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Description  |
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TECHNICAL FIELD
The present invention relates to base metal resistive paint, resistors made
from the resistive paint, and a method for making the resistive paint.
More particularly, this invention relates to thick film base metal
resistive paints comprising 20 to 25% tantala glass frit and 75 to 80% tin
oxide, well mixed with 25 to 35% screening agent, to form a base metal
resistive paint for firing in an inert atmosphere at a peak temperature of
about 900.degree. C. The tantala glass frit preferably comprises 5 to 25%
tantalum oxide. The tin oxide is preferably preheated at 450.degree. to
600.degree. C. in the presence of a reducing gas, prior to mixing with the
tantala glass frit. The screening agent preferably forms no carbon residue
when pyrolytically decomposed in an inert atmosphere.
BACKGROUND ART
Tin oxide compounds have been used as a major conductive material in
resistors for many years. Tin oxide films may be processed by spraying and
heating a tin chloride solution; by evaporation or sputtering technology,
by chemical vapor disposition, or by thick film technology.
Thick film technology has been used in the electronics industry for more
than 25 years. Thick film technology includes printing and firing a
resistive paint in a desired pattern upon a suitable substrate. Resistive
paints used in thick film technology typically include a conductive
material, a glass frit, and a screening agent.
Early thick film resistive paint patents varied only in the composition of
the conductive materials. The glass frit, after melting, was used
primarily as a bonding agent to bond the conductive material to the
substrate. The chemical composition of the glass frit was considered
important only in regard to its melting point which was required to be
below the melting point of the conductive material used. The screening
agent was selected for consistency and ease of printing. Commercially
available glass frits and screening agents were typically used.
Certain materials were typically mixed with tin oxide powder to obtain the
wide range of resistivity and low TCR (temperature coefficient of
resistance) desired.
Dearden, in an article published in Electronic Components Magazine in
March, 1967, entitled High Value, High Voltage Resistors discloses the use
of doped antimony oxide with tin oxide to make a binary resistive paint,
but the best TCR obtained was -1500 ppm/.degree.C. Kamigaito (U.S. Pat.
No. 3,915,721) patented a ternary conductive paint material including
powders of 2% tantalum oxide, antimony oxide and tin oxide. Kuden et al.
(U.S. Pat. No. 3,928,242) discloses the use of tantala glass frit for use
with ruthenium oxide resistors. Moriguchi et al. (U.S. Pat. No. 3,900,330)
patented a zinc-sealing glass containing 0.1 to 25% Ta.sub.2 O.sub.5 to
improve surface charge density. Wahlers et al. (U.S. Pat. No. 4,065,743)
patented the use of binary conductive materials of tin oxide and tantalum
oxide powders for use with standardized glass frit. Wahlers et al. (U.S.
Pat. No. 4,215,020) also patented ternary conductive materials for use
with tin oxide resistors. Recently Wahlers et al. (U.S. Pat. Nos.
4,378,409 and 4,397,915) claimed a tin oxide material for use with a 30 to
40% barium oxide glass frit, and a glass frit with more than 20% silicon
oxide.
Chemical compounds found in a typical glass frit are mineral and inorganic.
These chemicals typically exhibit a number of undesirable properties, such
as: high TCR; widely variable thermal stability; poor short time overload
characteristics; variable resistance values due to uneven mixing; and
visible cracks and fissures. A base metal resistive paint is a resistive
paint having no noble metals included in its composition.
DISCLOSURE OF THE INVENTION
The present invention discloses a tantala glass frit, preferably a tantala
strontium glass frit, for use in mixing with tin oxide and a screening
agent, preferably a non-carbon residue organic compound screening agent to
form a base metal resistive paint for screening upon a substrate for
subsequent firing in an inert atmosphere at a peak temperature of
approximately 900.degree. C. The tantala glass frit and tin oxide are
preferably ground to a particle size of 10 microns or less prior to
mixing.
The tantala glass frit powder is well mixed at a ratio of 20 to 25% with 75
to 80% tin oxide powder. A screening agent in the ratio of 25 to 35% is
added to the tantala glass frit and tin oxide powders, and well mixed to
form a base metal resistive paint for subsequent screening and firing in
an inert atmosphere at a peak temperature of 900.degree. C..+-.20.degree.
C. to form a resistive pattern upon a substrate with a TCR within .+-.300
ppm/.degree.C.
The disclosed resistive paint exhibits improved thermal stability and short
time overload; maintains a tight resistance value due to improved
homogeneous mixing of the chemical compounds; and improves physical
appearance by reducing the observable cracks and fissions in the fired
resistive paint screened upon a suitable substrate. Furthermore, a cost
savings in energy, furnace life and maintenance is realized by firing at a
peak temperature of 900.degree. C., as compared with firing at peak
temperatures of 1000.degree. C. to 1100.degree. C., as currently practiced
for firing most tin oxide resistive paints.
Therefore, one object is to provide an improved thick film resistive paint.
Another object is to provide a resistive paint mixed from tantala glass
frit, tin oxide powders, and a screening agent to form a resistive paint
whose TCR is within .+-.300 ppm/.degree.C.
Another object is to provide a method for manufacturing a resistor made
from tantala glass frit and tin oxide powders.
Another object is to provide a screening agent having no carbon residue
when pyrolytically decomposed in an inert atmosphere, for use with a base
metal resistive paint.
Yet another object is to provide a tantala glass frit and tin oxide
resistive paint suitable for firing at a peak temperature of approximately
900.degree. C.
Still another object is to provide a resistor made from a resistive paint
embodying any of the objects previously disclosed.
The above-mentioned and other features and objects to this invention and
the manner of attaining them will be best understood by reference to the
following description of an embodiment of the invention, when considered
in conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows a partial sectional view of a resistor prior to firing made
with the resistive paint of the present invention.
FIG. 1A is a graph comparing the effects of firing temperatures on TCR
(SnO.sub.2 reduced at 520.degree. C.).
FIG. 1B is a graph comparing the effects of firing temperatures on sheet
resistance on ohms/square (SnO.sub.2 reduced at 520.degree. C.).
FIG. 2A is a graph comparing the effects of firing temperatures on TCR
(SnO.sub.2 reduced at 450.degree. C.).
FIG. 2B is a graph comparing the effects of firing temperatures on sheet
resistance in ohms/square (SnO.sub.2 reduced at 450.degree. C.).
FIG. 3 is a graph comparing the ratios of SnO.sub.2 /tantala glass on sheet
resistance and TCR.
FIG. 4 is a flow chart showing the method of processing a resistor for
tantala glass frit, tin oxide, and a screening agent.
BEST MODE FOR CARRYING OUT THE INVENTION
The subject matter which I regard as my invention is particularly pointed
out and distinctly claimed in the claims. The structure and operation of
my invention, together with further objects and advantages, may be better
understood from the following description given in connection with the
accompanying drawings, in which:
FIG. 1 shows a base metal resistor of the present invention prior to
firing, generally designated 10. Resistor 10 comprises a substrate 12,
such as a ceramic substrate, having a layer of the resistive paint 14 of
the present invention screened or otherwise coated thereon for subsequent
firing. The resistive paint 14 comprises a mixture of tantala glass frit
16 and tin oxide 18 in a preferred ratio of 20 to 25% tantala glass frit
to 75 to 80% tin oxide. Note: all compositions disclosed herein are based
upon weight percentage.
The tantala glass frit 16 preferably has a melting point of 800.degree. C.
or less, and comprises from five to twenty-five percent tantalum oxide
(Ta.sub.2 O.sub.5). The tantala glass frit 16 preferably comprises, at
least in part, strontium oxide (SrO), or strontium peroxide (SrO.sub.2).
The glass 17 and tantala 19 are preferably ground to a particle size of
ten microns or less, then well mixed, remelted and ground to form the
tantala glass frit 16 of the present invention.
The tantala glass frit 16 and tin oxide 18 are preferably ground to a
particle size of ten microns or less, and the resulting powders are added
to a screening agent 20 in a ratio of 65 to 75% tantala glass frit and tin
oxide powders to 25 to 35% screening agent. The screening agent 20
preferably forms no carbon residue when pyrolytically decomposed in an
inert atmosphere, such as 10% butyl-methacrylate dissolved into 90% pine
oil.
The solvent used for making the screening agent can be pine oil, terpineol,
an ester alcohol of Texanol from Texas Eastman Company, butyl carbitol
acetate or the like. The resins used for binders can by
polyalkylmethacrylate available from DuPont or Rohm and Haas; or
polybutenes available as Amoco H-25, Amoco H-50, and Amoco L-100 from
Amoco Chemicals Corporation. A wetting agent may be added to wet the solid
powders for good paint rheology.
Some commercially available screening agents after firing in an inert
atmosphere at high temperature contain carbon residue, which is
conductive. Such carbon residue is not combined with oxygen to form a
carbon oxide during heating in an inert atmosphere, therefore the carbon
in the screening agent remains in the resistive paint, adversely affecting
the controlled performance characteristics of the resistor 10.
As shown in FIG. 1, the tantala glass frit 16 contains a well blended
mixture of tantala 19 and glass 17, which has been remelted and ground to
form the tantala glass frit 16 of the present invention. The tantala glass
frit 16 when mixed with tin oxide 18 provides a more homogenous mixture
than can be readily obtained by admixing tantalum oxide to the resistive
paint 14 during the mixing operation. Thus, as shown in FIG. 1, the
tantala glass frit 16 and the tin oxide 18 are well dispersed within the
layer of resistive paint 14. The resulting resistance characteristics are
thereby improved resulting in a more homogenous mixture of resistive paint
14, which provides improved and more controllable conductive
characteristics, as will hereinafter be disclosed.
As shown in FIG. 5, the tin oxide 18 is preferably preheated 22 in a
reducing gas to a temperature of from 450.degree. C. to 600.degree. C.,
for a time sufficient to reduce the oxide to a desired level. When
preheating 22 is done in a tube furnace, the tin oxide 18 is preheated for
10 minutes to one hour in a forming gas such as 2 to 7% H.sub.2 and 93 to
98% N.sub.2 atmosphere. The preheated 22 tin oxide 18 is then mixed 24
with the tantala glass frit 16 and the screening agent 20, preferably in a
three roll mill (not shown) to yield a consistent resistive paint 14
having uniform dispersions of tantala 19 and tin oxide 18 throughout, in
the desired proportions previously disclosed.
The screening agent 20 is preferably an organic compound which is free of
carbon residue when pyrolytically decomposed in an inert atmosphere. A
binding resin 21 may be incorporated into the screening agent 20 to
improve the binding properties of the mixed resistive paint 14, prior to
firing 28. Once the tantala glass frit 16, tin oxide 18, and screening
agent 20 are mixed 24 into a homogeneous resistive paint material 14, they
are subsequently screened 26, preferably through a silk or stainless steel
screen, onto a substrate 12. Substrate 12 is preferably an alumina
substrate that has been prefired onto a thick film copper conductor in an
inert atmosphere at about 900.degree. C. The screen aperture size affects
the resistive quality of the fired resistive paint, as will be
subsequently disclosed. The preferred screen aperture size is from 165 to
325 mesh.
After screening, the resistive paint 14 disposed upon substrate 12 is
preferably allowed to dry prior to firing 28. The resistive paint and
substrate are subsequently fired 28 at a peak temperature of 900.degree.
C..+-.20.degree. C., in an inert atmosphere such as nitrogen, to form a
vitreous enamel base metal resistor material fused to the substrate.
TABLE I
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Properties
Example 1 Example 2 Example 3
Example 4
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ohms/sq. 60K 31K 9K 15K
Cold TCR -13900 -2270 -285 -183
Hot TCR -6400 -1074 -279 -187
Thermal 2% 4% 0.3% 0.3%
Stability
STOL 3% 4% 0.05% 0.01%
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As shown in Table I, Examples 1 through 4 demonstrate the advantages of the
addition of Ta.sub.2 O.sub.5 (tantalum oxide) to the glass frit by
comparison of 0%; 2%; 5% and 20% Ta.sub.2 O.sub.5. Each example was tested
to determine the ohms/square; Hot and Cold TCR; Thermal Stability; and
STOL.
In each of the following examples the Cold TCR was tested at -55.degree. C.
to .+-.25.degree. C.; the Hot TRC was tested at +25.degree. C. to
+125.degree. C.; Thermal Stability was tested at 150.degree. C. for 48
hours; the STOL was tested at 500 volts or 5 watts maximum; and the
resistor size tested was 0.062.times.0.156 inches, or 2.5 squares.
Example 1 comprises a glass frit, having no (0%) Ta.sub.2 O.sub.5, with
(5%) SiO.sub.2 +(35%) SrO+(60%) B.sub.2 O.sub.3. These materials were
heated to 1200.degree. C. to form a homogeneous glass frit. This glass
frit was then ground into a fine powder having a particle size of ten
microns or less. SnO.sub.2 (tin oxide) was then added to the glass frit in
a ratio of (75%) SnO.sub.2 +(25%) glass frit. No Ta.sub.2 O.sub.5 was
present in Example 1. The materials were well mixed with (30%) no carbon
residue screening agent to yield a resistive paint, which was subsequently
screened and fired at 900.degree. C. in an inert atmosphere as previously
disclosed. As shown in Table I, Example 1, the ohms/square was 60K; the
hot and cold TCR values were extremely high; the thermal stability was 2%;
and the STOL was 3%.
Example 2 comprises the glass frit of Example 1, wherein (2%) Ta.sub.2
O.sub.5 was added to the glass frit prior to heating. After heating and
grinding, SnO.sub.2 was added to the glass frit in a ratio of (75%)
SnO.sub.2 +(25%) glass frit having (2%) Ta.sub.2 O.sub.5 therein. These
materials were well mixed with (30%) no carbon residue, organic screening
agent to yield a resistive paint, which was subsequently screened and
fired at 900.degree. C. in an inert atmosphere, as previously disclosed.
As shown in FIG. 4, Example 2, the ohms/square was 31 k; the Cold TCR was
-2270; the Hot TCR was -1074; the thermal stability was 4%; and the STOL
was 4%.
Example 3 comprises the glass frit of Example 1, wherein (5%) Ta.sub.2
O.sub.5 was added to the glass frit prior to heating. After heating and
grinding, SnO.sub.2 was added to the glass frit in a ratio of (75%)
SnO.sub.2 +(25%) glass frit having (5%) Ta.sub.2 O.sub.5 therein. These
materials were well mixed with (30%) no carbon residue, screening agent to
yield a resistive paint, which was subsequently screened and fired at
900.degree. C. in an inert atmosphere, as previously disclosed. As shown
in Table I, Example 3, the ohms/square was 9K; the Cold TCR was -285; the
Hot TCR was -279; the thermal stability was 0.3%; and the STOL was 0.05%.
Example 4 comprises the glass frit of Example 1, wherein (20%) Ta.sub.2
O.sub.5 was added to the glass frit prior to heating. After heating and
grinding, SnO.sub.2 was added to the glass frit in the ratio of (75%)
SnO.sub.2 +(25%) glass frit having (20%) Ta.sub.2 O.sub.5 therein. These
materials were well mixed with (30%) no carbon residue, screening agent to
yield a resistive paint, which was subsequently screened and fired at
900.degree. C. in an inert atmosphere, as previously disclosed. As shown
in Table I, Example 4, the ohms/square was 15K; the Cold TCR was -183; the
Hot TCR was -187; the thermal stability was 0.3%; and the STOL was 0.01%.
As shown in Examples 1 through 4, the addition of Ta.sub.2 O.sub.5 to the
glass frit varies the TCR considerably. The addition of 5% Ta.sub.2
O.sub.5 to the glass frit as shown in Example 3 brings the TCR to less
than .+-.300 ppm/.degree.C., which is considered acceptable for most thick
film applications. The addition of 20% Ta.sub.2 O.sub.5 to the glass frit
as shown in Example 4, brings the TCR to less than .+-.200 ppm/.degree.C.
which is prefered.
Another significant improvement is in thermal stability and STOL. 2 to 4%
variation shown in Examples 1 and 2, is unacceptable for most
applications. However, increasing the content of Ta.sub.2 O.sub.5 to 5% or
more brings the thermal stability and STOL to 0.3% or less, which is
within a preferred range of less than 0.5% required for most stringent
thick film resistor applications.
The addition of Ta.sub.2 O.sub.5 to the glass frit to form a tantala glass
frit provides a more homogeneous and uniform paint, compared with the
admixture of Ta.sub.2 O.sub.5 particles, tin oxide, non-tantala glass
frit, and screening agent. Using the same percentage composition, the CV
(coefficient of variance) of tantala glass frit is 8 to 9% as compared to
13 to 14% for admixture compositions.
Any improvement in print technology and paint rheology will improve the CV
in equal proportion. That is, an improvement in print-technology and paint
rheology reducing admixture compositions to a CV of 10 to 11%, will also
reduce tantala glass frit resistors to a CV of 5 to 6%, which remains a
significant improvement.
Typically, thick film base metal resistors using tin oxide are fired at
1000.degree. C. or above (ref. U.S. Pat. Nos. 4,137,519 and 4,065,743) to
improve thermal stability. At lower temperatures the glass frit is not
well sintered, reducing thermal stability. By addition of 5% or more
Ta.sub.2 O.sub.5 to the glass frit, the frit is well sintered at
900.degree. C..+-.20.degree. C.
Using the formulation disclosed in Table I Example 4 above, and varying the
firing temperature in an inert atmosphere from 800.degree. C. to
1000.degree. C., as shown in FIGS. 1A and 1B, the most stable Hot and Cold
TCR occur at approximately 900.degree. C..+-.20.degree. C.
As shown in FIG. 1A, using a firing temperature of 900.degree.
C..+-.20.degree. C., both Hot and Cold TCR values are less than .+-.200
ppm/.degree.C., which is desirable for high reliability resistor
applications.
As shown in FIGS. 2A and 2B, SnO.sub.2 was preheated in a reducing
atmosphere of 7% H.sub.2 and 93% N.sub.2 at a temperature of 450.degree.
C., prior to mixing the SnO.sub.2 with the tantala glass frit of Example 4
in a ratio of 3:1, (75%) SnO.sub.2 +(25) tantala glass frit. These
materials were well mixed with (30%) of a no carbon residue, screening
agent to yield a resistive paint, which was subsequently screened up a
substrate and fired in an inert atmosphere at various temperatures from
800.degree. C. to 1000.degree. C. shown in FIGS. 2A and 2B. As shown in
FIG. 2A, the most stable Hot and Cold TCR values are obtained at a peak
firing temperature of 900.degree. C..+-.20.degree. C. These TCR values
fall within .+-.100 ppm/.degree.C., which is most desirable for the most
stringent resistor applications.
FIG. 3 shows the effect on sheet resistance and Hot and Cold TCR when the
ratios of SnO.sub.2 to tantala glass frit are varied, and the resulting
mixture is screened and fired in an inert atmosphere at a peak temperature
of 900.degree. C. At ratios between 3:1 and 4:1, the resulting Hot and
Cold TCR are less than .+-.200 ppm/.degree.C.; and the sheet resistances
in ohms/square are less than 10K.
FIG. 3 also shows the effect of screening through a 325 mesh screen in
solid line; and the effect of screening through a 165 mesh screen in
dashed line. Thus, it is noted that the larger the screen mesh size, the
lower the resistance values and the more positive the TCR. However, as
shown in FIG. 3, any screen size from 165 mesh to 325 mesh may be used to
yield a TCR withn .+-.200 ppm/.degree.C., when the ratio of SnO.sub.2 to
tantala glass frit is within the preferred range of from 3:1 to 4:1.
Thus, from Examples 1 through 4, and FIGS. 1A through 3 and Table I, it is
disclosed that the preferred quantity of tantala glass frit is from 20 to
25% and the preferred quantity of tin oxide is from 75 to 80%, for best
TCR results. If tin oxide is present in excess of 80%, the adhesion of the
resistor paint to the substrate is weakened, and thermal stability is
impaired. If more than 25% tantala glass frit is present, the TCR becomes
too negative.
Thus, a base metal resistor paint for firing on a substrate to form a
controlled temperature coefficient of resistance within +300
ppm/.degree.C. is disclosed for use with high reliaiblity, thick film
resistor applications.
Therefore, while this invention has been described with reference to a
particular embodiment, it is to be understood that modifications may be
made without departing from the spirit of the invention or from the scope
of the following claims.
Industrial Applicability
This invention discloses a base metal resistive paint for subsequent
screening and firing on a substrate to make a base metal thick film
resistor for use in an electronic circuit.
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