# Patent

## Title
Conductor paste for ceramic substrate and electric circuit

## Abstract

A conductor paste for a ceramic substrate contains a) a conductive metal powder comprising a silver powder and a palladium powder; b) a glass powder; and c) an organic solvent, wherein the conductive metal powder has an average particle diameter of not more than 1.2 μm, and the glass powder is a Bi2O3—SiO2—B2O3 type glass powder, and the content of the glass powder is in a range of from 1 to 6 wt % based on the weight of the paste.

## Background
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a conductor paste used for forming circuitry on a ceramic substrate, and a circuit substrate having electric circuit formed thereon using the conductor paste.
2. Technical Background
Circuit substrates made of a ceramic, such as alumina (Al2O3) or aluminum nitride (AlN), are commonly used as circuit substrates for various electronic components.
As one of the methods for forming the circuit (a conductor layer) on such as a ceramic substrate, a conductor paste is used to form the electric circuit though the process of screen printing, drying and firing.
The conductor paste is generally one in which a conductive powder and a glass powder are dispersed in an organic medium. For example, in JP-A-10-340622, there is disclosed a conductor paste in which a conductive powder comprises silver and palladium, a glass powder has an average particle diameter of not more than 1.5 μm and a softening point in a range of from 750 to 900° C., and a silicon dioxide powder has an average particle diameter of not more than 0.1 μm are dispersed in an organic vehicle.
In US 2004-0245508, there is disclosed a conductor paste for an aluminum nitride substrate comprising a boron-containing compound and a metal oxide, this structure being designed to increase adhesive strength.
Meanwhile, in recent years, to reduce environmental impact, disuse of lead and of cadmium has been encouraged. Also, to keep down manufacturing cost, it is desirable to reduce the firing temperature (to not more than 650° C.). Note that the firing temperature to form a conductor film on a ceramic substrate has generally been approximately 850° C.
Retaining adhesion is important. It is not enough only to use glass frits having low softening points in the paste to promote good adhesion where the firing temperature is lower than 650° C. One problem that may occur at such low temperatures is that sintering may be deficient and strong bonds may not occur, limiting adhesion.
It is an object of the present invention to provide a conductor paste that is lead-free and cadmium-free, can be fired at a low temperature of not more than 650° C., and has excellent adhesive strength on a ceramic substrate. Further, it is an object of the present invention to provide an electric circuit substrate in which the adhesive strength between a ceramic substrate and a conductor film is adequate.
BRIEF SUMMARY OF THE INVENTION
One embodiment of the present invention is a conductor paste for a ceramic substrate comprising a) a conductive metal powder selected from the group consisting of silver, palladium and a mixture of silver and palladium; b) a glass powder; and c) an organic solvent, wherein the conductive metal powder has an average particle diameter of not more than 1.2 μm, and the glass powder is a Bi2O3—SiO2—B2O3 type glass powder, and a content of the glass powder is in a range of from 1 to 6 wt % based on the weight of the paste.
Another aspect of the present invention is as a conductor film formation method containing the following steps: a step of applying onto a ceramic substrate a conductor paste comprising a) a conductive metal powder selected from the group consisting of silver, palladium and a mixture thereof, b) a glass powder, and c) an organic solvent, in which the conductive metal powder has an average particle diameter of not more than 1.2 μm, and the glass powder is a Bi2O3—SiO2—B2O3 type glass powder, and the content of the glass powder is in a range of from 1 to 6 wt % based on the weight of the paste; and wherein the process has a step of firing the conductor paste that has been applied onto the ceramic substrate at not more than 650° C.
Yet another aspect of the present invention is an electric circuit substrate having a circuit comprising a conductor film obtained using the above conductor film formation method.
According to the present invention, there is provided an electric circuit substrate in which the adhesive strength between a ceramic substrate and a conductor film is excellent.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 consists of schematic view for explaining a method of forming a conductor film of the present invention; and
FIG. 2 is a schematic sectional view showing an example of an electric circuit substrate having a circuit comprising the conductor film of the present invention.



DETAILED DESCRIPTION OF THE INVENTION
Bonding between a ceramic substrate and a conductor film fired at low temperature may be insufficient. This may occur for several reasons. For example, at low temperature, sintering of Ag powder does not proceed sufficiently and strong bonds may not be formed between the glass layer in a conductor film and the ceramic substrate. The present invention has been found to avoid these problems.
The paste of the present invention comprises a) a conductive metal powder, b) a glass powder, and c) an organic solvent.
a) Conductive Metal Powder
The conductive metal powder herein contains silver or palladium or a mixture thereof. Preferably, the conductive metal powder comprises at least a mixture of a silver powder and a palladium powder, a silver/palladium co-precipitated powder, or a silver-palladium alloy powder. Two or more of these may also be used in combination. Moreover, a silver/palladium co-precipitated powder or silver-palladium alloy powder may be mixed with a silver powder. Examples of a co-precipitated powder are (1) one in which silver particles and palladium particles are aggregated together, and (2) one in which a double layer is formed with either silver or palladium being the core. An example of a method of manufacturing a co-precipitated powder of (1) is a method in which an aqueous solution of a silver salt and a palladium salt are reduced together so as to produce a silver powder and palladium powder of diameter approximately from 0.02 to 0.06 μm, and these fine powders are aggregated together in a predetermined compositional ratio, thus forming particles of apparent diameter not less than 0.1 μm. An example of a method of manufacturing a co-precipitated powder of (2) is a method in which the silver is first reduced to produce particles, and then the palladium is reduced, so as to precipitate palladium on the surface of the silver particles, and thus form a double layer with silver as the core.
The palladium content is preferably from 0.1 to 7 wt % relative to a total amount of silver and palladium of 100 wt %. Through the palladium content being within this range, adhesion strength between a ceramic substrate and a conductor film is improved.
The average particle diameter of the conductive metal powder is not more than 1.2 μm. By using a conductive metal powder having an average particle diameter of not more than 1.2 μm, sintering of conductive metal powder proceeds even at a low temperature and the adhesive strength between the conductor film and the ceramic substrate is improved. The average particle diameter of the conductive metal powder is preferably from 0.05 to 1.2 μm, more preferably from 0.05 to 0.8 μm.
As the method of measuring the average particle diameter, measurement is carried out using a Microtrac particle size distribution measurement apparatus, the D50 value is taken as the average particle diameter.
Any of various shapes, such as spheres or flakes, can be used as the shape of the conductive metal powder.
The conductive metal powder preferably comprises monodispersed particles. A fine particle powder has a high surface activity, and hence exists as an aggregate. “Monodispersed particles” means particles for which, upon applying mechanical force to this aggregate by a disperser such as a triple roll mill, a ball mill or a bead mill, the aggregate is broken up into primary particles, which can be dispersed uniformly in a vehicle (organic medium). Conversely, the case where the agglomeration strength between the fine particles is too strong so that dispersion into primary particles is not possible even upon applying such mechanical force is not referred to as “monodispersed particles”. By using monodispersed particles in the paste, the packing density of the dried film can be increased, and the contact area between particles can be increased, and hence the sintered density upon firing can be increased.
In addition to the above conductive metal powder contained as an essential component, other conductive metal powder may be contained. For example, gold or platinum may be included as a supplementary conductive metal powder. The amount included in the case of adding gold or platinum is preferably in a range of from 0.1 to 10 wt % where the paste has a weight of 100 wt %.
The content of the conductive metal powder is preferably in a range of from 60 to 92 wt % where the paste has a weight of 100 wt %.
b) Glass Powder
A Bi2O3—SiO2—B2O3 type glass powder is used as the glass powder. “Bi2O3—SiO2—B2O3 type glass powder” means a glass containing at least Bi2O3, SiO2 and B2O3 as composition thereof.
The content of bismuth oxide (Bi2O3) is preferably from 65 to 90 wt % based on the weight of the glass powder, and the content of silicon oxide (SiO2) is preferably from 0.5 to 8 wt % based on the weight of the glass powder. Moreover, the content of boron oxide (B2O3) is preferably from 3 to 10 wt % based on the weight of the glass powder.
Other components may be contained as required. Examples of such other components include ZnO, Al2O3, La2O3, TiO2, ZrO2, SnO2, CeO2, MgO, CaO, SrO, BaO, Li2O, Na2O, and K2O.
The glass powder in the paste may also be mixed with two or more type of glass powders used in-combination. In this case, from the view point of the adhesive strength between the ceramic substrate and the conductor film, of the glass powders contained, preferably not less than 50 wt %, more preferably not less than 80 wt %, yet more preferably not less than 90 wt %, most preferably 100 wt %, is the Bi2O3—SiO2—B2O3 type glass powder.
The transition point and softening point of the glass powder are preferably low so that firing at low temperature can be accomplished. Specifically, the transition point of the glass powder is preferably from 360 to 450° C. The softening point of the glass is preferably from 400 to 530° C.
The following Bi2O3—SiO2—B2O3 glass powder is a specific example of the composition of the glass powder. Note, however, that the glass powder used in the present invention is not limited to being such a Bi2O3—SiO2—B2O3 glass powder.

    Bi2O3: 65 to 90 wt %
    SiO2:0.5 to 8 wt %
    B2O3: 3 to 10 wt %
    ZnO: 0 to 8 wt %
    Al2O3: 0 to 3 wt %
    TiO2: 0 to 3 wt %
    alkali metal oxides selected from the group consisting of Li2O, Na2O, K2O and a mixture thereof: 0 to 5 wt %
    alkaline earth metal oxides selected from the group consisting of MgO, CaO, BaO, SrO and a mixture thereof: 0 to 5 wt %


The content of the glass powder is preferably from 1 to 6 wt %, more preferably from 1.5 to 5.0 wt %, where the paste has a weight of 100 wt %.
c) Organic Solvent
There are no particular limitations on the type of the organic solvent. Examples of the organic solvent include, but are not limited to, α-terpineol, butyl carbitol, butyl carbitol acetate, Texanol, octanol, 2-ethylhexanol, and mineral spirit.
An organic binder may also be added so as to make the organic solvent into a resin solution. Examples of such an organic binder include an ethyl cellulose resin, a hydroxypropyl cellulose resin, an acrylic resin, a polyester resin, a polyvinyl butyral resin, a polyvinyl alcohol resin, a rosin-modified resin, and an epoxy resin.
Furthermore, a dilution solvent may be added to the conductor paste for adjusting the viscosity. Examples of such a dilution solvent include terpineol and butyl carbitol acetate.
d) Glass Softening Point Adjuster
The paste may further contain a glass softening point adjuster. By adding a glass softening point adjuster, the softening point of the glass powder can be reduced. Examples of such a glass softening point adjuster include Cu2O3, CuO, P2O5, SnO2, alkali metal halides, and alkaline earth metal halides. Two or more of these may be used in combination.
The glass softening point adjuster content is from 0 to 5 wt % where the paste has a weight of 100 wt %.
e) Other Additives
Thickeners and/or stabilizers and/or other common additives (e.g. sintering aids etc.) may be added to the conductive paste of the present invention. Examples of other common additives include dispersants and viscosity modifiers. The amount of such an additive is determined depending on the properties of the conductor paste ultimately required. The amount of the additive can be determined by a person skilled in the art as appropriate. A plurality of additives may be added.
The ceramic substrate conductor paste of the present invention can be conveniently manufactured by mixing together the various components using a triple roll mill or the like.
Next, the method of the electric circuit forming with the paste on ceramic substrate of the present invention will be described with reference to FIG. 1.
The conductor film formation method of the present invention comprises a step of applying onto a ceramic substrate a conductor paste comprising a) a conductive metal powder comprising a silver powder and a palladium powder, b) a glass powder, and c) an organic solvent, wherein the conductive metal powder has an average particle diameter of not more than 1.2 μm, and the glass powder is a Bi2O3—SiO2—B2O3 type glass powder, and the content of the conductive metal powder is in a range of from 60 to 92 wt % based on the weight of the paste, and a step of firing the conductor paste that has been applied onto the ceramic substrate at not more than 650° C.
A ceramic substrate 102 is prepared as shown in FIG. 1A. The material of the ceramic substrate 102 may be AlN, Al2O3, a mixture of Al2O3 and glass, SiC, ZrO, crystallized glass, or the like.
Next, as shown in FIG. 1B, a conductor paste 104′ according to the present invention as described above is applied onto the ceramic substrate. The printing may be carried out by screen printing or the like.
Next, as shown in FIG. 1C, the printed conductor paste is fired, so as to obtain a conductor film 104. The firing is carried out at not more than 650° C., preferably from 480 to 600° C., using an apparatus such as a belt furnace.
The present invention also includes an electric circuit substrate having circuit comprising a conductor film obtained using the above conductor film formation method.
The circuit substrate of the present invention has, for example, a structure as shown in FIG. 2. Specifically, the circuit substrate comprises the ceramic substrate 102, circuit comprising the conductor film 104 formed thereon, a chip 106 such as an LCR component, and solder 108 that joins the conductor film 104 and the chip 106 together.
For this circuit substrate, first, a desired chip is mounted on the ceramic substrate 102 having thereon the conductor film 104 which has been obtained using the above conductor film formation method. The mounting of the chip on the ceramic substrate 102 may be carried out using mounting means suitable for the chip. Next, soldering is carried out using a solder (e.g. a lead-free solder of composition Sn/Ag/Cu=95.75/3.5/0.75), whereby the conductor film and the chip are connected together, so as to complete the manufacture. For the soldering, a conventional method suitable for the mounted chip may be used. Moreover, instead of an LCR component, an IC package, an LED chip, a filter, an antenna component or the like may be mounted using a similar method.
EXAMPLES
The present invention is now described in more detail through examples; however, the following examples are merely illustrative, and are not intended to limit the present invention.
1. Preparation of Conductor Paste
A conductor paste was prepared in accordance with the compositional ratio for each sample shown in Table 1. Each sample was prepared by weighing the components of the sample out into a container, mixing together using a stirrer, and the dispersing using a triple roll mill.









TABLE 1









Material compositional proportions (%)














Silver/
Average particle

Silver (80%)/




Paste
palladium ratio
diameter of silver

palladium (20%)


no.
(%)
powder (μm)
Silver powder
coprecipitated powder
Glass powder
Resin solution
















J-1
Ag:Pd = 95:5
0.3
63.75
21.25
1.5
13.5


J-2
Ag:Pd = 98:2
1.0
76.5
8.5
1.5
13.5


J-3
Ag:Pd = 95:5
0.3
63.75
21.25
3
12


J-4
Ag:Pd = 95:5
0.3
76.5
8.5
3
12


H-1
Ag:Pd = 100:0
0.3
85
0
0
15


H-2
Ag:Pd = 100:0
0.3
85
0
1
14


H-3
Ag:Pd = 100:0
0.3
85
0
2
13


H-4
Ag:Pd = 100:0
0.3
85
0
3
12


H-5
Ag:Pd = 98:2
0.3
76.5
8.5
0.5
14.5


H-6
Ag:Pd = 95:5
0.3
63.75
21.25
0.5
14.5


H-7
Ag:Pd = 98:2
1.6
76.5
8.5
1.5
13.5


H-8
Ag:Pd = 95:5
1.6
63.75
21.25
1.5
13.5


H-9
Ag:Pd = 95:5
0.3
63.75
21.25
7
8


H-10
Ag:Pd = 90:10
0.3
42.5
42.5
7
8


H-11
Ag:Pd = 95:5
1.6
63.75
21.25
7
8









Resin solution: A solution of an ethyl cellulose resin in butyl carbitol was used.
Glass powder: One of Bi2O3—SiO2—B2O3 composition having a glass transition point of 410° C. and a softening point of 450° C. was used.
Silver/palladium co-precipitated powder: A co-precipitated powder of 80 wt % silver and 20 wt % palladium, being a spherical powder of average particle diameter 0.6 μm was used.
2. Formation of Conductor Film
The conductor paste prepared as described above was printed onto an AlN or Al2O3 substrate using a screen mask (stainless steel, 300 mesh, emulsion thickness 15 μm) such that the thickness after firing would be from 13 to 16 μm.
Next, firing was carried out using a belt furnace under either of two types of firing conditions as follows.
(1) 550° C. firing: for Examples 1 to 5 and Comparative Examples 1 to 11, held for 10 minutes at 550° C., total 30 minute profile.
(2) 600° C. firing: for Examples 3 and 4 and Comparative Examples 1 to 4, held for 10 minutes at 600° C., total 30 minute profile.
The following were used as the AlN and Al2O3 substrates.
AlN (170 W/mK article) substrate: 1 inch size, thickness 0.6 mm.
Al2O3 (96%) substrate: 1 inch size, thickness 0.6 mm.
Evaluation
(A) Adhesive Strength Between AlN or Al2O3 Substrate and Conductor Film
(i) Measurement of Initial Adhesive Strength
The conductor film on substrate prepared in each of the Examples was immersed for 10 seconds at 240° C. in a lead-free solder of composition Sn/Ag/Cu=95.75/3.5/0.75. A tin-plated copper wire was attached onto a 2 mm2 silver conductor film using the lead-free solder, the copper wire was stood up perpendicular to the substrate, and the initial adhesive strength immediately after the soldering was measured using a tensile tester. The results are shown in Table 2.
(ii) Measurement of Adhesive Strength After Aging at High Temperature, and After Thermal Cycle
For the samples of Examples 3 and 4 (samples of two types, i.e. fired at 550° C. or fired at 600° C.) that had been subjected to soldering as described in (i) above, the adhesive strength after leaving for 500 hours at 150° C., and the adhesive strength after 100 thermal cycles of −40° C./125° C. were measured. The results are shown in Table 3.











TABLE 2









600° C.






firing



Paste

550° C. firing
Initial 


Example no.
no.
Substrate
Initial strength (N)
strength (N)



















Example 1
J-1
AlN
24.2
—


Example 2
J-2
AlN
16.1
—


Example 3
J-3
AlN
25
27


Example 4
J-3
Al2O3
25.1
28.3


Example 5
J-4
AlN
23.9
—


Compar. Ex. 1
H-1
AlN
0
0


Compar. Ex. 2
H-2
AlN
1.2
2.4


Compar. Ex. 3
H-3
AlN
0
0


Compar. Ex. 4
H-4
AlN
0
0


Compar. Ex. 5
H-5
AlN
0
—


Compar. Ex. 6
H-6
AlN
1.8
—


Compar. Ex. 7
H-7
AlN
4.9
—


Compar. Ex. 8
H-8
AlN
8.8
—


Compar. Ex. 9
H-9
AlN
0
—


Compar. Ex. 10
H-10
AlN
0
—


Compar. Ex. 11
H-11
AlN
2.1
—



















TABLE 3









550° C. firing
600° C. firing
















Strength after aging at

Strength after aging at






high temperature of
Strength after 100
high temperature of
Strength after 100


Example
Paste

150° C. for 500 hours
theram cycles of
150° C. for 500 hours
thermal cycles of


no.
no.
Substrate
(N)
−40° C./125° C. (N)
(N)
−40° C./125° C. (N)





Example 3
J-3
AlN
15.5
11.3
25.6
—


Example 4
J-3
Al2O3
17.2
16.2
22.2
21.7









As can be seen from the above results, a conductor paste of the present invention is lead-free and cadmium-free, can be fired at a low temperature of not more than 650° C., and gives markedly improved adhesive strength between a ceramic substrate and a conductor film. Moreover, it can be seen that for a conductor film produced using a conductor paste of the present invention, sufficient adhesive strength can be maintained even after aging at a high temperature, or after a thermal cycle.

## Claims

1. A conductor paste for a ceramic substrate comprising:
a) a conductive metal powder comprising a mixture of silver powder and palladium powder;
b) a glass powder; and
c) an organic solvent, wherein the conductive metal powder has an average particle diameter of not more than 1.2 μm, and the glass powder is a Bi2O3—SiO2—B2O3 type of glass powder, and the content of the glass powder is in a range of from 1 to 6 wt % based on the weight of the paste, wherein the content of the palladium powder is in the range of from 0.1 to 7 wt % based on the total weight of the silver and palladium.

2. The conductor paste according to claim 1, wherein the conductive metal powder comprises monodispersed particles.

3. The conductor paste according to claim 1, further comprising, as the conductive metal powder, from 0.1 to 10 wt % of gold or platinum based on the weight of the paste.

4. The conductor paste according to claim 1, wherein the glass powder has a transition point of from 360 to 450° C., and a softening point of from 400 to 530° C.

5. The conductor paste according to claim 1, wherein the glass powder has a composition as follows:
Bi2O3: 65 to 90 wt %
SiO2: 0.5 to 8 wt %
B2O3: 3 to 10 wt %
ZnO: 0 to 8 wt %
Al2O3: 0 to 3 wt %
TiO2: 0 to 3 wt %
alkali metal oxides selected from the group consisting of Li2O, Na2O, K2O and a mixture thereof: 0 to 5 wt %
alkaline earth metal oxides selected from the group consisting of MgO, CaO, BaO, SrO and a mixture thereof: 0 to 5 wt %.

6. The conductor paste according to claim 1, wherein the content of the conductive metal powder is in a range of from 60 to 92 wt % based on the weight of the paste.

7. The conductor paste according to claim 1, further comprising a glass softening point adjuster selected from the group consisting of Cu2O3, CuO, P2O5, SnO2, alkali metal halides, and alkaline earth metal halides.

8. A conductor film formation method comprising steps of: applying onto a ceramic substrate the conductor paste according to claim 1; and firing the conductor paste that has been applied onto the ceramic substrate at not more than 650° C.

9. An electric circuit substrate having a circuit comprising a conductor film obtained using the conductor paste according to claim 1.