GB2028873A - Gold Alloy Electroplating Bath and Method - Google Patents

Abstract

A gold alloy electroplating bath which is particularly useful in the production of electrical connectors or contacts for use in electronic apparatus contains a soluble gold cyanide, cadmium in the form of a bath soluble cadmium compound, complex or chelate, and to provide a small amount of carbon in the gold electrodeposit obtained from the bath, an amine or imine which will reduce the cathode efficiency of the bath by at least 15%. The amine may be an alkylpolyamine or polyalkyleneimine. Electrodeposits which can be obtained from the bath consist of a gold- cadmium-carbon alloy which has a stable low contact resistance which remains substantially constant even at elevated temperatures over prolonged periods, which renders said alloy of particular advantage in the manufacture of electrical connectors and electrical contacts. The bath may also contain an organo-phosphorus compound.

Description

SPECIFICATION Gold Alloy Electroplating Bath and Method This invention relates to the electrodeposition of gold alloys and is concerned with a gold alloy electroplating bath and method which are especially useful in the production of electrical connectors and contacts such as those used in electronic apparatus, for example telecommunications and computer equipment.

Electrodeposited gold is widely used as a contact material for sliding contacts of separable connectors in electronic low-energy systems requiring high reliability, such as telecommunications and computer equipment, since gold, because of its chemical stability, maintains the contact surface free of corrosion and other surface films.

From the point of view of optimum solderability, freedom from porosity and low contact resistance (i.e. electrical resistance of the contact surface), pure 24 carat gold electrodeposits are the most desirable. However, because constant movement of connectors with mated parts results in rapid wear of the electrodeposited coating and also because the pure 24 carat gold electrodeposits are soft and are subject to galling or seizing, pure gold electrodeposits are undesirable for sliding contacts. It is for this reason that, to improve wear and sliding characteristics, electrodeposited gold alloyed with cobalt or nickel has been used for many years in the telecommunication and computer fields for connectors and contacts.

Because of the necessity of maintaining constant low contact resistance over prolonged periods of time, particularly when the parts are subjected to operation at high ambient temperatures, the gold-cobalt and gold-nickel alloys have proved undesirable since under such conditions the contact resistance increases markedly. In addition electrodeposited alloys of gold with concentrations of 0.1 5-0.3% of cobalt or nickel pose difficult soldering problems and have a tendency for porosity.

British Specification No. 1,461,474 is concerned with the problems encountered with thin (less than 5 microns) electrodeposits of gold cobalt alloys and offers as an improvement the use of gold-cadmium, gold-tin, or gold-antimony alloys, wherein the alloy comprises 0.1-5% by weight of the alloying metal. The specification states that the most desirable alloy was the gold cadmium alloy and that it is especially effective when plated directly on copper. However, Specification No. 1,461,474 discloses that the gold-cadmium alloy was plated from proprietary gold sulphite-based solution. Gold-cadmium deposits obtained from such sulphite-based electroplating baths are however soft and have poor sliding and wear resistance, and are therefore unable to compete in this respect with the electrodeposited gold-cobalt alloys.Thus the gold-cadmium alloys as deposited from gold sulfite baths are not practical for electrical connectors which are subject to sliding motion.

Another publication disclosing gold sulfite electroplating baths in U.S. Patent 3,883,409.

U.S. Patent 2,967,135 discloses gold and gold-alloy cyanide electroplating baths containing amines. One of the stated purposes of adding these amines to gold or gold alloy baths is to improve the brightness and hardness of the deposits. Some of the amines disclosed are alkylpolyamines. Although the disclosure in this patent includes, broadly, gold alloys, goldcadmium alloys are not specifically mentioned.

An article by G.B. Munier, in PLATING, October, 1969, pp. 1151-1157, entitled "Polymer Codeposition With Gold During Electroplating" is also of interest, as the process there discloses carbon determination as a measure of the degree of polymer contamination in gold platings.

We have now found that if a small amount of carbon is present in a gold-cadmium alloy, electrodeposits of such a gold-cadmium-carbon alloy unexpectedly have a stable low contact resistance which, unlike the known gold-cobalt and gold-nickel alloys, remains substantially constant even at elevated temperatures over prolonged periods of time, which property renders such an alloy of particular advantage in the production of connectors and contacts to be used in electronic equipment.We have also found that such goid-cadmium-carbon alloy electrodeposits having said advantageous property can be obtained from a gold cyanide-cadmium electroplating bath by incorporating in the bath an amine or imine which will reduce the cathode efficiency of the bath by at least 1 5% compared with the cathode efficiency of the bath in the absence of the amine or imine, whereas the use of such a amine or imine in a gold sulphite-cadmium electroplating bath will not result in the formation of such a gold-cadmium-carbon alloy.

Thus in accordance with one aspect of the invention, there is provided an aqueous electroplating bath suitable for use in electrodepositing a gold alloy on to an electrically conductive substrate for an electrical connector or contact, said bath comprising a soluble gold cyanide, from 0.01 to 1.0 g/l of cadmium as a bath soluble cadmium compound, complex or chelate, and an amine or imine capable of and in an amount sufficient to reduce the cathode efficiency during electroplating by at least 15%.

In accordance with another aspect of the invention, there is provided a method of producing an electrical connector or contact, which comprises electrodepositing on to an electrically conductive substrate from an aqueous electroplating bath in accordance with the invention a gold-cadmium-carbon alloy containing from 0.1 to 5% by weight of cadmium and from 0.1 to 0.7% by weight of carbon.

In accordance with a further aspect of the invention, there is provided a novel gold alloy consisting essentially of from 0.1 to 5% by weight of cadmium, from 0.1 to 0.7% by weight of carbon and the balance gold apart from any identical elements and impurities.

The carbon content of the electrodeposited alloy is from 0.1 to 0.75% by weight. A carbon content within this range can be obtained by adding to a gold cyanide-cadmium electroplating bath an amine or imine which can reduce the cathode efficiency of the gold cyanide-cadmium bath by at least 15% and in an amount sufficient to ensure the carbon content desired. The preferred carbon range is from 0.2 to 0.35%.

Most advantageously the alloy contains about 0.3% of carbon.

The amines or imines useful according to this invention comprise those which are capable of reducing and will reduce the cathode efficiency of the gold cyanidecadmium bath by at least 15% and preferably by 25% and which are soluble in the bath, for example alkylpolyamines and polyalkyleneimines. Thus, assuming that an electroplating bath containing, for example, gold cyanide and cadmium sulphate has a cathode efficiency close to 100%, the amine or imine added should reduce the cathode efficiency to at least about 85%. Of course, the cathode efficiency should not be reduced to an extent where operation of the bath becomes impracticable but the efficiency can be quite low.For example, when polyethyleneimine is used at 5 ml/l in a bath containing 10 g/l of gold as a gold cyanide complex and 0.25 g/l of cadmium as cadmium sulphate, the cathode efficiency is reduced to about 28 percent. For practical reasons, however, it is advantageous to operate at higher cathode efficiencies and therefore alkylpolyamines, such as triethylene tetramine, which reduces cathode efficiency to about 75 to 80%, and tetraethylene pentamine are preferred. Other preferred amines are the reaction products of the amines, such as tetraethylene pentamine, with an epihalohydrin.

These types of reaction products, even though they contain oxygen, are intended to come within the scope of the terms amines, imines, alkylpolyamines and polyalkyleneimines as used herein.

The amount of the amine or imine added to the gold cyanide-cadmium baths should be sufficient to give a gold-cadmium deposit containing from 0.1 to 0.7% by weight of carbon. The use of 22 g/l of triethylene tetramine or tetraethylene pentamine, or 5 ml/l of polyethyleneimine will produce a gold-cadmium alloy containing about 0.3% carbon. The exact amount of the amine or imine required to produce the alloy in accordance with this invention can be determined by routine experimentation and will depend upon the particular gold-cadmium bath used, the amine or imine used, the presence of other additives, such as phosphates, organo phosphorus compounds, operating conditions, and so forth. Generally from 5 to 80 g/l of the amine or imine may be present.

The cadmium content of the electrodeposited alloy is from 0.1 to 5% by weight, preferably 0.7 to 2% and most advantageously about 1%.

The gold may be present in the baths of this invention in an amount of from 2 to 50 g/l, as an alkali metal gold cyanide, preferably potassium gold cyanide, or ammonium gold cyanide. The cadmium should be present in an amount of from 0.01 to 1.0 g/l, as a bath soluble cadmium compound, complex, or chelate, as is well known in the art. For example, cadmium may be present in the bath as the sulphate or chloride or as a complex or cheiate with ethylenediamine tetraacetic acid (EDTA) or phosphorus or aminophosphorus compounds.

The electroplating baths of this invention can contain other materials so long as they do not interfere with the production of the desired goldcadmium-carbon alloy. The conductivity of the electroplating baths can be enhanced by adding known electrically conductive compounds, for example boric acid and potassium dihydrogen phosphate. The compounds may be present in any amount up to saturation as required.

Organo-phosphorus compounds or water soluble salts thereof may advantageously be present in the electroplating baths since they also act as conductive compounds, but more importantly they render the plating baths more tolerant to metallic impurities which might build up in the bath during its use. The organophosphorus compound is preferably one which does not contain a hydrocarbon radical containing more than 5 carbon atoms directly linked to the phosphorus atom.

The organo-phosphorus compounds which can be present in the bath may be chosen from those represented by the general formula:

in which Fi' is hydrogen or a C 5 alkyl radical, R is a C15 alkylene radical, and n is an integer from 1 to 3, or an alkylenediaminotetra (alkylenephosphonic acid) or 1 -hydroxyalkylidene- 1, 1-diphosphonic acid, the alkylene and alkylidene groups of which contain from 1 to 12 carbon atoms, or a water-soluble salt thereof.

Specific examples of such organo-phosphorus compounds are ethylenediaminetetra (methylenephosphonic acid), aminotrimethylenephosphonic acid, 1 hydroxyethylidene-1, 1 -diphosphonic acid, and diethylenetriaminepenta (methylenephosphonic acid).

As an alternative to the organo-phosphorus compound, there may be used a conventional complexing agent, such as, for example, nitrilotriacetic acid, glycine, oxalic acid, gluconic acid, citric acid, or sulphamic acid.

The pH of the electroplating bath may be adjusted to a value which is advantageously in the range from 6 to 9 and preferably from 6.5 to 8.

High free cyanide content in the baths of this invention should generally be avoided. More advantageous results have been obtained when the free cyanide content is maintained below about 5 g/k and preferably close to zero g/l.

The electroplating process can be effected at conventional temperatures in the range from 30 to 800 C. and conventional cathode current densities in the range from 0.1 to 50 amps/dm2.

In addition to their stable contact resistance at elevated temperatures, the alloy electrodeposits obtained by this invention are compressively stressed so that they are not prone to spontaneous stress cracking. They also exhibit excellent wear resistance which is superior to or at least equal to that possessed by electrodeposits obtained with the conventional gold-nickel and gold-cobalt acid cyanide baths.

It should be noted that the main alloy components, gold, cadmium and carbon, may not in practice add up to 100%. With regard to the alloy electrodeposits produced as described in the following Examples, these minor differences are made up by the presence of impurities, such as nitrogen, oxygen, potassium, sodium, iron and lead. Other metals such as silver, lead, tin, copper, or zinc, may be deliberately added to the bath in minor amounts since they may improve the brightness somewhat. The presence of other elements in minor amounts, such as selenium and bismuth, even though they have little or no apparent effect on the alloy of this invention, are not excluded.

The following Examples illustrate the invention.

Example 1 An aqueous electroplating bath was prepared by dissolving in distilled water the following constituents: Gold (as KAu (CN)2) 10 g/l Cadmium (as 3CdSO4.8H2O) 0.25 g/l Boric acid 30 g/l Potassium dihydrogen phosphate 80 g/l Tetraethylene pentamine 22 g/l Ethylenediaminetetra (methylene phosphoric acid) 80 gIl The pH of the bath was adjusted by the addition of potassium hydroxide to a value of 7.0.

The foregoing electroplating bath was then used in a Hull cell fitted with a platinised titanium anode to form a lustrous gold alloy electrodeposit on a standard Hull cell panel at a temperature of 55-600C. with vigorous agitation, and with a current of 0.5 amps for 5 minutes. At a cathode current density of 1 amp/dm2, the cathode efficiency was about 75% and a lustrous gold alloy electrodeposit was formed to a thickness of 20 microns. This was found by assay to contain 98.0% by weight of gold, 1.0% by weight of cadmium and 0.3% by weight of carbon.

The deposit was tested for wear resistance against itself by submitting it to 250 reciprocal cycles along a 4 cm. track under a 200 g. load using a Post Office type probe and plaque. No measurable reduction in the thickness of the deposit could be observed by micro-section. The contact resistance remained substantially constant at high temperatures and for prolonged periods of exposure at this temperature. For example, when electrical contacts made by this method of this Example were subjected to a temperature of 1 25 C. for 1000 hours, the rise in contact resistance was negligible. The alloy electrodeposit had a contact resistance of about 2 milliohms under a 200 gram load and a hardness of about 1 60 on the Vickers Pyramid Number hardness scale (VPN).

Example 2 An aqueous electroplating bath was prepared by dissolving in distilled water the following constituents: Gold (as KAu(CN)2) 10 9/l Cadmium (as 3CdSO4.8H2O) 0.05 g/l Boric acid 30 g/l Potassium dihydrogen phosphate 80 g/l Tetraethylene pentamine 22 g/l The pH of the bath was adjusted to a value of 7.0 by the addition of potassium hydroxide. The bath had a cathode efficiency of 75% at 1 amp/dm2.

This bath, when used in the manner described in Example 1, gave a lustrous gold-cadmium alloy electro-deposit.

Example 3 An aqueous electroplating bath was prepared by dissolving in distilled water the following constituents: Gold (as KAu(CN)2) 10 g/l Cadmium (as 3CdSO4.8H2O) 0.05 g/l Boric acid 30 g/l Potassium dihydrogen phosphate 80 g/l Triethylene tetramine 22 g/l The pH of the bath was adjusted to a value of 7.0 by the addition of potassium hydroxide. The bath had a cathode efficiency of 75% at 1 amp/dm2.

This bath, when used in the manner described in Example 1, gave a lustrous gold-cadmium alloy electro-deposit.

Example 4 An aqueous electroplating bath was prepared by dissolving in distilled water the following constituents: Gold (as KAu(CN)2) 10 gIl Cadmium (as 3CdSO4.8H2O) 0.1 gIl Boric acid 30 g/l Potassium dihydrogen phosphate 80 g/l Tetraethylene pentamine 22 g/l Citric acid 100 gIl The pH of the bath was adjusted to a value of 7.0 by the addition of potassium hydroxide. The bath had a cathode efficiency of 70% at 1 amp/dm2.

The foregoing bath, when used in the manner described in Example 1, gave a lustrous goldcadmium alloy electrodeposit.

The gold deposits produced in each of the Examples 2, 3 and 4 contained amounts of gold, cadmium and carbon similar to those contained in the deposit of Example 1, and the electrical contacts produced had substantially the same advantageous properties as mentioned in Example 1.

Claims (16)

Claims

1. An aqueous electroplating bath suitable for use in electrodepositing a gold alloy on to an electrically conductive substrate for an electrical connector or contact, the bath comprising a soluble gold cyanide, from 0.01 to 1.0 g/l of cadmium as a bath soluble cadmium compound, complex or chelate, and an amine or imine capable of and in an amount sufficient to reduce the cathode efficiency during electroplating by at least 1 5%.

2. A bath as claimed in Claim 1, wherein the amine is an alkylpolyamine.

3. A bath as claimed in Claim 2, wherein the amine is triethylene tetramine, tetraethylene pentamine, or the reaction product of triethylene tetramine or tetraethyiene pentamine with an epihalohydrin.

4. A bath as claimed in Claim 1, wherein the imine is a polyalkyleneimine.

5. A bath as claimed in any preceding claim, wherein the amine or imine is present in an amount of from 5 to 80 g/l.

6. A bath as claimed in any preceding claim, wherein the soluble gold cyanide is an alkali metal or ammonium gold cyanide present in an amount of from 2 to 50 girl.

7. A bath as claimed in Claim 6, wherein the gold cyanide is potassium gold cyanide.

8. A bath as claimed in any preceding claim, the bath having a pH in the range from 6 to 9.

9. A bath as claimed in any preceding claim and further comprising an organo-phosphorus compound or a water-soluble salt thereof.

10. A bath as claimed in Claim 9, wherein the organo-phosphorus compound is one which does not contain a hydrocarbon radical containing more than 5 carbon atoms directly linked to the phosphorus atom.

11. A method of producing an electrical connector or contact, which method comprises electrodepositing on to an electrically conductive substrate from an aqueous electroplating bath as claimed in any preceding claim a gold-cadmiumcarbon alloy containing from 0.1 to 5% by weight of cadmium and from 0.1 to 0.7% by weight of carbon.

12. An electrical connector or contact whenever produced by the method claimed in Claim 11.

13. A gold alloy consisting essentially of from 0.1 to 5% by weight of cadmium, from 0.1 to 0.7% by weight of carbon and the balance gold apart from any incidental elements and impurities.

14. An aqueous electroplating bath substantially as described in any one of the foregoing Examples.

15. A method of electrodepositing a goldcadmium-carbon alloy on to an electrically conductive substrate, substantially as described in any one of the foregoing Examples.

16. A gold alloy whenever produced by a method substantially as described in any one of the foregoing Examples.