Abstract:
An electrode for use in a wire bonding apparatus is constructed of substantially pure iridium or an iridium alloy. The electrode also has a diameter that is chosen to aid in the self-cleaning of an end of the electrode during the electronic flame-off process.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]     The present application claims priority from U.S. Provisional Application No. 60/506,269, filed Sep. 26, 2003, which is incorporated herein by reference in its entirety. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The invention relates to the field of wire bonding, and particularly to electrodes used in a wiring bonding process.  
       BACKGROUND OF THE INVENTION  
       [0003]     A conventional wire bonding apparatus  100  is depicted in  FIG. 1 . The apparatus includes an electrical flame-off (EFO) wand  104 , a bonding wire  108 , which is fed through a wire damper  112  and a capillary  116 . The wire damper  112  and the capillary  116  are typically referred to collectively as a bond head assembly. A ball  120  is formed at the end of the wire  108  that protrudes from the narrow end of the capillary  116 . The apparatus  100  also includes a lead frame  122  having leads  124  and  128 . A semiconductor integrated circuit or die  132  is mounted on the lead frame  122  and forms die-lead frame assembly. The die  132  includes bond pads  136  and  140 . A wire  144  is bonded to the pad  140  and the lead  124 . The apparatus  100  also includes a lead frame holder  148 , which secures the die-lead frame assembly during wire bonding, and an electrical discharge generator  152  coupled to the wire damper  112  and the EFO wand  104 . The electrical discharge generator creates an electrical discharge in response to receiving a discharge signal.  
         [0004]     Bonding the wire  108  to the die  140  and the lead frame  122  is accomplished using the bond head. The bond head generally moves in the x, y, and z directions above the die-lead frame assembly.  
         [0005]     Conventionally, the ball  120  on the end of the wire  108  is formed by placing the electrical flame-off (EFO) wand  104  a predetermined distance from the end of the wire  108 . An electrical arc is emitted between the end of the wire  108  and the EFO wand  104 . The arc forms the ball  120  on the end of the spool of bonding wire  108 . By varying the intensity and the duration of the electrical arc, the size of the ball that is formed can be adjusted to specific dimensions.  
         [0006]      FIG. 2  depicts a prior art EFO wand  104 , which includes a mounting section  164 , epoxy  168 , and an electrode  172 . The mounting section  164  is both rigid and electrically conductive. The epoxy  168  is electrically conductive and secures the electrode  172  to the mounting section  164 . As discussed above, the electrode  172  emits an electrical discharge or arc from a tip  176  to the end of a bonding wire  108  to form a ball  120 . The electrode  172  is formed from a rigid and electrically conductive material. Typical electrodes  174  are made of platinum having a purity of 99.95%. The diameter of a typical electrode is 0.020 inches.  
         [0007]     It has been determined that, during use, a certain amount of carbon contamination forms on the EFO electrode each time a bonding ball  120  is formed. The carbon contamination acts as an insulator. As such, build up of carbon contamination interferes with the normal functioning of the electrode tip  176 . As the carbon contaminates the electrode, it gradually degrades the quality of the bonding balls that are formed.  
         [0008]     Mechanical scraping of the electrode tip  176  is one possible method of removing the contamination which has been deposited. However, this method has serious drawbacks in that continual mechanical adjustments would be required to avoid excessive damage to electrode tip  176 . Additionally, the mechanical apparatus needed to scrape the electrode tip would need to be located on the automated ball bonding machinery near the normal operating position of the electrode  172 , a location where space is already at a premium. Therefore, a need exists for an electrode with self-cleaning features.  
       SUMMARY OF THE INVENTION  
       [0009]     The invention relates to an electrode for use in a wiring bonding apparatus. The electrode is constructed of iridium or an iridium alloy, and has a diameter of a predetermined value. In various embodiments, the iridium alloy is a mixture of iridium and at least one other material such as rhodium, platinum, ruthenium, palladium, molybdenum, and tungsten. For example, the iridium alloy can be approximately 99 weigh percent iridium and 1 percent rhodium. In a preferred embodiment, the iridium alloy can be 80 weight percent iridium and approximately 20 weight percent rhodium. Alternatively, substantially pure iridium can be used. The diameter of the electrode is less than 0.020 inches and may be approximately 0.010 inches. In one embodiment, the electrode includes an insulating layer disposed on the iridium alloy. The insulating layer can be glass or another suitable material.  
         [0010]     In another aspect, the invention is directed to an electronic flame-off wand assembly having a mounting section and an electrode. The mounting section is fabricated of an electrically conductive material and has a bore of a first diameter through a portion of the section. The electrode has a second diameter and is disposed within the bore of the mounting section. The electrode is fabricated of an iridium alloy as described above.  
         [0011]     The foregoing and other features of the invention and advantages of the present invention will become more apparent in light of the following detailed description of the preferred embodiments, as illustrated in the accompanying figures. As will be realized, the invention is capable of modifications in various respects, all without departing from the invention. Accordingly, the drawings and the description are to be regarded as illustrative in nature, and not as restrictive.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]     For the purpose of illustrating the invention, there is shown in the drawings a form which is presently preferred; it being understood, however, that this invention is not limited to the precise arrangements and instrumentalities shown. The drawings are not necessarily to scale, emphasis instead being placed on illustrating the principles of the present invention.  
         [0013]      FIG. 1  is a prior art prior art wire bonding apparatus.  
         [0014]      FIG. 2  is an electrode used in the apparatus of  FIG. 1 .  
         [0015]      FIG. 3  is an electronic flame-off wand incorporating an electrode constructed according to the principles of the present invention.  
         [0016]      FIGS. 4A, 4B , and  4 C are electrodes constructed according to the principles of the present invention.  
         [0017]      FIG. 5  is an embodiment of the electrode of  FIG. 4  having an insulative layer disposed thereon. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0018]     In the drawings, in which like numerals indicate like elements, there is shown an electrode for use in a wire bonding apparatus and an EFO wand assembly including an electrode constructed according to the principles of the present invention.  
         [0019]      FIG. 3  illustrates an EFO wand  200  that incorporates an electrode  204  constructed according to the principles of the present invention. The wand  200  includes a mounting section  208  and the electrode  204 . The mounting section  208  is preferably rigid and electrically conductive. In one embodiment, the mounting section  208  is made from stainless steel. The mounting section  208  is preferably a tube having an inner diameter of sufficient size to receive the electrode  204 , which has an outer diameter, D. Preferably, the mounting section  208  is secured to the electrode  204  by crimping an end portion  210  of the mounting portion  208  as shown. It is not a requirement of the invention, however, that the mounting portion is secured to the electrode by crimping. Any suitable means could be used to secure the mounting portion  208  instead of crimping, such as adhesive, fastening, welding and the like.  
         [0020]     The electrode  204  is mounted into the bore of the mounting section  208  such that a tip  212  of the electrode  208  is exposed outside the mounting section  208 . The electrode  204  is in electrical communication with the mounting section  208  and emits an electrical discharge or arc from the tip  212 . The arc forms a ball at the end of the bonding wire.  
         [0021]     With reference to  FIGS. 4A, 4B  and  4 C, the electrode is shown in more detail. The electrode  204  is preferably formed from rigid construction and is electrically conductive. In one embodiment, the electrode is fabricated out of substantially pure iridium. In another embodiment, the electrode is fabricated out of an iridium alloy. The iridium alloy is a mixture of iridium and at least one other material such as rhodium, platinum, ruthenium, palladium, molybdenum, and tungsten. In one embodiment, the mixture is approximately 50 weight percent iridium and approximately 50 weight percent rhodium. In a preferred embodiment, the alloy is approximately 80 weight percent iridium and approximately 20 weight percent rhodium. However, depending on the use and efficiency desired, the combination of iridium and rhodium may include a percentage iridium between approximately 50% to 99% or more.  
         [0022]     The diameter of the electrode  204  can vary depending on the desired result. In one embodiment, diameter of the electrode  204  is less than approximately 0.020 inches. In a preferred embodiment, the diameter of the electrode  204  is approximately 0.010 inches. It should be readily apparent that the electrode may range in diameter from approximately 0.010 to 0.020 inches. The tip  212  can vary in shape. It is desirable that the tip  212  not be blunt. In one embodiment, the tip  212  is rounded. In alternate embodiments the tip  212  is cut at an angle (e.g., 45 degrees) to create a point  216 .  
         [0023]     An electrode constructed of an iridium alloy containing 80 weight percent iridium and 20 weight percent rhodium with a diameter of 0.010 inches having a rounded tip provides various advantages in wire bonding applications. The iridium alloy is harder and stiffer (i.e., it has a higher modulus) than a conventional platinum electrode. The harder iridium alloy increases the operational life of the electrode since the harder alloy is less easily damaged and less susceptible to electrical wear. In some applications, such an electrode is capable of forming in excess of twenty million balls on the end of the bonding wire before requiring replacement. Conventional electrodes typically are capable of forming one million balls before maintenance must be performed due to the contamination build up. Also conventional electrodes must be “burned-in” by creating as many as 100,000 balls before optimal performance of the electrode is achieved.  
         [0024]     The smaller diameter of 0.010 is also a factor in extending the usability of the electrode  204 . A smaller diameter electrode reduces the surface area of the tip  212 . During the arcing process, a small amount of plasma is created on the tip of the electrode. The plasma acts as a cleanser to assist in removing a portion of the oxidation that forms on the tip. In the present invention, the reduced tip surface area means that the plasma removes a larger amount of the oxidized material. As a result, a greater surface area of the tip  212  is self-cleaned. A clean tip electrode yields more consistent ball diameters on the end of the wire during the wire bonding processes.  
         [0025]     With reference to  FIG. 5 , in another embodiment a layer of insulative material  220  is disposed on the iridium alloy. In such an embodiment, it is preferred that the tip  212  be rounded. The insulative layer  220  constrains the plasma field during the arcing time. The constrained plasma field concentrates the spark to a limited location on the electrode  204 . As such, the plasma that is generated as close as possible to the tip, which is where the oxidation occurs. Thus the insulative material  220  increased the cleansing provided by the electrode, thereby producing more consistent balls over the life of the electrode. In one embodiment, the insulative material is a layer of glass having a thickness of approximately 0.002 inches. Alternatively, other materials such as, oxides, nitrides, silica, alumina, and other ceramics can be used. Additionally, high temperature polymers such as polyimide, bisbenzocyclobutene, polysulfones, and phenolics can be used.  
         [0026]     The narrower tip that is provided in the present invention through the use of the iridium materials also permits the electrode to be positioned closer to the bonding location than is possible using prior electrodes. Furthermore, the smaller tip is also lighter than conventional electrodes, thus reducing the weight of the entire bonding assembly.  
         [0027]     As noted above, a variety of modifications to the embodiments described will be apparent to those skilled in the art from the disclosure provided herein. Thus, the present invention may be embodied in other specific forms without departing from the spirit or essential attributes thereof and, accordingly, reference should be made to the appended claims, rather than to the foregoing specification, as indicating the scope of the invention.