JP2015214722A - Method for manufacturing copper fine particle sintered body and conductive substrate - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide new methods for manufacturing a conductive copper fine particle sintered body and a conductive substrate using the sintered body, which can be applied to a coating machine or screen printing using a calcination binder and avoid problems accompanying solder resist coating, without requiring a large amount of a protective agent.SOLUTION: The method for manufacturing a conductive copper fine particle sintered body comprises producing a sintered body of copper fine particles having a resin coating film on a substrate through at least the following processes. The processes are: (A) applying a conductive ink comprising copper fine particles and a calcination binder on the substrate; (B) oxidizing the ink to obtain a cuprous oxide (CuO) fraction of 7 mass% or more in the copper particles after applied; and (C) reducing the ink after the oxidation. The copper fine particles applied on the substrate have an average particle diameter of 100 nm or more; and the calcination binder has a thermal decomposition temperature higher than the temperatures of the oxidation process and the reduction process.

Description

  The present invention is a conductive copper fine particle sintered body useful in the fields of circuit forming materials such as printed circuit boards, other fine wiring materials, antistatic materials, electromagnetic wave shielding materials, infrared shielding materials, etc. The present invention relates to a method for manufacturing a conductive substrate.

  Printed electronics that draws conductive ink containing fine metal particles as a predetermined wiring pattern using existing printing technology eliminates harmful chemical substances without the need for exposure and etching used in conventional printed circuit board fabrication methods. Recently, it has been attracting attention as a clean manufacturing process. As printing techniques at that time, coating machines such as a doctor blade, a die coater, a bar coater, a flow coater, and a spin coater, screen printing, ink jet, and spray are known. The conductive ink is mainly composed of a dispersion stabilizer and an organic solvent in addition to the metal fine particles. In the case of a coater or screen printing, a baked binder is added in order to give appropriate fluidity and adhesion to the ink. As the baking binder, synthetic resins such as acrylic, phenol, and epoxy, celluloses such as acetyl cellulose and ethyl cellulose, rubbers, and the like are used.

  However, since the printed conductive ink is an aggregate of independent fine particles as it is, the conductivity is very low. For this reason, conductivity is imparted by heat treatment to form a sintered body of metal fine particles. Although general metal sintering is performed at a temperature of 1000 ° C. or higher, application to printed wiring materials requires sintering at a low temperature below the thermal decomposition temperature of 250 to 350 ° C. of the epoxy resin of the substrate material. . As a low-temperature sintering method for this purpose, it is known that atomization of atoms and molecules occurs at a temperature lower than that of a bulk material by making metal particles atomized to the order of nm, thereby facilitating sintering. And in the production | generation process of a metal microparticle, the organic protective agent for controlling a particle diameter is used, and the microparticles | fine-particles produced | generated in connection with this are covered with the organic protective layer. Since the sintering phenomenon does not occur unless the metal particles are in contact with each other, it is necessary to remove these organic protective layers during the sintering, and various methods have been devised so far. For example, a method of obtaining a conductive thin film by a two-stage firing method in which an organic protective layer covering the periphery of copper fine particles is decomposed in an oxygen-containing atmosphere and then reduced in a hydrogen-containing or inert atmosphere has been devised ( Patent Document 1, Non-Patent Document 1).

  The wiring material produced in this way is used with a solder resist applied to ensure electrical insulation and prevent ion migration in a humidity environment. The solder resist is formed by printing a resist agent on the entire surface of the substrate, then curing the resist forming surface with ultraviolet rays, and washing and removing uncured portions with an alkali cleaning liquid.

JP 2007-262446 A

Thin Solid Films 520 (2012) 2789-2793

  In the methods of Patent Document 1 and Non-Patent Document 1, the organic protective agent around the fine particles is removed by thermal decomposition in the first baking step. However, when the baking binder is added to the ink, a large amount of the organic protective agent is removed. Since the binder resin is developed around the fine particles, it is considered difficult to decompose and remove them. Therefore, it is difficult to apply this method to a coating machine or screen printing that requires a baking binder.

  The smaller the metal fine particles, the lower the sintering temperature can be expected, but the fine particles below submicron have a strong cohesive force and are difficult to disperse in the solution. In addition, the smaller the particles, the larger the surface area, and inevitably increases the amount of organic protective agent covering the periphery of the particles. A large amount of the protective agent makes heat decomposition difficult and sometimes causes gelation of the ink, which may cause clogging of nozzles such as ink jet and spraying in combination with particle characteristics that easily aggregate.

  And the big problem with the conventional technology that various proposals have been made is that, even in printed electronics, which is attracting attention as a clean manufacturing process that does not require chemical etching, it is necessary to apply a solder resist after wiring formation In this process, ultraviolet irradiation and use of harmful chemical substances are unavoidable as in the conventional printed circuit board.

  The present invention has been made in view of the circumstances as described above, and can be applied to a coating machine or a screen printing using a baked binder, without requiring a large amount of a protective agent, It is an object of the present invention to provide a new method for producing a conductive copper fine particle sintered body and a conductive substrate using the same, which is free from inconvenience associated with resist coating.

  The manufacturing method of the present invention is characterized by the following.

  That is, it is a method for producing a conductive copper fine particle sintered body characterized in that a copper fine particle sintered body having a resin film on a substrate is generated at least in the following processing steps.

(A) Application of conductive ink containing copper fine particles and baked binder on substrate (B) Cuprous oxide (Cu ₂ O) fraction of copper fine particles after application is 7% by mass or more (C) Reduction treatment after the oxidation treatment In the above production method, the average particle diameter of the copper fine particles applied on the substrate is preferably 100 nm or more.

  Moreover, it is preferable that the thermal decomposition start temperature of a baking binder is higher than the temperature of an oxidation process and a reduction process.

  It is also preferable to be an oxidation treatment by heating in air or a reduction treatment with hydrogen.

  And this invention also provides the manufacturing method of the electroconductive board | substrate characterized by forming with the above method.

  Also provided are a copper fine particle sintered body obtained by the above method and a conductive substrate.

  In the present invention as described above, copper fine particles having an average particle diameter of 100 nm or more can be used. The conductive ink is composed of copper fine particles having an organic protective layer, a dispersion stabilizer, an organic solvent, and a baked binder in a conventional manner, but the baked binder resin should be made of a material having a higher thermal decomposition start temperature than the baked temperature. preferable. After the produced copper fine particle ink is applied to the substrate, the copper fine particles are oxidized in air at a firing target temperature until the cuprous oxide content becomes 7% by mass or more. Thereafter, reduction is performed in an inert atmosphere containing a small amount of hydrogen to obtain a conductive thin film.

  According to the present invention, since the conductive ink contains the binder resin, appropriate fluidity and adhesiveness are imparted to the ink, and it can be applied to any printing technique. Copper fine particles exceeding 100 nm reduce the cohesive force of the particles and contribute to the improvement of the dispersibility of the ink. The copper fine particles coated on the substrate are heated and oxidized in air until the cuprous oxide content is 7% by mass or more, resulting in expansion of the particles due to the difference in density between copper and cuprous oxide. Even in the presence of a protective agent, it can come into contact with adjacent metal particles, facilitating sintering. In addition, since the cuprous oxide fine particles produced at this time are several tens of nanometers or less, the sintering is likely to proceed due to a melting point drop during the subsequent reduction to copper by heat treatment in an inert atmosphere containing a small amount of hydrogen. There is. On the other hand, since the thermal decomposition start temperature of the binder resin is higher than the firing temperature, some of it seems to be carbonized in a reducing atmosphere, but most of it remains unaffected by heating and is coated around the sintered copper fine particles. Form. By using a hydrophobic and heat-stable material such as an acrylic resin, a phenol resin, or ethyl cellulose as the binder resin, it is possible to impart the same function as the solder resist.

  The conductive substrate thus obtained is given high conductivity by sintering, and at the same time, the binder resin forms a film around the sintered body. By selecting a material excellent in hydrophobicity and thermal stability for the binder resin, a resist layer having the same function as a solder resist can be imparted to the produced conductive substrate. As described above, if this method is used, it is possible to simultaneously form the resist layer in a series of processes from wiring pattern printing to baking, which contributes to a significant reduction in the process and emits no harmful chemical substances at all. A clean manufacturing process that does not occur can be realized.

The vehicle TG measurement result in Example 1. 2 is a cross-sectional SEM image of the copper thin film obtained in Example 1. FIG. The XRD measurement result in Example 1. 2 is an SEM image after printing in Example 1. FIG. 2 is an SEM image after the oxidation treatment in Example 1. FIG. 2 is an SEM image after reduction treatment in Example 1. FIG. 2 is a TEM image of particle shapes in each step of Example 1. FIG. 3 is a high-magnification TEM image of particles after oxidation treatment in Example 1. FIG. The schematic diagram of the sintering mechanism of this invention. 2 is a cross-sectional SEM image of the copper thin film obtained in Example 2. FIG. The XRD measurement result in Example 2. 3 is an SEM image after the oxidation treatment in Example 2. FIG. FIG. 4 is an SEM image after reduction treatment in Example 2. FIG. XRD measurement result in Comparative Example 1. The SEM image after the reduction process in Comparative Example 1.

  As described above, the method for producing a conductive copper fine particle sintered body and a conductive substrate according to the present invention requires that a copper fine particle sintered body having a resin film on the substrate is generated at least in the following processing steps. Yes.

  (A) Application of conductive ink containing copper fine particles and a baking binder on a substrate.

(B) An oxidation treatment of copper fine particles in which the cuprous oxide (Cu ₂ O) fraction in the copper fine particles after coating is 7% by mass or more.

  (C) Reduction treatment after the oxidation treatment.

  First, the conductive ink in the processing step (A) will be described. It is essential to include copper fine particles and a fired binder. Here, the copper fine particles preferably have an average particle diameter of 100 nm or more. The method for producing and preparing such copper fine particles is not particularly limited, and examples thereof include a hydrazine reduction method, a polyol method, a laser ablation method, and a submerged plasma method.

  As described above, in the present invention, preferably, the average particle diameter is set to 100 nm or more in order to reduce the cohesive force of the copper particles. More preferably, the average particle size is in the range of 100 nm to 300 nm.

  Here, the average particle diameter of the copper particles can be obtained as a number average of the projected diameters (constant direction diameters) by randomly extracting 50 particles from the field of view by SEM photograph observation.

  And about a baking binder, the thermal decomposition start temperature is higher than the temperature of an oxidation process and a reduction process, and the material which has hydrophobicity and heat stability is preferable. Further, considering viscosity, handleability and the like, for example, a cellulose-based material is exemplified as a suitable material. Polymers such as ethyl cellulose, agar, polyvinyl butyral, polyacryl, PEEK, urethane, polyamideimide, polyethersulfone, and copolymers thereof. In addition, phenol resin materials, epoxy materials, and the like are also considered.

  In the present invention, high conductivity is imparted by sintering, and at the same time, the sintered binder resin forms a coating around the sintered body. By selecting a material excellent in hydrophobicity and thermal stability for the binder resin, a resist layer having the same function as a solder resist can be imparted to the produced conductive substrate. In a series of processes from wiring pattern printing to baking, it is possible to form the resist layer at the same time, contributing to drastic reduction of the process and realizing a clean manufacturing process that does not emit any harmful chemical substances. Become. In the (C) reduction treatment step in an inert atmosphere containing hydrogen, the surface of the binder resin containing the copper fine particle sintered body may be carbonized. In such a case, more hydrophobicity is imparted. This supplements that the function of the resist layer is strengthened.

  In addition, since ethyl cellulose softens when heated to about 150 ° C., it facilitates contact between the copper fine particles due to the formation of cuprous oxide in the pre-sintering process, and also rapidly flows and burns during the reduction of cuprous oxide. This is desirable because a film can be formed around the bonded body.

  In the conductive ink, a blend of a dispersant, a solvent and the like is further considered. Examples of the dispersant include polymer-type dispersants such as polyamine type and polycarboxylic acid type, and surfactants such as alkyl polyamines. Examples of the solvent include terpineol and ethylene glycol.

  As for the composition ratio of the conductive ink, the following range is generally exemplified when the whole is 100 mass%.

Copper fine particles: 10 to 70% by weight
Firing binder: 10% by weight or less Dispersant: 0% to 10% by weight
Solvent: 30% to 90% by weight
About this composition ratio, since it is preferable that the copper fine particle exceeds the presence level of a baking binder at the time of baking in processing process (B) (C), the compounding ratio of a copper fine particle and a baking binder is
33: 1-10: 1 (Copper fine particles: Binder weight ratio)
It is preferable to be in the range. In the treatment step (B), heating can be performed using air or a mixed gas with oxygen gas or an inert gas thereof.

  In the treatment step (C), heating can be performed using hydrogen gas or a mixed gas of hydrogen gas and inert gas.

In the treatment step (B), when considering the contact between adjacent copper fine particles (sheet resistance after oxidation treatment of 10 MΩ / sq or less in Example 3 described later), the cuprous oxide (Cu ₂ O) fraction in the copper fine particles is Make sure it is at least 7%. It is preferably within the range of 7% to 24%. In the case of heat treatment in air, the heating temperature can be 130 ° C. or more and 170 ° C. or less.

  If the cuprous oxide (Cu2O) fraction other than this range greatly exceeds 30%, the heating temperature under air will be as high as 200 ° C (from the results shown in Example 3). In order to reduce the oxygen concentration, it is necessary to separately adjust the oxygen concentration by introducing oxygen gas. In addition, with the generation of a large amount of cuprous oxide, it is considered that measures such as 1) increasing the reduction time, 2) raising the reduction temperature, and 3) raising the hydrogen concentration are necessary for the reduction treatment.

  In the present invention, for example, as described above, copper fine particles of 100 nm or more reduce the cohesive strength of the particles. By adding a baked binder resin to the conductive ink, appropriate fluidity and adhesion are imparted to the ink. The copper fine particles coated on the substrate are heated and oxidized in air until the cuprous oxide content is 7% by mass or more, resulting in expansion of the particles due to the difference in density between copper and cuprous oxide. Even in the presence of a protective agent, it comes into contact with adjacent metal particles. Since the cuprous oxide fine particles generated at this time are several tens of nm or less, sintering by a melting point drop proceeds in the process of reduction to copper by subsequent heat treatment in an inert atmosphere containing a small amount of hydrogen. Since the binder resin uses a material having a thermal decomposition start temperature higher than the firing temperature, most of the binder resin remains unaffected by heating and forms a film around the sintered copper fine particles.

  Hereinafter, examples will be shown and described in more detail.

<Example 1>
Copper particles were synthesized using gelatin (made by Nitta Gelatin) as an organic protective agent, copper (II) oxide (made by Nisshin Chemco) as a raw material, and hydrazine monohydrate (made by Kanto Chemical or Pure Chemical) as a reducing agent. . That is, first, 8 parts by weight of copper oxide was added to an aqueous solution in which gelatin and pure water were sufficiently dissolved at a ratio of 1:30, adjusted to pH 11 with aqueous ammonia, and then stirred at 80 ° C. After raising the temperature, 2.4 mol of hydrazine monohydrate was added to 1 mol of copper oxide and reacted for 2 hours. The copper particles obtained by removing the supernatant and washing with water by the decant method were mixed with 5 parts by weight of a dispersant, 53 parts by weight of a firing vehicle (EC vehicle manufactured by Nisshin Kasei) and 53 parts by weight of α-terpineol. Thereafter, an ink of about 48 wt% copper fine particles was prepared using an ultrahigh pressure disperser. The firing vehicle is composed of a solvent and ethyl cellulose which is a firing binder, and the solvent is almost completely volatilized at 150 ° C., and the decomposition start temperature of ethyl cellulose is found to be about 210 ° C. from the measurement result of TG ( Figure 1).

  The obtained ink was printed on an alumina substrate with a doctor blade with a coating thickness of 20 μm, dried in a solvent at 60 ° C. in nitrogen, and oxidized for 4 hours in air at 200 ° C., and 3% at 200 ° C. A thin film sintered body of copper particles was obtained by reduction treatment for 2 hours in a 97% hydrogen mixed atmosphere. The film thickness was about 2.7μm from the observation result of the SEM cross section (Fig. 2).

Conductivity is measured by a resistivity meter Loresta-GP, measuring 6 points in total, 3 points in length and width on a 2 cm square sample, with an average volume resistivity of 3 × 10 ⁻⁵ Ω · cm, 1.55 × 10 ^{−6 of} bulk copper Good conductivity was exhibited for Ω · cm.

  The XRD measurement result (FIG. 3) of the firing process shows that a) after printing, b) the sample was oxidized after the oxidation treatment, and c) re-reduced to copper after the reduction treatment. As a result of semi-quantitative analysis (RIR method) by XRD, after oxidation treatment of this sample, copper was 71 wt% and cuprous oxide was 29 wt%.

  In SEM observation in each process, in order to confirm the residue of organic components, as shown in FIGS. 4, 5, and 6, a) secondary electron images and b) reflected electron images were taken and compared. b) Since the backscattered electron image has an atomic number dependency, a large atomic number such as metallic copper is bright, and a small atomic number such as an organic component is a dark image. After printing, each is an independent particle covered with organic components (Figure 4). When heated in air, many particles of several tens of nanometers or less are generated and the particles come into contact with each other (Fig. 5). Furthermore, when heated in 3% hydrogen, fine particles are bonded to form a sintered body, and it can be confirmed that the periphery is covered with a binder resin (FIG. 6).

  In order to grasp the change of the particle shape in each step, a small amount of the synthesized copper fine particles, the oxidation treatment, and the reduction treatment sample were sampled by peeling and observed (FIG. 7). a) Copper particles (after synthesis) b) After oxidation treatment, fine particles of about 10 nm are generated around copper fine particles of about 100 to 200 nm. c) From the TEM observation image after the reduction treatment, it can be clearly confirmed that the particles are sintered. b) As a result of measuring the diffraction grating confirmed by high-magnification observation, the fine particles of about 10 nm generated after the oxidation treatment were confirmed to be cuprous oxide in both a) visual field 1 and b) visual field 2. (Figure 8).

From the above, it is considered that the firing mechanism in the present invention is as shown in FIG. a) Although copper fine particles after printing are independent of each other, volume expansion occurs due to the difference in density between copper and cuprous oxide during the cuprous oxide ultrafine particle formation process that occurs with the oxidation of copper fine particles in the oxidation treatment process. Adjacent copper fine particles contact with each other through the cuprous oxide ultrafine particles beyond the binder resin layer. It is considered that sintering is performed due to a melting point drop between the ultrafine particles by such a b) reduction treatment after the oxidation treatment. c) Since the volume shrinkage occurs after the reduction treatment, the sintered particles are encapsulated in the binder resin.
<Example 2>
The same ink as in Example 1 was printed on an alumina substrate with a doctor blade with a coating thickness of 40 μm, dried in a solvent at 60 ° C. in nitrogen, and oxidized in air at 150 ° C. for 4 hours. A reduction treatment for 8 hours in a 97% hydrogen mixed atmosphere was performed to obtain a thin film sintered body of copper particles. The film thickness was about 5.3 μm from the observation result of the SEM cross section (FIG. 10).

Conductivity is measured by a resistivity meter Loresta-GP, measuring 6 points in total, 3 points vertically and horizontally on a 2 cm square sample, with an average volume resistivity of 3.1 × 10 ⁻⁵ Ω · cm, 1.55 × 10 ^{−6 of} bulk copper Good conductivity was exhibited for Ω · cm.

  The XRD measurement result in the firing process (FIG. 11) shows that, as in Example 1, a) after printing, b) after oxidation treatment, and c) after reduction treatment, it was re-reduced to copper. As a result of semi-quantitative analysis (RIR method) by XRD, after the oxidation treatment of this sample, copper was 85 wt% and cuprous oxide was 15 wt%.

In the SEM observation in each process, in order to confirm the residue of the organic component, a) secondary electron image and b) reflected electron image were taken and compared in the same manner as in Example 1. When oxidation is performed by heating in air, many particles of several tens of nanometers or less are generated and each particle comes into contact (Fig. 12). Furthermore, when reduced by heating in 3% hydrogen, fine particles are bonded to form a sintered body, and it can be confirmed that the periphery is covered with a binder resin (FIG. 13).
Comparative Example The same ink as in Example 1 was printed on an alumina substrate with a doctor blade with a coating thickness of 40 μm, dried in a solvent at 60 ° C. in nitrogen, and heated for 4 hours at 200 ° C. in nitrogen (purity 99.99%). Then, reduction treatment was performed for 4 hours in a mixed atmosphere of 3% hydrogen and 97% nitrogen at 200 ° C.

  Conductivity was measured with a resistivity meter Loresta-GP, measuring a total of 6 points, 3 cm in length and width, on a 2 cm square sample, but it was impossible to measure and it was confirmed to be an insulator.

The XRD measurement results during the firing process (FIG. 14) show that there is no significant change in the composition of the sample a) after printing b) after heat treatment and c) after reduction treatment. The SEM observation image after the reduction treatment (FIG. 15) is almost the same as the SEM image after printing shown in FIG. 4, and each is an independent particle covered with an organic component. I can confirm. From this, it was clarified that in the system in which the oxidation treatment is not performed, the particles are not sintered and the conductivity is not exhibited.
<Example 3>
From Example 1, Example 2 and the comparative example, it was confirmed that the contact between the copper particles via the cuprous oxide after the oxidation treatment affects the sintering during the subsequent reduction treatment. Therefore, to what extent the oxidation progresses, the copper particles were in contact with each other.

  That is, the same ink as in Example 1 was printed on an alumina substrate with a doctor blade at a coating thickness of 40 μm, and a plurality of printed substrates obtained by drying the solvent at 60 ° C. in nitrogen were prepared. Oxidation treatment was performed in air at ℃, 150 ℃ and 200 ℃ for 4 hours, and semi-quantitative measurement of copper and cuprous oxide by XRD and conductivity measurement by resistivity meter Loresta-GP were performed. Further, as a comparative example, a sample that was heat-treated in nitrogen at 200 ° C. (purity 99.99%) for 4 hours was produced, and the same measurement was performed.

  The results are shown in Table 1. In the case of heat treatment in nitrogen, conductivity did not develop after the oxidation treatment. In air, when the heating temperature was 100 ° C., conductivity did not appear, but at 130 ° C., 150 ° C., and 200 ° C., conduction in MΩ order could be confirmed. From this result, in the air, there is little formation of cuprous oxide at 100 ° C, and it shows an insulator because it does not lead to sufficient contact between adjacent copper fine particles, but when heated to 130 ° C or more, resistance appears simultaneously with the formation of cuprous oxide. It can be seen that decreases. In view of the results of Example 1 and Example 2, it is considered that adjacent copper fine particles are in contact with each other through cuprous oxide at this stage. At 150 ° C, cuprous oxide increases and the resistance further decreases slightly. When the temperature is raised to 200 ° C, the resistance starts to rise. This is thought to be because the specific resistance of cuprous oxide becomes dominant as the ratio of cuprous oxide to the whole increases. Based on the above results, by further study, preferably, to make the sheet resistance in this system 10 MΩ / sq or less, the cuprous oxide fraction is 7% or more and 24% or less, and the heating temperature in air is It is considered that the standard is 130 ° C or higher and 170 ° C or lower.

Claims (8)

A method for producing a conductive copper fine particle sintered body, comprising producing a sintered body of copper fine particles having a resin film on a substrate through at least the following processing steps.
(A) Application of conductive ink containing copper fine particles and baked binder on substrate (B) Cuprous oxide (Cu ₂ O) fraction of copper fine particles after application is 7% by mass or more (C) Reduction treatment after the oxidation treatment   2. The method for producing a copper fine particle sintered body according to claim 1, wherein the average particle diameter of the copper fine particles applied on the substrate is 100 nm or more.   3. The method for producing a copper fine particle sintered body according to claim 1, wherein a thermal decomposition start temperature of the fired binder is higher than temperatures of the oxidation treatment and the reduction treatment.   4. The method for producing a copper fine particle sintered body according to any one of claims 1 to 3, wherein the method is an oxidation treatment by heating in air.   5. The method for producing a copper fine particle sintered body according to any one of claims 1 to 4, wherein the copper fine particle sintered body is a reduction treatment with hydrogen.   6. A method for producing a conductive substrate, which is formed by the method according to any one of claims 1 to 5.   A copper fine particle sintered body obtained by the method according to any one of claims 1 to 5.   7. A conductive substrate obtained by the method according to claim 6.