Patent Publication Number: US-2005121420-A1

Title: Electric welder, electrode and method of use thereof for spot-weld tacking eutectic solder to thin film depositions on non-conductive substrates

Description:
FIELD OF THE INVENTION  
      The present invention relates to apparatus and methods for electric welding. More specifically, the present invention relates to methods and apparatus for electric spot-welding multiple spots from one face of a heat sensitive material. Even more specifically, the present invention relates to methods and apparatus for spot-welding multiple spots from one face thereby tacking a heat-fusible conductive sheet to a thin-film conductive layer on an electrically non-conductive heat-tolerant substrate.  
     BACKGROUND  
      Using a variety of techniques such as vapor deposition, sputter coating, photolithography, electroplating, etc., articles made of non-conductive materials, such as glass or ceramic, are increasingly produced with swig of one or more very thin metal layers. These thin film layers, purposed as electrical conductors, heat conductors, nucleation substrates for continued electroplating, etc., can also be used as anchorage substrates in a eutectic bonding operation.  
      For example of such a use, referring to  FIG. 1 , it is conventional to hermetically seal an IC semiconductor chip (not Shown) in a cavity  3  of a ceramic package  1 . Such is disclosed in U.S. Pat. No. 3,946,190 to Hascoe for a “Method of Fabricating a Sealing Cover for an Hermetically Sealed Container,” the entirety of which is incorporated herein by reference. To make the seal a frame  7  of a eutectic solder such as gold-tin is sandwiched between a metal lid  2  and a gold plated ledge  6  within an indentation  5  in a package body  4 . The expensive gold based eutectic forming frame  7  is often brittle and frame  7  is of very small dimensions to conserve material. Consequently frame  7  is delicate and difficult to handle. Assembly and ration is improved by first tacking frame  7  to metal lid  2  using multiple spot welds. Then placing the lid-frame subassembly onto package body  4  in indentation  5  and heating the assembly to fuse frame  7  to lid  2  and ledge  6 ; thereby forming a hermetic seal between lid  2  and package body  4 .  
      To improve thermal conductivity properties, the industry began moving away from a metal lid  2  and began using a ceramic lid  2 ′ that was made of an electrically non-conductive material. Although ceramic lid  2 ′ can be formed with a thin-film deposit of gold on the mating surface matching frame  7 , the spot welding electrode arrays and methods used to tack frame  7  to metal lid  2  cannot spot weld frame  7  to ceramic lid  2 ′.  
      Since the industry is moving in the direction of utilizing ceramic lids in place of metal, it would be an important advance to have an apparatus and/or method for tacking a eutectic gold-tin frame material to the thin gold plating on the undersurface of a semiconductor package lid. In this regard it would be desirable to provide a method and apparatus for spot-welding a eutectic material to a thin-film on a non-conductive substrate.  
     OBJECTS AND SUMMARY OF THE INVENTION  
      It is an object of the present invention to overcome the problems of the prior art by providing a method and apparatus for spot-welding a eutectic material to a thin-film on a nonconductive substrate.  
      According to an embodiment of the invention, there is provided an electric welder for spot fusing a eutectic metal to a thin film electrical conductor on an electrically non-conductive substrate, including; at least one pair of an electrode, at least one the electrode having a terminal end that is tapered tangentially to a domed terminus, and the domed terminus having a radius sufficiently small to concentrate an electrical current to produce a heat cone and penetrate the eutectic metal and sufficiently large to prevent an intolerable damage to any of the eutectic metal, the thin film, and the substrate.  
      According to a feature of the invention, there is provided, for the above welder, that the substrate is a lid for an IC package made of ceramic or glass, the thin film is made substantially of gold, and the eutectic metal is a frame therefore made substantially of an alloy of 80% gold and 20% tin, and each the electrode is made of and alloy of copper tungsten.  
      According to a feature of the invention, there is provided, for the above welder, that the spot fusing is accomplished by pulsing the electrical current from a discharging capacitor through the electrode pair.  
      According to an embodiment of the invention, there is provided a method of using the welder for spot fusing a eutectic metal to a thin film electrical conductor on an electrically non-conductive substrate, including; placing the eutectic metal in registration contact with the thin film, pressing a pressure block against the eutectic metal, pressing at least one of the electrode pair of the welder into contact with the eutectic metal under sufficient pressure to displace a tiny bit of the eutectic metal against the thin film, pulsing an electrical current through the electrode pair of sufficient energy to cause a localized heating and melting of the eutectic metal, allowing the eutectic metal to cool and thereby solidify, and removing the electrode pair and the pressure block therefrom.  
      Briefly stated, the present invention includes an electric welder for spot fusing a eutectic solder matching a thin film deposition on an electrically non-conductive substrate. An example being a gold-tin solder fusing to a gold plated ceramic or glass surface. At least one pair of laterally displaced electrodes is resiliently engaged with the surface of the eutectic. The electrodes are tapered at the terminus to restrict the area of current flow. The engaging tip of the electrode is rounded tangentially to the taper and very slightly penetrates the surface of the eutectic. Once engaged the electrode pair is pulsed with sufficient current, from for example a capacitor discharge circuit, to create a local heat cone at the contact point sufficient to reach the eutectic point thereby locally fusing the eutectic to the plating.  
      The above, and other objects, features and advantages of the present invention will become apparent from the following description read in conjunction with the accompanying drawings, in which like reference numerals designate the same elements. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a perspective view of an IC semiconductor chip package with the lid placement illustrated  
       FIG. 2  is a perspective view of an electrode array portion of an embodiment of the present invention used to spot weld the four corners of a frame of weld material to a thin metal film on the underside of a ceramic IC packaging Lid.  
       FIG. 3  is a perspective view of an electrode array portion of an embodiment of the present invention with an inset detailing the positioning of one electrode.  
       FIG. 4  is a longitudinal cross section of the tip of the electrode of an embodiment of the present invention demonstrating the terminal portion shape thereof.  
       FIG. 5  is a longitudinal cross section of the tip of the electrode of an embodiment of the present invention demonstrating the positioning of one electrode in conjunction with the work pieces and detailing some typical dimensions in the art instant thereto.  
       FIG. 6  is a longitudinal cross section of an electrode array portion of an embodiment of the present invention with an inset detailing the presence of the heat cone resulting from a firing of the electrodes. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)  
      Referring to  FIG. 2 , an embodiment of an electrode array head  100  of a capacitor discharge spot welder includes four electrodes  101  slidably embedded in an electrode channel through an electrode block (Not Shown) and protrudable through a guide channel  102  in a pressure block  103 . Pressure block  103  has a push rod  104  attached thereto that is likewise slidably embedded in a central channel (also Not Shown) in the electrode block. Springs (also Not Shown), within the electrode channels and the central channel bias electrodes  101  and pressure block  103  in the direction of arrow  105 .  
      In this embodiment, the electrode array&#39;s intended use is to tack a frame  106  of eutectic solder to a thin metal film disposed on the undersurface of an IC package lid  107 . A pod  108  aligns frame  106  laterally with lid  107  while electrode array head  100  advances. Referring to  FIG. 3  in the context of  FIG. 2 , (pod  108  is removed for clarity and frame  106  is shown in contact with a thin metal film  109  on lid  107 ) electrode array head  100  has advanced far enough toward pod  108  to activate the pressure from the spring therein such that pressure block  103  is pressing against frame  106  stabilizing its position contacting lid  107 . Also, each electrode  101  has contacted a corner of frame  106  with enough pressure from the springs to cause a very tiny indentation therein.  
      Referring to  FIG. 4  in the context of  FIGS. 2 &amp; 3 , a longitudinal section of the tip of electrode  100 , demonstrating the terminal portion shape thereof has a straight cylindrical mass portion  110 , leading to a tapered portion  111  that terminates tangentially with a curved dome portion  112 . The amount of taper for tapered portion  111  and the radius of curvature of dome portion  112  are determined empirically to satisfy the conditions of the welding operation. The goal is to have a very small surface penetration by dome portion  112  without causing undue damage to frame  106 , thin film  109 , or lid  107 . If dome portion  112  is small or has a small radius electrode tip  100  will penetrate too far into frame  106 . At the least this would cause a flaw in the surface and at the worst cause frame  107  and or lid  17  to crack or break. on the other hand a large shallow dome portion results I lack of penetration and inability to form the weld.  
      Referring to  FIG. 5 , an example of a typical cross section of lid  107 , thin film  109 , frame  106  and electrode  100  arranged for welding with electrode  100  penetrating frame  106  a depth  120 . In this example depth  120  is approximately 0.0004″, ⅕ the thickness  121  of frame  106  which is 0.0018″-0.0020″. Thickness  122 , the dimensional thickness of thin film  109 , is very thin at 0.000020″ and thickness  123 , the dimensional thickness of lid  107 , is 0.01″.  
      Referring to  FIG. 6 , since lid  107  is an electrically non-conductive substrate current flow between the electrode  100  pairs is laterally directed through thin film  109  and flame  106 . The tapered shape of electrode  100 , at the terminus ending at an interior point in flame  106 , concentrates the current flow to a very small area resulting in a heat cone  124  at each electrode. Heat cone  124  is sufficient to locally fuse the eutectic metal of frame  106  with thin film  109 .  
      Having described preferred embodiments of the invention with reference to the accompanying drawings, it is to be understood that the invention is not limited to those precise embodiments, and that various changes and modifications may be effected therein by one skilled in the art without departing from the scope or spirit of the invention as defined in the appended claims. For example it is considered within the scope of the invention to substitute other eutectic solder/metal matched combinations in place of the gold-tin eutectic/gold. Such metals as silver, palladium, lead, copper, platinum, etc and their eutectic match could be substituted for the gold.