JP2007165305A - Joined copper particles and powder for conductive paste - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To obtain copper powder for conductive paste capable of forming a coating film excellent in electrical conductivity. <P>SOLUTION: The present invention provides jointed copper particles and powder for conductive paste composed of two or more unit particles, preferably 20 or less unit particles, joined through neck portions. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to metallic copper particles / powder for a conductive paste from which a conductive paste having good conductive performance can be obtained.

  For example, when a thick film circuit board is produced by screen printing a conductive paste on an insulating substrate, silver-based paste has been mainly used as the conductive paste, but recently, copper-based paste has also been used. This is because copper paste has the following advantages over silver paste.

(1) It is difficult for short-circuiting because migration is unlikely to occur.
(2) Since the conductor resistance and high-frequency loss are small, the circuit can be miniaturized.
(3) High reliability due to excellent solder resistance.
(4) Cost reduction is possible.

  A copper-based paste having such advantages can be obtained by dispersing copper powder having a particle size of about 0.1 to 10 μm in a vehicle (resin).

As a method for producing copper powder, a mechanical pulverization method, an atomization method in which molten copper is sprayed, an electrolytic deposition method on a cathode, an evaporation method, a wet reduction method, and the like are known. Each of these has advantages and disadvantages, but the wet reduction method can obtain a fine powder having a particle size suitable for paste relatively easily, and is therefore the mainstream in producing copper powder for conductive paste. Japanese Laid-Open Patent Publication No. 4-116109, Japanese Laid-Open Patent Publication No. 2-97012, and Japanese Laid-Open Patent Publication No. 62-99406 describe a method for producing copper powder by a wet reduction method.
JP-A-4-116109 Japanese Patent Laid-Open No. 2-197012 JP-A-62-99406

  The performance as a copper-based paste can be evaluated from various viewpoints, but when used as a conductive paste, it is basically important to have excellent conductivity. Even when metallic copper powder of the same purity is dispersed in a resin, the electrical resistance also shows different values due to differences in particle size distribution and particle shape. In order to reduce the electrical resistance, it is important that the particles are in close contact with each other, in other words, that the copper particles are dispersed with a high filling rate in the resin so that the contact interface between the particles increases. Naturally, it is possible. However, it is not easy in practice to realize this so as to maintain other properties required for the conductive paste, for example, proper fluidity and viscosity and appropriate strength.

  In the conventional copper powder manufacturing method using the wet reduction method, the particle size of the obtained copper powder is often determined empirically, and the particle size distribution usually increases as the particle size increases. Therefore, it was practically impossible to control the production method so as to obtain a copper powder that can be filled in the resin with a high filling rate and that the contact interface between the particles becomes large.

  An object of the present invention is to solve such problems and to provide copper particles / powder exhibiting high electrical conductivity (low electrical resistance) when dispersed in a resin.

  According to the present invention, there are provided bonded copper particles for a conductive paste in which two or more unit particles, preferably 2 or more and 20 or less unit particles are bonded with a neck portion. In particular, there are provided bonded copper particles for a conductive paste in which 2 to 20 unit particles having a particle size of about 0.5 to 10 μm are bonded to each other with a neck portion in an arbitrary three-dimensional direction.

  Here, the neck portion refers to a portion where unit particles and unit particles are joined, and the diameter of the neck is smaller than the particle size of at least one of the unit particles on both sides of the neck portion, preferably on both sides of the neck portion. It is smaller than the particle size of both unit particles. According to the present invention, there is provided a copper powder for a conductive paste composed of bonded copper particles having such a neck portion and unit particles having no neck portion. In this case, it is desirable that the number of bonded copper particles having a neck portion occupy 20 to 80% of the whole. By dispersing such metal copper powder in the resin, a conductive paste having good electrical conductivity can be obtained. The resin can be a thermosetting resin such as a phenol resin.

  Such a metal copper powder is obtained by adding a reducing agent to a suspension obtained by reacting an aqueous copper salt solution with an alkali agent to precipitate copper hydroxide, and performing intermediate reduction to cuprous oxide. In the method for producing a copper powder that is finally reduced to copper in water, the step of precipitating copper hydroxide is carried out in an oxygen-containing gas atmosphere, and the step of precipitating copper hydroxide is further performed with an Fe concentration of It can be advantageously produced by carrying out in an aqueous solution of 50 ppm or less, and further by blowing an oxygen-containing gas into the cuprous oxide suspension after the intermediate reduction.

  ADVANTAGE OF THE INVENTION According to this invention, the copper powder for electrically conductive paste which can form the coating film excellent in electrical conductivity can be provided.

  1 and 2 are electron micrograph images (SEM images) of metallic copper powder according to the present invention obtained in Examples described later, and FIG. 2 is an enlarged view of the central portion of FIG. The joint copper particles in the center of FIG. 2 can be seen as seven unit particles having a particle size of about 2.5 to 5 μm joined with a neck portion in a three-dimensional random direction. The diameter is smaller than the diameter of the unit particles on both sides. In addition to the seven unit particle bonded copper particles, as shown in FIG. 1, one unit particle (independent without bonding), two bonded, three There are n joined ones (n is about 20 at the maximum in FIG. 1). In any case, the neck part is different from the case where the particles are merely in point contact with each other, and the unit particles are joined with a certain amount of joint area. It has a neck strength that does not separate. It was found that when the conductive paste was made of copper powder containing bonded copper particles bonded with such a neck portion, the electrical resistance was remarkably reduced as compared with that containing no bonded copper particles.

  3 to 4 are electron micrograph images (SEM images) of metallic copper powder obtained by the comparison method described later, and FIG. 4 is an enlarged view of the center portion of FIG. 3 slightly above. It can be seen that this copper powder has no joint copper particles having a neck portion smaller than the particle diameter, and those having a particle diameter of 2 to 10 μm are distributed almost evenly in a substantially spherical shape (ball shape). The average particle size is about 6.0 μm.

When the electric resistance (volume resistance of the dried coating film) is measured under the same conditions for the paste in which both metal copper powders are dispersed in the same amount, the former paste is 3. It is 12 × 10 ⁻³ Ω · cm, and the latter paste is 2.76 × 10 ⁻² Ω · cm. The former is one order of magnitude smaller and the conductivity is remarkably good. Although the reason for this is not exactly known, the connection between the unit particles is assured as the number of bonded copper particles increases, and therefore, the contact interface between the particles increases, and such bonded copper It can be mentioned that the resin is satisfactorily filled into the resin as a whole by interposing smaller bonding copper particles or single unit particles between the particles. However, even in the former case, it is preferable that the bonded copper particles having a unit particle bonding number of more than 20 have too many necks and become inferior in dispersibility in the resin and become agglomerated. is not. Similarly, dispersibility may be inferior if it consists only of bonded copper particles. For example, if the number of bonded copper particles exceeds 80% of the total, the dispersibility deteriorates, and therefore it is better to have independent unit particles. The number of copper particles should be in the range of 20-80% of the total. The unit particles forming the bonded copper particles should have a particle size of about 0.5 to 10 μm. On the other hand, when the latter is composed of copper particles without a neck (ball-shaped ones), the contact interface between the particles is the same even if the particle size distribution is good for filling. As a result, it is expected to become much less conductive than the former.

  Metallic copper powder in which bonded copper particles having an appropriate neck number such as the former are distributed is obtained by controlling the atmosphere during the initial copper hydroxide formation process in the production process of metallic copper powder by the wet reduction method. It was found that it can be manufactured by controlling impurities during the formation of copper oxide. That is, a reducing agent is added to a suspension in which copper hydroxide aqueous solution and an alkali agent are reacted to precipitate copper hydroxide, and intermediate reduction is performed to cuprous oxide. In the conventional copper powder manufacturing method, the process of precipitating copper hydroxide was changed from a method that was conventionally performed in an inert atmosphere such as nitrogen to an oxygen-containing gas, typically in the air. If the concentration of impurities such as Mg, Ca, Zn, Na, Al, and Fe coexisting in the liquid for depositing copper hydroxide is lowered, bonded copper particles having the above-mentioned neck number may be obtained. all right. Furthermore, after an intermediate reduction to cuprous oxide and before final reduction to metallic copper, oxygen-containing gas, typically air, is blown into the cuprous oxide suspension to obtain It was also found that the particle size and particle size distribution of the resulting copper powder can be controlled.

  More specifically, the copper salt aqueous solution is most commonly used as the copper salt aqueous solution, and the NaOH aqueous solution is most commonly used as the alkaline agent in the step of precipitating copper hydroxide by first reacting the copper salt aqueous solution with the alkali agent. In some cases, the former may be an aqueous solution of copper chloride, copper carbonate, copper nitrate, etc., and the latter may be used as long as it is an alkaline agent that does not affect others, and a copper salt aqueous solution having a predetermined concentration. Separately, an alkaline aqueous solution of a predetermined concentration is prepared separately, and both solutions are mixed and immediately stirred vigorously, or the copper hydroxide aqueous solution is continuously added to the copper salt aqueous solution with stirring, and the copper hydroxide precipitation reaction proceeds. However, if this atmosphere is changed from a conventional inert gas atmosphere to an oxygen-containing gas atmosphere (air) to precipitate copper hydroxide, the intermediate reduction is relatively large. Cuprous oxide a particle size, typically cuprous oxide particle size of about 0.5~1.5μm is obtained. On the other hand, when copper hydroxide is deposited under the same conditions except for the nitrogen atmosphere, cuprous oxide with a small particle size of about 0.3 μm is obtained. In the case of cuprous oxide having such a small particle size, it is difficult to obtain metallic copper powder having the neck as described above.

  At that time, if the concentration of impurities such as Mg, Ca, Zn, Na, Al, and Fe in the liquid is high, cuprous oxide having such a large particle size is difficult to obtain. In particular, Fe in the liquid has a strong effect of preventing the obtaining of cuprous oxide having a large particle size. Therefore, the concentration of these impurities should be as low as possible, Fe should be 50 ppm or less, and the total impurities should be 70 ppm or less, preferably 50 ppm or less. Such impurities are usually accompanied by the starting copper salt. Therefore, it is recommended to use a copper salt with the highest possible purity.

  When a reducing agent is added to the precipitated copper hydroxide suspension to reduce it to cuprous oxide (intermediate reduction), glucose (glucose) can be used as the reducing agent. This intermediate reduction step is preferably performed while raising the temperature in an inert gas atmosphere. After the intermediate reduction treatment, the atmosphere gas is preferably replaced with an oxygen-containing gas and the oxygen-containing gas is bubbled into the liquid.

  By performing such oxidation treatment after the intermediate reduction, the pH of the liquid becomes 5-9, but as the amount of oxygen-containing gas blown is increased, the particle size of the copper unit particles when final reduction is increased There is a tendency. The flow rate of the oxygen-containing gas is determined by the flow rate and the blow time. By adjusting the flow rate and the blow time, the particle size of the unit particles can be controlled. It was found that unit particles having a uniform particle size were obtained by narrowing the particle size distribution of the particles, and the shape of the unit particles was ball-shaped. The amount of oxygen-containing gas necessary to obtain such a result is preferably adjusted so that the amount of oxygen is at least 0.1 mol or more per 1 mol of copper in the liquid. The upper limit of the blowing amount is not particularly restricted, but the effect is saturated even if the blowing amount is too large, so depending on the blowing method, the oxygen amount is 20 mol or less per 1 mol of copper in the liquid. Depending on the case, it may be 10 mol or less. Air is most convenient as the oxygen-containing gas to be blown. Unless otherwise specified, room temperature air may be blown into the room temperature suspension. Of course, oxygen-enriched air or pure oxygen gas can also be used.

  The suspension is then decanted under an inert gas atmosphere, and the supernatant is removed to collect a precipitate. The precipitate is suspended in fresh water, and hydrazine hydrate is added as a reducing agent. Used to final reduction to copper metal. The metallic copper in the liquid thus obtained is separated from the liquid and dried with or without surface treatment for imparting oxidation resistance to obtain a metallic copper powder having a neck according to the present invention. be able to.

  Therefore, according to the present invention, a reducing agent is added to a suspension obtained by reacting an aqueous copper salt solution with an alkali agent to precipitate copper hydroxide, and intermediate reduction to cuprous oxide is performed. In the method for producing a copper powder that is finally reduced in water, the step of precipitating copper hydroxide is carried out in an oxygen-containing gas atmosphere, the Fe concentration in the liquid for precipitating copper hydroxide is 50 ppm or less, Furthermore, the present invention provides a method for producing a copper powder comprising metallic copper particles having a neck characterized by performing an intermediate reduction to cuprous oxide followed by oxidation treatment by blowing an oxygen-containing gas.

[Example 1] The following aqueous copper sulfate solution A and alkaline aqueous solution B were prepared.
Copper sulfate aqueous solution A:
[CuSO ₄ .5H ₂ O: 0.6925 Kg] + [pure water: 2.20 Kg]
Alkaline aqueous solution B:
[48.1% NaOH aqueous solution: 0.545 kg] + [pure water: 4.15 kg]

  The entire amount of the copper sulfate aqueous solution A at a temperature of 29 ° C. is added to the alkaline aqueous solution B maintained at a temperature of 27 ° C. in an air atmosphere and vigorously stirred. The Fe concentration in the liquid is 50 ppm or less, and there are only traces of other impurities. The temperature of the A + B liquid rises to 32 ° C. due to heat generation, and a suspension in which copper hydroxide is precipitated is obtained. The pH of this solution is 13.2. The mixing ratio of the liquid A and the liquid B is 1.20 equivalent ratio of caustic soda to copper in the liquid.

  After adding a glucose solution in which 0.9935 kg of glucose was dissolved in 1.41 kg of pure water to the total amount of the obtained copper hydroxide suspension, the temperature of the solution was raised to 70 ° C. in 38 minutes after the addition. , Hold for 15 minutes. The pH of the liquid at this time is 7.8. This treatment is performed in a nitrogen atmosphere.

  Next, air is bubbled through the liquid at a flow rate of 0.7 liter / min for 420 minutes to oxidize the liquid. As a result, the pH of the liquid becomes 5.76. The particle size of cuprous oxide is approximately 0.7 μm.

  The suspension is allowed to stand in a nitrogen atmosphere for 2 days, after which the supernatant (pH 5.9) is removed, almost all of the cuprous oxide precipitate is collected, and 0.55 kg of pure water is added thereto. . 0.074 kg of hydrazine hydrate is added in several portions to the total amount of the cuprous oxide suspension. Due to the exothermic reaction, the temperature of the liquid is finally raised from 50 ° C. to 80 ° C. to complete the reaction. After the completion of the reaction, the suspension is subjected to solid-liquid separation, and powder is collected and dried in a nitrogen atmosphere at 120 ° C. to obtain copper powder.

  The electron microscope SEM images of the copper powder obtained by this method are shown in FIGS. As described in the text, this copper powder is a copper powder containing bonded copper particles in which unit particles having a particle size of 2.5 to 5.0 μm are bonded with a neck. The number of copper particles was approximately 40% of the total particles.

In accordance with the same treatment method as conventional copper-based conductive paste, 30 g of this copper powder and 7.5 g of phenol-based resin were kneaded to prepare a paste, which was coated on a glass substrate with a thickness of 30 μm and dried. After that, the electrical resistance was measured. As a result, the volume resistance of the coating film was 3.12 × 10 ⁻³ Ω · cm.

[Comparative Example 1] The following aqueous copper sulfate solution A and alkaline aqueous solution B 'were prepared.
Copper sulfate aqueous solution A:
[CuSO ₄ .5H ₂ O: 0.6925 Kg] + [pure water: 2.20 Kg]
Alkaline aqueous solution B ′:
[49.0% NaOH aqueous solution: 0.541 Kg] + [pure water: 4.15 Kg]

  The entire amount of the copper sulfate aqueous solution A at a temperature of 29 ° C. is added to the alkaline aqueous solution B ′ maintained at a temperature of 27 ° C. in a nitrogen gas atmosphere and stirred vigorously. The Fe concentration in the liquid is 50 ppm or less, and there are only traces of other impurities. Due to heat generation, the temperature of the A + B ′ liquid rises to 32.9 ° C., and a suspension in which copper hydroxide is precipitated is obtained. The pH of this solution is 12.9. The mixing ratio of the A liquid and the B 'liquid is 1.19 in terms of the equivalent ratio of caustic soda to copper in the liquid.

  After adding a glucose solution in which 0.9935 kg of glucose was dissolved in 1.41 kg of pure water to the total amount of the obtained copper hydroxide suspension, the temperature of the solution was raised to 70 ° C. in 38 minutes after the addition. , Hold for 15 minutes. The pH of the liquid at this time is 7.8. This treatment is performed in a nitrogen atmosphere.

  Subsequently, air is bubbled into this liquid at a flow rate of 0.7 liter / min for 420 minutes to oxidize the liquid. As a result, the pH of the liquid becomes 5.80. The particle size of cuprous oxide is approximately 0.3 μm.

  After leaving this suspension in a nitrogen atmosphere for 2 days, the supernatant (pH 6.02) is removed, almost all of the cuprous oxide precipitate is collected, and 0.55 kg of pure water is added thereto. . 0.074 kg of hydrazine hydrate is added in several portions to the total amount of the cuprous oxide suspension. Due to the exothermic reaction, the temperature of the liquid is finally raised from 50 ° C. to 80 ° C. to complete the reaction. After the completion of the reaction, the suspension is subjected to solid-liquid separation, and powder is collected and dried in a nitrogen atmosphere at 120 ° C. to obtain copper powder.

  The electron microscope SEM images of the copper powder obtained by this method are shown in FIGS. As described in the text, this copper powder is made of ball-like particles having an average particle diameter of approximately 6.0 μm and has no neck.

In exactly the same manner as in Example 1, 30 g of this copper powder and 7.5 g of phenolic resin were kneaded to prepare a paste, which was coated on a glass substrate with a thickness of 30 μm, dried, and then measured for electrical resistance. did. As a result, the volume resistance of the coating film was 2.76 × 10 ⁻² Ω · cm.

It is an electron micrograph image of the metallic copper powder obtained in Example 1 of the present invention. It is the electron micrograph image which expanded the particle | grain part of the center part of FIG. It is an electron micrograph image of the metallic copper powder obtained in Comparative Example 1 of the present invention. It is the electron micrograph image which expanded the particle | grain part in the center part of FIG. 3 slightly.

Claims (8)

  Bonded copper particles for a conductive paste formed by bonding two or more unit particles with a neck portion.   Joined copper particles for a conductive paste formed by joining two or more and 20 or less unit particles with a neck portion.   The bonded copper particles according to claim 1 or 2, wherein the unit particles have a diameter of 0.5 to 10 µm.   The bonded copper particles for conductive paste according to claim 1 or 2, wherein the diameter of the neck portion is smaller than the diameter of unit particles on both sides sandwiching the neck portion.   A copper powder for a conductive paste, comprising a bonded copper particle formed by bonding two or more unit particles with a neck portion and a unit particle having no neck portion.   The copper powder for conductive paste according to claim 5, wherein the number of bonded copper particles is 20 to 80% of the total.   A conductive paste obtained by dispersing the copper powder for a conductive paste according to claim 1 in a resin.   The conductive paste according to claim 7, wherein the resin is a phenol resin.