Note: Descriptions are shown in the official language in which they were submitted.
CA 0220~966 1997-0~-23
.
~rocess for Bonding Wires to Oxidation-Sensitive Metal
Substrates which can be Soldered
Speci~ication:
The invention relates to a process for bonding wires to
oxidation-sensitive metal substrates which can be
soldered.
The wire bonding technique has for a long time been used
in order to establish electrically conductive connections
between uncased semiconductor circuits (chips) and the
conductor tracks of the component carriers supporting the
circuits. For this purpose the connections are formed by
wire bridges, which are welded on both sides to the
connection points on the surface of the electronic
components.
Fusion welding processes cannot be used in this case, as
the high temperatures necessary for this would destroy
the components. Therefore cold and hot pressure welding
processes have been developed as microstructuring
techniques. With the named processes, connections with
high mechanical strength and low electrical resistance
can be achieved, such as are required when connecting two
metals or alloys during wire bonding. In order to weld
the wire to the material of the contact surface, a supply
of external energy is necessary, by means of which a
plastic deformation of the bonding wire and/or of the
metallising layer is produced in the connection zone, and
the formation of the largest possible actual contact
surface.
In cold pressure welding, the energy is principally
applied by the contact pressure. A preferred method
which is particularly considered for thermally sensitive
CA 0220~966 1997-0~-23
substrates is the ultrasonic welding or bonding technique
("Technologie des Drahtbondens", ATV Report,
Schriftenreihe fur Aufbau- und Verbindungstechnik,
VDI/VDE-Technologie-Zentrum Informationstechnik GmbH,
Heft 4, August 1991, Berlin, DE) ["Technology of Wire
Bonding", ATV Report, Bibliography for Construction and
Connection Technology, Association of German
Engineers/Electrical Engineers, Technology Centre for
Information Technology GmbH, volume 4, august 1991,
Berlin, Germany]. In this case the energy necessary for
compression and connection of the materials to be bonded
is supplied in the form of elastomechanical vibrations in
the ultrasonic frequency range with the action of
pressure (bonding force). The process needs no external
supply of heat.
According to the present view, the mutually-opposed
surfaces are firstly cleaned during the welding procedure
by means of the friction of the surfaces on one another,
and layers of oxide and other extraneous material are
removed. Moreover, peaks of roughness on the surfaces
are smoothed. In this way an intimate contact is made
possible between the surfaces. Local heating occurs in
the connection zone. During the continued welding
procedure, the contact surfaces are increasingly
plastically deformed, and the formation of a solid
connection begins. The procedure continues until a
compact welded connection has been formed.
The normal working frequency during ultrasonic bonding
lies at 60 kHz or 100 kHz and a power of 0.1 watt to 30
watts. The amplitude of oscillation generally comes to
about 0.5 um to 2 um, the direction of oscillation lying
parallel to the plane of the workpiece. Extremely
specific process conditions are required in order to
CA 0220~966 1997-0~-23
establish reproducible welded contacts. In particular,
the maximum possible definition of metal surfaces must be
present, in order to achieve reliable contacting of the
wire.
In the ultrasonic bonding technique, the contacts are
produced by wedges. The wire is offered up to the
bonding surface by means of a wedge-shaped tool, welded
at that point with the application of pressure and
ultrasonic action, drawn to the second bonding surface, a
loop of wire spanning from the first to the second
bonding surface, and is then again welded under identical
conditions, and finally separated behind the second
welding point.
The bonding process has normally been used in the past to
connect the uncased semiconductor circuits with
connections on the component carriers. The ensemble thus
formed comprising the uncased semiconductor circuit on a
component carrier was provided with a casing and then in
turn soldered on to printed circuit boards via connector
pins.
More recent techniques start by fixing the uncased
semiconductor circuits directly, without component
carriers, on the surface of the printed circuit board,
and electrically conductively connecting them to the
conductor tracks by wire bonding. In this case there is a
requirement also to provide suitable surfaces for wire
bonding on the conductor tracks of the printed circuit
boards. Normally, metal coatings of nickel and gold are
used.
Conductor tracks on printed circuit boards as a rule
consist of copper, if necessary covered by tin or tin-
CA 0220~966 1997-0~-23
.
lead coatings. The named materials are very restricted
in their suitability, or are not suitable at all, for
connecting wires thereto by wire bonding. Thick oxide
layers, which prevent the bonding procedure, easily form
on copperi tin and tin/lead coatings cannot be bonded.
Therefore until now further layers of nickel and gold
have been deposited for this purpose on the conductor
tracks. During bonding, the bonding wire is welded to
the nickel coating. The extremely thin gold coating
covering the nickel coating serves merely to prevent
oxidation of the nickel coating in order to maintain its
capacity for bonding. Such coatings are however
extremely expensive. Normally these metals are deposited
only after production of the conductor tracks, so that
there is an additional requirement i.e. that the metals
must be selectively deposited exclusively on the copper
conductor tracks, and not on the non-conductive areas of
the printed circuit board lying therebetween. Otherwise
there would be a risk that conductive bridges and thus
short circuits would form between the conductor tracks.
In a prior technique, the nickel and gold coatings are
deposited on the copper surfaces immediately after
metallisation of the whole surface of the printed circuit
board and of the bore holes. This however has the
disadvantage that on the one hand considerable quantities
of the expensive gold must be applied, and on the other
hand that subsequent removal of these coatings by means
of etching processes in order to produce the conductor
tracks is difficult.
There is known from the Patent DD 136 324 a method of
bonding on more intensely oxidising connector surfaces,
in which a thin protective layer, which tears off during
CA 0220~966 1997-0~-23
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bonding, is applied to the connector surfaces after they
have been cleaned and prepared. There is in particular a
description of how copper surfaces are treated with
chromating solutions comprising chromium trioxide and
sulphuric acid in water. In this case the result is an
extremely thin protective layer, if necessary only a few
atoms thick, on the copper surface.
For widespread application o~ the named technique it is
however necessary for the surfaces prepared for bonding
also to be capable of being soldered, in particular
without the use of aggressive fluxes. As a rule, not
only semiconductor circuits are to be directly mounted
and bonded on the printed circuit board surfaces. At the
same time other components, such for example as
resistors, condensers and cased semiconductor circuits
are also to be mounted and electrically connected by
soldering to these surfaces. However, the layers formed
according to DD 136 324, cannot be soldered, at least
without the use of aggressive soldering aids. Therefore
such coatings for protecting the copper from oxidation
are not suitable as protective coatings for subsequent
mounting of components by soldering. Moreover, these
coatings change upon lengthy storage times at increased
temperature, so that no reproducible results are possible
during bonding and soldering.
In addition, such processes are extraordinarily toxic due
to the high concentration of chromium trioxide in the
chromating solution, so that complex measures for health
and safety and for waste water processing must be
undertaken.
The problem underlying the present invention is therefore
to avoid the disadvantages of prior art and to find a
CA 0220~966 1997-0~-23
.
process for bonding wires on oxide-sensitive metal
substrates which will also permit subsequent soldering.
In particular, the purpose consists in finding a method
of producing printed circuit boards fitted with
components soldered to copper structures and with
directly electrically conductively contacted uncased
semiconductor circuits.
This ob~ect is achieved by claims 1 and 3. Preferred
embodiments of the invention are given in the sub-claims.
The invention consists in the fact that the wires to be
bonded are connected by a pressure welding process with
ultrasound to the metal substrate, after the metal
substrate has been provided with organic protective
coatings by bringing the metal substrates into contact
with a solution containing nitrogen compounds, before the
bonding procedure, in order to provide protection against
oxidation and other impurities. For reproducible
application of such a protective coating the surface
which obtains contact with the bonding wires is freed of
oxides and other coverings or also organic impurities
which might impair the bond connection.
The process is particularly considered for metal
substrates of copper or copper alloys. It is based on
the recognition that fresh copper surfaces not
contaminated with oxides or adsorbates, bond well. Due
to the high reactivity of copper it is however not
possible to guarantee the necessary purity of the
surfaces, even under industrial circumstances. The
inventive idea consists in protecting the copper surface
by means of an appropriately thin organic protective
coatin~ fro~ ~he attack of atmospheric oxygen and other
corrosive adsorbates. The thin organic coating must
CA 0220~966 1997-0~-23
.
however satisfy a further condition: the welded
connection between the wire and the metal substrate must
not be impaired by the presence of the organic protective
layer. Direct metallic contact between the bonding wire
and the surface of the metal substrate must necessarily
be established during bonding. Oxides and other surface
connections such for example as adsorbates, prevent this
contact. It has become apparent that during the bonding
process, the organic protective layer is penetrated or
removed in a suitable manner.
Surprisingly, the organic protective layer which is more
than 50 nm, preferably 100-500 nm thick, does not lead to
a situation in which the adhesion of the contact between
the bonding wire and the metal substrate is impaired.
Such organic protective coatings are in fact used when
soldering metal substrates. In this case however the
soldered connection is impervious to organic impurities
of the substrates to be connected, as the soldered
contact is not only formed on the surfaces of the
substrates. During soldering, inter-metallic phases
(adhesion) usually form between the substrates, so that
impurities have practically no disadvantageous influence
on the adhesive strength of the bond, where these have
not already been removed from the soldering contact
surface by the action of temperature. Contrary to this,
the contact between the bonding wire and the metal
substrate is formed (cohesion) only in the contact
surface which has dimensions in the atomic range, so that
impurities, particularly of an organic type, can exert an
intensely negative influence on the adhesive strength of
the bond. The fact that inorganic metallic minimum
layers have no negative influence is understandable for
metals which may be connected in a material-to-material
manner, but not the fact that the organic compounds
CA 0220~966 1997-0~-23
.
according to the invention also have no negative
influence.
The process therefore has various unexpected advantages
compared to prior art: wires may be bonded on oxidation-
sensitive surfaces, for example of copper or copper
alloys, without the necessity for these to be cleaned
immediately prior to the bonding process. As a rule it
is to be expected that the metal surfaces will be stored
for a lengthy period of time before the bonding process,
and under certain circumstances will be exposed to
corrosive gases. These waiting periods can be of
variable length. Moreover, there is also the necessity
of subjecting the substrates before wire bonding to a
heat treatment, so that thicker oxidation layers form on
the metal surfaces. Such contaminations hinder the
bonding procedure or entirely prevent formation of the
welded connection.
The formation of such oxides and other contaminations is
prevented by the organic protective layer. Waiting
periods and heat stresses on the substrate before bonding
have no significant influence on the bonding result.
The coatings may also be soldered. Therefore for example
uncased semiconductor circuits may be electrically
conductively contacted by bonding, and other components,
such for example as resistors, condensers and cased
semiconductor circuits may be electrically conductively
contacted by soldering.
In addition, the process is eco-friendly, as no dangerous
substances such for example as chromates are used. It is
also cost-effective and reliable, as the expensive
CA 0220~966 1997-0~-23
.
electroless metallising processes for depositing nickel
and gold can be omitted.
The organic protective layers are formed by bringing the
metal substrates into contact with a solution containing
nitrogen compounds. In particular, alkyl, aryl, or alkyl
aryl imidazoles or benzimidazoles with alkyl residues
with at least three carbon atoms are used as nitrogen
compounds. In this case the derivates substituted in the
2-position are preferably involved. Basically, the
nitrogen compounds described and the additives further
indicated are used as described in the following
publications: EP 0428383 A1, EP 0428260 A2, EP 0364132
A1, EP 0178864 B1, EP 0551112 A1, CA 2026684 AA, WO
8300704 A1, JP 3235395 A2, JP 4080375 A2, JP 4159794 A2,
JP 4162693 A2, JP 4165083 A2, JP 4183874 A2, JP 4202780
A2, JP 5025407 A2, JP 5093280 A2, JP 5093281 A2, JP
5098474 A2, JP 5163585 A2, JP 5202492 A2 and JP 6002158
A2.
The solution from which the organic protective layers are
deposited preferably has a pH value of less than 7 and in
particular between 2 and 5. The solution is adjusted to
the desired pH value by the addition of acids. Both
inorganic and organic acids are suitable for this
purpose, such for example as phosphoric acid, sulphuric
acid, hydrochloric acid and formic acid.
The process according to the invention serves in
particular to produce printed circuit boards fitted with
cased components soldered on copper structures and with
directly electrically conductively contacted uncased
semiconductor circuits.
CA 0220~966 1997-0~-23
.
For this purpose the following essential process steps
are necessary:
- production of the copper structures on the surfaces of
the printed circuit boards,
- formation of organic protective layers on the copper
structures,
- mechanical fixing and soldering of the cased components
on the surfaces of the printed circuit boards,
- attachment of the uncased semiconductor circuits on the
surfaces of the printed circuit boards,
- electrical contacting of bonding islands provided on
the uncased semiconductor circuits with the copper
structures by pressure welding of wires with ultrasound.
The process may also be used in the production of other
connection carriers, such for example as multi-chip
modules or chip carriers.
The process steps may be followed in the sequence shown,
or also in another sequence. In addition to those given,
further process steps are if necessary carried out, which
will be illustrated in the following.
As basic materials for the substrates carrying the copper
structures, in addition to FR4 or FR5 (flame-retarding
epoxy resin/glass fibre laminate), polyimide and other
materials, and also ceramics are used. Copper structures
are produced on these generally plate-shaped carriers by
known techniques (Handbuch der Leiterplattentechnik, Vol.
II, ed. G. Herrmann, Eugen G Leuze Verlag, Saulgau,
CA 0220~966 1997-0~-23
.
1991). If necessary, the carriers also contain bore
holes, which serve to connect individual planes of
conductor tracks, and for this purpose are metallised
with copper coatings.
After production of the copper structures on the outer
sides of the substrates, conventionally a solder mask is
applied to each side of the carrier, in order to cover
the areas of the copper structures at which no further
electrically conductive connections with cased components-
or uncased semiconductor elements are provided. Then the
organic protective coating can be formed on the copper
surfaces left exposed. For this purpose, these are
firstly cleaned and then superficially etched. Acidic
and alkaline solutions are used for cleaning, in order to
remove oxide layers and other impurities such for example
as greases and oils. Therefore, in addition to acids or
bases, the solutions if necessary also contain wetting
agents. After cleaning, the surfaces are etched in order
to produce a rough surface profile. Coatings
subsequently applied thus adhere better to the surface.
The etching solutions for copper usual in printed circuit
board technology are used. Between the treatment stages,
the surfaces are thoroughly rinsed in order to remove
adhering solutions.
Before the subsequent formation of the organic protective
coatings, the surfaces are as a rule once more dipped in
a sulphuric acid solution and immediately thereafter
brought into contact with the solution containing the
nitrogen compounds, without further treatment. Due to
this contact a coating of the nitrogen compounds forms on
the copper surface. The selected temperature of the
treatment solution is preferably between 30~C and 50~C.
Depending on the layer thickness required, treatment
CA 0220~966 1997-0~-23
.
times between 0.5 and 10 minutes are suitable. Layer
thicknesses between about 0.1 um and 0.5 um may be
reproducibly produced. Thinner layers do not
sufficiently prevent oxidation of the protected metal
surfaces. Thicker layers hinder the bonding capacity of
the surfaces.
The protective layer can be formed by dipping, spraying,
and flooding in a vertical or horizontal arrangement of
the substrates. The normal process procedure consists in
dipping the substrates, which are held vertically, into a
dip-bath. Recently, the horizontal technique has become
more widely used, in which the substrates are passed
through a treatment system in a horizontal position and
direction, and are flooded with the treatment solutions
from both sides via nozzles. The effectiveness of the
treatment in dip-baths is intensified by agitation of the
substrates and the additional action of ultrasound.
After formation of the protective layer, the latter is
dried and if necessary fired.
If components such as resistors, condensers and cased
semiconductor circuits are to be electrically
conductively connected to the copper structures,
thereafter solder deposits may also be applied at the
appropriate points by printing paste solders. Thereafter
the components themselves are mechanically fixed by
adhesion to the surface of the printed circuit board. By
means of a reflow soldering process, the connector
contacts of the components are conductively connected to
the copper structures. Infrared, vapour phase, and laser
soldering processes are considered for example as
reflowprocesses. The named process steps are for example
described in more detail in Handbuch der
CA 0220~966 1997-0~-23
Leiterplattentechnik, Vol. III, ed. G. Herrmann, Eugen G
Leuze Verlag, Saulgau, 1991.
Thereafter the uncased semiconductor circuits to be
connected to the substrate are secured by adhesion to the
copper structures.
The ultrasonic bonding process, classified among the
microjoining techniques, is used in particular ~or
electrically conductively contacting the connection
points on the semiconductor circuits with the copper
structures. This procedure needs no additional external
supply of heat during the bonding process. Aluminium
wire is preferably used for bonding. Copper, gold and
palladium wires are however also possible. Aluminium
wire with wire diameters between 17.5 ,um and 500 um in
particular is used. Thicker wires with a diameter of
above 100 um are made from hlghest grade aluminium.
Wires with thinner diameters cannot be drawn and
processed from highest grade aluminium for reasons of
strength. These are alloyed with 1% silicon or 0.5% to 1%
magnesium in order to harden them. With thinner wires,
connections can be formed on smaller contacting points on
the uncased semiconductor circuits and on the copper
structures. Thicker wires are used in component groups
in electronic power systems.
Characteristic process parameters in ultrasonic wire
bonding are the ultrasonic power, the bonding force and
the bonding time. In addition to the condition of
generating a reliable, i.e. solid connection between the
wire and the bonding surfaces, there is the further
necessity of achieving maximum productivity, i.e. the
shortest possible cycle time (production time of a wire
CA 0220~966 1997-0~-23
.
14
bridge). For this purpose the named parameters are
optimised.
The ultrasonic power is selected in dependence on the
wire diameter. The m~xi mum power of the ultrasonic
generators lies between 5 watts and 30 watts. The
pressure applied by the tool during bonding of the wire
on the connector surface is as a rule between 250 mN and
1000 cN. The bonding time preferably lies between 30
msec and 500 msec. The lower ranges of the named
parameters in turn respectively apply, in dependence on
the wire diameter, to thin wire bonding, and the upper
ranges to thick wire bonding.
For the wedge-on-wedge technique which is used in
ultrasonic bonding with aluminium wires, tools are used
in the form of a stamp with a wedge-shaped point (bonding
wedge). The tool is normally made of tungsten carbide, a
sintered hard metal with cobalt as a binder. Steel or
titanium carbide are also possible. The wire guidance
system is usually additionally integrated in the tool.
Manual and automatically controlled systems are used for
bonding. The system substantially comprises the bonding
head, which holds the tool, the ultrasonic unit, the tool
holder, an electronic control system and optical aids for
positioning the materials to be connected.
The quality of the bonded connection can be tested by
various methods. The tensile test is preferably used to
determine the adhesive strength of the bonded connection.
For this purpose a small hook is placed in the centre
between the two bonded contacts beneath the wire loop,
and drawn upwards at a constant speed until the wire loop
ruptures. The tensile force is measured with the aid of
CA 0220~966 1997-0~-23
.
a measuring device and the force leading to rupture of
the wire loop is recorded. It should however be noted
that under certain circumstances it is not the joint
position itself which ruptures, but, due to its high
strength, the wire. This is the case particularly when
the bonded connection is intensely deformed by high
bonding energy and the bonding wire is therefore notched
in the vicinity of the connection.
The following embodiments, given by way of example, serve
to explain the invention:
Example 1 and comparative example:
FR4 printed circuit boards (base material of the company
Isola, Duren, Germany) with a copper structure permitting
the bonding of wire bridges about 2.5 mm long, were
treated after structuring with the following process:
20 Process steps Treatment Temperature
Time (Minutes) (~C)
1. Cleaning of 2 - 3 40 - 50
copper surface in
25 a sulphuric acid
aqueous caroate
etching solution
2. Rinsing in
30 water
CA 02205966 1997-05-23
.
3. Cleaning in 2 - 3 40 - 50
alkaline aqueous
solution
containing wetting
agent
4. Rinsing in
water
5. Etch cleaning 2 - 3 30 - 40
in an aqueous
sulfuric acid
hydrogen peroxide
i5 solution
6. Rinsing in
water
7. 5% by volume 1 ambient
aqueous sulphuric temperature
acid solution
8. Rinsing in
water
9. Formation of 1 30 - 50
the organic
protective coating
~ :
10. Rinsing in
water
11. Drying 5 - 10 60 -80
CA 0220~966 1997-0~-23
.
In order to form the organic protective layer a solution
was used containing 10 g/l of 2-n-heptylbenzimidazole and
32 g/l formic acid in water.
A proportion of the printed circuit boards was then
stored at ambient temperature without further measures.
A further proportion was fired, in order to simulate the
hardening conditions during adhesion of the semiconductor
circuits, for 2 hours at 80~C, and a third proportion was
fired for 1 hour at 150~C.
Thereupon an AlSil bonding wire (aluminium with 1%
silicon) with a diameter of 30 um was bonded on to the
copper structures (manufacturers Heraeus, Hanau,
Germany). The wire had a pull-off strength of 19 to 21
cN at an elongation of 2 - 5%.
The individual bonded connections were carried out with a
manual wire bonder of the type MDB11 (manufacturers
Elektromat, Dresden, Germany). A standard bonding wedge
of the type DS 3102.02-18L (Elektromat, Dresden, Germany)
was used.
At a constant bonding force of about 40 cN and a bonding
time of about 200 msecs, the ultrasonic power was varied
in a range from 550 - 1000 scale divisions, corresponding
to about 0.5 watts to 1 watt. The illustration
reproduces results of the individual tests as a graphic
visualisation of the dependencies of the pull-off
strength determined by the tensile test on the ultrasonic
power at constant bonding force and time. Curve Fl gives
the average pull-off strength with bond connections
without further thermal storage of the substrate before
wire bonding, curve F2 the average pull-off strength
after thermal storage of 2 hours of the substrate before
CA 0220~966 1997-0~-23
18
wire bonding at 80~C, and curve F3 the average pull-off
strength after 1 hour of thermal treatment of the
substrate before wire bonding at 150~C.
The results show that the copper structures provided with
the organic protective layers may be bonded to the
printed circuit boards, and the wire bridges have an
acceptable strength. Each measurement point given in the
illustration lies clearly above the minimum pull-off
strength of 3cN required in the international standard
regulation MIL-STD-883C, method 2011, for wire bridges.
No failures occurred during bonding.
Contrary to this, printed circuit boards not provided
with the organic protective coating could not, or could
not be reliably bonded without immediately preceding
cleaning of the copper surfaces, as the strength of the
bond connections was insufficient, particularly with the
substrates with thermal stress before wire bonding. In
addition, the yield was low.
Examples 2 to 8:
Under the conditions of example 1, the operation was
carried out with the following solutions for forming the
organic protective coatings:
2. 10 g/l 2-n-heptylbenzimidazole
33 g/l formic acid
1.0 g/l copper (II) chloride (CuCl22 H2O)
in water
3. 5 g/l 2-n-heptylbenzimidazole
1 g/l lH-benzotriazole
1 g/l zinc acetate dihydrate
CA 0220~966 1997-0~-23
.
19
33 g/l glacial acetic acid
in water
4. 5 g/l 2,4-diisopropylimidazole
32 g/l glacial acetic acid
in water
4. 5 g/l 2,4-dimethylimidazole
5 g/l 2-n-nonylbenzimidazole
1 g/l toluol-4-sulphonic acid monohydrate
35 g/l formic acid
in water
5. 2-(1-ethylpentyl-benzimidazole)
32 g/l glacial acetic acid
in water
6. 10 g/l 2-phenylbenzimidazole
2 g/l ethylamine
60 ml/l hydrochloric acid, 37% by weight
in water
7. 10 g/l 2-phenylbenzimidazole
35 g/l glacial acetic acid
in water
8. 10 g/l 2-ethylphenylbenzimidazole
1 g/l zinc acetate dihydrate
33 g/l formic acid
in water
Perfect bonding results were obtained even after storage
at increased temperature of the metal surfaces to be
equipped with bonding wire.
