Feedthrough filter capacitor assemblies having low cost terminal pins

[0001] This application claims priority from provisional application Ser. No. 60/749,456, filed Dec. 12, 2005.

[0002] 1. Field of the Invention

[0003] This invention relates generally to a hermetic feedthrough terminal pin assembly, preferably of the type incorporating a filter capacitor. More specifically, this invention relates to terminal pins comprising palladium or palladium alloys for incorporated into feedthrough filter capacitor assemblies, particularly of the type used in implantable medical devices such as cardiac pacemakers, cardioverter defibrillators, and the like, to decouple and shield internal electronic components of the medical device from undesirable electromagnetic interference (EMI) signals. The terminal pin feedthrough assembly provides a hermetic seal that prevents passage or leakage of fluids into the medical device.

[0004] 2. Prior Art

[0005] Feedthrough assemblies are generally well known in the art for use in connecting electrical signals through the housing or case of an electronic instrument. For example, in an implantable medical device, such as a cardiac pacemaker, defibrillator, or neurostimulator, the feedthrough assembly comprises one or more conductive terminal pins supported by an insulator structure for passage of electrical signals from the exterior to the interior of the medical device. The conductive terminals are fixed into place using a gold brazing process, which provides a hermetic seal between the pin and insulative material. Conventionally, the terminal pins have been composed of platinum or a combination of platinum and iridium. Platinum and platinum-iridium alloys are biocompatible and have good mechanical strength, which adds to the durability of the feedthrough. However platinum is a precious metal that creates a manufacturing cost barrier.

[0006] The replacement of platinum and platinum alloys by palladium and its alloys offers several advantages. First, platinum has a density of 21.45 grams/cc. Palladium has a density of 12.02 grams/cc. These materials are priced by weight, but used by volume, which means that palladium has a significant cost advantage over platinum. Secondly, platinum and palladium have nearly equivalent mechanical properties. After high temperature brazing, there is no significant degradation in the mechanical properties of palladium, such as in strength and elongation, in comparison to platinum. Palladium also has comparable soldering and welding characteristics, and it has good radiopacity. Finally, previous research indicates that palladium is biocompatible under both soft tissue and bone studies. Palladium and additive materials that are typically combined with it to form alloys are regarded as chemically inactive.

[0007] In a preferred form, a feedthrough filter capacitor assembly according to the present invention comprises an outer ferrule hermetically sealed to either an alumina insulator or fused glass dielectric material seated within the ferrule. The insulative material is also hermetically sealed to at least one terminal pin. That way, the feedthrough assembly prevents leakage of fluid, such as body fluid in a human implant application, past the hermetic seal at the insulator/ferrule and insulator/terminal pin interfaces.

[0008] According to the invention, the terminal pin of a feedthrough assembly, and preferably of a feedthrough filter capacitor assembly, are composed of palladium. The terminal pin can be a uniform wire-type structure of palladium or an alloy thereof, or it can comprise an outer palladium coating over a core material. The core can be of platinum, tantalum, niobium or other electrically conductive materials commonly used in implantable medical devices. In that respect, palladium is an alternative corrosion resistant material that provides a considerably less expensive terminal pin than conventional platinum or platinum-iridium terminal pins while still achieving the same benefits of biocompatibility, good mechanical strength and a reliable hermetic feedthrough seal. Replacement of platinum and platinum-iridium terminal pins with a palladium-based material is done without employing complex and expensive manufacturing operations and, generally, without the addition of a secondary manufacturing process.

[0009] These and other objects and advantages of the present invention will become increasingly more apparent by a reading of the following description in conjunction with the appended drawings.

[0015] Referring now to the drawings, FIGS. 1 and 2 show an internally grounded feedthrough capacitor assembly 10 comprising a feedthrough 12 supporting a filter discoidal capacitor 14. The feedthrough filter assembly 10 is useful with medical devices, preferably implantable devices such as pacemakers, cardiac defibrillators, cardioverter defibrillators, cochlear implants, neurostimulators, internal drug pumps, deep brain stimulators, hearing assist devices, incontinence devices, obesity treatment devices, Parkinson's disease therapy devices, bone growth stimulators, and the like. The feedthrough 12 portion of the assembly 10 includes terminal pins 16 that provide for coupling, transmitting and receiving electrical signals to and from a patient's heart, while hermetically sealing the interior of the medical instrument against ingress of patient body fluids that could otherwise disrupt instrument operation or cause instrument malfunction. While not necessary for accomplishing these functions, it is desirable to attach the filter capacitor 14 to the feedthrough 12 for suppressing or decoupling undesirable EMI signals and noise transmission into the interior of the medical device.

[0016] More particularly, the feedthrough 12 of the feedthrough filter capacitor assembly 10 comprises a ferrule 18 defining an insulator-receiving bore surrounding an insulator 20. Suitable electrically conductive materials for the ferrule 18 include titanium, tantalum, niobium, stainless steel or combinations of alloys thereof, the former being preferred. The ferrule 18 may be of any geometry, non-limiting examples being round, rectangle, and oblong. A surrounding flange 22 extends from the ferrule 18 to facilitate attachment of the feedthrough 10 to the casing (not shown) of, for example, one of the previously described implantable medical devices. The method of attachment may be by laser welding or other suitable methods.

[0017] The insulator 20 is of a ceramic material such as of alumina, zirconia, zirconia toughened alumina, aluminum nitride, boron nitride, silicon carbide, glass or combinations thereof. Preferably, the insulating material is alumina, which is highly purified aluminum oxide, and comprises a sidewall 24 extending to a first upper side 26 and a second lower side 28. The insulator 20 is also provided with bores 30 that receive the terminal pins 16 passing there through. A layer of metal 32, referred to as metallization, is applied to the insulator sidewall 24 and the sidewall of the terminal pin bores 30 to aid a braze material 34 in hermetically sealing between the ferrule 18 and the insulator 24 and between the terminal pins 16 and the insulator 24, respectively.

[0018] Suitable metallization materials 32 include titanium, titanium nitride, titanium carbide, iridium, iridium oxide, niobium, tantalum, tantalum oxide, ruthenium, ruthenium oxide, zirconium, gold, palladium, molybdenum, silver, platinum, copper, carbon, carbon nitride, and combinations thereof. The metallization layer may be applied by various means including, but not limited to, sputtering, electron-beam deposition, pulsed laser deposition, plating, electroless plating, chemical vapor deposition, vacuum evaporation, thick film application methods, and aerosol spray deposition, and thin cladding. Parylene, alumina, silicone, fluoropolymers, and mixtures thereof are also useful metallization materials.

[0019] Non-limiting examples of braze materials include gold, gold alloys, and silver. Then, if the feedthrough 10 is used where it will contact bodily fluids, the resulting brazes do not need to be covered with a biocompatible coating material. In other embodiments, if the brazes are not biocompatible, for example, if they contain copper, they are coated with a layer/coating of biocompatible/biostable material. Broadly, the biocompatibility requirement is met if contact of the braze/coating with body tissue and blood results in little or no immune response from the body, especially thrombogenicity (clotting) and encapsulation of the electrode with fibrotic tissue. The biostability requirement means that the braze/coating remains physically, electrically, and chemically constant and unchanged over the life of the patient.

[0020] According to one embodiment of the invention, the terminal pins 16 consist of palladium and its alloys. Non-limiting examples include pure palladium and alloys comprising from about 50% to about 99% palladium along with other elements including those from the platinum group such as ruthenium, rhenium, and iridium, or refractory metals such as molybdenum, and boron, and combinations thereof.

[0021] Mechanical properties of the terminal pin 16 can be tailored to a desired mechanical performance by adjusting the amounts of the elemental additions in the palladium alloy. For example, age hardening can be improved by increasing the amount of ruthenium. Other additions to the palladium alloy such as platinum, gold, copper, and zinc, for example increase the alloy's ability to be cold worked to achieve a higher tensile strength or to allow the alloy to be annealed and to increase its elongation characteristics.

[0022] In another embodiment of the present invention, the terminal pins 16 comprise an exterior outer coating 16A of palladium and palladium alloys applied as a coating to a core 16B of a second, electrically conductive material other than palladium (FIG. 3). Preferably, the core material 16B is selected from the group consisting of niobium, tantalum, nickel-titanium (NITINOL®), titanium, particularly beta titanium, titanium alloys, stainless steel, molybdenum, tungsten, platinum, and combinations thereof. The means of coating may include sputtering, cladding, and or plating. The coating may be applied through a process of sputtering, electron-beam deposition, pulsed laser deposition, plating, electroless plating, chemical vapor deposition, vacuum evaporation, thick film application methods, aerosol spray deposition, and thin cladding.

[0023] For example, it is known that niobium readily oxidizes. This means that when it is used as a terminal pin material secondary operations are necessary in order to effect a hermetic braze with low equivalent series resistance (ESR). Providing a palladium outer coating 16A over a niobium core 16B in an evacuated atmosphere prior to formation of niobium oxide means that the thusly constructed terminal pin can be directly brazed into the insulator 20.

[0024] Although the terminal pin 16 is shown having a circular cross-section that is not necessary. The terminal pin 16 can have other cross-sectional shapes including square, triangular, rectangular, and hexagonal, among others. Nonetheless, the core 16B has a diameter of from about 0.002 inches to about 0.020 inches and the outer coating 16A has a thickness of from about 0.5μ inches to about 0.002 inches.

[0025] Up to now, terminal pins for feedthrough assemblies used in implantable medical devices, and the like, have generally consisted of platinum. However, replacement of platinum and platinum alloys by palladium and its alloys offers several advantages. For one, the density of platinum is 21.45 g/cc in comparison to palladium at 12.02 g/cc. Both of these materials are priced by weight, but used by volume. Therefore palladium has significant cost advantage over platinum. Secondly, palladium has comparable electrical conductivity to platinum (platinum=94.34 l/mohm-cm, palladium=94.8 l/mohm-cm and gold=446.4 l/mohm-cm). Thirdly, palladium and platinum have significantly eguivalent mechanical properties. After high temperature brazing, there is no significant degradation of mechanical properties such as strength and elongation. Fourthly, palladium is both solderable and weldable. Fifthly, palladium has good radiopacity characteristics. This is an important consideration for viewing the terminal pin during diagnostic scans such as fluoroscopy. Lastly, but every bit as important, palladium is biocompatible. Previous research indicates a variety of positive biocompatibility studies (both soft tissue and bone) for all elements used. Palladium and its alloy additives are regarded as chemically inactive.

[0026] As further shown in FIGS. 2, 4 and 5, the feedthrough filter capacitor 10 includes the filter capacitor 14 that provides for filtering undesirable EMI signals before they can enter the device housing via the terminal pins 16. The filter capacitor 14 comprises a ceramic or ceramic-based dielectric monolith 36 having multiple capacitor-forming conductive electrode plates formed therein. The capacitor dielectric 36 preferably has a circular cross-section matching the cross-section of the ferrule 18 and supports a plurality of spaced-apart layers of first or “active” electrode plates 38 in spaced relationship with a plurality of spaced apart layers of second or “ground” electrode plates 40. The filter capacitor 14 is preferably joined to the feedthrough 12 adjacent to the insulator side 26 by an annular bead 42 of conductive material, such as a solder or braze ring, or a thermal-setting conductive adhesive, and the like. The dielectric 36 includes lead bores 44 provided with an inner surface metallization layer. The terminal pins 16 pass there through and are conductively coupled to the active plates 38 by a conductive braze material 46 contacting between the terminal pins 16 and the bore metallization. In a similar manner, the ground plates 40 are electrically connected through an outer surface metallization 48 and the conductive material 42 to the ferrule 18.

[0027] It is appreciated that various modifications to the invention concepts described herein may be apparent to those of ordinary skill in the art without departing from the scope of the present invention as defined by the appended claims.