GB2153386A

GB2153386A - Gold alloy plating bath

Info

Publication number: GB2153386A
Application number: GB08402617A
Authority: GB
Inventors: Gerardus Antonius Somers; John Richard Lovie; Jan Jaques Marie Hendriks
Original assignee: OMI International Corp
Current assignee: OMI International Corp
Priority date: 1984-02-01
Filing date: 1984-02-01
Publication date: 1985-08-21
Also published as: FR2558853A1; NL189416B; NL8500279A; BE901632A; JPS60187696A; GB8402617D0; CA1272160A; FR2558853B1; GB2153386B; NL189416C; DE3502995A1; DE3502995C2; JPS6229517B2

Abstract

A citrate-free electroplating bath for the high speed deposition of gold alloy comprises a source of gold (for example gold (I) potassium cyanide), a source of alloying metal (for example nickel sulphate), oxalic acid and formic acid. As citrate is not used, higher plating speeds may be obtained and precipitates of certain citrate salts (for example nickel citrate) are avoided.

Description

SPECIFICATION Gold alloy plating bath This invention relates to a bath for electroplating gold alloys and more particularly to a bath for the high speed plating of a hardened gold alloy.

It is known to co-deposit various metals with gold during electrodeposition to give a harder electrodeposit than is achievable with pure gold. It is usual for a complexing agent for the alloying metal to be present and in a known gold-cobalt plating bath, citrate is present as a complexing agent and as a buffering agent for the bath.

It might be expected that a bath for the electrodeposition of a gold-nickel alloy might be derivable from a bath for the electrodeposition of a gold-cobalt alloy merely by replacing the source of cobalt with a source of nickel.

However, if this is tried, it is found that the plating speed is impaired and that unwanted precipitates are obtained in the bath.

It has surprisingly been found that citrate appears to be the agent responsible for these deficiencies, especially when the bath is for depositing gold-nickel alloys.

According to a first aspect of the invention, there is therefore provided a citrate-free bath for the electrodeposition of a gold alloy, the bath comprising a bath soluble source of gold, a bath soluble source of alloying metal, oxalic acid and formic acid.

It will be understood that throughout the specification reference to a weak organic acid (such as oxalic acid or formic acid) and its anion are used interchangeably: the nature of the species present will of course depend on the pH of the bath.

The bath soluble source of gold is preferably a gold (I) salt, which could for example be an alkali metal gold (I) cyanide or ammonium gold (I) cyanide.

The gold may be present in an amount of from 4 to 50 g/l, preferably 4 to 20 g/l, for example 8 to 12 g/l.

It is particularly preferred that the alloying metal be nickel. In such a case, the bath soluble source of alloying metal preferably comprises nickel sulphate. The nickel may be present in an amount of from 0.5 to 20 g/l, preferably 1 to 5, for example 2 to 3 g/l.

The oxalic acid is believed to have two primary functions in the bath. First, it is believed to act as a complexing agent for the nickel ions; secondly, it acts as a buffering agent for the bath. The oxalic acid will therefore normally be present in an amount sufficient for it to fulfil these functions. Whe choosing the amount of oxalic acid to be used, the relatively limited solubility of the acid at lower temperatures must be taken into account.

Preferred baths in accordance with the invention are those in which oxalic acid is present in an amount of from 20 to 100 g/l, preferably 30 to 80 g/l, for exmaple 40 to 60 g/l.

More specifically, the oxalic acid may be present in an amount from 45 to 55 g/l, for example 50 g/l, which was found to be the most suitable concentration for use. Small variations of oxalic acid concentration around this leval has hardly any effect on the plating speed at which the bath may be operated.

The formic acid is believed to be an essential ingredient for obtaining high plating speeds. It appeared to operate an an antiburning agent or an inhibitor for metal transport in the high current density areas. The formic acid may be present in an amount of from 20 to 100 ml/l, preferably 30 to 80 ml/l, although from 30 to 40 ml/l appeared to be the optimum concentration range. A particularly preferred concentration of formic acid was 35 ml/l.

A pH adjusting agent, for example potassium hydroxide or another alkali metal hydroxide, may be present in the bath, which preferably a final pH of from 3.9 to 5.1, more particularly 4.1 to 4.9.

It is not necessary for the bath to contain any further ingredients and, accordingly, in a second aspect the invention provides a citratefree bath for the electrodeposition of gold alloy consisting substantially only of a bath soluble source of gold, a bath soluble source of alloying metal, formic acid, oxalic acid, a pH adjuster if necessary and water.

According to a third aspect of the invention, there is provided a process for plating gold alloy, particularly gold nickel, on a substrate, especially at high speed, the process comprising immersing the substrate to be plated as a cathode in a bath in accordance with the first or second aspect of the invention and passing current between the cathode and an anode in the bath.

The bath may be operated at a temperature of from 20' to 80"C, preferably 30 to 70"C.

for example 40 to 60'C.

After plating, the duly plated substrate is preferably rinsed in softened or deionised water so as to avoid deposits of calcium oxalate.

In laboratory scale plating processes carried out in accordance with the invention in agitation equipment, it was found that plating speeds of 3.8 microns per minute at current densities of 1 3 ASD could be achieved without loss of brightness. With the use of suitable equipment with higher solution and handling capabilities, for example, jet plating, much higher plating speeds at much higher current densities (up to 200 ASD) may be achievable.

Baths in accordance with the present invention may be particularly suitable for use in high speed plating.

In the commercial use of baths of the invention, current densities of from 10,20,50 or 100 to 200 ASD may be used to take advantage of the high speed plating potential of the baths. When plating at high speeds, it is generally desirable to agitate the solution, preferably to achieve such a high level of agitation that the solution is turbulent.

For a better understanding of the invention, the following non-limiting examples are given.

EXAMPLE 1 A bath having the following composition was made up: INGREDIENT AMOUNT Gold as potassium gold (I) cyanide 10 g/l Oxalic acid. 2H2O 63 g/l Formic acid 40 Ml/I Nickel (as sulphate) 2 g/l Potassium hydroxide to pH 4.1 Distilled Water to 1 litre.

The bath formulated as above was placed in a laboratory scale turbulent agitation plating system. Electrolyte was pumped through two pipes into a 500 ml/l beaker and was directed through holes in the pipes onto the substrate, which was immersed as a cathode in the beaker. Electrolyte solution was pumped away through a third pipe in the beaker. The cathode is located between the two supply pipes and anodes are placed around the supply pipes at such a position that they do not disturb the solution flow.

The solution is pumped around the system at a flow rate of 5 litres per minute (measured with water at room temperature).

It was found that fully bright gold nickel electrodeposits up to about 5 microns in thickness could be deposited from the above bath a deposition rate of 3.8 microns per minute using a current density up to 1 3 ASD.

EXAMPLE 2 An electrolyte bath having the following composition was prepared: INGREDIENT AMOUNT Gold as potassium gold (I) cyanide 10 g/l Oxalic acid. 2H2O 50 g/l Formic Acid 35 ml/l Nickel (as sulphate) 3 g/l Potassium hydroxide to pH 4.4 Distilled water to 1 litre Under the conditions described from Example 1, it was found that fully bright goldnickel electrodeposits could be obtained at a deposition rate of 3.75 microns per minute using a current density of up to 12 ASD.

EXAMPLE 3 With a bath as described above in Example 2, except that 20 ml/l formic acid and 2 g/l nickel were used, fully bright deposits were obtained at a plating speed of 2.5 microns per minute at a current density of 8 ASD Fully bright deposits were also obtained at a plating speed of 2.8 microns per minute (9ASD) and 3.1 microns per minute (10 ASD). In all cases, the temperature of the bath was 50"C.

EXAMPLE 4 Using an electroplating bath as prepared in Example 2, except that the concentration of formic acid was 40 ml/l and the concentration of nickel was 2.0 g/l, fully bright deposits of gold-nickel alloy were obtained at a plating speed of 3.0 microns per minute with a current density of 10 ASD. Fully bright deposits were also obtained at plating speeds of 3.3 microns per minute (1 1ASD) and 3.55 microns per minute (12 ASD). In all cases the bath temperature was 50"C.

Claims

1. A citrate-free bath for the electrodeposition of a gold alloy, the bath comprising a bath soluble source of gold, a bath soluble source of alloying metal, oxalic acid and formic acid.

2. A bath as claimed in Claim 1, wherein the bath soluble source of gold comprises a gold (I) salt.

3. A bath as claimed in Claim 2, wherein the gold (I) salt is an alkali metal or ammonium gold (I) cyanide.

4. A bath as claimed in Claim 1, 2, or 3, in which the gold is present in a concentration of from 4 to 50 g/l.

5. A bath as claimed in any one of Claims 1 to 4, in which the gold is present in an amount of from 4 to 20 g/l.

6. A bath as claimed in any one of Claims 1 to 5, in which the gold is presenf at a concentration of from 8 to 1 2 g/l.

7. A bath as claimed in any one of Claims 1 to 6, wherein the alloying metal is nickel.

8. A bath as claimed in Claim 7, wherein the bath soluble source of nickel comprises nickel sulphate.

9. A bath as claimed in Claim 7 or 8 in which the nickel is present in an amount of from 0.5 to 20 g/l.

10. A bath as claimed in Claim 7, 8 or 9 in which the nickel is present in an amount of from 1 to 5 g/l.

11. A bath as claimed in any one of Claims 7 to 10, in which the nickel is present at a concentration of from 2 to 3 g/l.

1 2. A bath as claimed in any one of Claims 1 to 11 in which the oxalic acid is present in an amount of from 20 to 100 g/l.

1 3. A bath as claimed in any one of Claims 1 to 12, in which the oxalic acid is present in an amount of from 30 to 80 g/l.

14. A bath as claimed in any one of Claims 1 to 1 3, in which the oxalic acid is present in an amount of from 40 to 60 g/l.

1 5. A bath as claimed in any one of Claims 1 to 14 in which the oxalic acid is present in a concentration of 50 g/l.

1 6. A bath as claimed in any one of Claims 1 to 1 5 in which the formic acid is present in an amount of from 20 to 100 ml/l.

1 7. A bath as claimed in any one of Claims 1 to 1 6 in which the formic acid is present in an amount of from 25 to 50 m/l.

1 8. A bath as claimed in any one of Claims 1 to 1 7 in which the formic acid is present in an amount of from 30 to 40 ml/l.

1 9. A bath as claimed in any one of Claims 1 to 1 8 comprising a pH adjuster.

20. A bath as claimed in Claim 1 9 wherein the pH adjuster is potassium hydroxide.

21. A bath as claimed in any one of Claims 1 to 20 wherein the pH of the bath is from 3.9 to 5.1.

22. A bath as claimed in any one of Claims 1 to 21 in which the pH of the bath is from 4.1 to 4.9.

23. A bath as claimed in any one of Claims 1 to 22 in which the pH of the bath is from 4.3 to 4.5.

24. A citrate-free bath for the electrodeposition of a gold alloy consisting substantially only of a bath soluble source of gold, a bath soluble source of alloying metal, formic acid, oxalic acid, a pH adjuster if necessary and water.

25. A process for plating gold alloy on a substrate the process comprising immersing the substrate to be plated as a cathode in a bath as claimed in any one of Claims 1 to 24 and a passing current between the cathode and an anode in the bath.

26. A process as claimed in Claim 25 including the subsequent step of rinsing the substrate in deionised, distilled or softened water.

27. A bath substantially as described with reference to any one of the examples.

28. A process for plating gold alloy on a substrate comprising immersing the substrate to be plated as a cathode in a bath as described in any one of the examples and a passing current between the cathode and an anode in the bath.

29. An article or other product whenever plated by means of a bath as claimed in any one of Claims 1 to 24 and 27 and/or by a process as claimed in Claim 25, 26, or 28.