Method and apparatus for forming a ball at the end of a wire

A ball is formed at the tip of a wire by an arc. The arc is initiated and maintained across an air gap between the wire tip and an electrode by applying timed RF and DC voltages between them.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates generally to the field of joining the tip of 
a wire to another surface and, more particularly, to forming a ball at the 
tip of such a wire preparatory to such a joinder. 
2. Description of the Prior Art 
In the fabrication of electronic circuits incorporating integrated circuit 
chips, many secure electrical connections must be made to individual 
conducting pads on the integrated circuit so as to provide the means to 
interconnect its myriad circuits with other components. Ball bonding is a 
technique which has gained widespread acceptance in the electronics 
industry for effecting such interconnections. U.S. Pat. No. 3,767,101, 
issued to W. D. Genrich, and assigned to the present assignee, describes 
this technique and a bonder suitable to perform it. As described in that 
patent, a wire is fed through the tip of a capillary until it protrudes 
slightly beyond its end. A flame applied to the wire near the capillary 
tip terminates the wire at that point and causes it to form a small ball. 
The capillary is then moved against the conducting pad to which the wire 
is to be bonded until the ball-tipped wire makes contact therewith. By the 
combined application of pressure, heat and vibration, a bond is created 
between the conducting pad and the ball at the tip of the wire. The 
capillary is then withdrawn, allowing wire to be pulled from it to form a 
lead tailing from the bonded tip of the wire. The wire may then be 
terminated to form a free lead, or may be bonded to a second point on the 
circuit in a manner which is not relevant here. 
Gold is the metal of choice in such a bonding arrangement because it does 
not oxidize when the ball is formed on the wire being heated. Considerable 
research has been directed to discover ways in which non-precious metal 
wires can be successfully substituted for gold. Some of these efforts are 
described in "Ball Bonding Of Nonprecious Metal Wires" in the August 1982 
issue of Semiconductor International at pages 65 et seq. According to the 
article, it is impossible to form a ball at the end of an aluminum alloy 
wire by flaming off in air. Not only is ball formation difficult under 
those circumstances, but a subsequent bonding operation is made impossible 
according to the article by the formation of thick oxide layers on the 
surface of the ball. The article despribes as a possible solution the 
performance of the flaming off operation by performing the ball formation 
in an inert gas, such as argon. 
Another approach to forming a ball at the tip of a wire preparatory to ball 
bonding appears in British Pat. Nos. 1,536,872 and 1,600,021. Both of 
these patents address the goal of forming a ball at the tip of an aluminum 
wire and propose to do so by. means of a spark discharge. In Pat. No. 
1,536,872 the spark discharge is initiated by applying a voltage of about 
200 volts across a gap between the wire end and an electrode, and bringing 
the tip of the wire and the electrode into temporary contact, fusing the 
wire's tip and forming the spark discharge while feeding a shielding gas 
to the region of the spark discharge. A similar technique for forming a 
ball at the tip of an aluminum wire is described in Pat. No. 1,600,021, 
with the spark discharge, however, being initiated across the gap in a 
shielding atmosphere. This apparently was thought to be made possible by 
the use of a voltage between 350 volts and 10,000 volts. 
From the above it is apparent that previous approaches to the formation of 
a ball at the tip of an aluminum or other base metal wire were limited to 
the use of an inert atmosphere in order to prevent the formation of an 
oxide layer on the ball which would prevent subsequent bonding, and that 
the use of such an inert gas atmosphere was deemed necessary both where 
the ball was formed by flaming off and by spark discharge. To the best of 
our knowledge this approach has not produced balls of consistent shape and 
size, sufficiently free of oxides to allow reproducible bonding results, 
because the turbulence created by the moving bonder mechanism does not 
allow an inert gas envelope of consistent density to be maintained in the 
immediate area of the flame-off or spark discharge. 
It is a principal object of the present invention to form a substantially 
oxide-free ball at the tip of a wire by a method which results in 
consistent ball size, which is equally effective whether the wire be 
precious metal or base metal, and which does not necessitate the use of an 
inert gas atmosphere. 
A related object of the invention is to provide apparatus capable of 
developing and applying the necessary potentials and currents to form, in 
air, a ball of consistent and controllable size at the end of a wire even 
though the wire is of a base metal. 
SUMMARY OF THE INVENTION 
In keeping with the above object of the present invention, a ball is formed 
at the tip of a wire preparatory to its being joined to another surface by 
first initiating an electric arc to the wire's tip by subjecting it to an 
RF electric field after a DC voltage is applied and maintaining that arc 
after the DC is removed by subjecting the wire tip to an RF electric 
field. Advantageously, the wire tip is subjected to the DC electric field 
prior to, during and after the initiation of the electric arc by the RF 
electric field. Establishment of the DC electric field prior to initiation 
of the electric arc serves to establish conditions which are conducive to 
initiation of the arc by the RF field. Maintaining the DC electric field 
after initiation of the electric arc serves to create a ball of the 
desired size. 
The present invention also contemplates the provision of apparatus for 
implementing the above-described method by the provision of means whereby 
the wire may be supported adjacent to the surface to which it is to be 
joined, an electrode supported next to the tip of the wire and means for 
generating and applying across the gap between the wire tip and the 
electrode an RF voltage sufficient to initiate and maintain an arc across 
the gap and the DC voltage necessary to form the ball at the end of the 
wire. In keeping with this aspect of the invention, a tuned circuit is 
provided to apply the outputs of both the RF and the DC voltage sources 
across the gap between the wire and the electrode, with the tuned circuit 
being resonant at the frequency of the RF voltage so as to step up the RF 
voltage from a relatively low level at which it is generated to a 
significantly higher level at which it is applied across the gap. The RF 
voltage also serves to maintain a plasma across the gap after termination 
of the DC voltage, a phenomenon which retards oxide formation on the ball. 
With the use of the above-described method, particularly when implemented 
by the above-described apparatus, it has been found possible to 
consistently form balls of uniform size at the tip of a wire in air. 
Metals which have been successfully used include copper, nickel, aluminum 
and brass. 
Other objects and features of the invention will become apparent from the 
following detailed description with reference to the following drawings.

DETAILED DESCRIPTION OF THE INVENTION 
An apparatus suitable for implementing our improved method is disclosed in 
FIG. 1. It includes a conventional capillary 11 having an orifice 12 in 
its tip through which a wire 13 is fed. By means to be described, a ball 
15 is formed in accordance with the invention at the tip of the wire 13. 
This ball, which in the past was formed by flaming off or by spark 
discharge, is normally pressed by means of the capillary 11 against a 
conductive metal pad 21 which is one of many such pads on an integrated 
circuit 19 supported on a positioning table 17. Successive bonds may be 
made by moving a succession of the conductive pads 21 by translation of 
the table 17 coordinated with the lowering of the capillary 11 in a manner 
well known in the art. Spaced from the tip of the capillary 11 is an 
electrode 22 which is positioned (by conventional means not shown) between 
the capillary 11 and the conductive metal pad 21 during formation of the 
ball 15 and is swung out of position to allow the capillary 11 to be 
lowered so as to press the ball 15 against the conductive metal pad 21 
during the actual bonding step. 
The first step in the process of forming the ball 15 is to apply a DC 
voltage across the gap between the ball 15 and an electrode 22 so as to 
enhance conditions for the initiation of the arc across the gap which is 
to follow shortly. Toward this end, a control pulse 25 is generated by a 
control pulse source 23 and applied to a negative DC pulse generator 27 
whose output 29 is applied to the input node 38 of a tuned circuit 31. The 
tuned circuit 31 comprises a pair of branches 33 and 35 wherein the branch 
33 includes an inductor 37 connected in series with a capacitor 39, and 
the branch 35 comprises a capacitor 41 connected in series with a 
capacitor 43. One end of the tuned circuit 31 is connected to ground, as 
is the wire 13. The other end of the tuned circuit 31, output node 46, is 
connected to the electrode 22 through a DC current limiting resistor 44. 
Thus, the DC pulse 29 appears across the gap G during the time period T1 
as indicated in FIG. 2. 
During the period T1 the spark across the gap G is initiated by applying an 
RF pulse 59 across the gap G. The RF pulse 59 is derived in the exemplary 
embodiment illustrated in FIG. 1 from an RF source 47 whose output 49 is 
applied through a gate 51 to an RF power driver 52 under the control of a 
gating pulse 53 produced by the control pulse source 23. The resulting RF 
pulse 59 at the output of the RF power driver 52 is applied to the RF 
input node 45 of the tuned circuit 31. The tuned circuit 31 is designed to 
be resonant to the frequency of the RF pulse 59 so as to optimize the 
output which appears between ground and the RF output node 46. In the 
disclosed arrangement, the tuned circuit 31 serves to elevate the voltage 
of the RF pulse 59 which is applied at the RF input node 45 to a much 
higher level at the RF output node 46. This is accomplished by selecting 
the value of the capacitors 41 and 43 so as to make the reactive impedance 
of the capacitor 41 a substantial multiple of the reactive impedance of 
the capacitor 43. Since reactive impedance across a capacitor is inversely 
proportional to its size, the above selection requires that the 
capacitance of the capacitor 43 be a multiple of the capacitance of the 
capacitor 41. For reasons which will be explained shortly, the reactive 
impedance of the inductor 37 greatly exceeds that of the capacitor 39, so 
that the effect of the latter upon the tuned circuit's voltage 
multiplication function is negligible. 
In forming a ball at the tip of the wire there will be a number of 
variables whose optimum value depends on various circumstances. Therefore, 
it is not possible to give a particular set of values which will yield the 
best results in all cases. Instead these variables will be noted, their 
relationship will be explained, and the manner in which their values 
should be selected will be set forth. In implementing the invention, 
energy is delivered in the gap G so as to melt the tip of the wire 13 and 
form a ball 15 thereon. The amount of energy that needs to be delivered 
will be a function of the material and diameter of the wire 13, as well as 
the size of the ball 15 to be formed thereon. The energy which is 
delivered, in turn, will be a function of the duration and magnitude of 
the current pulse which is delivered in the gap G. Results to date 
indicate a preferable gap size of 30 mils, with a useful range of between 
20 and 40 mils. Wires which have been used have ranged between 0.0007 and 
0.002 inches in diameter. 
Thus, the magnitude and duration T1 of the DC pulse 29 are principally a 
function of the amount of energy that is to be delivered in the gap G. The 
total time period T1 is set by the negative DC pulse generator 27 which is 
operative to produce the DC pulse 29 for the time period T1 in response to 
a triggering pulse from the control pulse source 23. Time period T3 
represents the delay between the leading edges of the timing pulse 25 and 
of the RF pulse 59, set by means of the control pulse source 23. The RF 
pulse 59 corresponds in duration with the gating pulse 53, and the DC 
pulse 29 corresponds in timing with the duration of the control pulse 25. 
The time period T3 is selected to provide enough time for the DC voltage 
29 applied to DC input node 38 to rise substantially to its full value by 
the time the RF pulse 59 begins. The time period T2, during which the RF 
pulse 59 is maintained, is similarly designed to be long enough to insure 
that the RF pulse 59 will rise to the level at which it is sufficient to 
initiate an arc in the gap G before the end of the time period T2 and long 
enough to overlap the end of the DC voltage. In this way the circuit 
designer can assure that, by the end of the time period T2 the RF voltage 
59 will have risen beyond the minimum voltage value required to initiate 
an arc in the gap G and provide a covering plasma after the DC voltage is 
removed. The maximum desirable time for T2 is determined by the need to 
avoid excessive annealing of the wire 13 where it necks into the ball 15, 
a condition which could weaken the wire sufficiently to reduce its 
strength below that of the bond, and which might occur if the RF pulse 59 
were of excessive length. Finally, the time period T2 is selected so that 
it is sufficiently long to provide a low energy plasma arc to prevent 
excessive oxide formation on the ball. For a wire of a given material and 
diameter, ball size can be adjusted by varying either the amplitude 
(controllable by selecting the proper value for resistor 44) or the 
duration of the DC pulse, or both. 
It should be understood that the waveforms illustrated in FIG. 2 are 
idealized and that they do not rise or fall instantaneously as 
illustrated. Thus, both the DC voltage 29 and the RF voltage 59 rise 
gradually to their ultimate levels, rather than instantaneously as shown. 
This is why sufficient time must be allowed for them to reach these 
levels. It should also be understood that the formation of the ball 15 
begins as soon as the arc is initiated, which occurs sometime during the 
period T2, the precise time depending upon the instant when the RF voltage 
has reached the level at which the arc is initiated. Once the arc is 
formed, the RF voltage will level off at the value where the arc was 
initiated and will stay at that value until the end of the time period T2. 
The circuit values which determine the magnitude and duration of the RF 
pulse 59 may be derived through a series of design steps. It should be 
understood that these design steps are relevant only where the particular 
apparatus illustrated in FIG. 1 is utilized. Needless to say, other 
circuits may be designed to create the sequence of voltages illustrated in 
FIG. 2 which constitute the essence of the method aspect of the present 
invention. Assuming, however, that the preferred circuit of FIG. 1 is 
used, an initial step in selecting these values is to determine the value 
of the stray capacitance C.sub.S introduced by the lead wire which 
connects the tuned circuit 31 to the electrode 22. Once the value of 
C.sub.S is determined, the capacitance C1 of the capacitor 41 will be 
selected to be sufficiently large relative to the stray capacitance value 
C.sub.S to render the effect of the latter minimal in the operation of the 
circuit. Next, the capacitance C2 of the capacitor 43 is selected so as to 
be a multiple of the capacitance C1 so as to cause the voltage appearing 
between the output node 46 and ground to be a multiple of the voltage 
applied between ground and the RF input node 45 to the tuned circuit 31. 
Since it is desired to have the inductor 37 appear to be connected in 
parallel across the capacitors 41 and 43 at the frequency of the RF pulse 
59, the capacitance C3 of the capacitor 39 which is in series with the 
coil 37 is selected so as to make the reactive impedance of that capacitor 
a small fraction of the reactive impedance of the inductor. Completing the 
design of the tuned circuit 31, the inductance L of the inductor 37 is 
selected so that the resonant frequency of the circuit determined by the 
capacitance C1 and the inductance L will be in the vicinity of the 
frequency at which the RF source 47 is to operate. In selecting the 
inductor 37, its Q.sub.L also needs to be considered. The Q.sub.L of the 
inductor is a circuit characteristic of that component, and if it is 
sufficiently large will determine the Q.sub.in of the tuned circuit 31. 
The desired Q.sub.in of the tuned circuit 31, in turn, will be a function 
of the minimum input impedance Z between ground and the input node 45, 
expressed by Z=Q.sub.in .times.X.sub.C2. 
The minimum desirable Q.sub.in at 1.5 MHz was found to be on the order of 
10, a value which is readily attainable, since inductors with a Q of about 
100 are commercially available. 
In implementing the system of FIG. 1 it is desirable that the circuit 31 be 
tuned to resonate at the frequency of the RF pulse 59. This could be 
accomplished by making the capacitor 41 or the inductor L a variable 
component so as to adjust the resonant frequency of the circuit 31 to that 
of the RF pulse 51. It is preferable, however, to keep the components 41 
and 37 fixed and to make the frequency of the RF pulse variable instead. 
This is simply because it is much easier to make the RF source 47 variable 
in frequency than it is to make the capacitor 41 or the inductor 37 
variable in size. Toward this end the RF source 47 in the preferred 
embodiment was formed of a fixed frequency oscillator operating at a 
frequency of 60 MHz working into a programmable frequency divider 
constructed in accordance with Motorola Application Note AN-584, entitled 
"Programmable Counters Using the MC 10136 and MC 10137 MECL 10,000 
Universal Counters". FIG. 9 on page 5 of that Application Note shows the 
circuit suitable for creating the programmable frequency divider. A 
12-position switch was used to configure the frequency divider so as to 
give a choice of any one of twelve frequencies ranging from 1.395 to 1.875 
MHz. That is, the frequency division factor of the programmable frequency 
divider was changed by means of the 12-position switch so as to convert 
the 60 MHz input frequenc.y derived from the fixed frequency oscillator 47 
to any one of twelve values within the range of 1.395 to 1.875 MHz. With 
such a variable RF frequency range, the tuned circuit 31 can be designed 
to be resonant at a point within a relatively wide range so that minor 
variations in the resonant frequency of the circuit 31 due to a change in 
the stray capacitance C.sub.S can be readily accommodated by use of the 
frequency selector switch. 
An optional feature of the apparatus illustrated in FIG. 1 is a circuit 61 
for diverting an arc which might be struck between the electrode 22 and 
either the wire 13 or the capillary 11 in the event that, through machine 
malfunction, the wire 13 failed to protrude beyond the tip of the 
capillary 11 preparatory to application of the arc-inducing potentials 
between the electrode 22 and the wire 13. The circuit which is to be 
described presently is not the invention of the present inventors, but 
rather is the invention of Roger F. Cressey, by whom a patent application 
will be filed thereon and will be assigned to the present assignee. The 
invention is described herein because it is believed to enhance the 
operation of the apparatus which is the subject of the present 
application. The circuit 61 comprises a pair of auxiliary electrodes 63 
and 65, with the electrode 63 being connected to the electrode 22, and the 
electrode 65 being connected to ground through a resistor 67. The 
separation G' between the electrodes 63 and 65 relative to the separation 
G which exists between the electrode 22 and the tip of the wire 13 when 
the latter extends properly beyond the tip of the capillary is selected so 
that when the tip of the wire 13 projects properly beyond the tip of the 
capillary 11, the potential which is applied across both of the gaps G and 
G' will create an arc only across the gap G. Conversely, the gap 
separating the tip of the capillary 11 and the tip of the electrode 22, 
relative to the auxiliary gap G' is selected so that if the wire 13 should 
become arrested inside, and fail to protrude beyond, the tip of the 
capillary 11, the operating potentials applied across the gaps G and G' 
would result in the striking of an arc only across the auxiliary gap G'. 
In this way damage to the capillary, which might otherwise occur, is 
prevented. Such a condition can be readily detected by means of the 
circuit 61 by the provision of a sensing unit 69 connected so as to detect 
the potential which is generated across the resistor 67 only when an arc 
is struck across the electrodes 63 and 65, signifying failure of the wire 
13 to protrude beyond the tip of the capillary 11. 
If the geometries of the auxiliary electrodes 63 and 65 were similar to 
those of the electrode 22 and the tip of the wire 13 prior to it being 
formed into the ball 15, the auxiliary gap G' would be set to be 
intermediate in size between the gap G and the space between the 
capillary's tip and electrode 22. 
In fact, because of the sharp edges presented by the tip of the wire 13 
prior to arcing, and because the electrodes 63 and 65 may be flat-tipped 
rather than pointed, the auxiliary gap G' may have to be chosen to be 
somewhat smaller than the gap G to achieve the desired effect. 
When an arc is properly struck across the gap G in forming a ball 15, its 
magnitude and duration will be a function not only of the potentials which 
are applied across the gap, but also of the circuit impedance through 
which the arc must flow in the form of a current. This circuit impedance 
is made up principally of the gap, the DC current limiting resistor 44, 
and the series resonant circuit comprising the capacitor 41 and the 
inductor 37. The latter two components can be considered to be connected 
essentially to ground at the RF frequency because the capacitor 39 has a 
sufficiently large value to represent a direct connection from the 
inductor 37 to ground and because the capacitor 43 can be considered 
effectively to be shunted to ground through the output impedance of the 
high power driver 52, typically on the order of 25 ohms. The series 
resonant circuit formed by the capacitor 41 and the inductor 37 serves to 
smooth the arc-current which might otherwise fluctuate excessively due to 
variations in the impedance across the gap G. The latter impedance will be 
on the order of 100 ohms, whereas the output impedance of the series 
resonant circuit comprising the capacitor 41 and the inductor 37 will be 
on the order of 30 kilo-ohms, this assuming a frequency of 1.5 MHz and a 
circuit Q of 30. 
Thus, it may be seen that the size of the current represented by the arc 
across the gap G will be predominantly determined by the output impedance 
of the series resonant circuit and will be influenced only slightly by 
variations in the gap impedance. 
Furthermore, since the arc resonates at the tuned frequency of the series 
resonant circuit, even after the DC pulse 29 has terminated, oxide 
formation on the ball 15 is significantly reduced, thus helping make the 
process operational without the use of an inert gas in the gap G or around 
the wire. 
The following values and measurements represent one successful 
implementation of the method and apparatus in accordance with the present 
invention: 
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inductor 37 L = 75 .TM. H at 1.8 MHz 
Q.sub.L = 150 at 1.8 MHz 
capacitor 39 2000 picofarads 
capacitor 41 100 picofarads 
capacitor 43 3000 picofarads 
resistor 44 150 ohms 
wire material aluminum 
wire diameter .0012 in. 
wire ball diameter .003 in. (approx.) 
DC output voltage (29) 
550 volts 
DC (arc) output current 
3.5 amps 
RF driver output (59) 
350 volts 
Gap G .030 in. 
Gap G' .040 in. 
T1 230 .mu.sec 
T2 150 .mu.sec 
T3 150 .mu.sec 
______________________________________ 
Thus, there has been brought to the art of ball bonding a method, and the 
apparatus for its execution, whereby a ball can be formed at the end of a 
wire, whether it be of base or precious metal. The method yields balls 
which are substantially oxide-free and controllable in size without 
requiring the use of an inert gas to shield the ball during its formation.