JPH03291989A - Conductive copper paste, manufacture thereof and low temperature baked board - Google Patents

Abstract

PURPOSE:To obtain conductive copper paste having high bonding strength to a board and capable of forming a thick copper film conductor by incorporating the same components as those of glass and incorporating glass frit having specific average particle size and conductive copper powder therein. CONSTITUTION:Glass frit containing aluminum calcium borosilicate having 2mum or less of average particle size and conductive copper powder having 1-2mum of average particle size are vigorously agitated while applying an ultrasonic vibration thereto to be mixed. The mixture is dispersed in an organic vehicle, and kneaded to form conductive copper paste. The paste is screen printed in a predetermined shape on a low temperature based board formed of glass containing the same components as those of the glass frit contained in the paste and alumina, preliminarily dried, and baked at 750 deg.C or lower in a nitrogen atmosphere to obtain a thick conductor copper film.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Application Field) The present invention relates to a copper conductive paste (hereinafter also referred to as rCu conductor paste), its manufacturing method, and a low-temperature fired substrate. More specifically, the present invention provides a copper conductive paste that can be fired even at a lower temperature than before and that can produce a copper thick film circuit with high adhesive strength, a method for producing the same, and a method using the copper conductive paste. The present invention relates to low-temperature fired substrates.

(Conventional technology) Miniaturization, higher functionality, higher reliability, and
There is an extremely high demand for lower costs, and in order to meet these demands, semiconductor chips have made remarkable progress in the direction of higher integration density and higher speed. Along with this, alumina (41
Characteristics superior to those of gOJ substrates are now required.

In general, conventionally used alumina substrates have the following drawbacks: (1) Alumina requires high-temperature sintering of about 1500 to 1600°C, so the conductor material that is fired at the same time is tungsten. , molybdenum, etc. are limited to high melting point metal materials with relatively high specific resistance (5.5 to 5.6 , 62Xl0-'' Ω・C11) or Cu (1,72 if you did this,
It is not possible to increase the limit of miniaturization of wiring patterns.

(ii) What is the coefficient of thermal expansion of alumina? , 3X10-'de
g-', and the coefficient of thermal expansion of silicon (3,6X10-
Due to the large difference in temperature (deg-') and poor thermal matching, there are difficulties in mounting large silicon chips and in high-density packaging.

(tj) The dielectric constant of alumina is approximately 10 (at I MH
z), and the signal delay time is also large.

In order to overcome the above drawbacks, low-temperature fired substrates have been developed to replace alumina substrates, and some of them have even been put into practical use. Such low-temperature fired substrates can be broadly classified into those that use glass and those that do not. moreover,
Systems that use glass are classified into crystallized glass systems and composite systems that combine glass and filler (filler made of alumina, silica, etc.). In particular, in the case of composite systems, it is possible to obtain ceramics with various characteristics by appropriately selecting the type of glass, the composition of the filler, and their blending ratio. research is being conducted.

To form a conductor on the surface of this low-temperature fired substrate, the most commonly used technique is to screen print a conductive paste in a predetermined shape onto the substrate and then bake it, as in the case of an alumina substrate. As the conductor paste, Ag--Pd conductor paste, N1 conductor paste, Cu conductor paste, etc. are currently used. ^, - Pd-based conductor paste has the characteristics of being able to be fired in the atmosphere, and Ni conductor paste has excellent environmental resistance and migration resistance, and can provide high reliability. In consideration of cost reduction, Cu conductor paste is the most advantageous and can be said to be an ideal conductor material. When forming a circuit with cult paste, the sheet resistance is 1.5 to 2.
Because it is low at mΩ/mouth (4 to 4 with Ag-Pd conductor paste)
6-Ω/port, 8-10IlΩ/port with Ni conductor paste)
, thinning of the pattern can be realized. As a result, stray capacitance between patterns is reduced and high-density wiring becomes possible, making it possible to manufacture a substrate that is compatible with higher speed and higher integration density of semiconductor chips.

The copper conductive paste is made by dispersing copper conductive powder and binder powder in an organic vehicle consisting of a resin and a volatile solvent. In the firing process after printing the copper conductive paste onto the low-temperature firing substrate, the following reactions occur sequentially. First, the volatile solvent smokes, and then the resin burns between 200 and 400°C.

As the temperature increases further, the binder powder melts and begins to react with the low temperature fired substrate. At the same time, it also starts to react with the copper conductive powder, promoting sintering of the copper conductive powder.

At this time, the binder powder in the copper conductive paste has the effect of fixing the fired Cu thick film conductor on the substrate, and is mainly classified into the following three types.

(i) During glass bond firing, the glass that is melted and softened brings the substrate and the thick film conductor into close contact due to a wetting phenomenon.

(ii) Chemical Bond A metal oxide such as CuO causes a chemical reaction with the substrate to form a composite oxide, thereby bonding the substrate.

(iii) Mix bond glass and metal are combined in a well-balanced manner, and the effects of both are utilized to bring the substrate and the thick film conductor into close contact.

On the other hand, JP-A No. 60-35405 discloses a copper conductor composition containing CuO as one component of glass frit, and JP-A No. 61-107607 discloses a copper conductor composition containing a solid content and an organic binder. The conductor composition is further characterized in that the solid content is 80 to 95% by weight of Cu powder and 5 to 20% by weight of an additive consisting of V, O, alone or in combination with at least one of CugOlPbzOlSbgOa and Bi2O2. JP-A-1-128488 discloses a copper conductor ink containing a zinc-calcium-aluminum silicate glass frit, an aluminum-magnesium-barium-aluminum silicate glass frit, and a mixture thereof as a glass frit. Each is proposed.

(Problem M to be solved by the invention) By the way, the copper conductive paste currently on the market was developed mainly for use in aluminum-containing substrates, and is not reliable enough for application to various low-temperature fired substrates. There is no gender.

In particular, copper conductive paste containing binder powder for chemical bonding requires high-temperature firing at around 1000°C.
The yield point of glass for low-temperature fired substrates is usually about 800° C., so it cannot be applied to low-temperature fired substrates at all.

Furthermore, in the case of glass bond, the thermal expansion matching of glass to a low-temperature fired substrate is inferior to that of an alumina substrate.

In addition, in JP-A No. 61-107607, the firing temperature must be as high as 850 to 950°C;
In Japanese Patent No. 8488, micro-cranks occur due to the difference in linear expansion coefficient between the glass of the glass frit and the glass of the low-temperature fired substrate, which is still a problem.

Thus, there has been a desire for the development of a copper conductive paste containing binder powder for low-temperature firing substrates.

Here, an object of the present invention is to provide 75% on the surface of a low-temperature fired substrate.
The object of the present invention is to provide a copper conductive paste that can form a copper thick film conductor with high bonding strength to a substrate by firing at a low temperature of 0° C. or lower, and a method for producing the same.

Another object of the present invention is to provide a low-temperature firing substrate using the copper conductive paste.

(Means for Solving the Problems) As a result of intensive research in order to solve the above problems, the present inventors have found that, in order to increase the adhesive strength of copper thick film conductors to low-temperature fired substrates, binder powder can be applied to low-temperature fired substrates. By using a glass frit with the same composition as the glass component contained in the glass frit and pulverizing the glass frit to an average particle size of 2 μm or less, the number of bonding points between the copper thick film conductor and the low-temperature fired substrate is increased.
It has been found that it is possible to increase the adhesive strength between a low temperature fired substrate and a copper thick film conductor. Here, the pulverized glass flift is very bulky (1,50 cffl/g)
Therefore, the specific surface area is high (3,5rrf/g) and the aggregation property is high. For this reason, finely pulverized glass frit is
It is very difficult to uniformly disperse it in a viscous organic vehicle (apparent viscosity is about 1×10 to .Pa-5). In order to overcome this difficulty, the present inventors conducted further studies and found that the copper conductive powder and glass frit were uniformly mixed in advance using a wet mixing method using acetone as a dispersion. We learned that by reducing the apparent specific volume and specific surface area of the powder and then dispersing it in an organic vehicle, it was possible to obtain a copper conductive paste with a high degree of solid powder dispersion and a high solid content. The invention was completed.

Here, the gist of the present invention is to provide a copper conductive paste for low-temperature firing substrates that combines glass and filler, which has the same components as the glass and has an average particle size of 2μ or less. A copper conductive paste characterized by containing a certain glass frit and a copper conductive powder.

Here, the "average particle size" means 2
For each of 000 powders, the distance between two parallel lines in a certain direction is taken as the representative diameter of each powder, and 2000
means the maximum @ value of the representative diameters.

Note that the copper conductive paste according to the present invention described above, like conventionally known conductive pastes, includes the glass frit,
It is prepared by mixing resin and solvent in addition to copper conductive powder.

Various known resins can be used as the resin used in the present invention. For example, ethyl cellulose, nitrocellulose, hydroxycellulose, polybutyl methacrylate, etc. can be used.

Moreover, various known solvents can be used as the solvent used in the present invention. For example, terpineol, dibutylcarpitol, dibutyl phthalate, 2,2,4-trimethyl-1,3-bentanediol, monoisobutyrate, etc. can be used.

Further, in the present invention described above, it is preferable that the average particle size of the copper conductive powder is 1 to 2 μm for firing at 750° C. or lower.

In order to obtain the copper conductive paste according to the present invention, the glass frit and the copper conductive powder may be wet-mixed in advance, and then dispersed in an organic vehicle to form a paste.

In addition, examples of the above-mentioned method of making paste include Nigoo,
Conventionally known methods such as kneading with a crusher, three rolls, etc. can be exemplified.

In addition, from another aspect, the gist of the present invention is a low temperature firing substrate combining glass and filler,
The present invention is a low-temperature fired substrate characterized in that the copper conductive paste according to the present invention described above is applied onto a substrate and fired at a temperature of 750° C. or lower.

(Function) Hereinafter, the present invention will be explained in detail along with the function and effect. In this specification, "%" means "% by weight" unless otherwise specified.

Thermal strain (microcrank) caused by the difference in thermal expansion coefficient between the glass frit and the substrate is a possible cause of the decrease in bond strength between the substrate and the fired thick copper film, especially after being left at high temperatures. ).

Currently, the glass frit in the copper conductive paste commercially available has a thermal expansion coefficient of 7.2 × 10-' de
Lead borosilicate glass system close to g-' (thermal expansion coefficient 6-8
X10-'deg-'), the thermal expansion matching with the alumina substrate is good. However, the thermal expansion coefficient of the low temperature fired substrate is 3.6, which is the thermal expansion coefficient of silicon.
In order to get close to X10-'deg-', it is set at 4 to 6 The adhesive strength decreases after being left standing.

Therefore, in the present invention, a glass with the same acid content as the glass contained in the low-temperature fired substrate is selected, and this glass is used as a glass frit in the copper conductive paste, thereby completely matching the thermal expansion compatibility between the two. As a result, the glass in the copper thick film and the glass in the substrate are integrated, and a strong bond is obtained.

However, simply using a glass having the same composition as the glass contained in the low-temperature fired substrate as the glass frit in the copper conductive paste causes new problems. That is, the yield point of glass used for low-temperature fired substrates is a little less than 800°C, and the copper conductive paste containing the glass flint is heated to 750°C.
When firing at temperatures below .degree. C., the fluidity of the glass frit is insufficient, resulting in a situation where the copper conductive paste does not sufficiently wet the substrate and does not spread, making it impossible to obtain good adhesive strength. This is due to the fact that with conventional glass frits that are sieved to 20 μm or less, only a very small portion of the glass frit comes into contact with the substrate, like the tip of an iceberg. In other words, if the fluidity of the glass frit is low, the total contact area of the glass frit with the substrate will be low. Considering the case where glass is used as a glass frit, in the present invention, the glass frit is finely pulverized so that the average particle size is 2 μm or less to increase the number of contact points with the substrate. like,
By limiting the particle size of the glass frit, even if the glass frit itself lacks fluidity, a sufficient contact area with the substrate can be obtained, improving the adhesion between the substrate and the copper conductive paste. do.

Further, it is preferable that the average particle diameter of the copper conductive powder in the copper conductive paste according to the present invention is 1 μm or more and 2 μm or less. Copper powder with an average particle size of more than 2 μm will not be sufficiently sintered by firing at 750”C, while if it is less than 1 μm, the amount of vehicle required to make a paste will increase, desolvation properties will deteriorate, and eventually This is because it may not be possible to obtain a copper conductive paste with desired properties even if the method is used.

However, a new problem arises here. That is,
When glass frit is pulverized to an average particle size of 2 μm or less, the apparent specific volume and specific surface area are significantly increased. Therefore, the pulverized glass frit is dispersed in an organic vehicle together with copper conductive powder to form a paste. When preparing , a large amount of organic vehicle is required.

For this reason, the concentration of the solid matter (copper conductive powder and glass frit) in the paste decreases, and the degree of filling of the solid matter powder after printing the paste on the substrate decreases, making it difficult to obtain a dense baked thick film. However, as a result, the sheet resistance value becomes high. Conventionally, special wetting agents (a type of surfactant)
was coated on the surface of the solid powder and then dispersed in an organic vehicle.

In addition, it is ideal that the copper powder and glass frit are uniformly dispersed in the organic vehicle, but if the average particle size of the glass frit is less than 2 μm, aggregation tends to occur, and the apparent viscosity decreases to about 1 μm. It is difficult to break up the agglomerated state and disperse it uniformly in an organic vehicle where the pressure is around 10' mPa·S. Traditionally, special dispersants (a type of surfactant) have also been used to disperse them in the organic vehicle.

The present inventors have overcome these two problems, namely, (1) a decrease in the concentration of solids in the paste, and (2) non-uniform dispersion of the fine powder, which is a solid substance in the paste, caused by the pulverization of the glass frit. Therefore, it has been found that it is effective to premix the fine powder of the solid material by a mixed mixing method. That is, for example, by a wet mixing method using acetone as a dispersion,
By uniformly pre-mixing copper conductive powder and glass frit, the surface of copper fine powder (copper conductive powder) is coated with glass frit, and the specific surface area of the mixture of copper fine powder and glass frit is This makes it possible to lower the specific surface area by less than the arithmetic mean of the specific surface area.

Then, by uniformly mixing the copper fine powder and the glass frit, the acetone dispersion is completely a! After the fine copper powder and glass lift are dispersed in an organic vehicle consisting of a resin and a solvent and made into a paste, the paste concentration is high and the fine powder is uniformly dispersed in the solid material. It becomes possible to obtain a high-quality copper conductive paste.

Note that the pasting method may be any known method as described above, and is not limited in any way.

Furthermore, in order to form the copper conductive paste according to the present invention thus obtained as a conductor on the surface of a low temperature fired substrate,
As with conventional methods, screen printing, metal mask printing, direct drawing, etc. may be used. For example,
The copper conductive paste may be screen printed onto a substrate in a predetermined shape and then fired. At this time, the copper conductive paste according to the present invention can be fired at a low temperature, and the firing temperature can be set to 750°C or less.

As described above, in the copper conductive paste according to the present invention, the firing temperature can be suppressed to 750° C. or lower, so that a decrease in bonding strength after firing can be prevented.

In addition, when a silver conductor is used as an internal conductor of a low-temperature fired substrate, when the firing temperature is 779°C or higher, a eutectic reaction occurs when connecting the copper conductive paste according to the present invention and the silver conductor, Although there is a risk of connection failure, in the present invention, since the firing temperature can be set to 750° C. or lower, the occurrence of the above-mentioned failure can also be prevented.

In this way, according to the present invention, 7
By firing at a low temperature of 50℃ or less, the bond strength with the substrate is 3kgf/2mm or more even after being left at high temperatures.
It is possible to provide a copper conductive paste capable of forming a high copper thick film conductor, a method for producing the same, and a low-temperature firing substrate using the copper conductive paste.

EXAMPLES Hereinafter, the present invention will be described in more detail using Examples, but these are merely illustrative of the present invention and are not intended to limit the present invention.

(Example) Alumino-calcium borosilicate <A with an average particle size of 1 μm
l2O213,1%-Ca021.8%-3i0□57
．． 3%-BzCh 7.8%) (specific surface area 1.20 m"/g) and copper conductive powder (specific surface area 0.30 m"/g) with an average particle size of 2 μm.
5.0 g was placed in a beaker containing 15 cc of acetone. This mixed solution was vigorously stirred while applying ultrasonic vibration. This stirring was continued until the acetone was completely volatilized.

After stirring, this mixed powder was observed under an electron microscope, and it was confirmed that the surface of the copper conductive powder was uniformly coated with glass frit.

The specific surface area of the mixed powder is 0.28+g"/g, and the arithmetic average value of each powder alone ((1,20(+1"/g)
0.1 (g) +0.30 (m”/g)
g)) +(0,1+5.0)(g) ) ′io, 3
The value was lower than 21/g.

Next, 5.10 g of the thus obtained mixture of copper conductive powder and glass lift was mixed with 0.82 g of organic vehicle (
Dispersed and kneaded in 6% ethyl cellulose (terpineol) to achieve a viscosity of 200 Pa, which is ideal for screen printing.
A copper conductive paste having 5 was prepared.

Note that a three-roll mill was used for kneading the paste.

The copper conductive paste obtained in this way was mixed with a glass (A) having the same composition as the glass frit contained in the copper conductive paste.
1 zch 13.1%-Ca021.8%-5iO*
57.3%-fhOi 7.8%) and alumina ((40% A j! gos 6
0% glass), thermal expansion coefficient 5.5X 10-”deg-
', relative dielectric constant 7.7 (at IMHz)) was screen printed into a predetermined shape, pre-dried at 120°C for 10 minutes to remove the solvent (terpineol), and then dried at 750°C x 7
Belt furnace for 0 minutes (peak keeping conditions = 750℃ x 10
min) under a nitrogen atmosphere (20 μpm O, -N, bit
).

As a result, although the temperature of the bottom of the pit was as low as 750° C., it was possible to obtain a copper conductor thick film of the present invention with a thickness of 20 μm.

In order to evaluate the copper conductor thick film according to the example of the present invention thus obtained, sheet resistance, adhesive strength (initial stage, after heating and standing), and solder wettability were evaluated.

The evaluation method is as follows.

22-11: Evaluated by measuring 4-body resistance value.

The resistance value of a copper conductor thick film with a line width of 0.5 mm was measured by a four-terminal resistance measurement method, and the resistance value was calculated by converting it into a sheet resistance value when the thickness was 20 μm.

Bean strength: Evaluated by measuring beer strength using the Du Pont method. A 0.6-open tin plated copper wire was horizontally soldered onto a W4 conductor thick film with a 2-opening angle using the solder dip method (60Sn/40
After soldering with pb solder, dipping at 230℃ for 3 seconds,
9 at a position 1+g- from the end of the tin plated 5pIvA thick film
Bend it by 0 degrees so that it is perpendicular to the board, and with the board fixed, pull the tinted copper wire at a rate of 10 cm/min using a tensile testing machine to measure the peel S strength when the board and tin #iwA peel off. , this value was taken as the initial adhesive strength. The adhesive strength after heating and standing was the peel strength obtained when the substrate was stored in an oven controlled at 150"C for 500 hours after being soldered.

1.41d view 1. Proportion of solder coverage obtained on the copper test pattern after solder-dipping the fired parts (%)
), the solder wettability was measured.

The results are shown in Table 1.

Silicic acid (Pb065%-Cd010%-8iO!10%
- After pre-wet mixing the glass frit made from -Br (h15%) (specific surface area 1.15-/g) 0.1Og and the copper conductive powder used in the present example in the same manner as in Example 1,
Disperse and knead in 2g of organic solvent to obtain a viscosity of 200P.
A conductive paste of a-5 was created.

Comparative Example 3 was prepared by printing the conductive paste on a low-temperature fired substrate and applying it to the bottom of a hole in the same manner as in the present invention example, and various conductor properties were evaluated.

The conductor properties obtained in Examples of the Invention and Comparative Examples to Comparative Example 3 are summarized in Table 1.

On the other hand, as a comparative example, 5.0 g of the copper conductive powder and 0.10 g of glass lift used in the above-mentioned examples of the present invention were directly dispersed in an organic vehicle without premixing by wet mixing, and then dispersed using a three-roller mill. When kneaded with
0.95 g of organic vehicle was required to make a paste with a-5.

The conductive paste was printed on a low-temperature fired substrate and formed into a pit in the same manner as in the above-mentioned example of the present invention, and the various conductor properties described above were evaluated as Comparative Example 1.

In addition, 0 and 10 g of glass frit having the same acid content as the glass frit used in the present invention example and having an average particle size of 10 μm and 5.0 g of the copper fine powder used in the present invention example were added in the same manner as in the present invention example. After premixing by wet mixing using acetone as a dispersion medium, it was dispersed and kneaded in 0.78 g of an organic vehicle until the viscosity was 2.
A paste of 00 Pa-5 was prepared.

The paste was printed on a low-temperature fired substrate in the same manner as Comparative Example 1, and the bottom of the hole was formed, and various conductor properties were evaluated as Comparative Example 2.

Furthermore, as is clear from Table 1 of the lead-cadmium powder with an average particle size of 2 μm, the copper conductive paste according to the present invention can be used on low-temperature fired substrates even though the firing temperature is about 750°C, which is lower than usual. It was possible to obtain a highly reliable low-temperature fired substrate that had high adhesive strength, especially after being left to heat, and had sufficient conductivity and good solder wettability.

(Effects of the Invention) As detailed above, the present invention makes it possible to form a conductor on the surface of a low-temperature fired substrate. Therefore, it can greatly contribute to expanding the uses of low-temperature fired substrates. It is.

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

Claims (4)

[Claims] (1) A copper conductive paste for low-temperature firing substrates that combines glass and filler, which contains glass frit and copper conductive powder that have the same components as the glass and have an average particle size of 2 μm or less. A copper conductive paste characterized by: (2) The copper conductive paste according to claim 1, wherein the copper conductive powder has an average particle size of 1 to 2 μm. (3) The method for producing a copper conductive paste according to claim 1 or 2, wherein the glass frit and the copper conductive powder are wet-mixed in advance and then dispersed in an organic vehicle to form a paste. (4) A low-temperature fired substrate combining glass and filler, characterized in that the copper conductive paste according to claim 1 or 2 is applied onto the substrate and fired at a temperature of 750°C or less. Low-temperature firing substrate.