JP5527163B2 - Conductive paste and wiring board using the same - Google Patents

Description

  The present invention relates to a conductive paste that can suppress a decrease in conductivity due to oxidation of copper nanoparticles and can be used to form a conductive pattern.

  Currently, a conductive paste using silver nanoparticles as a conductive material is known as a material for forming a conductive pattern on a printed wiring board or the like. By applying and baking this conductive paste on a substrate such as ceramic, silver nanoparticles can be sintered to form a conductive pattern. However, the conductive paste using silver nanoparticles as a conductive material has a problem that ion migration occurs and an insulating layer is energized, causing a short circuit, and silver nanoparticles are expensive. Thus, conductive paste using copper as a conductive material, which is inexpensive and hardly causes ion migration, has attracted attention.

  Here, copper is more easily oxidized than silver. In the case of nano-sized copper nanoparticles used for the conductive paste, there is a problem that the conductivity is lowered due to this oxidation because the surface area is large and the copper is more easily oxidized. As a technique for preventing the oxidation of copper, a method using dibutylhydroxytoluene (hereinafter abbreviated as “BHT”) has been proposed (see, for example, Patent Document 1). However, the prevention of oxidation by BHT is not sufficient, and the decrease in conductivity due to the oxidation of copper is inevitable.

  Therefore, there is a need for an electrically conductive paste that is inexpensive, does not cause a short circuit due to the occurrence of ion migration, and suppresses a decrease in conductivity due to oxidation.

JP 2009-146890 A

  The problem to be solved by the present invention is to provide a conductive paste which is inexpensive and hardly causes a short circuit due to the occurrence of ion migration and suppresses a decrease in conductivity due to oxidation, and a wiring board using the same.

  As a result of intensive research, the present inventors have been able to greatly suppress the oxidation of copper nanoparticles by adding a specific sulfur compound to the paste in a conductive paste using copper nanoparticles as a conductive material. The present invention has been completed.

  That is, the present invention relates to a conductive paste containing copper nanoparticles and thiadiazole represented by the following general formula (1), and a wiring board using the same.

（式中、Ｒ^１及びＲ^２は、それぞれ独立に炭素原子数１〜１８のアルキル基を表す。）

(Wherein, ^{R 1} and ^{R 2} represents an alkyl group having 1 to 18 carbon atoms independently.)

  Since the conductive paste of the present invention uses copper nanoparticles as a conductive material, there is an advantage that it is inexpensive and hardly causes a short circuit due to the occurrence of ion migration. Moreover, since it can prevent the oxidation of the copper nanoparticles contained in the paste, it is an excellent conductive paste in which the decrease in conductivity due to oxidation is suppressed. Therefore, the conductive paste of the present invention can be suitably used as a material for forming a conductive pattern such as a printed wiring board.

  The conductive paste of the present invention contains copper nanoparticles and thiadiazole represented by the following general formula (1).

（式中、Ｒ^１及びＲ^２は、それぞれ独立に炭素原子数１〜１８のアルキル基を表す。）

(Wherein, ^{R 1} and ^{R 2} represents an alkyl group having 1 to 18 carbon atoms independently.)

  The copper nanoparticles used in the conductive paste of the present invention are metal copper fine particles having a nano-sized average particle diameter. The average particle diameter of the copper nanoparticles is preferably 1 to 500 nm, more preferably 10 to 300 nm, and even more preferably 30 to 200 nm because sintering at a lower temperature is possible.

  The average particle size of the copper nanoparticles is determined by the dynamic light scattering method using FPAR-1000 type dense particle size analyzer (manufactured by Otsuka Electronics Co., Ltd.) ) By laser diffraction / scattering method.

  A known method can be used as the method for producing the copper nanoparticles, and examples thereof include a polyol reduction method in which a copper compound is reduced with a polyol. Examples of the copper compound that can be used in the polyol reduction method include copper sulfate, copper nitrate, copper formate, copper acetate, copper chloride, copper bromide, and copper iodide. Examples of the polyol that can be used as a reducing agent in the polyol reduction method include diols such as ethylene glycol, propylene glycol, and butylene glycol. These copper compounds and polyols can be used alone or in combination of two or more.

  When producing copper nanoparticles by the above polyol reduction method, it is preferable to use a protective agent. As this protective agent, a polymer having a ligand having an affinity for copper is preferable, and examples thereof include nonionic polymers such as polyvinylpyrrolidone and methoxypolyethylene oxide methacrylate. Among these, polyvinylpyrrolidone is preferable. In addition, the protective agent can be used alone or in combination of two or more.

  In order to prepare the copper nanoparticles, the copper compound, polyol and protective agent are charged into a reaction vessel, and an inert gas such as argon gas or nitrogen gas is blown in order to prevent oxidation of the generated copper nanoparticles. It can be performed by heating to a temperature of ℃ or higher. In order to reduce the primary particle size, it is preferable to rapidly heat to the target temperature. Although heating temperature should just be 120 degreeC or more, the range of 180-200 degreeC is more preferable.

  The thiadiazole used for the conductive paste of the present invention is a compound represented by the following general formula (1).

（式中、Ｒ^１及びＲ^２は、それぞれ独立に炭素原子数１〜１８のアルキル基を表す。）

(Wherein, ^{R 1} and ^{R 2} represents an alkyl group having 1 to 18 carbon atoms independently.)

R ¹ and R ² in the general formula (1) are each independently an alkyl group having 1 to 18 carbon atoms, and the alkyl group may be linear or branched. Moreover, as R < ¹ > and R < ² > in the said General formula (1), a C6-C12 alkyl group is more preferable, and a specific example includes n-hexyl group, n-octyl group, 2-ethylhexyl group. And 1-methyloctyl group. Among the thiadiazoles represented by the general formula (1), those in which R ¹ and R ² are the same are preferable, and those in which both R ¹ and R ² are 1-methyloctyl groups are particularly preferable.

  The thiadiazole represented by the general formula (1) is “New Experimental Chemistry Lecture 14 Synthesis and Reaction III of Organic Chemistry” (pp. 1735-1741; edited by the Chemical Society of Japan, February 20, 1978). It can be produced by reacting an oxidizing agent with 2,5-dimercapto-1,3,4-thiadiazole and aliphatic sulfide as raw materials by a known synthesis method described in the publication.

  Since the blending amount of thiadiazole represented by the above general formula (1) in the conductive paste of the present invention can sufficiently suppress the oxidation of the nanocopper particles, it is 0.1 to 100 parts by mass of the nanocopper particles. The range of 50 parts by mass is preferable, and the range of 1 to 40 parts by weight is more preferable.

  In order to give applicability | paintability to the electrically conductive paste of this invention, it is preferable to mix | blend a solvent other than the said copper nanoparticle and the said thiadiazole. Examples of the solvent include methanol, ethanol, acetone, methyl ethyl ketone, methyl isobutyl ketone, cyclohexanone, ethyl acetate, butyl acetate, propylene glycol monomethyl ether acetate, dimethylformamide, dimethylacetamide, N-methylpyrrolidone, dimethyl sulfoxide, diethyl ether, Examples thereof include organic solvents such as diisopropyl ether, tetrahydrofuran, dioxane, toluene and xylene, and water. These solvents can be used alone, but two or more of them can be used in combination as long as they are not mixed and separated.

  In addition to the above copper nanoparticles, thiadiazole and solvent, the conductive paste of the present invention may contain various additives as long as the effects of the present invention are not impaired. Examples of such additives include a leveling agent, a dispersant, a thixotropic agent (thixotropic viscosity imparting agent), an antifoaming agent, a filler, a plasticizer, and a curing catalyst.

  The conductive paste of the present invention can be prepared by stirring and mixing the above-mentioned copper nanoparticles, thiadiazole and solvent, and various additives added as necessary, into a uniform liquid. However, if necessary, it may be prepared by stirring and dispersing using a three-roll, ball mill, attritor, sand mill or the like.

  The wiring board of the present invention is obtained by forming a conductive pattern on a substrate such as ceramic or glass using the conductive paste of the present invention. As a method of forming this conductive pattern on the substrate, first, a method of applying a conductive paste to the shape of the circuit wiring on the substrate on which the conductive pattern is to be formed, drying the solvent, and baking it. As a method for applying the conductive paste at this time, a plate printing method, an ink jet printing method, a dispensing method, or the like can be used.

  Examples of the plate printing method include letterpress printing, planographic printing, intaglio printing, and stencil printing. As the ink jet printing method, either a piezo method or a thermal method can be used. Furthermore, as a dispensing method, there is a method in which the amount of conductive paste applied is controlled by image processing to form wiring.

  The above baking can be performed usually at a temperature of 600 to 800 ° C. in an atmosphere of an inert gas such as argon gas or nitrogen gas. However, the conductive paste of the present invention is baked at a lower temperature. And baking at 450 to 600 ° C. is also possible.

  Hereinafter, the present invention will be described in more detail with specific examples. The average particle size of the copper nanoparticles was measured under the following conditions.

[Measurement of average particle diameter of copper nanoparticles]
A part of the dispersion of copper nanoparticles was diluted with ethylene glycol, and using a FPAR-1000 type dense particle size analyzer (manufactured by Otsuka Electronics Co., Ltd.), the measurement conditions were a temperature of 25 ° C., and the refractive index of the medium was 1.4306. The average particle diameter was measured as a viscosity of 17.4 mPa · s.

(Production Example 1)
A round bottom flask was charged with 240 mg of copper (II) acetate (manufactured by Kanto Chemical Co., Inc.), 240 ml of ethylene glycol (manufactured by Kanto Chemical Co., Ltd.) and 33 g of polyvinylpyrrolidone (manufactured by Wako Pure Chemical Industries, Ltd.). Thereafter, while blowing nitrogen gas, the temperature was raised to 180 ° C. in 5 minutes, and heated and stirred at 180 ° C. for 10 minutes. Then, this reaction mixture is circulated in a hollow fiber type ultrafiltration membrane module ("HIT-1-FUS1582" manufactured by Daisen Membrane Systems Co., Ltd .; surface area 145cm ² , molecular weight cut off 150,000), While adding the same amount of purified water, purification was performed by circulating until the filtrate from the ultrafiltration module was about 500 mL. When the purified water was stopped and concentrated as it was by ultrafiltration, 536 mg of an organic compound / nanocopper particle composite aqueous dispersion was obtained. The amount of non-volatiles in the dispersion was 30% by mass, and the content of copper nanoparticles in the non-volatiles was 95% by mass. The average particle diameter which measured the obtained copper particle by said dynamic light scattering method was 176 nm. Wide-angle X-ray diffraction of the dispersion confirmed that it was reduced copper.

Example 1
7.7 mg of an aqueous dispersion of copper nanoparticles obtained in Production Example 1 (2.2 mg as copper nanoparticles), 9 mg of thiadiazole (“DAILUBE R-300” manufactured by DIC Corporation) represented by the following formula (2) Then, 90 g of toluene and 10 g of ethanol were uniformly mixed to prepare a conductive paste (1).

(Comparative Example 1)
An aqueous dispersion of copper nanoparticles obtained in Production Example 1 (7.7 mg (2.2 mg as copper nanoparticles)), BHT (manufactured by Kanto Chemical Co., Ltd.) 0.9 mg, toluene 90 and ethanol 10 g were uniformly mixed. A conductive paste (2) was prepared.

  For the conductive pastes (1) and (2) obtained in Example 1 and Comparative Example 1 above, the plasmon resonance absorption spectrum is measured, the oxidation state of the copper nanoparticles is observed, and the oxidation of the conductive paste is prevented. Sex was evaluated.

[Measurement of plasmon resonance absorption spectrum]
Immediately after the preparation of each conductive paste, using an MV-2000 type photodiode array type UV-visible absorption spectrum measuring apparatus manufactured by JASCO Corporation, sweeping from 400 to 800 nm in 0.1 seconds, Measurement started. The measurement was performed twice immediately after the preparation of the conductive paste and 2 hours after the preparation.

  In addition, plasmon resonance absorption is a phenomenon in which electrons in a nanoparticle interact with light to cause light absorption. When the oxidation of the nanoparticle proceeds, the light absorption intensity (absorbance) decreases. By measuring this, the oxidation state of the copper nanoparticles can be grasped, and when there is no decrease in absorbance, it can be said that the progress of oxidation is small.

[Evaluation of antioxidant properties]
The change rate (%) of the maximum absorbance value immediately after the preparation of the conductive paste of the plasmon resonance absorption spectrum measured above and after 2 hours from the preparation was calculated by the following formula (1). Antioxidant properties were evaluated according to the criteria.
○: Change rate is less than 2%.
X: The rate of change is 2% or more.

  As a conductive paste, in order to exhibit excellent performance with little decrease in conductivity due to oxidation, the change rate per hour after preparation of the conductive paste of the maximum absorbance of this plasmon resonance absorption spectrum is less than 2% per hour. It is preferable that it is less than 1%.

  Table 1 shows the results of the above measurement and evaluation for the conductive pastes (1) and (2) obtained in Example 1 and Comparative Example 1.

  From the evaluation results shown in Table 1, the conductive paste (1) of the present invention obtained in Example 1 was extremely excellent, with the rate of change of the maximum absorbance of the plasmon resonance absorption spectrum being as extremely low as 0.88%. It was found to have antioxidant properties.

  On the other hand, Comparative Example 1 is an example in which thiadiazole which is an essential component in the conductive paste of the present invention was not blended, but this conductive paste (2) has a maximum absorbance change rate of 5 in the plasmon resonance absorption spectrum. It was found that the antioxidant property was insufficient, as large as .59%.

Claims (3)

An electroconductive paste comprising copper nanoparticles and thiadiazole represented by the following general formula (1).

(Wherein, ^{R 1} and ^{R 2} represents an alkyl group having 1 to 18 carbon atoms independently.)   The electrically conductive paste of Claim 1 which contains 0.1-50 mass parts of thiadiazole represented by the said General formula (1) with respect to 100 mass parts of said copper nanoparticles.   A wiring board, wherein a conductive pattern is formed using the conductive paste according to claim 1.