Patent Application
20110303458
1. Field of the Invention
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 a refractive metal core in which an electrically conductive second. metal is selectively coated to provide a cost effective terminal pin of increased solderability for incorporation into a feedthrough filter capacitor assembly, 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.
2. Prior Art
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 combination of platinum and iridium. Platinum and platinum-iridium alloys are biocompatible, have good mechanical strength, which adds to the durability of the feedthrough. However, platinum is a precious metal that creates a manufacturing cost barrier.
The replacement of platinum and platinum alloys by refractive metals such as niobium, molybdenum and tungsten offers several advantages. First, these refractive metals have a significant cost advantage over platinum. Secondly, these refractive metals are generally known to be biocompatible. Finally, previous research has shown that after high temperature brazing, there is no significant degradation in the mechanical properties of these refractive metals, in comparison to platinum.
However, these refractive metals are susceptible to surface oxidation. Surface oxidation generally inhibits the ability of these metals to be joined to other materials, particularly other electrically conductive metals. What is needed, therefore, is a biocompatible, mechanically robust, cost effective terminal pin that can be readily joined to other metals. The present invention provides embodiments by which a terminal pin of a cost effective core metal is selectively coated with a metal that can be more readily joined to other electrically conductive metals.
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.
According to the invention, the terminal pin of a feedthrough assembly, and preferably of a feedthrough filter capacitor assembly, is composed of a refractive metal core in which a layer of a non-refractive electrically conductive second metal is selectively contacted to the surface of the pin core. In a preferred embodiment, the terminal pin comprises a core of tantalum, niobium, molybdenum or alloy thereof. A layer of a second non-refractive electrically conductive metal, such as palladium, platinum, gold or silver, is selectively applied to a portion or portions of the surface of the core metal. In that respect, the application of the second electrically conductive metal is an alternative solderable, oxidation 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, solderability and a reliable hermetic feedthrough seal.
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.
Referring now to the drawings,
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 12 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.
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 20 and between the terminal pins 16 and the insulator 20, respectively.
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.
Non-limiting examples of braze materials include gold, gold alloys, and silver. Then, if the feedthrough 12 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 riot 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.
In an embodiment of the present invention, the terminal pins 16 (
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 achieve a higher tensile strength.
As previously mentioned, the terminal pin core 16B is comprised of a refractive metal. A refractory metal is herein defined as a metal that is resistant to heating and has a melting temperature greater than about 1,800° C. Non-limiting examples of refractory metals include niobium, molybdenum, tantalum, tungsten, rhenium, titanium, vanadium, zirconium, hafnium, osmium, iridium, and alloys thereof. In a more preferred embodiment, the terminal pin core 16B comprises niobium and niobium alloys. However, an alternative embodiment, the terminal pin core 168 may comprise nickel-titanium (NITINOL®, titanium, particularly beta titanium, titanium alloys, stainless steel, palladium and palladium alloys, and combinations thereof.
In a preferred embodiment, the external outer coating 16A comprises an alternative electrically conductive metal. Non-limiting examples of this alternative second conductive metal comprise platinum, gold, silver, nickel and combinations thereof.
In a preferred embodiment, this second electrically conductive metal may have a surface 25 that is readily joinable to other materials, particularly electrically conductive metals. These material-joining processes may include soldering, welding and/or brazing. Preferably, the surface 25 of the second metal is “wettable” to tin based solders, such as Sn63/Pb37 and the like. A “wettable” surface is herein defined as the ability of a material to adhere to the surface.
In a preferred embodiment, as shown in
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.
Such a preferred embodiment of selectively applying the exterior outer coating 16A enhances electrical conduction and retards oxidation of the surface 23 of the terminal, pin core 16B within these regions 17, 19, 21. Selectively applying the external outer coating 16A to the core 16B allows for improved design and manufacturing flexibility. For example, the coating 16A may be precisely applied to the surface 23 of the core 16B after high temperature processing. This feature is beneficial the external outer coating 16A can be tailored to meet the dimensions of the joining metal. Furthermore, the application of the external outer coating 16A to a discrete portion of the core 16B further reduces cost of manufacture.
In a preferred embodiment, as illustrated in
For example, it is known that refractive metals such as niobium, tungsten and molybdenum readily oxidize. 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) characteristics. Providing a palladium outer coating 16A over a niobium core 16B in an evacuated atmosphere prior to formation of niobium oxide ensures that the thusly constructed terminal pin can be directly brazed into the insulator 20.
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.
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 such alternative metals as 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 equivalent 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.
As further shown in
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.
This application claims priority from U.S. Provisional Patent Application Ser. No. 61/354,747 filed Jun. 15, 2010.
| Number | Date | Country | |
|---|---|---|---|
| 61354747 | Jun 2010 | US |
