Abstract:
The invention discloses apparatus and methods for the formation of bond wires in integrated circuit assemblies by attaching two separate wires using a dual capillary bond head. The separate wires are preferably non-identical, for example, being of different gauges and/or material composition. According to a preferred embodiment of the invention, dual capillary bond head apparatus includes a rotatable ultrasonic horn with a pair of capillaries for selectably dispensing separate strands of bond wire and for forming bonds on bond targets. According to another aspect of the invention, a method is provided for dual capillary IC wirebonding including steps for using two dual capillary bond heads for contemporaneously attaching non-identical bond wires to selected bond targets on one or more IC package assemblies.

Description:
TECHNICAL FIELD 
       [0001]    The invention relates to microelectronic semiconductor chips, manufacturing equipment, and manufacturing processes. More particularly, the invention relates to wirebonding methods and apparatus for the manufacture of IC (integrated circuit) packages. 
       BACKGROUND OF THE INVENTION 
       [0002]    Wirebonding is a widely used technique for electrically connecting contacts within a semiconductor package. Commonly, a precious metal wire, normally gold within the range of approximately 0.0010 and 0.0015 inches in diameter, has one end ball-bonded to a bond pad on an IC, and another end stitch (or wedge) bonded to a lead on a leadframe. In order to accomplish this, the wire is fed through a capillary associated with a moveable bond head. For a ball bond, a ball is formed on the exposed end of the wire using an electronic flame off (EFO) mechanism. The ball is pulled against the end of the capillary and is then pressed into position on a pre-heated bond pad where a combination of heat, pressure, and ultrasonic vibration is used to cause the ball to adhere to the surface of the bond pad. With the ball end of the wire secured to the bond pad, the gold wire is payed out through the capillary as the bond head moves into position at the appropriate lead. A stitch bond is formed on the lead, and a tail wire is payed out through the capillary, clamped, and severed. A new ball is then formed readying the wire end for the next ball bond, and the cycle is repeated. 
         [0003]    Bond head machinery typically includes a fixed ultrasonic horn, which includes the capillary and is equipped for movement along the z-axis. Various peripheral mechanical and electronic systems support the implementation of the general wirebonding procedure described. Associated wire handling machinery typically includes a spool, tensioner, clamp, ball detector, and gas-powered venturi for feeding wire to the capillary. Movement in the x- and y-axes is implemented primarily by moving the bond head assembly in order to position the horn over the bond target. 
         [0004]    In efforts to overcome various problems in the arts, dual bond head systems and techniques are sometimes used. In some applications, for example, two separate bond heads are oriented for making bonds perpendicular to one another. In such a dual bond head configuration, each bond head independently performs wirebonding with a capillary on a horn in a fixed orientation. Supposing, for instance, that a 20-pin IC required fourteen bonds oriented in one direction and six oriented in a direction perpendicular relative to the others. Using a dual head bonder known in the arts, each head may perform the first six of its bond wire installations simultaneously. One bond head would then be idled, while the other completed the remaining, in this case eight, bond wires. Such workload imbalances are relatively common among dual head bonding systems and methods using apparatus known in the arts. Thus, inefficiency is a problem with the dual bond head approach current in the art. 
         [0005]    A significant portion of IC package manufacturing costs is due to the expense of precious metals, thus there are ongoing efforts to reduce the precious metal content of IC packages. Many IC package applications may be characterized as having at least two clearly separable groups of bond wires that, in theory, may employ significantly different wire gauges and still maintain reliable function. For example, a buffer function is readily separable into supply and output stages with high current demands, while input stages operate at current demands that are very small in comparison. Despite the opportunity for precious metal reduction by using smaller wire gauges on the input side, predominant applications commonly deploy the same wire size throughout an assembly, using the wire gauge demanded by the worst-case current path. Previous attempts to reduce gold content by use of dual head tandem arrangements have found little acceptance because such arrangements tend to reduce overall throughput due to the workload imbalance between the heads. 
         [0006]    Due to these and other technological problems, improved apparatus and methods for wirebonding with more than one wire source would be useful and advantageous contributions to the art. The present invention is directed to overcoming, or at least reducing, problems present in the prior art, and also contributes one or more heretofore unforeseen advantages indicated herein. 
       SUMMARY OF THE INVENTION 
       [0007]    In carrying out the principles of the present invention, in accordance with preferred embodiments thereof, the invention provides apparatus and methods useful for forming bond wires using two or more separate wires with a single bond head. The separate wires may be identical, or may differ in size and/or composition. 
         [0008]    According to one aspect of the invention, a preferred method for wirebonding in an IC package includes steps for bringing a first capillary of a dual capillary bond head to bear on bond targets, dispensing a wire from the first capillary, and attaching bond wires to selected bond targets. In further steps, a second capillary of the dual capillary bond head is brought to bear on bond targets and a second wire is dispensed from the second capillary and is attached to selected bond targets to form bond wires. 
         [0009]    According to another aspect of the invention, a method for IC wirebonding includes providing a bond head having two capillaries adapted for dispensing wire for bonding to bond targets on an IC assembly. Each of the capillaries is operable in a bonding mode and an idle mode. In further steps, one of the capillaries is operated in a bonding mode and the other capillary is contemporaneously maintained in an idle mode. 
         [0010]    According to another aspect of the invention, in an example of a preferred embodiment, dual capillary bond head apparatus includes a horizontally moveable bond head assembly positioned on a bonding table. An ultrasonic horn extends over the bonding table with a pair of capillaries for selectably dispensing separate strands of wire and for forming bond wires on bond targets. 
         [0011]    According to yet another aspect of the invention, a dual capillary bond head assembly in a preferred embodiment includes a horizontally moveable bond head assembly positioned on a bonding table with an ultrasonic horn extending over the bonding table as described above. A pair of capillaries offset at an acute angle to one another are provided on an ultrasonic horn adapted for rotating either of the capillaries into position for wirebonding on a bond target. 
         [0012]    According to still another aspect of the invention, a method for dual capillary IC wirebonding includes steps for using two dual capillary bond heads for contemporaneously attaching bond wires to selected bond targets on one or more IC package assemblies. 
         [0013]    According to another aspect of the invention, an IC assembly manufactured using apparatus and methods of the invention includes both insulated and un-insulated bond wires. 
         [0014]    The invention has advantages including but not limited to one or more of the following: improved wirebonding methods and apparatus; improved wire deployment equipment and techniques, particularly in applications where it is desirable to use two or more wires having different characteristics; improved wirebonding process throughput; precious metal conservation; and decreased costs. These and other features, advantages, and benefits of the present invention can be understood by one of ordinary skill in the arts upon careful consideration of the detailed description of representative embodiments of the invention in connection with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]    The present invention will be more clearly understood from consideration of the following detailed description and drawings in which: 
           [0016]      FIG. 1  is a schematic diagram illustrating examples of preferred embodiments of methods and apparatus implementing dual capillary wirebonding according to the invention; 
           [0017]      FIG. 2  is a partial cutaway side view of an example of apparatus in an implementation of a preferred embodiment of the invention; 
           [0018]      FIGS. 3A and 3B  are cutaway side views of a portion of the apparatus shown in  FIG. 2  in an example of a preferred embodiment of the invention; 
           [0019]      FIGS. 4A and 4B  are cutaway side views of another portion of the apparatus shown in  FIG. 2  in an example of a preferred embodiment of the invention; 
           [0020]      FIG. 5  is a cutaway side view of another portion of exemplary apparatus implementing the invention as shown in the embodiment of  FIG. 2 ; 
           [0021]      FIGS. 6A through 6C  provide simplified top views depicting an example of the implementation of a preferred embodiment of the invention; and 
           [0022]      FIG. 7  is a combined simplified top view and process flow diagram depicting an example of the implementation of a preferred embodiment of the invention. 
       
    
    
       [0023]    References in the detailed description correspond to like references in the various drawings unless otherwise noted. Descriptive and directional terms used in the written description such as first, second, top, bottom, upper, side, etc., refer to the drawings themselves as laid out on the paper and not to physical limitations of the invention unless specifically noted. The drawings are not to scale, and some features of embodiments shown and discussed are simplified or amplified for illustrating the principles, features, and advantages of the invention. 
       DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0024]    The invention provides novel and useful methods and apparatus for IC wirebonding whereby two separate wires may be dispensed and installed using a single bond head. Alternative embodiments of the invention may be implemented, either using identical wires, or wires of various compositions. Methods and apparatus of the invention may also be combined in order to provide multiple bond head wirebonding with advantageous gains in productivity and cost savings. 
         [0025]    First referring primarily to  FIG. 1 , an overview of an example of the operation of preferred embodiments of the invention is shown in a schematic diagram. The operation of the invention is shown with reference to a first capillary  102  and second capillary  104 . As indicated in their two respective loops in  FIG. 1 , either of the first or second capillaries  102 ,  104 , may be active while the other is idle, as indicated by the alternative states  106 ,  108 , respectively, of the capillary selection loop  110 . As shown in each of the active loops  102 , 104 , of the diagram, a wire  120  is held in position (a) with a ball  122  formed at its end by exposure to a ball-forming mechanism, such as an electronic flame-off (EFO) electrode  124 . The ball  122  is bonded (b) to a first bond target surface, e.g., a bond pad  126  on an IC. With the ball bond  128  secure, the capillary  102 ,  104  is moved (c) in order to form (d) a bond wire spanning between the ball bond  128  and a second bond target, e.g., a lead  130  on a package leadframe. A stitch bond  132  is formed (e) at the second bond target  130 , the wire  120  is payed out (f) through the capillary,  102 , 104 , clamped and pulled (g). The pulling (g) severs the wire  120  between the stitch bond  132  and the capillary  102 ,  104 , and the tail of wire protruding (h) from the capillary  102 ,  104 , is then presented to the EFO electrode  124  for the formation of a new ball  122  (i) at the end of the wire  120  so that the cycle  100  may be repeated. The individual steps, (a) through (i), may be generally familiar to those skilled in the arts, however, the details and advantages inherent in the capability of executing the steps using two capillaries performing in concert, as indicated by the two intersecting process loops  102 ,  104  joined by capillary selection loop  110 , bears additional description and illustration. 
         [0026]    Now referring primarily to  FIG. 2 , a partial cutaway simplified side view of an example of a bond head assembly  200  according to the invention is shown. A bond head assembly housing  202 , preferably moveable in the x-y axis, resides on a bonding table  204 . The bonding table  204  also preferably supports a workpiece such as a partially assembled IC package  206  having an IC  208  affixed to a leadframe  210 . The IC  208  and leadframe  210  each have bond targets, such as bond pads and leads. An ultrasonic horn  212  extends from the housing  202 , accompanied by a boom  214 . A mechanism is provided for raising and lowering the horn  212 , preferably a galvo arm  216  and pivot  218 , although suitable alternatives may also be used. The horn  212  has two capillaries  102 ,  104 , for positioning above the bond targets of the workpiece  206 . Suitable duplicate bond system components such as wire clamps  220 , venturi  222 , wire spools (not shown), and possibly other associated components are preferably provided in order to accommodate independent feed control of each of the wires (e.g.,  FIG. 1 ,  120 ).  FIG. 2  depicts an example of the general placement of these peripheral components provided to support the operations of each of the independent capillaries  102 ,  104 . It should be appreciated that capillaries of various materials, dimensions, and configurations may be used in various combinations depending upon the requirements of the application at hand. For example, in some instances it may be desirable to use identical capillaries, while in other instances it may be preferable to use a combination of standard, fine pitch, ultra-fine pitch, bottlenose, ceramic, ruby, and or alumina capillaries having different hole diameters, tip diameters, chamfer diameters, and so forth. Capillary selection may be made based on factors such as bond pad pitch, wire diameter, space available, and durability. The selectability of the capillaries during wirebonding operations, preferably by rotation of the ultrasonic horn as further described, provides advantages heretofore unavailable in the art. 
         [0027]      FIGS. 3A-3B , and  4 A- 4 B, show additional details of aspects of the horn  212  in this example of a preferred embodiment of the invention. A rotating mechanism  300  is provided for rotating the ultrasonic horn  212  in order to bring the capillaries  102 ,  104  into alignment with the workpiece  206  as desired. Rotating mechanisms such as stepper motors or other mechanical, electromechanical, pneumatic, or other means for rotating the horn  212  to align the capillaries  102 ,  104  may be used. In the example depicted in  FIGS. 3A and 3B , horn  212  rotation is accomplished using a stator winding  302  with four poles  304  that may be used to apply torque to a permanent magnet  306 , rotating the horn  212  into a desired position. The stator windings  304  and switch  308  arrangements are configured such that only one pair of opposing poles are energized at a time, and the active pair produces a north pole at one side and a south pole at the other. Such a configuration causes the permanent magnet  306  to align itself with the electromagnetic field, and so causes the horn  212  to rotate the capillaries into the desired positions. Positioning of the electromagnet poles  304  is such that each pair may rotate its respective capillary  102 ,  104  into a bonding position, preferably providing ultrasonic horn  212  rotation for two possible stationary alignments. The range of rotation is sufficient to position the capillaries  102 ,  104 , to avoid interference with one another during bonding, while avoiding excessive rotation to prevent excessive wire deformation. The axes of the capillaries  102 ,  104  form an acute angle to one another, preferably about 45 degrees. In operation, rotation of the ultrasonic horn  212  serves to move one capillary into an active bonding position, e.g.,  FIG. 4A ,  102 , while the other  104  simultaneously rotates to an idle position. The rotation preferably is performed with the horn  212  in an ‘up’ state, lifted by suitable mechanisms. 
         [0028]    An example of a preferred capillary configuration is shown in  FIGS. 4A and 4B . In this example, the first capillary  102  and second capillary  104  are oriented at an acute angle, preferably about 45 degrees apart from one another. Rotation of the ultrasonic horn  212  causes one or the other of the capillaries  102 ,  104  to align with a bond target. The rotatable ultrasonic horn  212  of the invention is also capable of being locked into position during use of either of the capillaries  102 ,  104  for bond formation. It should be recognized by those reasonably skilled in the arts that various rotating and locking mechanisms may be used within the scope of the invention. For example, as shown in the bearing lock mechanism  500  of  FIG. 5 , solenoids  502 , or hydraulically-driven arrestor pins, or an electromagnet mechanism, may be used to firmly hold the horn  212  at a desired position during wirebonding. The ultrasonic horn  212  is preferably fitted into a dual-race  504 ,  506 , bearing  508 . This bearing  508  is fitted into the bond head assembly housing  202 , so that the horn  212  is laterally fixed, but is free to rotate about its axis upon the inner race  504 . The axes of the capillaries preferably form an acute angle to one another, as shown in  FIGS. 4A and 4B . In operation, rotation of the ultrasonic horn  212  serves to move one capillary into an active bonding position, while moving the other capillary into an idle position. Once the active capillary is selected and the initial horn  212  alignment is made, the bearing lock mechanism  500  is preferably engaged in preparation for wirebonding. The bearing  508  preferably includes a symmetrical array of holes about the central circumference of both the inner and outer races  504 ,  506  in order to accept arrestor pins  510  for locking the inner and outer bearing races  504 ,  506  together, firmly immobilizing horn  212  rotation when engaged. As shown, each arrestor pin  510  is preferably an extended solenoid  502  plunger, with sufficient travel for full engagement of the inner race  504 . At each pinning location, the arrestor pins  510  preferably pass through the bearing  508  into an inner race seat  512 . Each solenoid plunger  510  is loaded against a light spring  514 , so that in a deactivated state, each spring  514  decompresses and retracts the arrestor pins  510  fully from the inner race  504 , allowing free horn  212  rotation. Preferably, each arrestor pin  510  is tapered so that constraints of initial horn  212  orientation are less critical, and upon activation of the rotation mechanism, e.g.,  FIGS. 3A-3B ,  300 , precise capillary alignment occurs. Once the solenoids  502  energize, overcoming the spring  514  force and engaging the arrestor pins  510 , the current source to stator the windings  304  is deactivated, thus the stator  302  has no influence on ultrasonic horn  212  performance during wirebonding. The bearing lock  500  solenoids  502  remain activated throughout a bonding cycle, and ball-and-stitch operations then continue with the selected capillary in a manner similar to that used with conventional wire bond machinery. 
         [0029]    It should be appreciated that the capability of switching capillaries during a wirebonding cycle can be used to provide notable advances in workload efficiency. The disclosed rotatable ultrasonic horn enables installation of dual capillaries oriented as desired in a single horn. Of course, the dual capillary arrangement of the invention has flexibility inherent in its design. It may be used for single-wire bonding, for example, as with a conventional bond head. In this mode of operation, the rotatable horn is locked in a single position and the second wire is held in an idle state. Considering that some wirebonding applications do not require the use of dual capillaries, this capability may be an important feature in some instances. Some noteworthy advantages in efficiency may nevertheless be achieved in such applications using the invention. For example, changing from one capillary to another provides a rapid means of replenishing the wire supply, or of changing wire sizes when switching production between packages requiring different gauge wires. Using the invention, throughput interruption occurs only during the selection of the active capillary, accompanied by a short x-y table move to align the bond target. Changing the active capillary requires only a ‘z-up’ operation, preferably accompanied by this sequence: unlock horn; rotate horn; lock horn; deactivate stator. In a single bond head operation, this selection cycle may occur twice as bonding progresses from one IC assembly to another, but in order to minimize interruption, one of the capillary changes may be made during the transport index cycle. 
         [0030]    An aspect of the invention is the potential for conserving precious metals used in the wire bonds of an IC package. An example is shown in  FIGS. 6A  though  6 C, in which an IC assembly  600  has a sixteen-bit driver IC  602  affixed to a leadframe  604 , with forty eight pins  606  requiring wirebonding to the leads  608  of the leadframe  604 . The IC  602  includes twelve supply pins  610 , sixteen output pins  612 , and twenty input pins  614 . Using the invention, as shown in  FIG. 6A , the twenty input pins  614  are preferably wirebonded using one capillary of a bond head, in this case using gold bond wire  616  of approximately 0.6 mils in diameter in order to provide the capability of carrying up to approximately 18 mA of current. As shown in  FIG. 6B , the output  612  and supply pins  610 , in this example requiring 180 mA capacity, are preferably wirebonded with 1.3 mil diameter gold bond wires  618  using the other capillary of the bond head. The resulting wirebonded IC assembly  620 , shown in  FIG. 6C  prepared for encapsulation into an IC package, thus realizes a nearly one-third reduction in gold bond wire content compared to methods for using one wire gauge. This is but one example representative of the advantageous precious metal conservation aspect of the invention. Of course, many other examples abound, but cannot all be shown. The invention may be used with numerous types of IC assemblies with particular advantages in assemblies in which it is desirable to use wires of different sizes or composition. To cite a few more examples, the advantages of the invention may also be exploited further by using wires of different compositions according to application requirements, such as, gold and copper, or various alloys (e.g., different levels of gold purity), or combinations of insulated and un-insulated wires. It should be understood that according preferred embodiments of the invention, an IC package assembly may be partitioned into two bond wire groups, such as a small bond wire group and a large bond wire group, or one alloy group and another, or insulated and un-insulated bond wires. The different wirebonds may be made without significantly slowing throughput compared to common single bond head techniques, and significant advantages may be realized in terms of reducing precious metal content. Further advantages may be realized in packages for which the IC dimensions are influenced by the minimum bond pad size. In some cases, using the invention to reduce bond wire sizes for some of the bond pads may enable the use of smaller bond pads spaced more closely. As a result, smaller ICs and smaller packages may in turn be realized by implementing the invention. 
         [0031]    Revisiting the workload efficiency aspect of the invention, a conventional dual bond head approach for producing the IC assembly  620  of  FIG. 6C  would introduce a workload imbalance; the bond head installing the twenty smaller bond wires would run continuously while the bond head installing the larger bond wires would remain idle during the time that four of the small bond wires were being bonded. With the conventional approach, the greater the disparity in the number of each type of bond wire required, the greater the inefficiency. Using the invention as described in this single bond head example, regardless of the number of bond wires of each type, the nonproductive bond head time is limited to the minimal time required to rotate the horn in order to change the active capillary. 
         [0032]    The invention provides further new and unique aspects of improved work flow in wirebonding processes, such as that denominated “interlaced” wirebonding herein. It has been determined during the course of developing the invention, that by using two rotatable ultrasonic horn bond head assemblies in a dual bond head configuration, additional advantages may be obtained. Now referring primarily to  FIG. 7 , an example of dual capillary, dual bond head, interlaced wirebonding according to the invention is shown. The workflow  700  in a preferred embodiment of an interlaced bonding method is shown in terms of three time slots  702 ,  704 ,  706  for showing the progression of the process. The activities of two dual capillary bond heads  708 ,  710  may be seen as follows. For the sake of this example, it is assumed that a particular type of IC-leadframe assembly is to be mass-produced using two different gauges of bond wire. Since each dual capillary bond head possesses the capability of installing both wire gauges, as further described herein, each dual capillary bond head is preferably used to perform alternating small wire and large wire bonds while indexing from one IC assembly to another, e.g., from IC assembly  712 , to IC assembly  714 . With the exception of initial start-up in the first time slot  702 , when one bond head, e.g.  710  in this example, is idle, both bond heads,  708 ,  710 , perform identical bonding operations at any given time. As shown, beginning with the first time slot  702 , the first bond head  708  is brought to bear on the first IC assembly  712 . The first bond head  708  initially installs bond wires of a first gauge  716 , for example large output wires, on the first IC chip  718 , then a bond wire of a second gauge  720 , e.g., a reference pin bond with a single small wire, on the next IC  722 , and alternately repeating this sequence thereafter. Initially, as shown in the first time slot  702 , the second bond head  710  remains idle. In the second time slot  704 , the first bond head  708  alternately installs bond wires of two gauges  716 ,  720  on a second IC assembly  714 , but in this example, in reverse order compared to the sequence it used in the first time slot  702 . Also during the second time slot  704 , the second bond head  710  preferably performs identical operations on the first IC assembly  712 , alternately bonding large wires  716 ′ and small wires  720 ′ at the bond targets bypassed by the first bond head  708  during the previous timeslot  702 . As shown in the third time slot  706 , the first IC assembly  712  has now been completed and may be ejected, as indicted by arrow  722 , for further manufacturing processes such as encapsulation, and the second IC assembly  714  takes its position for further bonding by the second bond head  710 . Meanwhile, a third IC assembly  724  is moved into a position accessible to the first bond head  708 . The first bond head  708  completes large wire  716  and small wire  720  bonds on the third IC assembly  724 . While the second bond head  710  completes the wire bonds  716 ′,  720 ′ on the second IC assembly  714 , but in an order opposite to the previous, i.e., second, time slot  704  sequence. It may be seen that the operations occurring in the second and third time slots  702 ,  704  may be repeated cyclically numerous times until a desired number of IC assemblies are wirebonded. Thus, the interlaced wirebonding methods of the invention provide a remarkable and novel process flow with a workload very nearly evenly distributed between dual bond heads. It has been found that using adaptations of the disclosed apparatus and methods within the scope of the invention, interlaced wirebonding may be used to provide balanced throughput wirebonding workloads with any combination of differing wire sizes, wire compositions, and pin counts. It should be noted that the changes between capillaries of the dual capillary bond heads are preferably performed during the transport index, or x-y table move to an adjacent IC, making such changes transparent to throughput. 
         [0033]    The methods and systems of the invention provide one or more advantages including but not limited to improved workload efficiency and precious metal conservation. While the invention has been described with reference to certain illustrative embodiments, those described herein are not intended to be construed in a limiting sense. For example, variations or combinations of steps or materials in the embodiments shown and described may be used in particular cases without departure from the invention. Various modifications and combinations of the illustrative embodiments as well as other advantages and embodiments of the invention will be apparent to persons skilled in the arts upon reference to the drawings, description, and claims.