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 simplified feedthrough terminal pin sub-assembly which incorporates a capture flange to facilitate assembly by automation.
Feedthrough terminal assemblies are generally well known 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 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 electromagnetic interference (EMI) signals for transmission into the interior of the medical device. In prior devices, such as those shown in U.S. Pat. Nos. 5,333,095 and 4,424,551 (the contents of which are incorporated herein), the hermetic terminal pin sub-assembly has been combined in various ways with a ceramic feedthrough capacitor filter to decouple electromagnetic interference (EMI) signals into the housing of the medical device.
With reference to FIG. 1, in a typical prior art unipolar feedthrough filter assembly 20 (as described in U.S. Pat. No. 5,333,095), a round/discoidal (or rectangular) ceramic feedthrough filter capacitor 22 is combined with a hermetic terminal pin assembly 24 to suppress and decouple the undesired interference or noise transmission along a terminal pin 26. The feedthrough capacitor is coaxial, having two sets of electrode plates 28, 30 embedded in spaced relation within an insulative dielectric substrate or base 32, formed typically as a ceramic monolithic structure. One set of the electrode plates 28 is electrically connected at an inner diameter cylindrical surface of the coaxial capacitor structure to a conductive terminal pin 26 utilized to pass the desired electrical signal or signals. A second set of electrode plates 30 is coupled at an outer diameter surface of the discoidal capacitor 22 to a cylindrical ferrule 34 of conductive material, which is electrically connected in turn to the conductive housing of the electronic device. The number and dielectric thickness spacing of the electrode plate sets 28, 30 varies in accordance with the capacitor value and the voltage rating of the coaxial capacitor 22. The second set of electrode plates 30 (or "ground" plates) are coupled in parallel together by a metallized layer 36 which is either fired, sputtered or plated onto the ceramic capacitor. The metallized band 36, in turn, is coupled to the ferrule by conductive adhesive, soldering, brazing, welding, or the like. The first set of electrode plates 28 (or "active" plates) are coupled in parallel together by a metallized layer 38 which is either glass frit fired or plated onto the ceramic capacitor. This metallized band 38, in turn, is mechanically and electrically coupled to the leads wire(s) 26 by conductive adhesive, soldering or the like.
In operation, the coaxial capacitor 22 permits passage of relatively low frequency electrical signals along the terminal pin 26, while shielding and decoupling/attenuating undesired interference signals of typically high frequency (such as EMI from cellular telephones or microwave ovens) to the conductive housing. Feedthrough capacitors of this general type are available in unipolar (one), bipolar (two), tripolar (three), quadpolar (four), pentapolar (five), hexpolar (six), and additional lead configurations. The feedthrough capacitors (in both discoidal and rectangular configurations) of this general type are commonly employed in implantable cardiac pacemakers and defibrillators and the like, wherein the pacemaker housing is constructed from a biocompatible metal such as titanium alloy which is electrically and mechanically coupled to the hermetic terminal pin assembly 24, which is in turn electrically coupled to the coaxial feedthrough filter capacitor 22. As a result, the filter capacitor and terminal pin assembly prevents entrance of interference signals to the interior of the pacemaker housing, wherein such interference signals could otherwise adversely affect the desired cardiac pacing or defibrillation function.
One drawback of the feedthrough filter assembly 20 described above is that it does not lend itself well to automated assembly using robots. More specifically, U.S. Pat. No. 5,333,095 (FIG. 1) discloses an EMI filter capacitor 22 which is essentially surface mounted onto the hermetic seal or titanium casing 40 of an implantable medical device. In U.S. Pat. No. 5,333,095, connections from the perimeter or outside diameter of the capacitor 22 are generally made by hand dispensing a conductive thermal-setting material 42 such as a conductive polyimide around the circumference of said capacitor. In a similar manner, electrical contact is made between the lead wires 26 and the inside diameter of the feedthrough capacitor 22. This construction technique results in a highly reliable and high performance EMI feedthrough capacitor; however, the handwork is very time consuming and is therefore costly. The design of the feedthrough terminal assembly 20 of U.S. Pat. No. 5,333,095 does not lend itself to automation. That is because there is no reservoir to contain the conductive thermal-setting material 42 such that it could be easily dispensed using an automated epoxy dispenser or robot. In addition, the lack of a capacitor guide combined with the necessity to hand dispense the conductive thermosetting material 42 around the capacitor outside diameter results in a substantial number of visual rejects. These visual rejects are typically noted by the customer at receiving inspection and are related in variations in the amount and location of the hand dispensed connection material.
Referring to FIG. 2, U.S. Pat. No. 4,424,551 teaches the installation of a circular or discoidal feedthrough capacitor 22 completely inside of a cylindrical cavity. As taught in the '551 patent, it is possible to automate this assembly by flooding the entire top surface of the ceramic capacitor 22 with a conductive thermal-setting material 42 and then centrifuging this assembly such that the conductive thermal-setting material (through centrifugal acceleration forces) is moved into the inner and outer cavities between the capacitor outer diameter and ferrule 34, and between capacitor inner diameter and lead(s) 26. The outer cavities as defined in the U.S. Pat. No. 4,424,551 specifically mean the space between the capacitor 22 outside diameter and the inside of the cylindrical ferrule 34, and the inner cavity is the space between the lead wire 26 and the inside diameter of the feedthrough capacitor. There are a number of significant problems associated with the approach of U.S. Pat. No. 4,424,551 including the fact that the package is not particularly volumetrically efficient. That is, the cylindrical ferrule 34 extends well above the ceramic capacitor 22 and a nonconductive epoxy coating (cap) 44 is placed over the top of the capacitor. Not only is this top covering volumetrically inefficient, it also does not match the thermal coefficient of expansion of the relatively brittle ceramic capacitor. This can lead to both moisture and micro-cracking problems within the ceramic capacitor 22 structure itself. Another problem is that the epoxy cap 44 material tends to act as an adjunct hermetic seal which can mask a leaking gold braze or glass hermetic seal underneath. That is, the hermetic seal test is typically done with helium where such test is done in a few seconds. The bulk permeability of such epoxy coverings/caps 44 is such that it might take several minutes for the helium to penetrate through such a covering.
Accordingly, there is a need for a novel feedthrough filter terminal assembly that addresses the drawbacks noted above in connection with the prior art. In particular, a novel terminal assembly is needed which accommodates automated dispensing or robotic installation of assembly components. Additionally, the improved feedthrough filter terminal assembly should lend itself to standard manufacturing processes such that cost reductions can be realized immediately. Of course, the new design must be capable of effectively filtering out undesirable electromagnetic interference (EMI) signals from the target device. The present invention fulfills these needs and provides other related advantages.