US20070234540A1 - A method of attaching a capacitor to a feedthrough assembly of a medical device - Google Patents

Abstract

A method for attaching a capacitor to the feedthrough assembly of a medical device having a terminal pin comprises threading a first washer over the terminal pin, and placing a body of epoxy in contact with the first washer. The capacitor is positioned over the terminal pin such that the first washer and the body of epoxy are between the lower surface of the capacitor and a support surface. The body of epoxy is cured to couple the capacitor to the insulating structure. During curing, the body of epoxy is substantially confined between the upper surface and the lower surface by the ferrule, the insulating structure, the capacitor, and the first washer.

Description

TECHNICAL FIELD
• This invention relates generally to implantable medical devices and, more particularly, to a filtered feedthrough assembly for use with an implantable medical device.
BACKGROUND OF THE INVENTION
• Cardiac pacemakers and other such implantable medical devices (e.g., cochlear implants, defibrillators, neurostimulators, active drug pumps, etc.) typically comprise a hermetically sealed container and a feedthrough assembly having one or more terminals (e.g., niobium pins) that provide conductive paths from the interior of the device to one or more lead wires exterior to the device. In general, such feedthrough assemblies comprise a ferrule that is fixedly coupled (e.g., welded) to a container and an insulating structure disposed within the ferrule. The insulating structure may include joint-insulator sub-assemblies, each of which is disposed around a different terminal pin. For example, the insulating structure may include one or more braze joints, each of which comprises an insulator ring (e.g., glass, ceramic, etc.) that insulates the pin from the ferrule, a pin-insulator braze (e.g., gold) that couples the insulating ring to the pin, and an insulator-ferrule braze (e.g., gold) that couples the insulating ring to the ferrule. When the medical device is implanted, the braze joints may be exposed to body fluids. It is thus important that each of the braze joints forms a hermetic seal between the ferrule and its respective terminal pin. To ensure that a satisfactory seal has been formed, a gas may be introduced through an aperture provided through a wall of the ferrule proximate the braze joint or joints. The aperture is then plugged, and the feedthrough assembly is externally monitored for the gas by way of, for example, a mass spectrometer.
• To reduce the effects of stray electromagnetic interference (EMI) signals that may be collected by lead wires coupled to the feedthrough terminal pins, it is known to attach a discoidal capacitor to the feedthrough assembly that permits passage of relatively low frequency electrical signals along the terminal pin or pins while shunting undesired high frequency interference signals to the device's container. Typically, the attachment of such a capacitor includes the thermal curing of one or more non-conductive epoxy preforms to physically couple the capacitor to the insulating structure of the feedthrough.
• Although feedthrough filter capacitor assemblies of the type described above perform satisfactorily, the installation of such filter capacitor assemblies poses certain problems related to the curing of the epoxy preforms. For example, the epoxy preforms may wick into the annular cavities provided between the capacitor and the terminal pins during curing and thus occupy space that should be filled by a conductive material (e.g., epoxy, solder, etc.). This results in a degraded electrical connection between the terminal pins and the capacitors. Additionally, the non-conductive epoxy preforms may seep into the insulating structure and cover cracks that have formed through the braze joint. This may prevent gas from being detected during leak testing and, therefore, may create the impression that a satisfactory hermetic seal has been formed when, in fact, one has not.
• Considering the above, it should be appreciated that it would be desirable to provide a filtered feedthrough assembly utilizing an improved capacitor attachment technique that prevents the undesired travel of non-conductive epoxy. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention.
SUMMARY OF THE INVENTION
• A method for attaching a capacitor to the feedthrough assembly of a medical device is provided. The feedthrough assembly comprises a ferrule configured to be coupled to the medical device and an insulating structure disposed within the ferrule for insulatively guiding at least one terminal pin through the ferrule. The method comprises threading a first washer over the terminal pin, and placing a body of epoxy in contact with the first washer. The capacitor is positioned over the terminal pin such that the first washer and the body of epoxy are between the capacitor and the insulating structure, and the epoxy preform is cured to couple the capacitor to the insulating structure.
• A filtered feedthrough assembly having at least one terminal pin therethrough is also provided. The feedthrough assembly comprises a ferrule having a cavity therethrough for receiving the terminal pin, and insulating structure having an upper surface. The insulating structure is disposed within the cavity and around the terminal pin for electrically isolating the pin from the ferrule. A capacitor is disposed around the pin and electrically coupled thereto. The capacitor has a lower surface that is disposed proximate the upper surface, and at least one washer is disposed between the upper surface and the lower surface. To attach the capacitor to the insulating structure, a body of epoxy is substantially confined between the upper surface and the lower surface by the ferrule, the insulating structure, the capacitor, and the at least one washer.
BRIEF DESCRIPTION OF THE DRAWINGS
• The following drawings are illustrative of particular embodiments of the invention and therefore do not limit the scope of the invention, but are presented to assist in providing a proper understanding. The drawings are not to scale (unless so stated) and are intended for use in conjunction with the explanations in the following detailed descriptions. The present invention will hereinafter be described in conjunction with the appended drawings, wherein like reference numerals denote like elements, and:
• FIGS. 1 and 2 are isometric and cross-sectional views, respectively, of a known unipolar (single pin) feedthrough assembly prior to attachment of a discrete discoidal capacitor;
• FIGS. 3-5 illustrate a prior art method of attaching a discrete discoidal capacitor to the feedthrough assembly shown inFIGS. 1 and 2;
• FIGS. 6-8 illustrate a method of attaching a discrete discoidal capacitor to the feedthrough assembly shown inFIGS. 1 and 2 in accordance with a first exemplary embodiment of the present invention;
• FIG. 9 is an exploded view of a multipolar feedthrough assembly illustrating the attachment of a monolithic discoidal capacitor in accordance with a second embodiment of the present invention;
• FIG. 10 an exploded view of an implantable medical device; and
• FIG. 11 is an isometric cutaway view of an implantable medical device incorporating the feedthrough assembly shown inFIG. 9.
DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENT
• The following description is exemplary in nature and is not intended to limit the scope, applicability, or configuration of the invention in any way. Rather, the following description provides a convenient illustration for implementing an exemplary embodiment of the invention. Various changes to the described embodiment may be made in the function and arrangement of the elements described herein without departing from the scope of the invention.
• FIGS. 1 and 2 are isometric and cross-sectional views, respectively, of a known unipolar (single pin)feedthrough assembly100 having aterminal pin102 therethrough.Assembly100 comprises a generallycylindrical ferrule104 having a cavity therethrough through whichpin102 passes.Ferrule104 is made of an electrically conductive material (e.g., titanium alloy) and is configured to be fixedly coupled (e.g., welded) to the container of a medical device as described below in conjunction withFIG. 10. An insulating structure106 is disposed withinferrule104 to securepin102 relative toferrule104 and to electrically isolatepin102 fromferrule104. Insulating structure106 comprises a supportingstructure108 and a joint-insulator sub-assembly110, both of which are disposed aroundterminal pin102. As will be more fully described below, joint-insulator sub-assembly110 acts as an insulative seal and may take the form of, for example, a braze joint. Supportingstructure108 is made of a non-conductive material (e.g., polyimide) and rests on an inner ledge112 provided withinferrule104. As will be seen, a discrete discoidal capacitor may be threaded overterminal pin102 and fixedly coupled to supportingstructure108 to attach the capacitor tofeedthrough assembly100.
• As can be seen inFIG. 2,braze joint110 comprises three main components: an insulator ring114 (e.g., glass, ceramic, etc.) that insulatespin102 fromferrule104, a pin-insulator braze116 (e.g., gold) that couples insulatingring114 topin102, and an insulator-ferrule braze118 (e.g., gold) that couples insulatingring114 toferrule104. Brazejoint110 is exposed along the underside offerrule104. Whenferrule104 is fixedly coupled to the container of the medical device, the lower portion offerrule104, and thus the lower portion ofbraze joint110, may be exposed to body fluids. For this reason, it is important thatbraze joint110 forms a hermetic seal betweenferrule104 andterminal pin102. Brazejoint110 may be leak tested. To permit this test to be performed, an aperture120 (FIG. 1) is provided throughferrule104 to the inner annular cavity formed by the outer surface ofbraze joint110, the lower surface of supportingstructure108, and the inner surface offerrule104. A gas is delivered throughaperture120 into the inner annular cavity, andaperture120 is plugged. Preferably, a gas of low molecular weight (e.g., helium, hydrogen, etc.) is chosen that may penetrate small cracks inbraze joint110. Feedthrough100 is then monitored for the presence of the gasproximate braze joint110 by way of, for example, a mass spectrometer. If no gas is detected, it is concluded thatbraze joint110 has formed a satisfactory seal.
• Terminal pin102 provides a conductive path from the interior of a medical device (not shown) to one or more lead wires exterior to the medical device. As described previously, these lead wires are known to act as antennae that collect stray electromagnetic interference (EMI) signals, which may interfere with the proper operation of the device. To suppress and/or transfer such EMI signals to the container of the medical device, a discrete discoidal capacitor may be attached tofeedthrough assembly100. In particular, the capacitor may be disposed around and electrically coupled toterminal pin102 and fixedly coupled to supportingstructure108.FIGS. 3-5 illustrate a known manner of attaching a discretediscoidal capacitor150 tofeedthrough assembly100 shown inFIGS. 1 and 2. The attachment method commences as a ring-shapedpreform152 of non-conductive epoxy is threaded over terminal pin102 (indicated inFIG. 3 by arrow154).Capacitor150 is then threaded overpin102 and positioned againstepoxy preform152 such thatpreform152 is sandwiched betweencapacitor150 and supportingstructure108. Next,feedthrough assembly100 is placed within a curing oven and heated to a predetermined temperature (e.g., approximately 175 degrees Celsius) to thermally cure preform152 (indicated inFIG. 4 by arrows156) and thus physicallycouple capacitor150 to supportingstructure108.
• During curing, preform152 melts and disperses under the weight ofcapacitor150, which moves downward toward supportingstructure108.Preform152 disperses along the annular space provided between the bottom surface ofcapacitor150 and the upper surface of supportingstructure108 to physically couplecapacitor150 and supportingstructure108 as described above. In addition,preform152 may disperse upward into the annular space provided between the inner surface ofcapacitor150 and outer surface of terminal pin102 (shown inFIG. 5 at158). Dispersal ofpreform152 in this manner may interfere with the proper electrical coupling ofcapacitor150 toterminal pin102. Also during curing, preform152 may disperse downward into insulating structure110 (shown inFIG. 5 at160). This dispersal may result inpreform152 covering any cracks that have formed through braze joint110 and, consequently, prevent the accurate leak testing offeedthrough assembly100. The inventive attachment method, which is described below in conjunction withFIGS. 6-8, overcomes these drawbacks by physically confining the flow of the epoxy preform to the annular space between the capacitor and supportingstructure108.
• FIGS. 6-8 illustrate a method of attaching adiscoidal capacitor200 tofeedthrough assembly100 shown inFIGS. 1 and 2 in accordance with a first exemplary embodiment of the present invention. As was the case previously, attachment is achieved through the curing of anon-conductive epoxy preform202. In contrast to the attachment method described above, however, afirst washer208 is inserted overterminal pin102 beforepreform202 is threaded overpin102. In addition, asecond washer206 is inserted overpin102 afterpreform202 is so threaded. This results in the stacked configuration shown inFIG. 7 whereinpreform202 is sandwiched betweenwashers206 and208, which are, in turn, collectively sandwiched betweencapacitor200 and supportingstructure108.Washers206 and208 may be substantially identical, or may vary in dimensions, composition, etc. Each washer is preferably made from a material having moderate to high rigidity, that is non-conductive, and that may readily adhere toepoxy preform202 while not impeding the flow ofpreform202 during curing. Such materials may include, but are not limited to, various ceramics, polyimide, polyetheretherketone (PEEK), and alumina. The inner diameters ofwashers206 and208 are preferably only minimally larger than the outer diameter ofpin102. For example, ifpin102 has an outer diameter of approximately 0.0152 inch, the inner diameter ofwasher206 and/orwasher208 may be approximately 0.0155 inch. Lastly, and by way of example only,washer206 and/orwasher208 may have an outer diameter of approximately 0.050 inch and a thickness of approximately 0.007 inch.
• Afterwasher208,epoxy preform202,washer206, andcapacitor200 have been threaded overterminal pin102,feedthrough100 may be placed within a curing oven and heated to cure preform202 (indicated inFIG. 7 by arrows210). As was the case previously, this causespreform202 to melt and disperse between the lower surface ofcapacitor200 and the upper surface of supportingstructure108. If desired, a weight, such asweight212 shown inFIG. 7, may be placed atopcapacitor200 to promote the dispersal ofpreform202. Importantly,washer206 physically prevents preform202 from wicking upward into the annular space betweencapacitor200 andpin102, andwasher208 physically prevents preform202 from seeping into insulatingstructure110 during curing. The result of this controlled dispersal may be more fully appreciated by referring toFIG. 8, which illustratesfeedthrough assembly100 afterepoxy preform202 is fully cured. Thus, it should be appreciated, that the attachment method illustrated inFIGS. 6-8 overcomes the drawbacks associated with prior art attachment methods thereby preserving the integrity of the electrical connection betweencapacitor200 andterminal pin102 and maintaining the leak-testability offeedthrough assembly100. In addition, because the annular space between the bottom ofcapacitor200 and the upper surface of supportingstructure108 is a fixed value, the inventive capacitor attachment method allows the precise volume of epoxy required for capacitor attachment to be accurately calculated. The precise volume of epoxy required for attachment could not previously be accurately determined as the amount of epoxy that would wick upward into the space betweencapacitor200 and pin102 and/or the amount of epoxy that would seep downward into the space beneath supportingstructure108 was overly difficult to predict. By allowing the volume of epoxy required for attachment to be determined, the inventive method facilitates the automation of the attachment process.
• Attachment ofcapacitor200 to the remainder offeedthrough assembly100 may be completed in the following manner. First, a conductive material (e.g., epoxy, polyimide, solder, etc.)212 is dispensed into the annular cavity provided betweenpin102 andcapacitor200 toelectrically couple pin102 to the inner electrode plates ofcapacitor200. A second thermal curing step is then performed to cureconductive material212 ifmaterial212 comprises epoxy or polyimide or to reflowconductive material212 ifmaterial212 comprises solder. Next, anadhesive fillet214 may be disposed around the outer periphery ofcapacitor200proximate ferrule104 to further secure and electrically couple the outer electrode plates ofcapacitor200 toferrule104. Lastly, a non-conductive top coat216 (e.g., epoxy, polyimide, glass, etc.) may be applied to the upper surface ofcapacitor200 to decrease the likelihood of high-voltage breakdown. Once properly installed,capacitor200 functions to permit passage of relatively low frequency electrical signals alongterminal pin102 while shunting undesired high frequency interference signals to the container of a medical device to whichferrule104 is coupled.
• FIG. 9 illustrates the attachment of a monolithicdiscoidal capacitor300 to amultipolar feedthrough assembly302 in accordance with a second embodiment of the present invention.Feedthrough assembly302 comprises aferrule306 and an insulatingstructure304 disposed withinferrule306.Feedthrough assembly302 guides an array ofterminal pins305 through the container of a medical device to whichferrule304 is coupled (shown inFIG. 11). As described above,terminal pin array305 and the lead wires to which array205 is coupled may act as an antenna and collect undesirable EMI signals. Monolithicdiscoidal capacitor300 may be attached tofeedthrough assembly302 to provide EMI filtering.Capacitor300 is provided with a plurality of terminal pin-receivingapertures310 therethrough.Capacitor300 is inserted overterminal pin array305 such that each pin inarray305 is received by adifferent aperture310 and placed in an abutting relationship with insulatingstructure304. If desired, one terminal pin inarray305 may be left unfiltered as shown inFIG. 9 to serve as an RF antenna.Capacitor300 may be attached to insulatingstructure304 by thermally curing, for example, a plurality ofepoxy preforms308. As was the case previously, awasher310 may be disposed betweencapacitor300 and each of preforms308 to prevent the epoxy from wicking upward intoapertures310. Alternatively or conjunctively, awasher312 may be disposed between insulatingstructure304 and each of preforms to prevent the epoxy from seeping into insulatingstructure304.
• FIG.10 is an exploded view of an implantable medical device (e.g., a pulse generator)350 coupled to aconnector block351 and a lead352 by way of anextension354. The proximal portion ofextension354 comprises aconnector356 configured to be received or plugged intoconnector block351, and the distal end ofextension354 likewise comprises aconnector358 including internalelectrical contacts360 configured to receive the proximal end oflead352 havingelectrical contacts362 thereon. The distal end oflead352 includesdistal electrodes364, which may deliver electrical pulses to target areas in a patient's body (or sense signals generated in the patient's body; e.g., cardiac signals).
• After acapacitor300 has been attached tofeedthrough assembly302 in the manner described above,assembly302 may be welded to the housing of an implantablemedical device350 as shown inFIG. 11.Medical device350 comprises a container352 (e.g. titanium or other biocompatible material) having anaperture354 therein through whichfeedthrough assembly302 is disposed. As can be seen, each terminal pin inarray305 has been trimmed and is electrically connected tocircuitry356 ofdevice350 via a plurality of connective wires358 (e.g., gold), which may be coupled toterminal pin array305 by wire bonding, laser ribbon bonding, or the like. After installation,feedthrough assembly302 andcapacitor300 collectively function to permit the transmission of relatively low frequency electrical signals along the terminal pins inarray305 tocircuitry356 while shunting undesired high frequency EMI signals tocontainer352 ofdevice350.
• Although the inventive filtered feedthrough assembly and method for the assembly thereof have been described above as utilizing two washers per terminal pin, it should be appreciated that only one washer may be utilized and disposed either between the capacitor and the preform or the supporting structure and the preform. Indeed, in the case of a unipolar feedthrough assembly wherein the aperture through the supporting structure may be narrowly tailored to conform to the outer diameter of the terminal pin, disposing a washer between the supporting structure and the preform may be largely unnecessary. It should also be appreciated that, although the inventive method for attaching a capacitor to a feedthrough assembly has been described as utilizing one or more epoxy preforms to physically couple the capacitor to the feedthrough assembly, other adhesives may be used. If a liquid epoxy (e.g., UV cure epoxy) is employed, it may be desirable to physically couple a washer to the capacitor before threading the capacitor over the terminal pin or pins and immediately curing the assembly after the capacitor/washer combination is properly positioned. This will help prevent the liquid epoxy from being entirely forced out from between the capacitor/washer combination and the supporting structure or other washer. Lastly, it should be understood that, although the insulating structures in the exemplary feedthrough assemblies described above employed one or more braze joints, other types of joint-insulator sub-assemblies may be utilized providing that the sub-assembly or assemblies utilized are biocompatible and form a suitable hermetic seal between the insulator pin and ferrule.
• It should also be understood that curing processes other than the thermal curing described above may be utilized. For example, if a UV-curable epoxy is used for the liquid epoxy or the epoxy preform, a UV snap cure process may be performed to effectuate capacitor attachment. UV curing occurs as the UV-curable epoxy is exposed to a UV light source (e.g., emitting light having a wavelength of approximately 200-300 nanometers) for a predetermined period of time (e.g., approximately 30-40 seconds). Though the UV light may only directly reach the portions of the epoxy exposed through the annular cavity formed between the outer diameter of the capacitor and the inner wall of the ferrule, the UV-curing process will also affect neighboring portions of epoxy that are not directly exposed to the UV light. Exposure to UV light causes the epoxy to undergo a cross-linking process, harden, and consequently couple the capacitor or capacitors to the feedthrough assembly.
• It should thus be appreciated from the foregoing that there has been provided filtered feedthrough assembly utilizing an improved capacitor attachment technique that prevents the non-conductive epoxy from undesired travel thereby preserving the integrity of the electrical connection between the capacitors and the terminal pins and the leak testability of the feedthrough assembly. Although the invention has been described with reference to a specific embodiment in the foregoing specification, it should be appreciated that various modifications and changes can be made without departing from the scope of the invention as set forth in the appended claims. Accordingly, the specification and figures should be regarded as illustrative rather than restrictive, and all such modifications are intended to be included within the scope of the present invention.

Claims (13)

1. A method for attaching a capacitor to a feedthrough assembly of a medical device, the feedthrough assembly comprising a ferrule configured to be coupled to the medical device and an insulating structure disposed within the ferrule for insulatively guiding at least one terminal pin through the ferrule, the method comprising:

providing a capacitor support structure surface within the ferrule surrounding the terminal pin and overlaying the insulating structure;

threading a first washer over the terminal pin to overlay the capacitor support surface;

placing a body of epoxy in contact with the first washer;

threading a second washer over the terminal pin to abut the body of epoxy;

positioning the capacitor over the terminal pin such that the first washer, the body of epoxy, and the second washer are between the capacitor and the capacitor support structure overlaying the insulating structure; and

curing the body of epoxy to couple the capacitor to the insulating structure.

2. The method according toclaim 1 wherein the step of placing comprises placing an epoxy preform over the terminal pin to abut the first washer.

3. The method for attaching a capacitor according toclaim 2 wherein the step of threading a first washer over the terminal pin is performed prior to placing the epoxy preform over the terminal pin.

4. (canceled)

5. The method for attaching a capacitor according toclaim 1 further comprising placing a weight on the capacitor to facilitate the dispersal of the epoxy during the curing of the feedthrough assembly.

6. The method for attaching a capacitor according toclaim 1 further comprising:

determining a post-attachment spatial volume between the capacitor and the insulating structure; and

selecting a body of epoxy having substantially the same volume as the determined spatial volume.

7. The method for attaching a capacitor according toclaim 1 further comprising leak testing the feedthrough assembly after curing.

8-20. (canceled)

21. A method for attaching a capacitor to a feedthrough assembly of a medical device, the feedthrough assembly comprising a ferrule configured to be coupled to the medical device and an insulating structure disposed within the ferrule for insulatively guiding at least one terminal pin through the ferrule, the method comprising:

providing a capacitor support structure within the ferrule surrounding the terminal pin and overlaying the insulating structure;

threading a first washer over the terminal pin to overlay the capacitor support surface;

determining a post-attachment spatial volume between the capacitor and the insulating structure;

selecting a body of epoxy having substantially the same volume as the determined spatial volume;

placing the body of epoxy in contact with the first washer;

positioning the capacitor over the terminal pin such that the first washer and the body of epoxy are between the capacitor and the capacitor support structure overlaying the insulating structure; and

curing the body of epoxy to couple the capacitor to the insulating structure.

22. The method according toclaim 21 wherein the step of placing comprises placing an epoxy preform over the terminal pin to abut the first washer.

23. The method for attaching a capacitor according toclaim 22 wherein the step of threading a first washer over the terminal pin is performed prior to placing the epoxy preform over the terminal pin.

24. The method for attaching a capacitor according toclaim 21 further comprising placing a weight on the capacitor to facilitate the dispersal of the epoxy during the curing of the feedthrough assembly.

25. The method for attaching a capacitor according toclaim 21 further comprising leak testing the feedthrough assembly after curing.

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