Thick film varistor and method of producing same

A glass-free thick film varistor operable at operating voltages ranging from about 30 to 200 volts per mm of active varistor material is produced by providing a screen-printable paste comprised of a non-glass containing substantially homogeneous mixture of granular varistor materials which have ZnO as the main component thereof and an organic binder, applying such paste in a desired pattern onto an insulating substrate and sintering such applied paste at relatively high temperatures so as to convert the paste into thick film varistors.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention relates to thick film varistors and somewhat more 
particularly to glass-free thick film varistors and a method of producing 
the same wherein varistor materials having ZnO as a main component thereof 
are admixed with an organic binder to form a varistor paste, which is 
applied onto an insulating substrate and then converted into a desired 
thick film varistor by sintering such paste. 
2. Prior Art 
Varistors are voltage-dependent impedence devices which must exhibit the 
highest possible impedence up to a specific voltage, the so-called 
varistor threshold voltage. In such devices, when the voltage is increased 
past the varistor threshold voltage, a steep conductivity rise occurs. The 
current-voltage characteristics of a varistor can be expressed by the 
following equation: 
EQU I=(V/C).sup.n 
wherein I is the current flowing through the varistor, V is the voltage 
applied across the varistor and the exponent n is a numerical value 
characterizing the so-called "steepness" of the varistor. The numerical 
value of such steepness, n, should be as high as possible, as this 
steepness determines the degree to which the varistor departs from general 
ohmic characteristics. 
Known varistors are normally produced as discrete components, typically by 
pressing and sintering pulverized varistor materials which have various 
main components, such as silicon carbide, silicon dioxide, selenium, etc. 
U.S. Pat. No. 3,725,836 suggests thick film varistors having a main 
component of ZnO and producing such varistors via thick layer techniques 
and thereby directly integrate such varistors into thick layer integrated 
circuits. In order to produce such known thick film varistors, which, as 
indicated above, belong to the family of ZnO-varistors, the varistor 
materials are mixed with glass frit and an organic binding agent to form a 
screen-printable varistor paste and applied via screen printing techniques 
onto an insulating substrate, which is then subjected to sintering 
conditions in order to form the desired varistors. Electrodes required for 
contacting such varistor can also be mounted or applied on the surface of 
the varistor via thick layer techniques. The steepness, n, of thick film 
varistors produced in this manner has a magnitude ranging between 4 and 8, 
which is too low for most applications. 
SUMMARY OF THE INVENTION 
The invention provides a glass-free thick film varistor having improved 
steepness values, n, in relation to prior art glass-containing varistors 
and provides a method of producing such improved thick film varistors. 
In accordance with the principles of the invention, a thick film varistor 
consisting essentially of a glass-free thick film having finely divided 
particles of varistor materials with ZnO as a main component thereof is 
produced by admixing such varistor materials with an organic binder to 
form a screen-printable varistor paste, screen-printing such varistor 
paste into desired patterns onto an insulating substrate and converting 
such paste patterns into thick film varistors. 
In an exemplary preferred embodiment of the invention, a glass-free thick 
film varistor operable at a relatively high operating voltage of about 200 
V/mm of active varistor material is provided and consists essentially of a 
substantially homogeneous glass-free mixture containing, on a 100% by 
weight solid material bases, about 87.5 to 98.0% by weight ZnO, about 1.0 
to 5.0% by weight of Bi.sub.2 O.sub.3, about 0.3 to 2.0% by weight of 
Sb.sub.2 O.sub.3, about 0.2 to 1.0% by weight of Cr.sub.2 O.sub.3, about 
0.5 to 3.5% by weight of Co.sub.2 O.sub.3 and about 0.1 to 1.0% by weight 
of MgO.sub.2. 
In another exemplary preferred embodiment of the invention, a glass-free 
thick film varistor operable at a relatively low operating voltage of 
about 30 V/mm of active varistor material is provided and consists 
essentially of a glass-free mixture containing, on a 100% by weight solid 
material bases, about 87.5 to 96.5% by weight ZnO, about 2.0 to 7.0% by 
weight of Bi.sub.2 O.sub.3, about 0.2 to 1.0% by weight of Co.sub.2 
O.sub.3, about 0.1 to 0.5% by weight of SnO.sub.2 and about 1.0 to 3.0% by 
weight of TiO.sub.2. 
Glass frit is typically employed as a binding agent in known conductor path 
pastes, impedence pastes and in known varistor pastes utilized with thick 
layer techniques. During sintering of such glass frit-containing pastes, 
the glass frit forms a solid glass matrix which guarantees cohesion of 
other solid materials admixed with such frits and insures adhesion of the 
overall paste to the substrate. In accordance with the principles of the 
present invention, it has now been discovered that even without the 
presence of glass frit, a strong cohesion of solid materials is obtained 
and a proper adhesion to the substrate is obtained when a glass-free 
varistor paste having zinc oxide as a main component thereof is utilized. 
The electrical properties of a finished thick film varistor produced in 
accordance with the principles of the invention are considerably improved 
by the absence of glass frit, for example, the steepness of varistors 
produced in accordance with the principles of the invention may have a 
number value of the exponent n of above 20. 
In attaining varistors of the invention having desired electrical 
properties, it is particularly advantageous to utilize varistor pastes 
having, on a 100% by weight solid material bases, about 87.5 to 98.0% by 
weight of zinc oxide. Further, varistor pastes utilized in the practice of 
the invention advantageously contain, on a 100% by weight solid materials 
bases, about 1.0 to 7.0% by weight bismuth oxide, about 0.2 to 3.5% by 
weight of cobaltic oxide and about 0.1 to 1.0% by weight of manganese 
dioxide. The addition of the foregoing oxides to the zinc oxide appears to 
facilitate crystal formation during the production of thick film varistors 
and thus leads to additional improvements in the electrical properties of 
such varistors. Varistors containing the above oxides also include an 
oxide selected from the group consisting of antimony trioxide, chromic 
oxide, stannic oxide and titanium dioxide. 
In the practice of the invention, it is preferable to sinter the select 
glass-free varistor paste at a temperature ranging between about 
1100.degree. to 1360.degree. C. and so that a peak temperature during the 
sintering process is maintained for a period of time ranging between about 
5 and 20 minutes. The threshold or actuation voltage of the resulting 
thick film varistors can be influenced by the proper choice of sintering 
temperature. Further, additional improvements in crystal formation within 
thick film varistors of the invention and thus additional improvements in 
the electrical properties of such varistors, can be obtained by 
controllably cooling the thick film varistors so that, after sintering, a 
temperature drop ranging between about 2.degree. to 8.degree. C./min. 
occurs. 
DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In accordance with the principles of the invention, an improved thick film 
varistor and method of producing the same are attained by providing a 
screen-printable varistor paste comprising essentially of a glass-free 
homogeneous mixture of particulate varistor material having ZnO as a main 
component thereof and an organic binder, applying such paste in select 
patterns onto an insulating substrate and converting such paste patterns 
into thick film varistors via sintering. Conductor paths or electrodes may 
be applied or connected with such varistor via conventional techniques. 
Varistor pastes produced in accordance with the principles of the invention 
are applied as layers on insulating substrates and have a thickness, after 
sintering, ranging between about 100 and 200 .mu.m. 
Since all other known thick layer processes occur in a temperature range of 
approximately 500.degree. C. to 1000.degree. C., it is necessary that the 
thick film varistors of the invention be produced before other thick layer 
elements, such as conductor paths, impedences, etc. Accordingly, conductor 
paths, electrodes, etc. for contacting select thick film varistors of the 
invention may be printed or otherwise applied after conversion of the 
varistor pastes into thick film varistors is completed. 
In accordance with the principles of the invention, thick layer circuits 
having integrated thick film varistors of the invention are readily 
produced. Further, it is also possible to produce thick film varistors of 
the invention as discrete components. In such process, for example, a 
multiplicity of select varistor paste patterns are applied via a screen 
printing technique onto an insulating substrate and sintered to convert 
such paste patterns into discrete varistor elements. Conductor paths for 
contacting such discrete varistor elements are subsequently applied, as by 
a screen printing technique and dried and then sintered at suitable 
temperatures. Such substrate may then be perforated, for example with a 
laser, to separate the resultant elements into discrete electrical 
elements. These individual elements, so-called varistor chips, may then be 
soldered into select printed circuits or film circuits.

With the foregoing general discussion in mind, there is presented detailed 
examples which will illustrate to those skilled in the art the manner in 
which the invention is carried out. However, the examples are not to be 
construed as limiting the scope of the invention in any way. 
EXAMPLE I 
A glass-free thick film varistor operable at relatively high operating 
voltages was produced by providing a screen-printable varistor paste 
containing particulate varistor materials, which were weighed-in at the 
following amounts: 
ZnO: 76.66 gr. 
Bi.sub.2 O.sub.3 : 2.33 gr. 
Sb.sub.2 O.sub.3 : 1.46 gr. 
Cr.sub.2 O.sub.3 : 0.38 gr. 
Co.sub.2 O.sub.3 : 2.48 gr. 
MnO.sub.2 : 0.26 gr. 
After the weighing of these solid materials, they were admixed with water 
and milled for about 18 hours in a ball milling means. The resultant mass 
was subsequently freed from water via suction filters and dried in an oven 
at a temperature of about 150.degree. C. for about 24 hours. The average 
maximum grain diameter of the resultant powder mixture was about 1 .mu.m. 
One hundred grams of the above-prepared powder mixture was admixed with 75 
grams of an organic binder comprising a solution containing about 10% 
ethylene cellulose in 90% terpinol-isomer compound, which is typically 
used in thick layer techniques. This admixture was placed in a milling 
means and homogenized. Other known organic binding agents, such as, for 
example, a solution consisting of nitrocellulose in butyl carbitol acetate 
may be utilized in place of the above-identified binding agent. After 
homogenization, the viscosity and flow behavior of the varistor paste was 
adjusted so that it could be processed in a screen printing technique. The 
so-attained varistor paste was then printed on an insulating substrate 
consisting of Al.sub.2 O.sub.3 -ceramic via screen printing techniques at 
locations thereof designated for varistors. The varistor paste was applied 
as a layer having a thickness of approximately 150 .mu.m, and after 
application, was dried in an oven at a temperature of approximately 
60.degree. C. Thereafter, such applied varistor paste was converted into a 
varistor by sintering in an oxidizing atmosphere at a temperature ranging 
between 1100.degree. to 1200.degree. C. and the peak temperature during 
such sintering process was maintained for about 10 minutes. The average 
temperature rise during heating up was about 10.degree. C. per minute. 
After the sintering was completed, varistor was controllably cooled at a 
temperature drop of about 7.degree. C. per minute. During the sintering 
process, the solid materials within the varistor pastes were bound 
together into a solid mass and onto the substrate and the desired varistor 
properties were formed. 
Electrodes, based on gold-platinum, were applied on the resultant thick 
film varistor, which had a thickness of about 130 .mu.m, in a conventional 
thick layer technique. The thick film varistor produced in this manner had 
a steepness value, n, of 25 and was particularly suited for operating 
voltages in the range of about 200 volts per millimeter of active varistor 
material. 
EXAMPLE II 
A glass-free thick film varistor operable at relatively low operating 
voltages was produced by providing a screen-printable varistor paste 
containing particulate varistor materials, which were weighed-in at the 
following amounts: 
ZnO: 77.23 gr. 
Bi.sub.2 O.sub.3 : 4.66 gr. 
Co.sub.2 O.sub.3 : 0.415 gr. 
MnO.sub.2 : 0.435 gr. 
TiO.sub.2 : 1.598 gr. 
SnO.sub.2 : 0.151 gr. 
After the weigh-in, the powdered discrete varistor materials were then 
processed into a screen printable-varistor paste in the manner described 
in Example I and were printed via screen printing techniques onto an 
Al.sub.2 O.sub.3 -ceramic insulating substrate. The thickness of the 
varistor paste applied as a layer on such substrate was approximately 150 
.mu.m, after drying at a temperature of about 60.degree. C. Thereafter, 
such a layer was subjected to sintering at a temperature ranging between 
about 1100.degree. to 1200.degree. C., during which the peak temperature 
was held for about 10 minutes. Again, during the heating up for the 
sintering, the temperature rise amounted to about 10.degree. C. per minute 
whereas during the cooling process, at least to a temperature of about 
1000.degree. C., a temperature drop of 3.degree. C. per minute was 
maintained and below 1000.degree. C., a temperature drop of 6.degree. to 
7.degree. C. per minute was maintained. 
After cooling the foregoing thick film varistors, gold-platinum electrodes 
were applied in a known manner and the resultant thick film varistor had a 
thickness of 130 .mu.m and upon analysis exhibited a steepness value, n, 
of 25. Such thick film varistors are especially useful for operating 
voltages in the range of about 30 volts per millimeter of active varistor 
material. 
The foregoing is considered as illustrative only of the principles of the 
invention. Further, since numerous modifications and changes will readily 
occur to those skilled in the art, it is not desired to limit the 
invention to the exact compositions, processes and operations shown and 
described, and accordingly, all suitable modifications and equivalents may 
be resorted to, falling within the scope of the invention as claimed.