Abstract:
A welded wire termination for use in thermally and mechanically stressful environments is provided by forming a wire termination tab on a conductive pad to define a tubule for receiving a wire, either stranded or solid. The tubule is tweezer welded to form a welded termination joint between the wire and the conductive tab. The tubule easily captures all of the stranded wires and increases the surface area of the weld to a solid lead. A tweezer weld exerts pressure laterally on the tubule, and thus does not exert a force on the underlying substrate that could damage the substrate. This approach creates only a single electro-mechanical connection from the wire to the conductive pad, which is inherently more reliable than multiple connections such as those that are crimped first then welded.

Description:
This is a division of parent application filed on Dec. 12, 1996 with Ser. No. 08/764,801. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to welded wire termination and more specifically to a wire termination tab that is integrally formed with a conductive surface, and which is formable into a tubule for receiving a wire for welded termination. 
     2. Description of the Related Art 
     In many applications such as satellite solar arrays and printed circuit (PC) boards wires must be terminated to a conductive pad to interconnect circuitry and other electrical systems. Typically, the wire is soldered flat to the conductive pad. However, in high reliability thermally stressful environments solder is not a viable option. Because of the mismatch between the thermal coefficients of expansion (TCE) for the solder, the wire, the conductive pad and its underlying substrate, extended thermal cycling will fatigue the solder and may cause it to crack and eventually fail. A satellite, for example, will thermal cycle between −180° C. and +80° C. for approximately 1600 cycles in a geosynchronous orbit and will thermal cycle between −80° C. and +100° C. for approximately 30,000 cycles in a low earth orbit. Furthermore, direct exposure to an extremely high temperature may liquify the solder causing the joint to fail. The potential for extremely high temperatures exists in military satellites, space probes which utilize solar array aerobraking and some automotive applications. To satisfy the reliability requirements in these types of thermally stressful environments, a non-solder wire termination is required. 
     The wires, either stranded wire or solid wire, can be terminated to a conductive pad by direct welding. Welding stranded wire to the conductive pad is difficult because the strands separate and may remain loose, thus risking a short circuit. Ensuring each strand is welded is an expensive labor intensive process. Solid wires such as diode or other component leads are difficult to weld because they are typically round and relatively thick, approximately 0.1 cm. Because they are round, the leads present only a small surface contact area with the conductive pad, which weakens the weld joint. Furthermore, the leads cannot be pressed sufficiently to reduce their thickness without sacrificing material integrity. As a result, a high power weld that exerts an extremely high localized pressure, for example 10,000 lbs per square inch (psi), on the underlying substrate is required. The weld may damage the underlying solar panel substrate which is typically formed from a thin brittle material such as graphite or Kevlar®. 
     A known approach is to crimp a flat lug onto the end of the wire and then weld the flat lug onto the surface of the conductive pad. There are several disadvantages to this process. First, a crimp lug that is strong enough to mechanically hold the wire strands must be relatively thick, e.g. 0.015 to 0.025 cm. The crimp lug is welded to the conductive pad using a pincher weld, in which opposing weld tips are placed on the surface of the lug and underneath the conductive pad, respectively. Because the lug is so thick, a force of approximately 10,000 psi has to be exerted on the weld tips to successfully weld the crimp lug to the conductive pad. Once the conductive pad is mounted onto its substrate, any repairs must be done using a parallel gap weld from the top surface. The extreme force required during welding can damage the thin, brittle graphite or Kevlar® substrates below the conductive pad. Furthermore, crimping the wire strands is an expensive labor intensive process, in which it is difficult to capture every strand. Stray strands can potentially cause a short. Because this approach involves two connections, pad-to-lug and lug-to-wire, the overall reliability of the wire termination is reduced. In addition, the crimped lug increases the series resistance of the connection. 
     SUMMARY OF THE INVENTION 
     In view of the above problems, the present invention provides a welded wire termination device and method that reduces the cost and improves the reliability of the welded wire termination in thermally or mechanically stressful environments. 
     This is accomplished by forming a wire termination tab on a thin conductive pad to define a tubule for receiving a wire, either stranded or solid. The tubule is tweezer welded to form a welded termination joint between the wire and the conductive tab. The tubule easily captures all of the stranded wires and increases the surface area of the weld since it is welded on both sides. A tweezer weld exerts pressure laterally on the tubule, and thus does not exert a force on the underlying substrate that could damage the substrate. This approach creates only a single electro-mechanical connection from the wire to the conductive pad, which is inherently more reliable than multiple connections. 
     These and other features and advantages of the invention will be apparent to those skilled in the art from the following detailed description of preferred embodiments, taken together with the accompanying drawings, in which: 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a plan view of a welded wire termination blank in accordance with the present invention; 
     FIGS. 2 a  through  2   d  illustrate the process of forming the termination blank shown in FIG. 1 to provide a welded termination joint; 
     FIGS. 3 a  and  3   b  are sectional views of an alternate welded termination joint using pincher and parallel gap welding, respectively; 
     FIGS. 4 a  and  4   b  are sectional views of another alternate welded termination joint using pincher and parallel gap welding, respectively; 
     FIG. 5 is a plan view of an end termination tab for a solar array having integrally formed wire termination tabs; 
     FIG. 6 is a perspective view of a solar array in which the wires are connected using the welded termination joint of the present invention; 
     FIG. 7 is a plan view of a PC board having an integrally formed power bus with wire termination tabs; 
     FIG. 8 is a sectional view of the PC board with the wire termination tabs formed into tubules for receiving a wire; and 
     FIG. 9 is a perspective view of a pair of welded wire termination pad having tubules for terminating the solid diode leads. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention provides a high quality welded wire termination that exhibits a high reliability in a thermally stressful environment and can withstand exposure to extreme temperatures. A wire termination tab is formed on a conductive pad to define a tubule for receiving a wire, either stranded or solid. The tubule is tweezer welded to form a welded termination joint between the wire and the conductive tab. The tweezer weld does not exert a force on the underlying substrate that could damage the substrate. The tubule easily captures all of the stranded wires and increases the surface area of the weld to a solid lead. 
     FIG. 1 shows a termination blank  10  that includes a conductive pad  12  and a pair of wire termination tabs  14  that are integrally formed with the conductive pad  12  along an interior edge  16  and extend laterally therefrom. As detailed in FIGS. 2 a  through  2   d , the wire termination tabs  14  are formed into tubules above the surface of the conductive pad  12  for receiving and terminating wires to the conductive pad. A layout designer specifies the number, size and position of the tubules on the conductive pad  12  as well as the size and shape of the pad itself. For example, an end termination tab for a solar cell may include three tabs that extend out of the conductive pad for connection underneath the solar cell and include two wire termination tabs for forming tubules to interconnect adjacent end termination tabs. A flat sheet of metal such as silver plated copper, molybdenum, kovar or invar is punched or etched to pattern the termination blank  10  as specified. 
     To form the tubules and terminate the wires, a die  18  is placed on the surface of the conductive pad  12  adjacent to the interior edge  16  of the wire termination tab  14  as shown in FIG. 2 a  The die preferably has an oval or circular shape so that the resulting tubule will have smoothed, and thus stronger, shape. The die has a diameter that is slightly larger than the diameter of the wire that will be inserted into the tubule. 
     As shown in FIG. 2 b , the wire termination tab  14  is formed around the die  18  to define a tubule  20  between the wire termination tab  14  and the conductive pad  12 . In the preferred embodiment, the exterior edge  22  of the wire termination tab  14  is positioned proximate to the surface of the conductive pad  12 , either just touching or slightly spaced apart from its surface. 
     Once formed, the die  18  is removed and the conductive pad  12  is typically mounted onto a substrate  24  such as a solar panel as shown in FIG. 2 c . A wire having strands  28  is inserted into the tubule  20  so that all of the strands  28  are captured by the tubule. Thereafter, the tubule  20  is tweezer welded to form a welded termination joint  30  that electro-mechanically connects the strands  28 , and hence the wire, to the conductive pad. A tweezer weld entails placing a pair of electrodes  32  on opposites sides of the tubule  20  and forcing current through them while exerting an approximately lateral inward pressure of 15-20 lbs. The combination of heat and pressure produces the welded termination joint  30 . 
     As shown in FIG. 2 d , the pressure of the tweezer weld crushes the tubule inward which flattens the strands  28  and increases the surface area of the welded termination joint  30 . To lower its profile, and thus reduce the chance that it could get ripped off of the conductive pad, the crushed tubule  20  is preferably bent down onto the surface of the conductive pad  12 . A similar approach can be used for a solid wire. 
     Because the tweezer weld does not exert a downward force on the conductive pad  12 , the underlying substrate  24  is not subjected to a localized force that could damage the substrate. Furthermore, tweezer welding the tubule produces a large and strong welded termination joint  30  that exhibits high reliability under extreme thermal conditions. 
     FIGS. 3 a - 3   b  and  4   a - 4   b  depict alternate embodiments of the welded termination joint. In FIGS. 3 a  and  3   b , the wire termination tab  14  is formed so that it defines the tubule  20  and an exterior flange  34  on the surface of the conductive pad  12 . The exterior flange  34  is welded to the conductive pad using a pincher weld (FIG. 3 a ) or a parallel gap weld (FIG. 3 b ). As long as the conductive pad  12  is not integrally formed with its substrate, the exterior flange  34  can be welded before the conductive pad  12  is attached to its substrate, thus avoiding any damage to the substrate. This approach provides a stronger and more well defined tubule  20  at the cost of an additional weld. 
     In FIGS. 4 a  and  4   b , the wire termination tab  14  is formed so that it defines the tubule  20  and an interior flange  36  on one side of the tubule adjacent the interior edge  16  and an exterior flange  38  on the other side on the surface of the conductive pad  12 . The interior and exterior flanges  36  and  38 , respectively, are welded to the conductive pad using a pincher weld (FIG. 4 a ) or a parallel gap weld (FIG. 4 b ). A solid wire  39  is inserted into the tubule and tweezer welded. This approach provides an even stronger tubule that is offset from the edge of the conductive pad at the cost of two additional welds. 
     A particular application for the welded wire termination of the present invention is in interconnection of series connected solar cell strings in a satellite solar array. As shown in FIG. 5, an end termination tab blank  40  is patterned with a conductive pad  42 , three solar cell termination tabs  44  that extend laterally from one side of the pad for termination to a solar cell, and three wire termination tabs  46  that extend laterally from the other side of the pad for forming respective tubules to terminate welded wires. 
     The end termination tab blank  40  is formed into an end termination tab  48  having three tubules  50   a ,  50   b , and  50   c  and used to clamp the soldered wires in a satellite solar array  52  as shown in FIG.  6 . The solar array  52  includes a plurality of solar cells  54  that are connected in series to produce a desired voltage and in parallel to produce a desired current, and mounted on a solar panel substrate  56 . The substrate  56  provides a rigid support structure with sufficient axial and bending stiffness for carrying the solar cells through a dynamic launch environment into orbit and positioning them to receive illumination. The substrate  56  is suitably formed by bonding a pair of thin facesheets  60 , suitably Kevlarg, graphite or aluminum, onto opposite sides of an aluminum honeycomb core  61 . If the material used in the facesheets  60  is conductive, an insulation layer  62  is formed over the top facesheet to electrically isolate the solar cells  54 . The combination of the honeycomb core with the stiff facesheets provides a lightweight yet strong substrate  56 . The TCEs of graphite and Kevlar® are much lower than the TCEs of the end termination tab and strap. In fact graphite composite structures can have a negative TCE such that it expands when cooled and contracts when heated, which produces stress on the solar array&#39;s soldered wires. 
     The solar cells  54  are connected in series to form strings  63   a ,  63   b , . . .  63   n . Each string is terminated with an end termination tab  48  having tubules  50   a ,  50   b , and  50   c . The strings are connected in parallel by terminating one end of a wire  64   a  to via a hole  66  in the substrate to a terminal board (not shown), typically ground reference potential or the supply voltage. The other end of wire  64   a  is inserted into the opening defined by the tubule  50   a  on the end termination tab  48 . The ends of wire  64   b  are inserted into the tubule  50   c  on the first end termination tab and the tubule  50   a  on the second end termination tab. The center tubule  50   b  is unused in this particular application but can be used for redundant wiring or wire rework. The sequence is repeated for each of the n end termination tabs  48  until the last wire is terminated to the reference terminal. The tubules  50   a  and  50   c  are tweezer welded to form welded termination joints  68  that connect one end of the strings  63   a ,  63   b , . . .  63   n  in series/parallel. The other end of the strings  63   a ,  63   b , . . .  63   n  are similarly terminated to the other reference terminal to complete the series/parallel connection of the solar cells  52 . 
     FIG. 7 is a plan view of a PC board  70  prior to the insertior of a stranded wire. A power bus  72  is integrally formed on the surface of the PC board  70  for distributing electrical signals to the circuitry on the board. A plurality of wire termination tabs  74  are integrally formed with the power bus  72  and extend laterally from the edge of the PC board  70 . As shown in FIG. 8, the wire termination tabs  74  are formed on a die to define a tubule  76  over the surface of the power bus  72 . A stranded wire  77  is inserted into the tubule  76  and tweezer welded to form a welded termination joint for communicating externals to the circuitry on the PC board  70 . 
     The tubules can be used to rework terminated wires that have to be replaced. For example, if the weld joints between the leads of a diode and respective copper bus pads on a PC board become fatigued or fail, the terminated wires can be reworked by welding a pair of conductive pads formed with a tubule to the respective bus pads, inserting the leads into the tubules, and tweezer welding them to form a reliable weld termination joint. As shown in FIG. 9, a diode  78  is terminated between a pair of bus pads  80  and  82  on a PC circuit board  84  by welding its leads  86  and  88  to tubules  90  and  92 , respectively. The tubules are integrally formed on a pair of conductive pads  94 ,  96 , which are parallel gap welded to the bus pads  80  and  82 . In an alternate embodiment, the conductive pads could be adhered directly to the PC board using a transfer adhesive. 
     While several illustrative embodiments of the invention have been shown and described, numerous variations and alternate embodiments will occur to those skilled in the art. For example, instead of patterning the wire termination tab integrally with the end termination tab or PC bus, a discrete wire termination tab could be formed into a yoke shape and either parallel gap or pincher welded to the end termination tab/PC bus to define the tubule. Such variations and alternate embodiments are contemplated, and can be made without departing from the spirit and scope of the invention as defined in the appended claims.