JPH0760617B2 - Low temperature firing conductive paste and firing method - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a low-temperature calcinable conductive paste and a calcining method thereof, and more particularly to a low-temperature calcinable conductive paste containing ultrafine powder as a calcining accelerator. And its firing method.

(Prior Art) Currently, in a conductive paste, a noble metal such as Ag is mainly used as a conductive powder. Generally, Ag conductive powder, together with glass frit as a binder between the substrate and the conductor, is dispersed in a vehicle that is an organic solvent that gives viscosity necessary for printing to form an Ag conductive paste, and this Ag conductive paste is used. After being applied on a substrate such as ceramics by a method such as screen printing, it is fired at a high temperature (over 900 ° C) and used as electrodes for ceramic capacitors, piezoelectric elements, semiconductor ceramics, etc. or wiring conductors for electronic circuits. There is.

However, when Ag conductive paste is used, printed / sintered Ag ions elute / precipitate in the water contained in the ceramic and connect the wires to each other, causing a short circuit between wires. The metal atoms in the metallized layer diffuse into the solder when the substrate is immersed in a solder bath for soldering to the metallized layer formed on the ceramic substrate, and when the metallized layer is pulled up afterwards. In addition, a phenomenon in which the metallized layer disappears, that is, so-called solder cracking is likely to occur. In addition, Ag is expensive, and price fluctuations are severe,
There is a problem that it is difficult to use as an industrial material.

Therefore, as an alternative to the Ag conductive paste, a Cu conductive paste having excellent migration resistance and solder cracking resistance has been developed, and it has been partially commercialized.

However, since the Cu powder used in the industrialized Cu conductive paste is composed only of fine powder having an average particle size of about 1 to 10 μm, it is normally 900 for forming a conductive path by diffusion bonding between Cu powders. High temperature firing above ℃ is required. However, when a Cu conductive paste is baked on a substrate by high-temperature firing, it may cause reduction deterioration (reduction by Cu) in a dielectric paste or resistor paste that is mainly composed of an oxide and is printed on the same substrate. . Therefore, a Cu conductive paste that can be fired at a low temperature is desired.

Therefore, as it has become clear in recent years that the ultra-fine powder has excellent low-temperature sintering characteristics (for example, Masami Kikuchi,
54 (1984) 48), attempts have been made to use ultrafine powder for low-temperature firing conductive paste.

(Problems to be Solved by the Invention) However, there are the following problems in forming an ultrafine powder into a paste, and a low-temperature firing type conductive paste utilizing the low-temperature sinterability of the ultrafine powder has not been developed.

That is, a large amount of vehicle is required to mix the ultrafine powder into the vehicle to form a paste. The reason for this is that ultrafine powder has a very high apparent specific volume (about 1.83 cm ³ / g) and has viscosity (apparent viscosity: about 5 × 10 ⁴ mPa · s), so it is difficult to disperse in the vehicle. is there. For this reason, when the ultrafine powder is made into a paste, the density of the paste becomes low, and even if it is fired, a dense sintered body necessary to obtain sufficient conductivity cannot be obtained. Therefore, a low temperature firing type conductive paste utilizing the excellent low temperature sinterability of ultrafine powder could not be obtained.

Furthermore, it is also possible to add ultrafine powder to the Cu conductive paste as a firing accelerator, but in this case, it is possible to uniformly disperse the ultrafine powder around the conductive fine powder in the Cu conductive paste to ensure conductivity. Although important from the viewpoint, the ultrafine powder easily aggregates, so that it has a drawback that it is difficult to uniformly disperse it in the conductive paste.

That is, simply using the ultra-fine powder in the Cu conductive paste, that is, simply mixing the fine powder and the ultra-fine powder, conversely the apparent density is reduced, since the sintering densification does not occur, conversely the sheet resistance, Necessary characteristics such as adhesive strength are deteriorated.

In view of the above points, an object of the present invention is to provide a conductive paste that can be fired at a low temperature and a firing method thereof by rationally utilizing ultrafine powder as a firing accelerator.

(Means for Solving the Problems) In order to achieve the above-mentioned object, the present inventors have made various investigations and obtained the following findings.

That is, in order to add as much ultrafine powder as possible to the conductive paste, it is necessary to make the apparent specific volume of the ultrafine powder as low as possible. However, in general, the apparent specific volume of powder increases as the particle size of powder decreases,
It is not easy to reduce the apparent specific volume of ultrafine powder with a particle size of 0.3 μm or less.

Therefore, as a result of further intensive studies by the present inventors, as described above, rather than adding ultrafine powder to the Cu conductive paste, by pre-attaching the ultrafine powder to the surface of the conductive fine powder, It was found that the apparent specific volume increase can be suppressed to some extent while effectively utilizing the easy sinterability of the fine powder.

That is, it is possible to improve the sinterability by the ultrafine powder attached to the surface of the fine powder and to suppress the increase in the specific volume by using the fine powder as the main component of the constitution.

Furthermore, since this means is a means for preliminarily dispersing the ultrafine powder around the fine powder before kneading into the vehicle, it also has an effect of preventing the agglomeration of the ultrafine powder in the paste. It was also found that the effects of volume reduction and conductivity improvement are extremely rational and effective.

Based on such knowledge, the present inventors have made further studies and completed the present invention.

Here, the gist of the present invention is that the average particle size is 1 to 10
A low-temperature calcinable conductive paste, characterized in that a mixed powder in which ultrafine copper powder having an average particle size of 0.3 μm or less is uniformly adhered to the surface of copper fine powder of μm is dispersed in a vehicle together with a glass frit. .

Further, in the above-mentioned present invention, it is preferable that the mixed powder is obtained by wet mixing.

Furthermore, by the low-temperature sinterable conductive paste firing method, which is characterized in that the low-temperature sinterable conductive paste obtained by the present invention is applied on a substrate and then fired at a temperature of 400 ° C. or higher and 900 ° C. or lower. is there.

In the present invention, “wet mixing” means, for example, that the fine powder and ultrafine powder are put into a low boiling point liquid such as acetone, and ultrasonic vibration is applied to the liquid so that the ultrafine particles are aggregated in acetone. It is an operation in which the fine powder is disassembled, and the liquid is made into a paste and then kneaded to uniformly attach the ultrafine powder to the surface of the fine powder.

Further, in the present invention, "adhesion" refers to a state in which ultrafine powder is dispersed on the surface of the fine powder without impairing its shape, and the high cohesive force of the copper ultrafine powder and the dry adhesive force of the powder accompanying the volatilization of the liquid. The state is maintained by and, which is distinguished from so-called fusion.

(Operation) Hereinafter, the present invention will be described in detail together with operation effects. In the present specification, "%" means "% by weight" unless otherwise specified.

According to the study of the present inventors, by pre-mixing the ultrafine powder and the fine powder, an increase in apparent specific volume is suppressed, so that the amount of vehicle for forming a paste can be reduced, and a high concentration of It becomes possible to prepare a paste.

Therefore, in the present invention, it is preferable that the fine powder and the ultrafine powder are mixed by a wet mixing method using a low boiling point liquid such as acetone. That is, the reason for using the wet mixing method is that (1) it is possible to dismantle the strong aggregation state of the ultrafine powder and disperse the individual ultrafine powders, (2) without damaging the shape of the ultrafine powder, Capable of adhering to the surface of fine powder. (3) Copper ultrafine powder because the mixed powder is not heated above room temperature because the heat of vaporization is taken from the mixed powder when the low boiling point liquid volatilizes. Does not melt and adhere to the surface of the fine copper powder, and (4) sufficient adhesion can be realized with the high cohesive force of the ultrafine copper powder and the dry adhesive force of the powder accompanying the volatilization of the liquid that is the mixed medium. The reason is that

Next, the reason why the average particle size of each of the fine powder and the ultrafine powder used in the present invention is limited to 1 to 10 μm and 0.3 μm or less will be described.

The reason that the average particle size of the copper fine powder is limited to 1 μm or more and 10 μm or less is to enable low temperature firing while preventing a remarkable decrease in apparent density. That is, since this copper fine powder is the main component of the above-mentioned mixed powder, if its average particle size is less than 1 μm, when it is mixed into a vehicle to form a paste, a large amount of the above-mentioned powder is produced. Need a vehicle,
This is because the paste concentration will decrease, and it will not be possible to obtain sufficient conductivity even if firing is performed. On the other hand, if the average particle size exceeds 10 μm, the conductive path due to diffusion bonding between Cu powders will be used. This is because it is necessary to perform high temperature firing for forming.

Further, the reason for limiting the average particle size of the ultrafine copper powder to 0.3 μm or less is to ensure the easy sinterability of such fine powder.
That is, this is to obtain a dense sintered body necessary for obtaining sufficient conductivity even after low-temperature firing.

By performing the wet mixing as described above to such copper fine powder and copper ultra fine powder, the copper ultra fine powder having an average particle size of 0.3 μm or less was uniformly attached to the surface of the copper fine powder having an average particle size of 1 to 10 μm. A mixed powder can be obtained.

The mixing ratio of the fine copper powder and the ultrafine copper powder is preferably about 97: 3 to 70:30 from the viewpoint of low apparent specific volume, that is, high filling and uniform adhesion of the ultrafine copper powder. Moreover, in order to make the respective average particle diameters of the copper fine powder and the copper ultrafine powder fall within the above-mentioned ranges, for example, the average particle diameter of the copper fine powder should be 2 μm.
m, and the average particle diameter of the ultrafine copper powder may be 0.1 μm. Here, as the average particle diameter, a value obtained by an image counting method using an electron microscope may be used, but it goes without saying that it is not limited to this method.

Then, this mixed powder is made into a paste form that can be printed by imparting an appropriate viscosity together with a powdery glass frit used for adhering onto the substrate in the step of firing the mixed powder. A conductive paste capable of low-temperature firing can be obtained by dispersing it in the vehicle.

Here, as the glass frit, for example, Bi ₂ O ₃ −B ₂ O ₃
-PbO-SiO ₂ system may be used. Specifically, Bi _{2
O 3: 25~40%, B 2} O 3: 15~30%, PbO: 20~30%, SiO 2: 5~
A glass frit having a composition of about 15% can be exemplified. As the vehicle, for example, one made of ethyl cellulose and terpineol may be used.

Further, as a method of dispersing the mixed powder and the glass frit in the vehicle, in addition to the means of kneading using a roller type mixing mill, kneading with a ball mill, a liquor machine, or a mixer blade is exemplified, but particularly It goes without saying that the means is not limited to the above.

The mixing ratio of the mixed powder, the glass frit and the vehicle is, for example, 75 to 85: 3 to 5:20 to 10, preferably about 85: 5: 10.

Using the thus obtained conductive paste that can be fired at low temperature according to the present invention, for example, to form an electrode such as a ceramic capacitor, a piezoelectric element, a semiconductor ceramic, or a wiring conductor for an electronic circuit, the conductive paste may be a ceramic paste. It may be applied on the surface of an object to be coated such as a substrate by a method such as screen printing, dried if necessary, and then baked.

In the present invention, it is possible to perform firing at a low temperature (400 ° C. or more and 900 ° C. or less) as described above, without the need for firing at a high temperature of more than 900 ° C. as in the conventional case. Therefore, there is no possibility that the dielectric paste or the resistor paste on the same substrate will undergo reduction deterioration. If the firing temperature is lower than 400 ° C, the firing temperature is too low, and it becomes difficult to secure conductivity. Therefore, the firing temperature is preferably 400 ° C. or higher and 900 ° C. or lower. Needless to say, if firing does not occur as described above even if firing is performed at a temperature higher than 900 ° C, firing may be performed at a temperature higher than 900 ° C.

Further, the present invention will be described in detail with reference to examples, but this is merely an example of the present invention, and the present invention is not limited thereto.

Example 1 Spherical Cu fine powder having an average particle size of 1 to 4 μm and an average particle size of
Mixing of Cu ultrafine powder in the range of 0.1 μm or less was performed by wet mixing using acetone. That is, the total amount of Cu fine powder and Cu ultrafine powder pre-weighed and blended in 15 cc of acetone is
It was thrown in so that it might become 5g. However, at this time, the amount of Cu ultrafine powder added was changed in 7 levels in the range of 0 to 5 g, and the amount of ultrafine powder added (% by weight) calculated by the following formula was 0 to 100%.
To obtain a mixed solution.

Amount of ultrafine powder added = amount of ultrafine powder added / (amount of ultrafine powder added + amount of fine powder) x 100 (%) Next, this mixture is vigorously agitated while applying ultrasonic vibration, and is agglomerated in acetone. Dismantle Cu ultrafine powder, Cu
The ultrafine powder was dispersed in acetone. The ultrasonic vibration and stirring were carried out until the evaporation of acetone proceeded and the mixture became a paste having a viscosity of 1 × 10 ⁴ mPa · s.

Then, this mixed liquid was kneaded with a roller-type mixing mill to uniformly mix the Cu fine powder and the Cu ultrafine powder. This kneading was performed until the acetone was completely evaporated and pulverized to obtain a mixed powder in which Cu ultrafine powder was uniformly adhered to the surface of Cu fine powder.

The apparent specific volume value of this mixed powder is calculated by an apparent specific gravity measuring device, and the relationship between the apparent specific volume value and the addition amount of the ultrafine powder is shown in a graph in FIG.

From FIG. 1, the value of the apparent specific volume of the mixed powder of the fine powder and the ultrafine powder that has been subjected to the wet mixing treatment is calculated by comparing the apparent specific volumes of the conductive fine powder and the ultrafine powder shown by the dotted line in FIG. It was confirmed that the value decreased from the averaged value, that is, the increase in the apparent specific volume was suppressed.

Next, the inventors of the present invention performed an amount of vehicle (viscosity 1 × 10 ⁶ mPas) necessary to enable printing of the mixed powder obtained by performing the premixing treatment of such Cu fine powder and Cu ultrafine powder.・ The amount of vehicle for obtaining s was investigated.

That is, when (a) a mixed powder obtained by premixing conductive fine powder and ultrafine powder is dispersed in a vehicle together with glass frit and then made into a paste, (b) without premixing conductive fine powder and ultrafine powder, The minimum amount of vehicle required for printing when using paste was determined. The results are shown in Table 1.

The glass frit used in this example was borosilicate glass, and the vehicle was a mixture of ethyl cellulose and terpineol.

The results are shown in Table 1.

From this, it was confirmed that the premixing of the conductive fine powder and the ultrafine powder reduced the amount of the paste-forming vehicle and was effective for producing the high-concentration conductive paste.

Example 2 First, spherical finely ground Cu fine powder having an average particle size of 2 μm and particle sizes of 1 to 4 μm and Cu ultrafine powder having an average particle size of 0.3 μm or less were mixed by wet mixing with acetone. This procedure will be described below.

First, Cu, which was previously weighed and mixed in 15 cc of acetone
A total of 5 g of fine powder and Cu ultrafine powder was added to obtain a mixed liquid. The compounding ratio (powder composition) of the Cu fine powder and the Cu ultra-fine powder was as shown in Table 2.

Next, this mixed solution was vigorously stirred while applying ultrasonic vibration to disaggregate the agglomeration of Cu ultrafine powder and disperse the Cu ultrafine powder in the liquid. Ultrasonic vibration and agitation were applied until the evaporation of acetone proceeded and the apparent viscosity of the mixed solution became a paste with a viscosity of about 1 × 10 ⁴ mPa · s. After the mixed solution became a paste, it was kneaded by a three-roller type mixing mill to uniformly mix Cu fine powder and Cu ultra fine powder. This kneading was performed until the acetone was evaporated and pulverized.

Thus, when the acetone has completely evaporated, the Cu
It was possible to obtain a mixed powder in which Cu ultrafine powder was uniformly adhered to the surface of the fine powder, and the powder was used as the conductive powder of the conductive paste.

Then, the production of the conductive paste, the conductive powder 5g,
A vehicle consisting of ethyl cellulose and terpineol, and Bi ₂ O ₃ , B ₂ O ₃ , PbO, kneaded with a glass frit composed of SiO _{2 and the} like using a three-roller type mixing mill, and a conductive paste. did. The amount of the conductive powder added was 82% of the total amount.

The conductive paste prepared in this manner was printed on an alumina substrate in a predetermined shape using a screen printing method, and then left to stand for 10 minutes to perform leveling.
It was dried for 0 minutes.

Next, in a nitrogen gas atmosphere, at a low temperature of 900 to 500 ° C.
Bake for 12 minutes.

Table 2 shows the resistance value between both ends of the above-mentioned printed pattern fired at each firing temperature. In addition, Table 2 also shows the resistance value of the fired film of the Cu conductive paste, which is a conventional product containing no ultrafine powder.

As is clear from Table 2, the conductive paste according to the present invention exhibits a low resistance value even at low temperature firing. On the other hand, it is understood that the commercially available paste to which the ultrafine powder is not added does not show a satisfactory resistance value at low temperature firing.

(Effects of the Invention) Since the present invention is configured as described above, it is possible to perform firing at low temperature without requiring high-temperature firing, and thus prevent reduction degradation of resistors, dielectrics and the like. You can

In addition, it is economically relatively inexpensive because it does not use precious metal powders.
Since it can be made of Cu powder, its industrial value is great.

The significance of the present invention having such effects is extremely remarkable.

[Brief description of drawings]

FIG. 1 is a graph showing the relationship between the amount of fine powder and ultrafine powder added by wet mixing and the apparent specific volume.

Claims (3)

[Claims] 1. A mixed powder obtained by uniformly depositing ultrafine copper powder having an average particle size of 0.3 μm or less on the surface of fine copper powder having an average particle size of 1 to 10 μm, which is dispersed in a vehicle together with a glass frit. Conductive paste that can be fired at low temperature. 2. The low-temperature calcinable conductive paste according to claim 1, wherein the mixed powder is obtained by wet mixing. 3. A low-temperature calcinable conductive paste calcining method comprising applying the low-temperature calcinable conductive paste according to claim 1 or 2 and then calcining at a temperature of 400 ° C. or higher and 900 ° C. or lower. .