CA2014432A1 - Thick film low-end resistor compositions - Google Patents

Abstract

Title THICK FILM LOW-END RESISTOR COMPOSITION

Abstract A thick film low-end resistor composition comprising an admixture of finely divided particles of (a) silver, palladium, an alloy of palladium and silver, or mixtures thereof; (b) an admixture of (1) glass having a softening point of 350 to 500°C, which when molten is wetting with respect to the other solids in the composition, and (2) glass having a softening point of 550 to 650°C; and (c) 5-20% by volume, basis total solids, of sub-micron particles of RuO2, all of (a) through (c) being dispersed in (d) an organic medium.

Description

Title THICK FILM LOW-END RESISTOR COMPOSITIONS

5 Field of Invention The invention relates to improved thick fllm low-end resistor compositions having improved laser trim stability which are especially suitable for the manufacture of chip resistors.

Background of the Invention Chip resistors are typically screen printed as thiclc film 15 pastes on a large, square alumina substrate with as many as a thousand chip resistors on a single such substrate. The printed resistors are then flred to remove all of the organic medium from the printed pattern and to densify the solids. A first encapsulant glass layer is printed over the resistors and fired. The resistor 20 values at this point have a distribution of 3-5%. The once encapsulated resistors are trimrned with a laser beam directly through the encapsulant, and the printed resistor layer, and into the alumina substrate. The laser trimming increases resistance values about 50%, but reduces the distribution of resistance values to about 25 0.1%

After laser trimming through the flrst encapsulant layer and the reslstor, a second glass encapsulant ls printed over the trimmed reslstor and fired at 600C. After firing the second 30 encapsulant layer, the large substrate ls broken into strips and a conductive edge termination is applied by dlpping the edge of the strips into a conductive paste. The thusly terminated strips are then fired. After flring the edge terminations, the strips are broken into individual chips and the chip terminations are nickel and solder-35 plated. The flnished chip resistors are about the size of a large grainof sand. They are usually soldered to a printed wiring board for use.

Chip resistors such as those described above are frequently made in a wide range of resistances from 1 to 1,000,000 44~

ohms, and to be effective, they must have a reslstance shift upon encapsulation and trimming of no more than 0.5%. Reslstance stabilities such as this, however, are very difficult to achieve with low-end resistors, i.e., those having resistance values of only 1-100 5 ohms per square.

Low-end reslstance reslstors of the current state-of-the-art, such as those based on RuO2 alone, tend to have reslstance shifts exceeding 0.5% in 1000 hours after laser trlmming, whereas higher 10 resistance resistors are much more stable. In addition, the state-of-the-art low resistance resistors are traditionally difflcult to manufacture to a resistance of +10% and a temperature coefficlent of resistance (TCR) of +100 ppm/C because a dense, consistent, insensitive microstructure is difflcult to achleve. The relatively low 15 volume fraction of glass binder phase in such compositions makes it difflcult to achieve this desired dense, consistent microstructure.

Sumrnarv of the Invention The present invention solves these problems by using lngredients that provide a relatively dense, low-porosity and therefore stable microstructure. The low softenlng polnt glasses and alloying action of the Pd and Ag provide a microstructural activity durlng resistor flring which gives a dense microstructure for stable reslstor performance and conslstent lot-to-lot performance.
Additional benefits include the low reslstance reslstors' ablllty to carry power, whlch varies from 1.5 to 2 tlmes that of RuO2-based resistors. Thus, the present lnvention overcomes the many problems of the prior art.
In lts prlmary aspect, the lnventlon ls directed to a thick fllm low-end reslstor composltion comprising an admixture of flnely divlded particles of:

35 (a) an alloy of palladtum and silver, an admixture of oxides of palladium and sllver, or mixtures thereof, the proportions by weight of palladium and silver being respectively from 35 to 45% and from 65 to 55%;

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2Q~ 44 (b) an adm~ture of (1) 0.2 to 5.0% weight, basis total sollds, of glass having a softening point of 350 to 500C, which when molten is wetting with respect to the other solids in the composltion, and (2) glass having a softening polnt of 550 to 650C; and (c) 5-20% by volume, basis total solids, of sub-micron particles of RuO2, all of (a) through (c) being dispersed in (d) an organic medium.

In a secondary aspect, the invention is directed to a method for making low-end resistors comprising the sequential steps of:

(a) applying a patterned layer of the above-described thick fllm composition to an inert substrate; and (b) firing the layer at a peak temperature of 800-900C to effect volatilization of the organic medium therefrom and densification of the solids.
Detailed Description of the Invention A Conductive Metal The conductlve phase of the compositions of the invention is an alloy of palladium and silver or it can be a mixture of 30 palladium and silver metal particles. Mixtures of both can be used as well. The preferred ratio by weight of palladium to silver is 40:60 because of the sintering and alloying characterlstics of that particular ratio. However, palladium/silver ratios of as low as 35:65 and as high as 45:55 can also be used.
The particle size of the metal(s) is not particularly important so long as it is suitable for the method of application.

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However, it is preferred that the metal particles be within the range of 0.5-5 microns.

~ Inor~anic Binder The inorganic binder component of the invention is comprlsed of two glasses. One of the glasses must be low melting and be capable of wetting the surface of the other solids in the composition. The low melting glass must have a softening point 10 (Dilatometer) of 350-500C and must be capable of wetting the surface of the other solids in the composition, i.e., the second glass, the conductive metal and the RUO2. The wetting characteristics of the glass are readily determined by measuring the contact angle of the molten glass on a surface of each of the other solids, at the 15 expected flring temperature (800-900C). Suitable wettability for the purposes of the invention ls established if the contact angle of the low melting glass on the other solids is 30 or less and preferably no more than 10.

It is necessary that the softening point of the lower melting glass not exceed about 500C lest the glass flow during firing be insufficient to obtain proper melting of the other solid particles. On the other hand, if the softening point of the glass is below 350C, glass flow during flring rnay become excessive and result ln maldistribution of the glass throughout the flred resistor. It is preferred that the softening polnt of the lower melting glass be In the range of 375-425C for optimum performance.

The second essential component of the inorganic binder is the hlgher melting glass which has a softening point (Dilatometer) of 550-650C and preferably 575-600C. It is preferred that the softening polnt of the glass not be lower than about 550C for the reason that the temperature coefl'icient of expansion trCEl of such glasses tends to be excessive in comparison with conventional substrate materials. On the other hand, if the softening point significantly exceeds 650C, the microstructure of the flred resistor is less uniform and the reslstor becomes less durable.

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Provided that the physlcal properties of the two glasses are appropriate, the composition of the glasses is not by itself crltical except as it relates to the viscosity and wetting properties of the glass when the composition is fired. Thus a wide variety of oxide 5 glasses containing conventional glass-forming and glass-modifying components can be used, e.g., alumino borosilicates, lead sllicates such as lead borosilicate and lead silicate itself and bismuth silicates and the like. It is, however, necessary that the low softening point glass be non-crystallizing (amorphous) at firing temperatures in 10 order to get a proper amount of glass flow during the firing process.

The total amount of inorganic binder in the composition of the inventlon is in part a function of the desired resistor properties. For example, a 1 ohm/square resistor will require on 15 the order of 45% vol. inorganic blnder, a 10 ohm/square resistor will require about 65% vol. glass binder, and a 100 ohm/square resistor will contain about 75% vol. Bass blnder. Thus the amount of binder may vary by volume from as low as, say, 40% to as high as 80%, but will usually fall within the range of 50 to 65%.
The relative amount of low softening polnt glass in the inorganlc blnder is a function of the total solids in the composition and the wettabllity of the lower melting glass on the other solids. In particular, it has been found that at least 0.2% wt. and preferably at 25 least 0.5% wt. low melting glass is needed to get adequate wetting of all the solids. However, if more than about 5% wt. low melting glass is used, the composition tends to incur bllstering upon firlng.

The particle size of the lnorganic binder is not 30 particularly critical. However, the glass particles should be in the range of 0.1-10 microns (preferably 0.5-5 microns) and have an average particle size of 2-3 microns. Glass flnes below 0.1 micron have so much surface area that too much organlc medlum is needed to obtain the proper rheology of the paste for printing. On the other 35 hand, if the particles are larger than 10 microns, they lnterfere with screen printing.

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C Ruthenium Dlo~de A minor amount of ruthenium dioxide (RuO2) Is required in the composition of the invention ln order to lower the TCR of the 5 composition. The amount of Ru02 needed is related to the total volume of the composition solids. In particular, at least 5% vol.
RuO2 is needed, but up to 20% vol. Ru02 may be used in some instances. Below 5% vol. RuO2 it ls difficult to make resistors reproducibly and above about 20% vol. the total amount of 10 conductive phase becomes excessive and correspondingly the amount of glass is insufflcient to give a good microstructure.
However, the particle size of the RuO2 should always be less than 1 micron in order to give adequate TCR propertles.
The RuO2 can be added to the composition in either of two forms. It can be added as discrete RuO2 partlcles or It can be added in the form of RuO2 partlcles sintered onto the surface of glass particles. It Is preferred to introduce the RuO2 sintered onto the surface of glass particles in order to obtain more even partlcle 20 distributlon, better wetting and more even coating of the RuO2 particles and also to reduce catalytic action by the particles when they are dispersed in the organic medium. In the latter instance, the particles are prepared by admixing the RuO2 particles with glass particles, heating the admixture to above the softening polnt of the 25 glass so that the glass sinters but does not melt and flow, and then milllng the slntered product.

It is preferred that the glass used for RuO2 addltion have an intermedlate softenlng point range of 400-650C, which is 30 Intermedlate to the softenlng point range of the primary glass components of the inorganic blnder. The purpose of thls Is to obtaln good wetting and coating of the RuO2 without incurrlng too much dislocatlon of the glass during flring. It is also preferred that the intermedlate glass contain a minor amount of one or more transltlon 35 metal oxides such as MnO2, Co2O3, Fe3O4, CuO, Ni2O3 and the like to facilitate further TCR control. About 1% wt. is required to be effective and as much as 20% wt. might be used In some Instances.

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i 44~hd It is preferred, however, to use no more than 15% wt. transition metal oxide to avoid excessive moisture sensitivity.
D. Or~anic Medium The inorganic particles are mixed with an organic liquid medium (vehicle) by mechanical mixing to form a pastelike composition having suitable consistency and rheology for screen printing. The paste is then printed as a "thick film" on dielectric or 10 other substrates in the conventional manner.

The main purpose of the organic medium is to serve as a vehicle for d~spersion of the flnely divided solids of the composition in such form that lt can readily be applied to a ceramic or other 15 substrate. Thus, the organic medium must flrst of all be one in which the solids are dispersible with an adequate degree of stabillty.
Secondly, the rheological properties of the organic medium must be such that they lend good application properties to the dispersion.

Most thick fllm compositions are applied to a substrate by means of screen printing. Therefore, they must have appropriate viscosity so that they can be passed through the screen readlly. In addltlon, they should be thixotroplc In order that they set up rapidly after being screened, thereby glving good resolution. While the rheological properties are of primary lmportance, the organic medium is preferably formulated also to give appropriate wettability of the solids and the substrate, good drying rate, dried fllm strength sufflclent to withstand rough handling and good flring properties.
Satisfactory appearance of the flred composition is also important.
In view of all these criteria, a wide variety of Inert llqulds can be used as organic medium. The organic medlum for most thlck fllm compositions is typically a solution of resin in a solvent and, frequently, a solvent solution containing both resin and thlxotroplc agent. The solvent usually boils within the range of 130-350C.

By far, the most frequently used resln for thls purpose Is ethyl cellulose. However, resins such as ethylhydroxyethyl cellulose~

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wood rosin, mixtures of ethyl cellulose and phenolic resins, polymethacrylates of lower alcohols and monobutyl ether of ethylene glycol monoacetate can also be used.

The most widely used solvents for thick fllm applications are terpenes such as alpha- or beta-terpineol or mixtures thereof with other solvents such as kerosene, dibutylphthalate, butyl carbitol, butyl carbitol acetate, hexylene glycol and high boiling alcohols and alcohol esters. Various combinations of these and other solvents are formulated to obtain the desired viscosity and volatility requirements for each application.

Among the thixotropic agents which are commonly used are hydrogenated castor oil and derivatives thereof and ethyl cellulose. It Is, of course, not always necessary to incorporate a thixotropic agent slnce the solvent/resin propertles coupled with the shear thinning lnherent in any suspension may alone be suitable in this regard.

The ratio of organic medium to solids in the dispersions can vary considerably and depends upon the manner in which the disperslon ls to be applled and the kind of organic medium used.
Normally, to achieve good coverage, the dispersions will contain complementary by weight 60-90% solids and 40-10% organic medium. Such dlspersions are usually of semlfluid consistency and are referred to commonly as "pastes".

The viscoslb of the pastes for screen printlng is typically within the following ranges when measured on a Brookfield HBT*
Viscometer at low, moderate and h1gh shear rates:

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-g Shear Rate ~sec 1) Viscositv (Pa.S) 0.2 100-5000 300-2000 Preferred 600- 1500 Most Preferred 100-250 Preferred 140-200 Most Preferred 384~ 7-40 10-25 Preferred 12-18 Most Preferred ~Measured on HBT Cone and Plate Model Brookfield Viscometer The amount of vehicle utilized ls determined by the flnal desired formulaffon viscosity.

Test Procedures A Sample Preparation Samples to be tested for temperature coemcient of reslstance ~CR) are prepared as follows:

A pattern of the resistor formulation to be tested is 30 screen prlnted upon each of ten coded Alsimag614 lx 1" ceramic substrates and allowed to equilibrate at room temperature and then dried at 150C. The mean thickness of each set of ten drled fllms before flring must be 22-28 microns as measured by a Brush Surfanalyzer. The dried and printed substrate is then flred for about 35 60 minutes using a cycle of heating at 35C per minute to 850C, dwell at 850C for 9 to 10 minutes, and cooled at a rate of 30C per minute to ambient temperature.

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Resistance Measurement and Calculations Substrates prepared as described above are mounted on terminal posts within a controlled temperature chamber and 5 electrically connected to a digital ohm-meter. The temperature in the chamber is ad~usted to 25 C and allowed to equilibrate, after which the resistance of each substrate is measured and recorded.

The temperature of the chamber is then ralsed to 125C
10 and allowed to equilibrate, after which the reslstance of the substrate is again measured and recorded.

The temperature of the chamber is then cooled to -55C and allowed to equllibrate and the cold resistance measured 15 and recorded.

The hot and cold temperature coefficients of resistance (TCR) are calculated as follows:
Hot TCR = _125c _- B25c x (10,000) ppm/C
R25oc Cold TCR = _-550c - R~ x (-12,500) ppm/C

The values of R2sC and Hot and Cold TCR are averaged and R25C values are normallzed to 25 mlcrons dry printed thlckness, and reslstlvlty Is reported as ohms per square at 25 30 microns dry prlnt thlckness. Normallzation of the multlple test values Is calculated with the followlng relationshlp:

Avg. Measured Avg. Dry Print Normalized = Resistance x Thickness. microns Resistance 25 Microns , . . ~ . .: .
', "' '' '`' ' , `' ~ ' ' C Laser Trlm Stability Laser trimming of thick fllm resistors is an important technique for the production of hybrid microelectronic circults. lA
5 discussion can be found in Thick Film Hvbrid Microcircuit Technolog!r by D. W. Hamer and J. V. Biggers tWiley, 1972), p. 173 ff.l Its use can be understood by considering that the resistances of a particular resistor printed with the same resistor paste on a group of substrates has a Gaussian-like distribution. To make all the 10 resistors have the same design value for proper circuit performance, a laser is used to trim resistances up by removing (vaporizing) a small portion of the resistor material. The stability of the trimmed resistor is then a measure of the fractional change (drift) in resistance that occurs after laser trimm1ng. Low resistance drift 15 (high stability) is necessary so that the resistance remains close to its deslgn value for proper circult performance.

D. Wettability Wettability of the low softening point glass with respect to the other solids is determined by measuring the contact angle of a molten drop of the low softening point glass on a surface of the other solids. The equilibrium shape assumed by a liquid drop placed 25 on a smooth solid surface under the force of gravity is determined by the mechanical force equilibrium of three surface tensions: ~ (LV) at the liquid-vapor interface; ~ (SV) at the liquid-solld interface; and (SV) at the solid-vapor interface. The contact angle is in theory independent of the drop volume and in the absence of crystallization 30 or lnteraction between the substrate and the test liquid depends only upon temperature and the nature of the respective solid, liquid and vapor phases in equillbrium. Contact angle measurements are an accurate method for chracterizing the wettability of a solid surface since the tendency for the liquid to spread and "wet" the 35 solids surface increases as the contact angle decreases.

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-2Q~a ~3 E. Electrostatic Dlscharge Test Thls Electrostatic Discharge (ESD) test is a military standard designated MIL-STD-883C, Method 3015.6. It establishes 5 the means of classifying microcircults (and resistors on microcircuits) according to their susceptibility to damage or degradation by exposure to electrostatic discharge.

The electrostatic discharge, deflned as the transfer of 10 electrostatic charge between two bodies at different electrostatic potentials, used in tllis test has a rise time between 5 and 10 nanoseconds and a decay time of 150 + 20 nanoseconds. The test results include the peak voltage and the relative resistance change when the resistor is exposed to the electrostatic discharge.
Examples An admixture was formed by mixing 25.7 grams of RuO2 powder mixed with 4.8 grams of silver and 2.3 grams of palladium powders. This conductive powder was further mixed with 32.2 grams of a manganese alumino lead borosilicate glass with a softening point of 510C, 7.7 grams of an alumino lead borosilicate 25 glass with a softening point of 525C, 0.7 gram of a bismuth silicate glass with a softening point of 445C and 23.1 grams of a calcium alumino lead borosilicate with a softening point of 660C. All the powders were ground to surface areas in the range of 1 to 10 m2/gram.
This powder mixture was dispersed with 38 grams of a liquid medium composed of ethyl cellulose and beta-terpineol to fonn a viscous suspension with a ~rlscosity between 100 and 300 Pascal-seconds. In practice of the present Inventlon, the dlspersion 35 ls usually screen printed onto an lnsulating substrate and flred in air at a temperature of between 700 and 950C to produce a flred resistor fllm.

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This resistor, having a printed thickness of 25 microns was fired at 850C for 10 minutes. The flred resistor had a resistance of 9.8 ohm per square, and a temperature coefficient of resistance (TCR), measured between 25 and 125C, was 35 ppm/C.
5 Its resistance drift after laser trimming and storage in an 85C/85%
relative humidity environment was 0.08 + 0.06%. Its resistance changed 0.01 i 0.01% when exposed to a single 5000 V pulse in an electrostatic discharge test and had a maximum rated power of 864 mw/sq.mm.
Example 2 A further admixture was formed by mixing 20.8 grams of RuO2 with 15.0 grams of silver and 7.2 grams of Pd. These 15 conductlves were mixed wlth 26.1 grams of the manganese alumino boros~licate glass, 18.1 grams of the lead alumino borosilicate glass, 10.5 grams of the 600C softening point glass, and 2.3 grams of the bismuth silicate glass. After these powders were dispersed in an organic medium to form a paste which was printed in a resistor 20 pattern and fired as in the previous example. The resistance of the resistor was 3.0 ohms per square, and the TCR was 50 ppm/C. Its resistance drift after laser trimming and storage in an 85C/85%
relative humidity environment was 0.01 + 0.06%. Its resistance changed -0.01 i 0.04% when exposed to a single 5000V pulse in an 25 electrostatic discharge test at a maximum rated power of 888 mw/sq.mm.

Example 3 An admixture of flnely divided solids was formed by mixing 19.5 g of sllver and 16.4 g of RUO2. These conductives were mixed wlth 19.5 g of the above-referred alumino lead borosilicate glass and 2.3 g of titanla lead aluminoborosilicate glass and 6.3 g of bismuth lead aluminoboroslllcate glass. The powders were ground to a surface area of 1-10 m2/g as ln Example 1. The ground particles were then dlspersed in an organic medium to form a paste.
After printing and flring, the resistance of the fired layer was 32.5 ohms/square, HTCR was -47 ppm/C, and CTCR was -99 ppm/C.

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~ 4 Example 4 An admixture was formed by mixing 18.4 grams of RuO2 S with 11.0 grams of palladium and 19.7 grams of silver. The RuO2 particles were not sintered onto the surfaces of glass particles in this case. Thls mixture was further mixed with 12.3 grams of the manganese lead alumino borosilicate glass with a softening point of 510C, 1.9 grams of the bismuth silicate glass with a softening point 10 of 445C, 4.12 grams of a lead alumino borosilicate glass with a softening point of 600C, and 8.9 grams of a titania lead alurnino borosilicate glass with a softening point of 525C. Again all the glass surface areas were in the range of 1 to 10 m2/gram.

Thls powder rnixture was also dlspersed in the ethyl celulose and beta-terpineol liquid medium to form a viscous suspension with the same viscosity range as in the previous examples. After printing onto an insulating substrate and firing at 850C for 10 minutes, the resistance of the printed layer was 2.8 20 ohms and the temperature coefflcient of reslstance was 110 ppm/C.
Resistance drift after laser trimming and storage in at 85C/85%
relative humidlty for 500 hours was 0.21%.

Example 5 An admixture was formed by mixing 11.1 grams of silver/palladlum alloy powder with 1,3 grams of palladlum and 6.1 grams of RUO2. The alloy had a silver-to-palladium ratlo of 2.6. The Ru02 was sintered to the surfaces of a manganese alumino lead 30 borosilicate glass. The amount of this glass, with a softening point of 510C, was 18.7 grams. This mixture was further mixed with 0.7 grams of the calcium alumino lead borosilicate glass with a softening point of 660C, 1.7 grams of the alumino lead borosilicate glass with a softening polnt of 600C, and 4.73 grams of a titania alumino lead 35 borosilicate glass with a softening point of 525C. This powder mixture was dispersed in 23 grams of an organic medium containing ethyl cellulose and beta-terpineol. The resistor, after printing onto an insulating substrate and flring at 850C for 10 minutes, had a .

- 26~4 resistance of 8.3 ohms/square, and a temperature coefflcient of resistance between -55 and 25C of 88 ppm/C. Its resistance drift after laser trimming and storage in an 85C/85% relative humidity was 0.12%.

Next is an example of a resistor with a relatively high level of Pd. The Ag/(Pd + Ag) ratio is only 42%, compared with approximately 60% for most of the other examples.

10 Example 6 An admixture was formed by mixing 14.6 grams of Ru02 with 16.85 grams of palladium and 12.2 grams of silver. The Ag/(Pd + Ag) ratio in this case was only 42%, compared with approximately 15 60% for most of the other examples. Thls mL~cture is further mixed with 7.8 grams of manganese alumino lead borosilicate glass with a softening point of 510C and 24.4 grarns of the titanla alumina lead borosilicate glass with a softening point of 525C.

This powder mixture was dispersed with 27.2 grams of the ethyl cellulose, beta-terpineol liquld. The fired reslstor had a reslstivity of 85 ohms and a temperature coefficlent of resistance between -55 and 25C of -257 ppm/C. This negative coefflcient is coorectable to more positive values by balancing the relative amounts of the different glasses.

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Claims (5)

1. A thick film low-end resistor composition comprising an admixture of finely divided particles of:

(a) silver, palladium, an alloy of palladium and silver or mixtures thereof, (b) an admixture of (1) 0.2 to 5.0% weight, basis total solids, of glass having a softening point of 350 to 500°C, which when molten is wetting with respect to the other solids in the composition, and (2) glass having a softening point of 550 to 650°C; and (c) 5-20% by volume, basis total solids, of sub-micron particles of RuO2, all of (a) through (c) being dispersed in (d) an organic medium.

2. The composition of claim 1 in which component (a) is an alloy of palladium and silver.

3. The composition of claim 1 in which component (a) is a mixture of palladium and silver particles in alloying proportions.

4. The composition of claim 3 in which the palladium and silver are in the form of an alloy containing 40% silver.

5. The composition of claim 1 in which the RuO2 particles are sintered to the surface of particles of an intermediate glass having a softening point of 400-650°C.