Abstract:
A hermetic ground pin assembly for making an electrical connection between a hermetically sealed enclosure and an external circuit. A case has an aperture and a plug located within the aperture. A low temperature braze alloy is located between the plug and the case to attach the plug within the aperture. Two different pins are attached to opposite sides of the plug. A high temperature braze alloy attaches one of the pins to the plug and the low temperature braze alloy attaches the other pin to the plug.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a hermetic ground pin assembly for making an electrical connection between a hermetically sealed enclosure and an external circuit. 
     2. Description of the Related Art 
     Various devices are well known for conducting an electrical signal. One type of application for signal conduction that is very demanding is in the field of implantable electronic medical devices such as pacemakers, heart pumps and implantable defibrillators. These electronic packages must be of the highest reliability and corrosion resistance as any failure can have potentially lethal consequences. The electronics are hermetically sealed within a case typically made from titanium. The case needs to be grounded to protect the circuitry inside from electromagnetic transients and electrostatic discharge. At the same time, the circuitry inside the case must be electrically isolated from the case and yet be connectable to external circuits such as a battery or signal pins. 
     In order to insure reliable connections gold plated pins and/or terminals are used. The plating is done after any high temperature processing. Subjecting the plating to high temperatures will destroy the integrity of the plating. Unfortunately, apart from the high cost of gold plating, the case cannot be electroplated due to physiological compatibility problems during contact with human tissue. Therefore, the. case cannot be connected directly to the electrical connectors for electroplating. In prior art designs, a ceramic insert was used to isolate the pins from the case for electroplating. Other methods then had to be employed such as special glass to metals seals around a wire to provide a ground connection after the rest of the assembly was completed. A current unmet need exists for a reliable, low cost, simple and robust hermetic ground pin assembly for making an electrical connection between a hermetically sealed enclosure and an external circuit that allow other terminals to be electroplated and the electronic enclosure to be physiologically compatible with human tissue. 
     3. Related Art 
     Examples of patents that are related to the present invention are as follows, and each patent is herein incorporated by reference for the supporting teachings: 
     U.S. Pat. Ser. No. 5,871,513 is a centerless ground feedthrough pin for an electrical power source in an implantable medical device. 
     The foregoing patents reflect the state of the art of which the applicant is aware and are tendered with the view toward discharging the applicant&#39;s acknowledged duty of candor in disclosing information which may be pertinent in the examination of this application. It is respectfully stipulated, however, that none of these patents teach or render obvious, singly or when considered in combination, the applicants claimed invention. 
     SUMMARY OF THE INVENTION 
     It is a feature of the invention to provide a hermetic ground pin assembly for making an electrical connection between a hermetically sealed enclosure and an external circuit. 
     An additional feature of the invention is to provide a hermetic ground pin assembly for making an electrical path that includes a case or a case having an aperture therethrough and a plug located within the aperture. A low temperature braze alloy is located between the plug and the case for affixing the plug within the aperture. One or more pins are attached to the plug. A high temperature braze alloy is located between the plug and the outer pin for affixing the pin to the plug. 
     The invention resides not in any one of these features per se, but rather in the particular combination of all of them herein disclosed and claimed and it is distinguished from the prior art in this particular combination of all of its structures for the functions specified. Other features of the present invention will become more clear from the following detailed description of the invention, taken in conjunction with the accompanying drawings and claims, or may be learned by the practice of the invention. 
     There has thus been outlined, rather broadly, the more important features of the invention in order that the detailed description thereof that follows may be better understood, and in order that the present contribution to the art may be better appreciated. There are, of course, additional features of the invention that will be described hereinafter and which will form the subject matter of the claims appended hereto. Those skilled in the art will appreciate that the conception, upon which this disclosure is based, may readily be utilized as a basis for the designing of other structures, methods and systems for carrying out the several purposes of the present invention. It is important, therefore, that the claims be regarded as including such equivalent constructions insofar as they do not depart from the spirit and scope of the present invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a side cross-sectional view of the preffered embodiment of a hermetic ground pin assembly. 
     FIG. 2 is a side cross-sectional view of another embodiment of a hermetic ground pin assembly 
    
    
     It is noted that the drawings of the invention are not to scale. The drawings are merely schematic representations, not intended to portray specific parameters of the invention. The drawings are intended to depict only typical embodiments of the invention, and therefore should not be considered as limiting the scope of the invention. The invention will be described with additional specificity and detail through the use of the accompanying drawings. In the drawings like numbering represents like elements between the drawings. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 1, there is a hermetic ground pin assembly  10  shown. Assembly  10  has a titanium case  12  for containing electronic circuitry (not shown). Case  12  has a cavity  11 . Case  12  has an inside surface  13  and an outer surface  14 .: An aperture  15  passes through case  12 . An annular titanium plug  16  is located in aperture  15  and has a pair of opposed bores, an inner bore  18  and an outer bore  17 . Outer bore  17  has a flare  19  and inner bore  18  has a flare  20 . A cylindrical inner pin  25  and a cylindrical outer pin  24  are located and affixed into bores  17  and  18 , respectively. Outer pin  24  is held by a high temperature braze alloy  27 . Inner pin  25  is held by a low temperature braze alloy  28 . The outer pin is formed from an alloy of 90% platinum and 10% iridium. The inner pin is formed from 100% nickel so that it can be wire bonded to. Pin  25  has a head  26 . A wire  30  is attached to head  26  by a wire bond  29 . Wire  30  is an ultrasonically bondable wire such as aluminum or gold. High temperature braze alloy  27  is located in flare  19  between pin  24  and bore  17 . The high temperature braze alloy  27  is made up of an alloy that ranges in concentration of 10 to 30 percent nickel and 70 to 90 percent gold by weight. The plug  16  has an annular groove  21  disposed circumferentially and an annular step  22  at one end. The plug  16  is located and affixed into aperture  15  by a low temperature braze alloy  28 . Low temperature braze alloy  28  is located between case  12  and plug  16 . The pin  25  is located and affixed into bore  18  by low temperature braze alloy  28 . The low temperature braze alloy  28  is made up of an alloy that ranges in concentration of 20 to 40 percent copper and 60 to 80 percent silver by weight. Other pins (not shown) are used in the package that are insulated from the case  12  by a ceramic insert (not shown). These pins are the signal pins which are required to be electroplated with gold. 
     The hermetic ground pin assembly  10  can be assembled in the following manner. First the outer pin  24  is inserted into the plug and the high temperature braze alloy is placed as a preform or otherwise adjacent flare  19 . The plug and pin are placed in a fixture and then inserted into a vacuum furnace between 920 and 970 degrees Celsius, such that the high temperature braze melts. The pin and plug is then cooled to room or ambient temperature. The high temperature braze solidifies and pin  24  is fixed into bore  17 . Next, the pin  25  is inserted into bore  18  and plug  16  is inserted into aperture  15 . A low temperature braze alloy is applied as a preform in the aperture, between the case and the plug and around pin  25  adjacent flare  20 . The low temperature braze alloy has a concentration range of 20-40 percent copper and 60-80 percent silver by weight. The case and plug, other signal pins and ceramic spacers are then placed in a fixture and heated in a vaccuum oven to between 760 and 800 degrees Celsius, such that the low temperature braze alloy melts. The assembly  10  is then cooled. The low temperature braze alloy solidifies and the plug  16  and pins are fixed into the aperture. Next, all the other pins in the package besides the ground pin are electroplated with electrolytic nickel and then with gold plating. The package would then have circuitry placed in case  12  for connection to the ground pin using wirebond  29 . 
     Referring to FIG. 2, there is another embodiment of a hermetic ground pin assembly  50  shown. Assembly  50  has a titanium case  12  for containing electronic circuitry (not shown). Case  12  has a cavity  11 . Case  12  has an inside surface  13  and an outer surface  14 . An inner bore  52  and an outer bore  53  are located on inside surface  13  and outer surface  14 , respectively. Outer bore  53  has a flare  55  and inner bore  52  has a flare  56 . A cylindrical inner pin  25  and a cylindrical outer pin  24  are located and affixed into bores  52  and  53 , respectively. Outer pin  24  is held by a high temperature braze alloy  27 . Inner pin  25  is held by a low temperature braze alloy  28 . The outer pin is formed from an alloy of 90% platinum and 10% irdium. The inner pin is formed from 100% nickel so that it can be wire bonded to. Pin  25  has a head  26 . A wire  30  is attached to head  26  by a wire bond  29 . Wire  30  is an ultrasonically bondable wire such as aluminum or gold. High temperature braze alloy  27  is located in flare  55  between pin  24  and bore  53 . The high temperature braze alloy  27  is made up of an alloy that ranges in concentration of 10 to 30 percent nickel and 70 to 90 percent gold by weight. The pin  25  is located and affixed into bore  52  by low temperature braze alloy  28 . The low temperature braze alloy  28  is made up of an alloy that ranges in concentration of 20 to 40 percent copper and 60 to 80 percent silver by weight. Other pins (not shown) are used in the package that are insulated from the case  12  by a ceramic insert (not shown). These pins are the signal pins which are required to be electroplated with gold. Hermetic ground pin assembly  50  is assembled similar to assembly  10 , with the high temperature braze alloy melted around outer pin  24  first and then the low temperature braze alloy melted around inner pin  25 , next. 
     Remarks About the Preferred Embodiment 
     The grounded pin assembly  10  of FIG. 1 allows for an electronic package with the following attributes: A hermetically sealed package. A wire bondable pin on the inside of the case that is not electroplated. A solderable pin on the outside of the case. A titanium case that is free of electroplating. Signal terminals or pins attached to the case that are electroplated with gold. 
     Variations of the Preferred Embodiment 
     Although the illustrated embodiment shows the use of a platinum and iddium alloy pin, if desired other types of alloys could be used such as alloys of palladium, gold, silver, rhodium. 
     The ground pin assembly shown used a low temperature braze alloy and a high temperature braze alloy of a particular composition. 
     While the invention has been taught with specific reference to these embodiments, someone skilled in the art will recognize that changes can be made in form and detail without departing from the spirit and the scope of the invention. The described embodiments are to be considered in all respects only as illustrative and not restrictive. The scope of the invention is, therefore, indicated by the appended claims rather than by the foregoing description. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope.