double optimization of the integrity of the flaky structure and the densification of the plated layer through the synergistic effect of three specifications of ball milling and chemical nickel plating, solves the problems of poor long-term oxidation resistance, the passivation influence of residual coating on the conductivity of flaky copper powder, and the unsuitable use in special environments such as high temperature and high humidity in the prior art, reduces the resistance, improves the wet heat stability and uniformity of flaky copper powder, and improves the corrosion resistance, can be applied to the conductive paste for the electrode of the flaky multilayer ceramic capacitor, and prolongs the service life of the electrode of the flaky multilayer ceramic capacitor in harsh environments. BRIEF DESCRIPTION OF DRAWINGS

[0017] The embodiments of the present application are further described below with reference to the accompanying drawings, in which:

[0018] Figure 1 The scanning electron microscope photos of the flaky copper powder for the conductive paste obtained after ball milling and chemical nickel plating of example 1 are shown;

[0019] Figure 2 The transmission electron microscope photos of the flaky copper powder for the conductive paste of example 1 are shown (the marked part is the nickel plating layer). DETAILED DESCRIPTION

[0020] In order to make the purpose, technical scheme and advantages of the present application more clear, the present application is further described in detail below with reference to the accompanying drawings through specific embodiments. It should be understood that the specific embodiments described herein are only used to explain the present application, and are not used to limit the present application.

[0021] Example 1

[0022] S1: mix the PVD prepared spherical copper powder (purity 99.95%, D50=12μm), spherical degree ≥0.95, with process control agent oleic acid according to the mass ratio of 80:1;

[0023] S2: adding three specifications of zirconium oxide grinding balls into the mixture obtained in step S1 simultaneously, and performing ball milling treatment, the first Φ1 = 15 mm, the second Φ2 = 5 mm, and the third Φ3 = 0.8 mm, the volume ratio of the three grinding balls (15 / 5 / 0.8 mm) is 3:5:2, the mass ratio of the total amount of grinding balls and the material is 20:1, a vertical stirring ball mill is adopted, the rotation speed of the ball mill is 400 rmp, the ball milling time is 8 h, nitrogen is introduced during the process (oxygen content < 50 ppm), and flaky copper powder is obtained after ball milling;

[0024] S3: after screening the flaky copper powder obtained in step S2, a nickel plating solution NiSO4·6H2O 0.12 mol / L and a reducing agent NaH2PO2·H2O 0.35 mol / L are added, an ultrasonic assisted chemical nickel plating process is adopted to coat the surface of the copper powder, the temperature is 70°C (temperature control accuracy ± 2°C), the pH in the initial stage is 3.8 to accelerate nucleation, the pH in the middle stage is adjusted to 5.2, the pH in the later stage is adjusted to 4.8, the ultrasonic frequency is 30 kHz, the nickel plating time is 30 min (plating layer thickness 3-3.5 nm), the volume ratio of the flaky copper powder and the nickel plating solution is 1:20 to avoid excessive local reaction, the nickel ion concentration is monitored in real time by an ion selective electrode, the error is not more than 0.1 g / L by replenishment, and flaky nickel plated copper powder is obtained after plating;

[0025] S4: the flaky nickel plated copper powder obtained in step S3 is washed with alcohol for 6 times, then washed with deionized water for 3 times, and vacuum dried at 60°C for 5 hours to obtain flaky copper powder for conductive paste.

[0026] The flaky copper powder for conductive paste obtained has a flaky diameter of 15-20 μm, a thickness of 0.3-0.5 μm (aspect ratio 30-50:1), a nickel plating layer thickness of about 3-4 nm, a plating layer porosity of < 0.3%, and a volume resistivity of 3.2 x 10 -6 Ω·cm, and no surface oxidation spots after 500 hours of IPC-TM-650 2.6.25 humid heat test.

[0027] Example 2

[0028] S1: mixing PVD prepared spherical copper powder (purity 99.95%, D50 = 12 μm) and process control agent stearic acid according to a mass ratio of 100:1;

[0029] S2: adding three sizes of zirconia grinding balls into the mixture obtained in step S1 simultaneously, and performing ball milling treatment, the first size Φ1 = 15 mm, the second size Φ2 = 5 mm, and the third size Φ3 = 0.8 mm, the volume ratio of the three grinding balls (15 / 5 / 0.8 mm) is 3:5:2, the mass ratio of the total amount of grinding balls to the material is 20:1, a vertical stirring ball mill is used, the rotation speed of the ball mill is 400 rmp, the ball milling time is 8 h, nitrogen is introduced during the process (oxygen content < 50 ppm), and flaky copper powder is obtained after ball milling;

[0030] S3: after screening the flaky copper powder obtained in step S2, a nickel plating solution NiSO4·6H2O 0.15 mol / L and a reducing agent NaH2PO2·H2O 0.40 mol / L are added, an ultrasonic assisted chemical nickel plating process is used to coat the surface of the copper powder, the temperature is 90°C (temperature control accuracy ± 2°C), the initial stage pH = 2.5 to accelerate nucleation, the middle stage pH = 5.0, the later stage pH = 4.5, the ultrasonic frequency is 60 kHz, the nickel plating time is 20 min (plating layer thickness 2-3 μm), the volume ratio of flaky copper powder to nickel plating solution is 1:10 to avoid local overreaction, the nickel ion concentration is monitored in real time by ion selective electrode, the error is not more than 0.1 g / L by replenishment, and flaky nickel plated copper powder is obtained after plating;

[0031] S4: the flaky nickel plated copper powder obtained in step S3 is washed with alcohol 6 times, then washed with deionized water 3 times, and vacuum dried at 70°C for 5 hours to obtain flaky copper powder for conductive paste.

[0032] The flaky copper powder for conductive paste obtained has a flaky diameter of 15-20 μm and a thickness of 0.6-0.8 μm (aspect ratio 30-50:1), the plating layer porosity is < 0.3%, and the volume resistivity is 3.0 x 10 -6 Ω·cm, and no surface oxidation spots are found after 500 hours of IPC-TM-650 2.6.25 humid heat test.

[0033] Example 3

[0034] S1: mixing PVD prepared spherical copper powder (purity 99.95%, D50 = 12 μm) and process control agent oleic acid according to a mass ratio of 80:1;

[0035] S2: three specifications of zirconium oxide grinding balls are simultaneously added to the mixture obtained in step S1 for ball milling treatment, the first Φ1 = 25 mm, the second Φ2 = 8 mm, and the third Φ3 = 1 mm, the volume ratio of the three grinding balls (25 / 8 / 1 mm) is 4:5:2, the mass ratio of the total amount of grinding balls to the material is 30:1, a vertical stirring ball mill is used, the rotation speed of the ball mill is 600 rmp, the ball milling time is 12 h, nitrogen is introduced during the process (oxygen content < 50 ppm), and flaky copper powder is obtained after ball milling;

[0036] S3: after screening the flaky copper powder obtained in step S2, a nickel plating solution NiSO4·6H2O 0.12 mol / L and a reducing agent NaH2PO2·H2O 0.35 mol / L are added, an ultrasonic assisted chemical nickel plating process is used for surface coating of the copper powder, the temperature is 70°C (temperature control accuracy ± 2°C), the initial stage pH = 3.8 for accelerating nucleation, the middle stage pH = 5.2, the later stage pH = 4.8, the ultrasonic frequency is 30 kHz, the nickel plating time is 30 min (plating layer thickness 2-3 μm), the volume ratio of flaky copper powder to nickel plating solution is 1:20 to avoid local overreaction, the nickel ion concentration is monitored in real time by ion selective electrode, the error is not more than 0.1 g / L by replenishment, and flaky nickel plated copper powder is obtained after plating;

[0037] S4: the flaky nickel plated copper powder obtained in step S3 is washed with alcohol for 3 times, then washed with deionized water for 5 times, and vacuum dried at 60°C for 5 hours to obtain flaky copper powder for conductive paste.

[0038] The flaky copper powder for conductive paste obtained has a flake diameter of 17-23 μm, a thickness of 0.3-0.6 μm, a plating layer porosity < 0.3%, and a volume resistivity 3.8 x 10 -6 Ω·cm, and no surface oxidation spots after 500 hours of IPC-TM-650 2.6.25 humid heat test.

[0039] Example 4

[0040] S1: spherical copper powder (purity 99.95%, D50 = 12 μm) prepared by PVD and oleic acid as a process control agent are mixed according to a mass ratio of 80:1;

[0041] S2: three specifications of zirconium oxide grinding balls are simultaneously added to the mixture obtained in step S1 for ball milling treatment, the first Φ1 = 20 mm, the second Φ2 = 6 mm, and the third Φ3 = 0.6 mm, the volume ratio of the three grinding balls (20 / 6 / 0.6 mm) is 3:5:2, the mass ratio of the total amount of grinding balls to the material is 10:1, a horizontal stirring ball mill is used, the rotation speed of the ball mill is 200 rmp, the ball milling time is 15 h, nitrogen is introduced during the process (oxygen content < 50 ppm), and flaky copper powder is obtained after ball milling;

[0042] S3: After screening the flaky copper powder obtained in step S2, a nickel plating solution NiSO4·6H2O 0.10 mol / L and a reducing agent NaH2PO2·H2O 0.30 mol / L are added, and an ultrasonic-assisted chemical nickel plating process is used to coat the surface of the copper powder, with a temperature of 80℃ (temperature control accuracy ±2℃), a pH of 3.2 in the initial stage to accelerate nucleation, a pH of 5.1 in the middle stage, and a pH of 4.5 in the later stage, an ultrasonic frequency of 50 kHz, a nickel plating time of 20 min (plating layer thickness of 2-3 μm), a volume ratio of flaky copper powder to nickel plating solution of 1:20 to avoid excessive local reaction, and a nickel ion concentration monitored in real time by an ion selective electrode, with an error of no more than 0.1 g / L by replenishment, to obtain flaky nickel-plated copper powder;

[0043] S4: The flaky nickel-plated copper powder obtained in step S3 is washed with alcohol 6 times, then washed with deionized water 5 times, and dried in a nitrogen atmosphere at 70℃ for 5 hours to obtain flaky copper powder for conductive paste.

[0044] The obtained flaky copper powder for conductive paste has a flake diameter of 16-21 μm, a thickness of 0.3-0.5 μm, a plating layer porosity of <0.3%, and a volume resistivity of 3.9×10 -6 Ω·cm, and no surface oxidation spots after 500 hours of IPC-TM-650 2.6.25 humid heat testing.

[0045] Comparative Example 1

[0046] The steps are similar to those of Example 1, except that only the second Φ2 = 5 mm grinding ball is used in step S2, to prepare flaky copper powder for conductive paste.

[0047] The obtained flaky copper powder for conductive paste has a diameter of 12-15 μm, a thickness of 0.8-1.0 μm (aspect ratio of 30-50:1), and a copper powder in irregular shape, a plating layer porosity of <0.3%, and a volume resistivity of 8.8×10 -5 Ω·cm, and no surface oxidation spots after 100 hours of IPC-TM-650 2.6.25 humid heat testing, but surface oxidation spots after 500 hours.

[0048] Comparative Example 2

[0049] The steps are similar to those of Example 1, except that the pH is controlled at 4.8 throughout step S3, to prepare flaky copper powder for conductive paste.

[0050] The obtained flaky copper powder for conductive paste has a flake diameter of 15-20 μm, a thickness of 0.3-0.5 μm (aspect ratio of 30-50:1), a plating layer porosity of 0.5%, and a volume resistivity of 1.0×10 -5Ω-cm, surface oxidation spots appeared after 300 hours IPC-TM-650 2.6.25 damp heat testing.

[0051] The above describes some exemplary embodiments of the present application, it can be understood that the above-described embodiments are only used to explain the present application, and do not constitute a limitation on the scope of protection of the present application. The features in these embodiments can be recombined in a suitable manner, and the schemes obtained thereby are still within the scope of protection required by the present application. Based on the above-described embodiments, all other embodiments obtained by those skilled in the art without making creative efforts, i.e. all modifications, equivalent replacements and improvements, etc. made within the spirit and principles of the present application, are also within the scope of protection required by the present application.

Claims

1. A method for producing a flaky copper powder for a conductive paste, characterized by, The method comprises the following steps: S1: mixing spherical copper powder with process control agent in a mass ratio of 80-100:1; the process control agent is one or both of oleic acid or stearic acid; S2: adding three sizes of zirconium oxide grinding balls into the mixture obtained in step S1 simultaneously for ball milling treatment; the diameters of the three sizes of grinding balls decrease in turn, the ratio of the diameter of the former grinding ball to the diameter of the latter grinding ball is ≥3, the mass ratio of the total amount of grinding balls to the material is 10-30:1, inert gas is introduced during the ball milling process, and flaky copper powder is obtained after ball milling; S3: after screening the flaky copper powder obtained in step S2, adding nickel plating solution and reducing agent, and using ultrasonic-assisted chemical nickel plating process to coat the surface of the copper powder, the nickel plating solution is an acidic nickel-containing solution, the nickel concentration of the nickel plating solution is 0.08-0.15 mol / L, the temperature is 70-90℃, the initial stage pH is 2.0-4.0, the intermediate stage pH is adjusted to 5.0-5.5, the later stage pH is adjusted to 4.5-5.0, the ultrasonic frequency is 30-60 kHz, and flaky nickel-plated copper powder is obtained; S4: washing and drying the flaky nickel-plated copper powder obtained in step S3 to obtain flaky copper powder for conductive paste.

2. The method of claim 1, wherein the copper powder is prepared by the steps of: The content of copper element in the spherical copper powder in step S1 is ≥99%. ​ 3. The method for preparing flake copper powder for conductive paste according to claim 1, characterized in that, The three sizes of grinding balls in step S2 are: the first size Φ1=15-25 mm, the second size Φ2=5-10 mm, and the third size Φ3=0.5-1 mm, wherein the volume of the second size of grinding balls added is the largest, the volume of the first size of grinding balls added is the second largest, and the volume of the third size of grinding balls added is the smallest.

4. The method for preparing flake copper powder for conductive paste according to claim 1, characterized in that, In step S2, the ball milling treatment uses a vertical stirring ball mill, a horizontal stirring ball mill, or a planetary stirring ball mill, the rotation speed of the ball mill is 100-600 rpm, and the ball milling time is 2-15 h.

5. The method for preparing flake copper powder for conductive paste according to claim 1, characterized in that, In step S3, the nickel plating solution is nickel sulfate, the reducing agent is sodium hypophosphite, the concentration of sodium hypophosphite is 0.3-0.4 mol / L, the nickel plating time is 15-60 min, and the volume ratio of the flaky copper powder to the nickel plating solution is 1:10-30.

6. The method for preparing flake copper powder for conductive paste according to claim 1, characterized in that, In step S4, the washing process is first performed 3-6 times of alcohol washing, and then 3-5 times of deionized water washing, the drying temperature is 60-80℃, the drying time is 2-6 hours, and the gas atmosphere during the drying process is any one of vacuum, nitrogen, argon, or helium.

7. A flaky copper powder for conductive paste, produced according to the production method of any one of claims 1 to 6, characterized by, a volume resistivity of less than 5 x 10 -6 Ω-cm.

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