U.S. patent application number 10/441476 was filed with the patent office on 2004-11-25 for feed-through filter capacitor assembly.
Invention is credited to Devoe, Alan, Devoe, Lambert.
Application Number | 20040233016 10/441476 |
Document ID | / |
Family ID | 33449998 |
Filed Date | 2004-11-25 |
United States Patent
Application |
20040233016 |
Kind Code |
A1 |
Devoe, Lambert ; et
al. |
November 25, 2004 |
FEED-THROUGH FILTER CAPACITOR ASSEMBLY
Abstract
A feed-through filter capacitor assembly using an electrically
conductive adhesive modified with a filler of low coefficient of
thermal expansion (CTE) to provide a conductive relation between
the conductive pin and the electrode plates of the ceramic
capacitor. The conductive adhesive contains an organic
polymer-based adhesive component that has a CTE greater than the
CTE of the capacitor ceramic body and a conductive metal filler
having a CTE lower than the adhesive component. The conductive
adhesive is further provided with a CTE-lowering filler that has a
CTE lower than the CTE of the conductive metal filler, thereby
lowering the overall CTE of the adhesive to a value closer to the
CTE of the capacitor ceramic body.
Inventors: |
Devoe, Lambert; (San Diego,
CA) ; Devoe, Alan; (La Jolla, CA) |
Correspondence
Address: |
WOOD, HERRON & EVANS, LLP
2700 CAREW TOWER
441 VINE STREET
CINCINNATI
OH
45202
US
|
Family ID: |
33449998 |
Appl. No.: |
10/441476 |
Filed: |
May 20, 2003 |
Current U.S.
Class: |
333/182 |
Current CPC
Class: |
H01G 4/35 20130101 |
Class at
Publication: |
333/182 |
International
Class: |
H03H 007/01 |
Claims
What is claimed is:
1. A feed-through filter capacitor assembly comprising: a
conductive pin; a feed-through filter capacitor comprising a
ceramic body having a first coefficient of thermal expansion (CTE),
first and second sets of electrode plates and an internal
passageway through which the conductive pin passes in conductive
relation with the first set of electrode plates; and an
electrically conductive adhesive in the passageway providing the
conductive relation between the conductive pin and first set of
electrode plates, the conductive adhesive comprising an organic
polymer-based adhesive component having a second CTE greater than
the first CTE, a conductive metal filler having a third CTE lower
than the second CTE, and a CTE-lowering filler having a fourth CTE
lower than the third CTE.
2. The assembly of claim 1 wherein the first CTE is in the range of
about 3-15 ppm/.degree. C., the second CTE is at least about 50
ppm/.degree. C., the third CTE is in the range of about 14-20
ppm/.degree. C., and the fourth CTE is less than about 15
ppm/.degree. C.
3. The assembly of claim 1 wherein the adhesive comprises a volume
ratio of CTE-lowering filler to conductive metal filler in a range
of about 0.5:1 to about 3.5:1.
4. The assembly of claim 3 wherein the ratio is in the range of
about 0.5:1 to about 2:1.
5. The assembly of claim 1 wherein the CTE-lowering filler is a
particulate material having an average particle size in the range
of about 0.5-5 mils.
6. The assembly of claim 1 wherein the CTE-lowering filler and the
conductive metal filler together form a particulate material
wherein the conductive metal filler forms a coating on the
CTE-lowering filler.
7. The assembly of claim 6 wherein particulate material is fused
silica coated with silver.
8. The assembly of claim 1 wherein the organic polymer-based
adhesive component is silicone.
9. The assembly of claim 1 wherein the organic polymer-based
adhesive component is an epoxy.
10. The assembly of claim 1 wherein the conductive metal filler is
selected from the group consisting of silver, palladium, platinum,
copper, gold, and combinations thereof.
11. There assembly of claim 1 wherein the CTE-lowering filler is
selected from the group consisting of barium titanate, alumina,
glass, zirconia, quartz, and combinations thereof.
12. There assembly of claim 1 wherein the CTE-lowering filler is
fused silica.
13. The assembly of claim 1 wherein the CTE-lowering filler is a
binary or ternary alloy of about 30-70% iron and one or a
combination of nickel, cobalt and chromium, the alloy having less
than about 1% impurity.
14. The assembly of claim 13 wherein the alloy is selected from the
group consisting of Fe-30-60Ni, Fe--Ni--Cr, Fe--Ni--Co and
Fe--Cr--Co.
15. The assembly of claim 14 wherein the alloy is Fe-36Ni.
16. The assembly of claim 1 further comprising a conductive housing
surrounding the ceramic body in conductive relation to the second
set of electrode plates and in non-conductive relation to the
conductive pin.
17. The assembly of claim 16 further comprising a non-conductive
adhesive separating the conductive housing from the conductive pin
and conductive adhesive.
18. The assembly of claim 17 wherein the non-conductive adhesive
comprises a second organic polymer-based adhesive component having
a fifth CTE and a second CTE-lowering filler having a sixth CTE
lower than the fifth CTE.
19. The assembly of claim 1 further comprising a non-conductive
adhesive in a bottom portion of the passageway, the non-conductive
adhesive comprising a second organic polymer-based adhesive
component having a fifth CTE and a second CTE-lowering filler
having a sixth CTE lower than the fifth CTE.
20. A feed-through filter capacitor assembly comprising: a
conductive pin; a feed-through filter capacitor comprising a
ceramic body having a first coefficient of thermal expansion (CTE)
in the range of about 3-15 ppm/.degree. C., first and second sets
of electrode plates and an internal passageway through which the
conductive pin passes in conductive relation with the first set of
electrode plates; and an electrically conductive adhesive in the
passageway providing the conductive relation between the conductive
pin and first set of electrode plates, the conductive adhesive
comprising an organic polymer-based adhesive component having a
second CTE of at least about 50 ppm/.degree. C., a conductive metal
filler having a third CTE in the range of about 14-20 ppm/.degree.
C., and a CTE-lowering filler having a fourth CTE lower than the
third CTE, wherein the conductive adhesive comprises a volume ratio
of CTE-lowering filler to conductive metal filler in a range of
about 0.5:1 to about 3.5:1.
21. The assembly of claim 20 wherein the ratio is in the range of
about 0.5:1 to about 2:1.
22. The assembly of claim 20 wherein the CTE-lowering filler is a
particulate material having an average particle size in the range
of about 0.5-5 mils.
23. The assembly of claim 20 wherein the CTE-lowering filler and
the conductive metal filler together form a particulate material
wherein the conductive metal filler forms a coating on the
CTE-lowering filler.
24. The assembly of claim 23 wherein particulate material is fused
silica coated with silver.
25. The assembly of claim 20 wherein the organic polymer-based
adhesive component is silicone.
26. The assembly of claim 20 wherein the organic polymer-based
adhesive component is an epoxy.
27. The assembly of claim 20 wherein the conductive metal filler is
selected from the group consisting of silver, palladium, platinum,
copper, gold, and combinations thereof.
28. There assembly of claim 20 wherein the CTE-lowering filler is
selected from the group consisting of barium titanate, alumina,
glass, zirconia, quartz, and combinations thereof.
29. There assembly of claim 20 wherein the CTE-lowering filler is
fused silica.
30. The assembly of claim 20 wherein the CTE-lowering filler is a
binary or ternary alloy of about 30-70% iron and one or a
combination of nickel, cobalt and chromium, the alloy having less
than about 1% impurity.
31. The assembly of claim 30 wherein the alloy is selected from the
group consisting of Fe-30-60Ni, Fe--Ni--Cr, Fe--Ni--Co and
Fe--Cr--Co.
32. The assembly of claim 20 wherein the alloy is Fe-36Ni.
33. The assembly of claim 20 further comprising a conductive
housing surrounding the ceramic body in conductive relation to the
second set of electrode plates and in non-conductive relation to
the conductive pin.
34. The assembly of claim 33 further comprising a non-conductive
adhesive separating the conductive housing from the conductive pin
and conductive adhesive.
35. The assembly of claim 34 wherein the non-conductive adhesive
comprises a second organic polymer-based adhesive component having
a fifth CTE and a second CTE-lowering filler having a sixth CTE
lower than the fifth CTE.
36. The assembly of claim 20 further comprising a non-conductive
adhesive in a bottom portion of the passageway, the non-conductive
adhesive comprising a second organic polymer-based adhesive
component having a fifth CTE and a second CTE-lowering filler
having a sixth CTE lower than the fifth CTE.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to feed-through filter
capacitor assemblies, and more particularly to the conductive
adhesive connection between the ceramic capacitor and the
conductive pin passing through a feed-through passage in the
capacitor.
BACKGROUND OF THE INVENTION
[0002] Feed-through filters are utilized to separate unwanted
interference from a signal path, for example in connection with
implantable medical devices, such as heart pacemakers and the like.
These electronic devices are often constructed having an outer
housing in which the necessary electronic components are contained.
For implantable devices, the outer housing must be formed of a
material that is biocompatible and capable of shielding the
electronics within the housing from external sources of
electromagnetic interference (EMI). Titanium is often used to
satisfy these dual requirements of shielding and biocompatibility.
A conductive wire or pin extends from the electronics within the
outer housing to a desired external location, such as one inside
the body. The conductive wire may act as an antenna picking up
spurious radio frequency signals that interfere with proper
operation of the device, such that a filtering capacitor is
desirable. A small capacitor is fitted in an annular space formed
between a conductive housing (ferrule) and the conductive pin. The
capacitor is formed from a dielectric ceramic and two sets of
electrode plates, wherein one set of plates is electrically
connected to the conductive pin and the other set of plates is
electrically connected with the conductive housing. For some
applications, however, there is no outer housing, such as with a
connector cable. Regardless, the electrical connection between the
conductive pin and the capacitor electrode plates is generally
achieved by means of a solder, an electrically conductive adhesive
material or brazing. The use of an organic polymeric-based
conductive adhesive is preferable, but in use, cracking in the
capacitor body has been observed. Because polymeric materials
exhibit shrinkage during curing, tension is placed on the ceramic
body in the area of the feed-through passage. That tension, if
sufficiently high, can crack the capacitor body.
[0003] Thus, there is a need to enable the use of conductive
adhesives for electrically connecting the capacitor with the
conductive pin in a feed-through filter capacitor assembly without
cracking the capacitor body during curing.
SUMMARY OF THE INVENTION
[0004] The present invention provides a feed-through filter
capacitor assembly in which an electrically conductive adhesive
modified with a filler of low coefficient of thermal expansion
(CTE) is used to provide the conductive relation between the
conductive pin and the electrode plates of the ceramic capacitor. A
feed-through filter capacitor has a ceramic body and first and
second sets of electrode plates. At least one internal passageway
is formed through the ceramic body and a conductive pin passes
through each passageway in conductive connection with the first set
of electrode plates. An electrically conductive adhesive in the
passageway provides the conductive connection. The conductive
adhesive comprises an organic polymer-based adhesive component that
has a CTE greater than the CTE of the capacitor ceramic body and a
conductive metal filler to provide the electrical connection
between the conductive pin and the electrode plates. In accordance
with the present invention, the conductive adhesive is further
provided with a CTE-lowering filler that has a CTE lower than the
CTE of the conductive metal filler. This low CTE filler effectively
lowers the overall CTE of the adhesive to a value closer to the CTE
of the capacitor ceramic body. By lowering the CTE of the adhesive,
less shrinkage occurs during curing, and therefore, less tension is
placed on the capacitor body, thereby reducing the likelihood of
cracking the capacitor body.
BRIEF DESCRIPTION OF THE DRAWINGS
[0005] The accompanying drawings, which are incorporated in and
constitute a part of this specification, illustrate embodiments of
the invention and, together with a general description of the
invention given above, and the detailed description given below,
serve to explain the invention.
[0006] FIG. 1 is a perspective view depicting a multiple-hole
feed-through filter capacitor array of the present invention.
[0007] FIG. 1A is a cross-section taken generally along line 1A of
FIG. 1 and further depicting the adhesive bond between the
capacitor body and conductive pin.
[0008] FIG. 2 depicts in cross-section a single-hole feed-through
filter capacitor assembly of the present invention.
DETAILED DESCRIPTION
[0009] With reference to FIGS. 1 and 1A, a feed-through filter
capacitor assembly 10 is depicted in top perspective view in FIG. 1
and in cross-sectional view, taken generally along line 1A, in FIG.
1A. A capacitor 10 is shown having a ceramic body 12 with multiple
feed-through passages 14 thereby providing a feed-through capacitor
array. A conductive pin 16 carrying the electrical signal to be
filtered passes through each internal passage 14 of the
feed-through filter capacitor 10. The filter capacitors on each pin
16 includes a ceramic body 12 and a set of electrode plates 18
associated with each of the multiple internal passages extending
through the ceramic body 12 and in opposition to grounded plate 20
conductively connected to an electrical ground. The electrode
plates 18 associated with each respective internal passageway 14
are in conductive connection with the conductive pin passing
through the respective passageway 14. To provide the conductive
connection, the internal passageways 14 in the ceramic body 12 are
filled with an electrically conductive adhesive 22 in accordance
with the present invention.
[0010] With reference to FIG. 2, a feed-through filter capacitor
assembly 10' is depicted in cross-section. The capacitor is shown
having a single feed-through passage 14, and thus, it should be
understood that both single and multiple feed-through assemblies
are contemplated by the present invention. In the single
feed-through embodiment, a single conductive pin 16 carrying the
electrical signal to be filtered passes through a single internal
passage 14 of a feed-through filter capacitor 10'. As with the
embodiment in FIGS. 1-1A, the filter capacitor 10' includes a
ceramic body 12 and first and second sets 18, 20 of electrode
plates. FIG. 2 further depicts the ceramic body 12 residing in a
conductive housing 24 and the conductive pin 16 passes through that
housing 24 in a non-conductive relation, i.e., the conductive pin
16 is insulated from the conductive housing 24. The first set 18 of
electrode plates is in conductive relation with the conductive pin
16. The second set 20 of electrode plates is in conductive relation
with the conductive housing 24. In this embodiment, the internal
surface 12a of the ceramic body 12 is surfaced with a conductive
metal (metallization layer) 26a to provide electrical contact to
the first set 18 of electrode plates, and the external surface 12b
of the ceramic body 12 is also surfaced with a conductive metal
(metallization layer) 26b to provide an electrode contact to the
second set 20 of internal electrode plates. However, the conductive
housing 24 and metallization layers 26a, 26b are not essential to
the present invention, as shown in FIGS. 1 and 1A. To provide the
conductive relation between the first set 18 of electrode plates
and the conductive pin 16, the internal passageway 14 in the
capacitor is filled with an electrically conductive adhesive 22. A
non-conductive adhesive 28 may also be provided in the bottom of
the internal passageway 14, for example, to prevent shorting the
device, and outside the passageway 14 between the conductive
housing 24 and the conductive pin 16 and adhesive 22 to provide
insulation therebetween. For any non-conductive adhesive 28
residing in the internal passageway, the adhesive 28 advantageously
comprises an adhesive component and a CTE-lowering filler, as
described herein for the conductive adhesive 22. The CTE-lowering
filler has a CTE lower than the adhesive component to thereby lower
the total CTE for the non-conductive adhesive.
[0011] Conductive adhesives typically comprise an adhesive
component and conductive metal filler. The adhesive component is an
organic polymer-based material, such as thermoplastics (e.g.,
polyamides), thermosetting resins, Uv-light curing adhesives,
epoxies, silicones, etc. An exemplary adhesive component is epoxy.
Conductive metal fillers may include silver, palladium, platinum,
copper, gold or other noble metals, or combinations thereof. In
accordance with the present invention, the conductive adhesive is
modified by adding a low CTE filler to reduce the CTE of the
conductive adhesive. To this end, a filler, which is understood to
refer to relatively non-adhesive substances added to an adhesive to
improve one or more qualities of the adhesive, is selected that has
a CTE lower than the CTE of the adhesive component and of the
conductive metal filler. The total CTE of the conductive adhesive
is a volume fraction average of the materials contained in the
conductive adhesive, such that adding the low CTE filler has the
effect of lowering the total CTE for the adhesive. Because the CTE
of the ceramic body is significantly lower than that of the
adhesive component, the present invention achieves a decrease in
the thermal expansion mismatch between the adhesive and the ceramic
body, thereby decreasing the tension during curing, and
consequently, reducing the potential for cracking the ceramic
body.
[0012] In general, the CTE of ceramics typically used for the
capacitor body have a CTE generally in the range of about 3-15
ppm/.degree. C. For example, barium titanate has a CTE of about
10-12 ppm/.degree. C. In contrast, the adhesive component has a CTE
of at least about 50 ppm/.degree. C., and typically in the range of
about 50-150 ppm/.degree. C. For example, the CTE is on the order
of 100 ppm/.degree. C. for epoxy. The conductive metal fillers
typically used have CTE's that are lower than the CTE of the
adhesive component, generally in the range of about 14-20
ppm/.degree. C. By way of example, silver has a CTE of about 19
ppm/.degree. C. For a conductive adhesive containing 75 vol. %
epoxy (assume CTE=100 ppm/.degree. C.) and 25 vol. % silver (CTE=19
ppm/.degree. C.), the total CTE of the adhesive is about 79.75
(i.e., 100.times.0.75+19.times.0.25). Thus, adding the conductive
metal filler effectively lowers the CTE from that of the adhesive
component. By further modifying the adhesive with a filler having a
CTE lower than the CTE of the conductive metal filler, the total
CTE for the conductive adhesive is effectively lowered even further
from that of the adhesive component, which brings the total CTE
closer to the CTE of the capacitor body. In the example above of
epoxy with 25 vol. % silver, the further addition of 25 vol. %
fused silica having a CTE of 0.5 ppm/.degree. C. lowers the total
CTE to about 54.875 ppm/.degree. C.
(100.times.0.5+19.times.0.25+0.5.times.0.25).
[0013] The CTE of the CTE-lowering filler is less than the CTE's of
the adhesive component and conductive metal filler and is less than
about 15 ppm/.degree. C. Advantageously, the low CTE filler has a
CTE lower than the CTE of the ceramic body. In one embodiment of
the present invention, the low CTE filler is fused silica (a
specific type of glass), which has a CTE on the order of 0.4-0.6
ppm/.degree. C. Because fused silica has a CTE significantly lower
than that of the ceramic body, it is highly effective at lowering
the CTE of the entire adhesive toward that of the capacitor. In
another embodiment of the present invention, the low CTE filler is
a ceramic, such as barium titanate, alumina, zirconia or quartz, or
a glass. These materials generally have a CTE similar to that of
the capacitor body such that adding these materials to the
conductive adhesive moves the average CTE closer to that of the
capacitor. In yet another embodiment of the present invention, the
low CTE filler may be an alloy designed to have a low or no CTE
within a reasonable temperature range. Binary and ternary alloys of
iron may be suitable, in particular those having about 30-70 wt. %
iron alloyed with nickel, chromium, cobalt or a combination
thereof. The impurity content of these alloys is generally less
than about 1%, with typical impurities including manganese, silicon
and carbon. The binary Fe--Ni family having about 30-60 wt. %
nickel is exemplary. INVAR.RTM., which is Fe-36Ni, is designed to
have essentially no CTE, and thus would have a strong effect of
lowering the CTE of the entire conductive adhesive. Exemplary
ternary alloys having about 30-70% iron include Fe--Ni--Cr,
Fe--Ni--Co and Fe--Cr--Co alloys, such as 52Fe-36Ni-12Cr,
33-34Fe-32Ni-4-5Co, 54Fe-29Ni-17Co, and 36.5-37Fe-53-54.5Co-9-10Cr.
Other metal alloys may have low CTE's below about 15 ppm/.degree.
C., the selection of which is within the skill of one of ordinary
skill in the art. An additional advantage of the low CTE metal
fillers is that the particles are conductive, such that the
conductivity of the adhesive is not reduced when large quantities
of the low CTE filler are added.
[0014] The CTE-lowering filler may be in the form of a particulate
material separate and distinct from the conductive metal filler.
Alternatively, the CTE-lowering filler and the conductive metal
filler may together form a composite filler for the adhesive
component. The composite filler would advantageously comprise
CTE-lowering filler particles coated with the conductive metal. An
exemplary composite filler is fused silica coated with silver. One
advantage of this composite filler embodiment is that higher volume
fractions of filler may be used without degrading the conductivity
of the adhesive, since the conduction is occurring from the outside
of the particle. On the other hand, when the CTE-lowering filler
and the conductive metal filler are separate, distinct particulate
materials, the addition of a non-conductive CTE-lowering filler may
eventually lower the conductivity of the conductive adhesive. Thus,
the composite filler embodiment in which the conductive metal coats
non-conductive particles prevents lowering of the conductivity.
However, in both embodiments, the effect of lowering the CTE is the
same, because the total CTE of the conductive adhesive is based on
the volume fractions of the conductive metal, low CTE filler, and
adhesive component. While the form of a composite filler may be
used in the case of a metal CTE-lowering filler, such as a
silver-coated INVAR.RTM. filler, the advantages are not appreciated
because there is no risk of lowering the conductivity as in the
case of a non-conductive low CTE filler. Thus, the composite filler
embodiment is particularly beneficial when the CTE-lowering filler
is a non-conductive ceramic or glass material.
[0015] Advantageously, the amount of the adhesive component in the
conductive adhesive is as low as possible, given it contributes the
most to the high CTE. Of course, there must be sufficient adhesive
component to provide the desired bonding properties for the
adhesive, as well as the desirable handling properties, such as
viscosity. Advantageously, the fillers are added in amounts as high
as possible while still maintaining the desired workability and
adhesive force for the conductive adhesive. By way of example and
not limitation, the filler content may be as high as about 65 vol.
%. Advantageously, the filler materials have an average particle
size selected to enable the highest filler loading possible while
still maintaining the workability of the adhesive. For example, the
average particle size may be in the range of about 0.5-5 mils. The
conductive metal filler is present in an amount sufficient to
render the desired level of conductivity to the adhesive. By way of
example, silver is usually included in an amount of about 25 vol.
%. Advantageously, the CTE-lowering filler is present in an amount
such that the volume ratio of low CTE filler to conductive metal
filler is in the range of about 0.5:1 to about 3.5:1, and more
advantageously about 0.5:1 to about 2:1. Examples within the scope
of the present invention include 50 vol. % epoxy/25 vol. %
silver/25 vol. % glass and 35 vol. % epoxy/15 vol. % silver/50 vol.
% glass.
[0016] While the present invention has been illustrated by the
description of an embodiment thereof, and while the embodiment has
been described in considerable detail, it is not intended to
restrict or in any way limit the scope of the appended claims to
such detail. Additional advantages and modifications will readily
appear to those skilled in the art. The invention in its broader
aspects is therefore not limited to the specific details,
representative apparatus and method and illustrative examples shown
and described. Accordingly, departures may be made from such
details without departing from the scope or spirit of the general
inventive concept.
* * * * *