CN114340187A

CN114340187A - Method for preparing flexible printed circuit by using copper-nickel composite nano particles

Info

Publication number: CN114340187A
Application number: CN202111483069.0A
Authority: CN
Inventors: 王涛; 张亚兵
Original assignee: Tsinghua University
Current assignee: Tsinghua University
Priority date: 2021-12-07
Filing date: 2021-12-07
Publication date: 2022-04-12

Abstract

The invention discloses a method for preparing a flexible printed circuit by using copper-nickel composite nano particles, belonging to the technical field of flexible printed circuits. Firstly, preparing conductive ink from copper-nickel composite nano particles with a core-shell structure, and printing the conductive ink on the surface of a base material to prepare a line pattern; then preparing a copper coating on the circuit pattern by adopting a chemical plating method to obtain a high-conductivity copper printed circuit; the copper-nickel composite nano particles can realize magnetic separation and have a conductive function, and are used as catalyst seeds in a chemical plating process. The invention adopts the magnetic copper-nickel composite nano particles which can be magnetically separated, has excellent oxidation resistance and can obviously reduce the cost, and the chemical plating process is operated under the low-temperature atmospheric condition at the temperature of 20-60 ℃, and can be used for preparing the flexible printed circuit on the thermosensitive flexible base material.

Description

Method for preparing flexible printed circuit by using copper-nickel composite nano particles

Technical Field

The invention belongs to the technical field of flexible printed circuits, and particularly relates to a method for preparing a flexible printed circuit by using core-shell type copper-nickel composite nano particles.

Background

In recent years, flexible printed electronic technology is developed vigorously, and great progress is brought to advanced electronic product manufacturing, intelligent wearable equipment and information industry. Compared with the traditional electronic manufacturing technology, the advantages of the flexible printing electronic technology are mainly benefited from the advantages of the flexible printing electronic technology in the aspects of intellectualization and integration. Most importantly, the great flexibility, lightness and shape designability provide significant and unique advantages for flexible printed electronics. This is not possible with conventional etching techniques.

At present, the flexible printed electronic technology is mainly used for preparing electronic devices such as flexible displays, RFID labels, sensors, capacitors and the like. In these applications, a Flexible Printed Circuit (FPC) is its base and core component. The FPC is prepared by printing a conductive material or a precursor on a flexible substrate and performing post-treatment. The printing methods include inkjet printing, screen printing, and flexographic printing. Recently, the FPC manufacturing process has been widely studied and applied, including both conductive ink-based and electroless plating-based methods. The conductive ink is printed on the flexible substrate, and the conductive pattern is obtained through sintering. The conductive ink consists of metal (Ag or Cu) nanoparticles or metal organic complexes. In the electroless plating process, metal ions in a plating solution are reduced to metal and deposited on a substrate to form a conductive pattern. Conventionally, Pd, Sn, or Ag is used as a catalytic seed for electroless plating, and formaldehyde is used as a reducing agent for electroless plating. However, both the conductive ink process and the electroless plating process have some drawbacks. For conductive ink processes, high temperature (>200 ℃) sintering is generally required, and is not suitable for heat-sensitive substrates such as PET, fiber paper and the like. Therefore, many researchers have developed alternative sintering methods, such as plasma sintering, photo sintering, etc.; however, these sintering methods produce conductive patterns having relatively high electrical resistance because the non-conductive material contained in the ink is not completely removed. In addition, the thickness of the conductive pattern produced by printing with the conductive ink is small, which also results in a relatively high resistance of the conductive pattern, limited by the content of the conductive component in the ink. For the chemical plating process, the common reducing agent formaldehyde is easy to volatilize and cause carcinogenesis, and the waste liquid containing formaldehyde can cause disastrous damage to the environment. In addition, noble metal catalysts can increase the cost of the electroless plating process.

Compared with Ag, Cu is much cheaper and has less resistance difference. However, one significant drawback is that Cu is particularly susceptible to oxidation on a nanometer scale, which increases the resistance of the conductive copper pattern. In previous studies (Ind. Eng. chem. Res. 2018,57,2508-2516), silver-coated copper nanoparticle conductive ink with a core-shell structure was synthesized and a flexible printed circuit was successfully prepared. Silver-coated copper nanoparticles have good oxidation resistance, but the conductivity of the prepared printed circuit is poor compared to bulk copper (resistivity of 25.3 μ Ω cm, 14.88 times that of bulk copper). Fu-Tao Zhang et al (ACS appl. Mater. Interfaces 2018,10,2075-2082) prepared a Cu-PDMS conductor with high conductivity by using silver ions as an initiator and formaldehyde as a reducing agent of an electroless plating bath under the synergistic action of polydopamine, but the preparation process is complicated and formaldehyde which is easy to volatilize is used as the reducing agent. Jingxuan Cai and the like (ACS appl. Mater. interfaces 2018,10,28754-28763) take noble metal Pd nano-particles as a catalyst, formaldehyde as a reducing agent of an electroless plating bath, and a transparent electrode with light transmittance of more than 80 percent and square resistance of less than 1 omega/sq is prepared through the steps of photoetching, hot-press printing, electroless plating and the like.

Patent CN 108866521B developed formaldehyde/sodium hypophosphite electroless copper plating solution for copper plating of PCB boards. The process has the advantage that it does not contain cyanide stabilizers, but the reducing agent still contains formaldehyde. Patent CN 105821396 a developed a palladium-free electroless plating process, in which dopamine was used to reduce silver ions into silver particles as a catalyst seed layer, and conductive copper layers were prepared on the surfaces of various substrates, but the process steps are complicated, and noble metal silver was used as an activator.

By combining the existing research and report, the copper printed circuit resistor prepared by directly sintering the conductive copper ink is generally larger and can not be used for a heat-sensitive flexible substrate. Conductive patterns produced using electroless copper plating processes generally have a relatively low electrical resistance, but most of the current electroless plating studies use noble metals Pd, Pt, Ag, etc. as catalytic seeds and formaldehyde as a reducing agent for the electroless plating bath. This leads on the one hand to increased costs of the process and on the other hand to the health of humans and the environmental hazard caused by the use of volatile carcinogenic formaldehyde in large quantities.

Disclosure of Invention

In order to solve the above problems, the present invention provides a method for preparing a flexible printed circuit using copper-nickel composite nanoparticles, comprising the steps of:

firstly, preparing conductive ink from copper-nickel composite nano particles of core-shell structure copper-clad nickel, and printing the conductive ink on the surface of a base material to prepare a line pattern;

then preparing a copper coating on the circuit pattern by adopting a chemical plating method to obtain a high-conductivity copper printed circuit;

the copper-nickel composite nano particles can realize magnetic separation and have a conductive function and are used as catalyst seeds for chemical plating; the copper-nickel composite nano particles can utilize the conductive and magnetic functions of nickel metal, and are convenient to recover; the copper-clad structure also overcomes the defect that pure nickel is difficult to disperse uniformly;

the preparation method of the copper-nickel composite nano-particles comprises the following steps:

in liquid paraffin, carrying out thermal decomposition on a nickel acetate-oleylamine complex and a copper formate-oleylamine complex to prepare the copper-nickel composite nano-particles with the copper-nickel core-shell structure.

Further, the preparation method of the copper-nickel composite nano-particles comprises the following steps: mixing nickel acetate and oleylamine, adding into liquid paraffin, complexing for 1-2 h at 40-60 ℃ under the condition of introducing nitrogen, then heating to 220 ℃, reacting for 30min, naturally cooling to 80 ℃, adding a complex of copper formate and oleylamine, and reacting for 15 h; after the reaction is finished, separating the particles from the solution by magnetic force, and washing by using a mixed solvent of n-hexane and isopropanol with the same volume to obtain the copper-nickel composite nano particles with the copper-coated nickel core-shell structure, wherein the particle size of the copper-nickel composite nano particles is 30-60 nm, and the surfaces of the copper-nickel composite nano particles are coated by oleylamine.

The preparation method of the conductive ink comprises the following steps: dispersing copper-nickel composite nano particles in a solvent to prepare copper-nickel composite nano particle conductive ink with solid content of 0.1-10 wt%; the solvent is n-hexane, n-octane, petroleum ether, turpentine or mixture thereof.

The substrate comprises PET, PI or PEN;

the base material is pretreated before use: washing with deionized water and ethanol, drying at 50-60 ℃, soaking in 1-10 wt% ethanol solution containing silane for 1-6 h, washing with ethanol for multiple times to remove residues, and drying at 50-60 ℃ for 6 h;

the silane is 3-mercaptotriethoxysilane or 3-aminotriethoxysilane.

The printing method of the conductive ink comprises the following steps: inkjet printing, screen printing, gravure printing, letterpress printing, drop coating, blade coating, or coating.

Before chemical plating, placing the base material with the printed circuit pattern in 1-5 wt% formic acid solution, washing for 1-5 minutes, removing oleylamine coated on the surface of the copper nano-particles, taking out, washing with deionized water and drying; further, the solvent of the formic acid solution is ethanol or methanol.

The chemical plating method comprises the following steps: placing the base material printed with the circuit pattern and subjected to the deoiling amine treatment in chemical plating solution, carrying out chemical copper plating for 1-6 h at the temperature of 20-60 ℃, taking out the base material, washing the base material with deionized water for multiple times, and drying the base material for 6h at the temperature of 50-60 ℃ to obtain the high-conductivity copper printed circuit.

The chemical plating solution comprises the following components in percentage by concentration: dimethylamino borane (DMAB, 0.01-0.5 mol/L), ethylenediamine tetraacetic acid (EDTA, 0.001-0.1 mol/L), triethanolamine (TEA, 0.05-1.2 mol/L), copper sulfate pentahydrate (0.001-0.1 mol/L) and deionized water.

The chemical plating solution is recycled after being supplemented with copper salt and a reducing agent; preferably, the supplemental copper salt is copper sulfate pentahydrate and the reducing agent is dimethylaminoborane.

The invention has the beneficial effects that:

1. aiming at the defects of the existing conductive ink and chemical plating method, the invention provides a method for preparing a high-conductivity copper printed circuit by using cheap and magnetically separable magnetic copper-nickel-coated core-shell copper-nickel composite nano particles to replace the traditional noble metal as catalyst seeds for chemical plating and combining a process for preparing the high-conductivity copper printed circuit by non-formaldehyde chemical plating. The copper-nickel composite nano particles have low cost relative to noble metals, have oxidation resistance and magnetism, and can realize magnetic separation to replace centrifugal separation;

2. the dimethylamino borane provided by the invention replaces the traditional high-toxicity formaldehyde as a reducing agent, and can be used for carrying out chemical copper plating under the condition of weak alkalinity and mild condition. The chemical plating solution does not contain formaldehyde, can be kept stable for a long time, and can be recycled by supplementing reactants;

3. the prepared copper printed circuit has high conductivity, strong adhesion and good oxidation resistance;

4. the preparation process is simple, convenient and feasible, environment-friendly and suitable for large-scale production;

5. the preparation method adopts the operation under the conditions of medium and low temperature (20-60 ℃) and atmosphere, has simple equipment, avoids the influence of the operation such as high-temperature sintering and the like on the thermosensitive flexible substrate, and can be used for preparing the printed circuit on the thermosensitive flexible substrate.

Drawings

FIG. 1 is a transmission electron micrograph of copper-coated nickel nanoparticles;

FIG. 2 is an XRD pattern of copper-clad nickel nanoparticles;

FIG. 3 is an optical photograph of the printed circuit prepared;

FIG. 4 is a photograph of a conductivity test of the prepared printed circuit;

FIG. 5 is a scanning electron micrograph of a copper film of the printed circuit prepared;

FIG. 6 is a plot of sheet resistance versus time for the conductive copper film of case 1;

FIG. 7 is a plot of sheet resistance versus time for the conductive copper film of case 2;

FIG. 8 shows the replenishment of copper sulfate pentahydrate and DMAB during a cycling experiment using the conductive copper printed circuit obtained in example 3;

FIG. 9 is a block resistance of a cyclic experimental process using the conductive copper printed circuit obtained in example 3.

Detailed Description

The invention is described in further detail below with reference to the following figures and specific examples:

1) in liquid paraffin, thermally decomposing a nickel acetate-oleylamine complex and a copper formate-oleylamine complex to prepare core-shell type copper-coated nickel composite nano particles with the surface coated with oleylamine and the particle size of 30-60 nm;

2) dispersing copper-nickel composite nano particles coated by oleylamine in a solvent to prepare conductive ink with solid content of 0.1-10 wt%;

3) washing the heat-sensitive flexible base material with deionized water and ethanol for multiple times, drying at 50-60 ℃, soaking in an ethanol solution containing 1-10 wt% of silane for 1-6 h, washing with ethanol for multiple times to remove residues, and drying at 50-60 ℃ for 6 h;

4) printing the copper-nickel composite nanoparticle conductive ink on the surface of the modified base material by using an ink-jet printer or a coating method to prepare a printed pattern;

5) placing the printed pattern in a methanol or ethanol solution containing 1-5 wt% of formic acid, washing for 1-5 minutes, removing oleylamine coated on the surface of the nano-particles, taking out, washing with deionized water for multiple times, and drying;

6) placing the pattern treated in the step 5) in a chemical plating bath, carrying out chemical copper plating for 1-6 h at the temperature of 20-60 ℃, taking out the pattern, washing the pattern for multiple times by using deionized water, and drying the pattern for 6h at the temperature of 50-60 ℃ to obtain a flexible high-conductivity copper printed circuit;

and (4) supplementing copper salt and reducing agent to the chemical plating solution of the step 6) for recycling.

Example 1

1) 0.707g of nickel acetate and 4.76g of oleylamine are mixed, added into 70mL of liquid paraffin, complexed for 2 hours at 60 ℃ under the condition of introducing nitrogen, then heated to 220 ℃, reacted for 30 minutes, naturally cooled to 80 ℃, added with a complex of copper formate and oleylamine (obtained by complexing 0.154g of copper formate, 1.2g of oleylamine, 10mL of liquid paraffin at 60 ℃ for 1 hour), and then reacted for 15 hours. After the reaction is finished, magnetically separating out particles, and washing the particles once by using a mixture of 15mL of n-hexane and 15mL of isopropanol to obtain copper-clad nickel core-shell type copper-nickel composite nanoparticles;

2) weighing 0.2g of the copper-nickel composite nano-particles with the particle size of 39nm obtained in the step 1), adding the weighed particles into a mixture of 2mL of n-hexane and 2mL of n-octane to prepare copper-nickel composite nano-particle conductive ink with the volume concentration of 0.05 g/mL;

3) washing a PET substrate with deionized water and ethanol for multiple times, drying at 50-60 ℃, soaking in an ethanol solution containing 5 wt% of 3-mercaptotriethoxysilane for 3 hours, washing with ethanol for multiple times to remove residues, and drying at 50-60 ℃ for 6 hours;

4) 0.1mL of conductive ink is extracted and dropped on the surface of the PET substrate which is 5cm multiplied by 5cm and modified by 3-mercaptotriethoxysilane, and a printed pattern is prepared;

5) cutting the base material obtained in the step 4) into slices of 2cm multiplied by 2cm, then soaking in an ethanol solution containing 1 wt% of formic acid for 2min to remove oleylamine on the surface of the copper-nickel composite nano particles, taking out, cleaning with deionized water and drying;

6) the step 5) substrate was added to 10mL of plating solution (plating bath) (copper sulfate pentahydrate: 0.004mol/L, 0.028mol/L of ethylenediamine tetraacetic acid, 0.3mol/L of triethanolamine and 0.08mol/L of dimethylamine borane) at 20 ℃ for 1 to 6 hours, then taking out the solution to be washed by deionized water, and treating the solution in a 60 ℃ oven for 3 hours to obtain the conductive copper printed circuit.

As shown in fig. 1, the tem photograph of the cu-ni composite nanoparticle shows that the type of the prepared cu-ni composite nanoparticle is copper-clad ni-core shell type, and the cu-ni composite nanoparticle has magnetism, can realize magnetic separation, and has an average particle size of 39 nm. FIG. 2 is an XRD pattern of the copper-nickel composite nanoparticle from which Ni, Cu and Cu are seen₂Diffraction peak of O. Cu₂The small diffraction peak of O indicates that the surface of the particles was only slightly oxidized, and no peak of NiO was observed. An optical photo of the copper printed circuit obtained by chemically plating copper on the PET surface for 6 hours after the modification of the 3-mercaptotriethoxysilane is shown in figure 3, and the copper film has a smooth surface and good adhesion. The conductivity test of the copper printed circuit showed that the circuit had good conductivity and the lamp was lit as shown in fig. 4. The scanning electron micrograph of the copper layer is shown in fig. 5, and the copper film has a compact surface and a complete structure. The sheet resistance of the copper layer versus time is shown in figure 6. As the electroless plating time increased, the sheet resistance of the copper film decreased significantly until it stabilized. The minimum sheet resistance of the copper film was 43.6 m.OMEGA/sq, the maximum film thickness was 1.12. mu.m, and the minimum resistivity was 4.88. mu. OMEGA.cm (2.87 times the bulk copper resistivity).

Example 2

1) 0.707g of nickel acetate and 4.76g of oleylamine are mixed, added into 70mL of liquid paraffin, complexed for 2 hours at 60 ℃ under the condition of introducing nitrogen, then heated to 220 ℃, reacted for 30 minutes, naturally cooled to 80 ℃, added with a complex of copper formate and oleylamine (obtained by complexing 0.154g of copper formate, 1.2g of oleylamine, 10mL of liquid paraffin at 60 ℃ for 1 hour), and then reacted for 15 hours. After the reaction is finished, magnetically separating out particles, and washing the particles once by using a mixture of 15mL of n-hexane and 15mL of isopropanol to obtain the copper-nickel-coated core-shell copper-nickel composite nano particles;

2) weighing 0.2g (with the particle size of 39nm) of the copper-nickel composite nano particles obtained in the step 1), adding the weighed particles into a mixture of 2mL of n-hexane and 2mL of n-octane, and preparing the copper-nickel composite nano particle conductive ink with the volume concentration of 0.05 g/mL;

3) washing a PET substrate with deionized water and ethanol for multiple times, drying at 50-60 ℃, soaking in an ethanol solution containing 10 wt% of 3-mercaptotriethoxysilane for 1h, washing with ethanol for multiple times to remove residues, and drying at 50-60 ℃ for 6 h;

5) cutting the base material obtained in the step 4) into slices of 2cm multiplied by 2cm, then treating the slices in an ethanol solution containing 1 wt% of formic acid for 2min to remove oleylamine on the surface of the copper-nickel composite nano particles, taking out the slices, cleaning the slices with deionized water and drying the slices;

6) the step 5) substrate was added to a 10mL plating bath (copper sulfate pentahydrate: 0.024mol/L, 0.028mol/L of ethylene diamine tetraacetic acid, 0.3mol/L of triethanolamine and 0.08mol/L of dimethylamine borane) at 45 ℃ for 1 to 6 hours, then taking out and washing with deionized water, and carrying out oven treatment at 60 ℃ for 3 hours to obtain the conductive copper printed circuit.

FIG. 7 shows the sheet resistance of the copper film prepared in step 5) of example 3 as a function of time. The sheet resistance of the copper film gradually decreases as the electroless plating time increases. The square resistance of the copper film obtained by electroless plating for 6h is 5.25m omega/sq, the film thickness is 3.62 mu m, the resistivity is 1.90 mu omega cm (1.12 times of the resistivity of bulk copper), and the copper film has good conductivity.

Example 3

Carrying out chemical plating for 1h by adopting the plating bath in the step 6) according to the method of the embodiment 2, then taking out the plated copper film, washing the copper film by using deionized water, carrying out drying oven treatment at 60 ℃ for 3h to finally obtain a conductive copper film, and measuring the square resistance of the copper film by using a four-probe resistance meter;

the copper ion concentration and the dimethylamine borane concentration in the plating bath after the electroless plating in step 6) in example 2 were measured by flame atomic absorption and a potentiometric titrator, respectively; according to the measured copper ion concentration and dimethylamine concentration, corresponding content of copper sulfate pentahydrate and dimethylamine borane is supplemented, so that the concentration of the plating bath is equal to the concentration at the beginning of the step 1); repeating the chemical plating process, and drying the obtained copper film to measure the resistance;

after replenishment of copper ion concentration and reducing agent, the electroless plating bath was cycled repeatedly 5 times.

Claims

1. A method for preparing a flexible printed circuit by using copper-nickel composite nano particles is characterized by comprising the following steps:

firstly, preparing conductive ink from copper-nickel composite nano particles with a core-shell structure, and printing the conductive ink on the surface of a base material to prepare a line pattern;

the copper-nickel composite nano particles can realize magnetic separation and have a conductive function and are used as catalyst seeds in a chemical plating process;

in liquid paraffin, mixing nickel acetate-oleylamine complex and copper formate-oleylamine complex, and thermally decomposing to prepare the copper-nickel composite nano-particles with the copper-nickel core-shell structure.

2. The method for preparing a flexible printed circuit by using the copper-nickel composite nano-particles as claimed in claim 1, wherein the method for preparing the copper-nickel composite nano-particles comprises the following steps: mixing nickel acetate and oleylamine, adding into liquid paraffin, complexing for 1-2 h at 40-60 ℃ under the condition of introducing nitrogen, then heating to 220 ℃, reacting for 30min, naturally cooling to 80 ℃, adding a complex of copper formate and oleylamine, and reacting for 15 h; after the reaction is finished, separating the particles from the solution by magnetic force, and washing the particles by using a mixed solvent of n-hexane and isopropanol with the same volume to obtain the copper-nickel composite nano particles with the copper-clad nickel core-shell structure, wherein the particle size of the copper-nickel composite nano particles is 30-60 nm.

3. The method for preparing flexible printed circuit by using copper-nickel composite nano particles as claimed in claim 1, wherein the conductive ink is prepared by the following steps: dispersing copper-nickel composite nano particles in a solvent to prepare copper-nickel composite nano particle conductive ink with solid content of 0.1-10 wt%; the solvent is n-hexane, n-octane, petroleum ether, turpentine or mixture thereof.

4. The method for preparing a flexible printed circuit using copper-nickel composite nano-particles as claimed in claim 1, wherein the substrate comprises PET, PI or PEN;

the silane is 3-mercaptotriethoxysilane or 3-aminotriethoxysilane.

5. The method for preparing a flexible printed circuit using copper-nickel composite nano-particles as claimed in claim 1, wherein the printing method of the conductive ink comprises: inkjet printing, screen printing, gravure printing, letterpress printing, drop coating, blade coating, or coating.

6. The method for preparing a flexible printed circuit using copper-nickel composite nano-particles as claimed in claim 1, wherein the substrate with the printed circuit pattern is placed in 1 wt% -5 wt% formic acid solution before electroless plating, washed for 1-5 min, removed of oleylamine coated on the surface of the copper nano-particles, and then taken out and washed with deionized water for drying.

7. The method for preparing flexible printed circuit using copper-nickel composite nano-particles as claimed in claim 6, wherein the solvent of the formic acid solution is ethanol or methanol.

8. The method for preparing flexible printed circuit by using copper-nickel composite nano particles as claimed in any one of claims 1, 6 or 7, wherein the electroless plating method comprises the following steps: placing the base material printed with the circuit pattern and subjected to de-oiling treatment in a chemical plating solution, carrying out chemical copper plating for 1-6 h at 20-60 ℃, taking out the base material and washing the base material with deionized water for multiple times, and drying the base material for 6h at 50-60 ℃ to obtain the high-conductivity copper printed circuit.

9. The method for preparing a flexible printed circuit using copper-nickel composite nanoparticles as claimed in claim 1, wherein the electroless plating solution is composed of dimethylaminoborane, ethylenediaminetetraacetic acid, triethanolamine, copper sulfate pentahydrate, and deionized water; the component concentrations are 0.01 mol/L-0.5 mol/L of dimethylamino borane, 0.001 mol/L-0.1 mol/L of ethylene diamine tetraacetic acid, 0.05 mol/L-1.2 mol/L of triethanolamine and 0.001 mol/L-0.1 mol/L of blue vitriol respectively.

10. The method of claim 1, wherein the electroless plating solution is recycled after being supplemented with copper salt and a reducing agent; preferably, the supplemental copper salt is copper sulfate pentahydrate and the supplemental reducing agent is dimethyl amino borane.