Wire conveyance system having a contactless wire slacking device

A wire conveyance system is provided for a wire bonding machine having, in series, a first wire handler, a wire slacking device and a second wire handler. The wire slacking device includes a wire support chamber having a substantially enclosed bottom, opposing first and second vertical sidewalls, a substantially open top, a back opening and a front opening. The wire slacking device further includes a pressurized fluid source and first and second fluid orifices for receiving a pressurized fluid from the pressurized fluid source. The fluid orifices extend through the first or second sidewall of the wire support chamber with the second fluid orifice being positioned downstream of the first fluid orifice. First and second fluid paths are provided in fluid communication with the pressurized fluid source via the first and second fluid orifices. The first and second fluid paths extend upward between the first and second sidewalls of the wire support chamber and out the open top. A wire support path extends laterally through the wire support chamber above the first and second fluid orifices. The wire support path is substantially contactless with respect to the wire slacking device.

TECHNICAL FIELD 
The present invention relates generally to automated wire bonding, and more 
particularly to a series of devices which convey a bonding wire through a 
wire bonding machine. 
BACKGROUND OF THE INVENTION 
Miniature, electric power drawing elements, such as integrated circuits, 
semiconductors and the like, included within a larger host device 
typically have many electrically conductive pads. Each conductive pad 
requires electrical contact with one or more corresponding conductive pads 
on the same element or associated elements of the host device to form an 
electrically conductive connection between the pads. The electrically 
conductive connection is provided by a segment of fine, electrically 
conductive wire which extends between the pads and is bonded to a bonding 
site on each of the pads. Wire bonding is an automated operation performed 
during assembly of the host device using a machine having a high degree of 
speed and precision. The wire bonding operation is cyclic, comprising many 
bonding cycles integrated into a continuous unitary process. Each bonding 
cycle of the process comprises a number of discrete tasks performed 
sequentially by specific components of the wire bonding machine. 
In a simple case where an electrical connection is required between only 
two pads, the bonding cycle comprises the tasks of retaining an element or 
elements including the pad pair, conveying a bonding wire from a wire 
supply to the pad pair, positioning the wire at the first bonding site on 
the first pad of the pair, bonding the wire to the first bonding site, 
repositioning the wire at the second bonding site on the second pad of the 
pair, bonding the wire to the second bonding site, and breaking off the 
wire at the second bonding site to provide a wire segment which 
electrically connects the first and second pads. These tasks are repeated 
in the next bonding cycle at another pad pair. For more complex bonding 
cycles where electrical connection is required between a string of three 
or more pads, the wire is positioned and bonded to the bonding site on 
each pad of the string in series. The wire is not broken off until the 
bond is completed at the final bonding site to provide a wire segment 
which electrically connects every pad of the string. 
The task of conveying the bonding wire from the wire supply to the bonding 
site is critical to the performance of the wire bonding machine because 
any damage due to contamination, deformation, blemishing, or the like 
inflicted on the surface of the wire during the conveyance task can 
diminish the integrity of the ensuing wire bond. Accordingly, considerable 
effort in the prior art is directed to providing non-damaging means for 
conveying the bonding wire. For example, U.S. Pat. No. 5,318,234, 
incorporated herein by reference, discloses a wire de-spooler which 
conveys the bonding wire from a wire supply spool to a downstream wire 
handler. The wire de-spooler includes a spool drive motor to pay out the 
bonding wire from the spool and a wire slacking device comprising an air 
nozzle, a bail and a guide spindle positioned at the wire de-spooler 
outlet, which forms and maintains a desired slack length in the bonding 
wire. In operation, the wire slacking device suspends the bonding wire 
between a contact point on the spool and a contact point on the guide 
spindle as the bonding wire is paid out from the spool. The air nozzle and 
bail are positioned on the wire path between the spool and guide spindle. 
When the bonding wire passes through the bail, the air nozzle, which is 
positioned immediately above the bail, directs a downward jet of 
pressurized air directly onto the bonding wire. The force of the air jet 
on the bonding wire operates against the static force of the spool and 
guide spindle to create a slack length in the bonding wire between the 
spool and guide spindle. Maintenance of the slack length desirably reduces 
the risk of wire damage as the wire is being de-spooled. Maintenance of 
the slack length particularly reduces the risk of wire twisting during 
de-spooling which is extremely detrimental to subsequent wire bonding. 
Although the above-described prior art wire slacking device may effectively 
prevent wire twisting, direct application of the air jet to the bonding 
wire in turbulent flow as well as contact between the bonding wire and the 
guide spindle or bail create a significant risk of damage to the bonding 
wire. The present invention recognizes a need for a wire conveyance system 
which controls the degree of slack in the bonding wire with a diminished 
risk of damage to the bonding wire. Therefore, it is an object of the 
present invention to provide an effective wire conveyance system. More 
particularly, it is an object of the present invention to provide a wire 
conveyance system having a device which effectively controls the desired 
degree of slack in the bonding wire as the bonding wire is conveyed 
between wire handlers. It is another object of the present invention to 
provide such a wire slacking device which controls the rate at which the 
wire is conveyed between wire handlers. It is yet another object of the 
present invention to provide such a wire slacking device which operates in 
a non-damaging manner with respect to the bonding wire. It is still 
another object of the present invention to provide such a wire slacking 
device which operates in a contactless manner with respect to the bonding 
wire and wire slacking device. These objects and others are accomplished 
in accordance with the invention described hereafter. 
SUMMARY OF THE INVENTION 
The present invention is a wire conveyance system for a wire bonding 
machine. The wire conveyance system includes, in series, a first wire 
handler, a wire slacking device and a second wire handler. The wire 
slacking device comprises a wire support chamber having a substantially 
enclosed bottom and opposing first and second vertical sidewalls. The wire 
support chamber also has a substantially open top, a back opening and a 
front opening. The wire slacking device further comprises a pressurized 
fluid source retaining a pressurized fluid. First and second fluid 
orifices in fluid communication with the pressurized fluid source extend 
through the first or second sidewall of the wire support chamber, with the 
second fluid orifice being positioned downstream of the first fluid 
orifice. The central axes of the first and second fluid orifices are each 
aligned substantially perpendicular to the first and second sidewalls. 
First and second fluid paths are provided in fluid communication with the 
pressurized fluid source via the first and second fluid orifices, 
respectively. The first fluid path extends continuously through the wire 
support chamber, passing into the first fluid orifice, upward between the 
first and second sidewalls, and out the open top of the wire support 
chamber. The second fluid path follows a substantially similar route as 
the first fluid path, but passing into the second fluid orifice downstream 
of the first fluid path. A wire support path is provided in fluid 
communication with the first and second fluid paths. The wire support path 
extends continuously through the wire support chamber above the first and 
second fluid orifices, passing into the back opening, laterally between 
the first and second sidewalls, and out the front opening of the wire 
support chamber. The wire support path is substantially contactless with 
respect to the wire slacking device. 
The wire slacking device additionally comprises first and second internal 
wire sensors positioned in the wire support chamber below the wire support 
path and a wire sensor positioned above the wire support path. The first 
internal wire sensor is a proximity sensor positioned between the first 
and second fluid orifices. The second internal wire sensor is a contact 
sensor. 
The present invention is further a method of maintaining a slack length in 
a bonding wire utilizing the above-recited wire conveyance system in 
cooperation with the bonding wire. The method is initiated by feeding the 
bonding wire from a first wire handler to a wire support chamber at a wire 
feed rate. The wire is conveyed laterally through the wire support chamber 
along a wire support path while a pressurized fluid is injected into the 
wire support chamber at a fluid injection rate and at a substantially 
perpendicular angle to the first and second sidewalls of the wire support 
chamber. The pressurized fluid is injected via first and second fluid 
orifices which are in fluid communication with the wire support chamber 
and which are positioned below the wire support path. The pressurized 
fluid is directed upward through the first and second fluid paths past the 
wire, thereby suspending the wire in the wire support path in 
substantially contactless relation with the sides of said wire support 
chamber. The wire is withdrawn from the wire support chamber to a second 
wire handler at a wire withdrawal rate and a slack length is formed in the 
wire withdrawn from the wire support chamber as a function of the fluid 
injection rate, the wire feed rate and the wire withdrawal rate. The 
method further includes monitoring fluctuations in the position of the 
wire in the wire support path and varying the wire feed rate from the 
first wire handler to the wire slacking device in response to the position 
fluctuations to maintain the slack length substantially constant. 
The present invention will be further understood from the drawings and the 
following detailed description.

DESCRIPTION OF PREFERRED EMBODIMENTS 
Referring initially to FIG. 1, a wire conveyance system of the present 
invention is shown and generally designated 10. The wire conveyance system 
10 comprises, in series, a first wire handler 12, a wire slacking device 
14, and a second wire handler 16, which together define a wire conveyance 
path 18 for a bonding wire 20. The wire slacking device 14 comprises a 
primary housing 22 and a plurality of associated components including a 
pressurized fluid source 24, such as a high pressure vessel, and a fluid 
line 26 extending from the source 24 to the housing 22 which enables fluid 
communication therebetween. An external upstream sensor 28 is also 
included external to and upstream of the housing 22 beneath the wire 
conveyance path 18. An external downstream sensor 30 is included external 
to and downstream of the housing 22 above the wire conveyance path 18. The 
external upstream and downstream sensors 28, 30 are preferably each 
contact sensors which generate an electrical detection signal upon contact 
with the wire 20. The external upstream sensor 28 and external downstream 
sensor 30 are in electrical communication with a controller (not shown) of 
the first wire handler 12, such as a servo system. 
For purposes of defining "downstream" and "upstream", the terms are 
relative to the primary direction of travel of the bonding wire 20, which 
is deemed to originate at the first wire handler 12 and travel in the 
direction of the second wire handler 16. The first wire handler 12 is 
positioned in the wire conveyance path 18 upstream of the wire slacking 
device 14 and the second wire handler 16 is positioned in the wire 
conveyance path 18 downstream of the wire slacking device 14. As is 
appreciated by the skilled artisan, the wire conveyance system 10 is an 
integral part of a wire bonding machine, the remainder of which is not 
shown. The first and second wire handlers 12, 16 are substantially any 
known device which performs an operation on the bonding wire 20 as it is 
conveyed through the system 10. Thus, for example, the first wire handler 
12 can be a wire supply spool and spool drive motor, such as disclosed in 
U.S. Pat. No. 5,318,234, and the second wire handler can be a wire 
tensioning device located upstream of a bond head which is positioned at 
the termination of the wire conveyance path 18, such as disclosed in 
commonly assigned U.S. patent applications Ser. No. 09/113,722, entitled 
"Bond Head Having Dual Axes of Motion" and Ser. No. 09/113,666, entitled 
"Wire Tensioning Device for a Wire Bonding Machine", both filed 
concurrently with the present application and incorporated herein by 
reference. 
The first wire handler 12 is positioned on substantially the same 
horizontal plane as the housing 22 of the wire slacking device 14, while 
the second wire handler 16 is positioned below the housing 22 of the wire 
slacking device 14 at an angle .theta. approaching 90.degree. from the 
horizontal plane. Although the wire conveyance system 10 disclosed herein 
is not limited to the present configuration, the system 10 is typically 
configured such that the wire slacking device 14 redirects the wire 
conveyance path 18 at an acute angle or right angle from the horizontal 
plane of the first wire handler 12 as the wire conveyance path 18 proceeds 
from the wire slacking device 14 to the second wire handler 16. 
Referring additionally to FIGS. 2-4, the wire slacking device 14 of the 
system 10 is shown to comprise a wire support chamber 32 defined by the 
housing 22. The housing 22 is a rigid two-piece construction consisting of 
a body 34 and a cover 36. The wire support chamber 32 has three pairs of 
opposing sides, a top and a bottom 38, 40, a first lateral side and a 
second lateral side 42, 44, and a back and a front 46, 48. The top 38 is 
substantially open to the external environment while the bottom 40 is 
substantially enclosed by a base 50 of the body 34 which is joined to the 
lower edge of the cover 36. The first and second lateral sides 42, 44 are 
likewise substantially enclosed. The first lateral side 42 is enclosed by 
a first vertical sidewall 52 of the body 34, which is integrally 
constructed with the base 50, preferably from a durable metal or plastic. 
The second lateral side 44 is enclosed by a second vertical sidewall 54 
forming the cover 36, which is preferably fabricated from a transparent 
material such as a high-strength, clear, static-dissipating plastic 
enabling visual access to the wire support chamber 32. 
The first and second sidewalls 52, 54 have substantially matching profiles 
with the back border 56 of both sidewalls 52, 54 being substantially 
square at the top and bottom, and the front border 58 of both sidewalls 
52, 54 being square at the bottom, but cut away at an angle as the top is 
approached. The back border 56, front border 58 and top border 60 of both 
sidewalls 52, 54 are beveled to form a continuous chamfer on each sidewall 
52, 54. The first and second sidewalls 52, 54 extend vertically in 
opposing parallel relation to each other and are spaced a fixed horizontal 
distance apart, which corresponds to the width dimension of the wire 
support chamber 32. The preferred width dimension of the wire support 
chamber 32 is in a range of about 3 to 15 mil, whereas the bonding wire 
typically has a diameter in a range of about 0.5 to 10 mil. The first and 
second sidewalls 52, 54 also preferably have a length dimension which is 
substantially greater than their height dimension such that the wire 
support chamber 32 correspondingly has a substantially greater length 
dimension than height dimension. The preferred length dimension of the 
wire support chamber 32 is in a range of about 3 to 6 inches and the 
preferred height dimension is in a range of about 2 to 4 inches. 
The back 46 of the wire support chamber 32 is provided with a back opening 
62 which is continuous on its upper end with the open top 38 and is 
bounded by a back edge 64 on its lower end. The front 48 of the wire 
support chamber 32 is correspondingly provided with a front opening 66 
which is continuous on its upper end with the open top 38 and is bounded 
by a front edge 68 on its lower end. 
An internal central sensor 70 is mounted in the first sidewall 52 within 
the interior of the wire support chamber 32. The internal central sensor 
70 is positioned beneath the wire conveyance path 18 proximal to the base 
50 and approximately midway between the back 46 and front 48 of the wire 
support chamber 32. The internal central sensor 70 is preferably a 
proximity sensor which continuously monitors the proximity of the wire 20 
to the sensor 70. A pair of internal upstream and downstream sensors 72, 
74 are also mounted within the interior of the wire support chamber 32. 
The internal upstream and downstream sensors 72, 74 are each positioned 
beneath the wire conveyance path 18 upstream and downstream of the 
internal central sensor 70, respectively. The internal upstream and 
downstream sensors 72, 74 are positioned at the same height within the 
wire support chamber 32, which is preferably at about the same height or 
somewhat below the height of the internal central sensor 70. The internal 
upstream and downstream sensors 72, 74 are preferably each contact sensors 
which generate an electrical detection signal upon contact with the wire 
20. The internal upstream and downstream sensors 72, 74 are in electrical 
communication with the controller of the first wire handler 12. 
An upstream fluid orifice 76 is provided in the first sidewall 52 which 
opens into the wire support chamber 32 and extends completely through the 
thickness of the first sidewall 52. The upstream fluid orifice 76 
typically has a constant uniform diameter in a range of about 0.062 to 
0.125 inches. The upstream fluid orifice 76 is positioned at or near the 
base 50, below the wire conveyance path 18, and proximal to the back 46, 
upstream of the internal central sensor 70 and internal upstream sensor 
72. The central axis of the upstream fluid orifice 76 is aligned 
substantially perpendicular to the first and second sidewalls 52, 54. The 
upstream fluid orifice 76 is connected to the fluid line 26 to provide 
fluid communication between the pressurized fluid source 24 and the wire 
support chamber 32. A flow regulator (not shown), such as an adjustable 
valve, may be provided in the fluid line 26 at the upstream fluid orifice 
76 to set the flow rate of pressurized fluid entering the wire support 
chamber 32 through the upstream fluid orifice 76 at a predetermined 
constant flow rate. The pressurized fluid is preferably a 
contaminant-free, relatively inert gas, such as air or nitrogen, which is 
maintained in the pressurized fluid source 24 at a pressure significantly 
greater than the ambient external pressure. A typical pressurized fluid 
has a pressure in a range of about 30 to 80 psi. 
A downstream fluid orifice 78 having substantially the same dimensions as 
the upstream fluid orifice 76 is provided in the first sidewall 52 which 
also opens into the wire support chamber 32 and extends completely through 
the thickness of the first sidewall 52. The downstream fluid orifice 78 is 
positioned at or near the base 50 proximal to the front 48, downstream of 
the internal central sensor 70 and internal downstream sensor 72. The 
central axis of the downstream fluid orifice 78 is aligned substantially 
perpendicular to the first and second sidewalls 52, 54. The downstream 
fluid orifice 78 is connected to the fluid line 26, also providing fluid 
communication between the pressurized fluid source 24 and the wire support 
chamber 32. A separate flow regulator (not shown), such as an adjustable 
valve, may be provided in the fluid line 26 at the downstream fluid 
orifice 78 to set the flow rate of the pressurized fluid entering the wire 
support chamber 32 through the downstream fluid orifice 78 to a 
predetermined constant rate. 
Although the upstream and downstream fluid orifices 76, 78 have been shown 
in FIGS. 3 and 4 and described herein as being provided in the first 
sidewall 52, it is apparent to the skilled artisan that either the 
upstream fluid orifice 76 or the downstream fluid orifice 78 or both can 
alternatively be provided in the second sidewall 54 within the scope of 
the present invention. Similarly, it is within the scope of the present 
invention to alternatively mount the internal central sensor 70 in the 
second sidewall 54. FIG. 5 shows the downstream fluid orifice 78 and 
internal central sensor 70 alternatively positioned in the second sidewall 
54 while the upstream fluid orifice 76 remains positioned in the first 
sidewall 52. It is further noted that the present invention is not limited 
to the specific dual orifice configuration of the wire slacking device 14 
disclosed herein. Other fluid orifice configurations are possible within 
the scope of the present invention subject to the performance requirements 
recited herein. 
The base 50 is contoured to direct the pressurized fluid exiting the 
upstream and downstream fluid orifices 76, 78 into the upstream and 
downstream fluid paths 80, 82, respectively, within the wire support 
chamber 32. Specifically, the surface of the base 50 is substantially even 
with the bottom of the upstream fluid orifice 76. The base 50 has an 
upward incline 84 from the upstream fluid orifice 76 to the back edge 64 
on the upstream side of the orifice 76. The base 50 proceeds at a right 
angle from the upstream fluid orifice 76 on the downstream side of the 
orifice 76 to form a shelf 86 where the internal upstream sensor 72 is 
positioned substantially out of the upstream fluid path 80. Accordingly, 
the upstream fluid path 80 is relatively vertical, being defined as 
entering through the upstream fluid orifice 76, extending upward through 
the wire support chamber 32 between the first and second sidewalls 52, 54, 
and out the back opening 62 and open top 38 of the wire support chamber 
32. The path 80 is effectively bound on the back and front by the incline 
84 and shelf 86, respectively. 
The base 50 is similarly contoured with respect to the downstream fluid 
orifice 78. The surface of the base 50 is substantially even with the 
bottom of the downstream fluid orifice 78 and has an upward incline 88 
from the orifice 78 to the front edge 68 on the downstream side of the 
orifice 78. The base 50 proceeds at a right angle from the downstream 
fluid orifice 78 on the upstream side of the orifice 78 to form a shelf 90 
where the internal downstream sensor 74 is positioned substantially out of 
the downstream fluid path 82. Accordingly, the downstream fluid path 82 is 
relatively vertical, being defined as entering through the downstream 
fluid orifice 78, extending upward through the wire support chamber 32 
between the first and second sidewalls 52, 54, and out the front opening 
66 and open top 38 of the wire support chamber 32. The path 82 is 
effectively bound on the front and back by the incline 88 and shelf 90, 
respectively. 
It is noted that the upstream and downstream fluid paths 80, 82 are the 
enabling means for the wire support path 92, as will be described below in 
the method of operation. The wire support path 92 is the segment of the 
wire conveyance path 18 internal to the wire support chamber 32 which is 
essentially contactless with respect to the wire slacking device 14, i.e., 
the wire support path 92 does not intersect any structural components of 
the wire slacking device 14. Thus, the wire 20 being conveyed along the 
wire support path 92 preferably does not come into contact with any 
structural components of the wire slacking device 14 during normal 
operation. The wire support path 92 is substantially horizontal, being 
relatively orthogonal to the upstream and downstream fluid paths 80, 82 
and extending into the wire support chamber 32 through the back opening 62 
and out of the wire support chamber 32 through the front opening 66. The 
wire support path 92 is maintained substantially above the external 
upstream sensor 28, internal central sensor 70, internal upstream sensor 
72 and internal downstream sensor 74 and the upstream and downstream fluid 
orifices 76, 78, while being maintained substantially below the open top 
38 and external downstream sensor 30. 
METHOD OF OPERATION 
The method of operating the wire conveyance system 10 including the wire 
slacking device 14 is described below with reference to the figures. 
Operation is initiated by loading the first wire handler 12, wire slacking 
device 14 and second wire handler 16 with the bonding wire 20. Loading of 
the first and second wire handlers 12, 16 is performed in accordance with 
standard methods known in the art. The wire slacking device 14 is loaded 
by manually feeding the wire 20 into the wire support chamber 32. 
Specifically, the operator draws the wire 20 over the open top 38 and 
allows the wire 20 to fall under the force of gravity into the wire 
support chamber 32 using the beveled borders 56, 58 60 as a wire guide. 
Flow of pressurized fluid from the pressurized fluid source 24 into the 
wire support chamber 32 is then initiated. Substantially laminar flow is 
achieved in both the upstream and downstream fluid paths 80, 82 because 
the pressurized fluid is confined by the relatively small width dimension 
of the wire support chamber 32. Laminar flow within the wire support 
chamber 32 is indicated by the parallel directional flow arrows of FIGS. 3 
and 4. FIGS. 3 and 4 further show that laminar flow transitions to 
turbulent flow in the region above the wire support chamber 32 when the 
pressurized fluid exits the wire support chamber 32 as indicated by the 
circuitous directional arrows. 
The flow of pressurized fluid through the upstream and downstream fluid 
paths 80, 82 exerts an upward force against the wire 20, which drives the 
wire upward into the wire support path 92. The fluid flow rate is set by 
means of the flow regulators to balance the upward fluid force against the 
wire 20 with the downward gravitational force against the wire 20, thereby 
maintaining the wire 20 at equilibrium in the wire support path 92. The 
flow regulators may be controlled manually by the operator or 
alternatively an automated control system may be employed. 
The sensors 28, 30, 70, 72, 74 are provided to ensure the performance of 
the first and second wire handlers 12,16 and the wire slacking device 14. 
In general, the sensors function by monitoring the position of the wire 20 
and automatically transmitting electrical position notification signals to 
the controller of a given device. The controller automatically 
communicates an appropriate instruction to the device in response to the 
position notification signal. In accordance with one embodiment of the 
invention, the sensors ensure that the output requirements of the first 
wire handler 12 or the demand requirements of the second wire handler 16 
are satisfied by controlling the rate at which the wire 20 is fed into the 
wire slacking device 14 from the first wire handler 12. If the wire 20 
drops down from the wire support path 92 to a predetermined minimum 
threshold distance from the internal central sensor 70 due to an 
insufficient wire feed rate from the first wire handler 12, the sensor 70 
outputs an electrical position notification signal to the controller of 
the first wire handler 12, which instructs the first wire handler 12 to 
increase the wire feed rate to the wire slacking device 14. If the first 
and second wire handlers 12,16 are operating in opposition to one another, 
the wire 20 is drawn taut in contact across the internal upstream sensor 
72 or the internal downstream sensor 74 and an electrical position 
notification signal is transmitted to the controller of the first wire 
handler 12 upon contact with the wire 20. The controller in turn provides 
an appropriate instruction to the first wire handler 12. 
In accordance with another embodiment of the invention, the sensors ensure 
that the first and second wire handlers 12,16 and the wire slacking device 
14 properly maintain the wire 20 in the wire conveyance path 18. If the 
pressurized fluid force is insufficient to maintain the wire 20 elevated 
in the wire support path 92, the wire 20 drops from the wire support path 
and contacts the internal upstream sensor 72 or the internal downstream 
sensor 74. The sensor 72 or 74 automatically transmits an electrical 
position notification signal to the controller of the first wire handler 
12 which instructs the first wire handler 12 to interrupt or decrease the 
wire feed rate to the wire slacking device 14. If the second wire handler 
16 releases the wire 20 or the first wire handler feeds an excess of the 
wire 20 to the first wire handler 12, the wire 20 is driven upward out of 
the wire support path 92 and wire support chamber 32 into turbulent flow 
and contact with the external downstream sensor 30. The sensor 30 
transmits an electrical position notification signal to the controller of 
the bond head (not shown) to interrupt operation in response to contact 
with the wire 20. 
It is apparent that the skilled artisan can employ the wire slacking device 
14 to control the degree of slack in the wire 20 by controlling the wire 
feed rate from the first wire handler 12 in the above-described manner. It 
is generally desirable to maintain a substantially constant slack length, 
termed a service loop 94, in the bonding wire 20 downstream of the wire 
slacking device 14 to minimize damage to the bonding wire 20 in the wire 
conveyance system 10. The service loop 94 preferably has a sufficient 
length to satisfy the requirements of the second wire handler 16. The 
length of the service loop 94 also determines the degree of counter force 
that the bonding wire 20 exerts on the second wire handler 16 in 
opposition to the force exerted on the bonding wire 20 by the second wire 
handler 16. If the service loop 94 is maintained at a relatively long 
length, the counter force advantageously approaches a constant value which 
is relatively insensitive to small variations in length caused by 
fluctuations in the output demand of the first wire handler 12 or the 
input demand of the second wire handler 16. 
The internal central sensor 70 specifically enables cooperative operation 
of the first and second wire handlers 12, 16 to maintain the desired 
length of the service loop 94. For example, if the second wire handler 16 
is drawing the wire 20 from the wire slacking device 14 at a greater rate 
than the first wire handler 12 is feeding the wire 20 to the wire slacking 
device 14, the length of the wire 20 in the service loop 94 shortens. 
Consequently, the wire 20 drops below the predetermined minimum threshold 
distance of the internal central sensor 70. The sensor 70 transmits an 
electrical position notification signal to the controller of the first 
wire handler 12 in response to the depressed position of the wire 20 and 
the first wire handler 12 is instructed to increase the wire feed rate to 
the wire slacking device 14. If the wire support chamber 32 is too small 
to accommodate the increased wire supply, the level of the wire 20 drops 
upstream of the housing 22 and contacts the external upstream sensor 28. 
The sensor 28 transmits an electrical position notification signal to the 
controller of the first wire handler 12 upon contact with the wire 20 and 
the first wire handler 12 is instructed to interrupt or decrease the wire 
feed rate to the wire slacking device 14. 
While the forgoing preferred embodiments of the invention have been 
described and shown, it is understood that alternatives and modifications, 
such as those suggested and others, may be made thereto and fall within 
the scope of the invention.