Abstract:
A wire bonder has a capillary through which a wire passes. A discharge tip is positioned near a bottom section of the capillary and provides a flame to a distal end of the wire. A gas diffuser is positioned beside the capillary to diffuse a heated gas to the distal end of the wire.

Description:
FIELD OF THE INVENTION 
     This invention relates to wire bonder and, more specifically, to a wire bonder having a gas diffuser which reduces the hardness of a free air ball of a conductive wire in order to avoid damage to a bond pad of a semiconductor device and or a circuit pattern of a circuit board and to achieve improved bondability of the conductive wire. 
     BACKGROUND OF THE INVENTION 
     Generally, a semiconductor package is fabricated by attaching a semiconductor die to a circuit board (die bonding). The circuit board and the semiconductor die may then be electrically connected. In accordance with one method, conductive wires (wire bonding) may be used to electrically connect the circuit board and the semiconductor die. The semiconductor die and the conductive wires may then be encapsulated by an encapsulant (encapsulation). 
     The wire bonding process may includes the following steps: creating a free air ball (FAB) at one end of a conductive wire protruding downwardly through a lower end of a capillary using an electric flame-off (ER)) tip; moving the capillary toward a bond pad of a semiconductor die and primarily bonding the FAB to the bond pad (ball bonding); and moving the capillary toward a pattern of a circuit board and secondarily bonding the distal end of the conductive wire to the pattern (stitch bonding). 
     The conductive wire may be made of gold. In some cases, the gold wire is currently replaced by a cheaper copper wire. Since the Vickers hardness of the copper wire and its FAB is relatively high compared to that of the gold wire and its FAB, the use of the copper wire increases the probability of damage to the bond pad of the semiconductor die. That is, the bond pad is apt to crack when the relatively hard FAB of the copper wire is brought into close contact with the bond pad. Particularly, when the copper wire is applied to a low-dielectric constant (k) semiconductor device, weak active regions of the semiconductor device may lead to damage or cracking of the semiconductor device. Although the price of a copper wire is about one hundredth of that of a gold wire, the relatively high hardness of the copper wire increases the number of defects during wire bonding. 
     Therefore, a need existed to provide a system and method to overcome the above problem. The system and method would provide a wire bonder which reduces the hardness of a free air ball of a conductive wire. 
     SUMMARY OF THE INVENTION 
     A wire bonder has a capillary through which a wire passes. A discharge tip is positioned near a bottom section of the capillary and provides a flame to a distal end of the wire. A gas diffuser is positioned beside the capillary to diffuse a heated gas to the distal end of the wire. 
     A wire bonding method comprises: diffusing a heated gas to a conductive wire positioned at a lower end of a capillary; and providing an electric flame to a distal end of the conductive wire to create a free air ball. 
     A wire bonder has a capillary through which a conductive wire passes. A heater is positioned above the capillary to heat the conductive wire. 
     The present invention is best understood by reference to the following detailed description when read in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic view illustrating the construction of a wire bonder according to an embodiment of the present invention; 
         FIG. 2  is a schematic cross-sectional view illustrating a hot gas diffuser of a wire bonder according to the present invention; 
         FIG. 3  is a flow chart illustrating a wire bonding method of the present invention; 
         FIGS. 4A through 4E  are schematic views sequentially illustrating the individual steps of a wire bonding method according to the present invention; 
         FIGS. 5A and 5B  are graphs showing changes in ball shear and stitch pull with increasing temperature of a conductive wire after wire bonding in accordance with the present invention, respectively; 
         FIG. 6  is a schematic view illustrating the construction of a wire bonder according to a further embodiment of the present invention; and 
         FIG. 7  is a schematic view illustrating the construction of a wire bonder according to another embodiment of the present invention. 
     
    
    
     Common reference numerals are used throughout the drawings and detailed description to indicate like elements. 
     DETAILED DESCRIPTION 
     Referring to  FIG. 1 , a schematic view of a wire bonder  100  according to one embodiment of the present invention is shown. As illustrated in  FIG. 1 , the wire bonder  100  comprises a capillary  110 , an electric flame-off tip  120 , and a gas diffuser  130 . 
     The capillary  110  has a through-hole therein through which a conductive wire  140  passes. A transducer  150  is coupled to the capillary  110  to deliver ultrasonic energy to the capillary  110 . A clamp  160  is positioned above the capillary  110  to clamp or unclamp the conductive wire  140  during wire bonding. 
     A heater block  171  is installed below the capillary  110 . A circuit board  172  is securely mounted on the heater block  171 , and a semiconductor die  173  is attached to the circuit board  172 . The heater block  171  provides heat to a free air ball to be created at the distal end of the conductive wire  140 . The circuit board  172  is brought into close contact with the heater block  171  by a clamp  174 . 
     The electric flame-off tip  120  is installed beside the capillary  110 . The electric flame-off tip  120  provides an electric flame to the distal end of the conductive wire  140  positioned at the lower end of the capillary  110  to create a free air ball. That is, the electric flame-off tip  120  induces the creation of a free air ball at the distal end of the conductive wire  140  to enable ball bonding of the conductive wire  140 . 
     The gas diffuser  130  is installed beside the capillary  110  opposite to the electric flame-off tip  120 . The gas diffuser  130  supplies a hot forming gas to the conductive wire  140  during the wire bonding process. In accordance with one embodiment, the gas diffuser  130  supplies a hot forming gas around 25° C.-300° C. to the conductive wire  140 . If the temperature of the hot forming gas is lower than 25° C., a decrease in the hardness of a free air ball of the conductive wire  140  is insufficient and an improvement in the bondability of the conductive wire  140  is not significant. Meanwhile, a temperature higher than 300° C. in the present embodiment is substantially difficult to achieve in view of the characteristics of a heater, which will be described below. 
     In accordance with one embodiment, the conductive wire  140  may be made of a conductive material selected from, but not limited to, copper, gold, aluminum and equivalents thereof. The forming gas may be selected from nitrogen, hydrogen, air, mixtures thereof and equivalents thereof, but is not limited thereto. The listing of the above is given as an example and should not be seen to limit the scope of the present invention. 
     In the present embodiment, the hot forming gas is diffused from the gas diffuser  130  to the free air ball of the conductive wire  140  at the lower end of the capillary  110  to reduce the hardness of the free air ball. Therefore, the wire bonder can avoid damage to a bond pad of the semiconductor die  173  or a circuit pattern of the circuit board  172  and can achieve improved bondability of the conductive wire  140 . 
     In addition, direct supply of the hot forming gas to the free air ball of the conductive wire  140  eliminates the need for an excessive increase in the temperature of the heater block  171 . That is, there is no need for raising the temperature of the heater block  171  to heat the free air ball of the conductive wire  140 . Excessive heating of the heater block  171  has a bad influence on the circuit board  172  or the semiconductor die  173 . 
     Referring to  FIG. 2 , a schematic cross-sectional view of a hot gas diffuser  130  of a wire bonder according to one embodiment is shown. As illustrated in  FIG. 2 , the gas diffuser  130  includes a body  131 , a heater  136 , a power supply  137 , and a forming gas supply  138 . 
     The body  131  has an inner diameter surface  132  so as to define a space therein along the lengthwise direction thereof, a forming gas inlet port  134  in flow communication with the space defined by the inner diameter surface  132  at an upper end thereof, and a forming gas outlet port  135  in flow communication with the space defined by the inner diameter surface  132  at a lower end thereof. In accordance with one embodiment, the body  131  has an inner diameter surface  132  that defines a channel formed in an interior of and running a length of the body  131 . 
     A helical groove  133  is formed extending from the forming gas inlet port  134  to the forming gas outlet port  135  on the inner diameter surface  132  of the body  131 . With this configuration, a forming gas is introduced into the body  131  through the forming gas inlet port  134 , flows downwardly along the groove  133  on the inner diameter surface  132  to reach the forming gas outlet port  135 , and is sprayed out through the forming gas outlet port  135 . That is, the helical groove  133  serves to allow the forming gas to stay in the space of the body  131  as long as possible. 
     The heater  136  is inserted into the space defined by the inner diameter surface  132  of the body  131 . That is, the heater  136  is inserted into the space along the lengthwise direction of the body  131 . A gap is formed between the heater  136  and the inner diameter surface  132  of the body  131  to allow the forming gas to flow downwardly along the helical groove  133 . The heater  136  may take on a plurality of different configurations. There is no restriction on the structure and shape of the heater  136 . For example, the heater  136  may consist of a heating coil and a ceramic material surrounding the heating coil, and may be in the shape of a bar. The forming gas can be typically heated to 25-300° C. by the heater  136 . As a result, the temperature of the hot forming gas sprayed through the periphery of the heater  136  reaches 25-300° C. 
     The power supply  137  applies power to the heater  136 . The power supply  137  may be a direct or alternating current power supply. 
     The forming gas supply  138  is connected to the forming gas inlet port  134  of the body  131  to supply a forming gas at constant flow and pressure to the body  131 . The forming gas supplied from the forming gas supply  138  may be at different temperatures (including room temperature). 
     Due to this construction, the forming gas supplied from the forming gas supply  138  is heated to around 25-300° C. in the gas diffuser  130 . The hot forming gas is sprayed toward the free air ball of the conductive wire  140  ( FIG. 1 ) at the lower end of the capillary  110  ( FIG. 1 ). 
     The heating and spraying of the forming gas are explained in more detail below. First, a forming gas at room temperature is introduced into the body  131  through the forming gas inlet port  134  of the body  131 . Then, the heater  136  is operated by power applied from the power supply  137 . The forming gas introduced into the body  131  is heated to about 25-300° C. by the heater  136  while flowing downwardly along the helical groove  130  formed on the inner diameter surface  132 . Finally, the hot forming gas is sprayed out through the forming gas outlet port  135  of the body  131 . 
     Referring to  FIG. 3 , a flow chart of a wire bonding method of the present invention is shown. As illustrated in  FIG. 3 , the wire bonding method of the present invention comprises the following steps: diffusion of a hot forming gas (S 1 ), formation of a free air ball (S 2 ), ball bonding (S 3 ), looping (S 4 ) and stitch bonding (S 5 ). 
     Referring to  FIGS. 4A through 4E , there are sequentially illustrated schematic views for explaining the individual steps of the wire bonding method according to the present invention. As illustrated in  FIG. 4A , and in step S 1  of  FIG. 3 , a hot forming gas is diffused to the conductive wire  140  positioned at the lower end of the capillary  110 , through which the conductive wire  140  passes. In accordance with one embodiment, a hot forming gas at 25° C.-300° C. is diffused to the conductive wire  140  by heating a forming gas in the gas diffuser  130 . 
     In the case where the conductive wire  140  is made of copper or aluminum, nitrogen, or a mixed gas of nitrogen and hydrogen gases is desirable as the hot forming gas. The reason for the use of the nitrogen/hydrogen mixed gas is because the nitrogen gas protects a free air ball from oxidation and the hydrogen gas reduces the free air ball while protecting the free air ball from oxidation. In accordance with one embodiment, the nitrogen and hydrogen gases are substantially mixed in a ratio of 95:5. 
     Alternatively, in the case where the conductive wire  140  is made of gold, nitrogen or air is desirable as the hot forming gas. That is, since the free air ball of the gold wire does not substantially undergo oxidation, the use of hydrogen gas as the hot forming gas is excluded. Further, the hot forming gas may be continuously diffused to the conductive wire  140  without being stopped throughout steps S 2 , S 3 , S 4  and S 5  of  FIG. 3 . 
     As illustrated in  FIG. 4B , and in step S 2  of  FIG. 3 , the electric flame-off tip  120  provides an electric flame to the distal end of the conductive wire  140  positioned at the lower end of the capillary  110 , through which the conductive wire  140  passes, to create a free air ball  141 . That is, the free air ball  141  is created by applying power to the electric flame-off tip  120  to deliver a flame from the electric flame-off tip  120  to the distal end of the conductive wire  140  positioned at the lower end of the capillary  110 . It is known that the free air ball  141  is created at a temperature of about 1,000° C. At this time, the hot forming gas is still diffused to the free air ball  141  so as to maintain the free air ball  141  at the same temperature as the hot forming gas. 
     As illustrated in  FIG. 4C , and in step S 3  of  FIG. 3 , the free air ball of the conductive wire  140  is primarily bonded to the semiconductor die  173  (ball bonding). Specifically, the capillary  110  descends toward the semiconductor die  173 , and then ultrasonic energy is delivered to the capillary  110  in a state where the free air ball is pressed against a bond pad of the semiconductor die  173  to bond the free air ball of the conductive wire  140  to the bond pad of the semiconductor die  173 . 
     As illustrated in  FIG. 4D , and in step S 4  of  FIG. 3 , the capillary  110  moves toward the circuit board  172  (looping). 
     As illustrated in  FIG. 4E , and in step S 5  of  FIG. 3 , the distal end of the conductive wire  140  is secondarily bonded to the circuit board  172  (stitch bonding). Specifically, the capillary  110  is moved toward the circuit board  172 , and then ultrasonic energy is delivered to the capillary  100  in a state in which the distal end of the conductive wire  140  is pressed against a circuit pattern of the circuit board  172  to bond the distal end of the conductive wire  140  to the circuit pattern. 
     The following table shows changes in the hardness of the copper wire and the free air ball. The hardness tests were conducted at various temperatures between 100 to 250° C. From the results in the table, it can be confirmed that the copper wire had a hardness of 65 to 75 Hv at room temperature, 31 Hv at 100° C. and 20 Hv at 250° C. In addition, the free air ball was found to have a hardness of 45 to 55 Hv at room temperature. These results indicate that the free air ball will have lower hardness values at 100° C. and 250° C. than the hardness at room temperature. 
     
       
         
               
               
               
             
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                   
                   
               
               
                   
                   
                 Cu wire 
               
             
          
           
               
                   
                 Wire diameter: 1.0 mil 
                 Room Temp. 
                 100° C. 
                 250° C. 
               
               
                   
                   
               
             
          
           
               
                   
                 Hardness (Hv) 
                 FAB 
                 45-55 
                   
                   
               
               
                   
                   
                 Wire 
                 65-75 
                 31 
                 20 
               
               
                   
                   
               
             
          
         
       
     
     Referring to  FIGS. 5A and 5B , there are shown changes in ball shear and stitch pull versus temperature of the conductive wire  140  after wire bonding in accordance with the present invention, respectively. The temperatures of the hot forming gas supplied by the gas diffuser were varied from about 90° C. to about 230° C. while maintaining the temperature of the heater block at 100° C. 
     The x- and y-axis in  FIG. 5A  show temperature and ball shear, respectively. The ball shear of the conductive wire was determined by measuring a force applied when the ball bonding region of the conductive wire formed on the bond pad was pushed in the lateral direction using a tool equipped with a sensor until the ball bonding region was separated from the bond pad. As illustrated in  FIG. 5A , the ball shear increased by about 0.9 gr whenever the temperature of the hot forming gas was raised by about 10° C. In conclusion, the supply of the hot forming gas during wire bonding increased the ball shear of the conductive wire in the ball bonding region with increasing temperature. 
     Referring to  FIG. 5B , the x- and y-axis in  FIG. 5B  show temperature and stitch pull, respectively. The stitch pull of the conductive wire was determined by measuring a force applied when a hook equipped with a sensor was tied to the conductive wire after wire bonding and was then raised at a predetermined speed until the wire was cut. As illustrated in  FIG. 5B , the stitch pull increased by about 0.02 gr whenever the temperature of the hot forming gas was raised by about 10° C. In conclusion, the supply of the hot forming gas during wire bonding increased the stitch pull of the conductive wire with increasing temperature. 
     Referring to  FIG. 6 , a schematic view of a wire bonder  200  according to a further embodiment of the present invention is shown. As illustrated in  FIG. 6 , the wire bonder  200  comprises a capillary  110 , an electric flame-off tip  120  and a heater  210 . 
     A conductive wire  140  penetrates the capillary  110 . A transducer  150  is coupled to the capillary  110  to deliver ultrasonic energy to the capillary  110 . A clamp  160  is installed above the capillary  110  to clamp or unclamp the conductive wire  140 . A heater block  171 , a circuit board  172 , a semiconductor die  173  and a clamp  174  pressing the circuit board  172  are installed below the capillary  110 . 
     The heater  210  is positioned between the clamp  160  and the capillary  110  and has a substantially circular tubular shape. The conductive wire  140  penetrates the heater  210  and is heated by the heater  210 . The heater  210  may take on a plurality of forms. For example, the heater  210  may be a thermoelectric element. However, this is given as an example and should not be seen to limit the scope of the present invention. A power supply unit is connected to the heater  210  to supply power to the heater  210 . In the present embodiment, the heater  210  provides a temperature of 25 to 300° C. to the conductive wire  140 . As a result, the heat energy provided by the heater  210  enables effective wire bonding and improves the bondability of the conductive wire. 
     Referring to  FIG. 7 , a schematic view of a wire bonder  300  according to another embodiment of the present invention is shown. As illustrated in  FIG. 7 , the wire bonder  300  may comprise a capillary  110 , a gas diffuser  130  for supplying a hot forming gas to a free air ball created at the lower end of the capillary  110 , and a heater  210  positioned above the capillary  110  to heat a conductive wire  140 . Due to this construction, the hot forming gas is directly diffused to the free air ball to improve the ball shear or stitch pull of the conductive wire  140 . In addition, the conductive wire  140  penetrating the capillary  110  is preheated to achieve improved bondability upon wire bonding. 
     This disclosure provides exemplary embodiments of the present invention. The scope of the present invention is not limited by these exemplary embodiments. Numerous variations, whether explicitly provided for by the specification or implied by the specification, such as variations in structure, dimension, type of material and manufacturing process may be implemented by one of skill in the art in view of this disclosure.