Note: Descriptions are shown in the official language in which they were submitted.
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PHN.9872 1 27.4.81
"Method of forming a wire bond"
The invention relates to a mcthod of forming a
wire bond between a contact place on an electric micro-
circuit and a connection conductor, in which a wire of
aluminium or an aluminium alloy is used which is passed
through a capillary in which a ball is formed at the end
of the wire by means of a spark discharge between the
wire and an electrode, which spark discharge takes place
in a protecting gas atmosphere, after which the wire is
bonded by means of the capillary to a contact place on
the electronic microcircuit and thereafter the wire is
bonded to the connection conductor.
For making a w~ire bond between a contact place
on, forexample, a semiconductor body and an electric
conductor it has proved favolrable to use a ball bond
in adhering the wire to the semiconductor body. The ball
can be connected to the contact place by means of an
ultrasonic welding tool or by means of a thermocompression
bond, possibly with a combination o~ the two. In the case
of a wire of gold the ball can preferably be ~orrned by
means of an electric spark discharge. The formation of
a ball at a wire of aluminium or an aluminium alloy,
however, presents difficulties.
It has already been suggested to form~a ball
at the end of an aluminium wire by means of an electric
- spark discharge by contacting the wire and the electrode
with each other for a short period of time at a voltage
difference between the wire and the electrode of less
than 200 V and in a protective gas atrnosphere. The end
of the wire melts and the contact is interrupted, as a
result of which a spark discharge takes place and the
ball is formed. The protective gas serves to prevent that
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P~ 9872 2 28.4.81
in forming the ball oxidation phenomena occur. This method
of providing the ball in which contact between the wire
and the electrode is necessary~ is complicated *or mass
production. Furthermore excessive wear of the electrode
occurs which hence has often to be replaced.
It has ~ur-thermore been suggested to arrange that
the end of the wire and the electrode have a small
separation and a voltage difference of 350 to 10,000 Volts,
so a~ to obtain a spark discharge. The ohrnic resistance in
the discharge current circuit i3 chosen to be so that the
peak value of the current density in the wire cross-
section is 1.2 x 109 A/m2 to 13.5 x 109 A/m2. However, it
is to be preferred to cause the spark discharge to tal~e
place at a l~er voltage. In addition, in this known
method, a very small distance, approximately 0.125 mm,
must be adjusted rather accurately between the wire end and
the electrode. However, in mass production i-t is undesirable
to depend on an accurate adjustment and the distance
between the wire end and the electrode is furthermore
preferably chosen to be considerably larger than in -the
known method.
It is -the object of the invention to provide
a method of the kind mentioned in the opening paragraph
in which a spark discharge is obtained wi-th the use o~
a comparatively small voltage di~-ference between the
wire end and the electrode, and in which the mutual
distance between -these two may be comparatively large
and need not be accurately adjusted. For that purpose,
according to the invention, an electric spark discharge
is produced between two auxiliar~ electrodes, a plasma
being formed by ionisa-tion of the protective gas, an
electric spark discharge is produced between the electrode
and the wire at a vol-tage be-tween 25 and 20V V due to the
low resistance in -the plasma, by which spark discharge
a ball is formed at the end of the wire.
The discharge between the auxiliary electrodes
between which a rather large voltage difference may be
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PHN.9872 3 27.4.81
formed produces a pl~sma in the protective gas. The
resistance in the plasma i9 very low as compared with
the resistance in the unionised gas. As a result of this,
with a comparatively small voltage difference between
the electrode and the wire, a spark discharge is produced
which forms the ball at the wire. The spacing between
electrode and wire end is not critical9 when the
resistance of the gas becomes su~ficiently low the ball-
forming spark discharge will take place automatically.
In this manner which is suitable ~or seriesproduction a ball of a readily reproducible size is
formed at a wire of aluminium or an aluminium alloy.
This size depends on the voltage difference between the
electrode and the wire and on the electric charge; it
has been found that in order to obtain a good shape of
the ball the voltage difference should preferably be
smaller than 200 V.
In a favourable embodiment of the method in
accordance with the invention the plasma is formed by
a spark discharge by means of an ignition coil in which
the voltage between the auxiliary electrodes is in the
order of magnitude of 10,000 - 20,000 Volts. In that
case only simple means are necessary to form the plasma
in the protective gas.
It is recommendable for the spark discharge
between -the electrode and the wire to be obtained by
discharge of an electric capacitor at a voltage from
5 - 100 V.
In a preferred embodiment of the me-thod in
accordance with the invention the spacing be-t~een -the
electrode and the end of the wire is kept a-t a value in
the order of magni-tude of 2 mm during the forrrlation of
the ball. Although the spacing can be chosen to be larger
or smaller, the spacing of approximately 2 mm has proved
-to be extremel-y suitable both in behalf of series
manufacture and to obtain a favourable ball shape.
The invention will now be described in greater
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PHN.9872 4 ~7.4.81
detail b~ way o~ example, with r~fer~nce to an embodiment
shown in the drawing.
Fig. 1 shows diagrammatically a device for
providing the wire bond,
Figs. 2 to l~ are a longitudinal sectional view,
a plan vi0w and a front elevation, respectively, of an
apparatus in which the ball is formed at the wire,
Fig~ 5 shows an electric circuit to obtain
the spark discharge,
FigsO 6-8 show the bonding of the wire to the
electronic microcircuit and to a current conductor~
respectively,
Fig 1 shows an ultrasonic generator, which
generator i9 pivotable about an axis 2 which is incorpora-
ted in a support 3. The welding arm 4 of the generator 1
has a capillary 5 through which a wire 6 of aluminium
or an aluminium alloy is passed. A ball is to be formed
at the end of the wire 6. For that purpose, the wire is
guidedinto aslot 7 of a spark unit g (see also ~igs. 2,
3 and 4). The body of the spark unit consists of an
insulating material, for exampLe a synthetic resin. A
bore 9 opens in-to the slot 7 -through which a protective
ga.s, for example argon, is passed via a hose 10. An
electrode 11, as well as -two au.Yiliary elec-l;ro~es l2 arld
13, are incorporated in the spark UIlit 8. The spacing
between -the ends of the auxiliary electrodes i9 preferably
approximately 2 mm. The spacing between -the electrode 11
and the end of the wire 6 is also approximately 2 mm.
30 The spark unit can be rotated about a shaft l9 and can
thus be moved towards and also away ~rom the capillary 5.
The ~ig 1 device comprises furthermore a
support 1~ on which a slide lS is present. A conductor
grid may be placed on the slide 1~. A semicond-uctor
35 element 17 is present on a supporting part 16 of the
conductor grid, which element has con-tact places -to provide
an electrically conductive wire. The wire is guided from
a contact place of the serniconductor element 17 to a conduc-
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P~N.9872 5 27,4.g1
tor 18 of the conductor grid.
The for~lation of a ball at the wire 6 o~
aluminium or an aluminium alloy will be e~plained with
reference to Figs, 2 to 5. The end of the wire 6 is
provided in the slo-t 7 of the spark unit 8. A protective
gas, for example argon, is passed into the slo-t via a
bore 9; the gas flow preferably takes place for a short
period of time, namely o~ly during the formation of the
ball. Between the auxiliary el0ctrodes 12 and 13 a voltage
difference is generated, preferably of 10,000 - 20,000
~olt by means of an ignition coil so that a spark
discharge takes placeO Said spark discharge causes a
plasma in the protective argon gas. As a result of this
the electrie resistance in said gas drops to a very
low value. A voltage difference of 200 V or less,
preferably approximately 70 V, i9 maintained between
the electrode 11 and the end of the wire 6.
As a result of the low value of the electric
resistance in the plasma a spark discharge may take place
between the electrode 11 and the end of -the wire 6,
inspite of the fact that the spacing between the two
may be comparatively large, for e~ample may be 2 mm.
As a result of the spark discharge a ball is formed at
the end of the wire, the size of said ba}l bei~lg very
readily reproducible.
Fig. 5 shows diagrammatically a circuit
arrangement -to generate a spark to form a ball at the
aluminium wire, A pulse 20 originating from a ~onostable
multivibrator not shown brings the base of a transistor 21
at a sufficiently high voltage to cause current to flow
through the transistor. ~s a result of said current the
base of a transistor 22 is brought at such a voltage
that current also flows through transistor 22. The
current through transistor 22 is sufficiently large to
control a high-voltage transistor 23; cllrrent flows
through the primary 24 of an ignition coil 2~, At the
end of the short pulse 20, the transistors 21, 22 and 23
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PHN 9872 6 28.4.81
successively switch o~ and the current in the primary 24
of the ignition coil suddenl~ drops to a value zero.
By induction, a high voltage~ for example 20,000 V~ is
generated in the secondary 25 of the ignition coil. As a
result of this the electric spark discharge is formed
between the electrodes 12 and 13 and a plasma is formed
in the protective argon gas.
An electric capacitor 27 is connected between
the wire 6 and the electrode 11. The capacitor is
connected to a voltage source and is hence charged. As
a result of the low resistance in the plasma the capacitor
27 will discharge while forming a spark between the
electrode 11 and the end of the wire 6. The ball is thus
formed at the end of the wire.
The voltage across and the capacitance of the
capacitor 27 can be chosen depending on the diameter of
the wire at which the ball is formed. For example, it
has proved to be very advantageous with a wire having a
diameter of 200/um to use a capacitor of 500/uF with a
voltage difference of 70 V. l~ith a wire having a diameter
of 40/um a favourable ball shape was obtained by discharge
of a capacitor of lS/uF with a voltage difference of 70 V.
Figs.6 to 8 show the bonding of the wire at
one end to the electronic ~licrocircuit and a-t thc o-ther
end to a current conductor.
On the slide 15 also shown in Fig. 1 a conductor
grid is placed having a supporting part l6 on which a
semiconductor element 17 is connected. A curren-t conductor
30 is referenced 18. The capillary 5 having therein the wire 6
at which a ball has been ~ormed is present above a contact
place on the semiconductor element 17.
The capillary is moved towards the semiconductor
element, for example by pivoting the ultrasonic generator
35 1 about shaft 2 (Fig. 1). When the ball presses against
the contact place on the semiconductor element the bond
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PHN.9872 7 27.4.81-
is produced by means of ultrasonic vibrations (Fig. 7),
the ball being formed into a flat head~ The capillary
is then raised and moved towards the current conduc-tor 18.
The wire is clamped there between the conductor 18 and
the lower side of the capillary and bonded to conductor
18 by means o~ ultrasonic energy. Fig. 8 shows the
ultimate wire bond. The bond with the connection conductor
has not necessarily to be made with the capillary, but
can be done in any desired manner.
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