Cyanide-free electroplating bath for the deposition of gold and

Details Cyanide-free electroplating bath for the deposition of gold and Cyanide-free electroplating bath for the deposition of gold and

1998-06-02

2000-12-26

Mayekar, Kishor

Electrolysis: processes, compositions used therein, and methods

Electrolytic coating

Depositing predominantly alloy coating

205250, 205251, 205267, 106 124, C25D 348

Patent

active

06165342&

DESCRIPTION:

BRIEF SUMMARY
DESCRIPTION

This invention relates to a cyanide-free, electroplating bath for the deposition of gold and gold alloy coatings, containing 0.5 to 30 g/l of gold in the form of a complex of a sulphurous compound, 0 to 50 g/l of an alloy metal in the form of water-soluble compounds of silver, copper, indium, cadmium, zinc, tin, bismuth, arsenic and/or antimony, 1 to 200 g/l of the free sulphurous compound, 0 to 200 g/l of conductive and buffer salts in the form of alkali metal borates, phosphates, citrates, tartrates and/or gluconates and optionally wetting agents and brighteners.
Today, electrodeposition of gold is primarily performed using electrolytes based on gold cyanide complexes which, at least under alkaline conditions, also contain relatively large quantities of toxic alkali metal cyanides. Under acidic and neutral conditions, the cyanide liberated on electrolysis escapes at least in part as highly toxic hydrocyanic acid. Apart from severe toxicity, baths containing cyanide also occasion problems when detoxifying the cyanide, which, in practice, is predominantly performed with alkali metal hypochlorite. This may result in the formation of so-called adsorbable halogen compounds (AOX) which cause waste water treatment problems. Efforts have accordingly long been made to produce gold electroplating baths without using the toxic complexing agent cyanide. However, with the exception of baths based on gold sulphite complexes, it has not hitherto proved possible to produce an industrially viable bath.
However, such gold/sulphite complexes have the disadvantage of low stability and, even with a large excess of free sulphite ions, form elemental gold once the solution has stood for a relatively extended period, so rendering the solution unusable.
Electroplating baths which contain the gold in the form of a thiosulphate complex (DE-PS 24 45 538) are also not substantially more stable. Like other known gold complexes with sulphurous compounds, they decompose partially if they are kept for a relatively extended period. In published application EP 0 611 840, the gold thiosulphate complexes are thus stabilised by the addition of sulphinates. The current density usable in these baths is limited and decomposition generally occurs at current densities of above 1 A/dm.sup.2. Moreover, these baths usually cause an odour nuisance.
The object of the present invention was accordingly to provide a cyanide-free electroplating bath for the deposition of gold and gold alloy coatings, containing 0.5 to 30 g/l of gold in the form of a complex of a sulphurous compound, 0 to 50 g/l of an alloy metal in the form of a water-soluble compound of silver, copper, indium, cadmium, zinc, tin, bismuth, arsenic and/or antimony, 1 to 200 g/l of the free sulphurous compound, 0 to 200 g/l of conductive and buffer salts in the form of alkali metal borates, phosphates, citrates, tartrates and/or gluconates and optionally wetting agents and brighteners, which was also to be stable over a relatively extended period, to be operated at current densities of above 1 A/dm.sup.2 and, to the greatest extent possible, to be neutral in odour.


SUMMARY OF THE INVENTION

This object is achieved according to the invention by the bath's containing the gold complex of a mercaptosulphonic acid, a disulphidesulphonic acid or mixtures of these compounds as the sulphurous compound.
The salts, preferably the alkali metal salts, of these compounds are also suitable.
The baths preferably contain 1 to 200 g/l of the free sulphurous compound or the alkali metal salts thereof in excess of the stoichiometric composition of the corresponding gold complex.
It is furthermore advantageous for the baths to contain 0.01 to 10 g/l of wetting agents in the form of surfactants and 0.1 to 1000 mg/l of brighteners in the form of selenium and/or tellurium compounds.
The bath is advantageously operated at a pH value of 7 to 12.
The sulphurous compounds which are suitable for the baths according to the invention exhibit good solubility in water and elevated stability, combined with a

REFERENCES:
patent: Re35513 (1997-05-01), Nobel et al.
patent: 3057789 (1962-10-01), Smith
patent: 3929595 (1975-12-01), Biberbach et al.
patent: 4192723 (1980-03-01), Laude et al.
Chemical Abstract 89: 119754, Oct. 1978.

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