Patent Publication Number: US-7584881-B2

Title: Low loop height ball bonding method and apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional application of U.S. patent application Ser. No. 10/988,053 filed Nov. 12, 2004, now U.S. Pat. No. 7,347,352 which claims priority to U.S. Provisional Patent Application No. 60/525,305 filed Nov. 26, 2003 the content of which is incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The invention pertains to wire bonding on semiconductor devices. 
     BACKGROUND OF THE INVENTION 
     Ball bonding is a common technique for interconnecting the bond pads on a semiconductor die with the contact points on a lead frame or other substrate on which the die is mounted. Electrical interconnect wires typically are run from the bond pads on the top of the die to lead fingers on a lead frame in order to electrically connect the circuitry on the die to the pins of the lead frame that will extend from the package after the die has been encapsulated. The wire bonds between the bond pads of the die and the lead fingers commonly are formed using a ball bonding machine.  FIGS. 1A-1I  demonstrate the steps in a conventional technique of ball bonding. The conventional looping technique (herein termed forward looping) involves ball bonding one end of a gold wire to a bond pad on a die and stitch bonding the other end of the wire to the lead frame. More particularly, using a ball bonding machine, the wire  17  is passed through a set of clamps  18  and through a center bore of a capillary  11 . At the beginning of the process, a wire “tail”  23  is protruding from the tip of the capillary  11 , as shown in  FIG. 1A . The tail  23  at the end of the wire  17  is heated by means of an electric spark  16  termed an electric flame off (EFO) from an EFO wand  24 . The spark melts the end of the wire, which, in turn, forms into a ball  19  when melted, as shown in  FIG. 1B . The clamps  18  are closed during EFO in order to provide a current return path through the clamps and then are opened to allow the ball to seat itself in the capillary tip. The capillary  11  is then moved to a position above the bond pad  13  of the die  15 , as shown in  FIG. 1C . 
     The capillary  11  is then moved downwardly with the clamps  18  still open during the initial acceleration of the capillary and then are closed during deceleration of the capillary so that the ball remains seated during the downward motion of the capillary. The clamps then open just before the ball contacts the bond pad  13 . The ball  19  comes into contact with the bond pad  13  on the die  15  with the clamps  18  still open, as shown in  FIG. 1D . Heat and/or ultrasonic energy are applied to the die to cause the ball to become bonded to the bond pad  13 . This bond typically is termed a ball bond or first bond. The capillary  11  is then raised with the clamps  18  opened to pay out a short length of wire that is still attached to the top of the ball bond, as shown in  FIG. 1E . Next, with the clamps  18  open, the capillary  11  is moved through a predetermined looping motion with the wire (which is still connected to the ball bond) and trailing out of the capillary  11  to a position generally near and above the lead finger  21 . With the capillary  11  positioned above the lead finger  21 , the clamps  18  are closed, as shown in  FIG. 1F . The capillary  11  is then lowered to pinch the wire between the capillary and the surface of the lead finger  21 , as shown in  FIG. 1G . Again, heat and/or ultrasonic energy may be applied to bond the pinched portion of the wire to the lead finger  21 . This bond is termed a stitch bond or second bond. The clamps  18  are now opened again and the capillary  11  is then raised with the wire still attached to the stitch bond such that an additional wire “tail”  23  pays out of the capillary, as illustrated in  FIG. 1H . The clamps  18  are then closed and the capillary  11  is raised further to snap the wire tail  23  at the weakest point, which is at the stitch bond location. The completed connection  22  is termed a wire loop and is illustrated in  FIG. 11 . 
     At this point, the capillary is moved near the next bond pad on the die  15  for commencing the wire looping process for the next bond pad on the die. The wire tail  23  that remains protruding from the tip of the capillary after the conclusion of the formation of the preceding wire loop will be melted by EFO, as previously described, to form the next ball for commencing the next ball bonding operation. The above-described conventional forward ball bonding technique is fast, reliable, and inexpensive. However, it has limitations. Most notably, the minimal loop height is normally over 150 microns. Loop height is defined as the maximum height of the wire above the bonding surface, e.g., the top surface of the bond pad. Attempting to achieve lower loop height can cause neck damage to the wire loop. The neck is the portion of the wire loop directly adjacent to the ball bond. Reducing the loop height below 150 microns tends to weaken or break the neck. 
     There is an increasing demand for smaller and smaller integrated circuit packaging. One of the significant aspects of reducing the size of the integrated chip packaging is reducing its thickness or height. The thinner packages are generally referred to in the trade as low profile packages. Commensurate with the desire to reduce the height of the package is the desire to reduce the height of the highest point of the wire loops, which, in many instances, is the limiting factor as to the height of an integrated circuit package. 
     In order to reduce loop heights for integrated circuit packaging and other purposes, a wire looping technique known as reverse looping was developed. The premise behind reverse looping is that, because the highest point of the wire loop is adjacent the ball bond, it would be desirable reverse the looping process so as to make the first, ball bond on the lead frame (or other substrate) and make the second, stitch bond on the bond pad of the die because the surface of the lead frame is lower than the surface of the die. Hence, the highest point of the wire loop is near the lower bonding surface, thus reducing the overall height. 
     However, simply reversing the direction of the looping process would not be possible because, the stitch bond requires the capillary to come in contact with the bonding surface. The bond pads on a die usually are very small and, thus, it is difficult to make a stitch bond on a bond pad on a die without the capillary contacting and, hence, damaging surrounding circuitry on the die. Furthermore, the wire loops tends to sag to their lowest points close to the stitch bond. Thus, if the stitch bond site is higher than the ball bond site, the wire might contact the edge or the top surface of the die. This could lead to electrical shorts or breakage of the wire. 
     Thus, a reverse looping technique was developed, such as illustrated in  FIGS. 2A through 2C , in which the first step is to form a ball bond  25  on top of the bond pad  27  on the die  29  essentially in accordance with standard techniques for forming ball bonds. However, instead of paying out the wire  17  as would be the case after making the ball bond in a conventional forward looping technique, the capillary  11  is raised, the clamps  18  are closed, and the capillary is raised further to snap the wire off from the ball bond leaving just the ball bond (or bump)  25  on the bond pad  27 , as illustrated in  FIG. 2A . Then a complete wire looping process is performed in the reverse direction, i.e., from the substrate to the bond pad. That is, a second ball bond  37  is then formed on the lead frame  39 , the capillary  11  is then moved through a series of motions to a position above the first ball bond  25  to create the desired wire loop shape, as illustrated in  FIG. 2B . Then, a bond  43  is formed on top of the first ball bond (or bump)  25 . The completed wire loop is illustrated in  FIG. 2C . 
     This reverse looping process can provide low loop heights for low profile packaging. However, it is a much slower process than forward looping because it requires the formation of two ball bonds per loop. Furthermore, the die must suffer greater impact because the capillary must form a bond on the die twice per wire loop (i.e., once to create the first ball bond and a second time to create the bond on top of the ball bond). Another limitation of reverse looping is that it often is the limiting factor on how fine the pitch of the bond pads on the die. Particularly, the bump  25  on top of the die bond pad must be large enough to provide support for a bond. In addition, the diameter of the bump will increase in the lateral direction when the bond is made on top of it. 
     Accordingly, it is an object of the present invention to provide an improved wire loop formation method and apparatus. 
     It is another object of the present invention to provide a wire loop interconnect with very low loop height. 
     SUMMARY OF THE INVENTION 
     In accordance with the invention, a bump is formed on the die bond pad by forming a ball bond thereon. Then, without severing the wire and with the clamps open, the capillary undergoes a set of coordinated xyz motions to fold the wire on top of the ball bond. Then the wire is bonded on top of the bump without breaking off the tail. This is followed by a further set of coordinated xyz motions to bring the capillary to the second bond site (e.g., the lead frame or other substrate) and perform a, stitch bond on the substrate and then break off the tail to complete the process. 
     In accordance with one particular embodiment of the invention for making the aforementioned wire fold, the ball bond is made and then the capillary is raised in the z direction a designated height (herein termed the separation height). It is then moved horizontally (in the xy plane) a specified distance in a direction generally away from the second bond location (herein termed the fold offset distance). The capillary may or may not be raised again in the z direction by another distance (herein termed the fold factor). This is followed by another motion in the xy plane to generally bring the capillary back to the top of the bump for formation of the aforementioned bond on top of the bump. This is then followed by another coordinated xyz motion to bring the capillary to the location of the second bond site (e.g., on the lead frame) where the stitch bond will be made. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A through 1I  are elevation views illustrating the steps of a conventional forward looping operation. 
         FIGS. 2A through 2C  are elevation views illustrating steps of a reverse looping operation. 
         FIG. 3  is a pictorial elevation view of a folded wire loop interconnect in accordance with the present invention. 
         FIG. 4  is a pictorial elevation view showing the various components of the set of xyz motions involved in forming a folded bump in accordance with the present invention. 
         FIGS. 5A through 5O  are elevation views illustrating the steps of a folded forward looping operation in accordance with the present invention. 
         FIG. 6  is a pictorial elevation view showing the various components of the set of xyz motions involved in forming a folded bump in accordance with particularly preferred exemplary embodiment of the invention. 
         FIG. 7A  is a side view of a bump and a fold of a wire formed on the bump formed using the exemplary parameters set forth in  FIG. 6 ; and  FIG. 7B  is a stitch bond. 
         FIGS. 8A and 8B  are scanning electron micrographs of a completed loop produced after forming the bump shown in  FIG. 7A . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 3  is a side elevation pictorial of a folded forward wire loop  45  formed in accordance with the present invention.  FIG. 3  shows an integrated circuit die  51  including a bond pad  53  on its top surface and a lead frame substrate  55  with a wire loop interconnect between the bond pad  53  and the lead frame  55 . The wire loop  45  is formed in accordance with the technique of the present invention may be considered to comprise five general components. With reference to  FIG. 3 , they are (1) a bump  56 , (2) a wire fold  57  on top of the bump, (3) a bond on top of the bump  58 , (4) a wire loop  59  that interconnects the first and second bond sites, and (5) a stitch bond  60  at the second bond site. 
     The five general components described above may be formed using a ball bonding machine. 
       FIGS. 5A through 5O  illustrate the position of the capillary of the ball bonding machine and the condition of the wire at various stages of an exemplary process for forming a folded forward wire loop in accordance with one particular embodiment of the present invention. In the terminology used in this specification, the vertical direction is termed the z direction and the horizontal direction is termed the xy direction. The Figures in this application are, of course, two dimensional so that all xy motions are illustrated as being in the plane of the page and thus could be considered simply as one dimensional movements, i.e., x or y, rather than xy. However, because, in the real world, the wire loops on a given die are not all parallel to each other, all lateral motion must be defined as xy motions in the machine code that controls the motion of the capillary of a ball bonding machine. Thus, we use the same terminology in this specification. In addition, the terms vertical and horizontal are merely exemplary based on the assumption that the top surface of the die is oriented horizontally, which is typical, but not necessarily always an accurate assumption. 
     As shown in  FIG. 5A , the folded forward loop formation process begins with the capillary  11  at the end of the preceding looping process with a wire tail  23  extending from the capillary and the clamps  18  closed. The capillary is positioned in the vicinity of the bond pad  61  of the die  63  and the associated lead finger  65  between which the next wire loop interconnect is to be made. 
     The wire is melted with the electric-flame-off  67  to cause the end of it to melt. Upon melting, it inherently forms into a ball  72 , as shown in  FIG. 5B . The capillary is lowered and moved toward the bond pad  61 , as shown in  FIG. 5C . The capillary is then lowered to contact the ball  72  against the bond pad  61 . Heat and/or ultrasonic energy is applied to bond the ball to the bond pad.  FIG. 5D  shows the process at this point, which essentially is the end of the formation of the first portion of the folded forward wire loop, i.e., the bump  56 . 
     Next, the capillary  11  rises with the clamps  18  open in order to pay out a length of wire extending from the top of the bump  56 . The distance of this rise is herein termed the separation height, and is illustrated at  75  in  FIG. 4 .  FIG. 5E  shows the capillary position after this step. The capillary  11  then moves in the xy plane in a first direction generally opposite the direction to the second bond site  65  (to the left in  FIGS. 5A through 5O ). Preferably, the direction of the fold offset is exactly opposite the xy direction toward the second bond site  65 , however, it can be in any direction. In fact, there may be cases in which it is zero. The distance of this xy motion is herein termed the fold offset and is shown in  FIG. 4  at  76 . The position of the capillary after the fold offset motion is shown in  FIG. 5F . 
     In some embodiments of the invention, at the end of the fold offset motion, the capillary  11  is raised again (in the positive z direction) a short distance (herein termed the fold factor) to pay out additional wire. The fold factor is shown at  77  in  FIG. 4 . However, in some applications of the invention, a fold factor of zero will be adequate.  FIG. 5G  shows the position of the capillary and wire at this point in the process. The fold offset and the fold factor control the amount of wire in the wire fold. 
     The capillary  11  is then moved back toward the bump  56  to fold the wire back over on top of itself to begin forming the wire fold on top of the bump.  FIG. 5H  shows the position of the capillary  11  and the condition of the wire as the capillary is moving to form the fold. This last-mentioned motion is herein termed the fold return motion as is illustrated at  78  in  FIG. 4 . Preferably, the fold return motion is in the xy direction exactly opposite the xy direction of the fold offset motion. In those embodiments in which the fold factor  77  is zero, the fold return motion preferably is a purely horizontal (i.e., xy) motion. If the fold factor  77  was not zero, the fold return motion  78  may include a negative z component to return the capillary to the same height that it was during the ball bonding. Either case returns the capillary to the separation height at the end of the fold return motion. However, this is not a requirement. In fact, in at least some preferred embodiments of the invention, as discussed further below, the capillary returns to a height below the separation height In fact, the fold return motion  78  may include a positive (upward) or negative (downward) z component, even if the fold factor  77  was zero. The important aspect is that a wire fold is formed on top of the bump  56  preferably extending in a direction directly away from the second bond site  65 . The difference between the separation height and the position of the capillary at the end of the fold return motion  78  is herein termed the bump height. It may be a positive or a negative number. However, as noted previously, in some implementations of the invention, there will be no z motion of the capillary between reaching the separation height and completion of the wire fold and/or the fold factor and the z component of the fold return motion will be exactly equal and opposite so that the bump height will be zero. 
     Furthermore, the horizontal component of the fold return motion  78  need not be the same distance as the fold offset motion. Depending on the particular application, it may be desirable to return to a position for purposes of performing the bond at a position that is short of (as illustrated by path  78   a  in  FIG. 4 ), exactly the same as (as illustrated by path  78   b  in  FIG. 4 ), or long of (as illustrated by path  78   c  in  FIG. 4 ) the original xy coordinates of the capillary prior to the fold offset motion. 
     In those rare embodiments in which the fold offset is zero, the horizontal component of the fold return motion also may be zero or very small. However, it need not be since, as just noted, it may be desirable in certain circumstances that the bond on top of the bump be made short of or long of the original xy coordinates of the capillary prior to the fold offset motion (i.e., the original xy coordinates of the capillary when it made the bump when it made the bump). Generally, however, there should be at least either a non-zero offset motion or a non-zero horizontal component to the fold return motion in order to cause the loop to form in some specified direction. In the absence of both, the direction of the fold that the wire takes may be unpredictable and/or may vary from bond to bond. In the absence of any significant horizontal motion in both the fold offset motion and the offset return motion, there may be no folded loop formed at all or a negligible one. This may actually be desirable in some circumstances. 
     The horizontal difference between the xy coordinates of the capillary at this point relative to the xy position of the capillary when the bump was formed is termed the fold return offset. In other words, the fold return offset is the difference in the xy dimension between the fold offset motion and the horizontal component of the fold return motion. If the xy component of the fold return motion  78  is less than the fold offset motion  76 , the fold return offset is represented as a positive number. If the horizontal component of the fold return motion  78  is longer than the fold offset motion  76 , the fold return offset is represented as a negative number. If the fold offset motion  76  and the fold return motion  78  have the same xy (i.e., horizontal) magnitudes, then the fold return offset is zero. 
     At the end of the fold return motion, the capillary is contacting the top of the bump  56  and pinching the wire between the tip of the capillary  11  and the bump, as shown in  FIG. 5I . At this point, the wire has been pinched, but not severed. Also, at this point, the second of the five aforementioned portions of the overall folded forward wire loop (i.e., the fold  57 ) is completed. As the wire has not been severed since the beginning of the process, the fold  57  is continuous with the bump  56 . 
     The wire is then bonded to the top of the bump  56 . Heat and/or ultrasonic energy may be applied to facilitate bonding of the compressed portion of the wire to the top of the bump  56 . However, the bonding process need comprise nothing more than simply pinching the wire between the tip of the capillary and the ball without severing the wire. One may call this bond a stitch bond or a stitch-like bond, but the wire should not be severed. At this point, the third of the five aforementioned portions of the overall loop (i.e., bond  58 ) is completed.  FIG. 53  shows the process at this point, which is essentially the same position as shown in  FIG. 51  since the capillary typically does not move during bonding. 
     The capillary is now moved toward the second bond site through a set of coordinated xyz motions to form the desired wire loop shape (the fourth portion of the overall loop) and to position the capillary above the second bond site  65 .  FIGS. 5K and 5L  show the position of the capillary  11  at two points along an exemplary trajectory to the second bond site  65 . 
     The coordinated xyz motion can be relatively simple, comprising a straight xy motion toward the second bond site. However, typically there is motion in the z direction to help form the wire loop into the most preferable shape. As the wire has still not been severed since the beginning of the process, the wire loop  59  is continuous with the bump  56 , fold  57 , and bond  58  at the first bond site. 
     Next, the capillary is lowered to pinch the wire between the second bond surface  65  and the tip of the capillary  11  and the wire is stitch bonded to the second bond site. Heat and/or ultrasonic energy may be applied to facilitate bonding. At this point, the wire has been pinched, but has not been fully severed, as shown in  FIG. 5M . The capillary  11  is then raised with the clamps  18  still open and with the wire still attached to the stitch bond  60  such that a wire tail  69  pays out of the capillary  11 .  FIG. 5N  illustrates the position of the capillary at this point in the process. The clamps  18  are then closed and the capillary  11  is raised further to snap the wire at the stitch bond  60  as shown in  FIG. 5O . The fifth portion of the overall folded forward wire loop, i.e., the stitch bond  60 , is now completed and the entire folded forward looping process is concluded. 
     The main purposes of the bump  56  are to prevent direct contact of the capillary with the bond pad and to raise the height (z direction) from which the wire exits the first bond site. Specifically, the wire exits the first bond site at so low of an angle that, if not for the extra height above the die surface provided by starting the wire loop on top of a bump, the wire loop  59  might otherwise contact the die surface intermediate the first and second bond sites. 
     The primary purpose of forming the fold  57  on top of the bump is so that the wire exits the first bond site generally horizontally and pointing toward the second bond site, thus creating a low loop height. The fold is generally elliptical in shape with its major axis in the xy plane and the wire exits the end of the fold generally horizontally and generally pointing directly toward the second bond site. This orientation is to be contrasted with the generally straight upward orientation of the wire as it exits the first bond site in a conventional forward wire loop. Accordingly, the looping technique of the present invention provides ultra-low loop heights because the wire exits the ball bond site pointing horizontally rather than vertically upward. 
     In addition, the impact to the die is reduced relative to conventional reverse ball bonding because it does not perform a normal second bond on top of the bump, as is the case with reverse looping. The wire is only slightly compressed to form the loop and bond on top of the ball as in  FIG. 7A  in comparison to a normal stitch bond similar to  FIG. 7B  as in reverse bonding. 
     Further, finer pitches can be achieved with the present invention relative to reverse ball bonding because the bump is not compressed and squished out laterally as much as in the reverse wire looping technique. 
     The wire bonding machine is controlled by motion control system comprising control circuitry that can cause the machine to perform the processes described herein. Commonly, the circuitry comprises a digital processing device such as a programmed general purpose computer, a digital signal processor, a state machine, a combinational logic circuit, a microprocessor, an application specific integrated circuit, or any other known digital processing means. If the circuitry comprises a computer, the invention may reside largely, if not exclusively, in the software for programming the computer to control the wire bonding machine to perform the processes described herein. 
     By optimizing the various motion components described in connection with  FIG. 4 , we can achieve desired shape with low loop height and no neck damage. As an example,  FIG. 6  shows exemplary parameters that were used to form the fold shown in  FIG. 7A  and subsequently the loop profile shown in  FIGS. 8A and 8B . The first three motions (separation height  75 , fold offset  76  and fold factor  77 ) collectively determine the amount of the wire in the fold and the shape of the fold. If these motions are too large, a larger fold than desired may be formed. If these motions are too small, a fold may not be formed at all and the neck region of the wire could be damaged. 
     We have found through experimentation that a separation height and a fold offset slightly larger than the wire diameter (e.g., about 1.01 to 1.55 times the wire diameter) achieves excellent results. Furthermore, the fold factor should be a positive value. In the example of  FIG. 7A , the separation height  75  is 1.5 mil, the fold offset  76  is 1.3 mil and the fold factor  77  is 1 mil for a wire of a diameter of 1 mil). These settings pay out the proper amount of wire and angle the wire around 30 degrees from the vertical before the fold return motion  78  starts. 
     The fold return motion  78  determines the final shape of the bump. In the particular machine we used, the fold return motion  78  is specified by two parameters. Specifically, (1) the aforementioned fold return offset parameter (which, along with the fold offset, dictates the horizontal component of the fold return motion) and (2) the bump height (which, along with the fold factor, dictates the vertical component of the return motion). 
     To form a good fold with enough deformation to sustain the fold without overly flattening the bump, bump height normally should be a negative value (bump height being a vertical distance from the separation height). In this example, the bump height is −1.25 mil and the separation height is 1.5 mil. Thus, the final position of the capillary is about 0.25 mil above the bump  56 . This provides just enough flattening of the wire to form the fold without flattening the bump significantly. The fold return offset in this example is 0.5 ml, which means that the final position of the capillary after the fold return motion  78  is offset 0.5 mil horizontally from the center of the bump (away from the second bond  60 ). These settings ensure that the site of the bond  58  is centered on top of the bump  56 . 
     The wire loop resulting from bonding using these parameters is shown in  FIG. 8 . Over multiple experimental samples, the average loop height using these parameters was 2.3 mil. The maximum loop height was 2.5 mil. 
     Having thus described a few particular embodiments of the invention, various alterations, modifications, and improvements will readily occur to those skilled in the art. Such alterations, modifications and improvements as are made obvious by this disclosure are intended to be part of this description though not expressly stated herein, and are intended to be within the spirit and scope of the invention. Accordingly, the foregoing description is by way of example only, and not limiting. The invention is limited only as defined in the following claims and equivalents thereto.