Abstract:
A capillary tip for deforming a bonding wire during bonding of the wire to a bonding surface comprises a bottom face along an inner periphery of the capillary tip for pressing the bonding wire against a bonding surface, an outer radius along an outer periphery of the capillary tip, and includes a first inclined face adjacent to the bottom face and extending obliquely to the bottom face as well as a second inclined face adjacent to the first inclined face and extending obliquely to the first inclined face.

Description:
FIELD OF THE INVENTION 
     The invention relates to a capillary for delivering a bonding wire, and in particular to a capillary for use in connecting a wire to a semiconductor device by bonding, for example, by the application of ultrasonic energy. 
     BACKGROUND AND PRIOR ART 
     During the packaging of the semiconductor devices, it is typically necessary to place a semiconductor chip or integrated circuit die onto a substrate such as a leadframe, and then electrically connect the die and substrate with conductive bonding wires. In high-power integrated circuit packages, heavy aluminum wire is commonly used to make the connection and carry current between the die and the substrate. These aluminum wires typically have diameters of 5 mils and above, and can be as wide as 20 mils in diameter. The aluminum wires are preferably bonded to bonding pads of the respective die and substrate using wedge bonding. 
     There are several disadvantages associated with existing wedge bonding of heavy aluminum wires. Firstly, the cost of the heavy wire wedge bonder machines are expensive, which can be up to three times the cost of equivalent ball bonder machines. Secondly, the throughput of wedge bonding machines is very low, and the time it takes to bond a single wire by wedge bonding is up to three times longer as compared to an equivalent ball bonder machine. Hence, it makes economic sense for integrated circuit assembly houses to use copper wire instead of aluminum wire, because copper wire is cheaper and more suitable to ball bonding. Therefore, there are economic and other benefits to replace heavy aluminum wedge bonding machines with copper ball bonding machines. 
     In wedge bonding, since both the first and second bonds are formed in an identical manner, there is no substantial variation in the current-carrying capacity of the wire throughout the whole wire length as the cross-sectional area of the wire is about the same throughout the wire. Consequently, there is no significant difference in the pull strength of the first wedge bond at a first bonding pad, as compared to the pull strength of the second wedge bond at a second bonding pad. However, in ball bonding, the first bond is formed from a ball and the second bond is effected by pressing the wire between the capillary and the bonding surface resulting in a flattened area with diminished cross-sectional area now referred to as a stitch bond. The current-carrying capacity of the wire at a ball bond area of the first bond is thus higher than the current-carrying capacity of the wire at the stitch bond area of the second bond, where the cross-sectional area of the wire is at the lowest. There is thus a current-carrying bottleneck at the stitch bond. Furthermore, the smaller cross-sectional area means that the lowest bond pull strength of the wire is at the stitch bond. 
     Presently, copper ball bonding is generally confined to wire diameters of around 2 mils (about 50 microns) and below. For copper ball bonding of wires with wire diameters of more than 2 mils, the lack of stitch pull strength and non-uniformity of the wire causing greater electrical resistance at the stitch bond would pose greater operational issues. It would be desirable to increase the cross-sectional area of the wire at the stitch bond position so as to decrease the bottleneck effect and to increase the pull strength at the stitch bond position. 
       FIG. 1  is a cross-sectional side view of a capillary  100  according to the prior art. The capillary  100  has a capillary tip  102  that feeds a bonding wire  104  through a capillary hole  106  at the center of the capillary tip  102 . There is a bottom face  108  at the base of the capillary tip  102  that is instrumental in pressing the bonding wire  104  onto a bonding surface. Adjacent to the bottom face  108  is a sloping capillary tip face  110 , which leads to an outer radius  112  of the capillary tip  102 . The sloping capillary tip face  110  forms a face angle A 0  with respect to a horizontal bonding surface. 
     A stitch bond is formed by the capillary tip  102  deforming the wire  104  against the surface to be bonded, typically a “lead” or “second bond surface”, thereby producing a wedge-shaped bond. The top part of the stitch bond follows the contour of the sloping capillary tip face  110  and outer radius  112  of the capillary tip  102 . The actual area welded or bonded under the stitch is dependent upon the capillary tip face design, the bonding parameters used (ultrasonic power, bonding time, bonding force and bond stage temperature) and the bondability of the material to be bonded to. A smaller face angle A 0  of the sloping capillary tip face  110  will result in a stitch that is thinner than a sloping capillary tip face  110  with a larger face angle. Nevertheless, simply increasing the face angle A 0  to obtain a thicker stitch would reduce bond strength and lead to an unreliable bond. 
       FIG. 2  is a cross-sectional side view of a stitch bond formed by the capillary of  FIG. 1 . The wire thickness X-X at a selected point of the stitch bond where the wire first meets the bonding surface is 39 microns. With this wire thickness for a heavy wire, problems with pull strength and current-carrying capacity as explained above may be experienced. 
     One prior art method of increasing the pull strength of the wire at the stitch bond position is described in Japanese patent publication number JP2001-291736 entitled “Capillary for Wire Bonding”. The publication discloses a capillary with a cone shape facing downward. A leading edge of the capillary is formed in two stages. The leading edge has a bottom face for leading out a fine wire, and an edge of the bottom face is used to cut the fine wire. There is also a step-like peripheral region which is located next to the edge. When the fine wire is cut by ultrasonic bonding, a part of the fine wire which is near the cut end is simultaneously pressed by the step-like peripheral region. 
     However, this invention is said to be applicable for bonding thin gold wires, specifically gold wires of 10-20 microns in diameter, not heavy wires. Although the step-like peripheral region with orthogonal orientations tends to press the wire at the wedge bond and deforms it to help in improving bond adhesion towards the end of the wire, the relatively flat contact at the bottom face of the capillary cuts the wire rather abruptly. Since the contact area between the stitch bond and the bonding surface is one of the factors that determine stitch pull strength, the flat bottom face of the capillary limits the contact area between the stitch bond and the bonding surface of the die or substrate. The end of stitch profile of the stitch bond is still relatively thin. 
     Moreover, the stitch bond that is formed has a stepped shape since the peripheral surfaces are parallel and perpendicular to the bottom face respectively, which decreases the uniformity of the wire. There may be a mechanical weakness formed in the bond because the sharp orthogonal edge of the step initiates micro-cracks in the stitch bond which will result in fractures during subsequent operational cycles. Sharp edges of the capillary will lead to very fast build up of wire material resulting in bonds with lower and inconsistent stitch pull strengths. Uniformity of current-carrying capacity at the stitch bond is lacking. 
     SUMMARY OF THE INVENTION 
     The invention seeks to provide a capillary for wire bonding which offers improved bond pull strength and current-carrying capacity as compared to the aforementioned prior art. 
     Accordingly, the invention provides a capillary tip for deforming a bonding wire, comprising: a bottom face along an inner periphery of the capillary tip for pressing the bonding wire against a bonding surface; an outer radius along an outer periphery of the capillary tip; a first inclined face adjacent to the bottom face and extending obliquely to the bottom face; and a second inclined face adjacent to the first inclined face and extending obliquely to the first inclined face. 
     It would be convenient hereinafter to describe the invention in greater detail by reference to the accompanying drawings which illustrate one embodiment of the invention. The particularity of the drawings and the related description is not to be understood as superseding the generality of the broad identification of the invention as defined by the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Examples of preferred embodiments of capillaries in accordance with the invention will now be described with reference to the accompanying drawings, in which: 
         FIG. 1  is a cross-sectional side view of a capillary according to the prior art; 
         FIG. 2  is a cross-sectional side view of a stitch bond formed by the prior art capillary of  FIG. 1 ; 
         FIG. 3  is a cross-sectional side view of a capillary according to a first preferred embodiment of the invention; 
         FIG. 4  is a cross-sectional side view of a stitch bond formed by the capillary of  FIG. 3 ; and 
         FIG. 5  is a cross-sectional side view of a capillary according to a second preferred embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 3  is a cross-sectional side view of a capillary  10  according to a first preferred embodiment of the invention. The capillary  10  comprises a capillary tip  12  that feeds bonding wire  14  to a bonding location through a chamfered hole  16 . A bottom face  18  of the capillary tip  12  is operative to press and deform the bonding wire  14  after a stitch bond is formed at the bonding location. The bonding wire preferably has a diameter of at least 3 mils (76.2 microns) and is preferably made of copper material. 
     Instead of a single sloping capillary tip face  110  (see  FIG. 1 ) that is found in the prior art, the capillary tip  12  includes a first inclined face  20  adjacent to the bottom face  18  that extends obliquely to the bottom face  18 , and a second inclined face  22  that is adjacent to and extends obliquely to the first inclined face  20 . In this embodiment, the second inclined face  22  shown in cross-section in  FIG. 3  appears as two straight lines. An outside radius  24  along an outer periphery of the capillary tip  12  is adjacent to the second inclined face  22 . 
     The height D 1 , which is a height of the intersection point of the first and second inclined surfaces  20 ,  22  from a plane that is coplanar with the bottom face  18 , is preferably between 0.1 to 0.5 of the wire diameter, WD of the bonding wire  14 , such that: 0.1 WD≦D 1 ≦0.5 WD. More preferably, the height D 1  is configured such that: 0.3 WD≦D 1 ≦0.4 WD. The diameter D 3  of the outer circumference of the bottom face  18  is preferably between 1½ WD and 3 WD, such that: 1½WD≦D 3 ≦3 WD. More preferably, the diameter D 3  is configured such that: 2 WD≦D 3 ≦2½ WD. 
     The outer radius  24  comprising a curved surface preferably has a radius R, where 0.4 WD≦R≦1 WD. More preferably, the radius R is configured such that 0.5 WD≦R≦0.8 WD. 
     An angle A 1  of the second inclined face  22  with respect to a bonding surface, which is typically horizontal, is preferably between 4 degrees to 11 degrees. More preferably, the angle A 1  is between 6 degrees to 10 degrees. An interfacial angle A 2  between opposite faces of the first inclined face  20  is preferably between 70 degrees to 120 degrees. More preferably, the angle A 2  is between 80 degrees to 100 degrees. 
       FIG. 4  is a cross-sectional side view of a stitch bond formed by the capillary of  FIG. 3 . The wire thickness X′-X′ at a selected point of the stitch bond at the point where the bonding wire  14  first meets the bonding surface (which, for comparison, is the same point as that selected in  FIG. 2 ) is 61 microns. Thus, with the capillary  10  according to the preferred embodiment of the invention, the wire thickness at the selected point has been increased from 39 microns to 61 microns. With this wire thickness, the problems with pull strength and current-carrying capacity are significantly reduced. 
       FIG. 5  is a cross-sectional side view of a capillary  30  according to a second preferred embodiment of the invention. The capillary  30  also has a capillary tip  12  including a chamfered hole  16  for feeding the bonding wire  14  to a bonding location. The bottom face  18  is located next to the chamfered hole  16 . 
     Adjacent to the bottom face  18 , there is a first inclined face  20  that is adjacent to and extends obliquely to the bottom face  18 . A second inclined face, which in this embodiment is in the form of a groove  26 , is adjacent to and extends at an oblique angle to the first inclined face  20 . The second inclined face or groove  26  is formed adjacent to an outside radius  24  of the capillary tip  12 . The main difference between the first and second embodiments of the capillary  10 ,  30  is that the second inclined face is in the form of a groove  26  rather than a sloping surface  22  appearing in cross-section in  FIG. 3  as two straight lines. 
     The groove  26  preferably has a radius R 1 , where 0.25 WD ≦R 1  ≦0.75 WD. More preferably, the radius R 1  of the groove  26  is configured such that 0.35 WD ≦R 1  ≦0.55 WD. The outer radius  24  preferably has a radius R 2 , where 0.25 WD ≦R 2  ≦0.75 WD. More preferably, the radius R 2  of the outer radius  24  is configured such that 0.35 WD ≦R 2  ≦0.55 WD. If R 2  is too low, a weakened stitch bond may result. 
     The height D 2  of the lowest point of the outer radius  24  from a plane that is coplanar with the bottom face  18  is preferably from 0.2 times the wire diameter WD, and up to 0.4 times of WD, such that: 0.2 WD≦D 2 ≦0.4 WD. More preferably, the depth D 2  is configured such that 0.2 WD≦D 2 ≦0.3 WD. The diameter D 4  of the outer circumference of the bottom face  18  is preferably between 1½ and 3 times of WD, such that: 1½ WD≦D 4 ≦3 WD. More preferably, the diameter D 4  is configured such that: 2 WD≦D 4 ≦3 WD. With the capillary  10  according to the first embodiment of the invention, an electrical resistance comparison was done on a bonding wire with a diameter of 6 mils and a cross-sectional area of 18232.22 μm 2 . The wire had a length of 5 mm and an average resistance of 0.9269 μOhm. It was found that the stitch bond made from the capillary  10  had an electrical resistance that was 27% lower than a prior art capillary  100  illustrated in  FIG. 1 , as demonstrated in Table A below: 
     
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE A 
               
               
                   
               
               
                 Capillary 
                 Width of 
                 Thickness of 
                 Cross-sectional 
                 Resistance 1 
               
               
                 Design 
                 Stitch (μm) 
                 Stitch (μm) 
                 area (μm 2 ) 
                 (R) μ Ohm 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Conventional 
                 275.89 
                 39.39 
                 10,867.46 
                 1.5551 
               
               
                 Invention 
                 245.695 
                 60.61 
                 14,891.57 
                 1.1349 
               
               
                   
               
             
          
         
       
     
     Further, the capillary  10  was found to have a current-carrying capacity that was 37% higher than a prior art capillary  100 , as demonstrated in Table B below: 
     
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE B 
               
               
                   
               
               
                   
                 Stitch Cross- 
                   
                   
                   
               
               
                 Capillary 
                 sectional 
                 Wire 
                 Fusing 
                 Fusing 
               
               
                 Design 
                 area (μm 2 ) 
                 Length (mm) 
                 Time (sec) 
                 Current (A) 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Conventional 
                 10,867.46 
                 5 
                 5 
                 28.72 
               
               
                 Invention 
                 14,891.57 
                 5 
                 5 
                 39.36 
               
               
                   
               
             
          
         
       
     
     It was also found that on a stitch pull tests conducted on a total of thirty-six wires for each of the conventional design and the design according to a preferred embodiment of the invention, the capillary  10  was able to achieve an average stitch pull strength of 360.077 g, whereas the prior art capillary  100  was able to achieve an average stitch pull strength of 260.427 g. The average increase in stitch pull strength by using a capillary  10  according to the preferred embodiment of the invention was therefore about 100 g. 
     The invention described herein is susceptible to variations, modifications and/or additions other than those specifically described and it is to be understood that the invention includes all such variations, modifications and/or additions which fall within the spirit and scope of the above description.