Note: Descriptions are shown in the official language in which they were submitted.
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
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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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(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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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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Shear Rate ~sec 1) Viscositv (Pa.S)
0.2 100-5000
300-2000 Preferred
600- 1500 Most Preferred
4 40-400
100-250 Preferred
140-200 Most Preferred
10
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
2 5 R25C
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
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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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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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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
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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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