JP4524477B2 - Copper powder for conductive paste - Google Patents

Description

  The present invention relates to a copper powder from which a low-viscosity conductive paste can be obtained even at a high filling rate.

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

The copper powder production method by the wet reduction method is primarily intended to primarily reduce copper hydroxide precipitated in water to cuprous oxide, and then to secondary reduction of this cuprous oxide to metallic copper. Glucose is used as the reducing agent, and hydrazine hydrate is used as the secondary reducing agent. At that time, by setting the conditions for the copper hydroxide precipitation process, primary reduction process and secondary reduction process, the particle size and particle shape of the obtained copper powder can be controlled. There is an advantage that it can be manufactured stably. The present inventors previously proposed a method for performing an oxidation treatment by blowing oxygen-containing gas between the primary reduction step and the secondary reduction step in Japanese Patent Application No. 10-323866. By this oxidation treatment, copper powder with uniform particle size can be obtained, and particle size control and particle shape control can be refined further.
JP-A-4-116109 Japanese Patent Laid-Open No. 2-197012 JP-A-62-99406

  Even if the wet reduction method can produce copper powder having a particle size suitable for a conductive paste, the copper powder has a problem in obtaining a conductive paste having an appropriate viscosity. The viscosity of the conductive paste is related to the viscosity of the resin itself, the filling rate of the copper powder (filler value), the particle size distribution of the copper powder, etc., but the copper paste by the wet reduction method tends to increase the viscosity of the conductive paste. There is. In other words, it was found that the copper powder produced by the wet reduction method has a limit in reducing the viscosity of the conductive paste, even if the particle size can be controlled appropriately.

  Accordingly, an object of the present invention is to solve such problems and to obtain a copper powder that can ensure a viscosity necessary for a conductive paste even when a copper powder obtained by a wet reduction method is used.

  In order to solve the above-mentioned problems, the present inventors have made extensive studies and applied a process of mechanically colliding the copper powders obtained by the wet reduction method with each other to obtain a particle size and a particle size distribution. It was found that the viscosity of the conductive paste can be significantly reduced by smoothing the particle surface without changing the specific surface area so much. In other words, unevenness and angular portions on the particle surface are smoothed by collision between particles without substantially changing the particle size or particle size distribution, and this treatment mechanically fluidizes the particles. Can be performed using an apparatus capable of

  Therefore, this invention provides the copper powder for electrically conductive paste by which the surface smoothening process which makes particles collide mechanically with the copper powder manufactured by the wet reduction method was performed. The copper powder of the present invention has an average particle size of 0.1 to 10 μm, and an epoxy equivalent of glycidyl ester of dimer acid is 446, and an epoxy resin having a viscosity of 730 cps at 25 ° C. is 8% by weight. When 92 weight% of copper powder is kneaded and the viscosity of this kneaded product is measured at 10 rpm using an E-type viscometer, it shows a viscosity of 300 Pa · sec or less.

  According to the present invention, a copper powder that can be made into a paste having a low viscosity even when kneaded into a resin at a high filling rate can be produced by a wet reduction method, and as a result, a high-quality copper paste can be stably obtained. be able to.

As described above, a reducing agent is added to a suspension in which copper hydroxide is suspended in water to perform primary reduction to cuprous oxide, and the reducing agent is added to the suspension in which cuprous oxide is suspended in water. The copper powder produced by the so-called wet reduction method in which the secondary reduction to metallic copper is added to obtain a suitable particle size and particle shape for the conductive paste. For example, the average particle size is 0.1 to 10 μm, preferably 3 to 10 μm, more preferably 4 to 8 μm, and the specific surface area (measured by the BET method) is 0.1 to 10 m ² / g, preferably 0.1 to 1. A product of 0.0 m ² / g can be obtained stably. However, even if the particles have a nearly spherical shape, they actually have a polyhedral shape in which flat crystal faces are exposed in a multifaceted manner. As a whole, the particle surface is uneven. Such an angular surface state is fundamentally different from that obtained by melting treatment like atomized powder.

  And it was found that such squareness (unevenness) hinders the viscosity of the conductive paste from being lowered. That is why the copper powder obtained by the wet reduction method cannot lower the viscosity of the conductive paste. The present inventors have found that the viscosity of the conductive paste can be remarkably lowered by making the squared portion a smooth curved surface without changing the particle size, specific surface area, etc. of the copper powder. In other words, pre-dispersion of particles mechanically colliding with each other before dispersing in the resin, reducing the corners to form particles with a smooth curved surface, and then dispersing in the resin results in pre-treatment. It was found that the viscosity can be remarkably reduced as compared with that.

  This treatment can be performed by fluidizing the powder, and this fluidization is conveniently performed by an apparatus that mechanically fluidizes the powder, such as a cylindrical high-speed stirrer (fluid mixer). In other words, by applying a momentum to each particle and causing the moving particles to collide with each other to smooth the angular portions of the particle surface, the particle size and specific surface area are hardly changed. , The surface of each particle can be smoothed. The cylindrical high-speed stirrer can give centrifugal force and levitation force to the powder by rotating blades provided inside the cylindrical sealed container (cylindrical container with the shaft vertical). Since the powder flows, the surface is smoothed during this flow.

  At the final stage of the wet reduction method, the metal copper fine powder formed in the liquid is separated from the liquid, and the water is removed from the separated solids. It is necessary to crush this with a crusher and isolate the particles apart. In the crusher, the adhering particles are impacted and dissociated from each other, but the dissociated particles are almost restored to the final reduced particle shape, and this crushing process removes the irregularities on the particle surface. Therefore, it cannot be expected that the surface becomes smooth. For this reason, the copper powder obtained by pulverizing the copper powder cake exhibits high viscosity when dispersed in the resin. For example, as shown in the examples described later, 92% by weight of the crushed copper powder is added to 8% by weight of an epoxy resin having a glycidyl esterification of dimer acid and an epoxy equivalent of 446 g / eq and a viscosity at 25 ° C. of 730 cps. When the viscosity of this kneaded product is measured at 10 rpm using an E-type viscometer, it usually shows a viscosity of 400 Pa · sec or more, which is 300 Pa · sec or less, and in some cases 200 Pa · sec or less. A low viscosity cannot be expected.

  On the other hand, in the case of copper powder that has been subjected to surface smoothening treatment by mechanically colliding particles with each other as described above, even if it is obtained by the same wet reduction method, the dimer acid is the same as described above. The surface-smoothed copper powder (92% by weight) was kneaded with 8% by weight of epoxy resin having a glycidyl esterified epoxy equivalent of 446 g / eq and a viscosity at 25 ° C. of 730 cps. When the viscosity of the kneaded product was measured at 10 rpm, it was found that the viscosity was usually 300 Pa · sec or less, more preferably 250 Pa · sec or less, and in some cases 200 Pa · sec or less.

  It was also found that the copper powder produced by the wet reduction method was coated with an inorganic or organic substance and was subjected to a surface smoothing treatment that caused the particles to mechanically collide with each other. . In copper powder for conductive paste, the surface of copper powder may be coated with a metal such as silver to further improve the conductivity, or it may be coated with an organic compound such as carboxylic acid such as stearic acid to prevent surface oxidation. Advantageously, such a coating treatment can be carried out at the final stage in the case of producing copper powder by a wet reduction method. Then, when the surface smoothing treatment is performed on the copper powder subjected to the coating treatment by mechanically colliding particles with each other in the same manner as described above, the surface is smoothed without damaging the coated portion. Therefore, it was found that a low-viscosity conductive paste can be obtained while maintaining the characteristics of the coating.

  An aqueous copper sulfate solution and an aqueous caustic soda solution are mixed at an equivalent ratio of 1.25 mol of caustic soda to 1 mol of copper to obtain a suspension in which copper hydroxide is precipitated. An equivalent amount of glucose solution is added to this suspension, and the temperature of the solution is raised to 70 ° C. in 30 minutes after the addition, and then maintained for 15 minutes to primarily reduce copper hydroxide to cuprous oxide. All the processing operations so far are performed in a nitrogen atmosphere. After bubbling air into this solution and oxidizing, it was left in a nitrogen atmosphere for 2 days, and then the supernatant was removed to collect almost all the precipitate, and pure water was added to this precipitate. A hydrazine hydrate is added to the suspension in an equivalent amount or more and secondarily reduced to copper metal. The suspension after the reaction is separated into solid and liquid, and the solid content is dried in a nitrogen atmosphere at 120 ° C. to obtain a copper powder cake.

  In the above copper powder manufacturing method by the wet reduction method, only the time for the air bubbling oxidation treatment was changed, and three types of copper powder cakes A, B and C were obtained. Each of the obtained cakes was divided into two, and one was charged into a crusher and crushed in a nitrogen atmosphere to obtain copper powders A1, B1 and C1. The other was charged into a cylindrical high speed stirrer and fluidized in a nitrogen atmosphere to obtain copper powders A2, B2 and C2.

  The crusher used in the crushing treatment is an impact crusher with a swinging hammer inside, and crushes the coagulated and dried copper powder cake into fine particles obtained in the final step of the wet reduction method. There is almost no function for smoothing. The cylindrical high-speed stirrer used for the fluidization treatment is a mixer having two rotating blades at the bottom of a cylindrical container with a vertical axis, and the powder to which centrifugal force is applied by the rotation of the blades is upward. As the particles flow and the particles repeatedly collide with each other during the flow, the irregularities on the particle surface are smoothed.

  1 and 2 show electron microscope SEM images (a is 2000 times and b is 5000 times) of copper powder A1 obtained by pulverizing copper powder cake A and fluidized copper powder A2. Similarly, copper powders B1 and C1 obtained by pulverizing the copper powder cakes B and C, and electron microscopic SEM images (a is 2000 times and b is 5000 times) of the fluidized copper powders B2 and C2 are shown in FIGS. Shown in FIGS. In addition, the average particle size of each copper powder was investigated from these SEM images, and the specific surface area, bulk density and TAP density were measured by the BET method, and the results are shown in Table 1.

  Further, 92% by weight of each copper powder was kneaded with 8% by weight of an epoxy resin with a vibration mixer, and the viscosity of the obtained paste was measured. As the epoxy resin, an epoxy resin having a glycidyl ester of dimer acid and an epoxy equivalent of 446 g / eq and a viscosity of 730 cps at 25 ° C. is used, the kneading conditions are constant for each copper powder, and the viscosity of each paste is an E type viscosity. The measurement was performed at 25 ° C. using a meter at a rotation speed of 10 rpm. The results are also shown in Table 1.

  From the results of Table 1, the fluidized copper powders A2, B2 and C2 have the same average particle size, specific surface area, bulk density and TAP density as compared to the copper powders A1, B1 and C1 which are not fluidized. However, it can be seen that the viscosity of the paste when kneaded with the resin is significantly reduced. The fact that the viscosity of the paste was reduced even though the particle size and specific surface area did not change so much was apparent from the comparison of FIGS. 1, 2, 3, 4, 5, and 6. It can be seen that this is because the angular surface of the particle surface has been removed, resulting in a smooth curved surface.

It is an electron microscope SEM image of copper powder A1 obtained by crushing copper powder cake A, (a) of FIG. 1 is 2000 times, (b) of FIG. 1 is 5000 times. It is an electron microscope SEM image of copper powder A2 obtained by fluidizing copper powder cake A, (a) of FIG. 2 is 2000 times, (b) of FIG. 2 is 5000 times. It is an electron microscope SEM image of copper powder B1 obtained by crushing copper powder cake B, (a) of FIG. 3 is 2000 times, (b) of FIG. 3 is 5000 times. It is an electron microscope SEM image of copper powder B2 obtained by fluidizing copper powder cake B, (a) of FIG. 4 is 2000 times, (b) of FIG. 4 is 5000 times. It is an electron microscope SEM image of the copper powder C1 obtained by crushing the copper powder cake C, (a) of FIG. 5 is 2000 times, and (b) of FIG. 5 is 5000 times. It is an electron microscope SEM image of the copper powder C2 obtained by fluidizing the copper powder cake C, (a) of FIG. 6 is 2000 times, and (b) of FIG. 6 is 5000 times.

Claims (5)

A copper powder produced by a wet reduction method and subjected to a surface smoothing treatment for mechanically colliding particles with each other , and an epoxy equivalent of glycidyl ester of dimer acid is 446 g / eq and 25 ° C. Copper powder for conductive paste showing a viscosity of 300 Pa · sec or less when 92 wt% is kneaded with 8 wt% of an epoxy resin having a viscosity of 730 cps and the viscosity of the kneaded product is measured at 10 rpm using an E-type viscometer . Copper powder produced by a wet reduction method, coated with inorganic or organic matter, and subjected to surface smoothening treatment that mechanically collides particles with each other . The epoxy equivalent of dimer acid is glycidyl esterified. A conductive material exhibiting a viscosity of 300 Pa · sec or less when 92 wt% is kneaded with 8 wt% of an epoxy resin having a viscosity of 446 g / eq and a viscosity of 730 cps at 25 ° C., and the viscosity of the kneaded product is measured at 10 rpm using an E-type viscometer. Copper powder for paste. The copper powder for conductive paste according to claim 1 or 2 , wherein the average particle size is 0.1 to 10 µm. In the wet reduction method, a reducing agent is added to a suspension in which copper hydroxide is suspended in water to perform primary reduction to cuprous oxide, and the reducing agent is added to the suspension in which cuprous oxide is suspended in water. The copper powder for electrically conductive pastes in any one of Claims 1-3 which is the method of adding a secondary reduction to metallic copper. The copper powder for electrically conductive paste of Claim 4 which has an oxidation process between a primary reduction process and a secondary reduction process.