Electricity: electrical systems and devices – Electrostatic capacitors – Fixed capacitor
Reexamination Certificate
2003-02-27
2004-07-20
Reichard, Dean A. (Department: 2831)
Electricity: electrical systems and devices
Electrostatic capacitors
Fixed capacitor
C361S306100, C607S005000
Reexamination Certificate
active
06765779
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates generally to feedthrough capacitor terminal pin subassemblies and related methods of construction, particularly of the type used in implantable medical devices such as cardiac pacemakers and the like, to decouple and shield undesirable electromagnetic interference (EMI) signals from the device. More specifically, this invention relates to a method of providing a conductive coating on the flanges of human implantable hermetic seals for reliable EMI filter attachment, and a method of electrical connection of the feedthrough capacitor to the feedthrough lead wires at the hermetic gold braze. This invention is particularly designed for use in cardiac pacemakers (bradycardia devices), cardioverter defibrillators (tachycardia), neuro-stimulators, internal drug pumps, cochlear implants and other medical implant applications. This invention is also applicable to a wide range of other EMI filter applications, such as military or space electronic modules, where it is desirable to preclude the entry of EMI into a hermetically sealed housing containing sensitive electronic circuitry.
Feedthrough terminal pin assemblies are generally well known in the art for connecting electrical signals through the housing or case of an electronic instrument. For example, in implantable medical devices such as cardiac pacemakers, defibrillators or the like, the terminal pin assembly comprises one or more conductive terminal pins supported by an insulator structure for feedthrough passage from the exterior to the interior of the medical device. Many different insulator structures and related mounting methods are known in the art for use in medical devices wherein the insulator structure provides a hermetic seal to prevent entry of body fluids into the housing of the medical device. However, the feedthrough terminal pins are typically connected to one or more lead wires which effectively act as an antenna and thus tend to collect stray EMI signals for transmission into the interior of the medical device. In the prior art devices, the hermetic terminal pin subassembly has been combined in various ways with a ceramic feedthrough filter capacitor to decouple interference signals to the housing of the medical device.
In a typical prior art unipolar construction (as described in U.S. Pat. No. 5,333,095), a round/discoidal (or rectangular) ceramic feedthrough filter capacitor is combined with a hermetic terminal pin assembly to suppress and decouple undesired interference or noise transmission along a terminal pin.
FIGS. 1-6
illustrate an exemplary prior art feedthrough filter capacitor
100
and its associated hermetic terminal
102
. The feedthrough filter capacitor
100
comprises a unitized dielectric structure or ceramic-based monolith
104
having multiple capacitor-forming conductive electrode plates formed therein. These electrode plates include a plurality of spaced-apart layers of first or “active” electrode plates
106
, and a plurality of spaced-apart layers of second or “ground” electrode plates
108
in stacked relation alternating or interleaved with the layers of “active” electrode plates
106
. The active electrode plates
106
are conductively coupled to a surface metallization layer
110
lining a bore
112
extending axially through the feedthrough filter capacitor
100
. The ground electrode plates
108
include outer perimeter edges which are exposed at the outer periphery of the capacitor
100
where they are electrically connected in parallel by a suitable conductive surface such as a surface metallization layer
114
. The outer edges of the active electrode plates
106
terminate in spaced relation with the outer periphery of the capacitor body, whereby the active electrode plates are electrically isolated by the capacitor body
104
from the conductive layer
114
that is coupled to the ground electrode plates
108
. Similarly, the ground electrode plates
108
have inner edges which terminate in spaced relation with the terminal pin bore
112
, whereby the ground electrode plates are electrically isolated by the capacitor body
104
from a terminal pin
116
and the conductive layer
110
lining the bore
112
. The number of active and ground electrode plates
106
and
108
, together with the dielectric thickness or spacing therebetween, may vary in accordance with the desired capacitance value and voltage rating of the feedthrough filter capacitor
100
.
The feedthrough filter capacitor
100
and terminal pin
116
is assembled to the hermetic terminal
102
as shown in
FIGS. 5 and 6
. In the exemplary drawings, the hermetic terminal includes a ferrule
118
which comprises a generally ring-shaped structure formed from a suitable biocompatible conductive material, such as titanium or a titanium alloy, and is shaped to define a central aperture
120
and a ring-shaped, radially outwardly opening channel
122
for facilitated assembly with a test fixture (not shown) for hermetic seal testing, and also for facilitated assembly with the housing (also not shown) on an implantable medical device or the like. An insulating structure
124
is positioned within the central aperture
120
to prevent passage of fluid such as patient body fluids, through the feedthrough filter assembly during normal use implanted within the body of a patient. More specifically, the hermetic seal comprises an electrically insulating or dielectric structure
124
such as a gold-brazed alumina or fused glass type or ceramic-based insulator installed within the ferrule central aperture
120
. The insulating structure
124
is positioned relative to an adjacent axial side of the feedthrough filter capacitor
100
and cooperates therewith to define a short axial gap
126
therebetween. This axial gap
126
forms a portion of a leak detection vent and facilitates leak detection. The insulating structure
124
thus defines an inboard face presented in a direction axially toward the adjacent capacitor body
104
and an opposite outboard face presented in a direction axially away from the capacitor body. The insulating structure
124
desirably forms a fluid-tight seal about the inner diameter surface of the conductive ferrule
118
, and also forms a fluid-tight seal about the terminal pin
116
thereby forming a hermetic seal suitable for human implant. Such fluid impermeable seals are formed by inner and outer braze seals or the like
128
and
130
. The insulating structure
124
thus prevents fluid migration or leakage through the ferrule
118
along any of the structural interfaces between components mounted within the ferrule, while electrically isolating the terminal pin
116
from the ferrule
118
.
The feedthrough filter capacitor
100
is mechanically and conductively attached to the conductive ferrule
118
by means of peripheral material
132
which conductively couple the outer metallization layer
114
to a surface of the ferrule
118
while maintaining an axial gap
126
between a facing surface of the capacitor body
104
, on the one hand, and surfaces of the insulating structure
124
and ferrule
118
, on the other. The axial gap
126
must be small to preclude leakage of EMI. The outside diameter connection between the capacitor
100
and the hermetic terminal ferrule
118
is accomplished typically using a high temperature conductive thermal-setting material such as a conductive polyimide. It will also be noted in
FIG. 5
that the peripheral support material
132
is preferably discontinuous to reduce mechanical stress and also allow for passage of helium during hermetic seal testing of the complete assembly. In other words, there are substantial gaps between the supports
132
which allow for the passage of helium during a leak detection test.
In operation, the coaxial capacitor
100
permits passage of relatively low frequency electrical signals along the terminal pin
116
, while shielding and decoupling/attenuating undesired interference signals of typically high frequency to the conductive housing. Feedthrough capacitors of this general type
Brendel Richard L.
Frysz Christine A.
Stevenson Robert A.
Woods Jason
Zeng Haitong
Greatbatch-Sierra, Inc.
Kelly Bauersfeld Lowry & Kelley LLP
Reichard Dean A.
Thomas Eric
LandOfFree
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