WO2013050258A2

WO2013050258A2 - Selective hard gold deposition

Info

Publication number: WO2013050258A2
Application number: PCT/EP2012/068604
Authority: WO
Inventors: Bernd Weyhmueller; Uwe Manz; Klaus Bronder; Mario Tomazzoni; Frank Oberst; Sascha Berger
Original assignee: Umicore Galvanotechnik Gmbh
Priority date: 2011-10-06
Filing date: 2012-09-21
Publication date: 2013-04-11
Also published as: WO2013050258A4; TW201333275A; DE102011114931A1; DE102011114931B4; WO2013050258A3

Abstract

The present invention relates to a process for the electrolytic deposition of gold on, in particular, electric contacts. The process is characterized in that particular additives which allow undesirable deposition of gold in regions of low current density to be prevented are added to the electrolyte.

Description

Selective hard gold deposition

The invention relates to a process for the electrolytic deposition of gold on, in particular, electric contacts. The process is characterized in that particular additives are added to the electrolyte to prevent undesirable deposition of gold in regions of low current densities.

The use of specific additives in the field of metal electroplating is adequately known. In the case of gold and gold alloys, in particular hard gold (e.g. AuNi, AuCo, AuFe) , there are a number of additives both for electrochemical and electrolytic processes. Commercial gold electroplating baths therefore contain not only gold and optionally one or more alloying elements in dissolved form but also generally conductive salts and buffer salts and various inorganic and/or organic substances for regulating metal deposition and formation of the layer, as brighteners or for other auxiliary purposes.

Fine and hard gold coatings are used on a large scale for industrial applications in which, in particular, abrasion resistance, chemical resistance, bondability, solderability or wear behaviour are important. For the electroplating of gold on electric contacts, masking techniques (Edelmetallschichten; H. Kaiser; Eugen G. Leuze Verlag, 1st edition 2002, pp. 74 - 89 and Selektive Hochgeschwindigkeitsabscheidung von

Edelmetallen auf Bandanlagen; P. Wingenfeld, Galvanotechnik, 2/2004, p. 335ff, Eugen G. Leuze Verlag) are employed in the prior art in continuous plants in order to apply the metal deposits selectively in the functional region of the contact pins. However, in the runout zones (gap) under the mask, there are regions of low current density in the case of electrolytic deposition. Here, deposition of coatings from the hard gold electrolytes should be avoided as much as possible in order to reduce costs. For this purpose, additives, which selectively inhibit deposition of gold in this region should be found.

Thus, for example, WO0028108 discloses a process for the reductive deposition of gold, in which anti-gold deposition agents are used. The compounds used here are said to prevent, inter alia, etching or corrosive secondary reactions between the metal surface and the gold during the deposition process. Such compounds are considered to be, in particular, certain amines or reaction products thereof with compounds bearing epoxy groups .

According to GB 2028873, the addition of alkyl polyamines such as tetraethylenepentamine to electrolytic gold-cadmium baths leads to unusually stable gold deposits which have a low contact resistance. Furthermore, the polyamines ensure that the carbon content in the gold deposit assumes a particular value and the cathode efficiency is reduced by up to 80%. However, the use of corresponding gold-cobalt or gold-nickel alloys is advised against here.

US 3642589 discloses additives which are added to electrolytic baths containing gold and silver. These baths are employed for depositing gold-based alloys. The additives referred to above consist of polymeric condensates of epichlorohydrin and alkylenepolyamines . For example, a condensate of epichlorohydrin and tetraethylenepentamine is added to electrolytes containing gold and other alloy constituents such as nickel and cobalt. Very thick gold alloy layers can in this way be deposited without defects. However, the electrolytes discussed here are used in a basic pH range since the electrolyte is to contain free cyanides. This appears to be a disadvantage from an occupational hygiene point of view.

DE 3432784 is likewise concerned with an electrolytic bath from which hard coatings of gold-cobalt alloys can be deposited. The process described here operates in a current density range of 0.1-3.0 A/dm² and at a pH of from 3.5 to 6.5 and advises the addition of a reaction product of about one part of epichlorohydrin and two parts of a diaminoalkane and also one part of an α, ω- dihaloalkane . The objective of this text is to provide high-alloy steel products having firmly adhering coatings and to increase the upper current density range for the process. Gold or gold alloys can also be electrolytically deposited from baths which, as mentioned in US 4062736, comprise a cyanide complex of gold and a condensation product of a polyamine and epichlorohydrin in a ratio of 1 : 2.5. The bath operates in a pH range from 6 to 10 and additionally contains arsenic compounds and mild reducing agents. Such baths are said to ensure improved uniformity of the coating and better colour stability.

According to EP 37535, pure amines are also suitable for producing bright and highly ductile deposits in electrolytic gold(III) and gold(III) alloy baths when aminocarboxylic acids or phosphonic acids are added to the bath in addition to the alloy constituents and a pH of <3 is set in the bath. As amine, for example, tetraethylenetetramine is used.

In FR2414082, too, polyamines and epichlorohydrin are used in electrolytic baths which have arsenic compounds and reducing agents as further constituents. The bath used should have a pH of 6 - 10. Gold deposits produced in this way are said to have particularly uniform thicknesses . EP2014801 describes gold electrolytes which can be employed for plating, inter alia, contact materials. Such electrolytes contain many additives. Apart from hexamethylenetetramine, complexing agents and brighteners based on nitrogen- and sulphur-containing carboxylic acids or alcohols are used. Deposition of gold in regions of low current density is said to be prevented by means of these electrolytes. It is therefore an object of the present invention to indicate how the excess deposition of gold or a gold alloy in the runout zones of the masking can be avoided or at least reduced. The process proposed should be able to operate using electrolytes having a very simple make-up and be superior to the processes established in the prior art both from economic and ecological points of view.

This object and further objects which can be seen in an obvious way from the prior art by a person skilled in the art can be achieved by a process having the characterizing features of the present Claim 1. Preferred embodiments of the process of the invention are indicated in the claims dependent on Claim 1.

Using an aqueous electrolyte having a pH of from 3 to 6 and comprising: gold ions in the form of a soluble complex; optionally further ions selected from the group consisting of cobalt, nickel, iron or mixtures thereof; aliphatic polyamines which are soluble in the electrolyte and have at least 4 ethylene units and at least 2 primary or secondary amine groups and/or reaction products exclusively of aliphatic or aromatic amines with compounds bearing epoxy groups, in particular epichlorohydrin, in a molar ratio of from 1:1 to 1:5; and producing a corresponding deposit by dipping the substrate as cathode into the electrolyte and setting a sufficient current between an anode which is in contact with the electrolyte and the cathode in a process for the selective, electrolytic deposition of gold or a gold alloy, in particular hard gold, completely surprisingly but no less advantageously achieves the stated object. The process presented here makes it possible for the first time to reduce the deposition of gold or gold alloys in the low current density areas and at the same time leave the deposition factor unaffected in the current density range which is preferred from a process engineering point of view. This leads to savings of about 10-30% of the gold used.

The gold will be present in the form of its ions dissolved in the electrolyte. As gold source, it is possible to use all cyanide, chloride, sulphite and nitrilo complexes of gold which a person skilled in the art would consider for this purpose. Known gold compounds may be found in the following reference (Edelmetallschichten; H. Kaiser; Eugen G. Leutze Verlag, 1st edition 2002, pp. 35 - 45) . Cyanoaurate ( I ) complexes are preferably employed for this purpose. Very particular preference is given to the potassium salt of the gold cyanide complex in this context: K[Au(CN)2] · The gold compound is used in a concentration of 0.005 - 0.2 mol/1, preferably 0.025 - 0.1 mol/1 and very particularly preferably 0.05

0.08 mol/1, in the electrolyte to be used. It may be pointed out that the electrolyte ideally contains no additional free cyanide. All the cyanide used is introduced in the form of the complex indicated above into the electrolyte.

The deposit produced according to the invention can likewise consist of a gold alloy. As alloy, hard gold has been found to be useful in this respect since corresponding contacts should withstand certain mechanical stresses. Hard gold is generally understood as being an alloy of gold and one of the metals selected from the group consisting of iron, cobalt, nickel or mixtures thereof. The latter are likewise advantageously present in the form of their dissolved ions in the electrolyte. As compounds from which the corresponding ions can be dissolved in the electrolyte, particular mention may be made of their readily water- soluble salts with anions selected from the group consisting of chloride, bromide, carbonate, hydrogencarbonate, phosphate, hydrogenphosphate, sulphate, citrate, methanesulphonate, tartrate, oxalate and nitrate. The salts mentioned here are advantageously used in a concentration of 0.0001 0.1 mol/1, particularly preferably 0.0025 - 0.02 mol/1 and very particularly preferably 0.005 - 0.015 mol/1, in the electrolyte. It may be remarked that it is not ruled out according to the invention that further ions of other metals and nonmetals may be present in the electrolyte. Nevertheless, the electrolyte should for cost reasons be kept as simple as possible. From this point of view, inter alia, the addition of further salts or other inorganic compounds can be dispensed with. In particular, no further (non) metals apart from those mentioned here as optional should be present in the electrolyte. Addition of arsenic, silver and/or cadmium can very particularly preferably be dispensed with.

Aliphatic polyamines which are in the present case possible as current density-specific inhibitors for the deposition of gold or gold alloys from electrolytic baths are particularly advantageously those which have at least 4 ethylene units and at least 2 primary or secondary amine groups. This definition encompasses both polyamines which have at least 4 ethylene units in a row between terminal amine groups and polyamines in which all or at least some of the ethylene units are flanked by amine groups. Two general formulae for the latter are shown by way of example below:

1

NH₂ - (CH₂ - CH₂ - NRi -)_n CH₂ - CH₂ - NH₂ 2

n >3 Ri,2,3,4 = H or an aliphatic radical (e.g. methyl, ethyl, (iso)propyl, (n-, sec- iso)butyl)

It may be pointed out that these aliphatic polyamines are ones which are soluble in the amounts indicated below in the present electrolyte. Accordingly, there is an upper limit to the chain length or the number of amine groups in the polyamine at a given pH of the electrolyte of 3-6, in particular 4-5. The upper limit to the chain length between 2 amine groups of the aliphatic amines is expected to be <8 carbon atoms, preferably ≤6 carbon atoms and particularly preferably ≤4 carbon atoms. The number of secondary and/or primary amine groups can likewise have an upper limit of 10. Particular preference is given to polyamines which have from 3 to 9, preferably from 4 to 7 and particularly preferably 5 or 6, amine groups in the skeleton. Advantageous polyamines are those which may be found in the literature cited at the outset and obey the present criteria. Particular preference is given to using polyamines selected from the group consisting of tetraethylenepentamine and pentaethylenehexamine . The aliphatic polyamines are used in a concentration of 0.001 - 0.5 mmol/1, particularly preferably 0.005 0.4 mmol/1 and very particularly preferably 0.01 0.03 mmol/1, in the electrolyte used.

As an alternative or in addition, reaction products of aliphatic or aromatic amines with compounds bearing epoxy groups, in particular epihalohydrins such as epichlorohydrin, can likewise be employed as additive in the electrolyte. It may be pointed out that the reaction product spoken of here is formed exclusively from the amine and the compound bearing epoxy groups . Such compounds are already known from the prior art. In this respect, reference may be made to the literature cited at the outset, in particular WO 0028108, whose disclosures are incorporated by reference here. The reaction product between the two classes of compound can advantageously be formed at a ratio of epoxy derivative to amines of from 1 : 1 to 1 : 5. Particular preference is given to a range from 1 : 2 to 1 : 4, very particularly preferably about 1 : 3 (based on molar amounts) . As aliphatic or aromatic amines, it is here possible to select all compounds which a person skilled in the art would consider in this context. Those which are disclosed in the literature cited at the outset are particularly suitable. As such amines, particular preference is given to those selected from the group consisting of ethylenediamine, diethylenetriamine, triethylenetetramine, pentaethylenehexamine, tetraethylenepentamine, hexaethyleneheptamine, heptaethyleneoctamine, imidazole, piperazine, diaminopropylamine . The reaction products under consideration here are used in a concentration of 0.00001 - 2 g/1, particularly preferably 0.0001 - 0.5 g/1 and very particularly preferably 0.0001 - 0.1 g/1 and most preferably 0.0001 - 0.05 g/1, in the electrolyte used. It is advantageous for the reaction product of a polyamine containing 4 - 6, in particular 5, amino groups with epichlorohydrin in a molar ratio of about 1 : 3 or of a tertiary amine with epichlorohydrin in a molar ratio of about 1 : 1 to be present in the electrolyte.

The current density established in the electrolyte between the cathode and the anode during the deposition process can be selected by a person skilled in the art according to the efficiency and quality of the deposition. Depending on the application and the type of plating plant, the current density in the electrolyte is advantageously set to from 0.5 to 100 A/dm². The current densities can optionally be increased or decreased by matching to the plant parameters such as structure of the coating cell, flow rates, anode and cathode conditions, etc. A current density of 1 - 100 A/dm², preferably 2 - 90 A/dm² and very particularly preferably 4 - 80 A/dm², is advantageous. The pH range can be set within the region mentioned at the outset by a person skilled in the art. A pH of the electrolyte of from about 4 to 5, particularly preferably about 4.5, is advantageous.

When using the electrolyte, it is possible to employ various insoluble anodes. As insoluble anodes, preference is given to using ones composed of a material selected from the group consisting of platinated titanium, graphite, iridium-transition metal mixed oxide and a specific carbon material ("diamond- like carbon", DLC) or combinations of these anodes. Particular preference is given to mixed oxide anodes composed of iridium-ruthenium mixed oxide, iridium- ruthenium-titanium mixed oxide or iridium-tantalum mixed oxide. Further suitable anode materials may be found in Cobley, A.J. et al . (The use of insoluble Anodes in Acid Sulphate Copper Electrodeposition Solutions, Trans IMF, 2001, 79(3), pp. 113 and 114) . If insoluble anodes are used, a particularly preferred embodiment of the process is obtained when a direct contact membrane anode as described in DE 102010055143 is used. In the electrolyte according to the invention, anionic and non-ionic surfactants, e.g. polyethylene glycol adducts, fatty alcohol sulphates, alkylsulphates , alkylsulphonates , arylsulphonates , alkylarylsulphonates and heteroarylsulphates and salts and derivatives thereof, can typically be used as wetting agents (see also: Kanani, N: Galvanotechnik; Hanser Verlag, Munich Vienna, 2000; page 84ff) .

As suitable brighteners, it is possible to use, for example as described in DE 2355581 and DE1000 7325 Al, 3- (4-imidazolyl) acrylic acid, 3- pyridylhydroxymethanesulphonic acid, pyridine, pyridinesulphonic acids, pyridinecarboxylic acids, nicotinamide, 3- ( 3-pyridyl ) acrylic acid, quinolinesulphonic acid, 3-aminopyridine, 2,3- diaminopyridine, 2, 3-di (2-pyridyl) pyrazine, 2-

(pyridyl) -4-ethanesulphonic acid, 1- (3- sulphopropyl ) pyridinium betaine, 1- (3- sulphopropyl ) isoquinolinium betaine and salts and derivatives thereof.

It may be pointed out that the process of the invention preferably ensures exclusively a deposit of gold or gold alloys by the use of electric current. The use of further reducing agents can therefore be entirely dispensed with, which is advantageous. Likewise, it is not necessary to use any compounds selected from the group consisting of arsenic compounds, salicylates, polyvinylpyrrolidines , nitrogen- or sulphur-containing carboxylic acids, phosphonic acids and alcohols and aromatic nitro compounds in the process of the invention . Figures :

Fig. 1: shows the deposition rate in [mg/Amin] in the hard gold electrolyte using various concentrations of the additive according to the invention (reaction product of epihalohydrin with an aliphatic amine = epichlorohydrin and pentaethylenehexamine) .

Fig. 2: shows the deposition rate [mg/Amin] from a hard gold electrolyte with and without additive (working range expansion by means of the reaction product of epichlorhydrin and dimethylaminopropylamine) .

Fig. 3: shows the deposition rate [mg/Amin] from a hard gold electrolyte with and without addition of a polyamine having 4 ethylene units ( tetraethylenepentamine) .

Fig. 4: shows the deposition rate [mg/Amin] from a hard gold electrolyte with and without addition of a polyamine having 3 ethylene units ( triethylenetetramine) .

Fig. 5: shows the comparison between polyamines having 3 and 4 ethylene units from Examples 3 and 4.

It is clear from the graphs depicted that in the low current density range below 4 A/dm² the additives have a tremendous influence on the deposition rate of the hard gold electrolyte. Here, both the appropriate polyamines alone and the reaction products of epichlorohydrin and an amine have an inhibiting effect on hard gold deposition. The use of the optimal amount of inhibiting substance in the low current density range from 0.5 to 4 A/dm² enables the deposition rate to be reduced by up to 80% from about 80 to 15 mg/Amin. This leads to less to no hard gold being deposited in these regions, which ultimately helps to save expensive noble metal and thus reduce the cost of applying the coating. The process described here and that described with all its advantageous and preferred embodiments is therefore suitable for producing electric contacts.

Examples :

Illustrative electrolyte 1: Aqueous hard gold electrolyte with reaction product of epihalohydrin and an aliphatic amine

100 g/1 of a mixture of citrate and citric acid

200 mg/1 of Co as Co sulphate

12 g/1 of Au as [K[Au(CN)₂]

1 ml/1 of wetting agent

2 g/1 of brightener

0 - 0.04 g/1 of the reaction product of epichlorohydrin and pentaethylene- hexamine

pH 4.5 / 60°C

(see Fig. 1)

Illustrative electrolyte 2:

Aqueous hard gold electrolyte with expansion of the working range by means of a reaction product

180 g/1 of a mixture of citrate and citric acid

500 mg/1 of Co as Co citrate

15 g/1 of Au as [K[Au(CN)₂]

1 ml/1 of wetting agent

6 g/1 of brightener

0 - 0.6 g/1 of the reaction product of epichloro- hydrin and dimethylaminopropylamine pH 4.0 / 60°C

(see Fig. 2) Illustrative electrolyte 3:

Aqueous hard gold electrolyte with polyamine

120 g/1 of a mixture of citrate and citric acid

100 mg/1 of Co as Co sulphate

10 g/1 of Au as [K[Au(CN)₂]

1 g/1 of brightener

0 - 0.03 mmol/1 of polyamine: tetraethylenepentamine pH 4.2 / 60°C

(see Fig. 3)

Illustrative electrolyte 4 (not according to the invention) :

Hard gold electrolyte with ineffective polyamine

120 g/1 of a mixture of citrate and citric acid

100 mg/1 of Co as Co sulphate

10 g/1 of Au as [K[Au(CN)₂]

1 g/1 of brightener

0 - 0.07 mmol/1 of polyamine: triethylenetetramine pH 4.2 / 60°C

(see Fig. 4)

General experimental description:

Process sequence for the plating experiments in the abovementioned illustrative electrolytes:

Polished brass plates, preplated with gold, were used as substrates. These were cleaned and coated as follows : 1. Cathodic degreasing 30 sec / 5V

2. Rinsing 3. Dipping in acid 10 sec

4. Rinsing

5. Coating of the substrates in the hard gold electrolyte was carried out on a 1 1 scale with horizontal movement of the goods (5 m / min) and agitation of the electrolyte (200 rpm / 6 cm stirrer) at various current densities

6. Rinsing

7. Drying of the coated substrates using compressed air

After the drying operation, the deposition rate was determined via the weight difference of the substrates (weight before and after coating) and is depicted in the following graphs. (See Fig. 1 - 5)

Claims

Process for the selective, electrolytic deposition of gold or a gold alloy using an aqueous electrolyte having a pH of from 3 to 6 and comprising : gold ions in the form of a soluble complex; optionally further ions selected from the group consisting of cobalt, nickel, iron or mixtures thereof; - aliphatic polyamines which are soluble in the electrolyte and have at least 4 ethylene units and at least 2 primary or secondary amine groups and/or reaction products exclusively of aliphatic or aromatic amines with compounds bearing epoxy groups in a molar ratio of from

1:1 to 1:5; by dipping the substrate as cathode into the electrolyte and setting a sufficient current between an anode which is in contact with the electrolyte and the cathode so that corresponding deposition takes place.

Process according to Claim 1, characterized in that the aliphatic polyamines which are soluble in the electrolyte are used in a concentration of 0.001 - 0.5 mmol/1 and/or the reaction products of aliphatic or aromatic amines with epichlorohydrin are used in a concentration of 0.00001 - 2 g/1 in the electrolyte. Process according to Claim 2, characterized in that the reaction product of a polyamine containing 4-6 amine groups with epichlorohydrin in a molar ratio of about 1:3 or of a tertiary amine with epichlorohydrin in a molar ratio of about 1:1 is present in the electrolyte.

Process according to one or more of the preceding claims , characterized in that the current density is set to from 1 to 100 A/dm^'

Process according to one or more of the preceding claims , characterized in that the pH of the electrolyte is set to a value of from about 4 to 5.