CA2205966A1 - Process for bonding wires to oxidation-sensitive metal substrates which can be soldered - Google Patents

Abstract

In order to bond unhoused semiconductor circuits (chips) electroconductively on circuit boards, the surfaces of the copper structures are generally coated with a nickel layer which is then coated with a layer of gold in order to be able to produce wire connections between the circuit and the conductive structures on the circuit board. The connection between the wire and the connection sites on the circuit, on the one hand, and the conductive structures on the circuit board, on the other, is formed by pressure welding, in particular ultrasound bonding. A novel process by means of which organic protective layers are deposited on the copper surfaces on the circuit board prevents the formation of oxide layers and other impurities on the copper surfaces and thus permits direct connection of the wire by ultrasound bonding to the copper surfaces without impairing the ability of the surfaces to be soldered.

Description

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 .

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 .

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 .

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 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 .

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.

Claims (7)

1. Process for bonding wires by means of ultrasonic welding on a surface of an oxidation-sensitive metal substrate which is equally suitable for soldering, wherein, before bonding, the surfaces freed of oxides and/or other organic or inorganic coverings at least on areas to be contacted by bonding wires, thereafter an organic protective coating with a thickness of 100 - 500 nm is applied by contact with a solution containing nitrogen compounds, and thereafter the wires are bonded on to the metal substrate.

2. Process according to claim 1, characterised in that metal substrates of copper or copper alloys are used.

3. Method of producing printed circuit boards fitted with cased components soldered on copper structures and with directly electrically conductively contacted uncased semiconductor circuits, with the following essential process steps:

- production of the copper structures on the surfaces of the printed circuit boards, - removal of oxides and/or other organic or inorganic coverings from the surfaces and subsequent contact the surface by a solution containing nitrogen compounds until an organic protective coating with a thickness of 110 -500 nm is applied, - 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.

4. Process according to one of the preceding claims, characterised in that alkyl, aryl or alkyl aryl imidazoles or benzimidazoles with alkyl residues with at least three carbon atoms are used as nitrogen compounds.

5. Process according to one of the preceding claims, characterised in that the solution containing the nitrogen compounds has a pH value of less than 7, preferably between 2 and 5.

6. Process for bonding wires, characterised by individual or all the new features and combinations of disclosed features.

7. Process for manufacturing printed circuit boards fitted with cased components soldered on copper structures and with directly electrically conductively contacted uncased semiconductor circuits, characterised by individual or all the new features and combinations of disclosed features.