Bonding head for an ultrasonic bonding machine

A bonding head for an ultrasonic bonding machine comprises a wedge operatively associated with an ultrasound transducer by means of which an electrically conducting wire can be pressed against a contact surface of an electrical or electronic component and bonded thereto by ultrasonic excitation of the wedge. A wire clamp is located adjacent to and ahead of the wedge and can be moved back and forth by drive means in the direction of the wire being fed to the wedge. The wedge can be moved up and down together with the wire clamp and its drive means in a direction substnatially perpendicular to the contact surface. The bonding head as a whole is also disposed so that it can be swung about an axis perpendicular to the contact surface. The drive means for wire clamp comprises a translational drive which can move the wire clamp exactly parallel to the direction of the wire being fed to the wedge.

FIELD OF THE INVENTION 
The present invention relates to a bonding head of the type used in an 
ultrasonic wedge bonding machine. 
DESCRIPTION OF THE PRIOR ART 
In a wedge bonding machine for ultrasonic bonding, a wedge operatively 
associated with an ultrasound transducer is used to press an electrically 
conducting wire, typically aluminum wire, against a contact surface of an 
electrical or electronic component and bond it thereto under ultrasonic 
excitation. A wire clamp is positioned ahead of the wedge and the wedge 
together with the wire clamp and its driving mechanisms can be moved up 
and down approximately perpendicular to the contact surface. The bonding 
head as a whole is also so disposed as to be rotatable about an axis 
perpendicular to the contact surface. 
A bonding head of this type is described in U.S. Pat. No. 4,202,482. The 
bonding head described herein is distinguished by the fact that the wire 
clamp positioned ahead of the wedge is so disposed as to be rotatable 
about a horizontal axis, with the consequence that as the wire is 
advanced, both bending torques and transverse forces are exerted on the 
wire. That is, the part of the wire gripped by the wire clamp must follow 
the circular movement made by the wire clamp or the jaws of the clamp. 
This results in abrasion of the wire in the wedge bore through which the 
wire is guided to the contact surface. After a relatively brief period of 
operation, the abrasion of the wire within the wedge bore causes increased 
frictional resistance, with the consequence that it is no longer possible 
to form a good wire loop with the apparatus. 
SUMMARY OF THE INVENTION 
The object of the present invention is to overcome or substantially 
mitigate the problem of increased frictional resistance in the wedge bore. 
According to a first aspect of the present invention there is provided a 
bonding head for an ultrasonic bonding machine comprising a wedge for 
pressing and bonding wire to a contact surface by ultrasonic excitation, a 
transducer for ultrasonic excitation of the wedge, a wire clamp located 
adjacent the wedge for releasably clamping wire being fed to the wedge, 
first means for moving the wedge and the wire clamp up and down in a 
direction substantially perpendicular to the contact surface, second means 
for rotating the bonding head as a whole about an axis perpendicular to 
the contact surface, and a first translational drive means for moving the 
wire clamp back and forth in a direction parallel to the direction of the 
wire being fed to the wedge. 
Thus, the translational drive means moves the wire clamp exactly parallel 
to the bonding wire so that in this invention the wire no longer makes any 
circular movement. The wire is introduced into the wedge bore exactly 
parallel to the longitudinal direction of this bore. Wire abrasion is 
thereby kept to a minimum, with the consequence that even after prolonged 
operation there is no increase in the friction resistance between the 
bonding wire and wedge bore, and wire loops can continue to be produced 
with no deterioration in their quality. Furthermore, a constant tail 
length is also ensured over a long period of operation of the bonding 
head. 
Preferably, the bonding head comprises a flexible tube within which the 
wire can be located and thereby guided to the wire clamp. 
This feature further protects the wire against wear and an additional 
benefit of this arrangement is that the wire is not deflected either 
upward or downward before being gripped by the wire clamp and hence does 
not, as a result of such deflection, adopt a position wherein it cannot be 
properly gripped, or gripped at all, by the wire clamp. 
Preferably also, the bonding head comprises a third means for rotating the 
transducer and the wedge about an axis which is parallel to the contact 
surface and perpendicular to the longitudinal axes respectively of the 
transducer and the wedge, said third means comprising a means for 
generating a magnetic field giving rise to a force in a first direction 
which acts against an opposing force generated by a spring bias so that 
said third means rotates the transducer and the wedge in such a way that 
the wedge can be lifted up from the contact surface against the force of 
the spring bias and thereby the contact pressure of the wedge on the 
contact surface is adjustable by relative variation of said forces 
generated by said magnetic field and said opposing spring bias. 
The contact pressure or so-called "bonding weight" can thus be set and 
adjusted by simple constructional means. Bonding weight is understood to 
designate the contact pressure of the wedge i.e. the force with which the 
wedge presses the bonding wire against the contact surface of the 
electrical or electronic component. 
According to a second aspect of the present invention there is provided a 
bonding head for an ultrasonic bonding machine comprising a wedge for 
pressing anti bonding wire to a contact surface by ultrasonic excitation, 
a transducer for ultrasonic excitation of the wedge, a wire clamp located 
adjacent the wedge for releasably clamping wire being fed to the wedge, 
first means for moving the wedge and the wire clamp up and down in a 
direction substantially perpendicular to the contact surface, second means 
for rotating the bonding head as a whole about an axis perpendicular to 
the contact surface, and a flexible tube within which the wire can be 
located and thereby guided to the wire clamp. 
According to a third aspect of the present invention there is provided a 
bonding head for an ultrasonic bonding machine comprising a wedge for 
pressing and bonding wire to a contact surface by ultrasonic excitation, a 
transducer for ultrasonic excitation of the wedge, a wire clamp located 
adjacent the wedge for releasably clamping wire being fed to the wedge, 
first means for moving the wedge and the wire clamp up and down in a 
direction substantially perpendicular to the contact surface, second means 
for rotating the bonding head as a whole about an axis perpendicular to 
the contact surface, and a third means for rotating the transducer and the 
wedge about an axis which is parallel to the contact surface and 
perpendicular to the longitudinal axes respectively of the transducer and 
the wedge, said third means comprising a means for generating a magnetic 
field giving rise to a force in a first direction which acts against an 
opposing force generated by a spring bias so that said third means rotates 
the transducer and the wedge in such a way that the wedge can be lifted up 
from the contact surface against the force of the spring bias and thereby 
the contact pressure of the wedge on the contact surface is adjustable by 
relative variation of said forces generated by said magnetic field and 
said opposing spring bias. 
An example of a bonding head according to the various aspects of the 
present invention will now be described with reference to the accompanying 
drawings.

DESCRIPTION OF A PREFERRED EMBODIMENT 
FIGS. 1 and 2 show a bonding head 26 for an ultrasonic wedge bonding 
machine with a wedge 11 operatively associated with an ultrasound 
transducer 10, by means of which an electrically conducting wire 12, in 
particular aluminum wire, can be pressed against a contact surface 25 of 
an electrical or electronic component (not further illustrated) and bonded 
thereto by ultrasonic excitation. The bonding head 26 also comprises a 
wire clamp 13 with clamp jaws 29, 30 that is positioned adjacent and ahead 
of the wedge 11 and can be moved back and forth in the direction of the 
wire 12 as is indicated by the double-headed arrow 27 in FIG. 1. In 
addition, the bonding head 26 as a whole is so disposed that it can be 
swung about an axis 17 perpendicular to the contact surface 25. This 
rotational movement is indicated by the double-headed arrow 28 in FIG. 1. 
Furthermore, the wedge 11 can be moved up and down in a direction 
substantially perpendicular to the contact surface 25. For this purpose, 
the wedge 11 and the transducer 10 are so disposed that they can be swung 
about an axis 22 that extends parallel to the contact surface 25 and 
perpendicular to the longitudinal axes of the wedge 11 and the transducer 
10. This rotational movement is indicated by the double-headed arrow 31 in 
FIG. 1. 
A driving mechanism 14 for the back-and-forth movement 27 of the wire clamp 
13 in the direction of the wire 12, i.e. parallel to the wire 12, is a 
translational drive 16 in the form of a solenoid, which can move the wire 
clamp 13 exactly in parallel to the wire 12. The solenoid 16 thereby moves 
the wire clamp 13 in the wire-feed direction Z, against the action of a 
spring bias in the form of a retracting spring 18 (see FIG. 2). Arranged 
in parallel to the translational drive 16 for the back-and-forth movement 
of the wire clamp 13 is a separate translational drive 19, also connected 
to the wire clamp 13, the maximally advanced position of which defines the 
position of the wire clamp 13 at which the wire breaks off. The 
translational drive 19 is also comprises a solenoid, the plunger of which, 
like the plunger of the translational drive 16 for the back-and-forth 
movement of the wire clamp 13, is connected to the latter, by way of a 
common connecting yoke 20. When the wire clamp 13 is moved in the 
wire-feed direction Z by the solenoid 16 specified for that purpose, the 
plunger of the solenoid 19 that defines the wire-breaking position of the 
wire clamp 13 is moved along with it, beyond the maximally advanced 
position of solenoid 19 that defines the position for breaking of the 
wire. To the clamp jaws 29, 30 of the wire clamp 13 is attached yet 
another separate drive mechanism 15, preferably also in the form of a 
solenoid (see FIG. 2). 
In use, activation of the various drives and the movements of the wire 
clamp 13 thereby brought about are as follows: 
1. Activation of the solenoid 15 with the consequence that the clamp jaws 
29, 30 are moved into a clamping position and grip the bonding wire 12. 
2. Activation of the solenoids 16 and 19 with the consequence that the wire 
clamp 13 is moved in the wire-feed direction Z, carrying the bonding wire 
12 with it, until a so-called feed position has been reached. 
3. Lowering of and ultrasonic excitation of the wedge 11, during which the 
wire 12 is pressed against the contact surface 25 and bonded to the 
latter. 
4. Deactivation of the solenoid 15 and hence opening of the clamp jaws 29, 
30. 
5. Loop formation of the wire 12 in a conventional manner, by relative 
movement between the wedge 11 and the contact surface 25 of the electronic 
component in such a way that a new neighboring contact surface 25 is 
brought into association with the wedge 11, the solenoid 16 being 
deactivated during the loop formation with the consequence that the wire 
clamp 13 travels back into a position determined by the maximally advanced 
position of the solenoid 19. 
6. Lowering of the wedge 11 onto the new contact surface 25, during which 
the wire 12 is pressed against and bonded to the latter, the solenoid 15 
being activated during the lowering of the wedge 11 so that the wire 12 is 
clamped firmly between the clamp jaws 29, 30. 
7. Deactivation of the solenoid 19 with the consequence that under the 
action of the retracting spring 18 the wire clamp 13 is jerked still 
further back into a null or starting position and the wire 12 
simultaneously breaks directly at the bonding site, the thereby ensuing 
retraction of the wire 12 being adjusted so that the free end of the wire 
is still situated within the wedge bore after breaking of the wire has 
occurred. This retraction distance is also called the break-off length. 
The break-off length must thus be smaller than the length of the wedge 
bore. 
8. Deactivation of the solenoid 15 with release of the wire 12 by the clamp 
jaws 29, 30 of the wire clamp 13. The wire clamp 13 is again in the null 
or starting position. A new advancement of the wire with preset tail 
length can now occur. The translational movement of the drives 16 and 19, 
and hence the so-called tail length and break-off length, respectively, 
can be individually adjusted by corresponding set screws 32, 33, as shown 
in FIG. 1. 
As FIG. 1 further shows, the solenoid 19 that defines the wire-breaking 
position of the wire clamp 13 is offset in the wire-feed direction Z from 
the solenoid 16 that is responsible for feeding the wire 12. 
The wire 12 is guided within a flexible tube up to a position wherein it is 
gripped by the wire clamp 13. This arrangement protects the wire 12 from 
damage. In addition, it prevents the wire 12 from moving out of the region 
of the clamp jaws 29, 30 of the wire clamp 13 which would prevent the jaws 
29,30 from gripping the wire 12. 
The rotational movement of the wedge 11 at the transducer 10 and of the 
wire clamp 13 about the axis 22 is determined by the tension exerted by a 
draw spring 23 on one hand and tile magnetic field generated by a solenoid 
24 on the other hand. The spring 23 and the solenoid 24 act between a 
swing component 34 and a non-rotatable component 35 of the bonding head 
26. The spring 23 and the magnetic field generated by the solenoid 24 
determine the contact pressure of the wedge 11, i.e. the so-called bonding 
weight as described previously.