U.S. patent application number 10/651367 was filed with the patent office on 2004-05-27 for co-fired capacitor and method for forming ceramic capacitors for use in printed wiring boards.
Invention is credited to Borland, William J..
Application Number | 20040099999 10/651367 |
Document ID | / |
Family ID | 32030982 |
Filed Date | 2004-05-27 |
United States Patent
Application |
20040099999 |
Kind Code |
A1 |
Borland, William J. |
May 27, 2004 |
Co-fired capacitor and method for forming ceramic capacitors for
use in printed wiring boards
Abstract
A capacitor structure is fabricated by forming a pattern of
first dielectrics over a foil, forming first electrodes over the
first dielectrics, and co-firing the first dielectrics and the
first electrodes. Co-firing of the dielectrics and the electrodes
alleviates cracking caused by differences in thermal coefficient of
expansion (TCE) between the electrodes and the dielectrics.
Co-firing also ensures a strong bond between the dielectrics and
the electrodes. In addition, co-firing allows multi-layer capacitor
structures to be constructed, and allows the capacitor electrodes
to be formed from copper.
Inventors: |
Borland, William J.; (Cary,
NC) |
Correspondence
Address: |
E I DU PONT DE NEMOURS AND COMPANY
LEGAL PATENT RECORDS CENTER
BARLEY MILL PLAZA 25/1128
4417 LANCASTER PIKE
WILMINGTON
DE
19805
US
|
Family ID: |
32030982 |
Appl. No.: |
10/651367 |
Filed: |
August 29, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60418045 |
Oct 11, 2002 |
|
|
|
Current U.S.
Class: |
264/618 ; 29/832;
361/321.2 |
Current CPC
Class: |
H05K 2201/09509
20130101; H05K 3/4623 20130101; H05K 1/092 20130101; H05K 3/4611
20130101; Y10T 29/4913 20150115; H05K 2201/09763 20130101; H05K
3/429 20130101; H01G 4/01 20130101; H05K 2201/09718 20130101; H05K
1/162 20130101; H05K 2201/0355 20130101 |
Class at
Publication: |
264/618 ;
361/321.2; 029/832 |
International
Class: |
B28B 001/00; C04B
033/32; H01G 004/06; H05K 003/30 |
Claims
What is claimed is:
1. A method for making a fired-on-foil capacitor structure,
comprising: providing a metallic foil; forming at least one first
dielectric over the foil; forming at least one first electrode over
the first dielectric; and co-firing the first dielectric and the
first electrode.
2. The method of claim 1, wherein forming the first electrode
comprises: forming the first electrode comprising a metal, wherein
the metallic foil also comprises the metal.
3. The method of claim 2, wherein the metal is copper.
4. The method of claim 1, comprising: forming at least one second
dielectric over the first electrode; and forming at least one
second electrode over the second dielectric.
5. The method of claim 4, comprising: co-firing the second
dielectric and the second electrode.
6. The method of claim 4, wherein co-firing comprises: co-firing
the second dielectric and the second electrode along with the first
dielectric and the first electrode.
7. The method of claim 4, comprising: forming a trench in the foil
to electrically isolate the first and second electrodes.
8. The method of claim 4, comprising: laminating a side of the foil
containing the first dielectric and the first electrode.
9. The method of claim 4, comprising: forming at least one third
dielectric over the second electrode; and forming at least one
third electrode over the third dielectric.
10. The method of claim 9, comprising: co-firing the third
dielectric and the third electrode.
11. The method of claim 9, wherein co-firing comprises: co-firing
the third dielectric and the third electrode along with the first
and second dielectrics and the first and second electrodes.
12. The method of claim 9, comprising: forming a trench in the foil
to electrically isolate the first and third electrodes from the
second electrode.
13. The method of claim 1, wherein providing a foil comprises:
treating the foil with an underprint; and firing at a temperature
below the softening point of the foil.
14. The method of claim 1, wherein forming the first dielectric
comprises: screen-printing a layer of dielectric ink over the foil;
and drying the dielectric ink.
15. The method of claim 14, wherein forming the first electrode
comprises: screen-printing a layer of metallic ink over the first
dielectric; and drying the metallic ink.
16. The method of claim 1, wherein: forming at least one first
dielectric comprises forming a pattern of a plurality of first
dielectrics over the foil; and forming at least one first electrode
comprises forming a pattern of a plurality of first electrodes over
the first dielectrics.
17. A capacitor structure, comprising: a metallic foil; at least
one dielectric disposed over the foil; at least one first electrode
disposed over a portion of the dielectric; and at least one second
electrode disposed over a portion of the dielectric and over a
portion of the first electrode, wherein a portion of the dielectric
is disposed between the first and second electrodes.
18. The capacitor structure of claim 17, wherein the foil
comprises: a trench that electrically isolates the first electrode
from the second electrode.
19. The capacitor structure of claim 17, wherein: the at least one
dielectric comprises a plurality of dielectrics; the at least one
first electrode comprises a plurality of first electrodes; the at
least one second electrode comprises a plurality of second
electrodes; and the dielectrics, the first electrodes, and the
second electrodes are arranged as a plurality of stacks on the
metallic foil, each stack comprising a first electrode, a second
electrode, and a dielectric.
20. The capacitor structure of claim 19, comprising: a plurality of
third electrodes, one in each stack, wherein each third electrode
is disposed over a portion of a corresponding dielectric and over a
portion of a corresponding first electrode and is electrically
connected to the corresponding first electrode.
21. The capacitor structure of claim 19, wherein the metallic foil
and the first and second electrodes comprise copper.
Description
BACKGROUND
[0001] 1. Technical Field
[0002] The technical field is ceramic capacitors. More
particularly, the technical field includes co-fired ceramic
capacitors that may be embedded in printed wiring boards.
[0003] 2. Background Art
[0004] Passive circuit components embedded in printed wiring boards
formed by fired-on-foil technology are known. Known components are
separately fired-on-foil. "Separately fired-on-foil" capacitors are
formed by depositing a thick-film dielectric material layer onto a
metallic foil substrate and firing under thick-film firing
conditions, and subsequently depositing a top electrode material
over the thick-film dielectric material layer. U.S. Pat. No.
6,317,023 B1 to Felten discloses such a process.
[0005] The thick-film dielectric material should have a high
dielectric constant (K) after firing. A high K thick-film
dielectric is formed by mixing a high dielectric constant K powder
(the "functional phase") with a glass powder and dispersing the
mixture into a thick-film screen-printing vehicle. High K glasses
can be wholly or partially crystalline, depending on their
composition and the amount of high K crystal they precipitate.
These glasses are often termed "glass-ceramics."
[0006] During firing of the thick-film dielectric material, the
glass component of the dielectric material softens and flows before
the peak firing temperature is reached, coalesces, encapsulates the
functional phase, and subsequently crystallizes, forming the
glass-ceramic. The glass-ceramic, however, does not re-soften and
flow on subsequent firings, and its surface is often difficult to
adhere to.
[0007] Silver and silver-palladium alloys are preferred metals for
forming capacitor electrodes because of their relatively small
differences in thermal coefficient of expansion (TCE) from the
dielectrics used in fired-on-foil capacitors. Small TCE differences
result in low stress in the electrode upon cooling from peak firing
temperatures. However, silver and silver-containing alloys may be
undesirable in some applications because of the possibility of
silver migration. In addition, the relatively low melting points of
silver and silver alloys preclude their use at higher firing
temperatures.
[0008] Copper is a preferred material for forming electrodes, but
the large TCE differences between copper and thick-film capacitor
dielectrics lead to post-firing stresses in the electrodes. The
stresses result in electrode cracking. In addition, because
pre-fired glass ceramics do not re-soften and flow on subsequent
firings, a copper electrode fired on a pre-fired glass-ceramic
surface may not adhere well to the glass-ceramic. The electrode may
therefore separate from the dielectric. Both cracking and
separation result in high dissipation factors.
SUMMARY
[0009] According to a first embodiment, a method for making a
fired-on-foil ceramic capacitor structure comprises forming first
dielectrics over a metallic foil, forming first electrodes over the
first dielectrics, and co-firing the first dielectrics and the
first electrodes. In the first embodiment, cracking and separation
of the electrode from the dielectric caused by differences in
thermal coefficient of expansion (TCE) between the electrodes and
the dielectrics is avoided by co-firing the electrodes and the
dielectrics. Alleviation of the TCE problem also allows the use of
preferred materials, such as copper, to form the electrodes.
[0010] According to a second embodiment, a two-layer capacitor
structure comprises a metallic foil, dielectrics disposed over the
foil, first electrodes disposed over the first dielectrics, and
second electrodes disposed over the dielectrics and over the first
electrodes. In the second embodiment, the capacitance density of
the capacitor structure is increased because of the additional
dielectric/electrode layer. Additional layers may also be added,
further increasing capacitance density. Also according to the
second embodiment, the capacitor structure may comprise a copper
foil and copper electrodes.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] The detailed description will refer to the following
drawings, wherein like numerals refer to like elements, and
wherein:
[0012] FIGS. 1A to 1D schematically illustrate steps in
manufacturing a first embodiment of a capacitor structure shown in
front elevation;
[0013] FIG. 1E is a top plan view of the first capacitor structure
embodiment;
[0014] FIGS. 2A to 2J schematically illustrate steps in
manufacturing a second embodiment of a capacitor structure shown in
front elevation;
[0015] FIG. 2K is a top plan view of the second capacitor structure
embodiment; and
[0016] FIG. 3 illustrates a third embodiment of a capacitor
structure.
DETAILED DESCRIPTION
[0017] FIGS. 1A-1D illustrate a general method of manufacturing a
capacitor structure 100 (FIG. 1E) having a single-layer capacitor
on metallic foil design. FIG. 1E is a plan view of the finished
capacitor structure 100. Specific examples of the capacitor
structure 100 are also described below.
[0018] FIG. 1A is a side elevational view of first stage of
manufacturing the capacitor structure 100. In FIG. 1A, a metallic
foil 110 is provided. The foil 110 may be of a type generally
available in the industry. For example, the foil 110 may be copper,
copper-invar-copper, invar, nickel, nickel-coated copper, or other
metals that have melting points in excess of the firing temperature
for thick film pastes. Preferred foils include foils comprised
predominantly of copper, such as reverse treated copper foils,
double-treated copper foils, and other copper foils commonly used
in the multilayer printed circuit board industry. The thickness of
the foil 110 may be in the range of, for example, about 1-100
microns, preferably 3-75 microns, and most preferably 12-36
microns, corresponding to between about 1/3 oz and 1 oz copper
foil.
[0019] The foil 110 may be pretreated by applying an underprint 112
to the foil 110. The underprint 112 is a relatively thin layer
applied to a component-side surface of the foil 110. In FIG. 1A,
the underprint 112 is indicated as a surface coating on the foil
110. The underprint 112 adheres well to the metal foil 110 and to
layers deposited over the underprint 112. The underprint 112 may be
formed, for example, from a paste applied to the foil 110, and is
then fired at a temperature below the softening point of the foil
110. The paste may be printed as an open coating over the entire
surface of the foil 110, or printed on selected areas of the foil
110. It is generally more economical to print the underprint paste
over selected areas of the foil. When a copper foil 110 is used in
conjunction with a copper underprint 112, glass in the copper
underprint paste retards oxidative corrosion of the copper foil
110, and it may therefore be preferable to coat the entire surface
of the foil 110 if oxygen-doped firing is utilized.
[0020] In FIG. 1A, a dielectric material is screen-printed onto the
pretreated foil 110, forming a first dielectric layer 120. The
dielectric material may be, for example, a thick-film dielectric
ink. The dielectric ink may be formed of, for example, a paste. The
first dielectric layer 120 is then dried. In FIG. 1B, a second
dielectric layer 125 is then applied, and dried. In an alternative
embodiment, a single layer of dielectric material may be deposited
through a coarser mesh screen to provide an equivalent thickness in
one printing.
[0021] In FIG. 1C, an electrode 130 is formed over the second
dielectric layer 125 and dried. The electrode 130 can be formed by,
for example, screen-printing a thick-film metallic ink. In general,
the surface area of the dielectric layer 125 should be larger than
that of the electrode 130.
[0022] The first dielectric layer 120, the second dielectric layer
125, and the electrode 130 are then co-fired. The thick-film
dielectric layers 120, 125 may be formed of, for example, a high
dielectric constant functional phase such as barium titanate and a
dielectric property-modifying additive such as zirconium dioxide,
mixed with a glass-ceramic frit phase. During co-firing, the
glass-ceramic frit phase softens, wets the functional and additive
phases and coalesces to create a dispersion of the functional phase
and the modifying additive in a glass-ceramic matrix. At the same
time, the copper electrode powders of the layer 130 are wetted by
the softened glass-ceramic frit phase and sinter together to form a
solid electrode. The layer 130 has a strong bond to the high K
dielectric 128 that results from the co-firing. The post-fired
structure is shown in front elevation in FIG. 1D.
[0023] FIG. 1E is a plan view of the finished capacitor structure
100. In FIG. 1E, four dielectric/electrode stacks 140 on the foil
110 are illustrated. Any number of stacks 140, in various patterns,
however, can be arranged on a foil 110 to form the capacitor
structure 100.
[0024] Examples 1-3 illustrate particular materials and processes
used in practicing the general method illustrated by FIGS.
1A-1E.
[0025] FIGS. 2A-2J illustrate a method of manufacturing a capacitor
structure 200 having a double-layer capacitor on metallic foil
design. FIG. 2K is a plan view of the finished capacitor structure
200.
[0026] FIG. 2A is a front elevational view of first stage of
manufacturing the capacitor structure 200. In FIG. 2A, a metallic
foil 210 is provided. The foil 210 may be pretreated by applying
and firing an underprint 212, as discussed above with reference to
FIG. 1A. A dielectric material is screen-printed onto the
pretreated foil 210, forming a first dielectric layer 220. The
first dielectric layer 220 is then dried.
[0027] In FIG. 2B, a second dielectric layer 225 is then applied,
and dried. A single layer of dielectric material may alternatively
be used.
[0028] In FIG. 2C, a first electrode 230 is formed over the second
dielectric layer 225 and dried. The first electrode may be formed
by, for example, screen-printing a thick-film metallic ink. The
first electrode 230 is formed to extend to contact the foil
210.
[0029] The first dielectric layer 220, the second dielectric layer
225, and the first electrode 230 are then co-fired. The dielectric
layers 220, 225 may have similar compositions to the materials
discussed above with reference to FIGS. 1A-1E, and the co-firing
process imparts the advantages of adhesion and defect-free
processing discussed above. A resulting dielectric 228 is formed
from the co-firing step, as shown in FIG. 2D.
[0030] In FIG. 2E, a third layer of dielectric material is
screen-printed onto the co-fired structure of FIG. 2D, forming a
third dielectric layer 240. The third dielectric layer 240 is then
dried. In FIG. 2F, a fourth dielectric layer 245 is applied and
dried. A single layer of dielectric material may alternatively be
used.
[0031] In FIG. 2G, a second electrode 250 is formed over the fourth
dielectric layer 245 and dried. The second electrode 250 extends to
contact the foil 210. The structure is then co-fired. FIG. 2H
illustrates the structure after co-firing, with the resulting
dielectric 260 and dielectric/electrode stack 265. After co-firing,
the dielectric 260 securely adheres to both electrodes 230, 250,
and the electrodes 230, 250 are crack-free.
[0032] As an alternative to two separate firing steps as discussed
with reference to FIGS. 2D and 2H, a single co-firing can be
performed after forming the second electrode 250. A single
co-firing is advantageous in that production costs are reduced. Two
separate firings, however, allow inspection of the first electrode
230 for defects such as cracks and for printing alignment issues
after the first firing.
[0033] In FIG. 21, the structure may be inverted and laminated. For
example, the component face of the foil 210 can be laminated with
laminate material 270. The lamination can be performed, for
example, using FR4 prepreg in standard printing wiring board
processes. In one embodiment, 106 epoxy prepreg may be used.
Suitable lamination conditions are 185.degree. C. at 208 psi for 1
hour in a vacuum chamber evacuated to 28 inches of mercury. A
silicone rubber press pad and a smooth PTFE filled glass release
sheet may be in contact with the foil 210 to prevent the epoxy from
gluing the lamination plates together. A foil 280 may be applied to
the laminate material 270 to provide a surface for creating
circuitry. The embodiments of the capacitor structure 100 discussed
above with reference to FIG. 1E may also be laminated in this
manner. The dielectric prepreg and laminate materials can be any
type of dielectric material such as, for example, standard epoxy,
high Tg epoxy, polyimide, polytetrafluoroethylene, cyanate ester
resins, filled resin systems, BT epoxy, and other resins and
laminates that provide insulation between circuit layers.
[0034] Referring to FIG. 2J, after lamination, a photo-resist is
applied to the foil 210 and the foil 210 is imaged, etched and
stripped using standard printing wiring board processing
conditions. The etching produces a trench 215 in the foil 210,
which breaks electrical contact between the first electrode 230 and
the second electrode 250. FIG. 2K is a top plan view of the
completed capacitor structure 200. A section 216 of the foil 210 is
one electrode of the resulting capacitor structure 200, and may be
connected to other circuitry by a conductive trace 218. A section
227 is coupled to the second electrode 230 and may be connected to
other circuitry by a conductive trace 219.
[0035] The capacitor structure 200 discussed above has high
capacitance density due to its two-layer capacitor structure. In
addition, the capacitor structure 200 can be produced crack-free by
co-firing of the dielectric layers and the electrodes.
[0036] FIG. 3 illustrates a third embodiment of a capacitor
structure. The capacitor structure 300 is a three-layer embodiment
having a high capacitance density. The capacitor structure 300
comprises a foil 310 and a plurality of dielectric/electrode stacks
365 (only one stack 365 is illustrated). The dielectric/electrode
stack 365 include a first electrode 330 and a second electrode 350
separated by a dielectric 360, similar to the first and second
electrodes 230, 250 of the capacitor structure 200 discussed above.
Each dielectric/electrode stack 365 also has a third electrode 335
formed over the dielectric 360. A trench 315 breaks electrical
contact of a portion 316 of the foil 310 and the electrode 350,
from a portion 317 of the foil 310, the first electrode 330, and
the third electrode 335. A laminate material 370 and a second foil
380 may be included in the capacitor structure 300.
[0037] The capacitor structure 300 can be manufactured in a manner
similar to the capacitor structure 200. The third layer portion of
the dielectrics 360 in the stacks 365 may be formed from one or
more dielectric ink layers, as discussed above, and the electrodes
335 can be formed over the dielectrics 360.
[0038] The dielectric/electrode stacks 365 can be co-fired in three
individual steps, or in a single step. Firing of each
electrode/dielectric layer allows inspection of the product for
defects. A single firing, however, reduces the cost of producing
the capacitor structure 300.
[0039] The additional layer in the dielectric/electrode stacks 365
provides a high capacitance density for the capacitor structure
300. Co-firing of the dielectric layers and the electrode provides
a low dissipation factor and crack-free structure.
[0040] In other embodiments, four or more layer capacitor
structures can be produced by alternatively forming dielectric and
electrode layers, and co-firing the layers.
[0041] In the embodiments discussed in this specification, the term
"paste" may correspond to a conventional term used in the
electronic materials industry, and generally refers to a thick-film
composition. Typically, the metal component of the underprint paste
is matched to the metal in the metal foil. For example, if a copper
foil were used, then a copper paste could be used as the
underprint. Examples of other applications would be pairing silver
and nickel foils with a similar metal underprint paste. Thick film
pastes may be used to form both the underprint and the passive
components.
[0042] Generally, thick-film pastes comprise finely divided
particles of ceramic, glass, metal or other solids dispersed in
polymers dissolved in a mixture of plasticizer, dispersing agent
and organic solvent. Preferred capacitor pastes for use on copper
foil have an organic vehicle with good burnout in a nitrogen
atmosphere. Such vehicles generally contain very small amounts of
resin, such as high molecular weight ethyl cellulose, where only
small amounts are necessary to generate a viscosity suitable for
screen-printing. Additionally, an oxidizing component such as
barium nitrate powder, blended into the dielectric powder mixture,
helps the organic component burn out in the nitrogen atmosphere.
Solids are mixed with an essentially inert liquid medium (the
"vehicle"), then dispersed on a three-roll mill to form a
paste-like composition suitable for screen-printing. Any
essentially inert liquid may be used as the vehicle. For example,
various organic liquids, with or without thickening and/or
stabilizing agents and/or other common additives, may be used as
the vehicle.
[0043] High K thick-film dielectric pastes generally contain at
least one high K functional phase powder and at least one glass
powder dispersed in a vehicle system composed of at least one resin
and a solvent. The vehicle system is designed to be screen-printed
to provide a dense and spatially well-defined film. The high K
functional phase powders can comprise perovskite-type ferroelectric
compositions with the general formula ABO.sub.3. Examples of such
compositions include BaTiO.sub.3; SrTiO.sub.3; PbTiO.sub.3;
CaTiO.sub.3; PbZrO.sub.3; BaZrO.sub.3 and SrZrO.sub.3 Other
compositions are also possible by substitution of alternative
elements into the A and/or B position, such as
Pb(Mg.sub.1/3Nb.sub.2/3)O.sub.3 and
Pb(Zn.sub.1/3Nb.sub.2/3)O.sub.3. TiO.sub.2 and
SrBi.sub.2Ta.sub.2O.sub.9 are other possible high K materials.
[0044] Doped and mixed metal versions of the above compositions are
also suitable. Doping and mixing is done primarily to achieve the
necessary end-use property specifications such as, for example, the
necessary temperature coefficient of capacitance (TCC) in order for
the material to meet industry definitions, such as "X7R" or "Z5U"
standards.
[0045] The glasses in the pastes can be, for example, Ca--Al
borosilicates, Pb--Ba borosilicates, Mg--Al silicates, rare earth
borates, and other similar glass compositions. High K glass-ceramic
powders, such as lead germanate (Pb.sub.5Ge.sub.3O.sub.11), are
preferred.
[0046] Pastes used to form the electrode layers may be based on
metallic powders of either copper, nickel, silver,
silver-containing precious metal compositions, or mixtures of these
compounds. Copper powder compositions are preferred.
[0047] The capacitor structure embodiments described in this
specification have many applications. For example, the capacitor
structure embodiments can be used within organic printed circuit
boards, IC packages, applications of said structures in decoupling
applications, and devices such as IC modules or handheld device
motherboards.
[0048] In the above embodiments, the electrode layers are described
as formed by screen-printing. Other methods, however, such as
deposition by sputtering or evaporation of electrode metals onto
the dielectric layer surface may also be used.
[0049] The foregoing description of the invention illustrates and
describes the present invention. Additionally, the disclosure shows
and describes only the preferred embodiments of the invention, but
it is to be understood that the invention is capable of use in
various other combinations, modifications, and environments and is
capable of changes or modifications within the scope of the
inventive concept as expressed herein, commensurate with the above
teachings, and/or the skill or knowledge of the relevant art. The
embodiments described hereinabove are further intended to explain
best modes known of practicing the invention and to enable others
skilled in the art to utilize the invention in such, or other,
embodiments and with the various modifications required by the
particular applications or uses of the invention. Accordingly, the
description is not intended to limit the invention to the form
disclosed herein. Also, it is intended that the appended claims be
construed to include alternative embodiments.
EXAMPLES
Example 1
[0050] Referring to FIGS. 1A-1E, a specific embodiment of the
capacitor structure 100 was described. In this embodiment, the foil
110 was a copper foil. The type of copper foil 110 can be any
commercial grade of copper foil used in the printed wiring board
industry, and may be in the range of 1/3 oz copper foil
(approximately 12 microns thickness) to 1 oz copper foil
(approximately 36 microns thickness). The copper foil 110 was
pretreated by applying a copper underprint paste over selected
areas of the foil 110. The resulting product was then fired in
nitrogen at 900.degree. C. for 10 minutes at peak temperature, with
a total cycle time of approximately 1 hour, forming the underprint
112.
[0051] In FIG. 1B, a thick-film dielectric ink was screen-printed
onto the pretreated copper foil 110 through 400 mesh screen to
create a pattern of 1/2 inch by 1/2 inch first dielectric layers
120. The wet printed thickness of the first dielectric layers 120
is approximately 12-15 microns. The first dielectric layers 120
were dried at 125.degree. C. for approximately 10 minutes, and
second dielectric layers 125 were applied by screen-printing,
followed by another drying step at 125.degree. C. The thick-film
dielectric ink included a barium titanate component, a zirconium
oxide component, and a glass-ceramic phase.
[0052] Referring to FIG. 1C, thick-film copper electrode ink layers
130 was printed through 400 mesh screens onto the dielectric
squares 120, and dried at 125.degree. C. for approximately 10
minutes to form a 0.9 cm by 0.9 cm square electrode. In general,
the printed electrode 130 thickness was limited only by the need
for a pinhole-free film, and was typically in the range of 3 to 15
microns. The resulting structure was co-fired to 900.degree. C. for
10 minutes at peak temperature using a thick film nitrogen profile.
The nitrogen profile included less than 50 ppm oxygen in the
burnout zone, and 2-10 ppm oxygen in the firing zone, with a total
cycle time of 1 hour. Co-firing resulted in the
dielectric/electrode stacks 140 illustrated in FIG. 1E.
[0053] In this example, the thick film dielectric material had the
following composition:
1 Barium titanate powder 64.18% Zirconium oxide powder 3.78% Glass
A 11.63% Ethyl cellulose 0.86% Texanol 18.21% Barium nitrate powder
0.84% Phosphate wetting agent 0.5%. Glass A comprised: Germanium
oxide 21.5% Lead tetraoxide 78.5%.
[0054] The Glass A composition corresponded to
Pb.sub.5Ge.sub.3O.sub.11, which precipitated out during the firing,
and had a dielectric constant of approximately 70-150. The thick
film copper electrode ink comprised:
2 Copper powder 55.1% Glass A 1.6% Cuprous oxide powder 5.6% Ethyl
cellulose T-200 1.7% Texanol 36.0%.
[0055] After firing, the capacitor structure was crack free and had
the following electrical characteristics:
3 capacitance density approximately 150 nF/in.sup.2 dissipation
factor approximately 1.5% insulation resistance >5 .times.
10.sup.9 Ohms breakdown voltage approximately 800 volts/mil.
[0056] In this example, the use of copper as the material to form
the foil 110 and the electrodes 130 was advantageous because copper
was not subject to a large degree of migration. In conventional,
separately fired-on-foil methods, the large TCE difference between
copper and dielectric materials leads to cracking and separation of
the electrode from the dielectric, and high dissipation factors.
However, by co-firing the electrodes and dielectrics, cracking did
not occur and low dissipation factors were achieved.
Example 2
[0057] A process as described in Example 1 was repeated, except
that the thick-film dielectric 128 was printed through 325 mesh
screen, with a wet thickness of each of the two layers of
approximately 15-20 microns. Results were similar to the embodiment
of Example 1, except that the capacitance density was approximately
120 nF/inch.sup.2.
Example 3
[0058] A process as described in Example 2 was repeated using a
variety of dielectric and electrode dimensions shown in the table
below:
4 Dielectric Electrode Dielectric Electrode Size mils Size mils
Size mils Size mils 250 .times. 250 210 .times. 210 36 .times. 338
20 .times. 320 56 .times. 340 40 .times. 320 96 .times. 340 80
.times. 320 176 .times. 340 160 .times. 320 36 .times. 178 20
.times. 157 96 .times. 180 80 .times. 158 336 .times. 180 320
.times. 158 26 .times. 180 10 .times. 159 56 .times. 180 40 .times.
158 176 .times. 180 160 .times. 158 26 .times. 100 10 .times. 74 36
.times. 98 20 .times. 77 56 .times. 100 40 .times. 78 56 .times.
100 40 .times. 78 96 .times. 100 80 .times. 78 176 .times. 100 160
.times. 78 26 .times. 60 10 .times. 39 36 .times. 58 20 .times. 37
56 .times. 60 40 .times. 38 96 .times. 60 80 .times. 38 26 .times.
40 10 .times. 18 36 .times. 38 16 .times. 17 56 .times. 40 40
.times. 18 26 .times. 30 10 .times. 9 36 .times. 28 20 .times. 7 26
.times. 340 10 .times. 318 336 .times. 340 320 .times. 318 90
.times. 90 70 .times. 70 170 .times. 170 150 .times. 150 330
.times. 330 310 .times. 310 240 .times. 240 229.5 .times. 229.5
119.5 .times. 119.5 109.5 .times. 109.5
[0059] Capacitance in these embodiments was proportional to the
area of the printed copper electrode, but the calculated
capacitance densities were essentially identical to that of Example
1.
* * * * *