Abstract:
A resistance welding electrode that is particularly suited for resistance spot welding a workpiece to a substrate without cross-contaminating specific portions of the workpiece is described. This is done by sheathing an internal surface of the welding electrode with a thermoplastic polymeric material. Then, there is only contact between the welding electrode and the workpiece at the very distal end of the electrode adjacent to where the workpiece is to be connected to the substrate. Contamination in this area is not detrimental because subsequent workpiece plating does not take place there.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority based on provisional application Serial No. 60/342,294, filed Dec. 20, 2001. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to electrodes for resistance or elective spot welding, and particularly to construction of such electrodes. 
     2. Prior Art 
     In resistance spot welding, it is often difficult to prevent cross contamination of the material of the welding electrode onto the workpiece being welded. In those situations when the welded workpiece is subjected to further processing step, such as plating, cross contamination is undesirable. Often the contamination makes it difficult to plate over the workpiece. An example of this is when a nickel terminal pin is spot welded to the casing of an electrical energy power source for an implantable medical device. Copper is typically used for the welding electrode. However, copper contamination of the nickel surface makes it extremely difficult, if not impossible, to gold plate over the pin. In an implantable power source, the nickel pin welded to the battery or capacitor casing serves as one of the casing terminals. A second terminal pin electrically insulated from the casing by a glass-to-metal seal is the opposite polarity terminal. 
     What is needed, therefore, is a welding procedure for connecting a workpiece material to a substrate, such as a terminal pin to a casing for an electrical energy power source, that does not result in cross-contamination of the welding electrode material onto the workpiece material. 
     SUMMARY OF THE INVENTION 
     The present invention describes a resistance welding electrode that is particularly suited for spot welding a workpiece to a substrate without cross-contaminating specific portions of the workpiece. This is done by sheathing an internal surface of the welding electrode with a thermoplastic polymeric material. Then, there is only contact between the welding electrode and the workpiece at the very distal end of the electrode adjacent to where the workpiece is to be connected to the substrate. Contamination in this area is not detrimental because subsequent workpiece plating does not take place there. 
     These and other aspects of the present invention will become increasingly more apparent to those skilled in the art by reference to the following description and the appended drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a side elevational view and end view of a tip  10  for a welding electrode  12  according to the present invention. 
     FIG. 2 is a side elevational view and end view of the welding tip  10  shown in FIG. 1 provided with a polymeric sleeve  22 . 
     FIG. 3 is a side elevational view of welding electrode  12  including the tip  10  shown in FIG.  2 . 
     FIG. 4 is an exploded, cross-sectional view of the welding electrode  12  shown in FIG. 3 provided with a workpiece pin  42  that is intended to be spot welded to a substrate. 
     FIG. 5 is a schematic of an electrical energy storage device having the pin  42  welded to its casing to thereby serve as a terminal lead. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Turning now to the drawings, FIGS. 1 and 2 show a tip  10  for a spot welding electrode  12  (FIGS. 3 and 4) according to the present invention. The welding tip  10  is of a conductive material, such as of copper, stainless steel, titanium, aluminum, platinum, tantulum, and alloys thereof having a proximal, threaded section  14  extending to a first cylindrically-shaped section  16  of a reduced diameter. The first cylindrical section  16  steps up to a second cylindrically-shaped section  18  that is not threaded. A cylindrically-shaped bore  20  is provided through the tip  10  centered along its longitudinal axis. 
     FIG. 2 shows the tip  10  having a relatively thin-walled polymeric sleeve  22  in the shape of tubing received in the bore  20 . The sleeve  22  is preferably of a polyimide and of a diameter sufficient to snuggly fit inside the bore  20 . An industrial adhesive  24 , such as PERMABOND®, secures the sleeve in place. A distal end  26  of the sleeve is provided flush with the distal end wall  28  of the untreaded section  18 . However, the sleeve  22  is of a sufficient length to extend beyond the proximal end  30  of the threaded section  14 . 
     As shown in FIGS. 3 and 4, the welding tip  10  including the polymeric sleeve  22  is threadingly mated to the welding electrode  12 . The electrode comprises a cylindrically-shaped main body  32  having a frusto-conical shaped nose  34 . A first, threaded bore  36  is aligned along the longitudinal axis of the electrode  12 , extending from the nose  34  and a portion of the distance through the main body  32  where it widens into a second bore  38 . The second bore  38  extends to the proximal end  40  of the main body  32  where it is threaded for connection to a welding apparatus (not shown). 
     In use, a workpiece pin  42  is loaded into the welding tip  10 . As shown in FIG. 4, the pin  42  has a cylindrically-shaped shaft portion  44  connected to an enlarged head  46 . The shaft  44  is sized to easily move into and out of the polymeric sleeve  22 . In the position shown in the figure, the only contact between he welding tip  10  and the pin  42  is at the distal end wall  28  of the tip  10 . This is sufficient contact such that when the welding electrode  12  is electrically energized, the distal surface or projection  46 A of enlarged head  46  becomes molten. The molten material is then contacted to a substrate support and the electrical current removed from the welding electrode  12 . The welding electrode is moved away from the substrate with the polymeric sleeve  22  sliding over the shaft  44  of the pin  42 . In this manner, the pin is left behind, welded to the substrate. 
     FIG. 5 shows the pin  42  after it has been welded to the lid  48  for a casing  50  of an exemplary electrical energy storage device  52 . Nickel is a commonly used material for the pin  42 . Before the pin is connected to its associated device, however, it is desirable to coat it with a cover material, such as gold. Should copper from the welding electrode contaminate the pin shaft, the gold plating is often of poor quality. The provision of the polymeric sleeve  22  prevents any such cross-contamination. The uncontaminated pin  42  is readily plated along its shaft  44 . What little contamination there is at the enlarged head  46  is not a problem because plating does not take place there. 
     In that respect, the present invention is applicable for use with any type of electrical energy storage device housed inside of a casing. This includes low rate, medium rate, high rate, case negative and case positive electrochemical cells of both primary and secondary chemistries. Examples of such cells include lithium iodine cells, lithium thionychloride cells, lithium silver vanadium oxide cells, lithium carbon monofluoride cells, lithium manganese dioxide cells, and secondary cells containing lithium cobalt oxide, and the like. 
     In any event, these types of electrical energy storage devices are typically constructed with the casing serving as the terminal for one of the electrodes, such as the negative electrode, and a terminal lead  54  connected to the other electrode, such as the positive electrode. This is referred to as a case negative design. In that respect, the pin  42  directly connected to the casing  50  provides a structure for connecting to a device to be powered by the electrical energy storage device. 
     As shown in FIG. 3, the welding electrode  12  is provided with a cut-out portion  56 . The cut-out  56  is so that the electrode  12  does not contact the therminal lead  54  as the pin  42  is being welded to the lid  48 . The electrical energy storage device is complete by a fill opening  58  sealed with a closure member, such as a metal ball  60 . The opening  58  is for filling an electrolyte into the casing for activating the negative and positive electrodes. 
     It will further be recognized that such electrical energy storage devices may take one of various configurations. For example, depending on the type the configuration of the anode, cathode, terminal lead, and fill opening, etc. will vary. Also, when the electrical energy storage device is an electrochemical cell, the materials housed inside the casing will vary. Such materials may take the form of a liquid or a solid depending on the type of cell. Therefore, it should be clear that the present invention is in no manner limited to a specific type of electrochemical chemistry. 
     The present invention is also applicable to connecting pins to capacitors, such as those described in U.S. Pat. Nos. 5,926,362 and 6,334,879, both to Muffoletto et al. These patents are assigned to the assignee of the present invention and incorporated herein by reference. 
     It is appreciated that various modifications to the inventive concepts described herein may be apparent to those of ordinary skill in the art without departing from the spirit and scope of the present invention as defined by the appended claims.