Copper flake powder, method for producing copper flake powder, and conductive paste using copper flake powder

Abstract

The present invention is mainly for providing a flake copper powder for a conductive paste with particle properties which are defined by that the particle thickness is thinner being possible for use to form an electrode or a circuit, and a production process thereof. To achieve the object of producing the following particles, each powder particles shape is plastically processed into a flake-shape, wherein the flake copper powder has a cumulative particle diameter D50 of 10 μm or smaller measured with laser diffraction scattering particle size distribution method. The D10, D50 and D90 measured with the laser diffraction scattering particle size distribution method are illustrative and the SD/D50 value measured by the standard deviation of particle distribution with the laser diffraction scattering particle size distribution, is 0.55 or larger and/or a D90/D10 of 4.5 or smaller. The flake copper powder is compressed with a high energy ball mill whose media beads having fine particle diameter which plastically deforms the copper particles into flake-shaped particles, so that stable flake copper powder is produced.

Description

TECHNICAL FIELD

The present invention relates to flake copper powder, a method of producing flake copper powder and a conductive paste using the flake copper powder.

BACKGROUND ART

Conventionally flake copper powder has widely been used as raw material for a conductive paste. Also, conductive pastes have been typically applied to various electrical contacts in order to form a circuit of a printed-wiring board and an external electrode of a ceramic capacitor in order to ensure electrical conduction.

Normal shape of flake copper powder is substantially spherical. When flake copper powder is processed to a conductive paste, some properties are required for such flake copper powder, in which viscosity of a conductive paste can be controlled to form a thinner electrode of chip material and enhance filling capability for a via-hole. When a conductive circuit is formed using a method of sintering and solidifying the circuit by drawing a conductive pattern with the conductive paste, a high-layer-density is required that prevents an increase in the electric resistance in an electric circuit. Simultaneously it is desired to have the ability of maintaining a configuration of the formed conductive circuit.

To meet the market demands as above-mentioned, the copper powder used for producing a conductive paste is sometimes a copper powder formed by not using the spherical particles of copper powder but using flaky particles of powder (hereinafter called “flake copper powder” in the present description) has been considered. As apparent from the shape of each of the particles of the flake copper powder, the shape is fish-scale-shaped or flat, resulting in that a specific surface area of each of the powder particles becomes larger. Thus, the contact area between the powder particles has also become wider resulting in that the flake copper powder is very effective in reducing electric resistance and enhance properties to maintain the configuration of the conductive circuit. The above-mentioned details are referred in Japanese Patent publications Nos. H06(1994)-287762 and H08(1996)-325612. These publications clarify the above-mentioned description.

However, such conventional flake copper powder has not had evenly-sized particle diameter and even thickness. Further, there has been no existence of flake copper powder having fine particles while containing coarse particles at a constant ratio. Besides, some cracks could appear on a surface of each of particles of the flake copper powder. Also the conventional flake copper powder has been a product having the above-mentioned quality in addition to having a very broad particle size distribution.

The viscosity of flake copper powder having such above-mentioned quality has been very difficult to control when the flake copper powder is processed into a conductive paste, and the conductive paste is difficult to handle. Also the viscosity of the conductive paste has been unstable. The conventional flake copper powder has had defects regarding the instability of the thixotropic property of the conductive paste. The thixotropic property is particularly important when forming an electrode of a chip part using a dipping method. For example, upon producing an external electrode of a chip part for a multilayer ceramic capacitor, first the chip itself is dipped into a conductive paste and secondly the chip is lifted up from the conductive paste in order to apply a conductive paste onto the surface of the chip to form external electrodes.

In recent years, as a chip part becomes miniaturized, there is an increase in demand for a thinner layer of external electrode. To meet the demands of forming the thinner layer, the quality of a conductive paste is required as follows. More specifically, when a chip part is dipped into a conductive paste, the conductive paste is thinly applied onto a surface of the chip part with excellent wettability. It has an evenly coated layer formed using the conductive paste. The layer is lifted from the conductive paste, and thereafter the surface of the chip part shows a superior thixotropic property that can prevent flowing of the coated layer formed by the conductive paste. Further, other properties are required to maintain a condition of the coated layer as it stands during the above-mentioned process from the dipping step to a sintering step.

The conductive paste using the conventional flake copper powder can also have the superior thixotropic property. However, the conventional flake copper powder can arrive only at a certain level regarding the properties. However, when the conventional flake copper powder is processed into a conductive paste, a target of a certain level is required. The target is to enhance resistance on a sintered circuit using the conductive paste obtained from the conventional flake copper powder. Even if the target is achieved although the conductive paste having a limitation for lowering the electric resistance on the sintered circuit, it is impossible for the conventional flake copper powder to enhance the resistance because its layer density cannot be increased. Further, in case of drawing a circuit using the conductive paste from the conventional flake copper powder, or applying the dipping method thereto, in order to form an electrode of a chip device, a conductive circuit having an electrode in the sintered process to be a fine-pitched circuit or a thinner layer formation cannot be obtained. Therefore, the configuration stability and the surface condition regarding the conductive circuit or the electrode are a problem. Therefore, the conductive paste using the conventional copper powder has been used only for forming the conductive circuit having a thick and rough pattern of a circuit.

As apparent from the above-mentioned matters, the flake copper powder can be utilized not only for the conventional conductive circuit but also for a thinner and fine-pitched conductive circuit. Therefore, there has been a demand in the market for such type of flake copper powder.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows an observed image of a flake copper powder relating to the present invention through a scanning electron microscope.

FIG. 2 shows a conventional observed image of a conventional copper powder in order to compare the present invention powder with the conventional powder through a scanning electron microscope.

DISCLOSURE OF THE INVENTION

Accordingly, the inventors have developed subsequent flake copper powder based on following reasons. Coarse particles, each having a principal axis are mixed with the conventional flake copper powder, in which the principal axis of each of the coarse particles is five times or more longer than the diameter of a particle of the conventional flake copper powder. Further, the thickness of each of the powder particles is uneven and the particle distribution is uneven. The inventors have focused on the above-mentioned defects. In view of the relationship between the properties of the powder and a process of thinning the above-mentioned conductive circuit the inventors have developed flake copper powder as follows. Below the present invention will be described.

Flake Copper Powder Relating to the Present Invention

The inventors have researched the conventional copper powder which already existed, resulting in the various properties of the conventional copper powder shows in Table 1. Herein, D₁₀, D₅₀, D₉₀ and Dmax are defined by particle diameter sizes by each of 10%, 50% and 90% and maximum particle size regarding the volume cumulation, which can be obtained using a laser diffraction scattering particle size distribution measurement method. Then, 0.1 g of flake copper powder was mixed with 0.1%-aqueous solution of SN Dispersant 5468 (manufactured by San Nopco Limited). After dispersing them by an ultrasonic homogenizer (manufactured by Nippon Seiki Co., Ltd. US-300T) for five minutes, they were measured using a laser diffraction scattering particle size distribution apparatus, Micro Trac HRA 9320-X100 type (manufactured by Leeds and Northrup Limited).

	TABLE 1
	D10	D50	D90	Dmax	SD
SAMPLE	(μm)	SD/D50	D90/D10
1	10.13	26.15	46.77	104.70	18.31	0.70	4.62
2	2.88	6.28	14.09	44.00	4.15	0.66	4.89
3	2.71	5.87	13.14	52.33	3.86	0.66	4.85
4	2.81	8.20	21.38	52.33	7.17	0.87	7.61

As far as observing the results of Table 1, the results show that the conventional flake copper powder also has various characteristics of the powders, and it seems that the conventional flake copper powder can be changed depending on various properties of powder particles of raw materials and methods of process. In Table 1, first, a value of standard deviation (SD) is notable. The standard deviation (SD) is an index to indicate scattering of the data of indicators of all particle diameters, which can be obtained with the laser diffraction scattering particle size distribution measurement method. As the values of the data become larger, variation of the data also becomes larger. Therefore, the value of lots with a standard deviation (SD) measured therein can be shown by scattering from 3.86 μm to 18.31 μm. Also apparent is, that there is a significant scattering of particle size between lots. Focusing on a result of SD/D₅₀ value being a coefficient of variation, the result of scattering from 0.66 to 0.87 is obtained, and D₉₀/D₁₀ value shows scattering from 4.62 to 7.61. Further, the Dmax value can be obtained using the laser diffraction scattering particle size distribution measurement method, which is the maximum particle diameter and it can be noted that coarse particles as much as 104.70 μm in diameter are included. FIG. 2 shows the conventional flake copper powder (three types) observed by a scanning electron microscope. As apparent from the FIG. 2, thickness of conventional flake copper powder is thin and also the thickness is uneven; particularly the powder particle size is not only uniform but also unstable. Of course it depends on to what extend the flake is formed. Some spherical copper powder seems to remain, which had not been processed into flake copper powder. As a result, distribution of the conventional particle size shown in FIG. 2 becomes extremely broad.

When a conductive paste is produced, external electrodes of a ceramic capacitor and a sintered circuit made of a low temperature sintered ceramic material using the conventional flake these copper powder having copper powder properties, the following matters occur. Namely the shape is not uniform, additionally, it is impossible to make thinner the thickness of the external electrode and the sintered circuit mentioned above.

The inventors have devoted themselves to study and shown some facts as follows. If properties of a particle of flake copper powder as defined by a cumulative particle diameter D₅₀ of 10 μm or smaller; a SD/D₅₀ value of 0.55 or smaller; and a D₉₀/D₁₀ value of 4.5 or smaller; in which the SD is a standard deviation of particle distribution measured by a laser diffraction scattering particle size distribution method, and D₁₀, D₅₀ and D₉₀ are cumulative particle diameters measured thereby”, as recited in the claims, and when the flake copper powder is processed into a conductive paste then layer thickness of the circuit can be thinner drawing a circuit under the conditions described above. Further, the layer density is excellent and it is possible to acquire a well-quality-balanced thixotropic property which can easily remove a binder contained in a conductive paste. In case of using such a conductive paste, the following can be shown: it can prevent on increase in conductivity resistance; simultaneously the conductivity can be enhanced by its shape without increasing conductivity resistance. In FIG. 1, the flake copper powder (two types) relating to the present invention is shown, which is observed using the scanning electron microscope. As apparent from comparing FIG. 1 with FIG. 2, the powder particle sizes of flake copper powder in FIG. 1 are uniform and have more microscopical shape in comparison with flake copper powder in FIG. 2. Even at a level being clearly visible by the scanning electron microscope, it is easy to understand that the particle distribution may be sharp.

Here, the reason why “cumulation particle size D₅₀ is 10 μm or smaller with the laser diffraction scattering particle size distribution measurement” has been proved by eager researching and developing by the inventors, owing that if a cumulation particle size D₅₀ is 10 μm or smaller, it is impossible to stably decrease the thickness of conductor of a circuit drawn by the conductive paste using the flake copper powder and also it is impossible to enhance a filing ability of a via-hole. In particular, in the case where a cumulation particle D₅₀ is 7 μm or smaller, it can be possible to acquire an adequate thixotropic property when the copper powder is processed into a conductive paste. In a case of drawing a circuit after the copper powder is processed into a conductive paste, the thickness of a layer can be thinner and the layer density is superior, further, it has well-quality-balance that is able to perform a binder removing method as a conductive paste. It is noted that it has especially high stability as a conductive paste optionally, it is noted that thickness of the conductive figure cannot be thinner resulting in increasing electric resistance of a formed sintered circuit due to the inferiority of the layer density inside of the conductor, breaking linearity of the edge surface of the sintered circuit and roughening of the surface condition of a sintered circuit, even if a thinner conductor is formed using a conductive paste, the thin layer is not formed successfully because of existence of coarse particles and inferior thixotropic property. Additionally, one can consider that measurement of cumulative particle diameter D₅₀ using this method of measuring the laser diffractive scattering particle size distribution may be a reflection of the length at a major axis of the particle of flake copper powder affected and flattened by plastic deformation.

It is more preferable that an aspect ratio (average major axis/average thickness) of the powder particle is from 3 to 200. The aspect ratio herein is determined depending on a processing degree of the powder particle. Generally, as an aspect ratio is higher, a thickness of flake copper powder tends to become thinner. On the other hand, when the aspect ratio is smaller, the flake copper powder tends to become thick and large. Therefore, it is remarkable that if the range of the aspect ratio (average major axis/average thickness) is 3 or shorter, the thixotropic property will be apparently lacking with respect to the viscosity property when the flake copper powder is processed into a conductive paste. On the other hand, in times when the aspect ratio (average major axis/average thickness) exceeds 200, the shape of the powder particle itself folded, a defective shape occurs, and cracks on the surface of the powder particle occur. The range of the particle distribution will be very broad and the thickness of flake copper powder will be much too thin. This thin flake copper powder cannot be mixed evenly with organic vehicles when a flake copper powder is processed as a conductive paste.

Additionally, a property of the flake copper powder of the present invention, when the cumulative particle diameter D₅₀ through the laser diffraction scattering particle size distribution measurement method is defined by a standard value, has a maximum cumulative particle diameter Dmax value which will never exceed the standard value. The Dmax value is never more than five times the D₅₀ value. Namely, the Dmax/D₅₀ of a ratio of a cumulative particle diameter D₅₀ to the maximum cumulative particle diameter Dmax determined through the laser diffraction scattering particle size distribution method is 5 or smaller. As a result, the product (the flake copper powder relating to the present invention) is a narrow distribution of particles, because there is no coarse particle that could be observed in the conventional flake copper powder.

Also the above-mentioned flake copper powder is obtained through a process where the conventional copper powder particle having a substantially spherical shape is mechanically changed to be flake-shaped by plastic deformation. As a result, scattering upon producing will generally occur at a certain rate. Then the inventors have studied as follows. If the product contains 70% or more of the flake copper powder with above-mentioned fine powder properties, even if the powder properties of the other remaining flake copper powder do not meet above-mentioned assumption, the flake copper powder produces the properties sufficiently by maintaining stability of the circuit configuration by processing a conductive paste and reducing the thickness of drawing a circuit.

Method of Producing Flake Copper Powder Relating to the Present Invention

It is impossible to stably produce the above-mentioned flake copper powder, even if the conventional production process is used. In other words, the conventional flake copper powder, the substantially spherical copper powder obtained with wet method such as the typified hydrazine reduction method with dry method and a typified atomizing method, is directly milled with a mill such as a ball mill, a beads mill or the like. Then the processed powder particle is changed by plastic deformation to be flattened and flake-shaped.

However, in the case of performing such production process, usually-spherical copper powder itself used at the first step, is under a predetermined agglomerate condition, and even if conducting compressed deformation without destroying the agglomerate condition, the condition will be compressed by deformation, maintaining the agglomerate condition. As a result of the production process, the flake copper powder an produced under the same agglomerate condition as above-mentioned, and additional powder particles will not be mutually dispersed.

Therefore, the inventors have reached a conclusion that at first the agglomerate condition of substantially spherical powder particle be dispersed, and secondly the powder particle is forced to change by compressing and deformation to be flake-shaped. The corresponding method according to claim is that “a production method of the flake copper powder comprises the steps of: dispersing a copper powder under an agglomerate condition; using the copper powder having superior dispersity whose agglomerate degree is 1.6 or smaller after completion of dispersion; and forming and plastically deforming particles of the copper powder in a flake manner by compressing the particles of the copper powder with a high energy ball mill using media beads, each of which the particle diameter is 0.5 mm or smaller”.

Copper powder under agglomerate condition is defined such that even if the inventors use the wet method being the typified hydrazine reduction, or the dry method being typically atomizing method, a certain agglomerate condition of copper powder will be formed, which is the reason why the term “agglomerate condition” is used in the descriptions. Especially, applying wet method tends to induce of the agglomerate condition particle of copper powder. This, because in general the production method of copper powder with wet method uses copper sulfate solution as starting material. Then a sodium hydroxide solution is utilized to be reacted in order to obtain copper oxide. This copper oxide goes through the so-called hydrazine reduction, and then produces with the methods below, such as cleaning, filtering and drying. The method will provide copper powder to be under dry condition, though if the wet method is used in order to gain particles of copper powder, it will tend to produce a certain agglomerate condition in the producing process. Additionally, a copper powder slurry as below is defined by, that copper powder comes up in the so-called hydrazine reduction and such copper powder slurry conditions are established containing the above-mentioned copper powder. The operation that agglomerate particles are dispersed under initial particles as much as possible is so-called “dispersing”.

If an object is merely to disperse copper into particles using several methods, it seems to be possible to use such methods as a high-energy ball mill, a high-speed conductive jet mill, an impact mill, a gauge mill, a media agitating mill, a high-pressured water type mill and so on. However, according to study by the inventors, the two types of dispersing methods mentioned below are preferable from an aspect of reliability in dispersing procedure. A common point between the two methods is to inhibit minimum that the particles of copper powder touch inside of the device, impeller and media to mill, which occurs when powder particles under agglomerate condition an crashed on each other in order to disperse them into individual powder particle form the agglomerate condition. In the other words, this can restrain if at all possible the touching of the inside of the device, impeller and media to mill, injuring the surface of powder particle and increasing the roughness of the surface of powder particle. Further, when a sufficient crash between each powder particle occurs, this can result in dispersing the powder particle under agglomerate condition, at the same time it can produce a smooth surface of powder particle through the crash of each powder particle.

As one of the methods for the dispersing procedure, of dried copper powder under agglomerate condition can be dispersed into each particle of copper powder with a wind power circulator. The “wind power circulator utilizing centrifugal force” defined herein, first of all, to blow air and then blow up copper powder in concentrated condition like drawing an orbit of circumference of track in order to circulate. To use the centrifugal force occurred as set forth, each powder particle is forced to be crushed in the air in order to be dispersed. In this case, it is possible to use commercial force of a wind classification machine. The object of the machine is not to classify but the object is to take a role as a circulator to blow up air and then in concentrated status the copper powder is blown up in the air like drawing circumference of track.

Another method of dispersing copper powder into particles, was copper powder slurry containing copper powder under agglomerate condition in a procedure with a fluid mill using centrifugal force. The object is to use the “fluid mill used centrifugal force” here in, first of all, to flow copper powder slurry in high speed to drawn orbit of circumference, and then each particle of copper powder to crash these with each other in a solvent with centrifugal force which occurred at the time due to the dispersing procedure.

Accordingly, the above-mentioned dispersing procedure, can be conducted repeatedly to meet the requirements, and also to meet the products quality, and level for dispersing procedure of particle can be selected optionally. The copper powder finished in a dispersing procedure, has new properties as a powder particle after the concentrate condition is destroyed. Following is an explanation about the agglomerate value set in the description. Using the D₅₀ obtained value, with the laser diffraction scattering particle size distribution measurement and an average particle diameter which is a D_IA, defined by calculation from a size of the picture image by SEM, and then an agglomerate value of D₅₀/D_IA shown by the above value, D₅₀ and D_IA, that should be 1.6 or smaller is the most preferable value to be settled. That's why an almost perfect condition of mono-disperse could be established, even if the agglomerate value became 1.6 or smaller.

The D₅₀ value, obtained through the laser diffraction scattering particle size distribution measurement method, will not be considered to really observe a powder particle one by one. Most copper powder particles are not individual perfectly, so-called mono-dispersed. The copper powder are comprised of several particles of the agglomerate condition. The laser diffraction scattering particle size distribution measurement method is to regard each of the agglomerate powder particles as a single particle, and then calculates the value of cumulative particle diameter.

On the contrary, an average diameter value D_IA with SEM (Scanning Electron Microscope) observes a copper powder image and processes the observation image into image data, as is directly obtained from the SEM observation image. An image of initial particle can be perceived surely using the laser diffraction scattering particle size distribution measurement. On the other hand, it is not reflected thereon that there exist powder particles under agglomerate condition.

In view of the above-mentioned content, the inventors used D₅₀ being the value of cumulative particle diameter to get the laser diffraction scattering particle size distribution measurement and an average particle diameter D_IA obtained through an image analysis to determine the agglomerate value, which can be calculated as D₅₀/D_IA. In other words, the inventors presume that in copper powder from the same lot, the D₅₀ and D_IA values can be measured with the same accuracy, considering the above-mentioned theory. The D₅₀ value is meant by reflection of the concentrated condition over a value to be measured, so that D₅₀ value may be higher than D_IA value.

If the agglomerate particles of copper powder become perfectly individual, the D₅₀ value will be infinitely closer to the D_IA value and the concentrated degree D₅₀/D_IA will be close to 1. When the concentrated value becomes 1, then it can be said that there is completely no agglomerate condition of powder particles, and as a result, those particles are completely dispersed. However, in reality, sometimes the concentrated value is indicated as smaller than 1. Theoretically, when considering a particle is completely spheroid, in fact the value is not smaller than 1. However, if a particle whose shape is not spheroid, a value being smaller than 1 can be obtained. Further, the image analysis using scanning electron microscope in the description relating to the present invention, using IP-1000PC manufactured by Asahi Engineering, which sensitivity threshold value of 10, overlaps extending the value which is regarded as 20 by circular-shaped-particle analyzing. The average particle diameter is obtained as D_IA.

The substantially spherical copper powder after completion of the dispersion is processed with a high energy ball mill. The particle of copper powder is formed by plastic deformation and produces flake copper powder. Therefore the cumulated particle diameter D₅₀ of laser diffraction scattering particle size distribution measurement of the flake copper powder as the final product on the procedure mentioned above is 10 μm or smaller. First of all, D₅₀ can be employed as a standard using the laser diffraction scattering particle size distribution measurement of the flake copper powder as before compressive deformation and after the dispersion treatment (hereinafter referred as “original powder”), compared with a processed flake of copper powder. To consider these matters, D₅₀ can be used as an estimation index.

The “high energy ball mil” herein is a generic term used to refer to a device which employs media beads to compress copper powder into plastic deformation, e.g., using a ball mill, agitator and so on, regardless whether under wet condition or under slurry copper powder condition. Regarding the present invention, selecting a particle diameter of each of the media beads and quality of material is very important.

Firstly, media beads should be used which particle diameter is 0.5 mm or smaller. The reasons why a diameter of each of the media beads is defined based on the following. If the size of diameter of media beads is over 0.5 mm, it is easy that flake copper powder can be easily agglomerate when the media beads are compressed by plastic deformation. As a result, coarse flake powder particles are generated due to a change in shapes of agglomerate particles by compressive plastic deformation. The flake copper powder cannot be obtained that has a narrow and superior dispersibility particle size distribution, because the particle size distribution became broad.

Further, it is preferable to use media beads, wherein the gravity of each of the media beads is from 3.0 g/cm³ to 6.5 g/cm³. In case of a specific gravity of the media beads is smaller than 3.0 g/cm³, so that it takes a long time for compressive deformation because the gravity of media beads is too light. Considering productivity of flake copper powder, this is not a reasonable condition for production. On the contrary, in a case of specific gravity of the media exceeds 6.5 g/cm³, the gravity of media beads becomes heavier, so that compressive deformation force of each of particles of copper powder becomes large and it becomes easy to condensate each powder particle. Additionally, it is unable to have uniformed thickness of flake copper powder after the deformation. By obtaining flake copper powder using the above-mentioned method, products can be produced effectively providing powder properties relating to flake copper powder of the present invention. In addition, the producing conductive paste for which this flake copper powder is used has excellent performance. Therefore, when a conductor is produced using such flake copper powder, even if the thickness of conductor becomes thinner, such flake copper powder can maintain lower electronic resistance, and also its stability in a conductive configuration will be superior. Accordingly, it will be a suitable method for yielding sintering circuit of PWB, a sintered configuration of ceramic capacitor.

Conductive Paste

When producing conductive paste with the above-mentioned flake copper powder of the present invention, controlling viscosity is easy. Simultaneously, the changing because of aging of conductive paste is diminished and easily providing thixotropic property being superior to a conductive paste. Therefore, regarding conductive paste using flake copper powder of the present invention, if kinds of organic vehicles in the conductive paste and the containing amount of the flake copper powder are as same as those of the conventional flake copper powder content, the quality of the present conductive paste is incomparably better than that of the conventional one.

The level of thixotropic character of conductive paste should depend on the intended purpose and usage. In general, appropriate measures are determined with the consideration over variations of organic vehicles in the conductive paste, flake copper powder content, and the diameter of the particle of flake copper powder.

BEST MODE FOR CARRYING OUT THE INVENTION

The following examples specifically show the present invention.

EXAMPLE 1

In this example, copper powder obtained from raw material powder by a below-mentioned method is used, as starting powder for the production process of the present invention to produce flake copper powder.

The powder properties of the original powder utilized in this example are defined in that, the cumulative particle diameter; D₅₀ was 0.35 μm, which was obtained using a laser diffraction scattering particle size distribution measurement method and average particle diameter; D_IA was 0.20 μm obtained by an image analysis. Accordingly, an agglomerate value calculated on D₅₀/D_IA was 1.75.

The above-mentioned original powder under agglomerate condition was circulated at 6500 rpm, with a Turbo classifier manufactured by Nissei Engineering Limited, which is a commercial pneumatic classification device to perform an operation by which agglomerate particles were made to be singular by colliding the powder particles against each other.

As a result, the cumulative particle diameter of copper powder (starting powder) completed as single particles, i.e., D₅₀ was 0.30 μm using the laser diffraction scattering particle size distribution measurement method and an average diameter D_IA was 0.20 μm obtained from the image analysis so that the agglomerate value calculated on D₅₀/D_IA was 1.50. This fact showed that the above-mentioned dispersion operation was performed sufficiently.

Next, 300 g of starting powder containing a singular particle was used in using a DISPERMAT D-5226 manufactured VMG-GETZMANN with 800 g of zirconia's beads, as media beads, each whose specific gravity of zirconia's beads was 5.8 g/cm³, and its diameter was 0.3 mm. As a solvent, methanol 120 g mixed with cupric acid 5 g was used and then these were treated them using Turbo classifier, under a condition of 2000 rpm for 3 hrs., and then particles of original powder were converted by compression through plastic deformation, resulting in changing the spherical starting powder into the flake copper powder.

The properties of the flake copper powder obtained as described above, are that the maximum particle diameter is 1.64 μm, and the below mentioned Dmax/D₅₀, a ratio of average particle diameter D₅₀ equals 4.1, and there are no observances of particles over 5; and the number of SD/D₅₀ is 0.38 calculated with laser deffraction scattering particle size distribution measurement method of weight accumulation of D₁₀ (0.26 μm), D₆₀ (0.40 μm), D₉₀ (0.67 μm), and using the particle distribution, normal deviation SD (0.15 μm) calculated through laser diffraction scattering particle size distribution measurement method, and number represented with D₉₀/D₁₀ is 2.58.

And the average thickness of powder particle of the flake copper powder was 0.05 μm. The thickness was significantly determined using the following method having the steps of producing a sample made of flake copper powder being solidified using epoxy resin and observing that sample with the scanning electron microscope (at X10000-magnification) to monitor the sample in order to determine the thickness directly. Then, the total of thickness of the flake copper powder in the field of microscope view was divided into the total number of flake of copper powder. And yet, in the below-mentioned examples and the comparative example, magnification of the microscope was applied up to the thickness of copper powder for monitoring being available to determine the thickness as well as the above-mentioned methods. Further, the average particle diameter (major axis) being observed directly of this flake copper powder was 0.39 μm. Here, the powder particle was observed using the scanning electron microscope (at X5000-magnification), and then the average value of major axis for the flake copper powder, which could be confirmed from observation of the image obtained using the above-mentioned method was required. Comparing he magnification of the major axis of flake copper powder, by which the major axis of the flake copper powder could be observed at pleasure, the following can be viewed in examples and the comparative example. The average aspect ratio was 7.8. The average aspect ratio was required in the above-mentioned [average particle size]/[average thickness]. Accordingly, it could be shown that the requirement were satisfied which the flake copper powder of the present invention should meet.

Additionally, the inventors produced conductive paste which belonged to a terpineol group used for flake copper powder, and measured the change rate of viscosity of a conductive paste. The composition of the conductive paste belongs to a terpineol group produced in the present invention constituted by 65 wt % of flake copper powder and the rest of a composition which is an organic vehicle used as binder resin, and milling those in order to gain the conductive paste of the terpineol group. The organic vehicle utilized in this method had the composition constituted by terpineol 93 wt % and ethylcellulose 7 wt %. The viscosity of conductive paste of terpineol group being obtained using the above-mentioned method, was measured immediately after produced.

The viscosity in this description, was measured using RE-10 which was a viscometer manufactured from Toki Sangyo Co., Ltd. at 0.1 rpm and 1.0 rpm. The following, measured at 0.1 rpm, is called [A viscosity], and measured at 1.0 rpm is called [B viscosity]. A viscosity was 380 Pas and B viscosity was 160 Pas. Besides, in order to require the viscosity ratio ([A viscosity]/[B viscosity]), used for the index to show the thixotropic property of a conductive paste, as defined by 2.4. It can be said that the larger the viscosity ratio, the thixotropic property of the conductive paste might be preferable.

EXAMPLE 2

In this example, copper powder obtained from raw material powder with the below-mentioned method was used, as starting powder in a production process of the present invention to produce flake copper powder.

Powder properties of the original powder utilized in this example are defined in that, the cumulative particle diameter, i.e., D₅₀ value was 0.85 μm, in which the value was obtained using the laser diffraction scattering particle size distribution measurement method, and an average particle diameter, i.e., D_IA value was 0.48 μm, which was obtained by image analysis. Accordingly, an agglomerate value calculated based on D₅₀/D_IA value was 1.77.

With respect to the above-mentioned original particles powder under agglomerate condition, the powder was used in purified water as a copper powder slurry, and then circulated at 3000 rpm, with a fine flow mill manufactured by Pacific Machinery & Engineering Co., Ltd. which is a commercial fluid mill using a centrifugal force to perform an operation that converts agglomerate powder particles into singular particles by colliding the powder particles against each other.

As a result, the cumulative powder particle diameter of the copper powder (starting powder), completed after conversion to a single particle, the D₅₀ value was 0.73 μm measured with the laser diffraction scattering particle size distribution method, and an average diameter D_IA was 0.49 μm obtained by image analysis. Thus the agglomerate value calculated for the D50/D_IA value was 1.49. This fact showed that the above-mentioned operation was conducted sufficiently.

Next, 500 g starting powder, treated after dispersion of particles, was used in the same method as in Example 1, wherein the powder particles of the starting powder were compressed and plastically deformed, so that the spherical starting powder was converted to flake copper powder. However, in the media dispersion mill, the DISPERMAT D-5226, manufactured by VMG-GETAMANN in Example 1, only the processing time was changed to 10 hours for this treatment, and thus compressing the powder particles of the starting powder by plastic deformation, and finally the substantially spherical starting powder particular were compressed and plastically deformed into flake copper powder.

The obtained flake copper powder's properties using the above-mentioned method, are that the maximum particle diameter was 15.56 μm, and there were no coarse particle such that the Dmax/D₅₀ was equal to or larger than 4.7 but also equal to or smaller than 5 as mentioned below. The agglomerate values show a D₁₀ value (1.51 μm), a D₅₀ value(3.33 μm) and a D₉₀ value(6.03 μm) measured with the laser diffraction scattering particle size distribution method. The SD/D₅₀ value was 0.50 and the D₉₀/D₁₀ value was 3.99 showing a standard deviation SD (1.68 μm) of the particle size distribution measured with the laser diffraction scattering particle size distribution method. The average thickness of the powder particle of the flake copper powder was 0.02 μm, the average particle diameter (major axis) directly observed of this flake copper powder was 2.8 μm, and an average aspect ratio was 140. Accordingly, the fact that flake copper powder of the present invention met the requirements.

Furthermore, the inventors produced a conductive paste having a terpineol group using flake copper powder, and providing an organic vehicle with mix at ratio in the same way as in Example 1. The rate of viscosity of the conductive paste was then measured. As a result, the A viscosity was 600 Pa.s, the B viscosity was 143 Pa.s. Therefore the viscosity ratio ([A viscosity]/[B viscosity]) was 4.2.

EXAMPLE 3

In this example, copper powder obtained from raw material powder with a below-mentioned method was used, as starting in a production process of the present invention to produce flake copper powder. The raw material and starting powder utilized in this example were produced in the same way as in Example 2. Therefore, to avoid duplicate explanation about the properties of powder particle and similarly the properties after finishing the treatment, the explanation is omitted.

Next, 500 g of starting powder constituted by single particles in the same way as in Example 1 were used to compress powder particles of the starting powder and deform them by plastic deformation, so as to obtain from substantially spherical starting powder particles and the flake copper powder. However, in the media dispersion mill called the DISPERMAT D-5226 manufactured by VMG-GETAMANN used the processing time was only changed from Example 1 to 7 hours in this treatment. Then the powder particles of the starting powder were compressed to be converted by plastic deformation, to finally convert the substantially spherical starting powder particles into flake copper powder particles.

The obtained flake copper powder's properties using the above-mentioned method are, that the maximum particle diameter was smaller than 5.36 μm, and there were no coarse particle whose average particle diameter was defined as D₅₀, The resulting Dmax/D₅₀ value was larger than 3.6 but smaller than 5 as mentioned below, and the results show a D₁₀ value(0.67 μm), a D₅₀ value (1.50 μm) and D₉₀ value(2.80 μm) measured with the laser diffraction scattering particle size distribution method. The SD/D₅₀ value was 0.53 and the D₉₀/D₁₀ value was 4.18 using the standard deviation SD (0.79 μm) of particle size distribution measured with the laser diffraction scattering particle size distribution method. The average thickness of the powder particle of the flake copper powder was 0.08 μm, the average particle diameter (major axis) observed directly for this flake copper powder was 1.3 μm, and an average aspect ratio was 18.8. Accordingly, these facts show that the flake copper powder of the present invention met the requirements.

Further, the inventors produced a conductive paste which has a terpineol group using flake copper powder, providing an organic vehicle at mixed ratio similar to that in Example 1. The rate of viscosity of the conductive paste was then measured. As a result, the A viscosity was 420 Pa.s and the B viscosity was 130 Pa.s. Therefore, the viscosity ratio ([A viscosity]/[B viscosity]) was 3.2.

EXAMPLE 4

In this example, copper powder obtained from raw material powder with a below-mentioned method was used, as starting powder in a production process of the present invention to produce flake copper powder. The raw material and starting powder utilized in this example were produced in the same way as in Example 2. Therefore, to avoid duplicate explanation about the properties of the powder particle and similarly the properties after finishing treatment, the explanation is omitted.

Next, 500 g of starting powder constituted by single particles, provided by the same method as in Example 1 were used to compress powder particles of the starting powder and were converted by plastic deformation, so as to convert spherical starting powder into flake copper powder. However, in the media dispersion mill called the DISPERMAT D-5226 manufactured by VMG-GETAMANN in Example 1, only the treatment time was changed to 7 hours, followed by compressing the powder particles of starting powder and converting them by plastic deformation, finally converting the spherical starting powder particles to flake copper powder.

The obtained flake copper powder's properties using the above-mentioned method were as follows, the maximum particle diameter Dmax was 1.44, and there were no coarse particles having an average particle diameter D₅₀. The Dmax/D₅₀ value was 1.5, but there were no coarse particles whose Dmax/D₅₀ value was 5 or larger as mentioned below, and the agglomerate values show a D₁₀ value (0.51 μm), a D₅₀ value (0.95 μm) and a D₉₀ value (1.43 μm) measured with the laser diffraction scattering particle size distribution measuring method. The SD/D₅₀ value was 0.45 and the D₉₀/D₁₀ value was 2.80 observed by using the standard deviation SD (0.79 μm) of a particle size distribution measured with the laser diffraction scattering particle size distribution measuring method. The average thickness of the powder particle of the flake copper powder was 0.19 μm. The average particle diameter (major axis) obtained directly using this flake copper powder was 0.9 μm, and the average aspect ratio was 4.7. Accordingly these facts show that flake copper powder of the present invention met the requirements. Further the inventors produced a conductive paste which has a terpineol group using the flake copper powder, and applying organic vehicle and mixed ratio similar to that in Example 1. The rate of viscosity of the conductive paste was then measured. As a result, the A viscosity was 350 Pa.s and the B viscosity was 125 Pa.s. Therefore, the viscosity ratio ([A viscosity]/(B viscosity]) was defined by 2.8.

EXAMPLE 5

In this example, copper powder obtained from raw material powder using the method mentioned below was used, as starting powder in a production process of the present invention to produce flake copper powder.

With respect to the powder properties of starting powder utilized in this example, the cumulative particle diameter, i.e., D₅₀, was 6.84 μm, which was obtained using the laser diffraction scattering particle size distribution measurement method, and the average particle diameter; D_IA was 4.20 μm. This value was obtained by image analysis. Accordingly, an agglomerate value was calculated, so that D₅₀/D_IA value was 1.63.

The above-mentioned starting powder under agglomerate condition was circulated at 6500 rpm, in a Turbo classifier manufactured by Nissei Engineering Limited using a commercial pneumatic classification device to perform an operation that made the agglomerate particles singular by colliding the powder particles against each other.

As a result, the cumulative particle diameter of copper powder (starting powder) was measured after completion of the conversion to singular particles, the D₅₀ value was 4.92 μm as measured with the laser diffraction scattering particle size distribution method, and an average diameter D_IA was 4.10 μm obtained from image analysis, so that the agglomerate value calculated on D₅₀/D_IA value was 1.20. This fact shows that the above-mentioned operation was conducted sufficiently.

Next, 500 g of starting powder comprising single particles was treated in the same way as in Example 1. The compression of the powder particles of the powder converts these by plastic deformation, so as to convert spherical starting powder to flake copper powder. However, in the media dispersion mill called DISPERMAT D-5226 manufactured by VMG-GETAMANN as was used in Example 1 the only change made was the processing time to 10 hours for this treatment, following by compression of starting powder particles converting them by plastic deformation, thereby changing spherical starting powder to flake copper powder.

The obtained flake copper powder's properties using the above-mentioned method are that, the maximum particle diameter, Dmax was smaller than 40.00 μm, and there were no coarse particles having an average particle diameter of D₅₀. The Dmax/D₅₀ value was 4.2 and there is no coarse particle whose size is 5 or larger, and the agglomerate values show a D₁₀ (4.75 μm), a D₅₀ (9.50 μm) and a D₉₀ (12.83 μm) using the laser diffraction scattering particle size distribution measurement method. The SD/D₅₀ value was 0.34 and the D₉₀/D₁₀ value was 2.70 using a standard deviation SD (3.23 μm) of the particle size distribution measured with the laser diffraction scattering particle size distribution method. The average thickness of the powder particle of the flake copper powder was 0.80 μm and the average particle diameter (major axis) observed directly by this flake copper powder was 9.2 μm, and the average aspect ratio was 11.5. Accordingly, these facts show that the flake copper powder of the present invention met the requirements.

Additionally, the inventors produced a conductive paste, which has a terpineol group using flake copper powder, and providing an organic vehicle at a mix ratio in the same way as in Example 1. The rate of viscosity of the conductive paste was then measured. As a result, the A viscosity was 90 Pa.s and the B viscosity was 60 Pa.s. Therefore the viscosity ratio ([A viscosity]/[B viscosity]) was 1.5.

EXAMPLE 6

In this example, copper powder obtained from raw material powder was used in the method below, wherein as starting powder was used in the production process of the present invention to produce flake copper powder.

The powder properties of starting powder used in this example were, that the cumulative particle diameter; D₅₀ was 4.24 μm, in which the value was obtained with the laser diffraction scattering particle size distribution measurement method and the D_IA of the average particle diameter was 2.10 μm, in which the value was obtained by image analysis. Accordingly, the agglomerate value obtained by D₅₀/D_IA was 2.02.

The above-mentioned starting powder under agglomerate condition was circulated at 6500 rpm, Turbo classifier from Nissei Engineering Limited, used for a commercial pneumatic classification device to perform an operation that made the agglomerate powder particles singular by colliding the powder particle against each other.

As a result, the cumulative particle diameter of copper powder (starting powder) completed after conversion conducting to single particles was measured. The D₅₀ value was 2.80 μm using the laser diffraction scattering particle size distribution measurement method, and the average diameter D_IA was 2.00 μm obtained from image analysis, so that the agglomerate value calculated by D₅₀/D_IA value was 1.40. These facts show that the above-mentioned operation was sufficiently performed.

Next, 500 g of the starting powder constituting single particles was provided in the same way as in Example 1 to compress powder particles of starting powder converting them by plastic deformation, so as to convert spherical starting particles powder into flake copper powder. However, in the media dispersion mill, the DISPERMAT D-5226 manufactured by VMG-GETAMANN as in Example 1, only the processing time was changed to 7 hours for this treatment, followed by compression of the powder particles of the starting powder and converting them by plastic deformation, resulting in that the substantially spherical starting powder was changed into flake copper powder.

The obtained flake copper powder's properties using the method as mentioned above, has a maximum particle diameter, Dmax of 20.73 μm or smaller, and there were no coarse particles having an average particle diameter of D₅₀. The Dmax/D₅₀ ratio was 2.8 but there were no coarse particles whose D₅₀ was 5 or larger as described below, and the agglomerate values show a D₁₀ (3.87 μm), a D₅₀ (7.30 μm) and a D₉₀ (8.51 μm) measured with the laser diffraction scattering particle size distribution method. The SD/D₅₀ value was 0.50 and the D₉₀/D₁₀ value was 2.20 using the standard deviation SD (2.34 μm) of the particle size measured distribution with the laser diffraction scattering particle size distribution method. The average thickness of the powder particle of the flake copper powder was 0.70 μm, the average particle diameter (major axis) observed directly of this flake copper powder was 7.2 μm, and the average aspect ratio was 10.3. The facts show that the flake copper powder of the present invention met the requirements.

Accordingly, the inventors produced a conductive paste having terpineol groups using the flake copper powder, and applying an organic vehicle at a mix ratio in the same way as in Example 1. The rate of viscosity of the conductive paste was then measured. As a result, the A viscosity was 112 Pa.s and the B viscosity was 70 Pa.s. Therefore, the viscosity ratio ([A viscosity]/[B viscosity]) was shown to be 1.6.

COMPARATIVE EXAMPLE

In this comparative example, dried material powder under agglomerate condition was employed as in Example 1, without a dispersing operation similar to that in Example 1, using a Dyno-mill manufactured by Willy A. Bachofen A G Maschinenfabrik, KDL type, followed by compressing the powder particles of the starting powder and converting them by plastic deformation with 0.7 mm diameter beads, thus converting spherical starting powder into flake copper powder. As a result, the obtained powder properties of flake copper powder were shown as mentioned above in Table 1, labeled as sample number 4. This flake copper powder contains coarse particles, in which the maximum diameter was five times as long as the average diameter D₅₀.

Now, the powder properties of the flake copper powder labeled as sample number 4 will be described. The agglomerate values show a D₁₀ (2.81 μm), a D₅₀ (8.20 μm), a D₉₀ (21.38 μm) and the maximum particle diameter size Dmax (52.33 μm), a resulting Dmax/D₅₀ was 6.4 and the value of it was 5 or larger. Further, the SD/D₅₀ value was 0.87 with the value of the standard deviation, SD (7.17 μm), and the D₉₀/D₁₀ value was 4.04. The average thickness of the powder particle of the flake copper powder was 0.75 μm, and an average particle (major axis) to be observed directly was 7.8 μm, an average ratio was 10.4. Alternatively, these facts show that only the flake copper powder of the present invention meets the requirements. Using such above flake copper powder to produce a conductive paste, even if the composition of an organic vehicle were altered in order to control the viscosity of the conductive paste is difficult wherein, such flake copper powder could not be applied to drawing of a printed circuit by a high density.

Therefore, the inventors measured the viscosity of conductive paste utilizing this flake copper powder, of sample number 4, and applying an organic vehicle and mixing thereof to produce conductive paste with a terpineol group. As a result, the A viscosity was 250 Pa.s and the B viscosity was 227 Pa.s. The viscosity ratio ([A viscosity]/[B viscosity]) was defined by 1.1. Owing to this result, the thixotropic property alone thereof seemed to be especially inferior in comparison with the above-mentioned conductive paste, though there might be no extraordinary difference between both. This is to say, that the conventional flake copper powder was acquiring a thixotropic performance by diminishing the thickness of the particles of flake copper powders, but because the particle distribution of particles has broadened, and since it included especially large particles based on the average particle diameter, it can not be used for forming thin, electrode and small circuits having high layer density.

INDUSTRIAL APPLICABILITY

The viscosity of conductive pastes can be controlled by using flake copper powder of the present invention, and thereby it can provide a thixotropic property having a good balance with respect to the viscosity, forming conductive pastes which are thinner, and enhancing the layer density, without losing electrical resistance. Also the conductor shape is more easily controlled, resulting in that a thinner and/or fined circuit pattern can be established can be obtained. Further, usage of the production method of the present invention makes possible to produce flake copper powder efficiently. Also, through the flake copper powder having powder properties of the present invention, the particle distribution of fine particle is excellent, which before did not exist. Also, the production yield of the flake copper powder having the excellent powder properties can be enhanced much more.

It is apparent from the above-mentioned descriptions, that the flake copper powder of the present invention has a particle distribution which is much narrower than that of the conventional copper powder and the aspect ratio of the flake copper powder can be easily changed using the producing method of the present invention. As a result, the most preferable thixotropic characteristics can be designed of flake copper powder.

Claims

1. Flake copper powder processed by plastic deformation of each of particles of copper powder characterized in that:

a cumulative particle diameter D50 is 10 μm or smaller;

a SD/D50 value is 0.55 or smaller; and

a D90/D10 value is 4.5 or smaller;

in which SD is a standard deviation of a particle distribution measured by a laser diffraction scattering particle size distribution method, and D10, D50 and D90 are cumulative particle diameters measured thereby.

2. The flake copper powder according to claim 1, wherein an aspect ratio (average major axis/average thickness) of said powder particle is from 3 to 200.

3. The flake copper powder according to claim 1, wherein Dmax/D50 of a ratio of a cumulative particle diameter D50 to the maximum cumulative particle diameter Dmax by the laser diffraction scattering particle size distribution method is 5 or smaller.

4. Flake copper powder including said flake copper powder according to claim 1 by 70 wt % or larger in existence rate.

5. A manufacturing method of the flake copper powder according to claim 1 comprising the steps of:

dispersing a copper powder under an agglomerate condition;

using the copper powder having superior dispersity whose agglomerate degree is 1.6 or smaller after completion of dispersion; and

forming and plastically deforming particles of said copper powder in a flake manner by compressing said particles of said copper powder with a high energy ball mill using media beads, each whose particle diameter is 0.5 mm or smaller.

6. The manufacturing method of flake copper powder according to claim 5 wherein a gravity of each of the media beads is from 3.0 g/cm3 to 6.5 g/cm3.

7. A conductive paste produced using said flake copper powder according to claim 1.