GB2225026A

GB2225026A - Electroless gold plating composition

Info

Publication number: GB2225026A
Application number: GB8921796A
Authority: GB
Inventors: F Zoltan Mathe; Augustus Fletcher
Original assignee: American Chem & Refining Co; American Chemical and Refining Co Inc
Current assignee: American Chem & Refining Co; American Chemical and Refining Co Inc
Priority date: 1988-11-22
Filing date: 1989-09-27
Publication date: 1990-05-23
Also published as: GB8921796D0; JPH02159383A; DE3938653C2; FR2639654B1; FR2639654A1; DE3938653A1; JPH031383B2

Description

1 1 2.2 2 5 0 2 6 1 ELECTROLESS GOLD PLATING BATH AND METHOD OF USING THE

SAME The present invention is directed to electroless gold plating baths, and more particularly to -providing. reasonably stable electroless gold plating baths and methods for using and replenishing the same.

Autocatalytic or electroless gold plating baths are widely employed for the development of gold deposits on both conductive and non- conductiv substrates, particularly for electronics applications where optimum electrical properties in the deposit are desirable. To achieve the optimum electrical properties, it is desirable that the gold be of high purity, i.e., 99.9% or better, and that the deposit be substantially uniform over the surface of the workpiece.

Generally such electroless gold plating solutions have utilized alkali metal gold cyanide and free cyanide, and a water soluble borohydrate or a amine borane as the reducing agent. As the bath\is replenished, the cyanide concentration increases, and this has an unfavorable effect upon the deposition rate and the stability of the composition. In an article by Martin Ulrich Kittel and Christoph Julius Raub entitled "Elektrochemische Stabilitaetsbestimmung Reduktiv Arbeitender Goldelektrolyte" 2 published in Metalloberflaeche, Volume 41 (1-987) at pages 309-313, there is discussed the effect of various compounds as stabilizers in gold plating compositions. None of the compounds reported by the authors serves effectively to provide a stable electroless gold plating bath which could be replenished a number of times without adverse effect upon its performance-.

Accordingly, it is an object of the present invention to provide a novel and highly effective electroless gold plating composition which provides a useful rate of autocatalytic deposition of the gold upon the substrate, and which can be replenished a number of times without significantly adverse effect upon the plating rate or properties.

It is also an object to provide such an electroless gold plating composition which may be formulated readily and which is relatively Stable in an industrial plating environment.

Another object is to provide a method for autocatalytic deposition of substantially pure gold upon a workpiece utilizing a relatively stable composition which can be replenished easily a number of times without significantly adverse effect upon the plating rate.

SUMMARY OF THE INVENTION

It has now been found that the foregoing and related objects and advantages may be readily attained in an electroless gold plating composition comprising an aqueous solution of alkali metal gold cyanide sufficient to provide gold (calculated as 4 3 metal) in the amount of 1.0-16.6 grams per liter, and alkali metal cyanide in the amount of 3-110 grams per liter. These are also included in a boron compound selected from the group consisting of alkyl amine boranes, alkali metal borohydrides, and mixtures thereof, in the amount of 2-10 grams per liter, and alkali metal hydroxide in the amount of 10- 1100 grabs per liter. Lastly, there is provided 0.1-0.3 gramg per liter of a stabilizer having the formula 9-2 t02 wherein R, is - COOH, -OH, -CH 2 OH, or -SO 3 H (or an alkali metal salt thereof) R 2 is - COOH, -OH, -Cl, -H, (or an alkali metal salt thereof) and is disposed in the 2, 5, or 6 ring position -N02 is in the 3 or 4 ring position. The composition has a pH of 12.5-14.0, the weight ratio of OH_/CN7 is 4.0-10.0, and the oxidation reduction potential of the solution is -550 to -700 millivolts. Preferably, the boron compound is dimethyl amine borane i the amount of 4-7 grams per liter, and the stabilizer is m-nitrobenzene sulfonic acid or an alkali metal salt thereof.

4 Desirably, the composition has, at initial makeup, alkali metal ey&ftide in -the amoun.t, of 4.0-6;0.grams per litir-, and alkali zketal hydroxide '-in the amount of 40-50 grams per liter. Tho gold-'is prosent- in the amount of 4-5 grams per liter as calculated as gold metal. Desirably, the solution has a pH of about 13.4-14.0. The stabilizer is desirably added in small increment& during the use of the. -plating solution with the preferre7d condensation being 0.15-0.25 gram per liter.

In the method for use thereof, there is immersion plated upon the surface of a workpiece a thin deposit of immersion gold. The plated workpiece is then immersed in the alorementi oned el-ectroless gold plating composition for a period of time sufficient to plate thereon high purity gold in the desired thickness. Most desirably, the solution is mairktained at a temperature of about 85-95 OC.

The composition may be replenished when the gold content (as metal) has decreased to 1.5-3 grams per liter, with a replenisher formulation comprising:

(a) alkali metal gold cyanide in the amount of 70-90 grams-per liter (as metal); - (b) - alkali metal hydroxi-de in the amount of 1-10 grams per liter; and (C) stabilizer in the amount of 2-6 grams per liter.

T_ Desirably, the gold plating composition is prepared by first preparing an aqueous solution of the alkali metal hydroxide, alkali metal cyanide, alkali metal gold cyanide, stabilizer, and boron compound. This is heated to the operating temperature while monitoring the oxidation/reduction potential until a value of -550 to -700 millivolts is obtained after which the workpiece may be placed therein.

During the plating operation, the oxidation/reduction potential is monitored and stabilizer is desirably added in small increments of 0.05-0. 1 gram per liter of the plating solution to maintain the potential of the solution within the range of -550 to -700 millivolts.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

As previously indicated# the bath of the present invention essentially requires an alkali metal gold cyanide, alkali metal cyanide, alkali metal hydroxide sufficient to maintain the desired pH and to stabilize the cyanide ion, a boron compo und reducing agent, and a nitroaromatic compound as a stabilizer. These components must be maintained within certain ranges and/or ratios in order to maintain a stable composition and reasonably uniform plating rate.

Turning first to the gold component, potassium gold cyanide is preferred although sodium salt may also be employed. Lithium compounds generally involve unnecessary costs. The amount of the gold cyanide may vary within the range of 1-16.6 grams per liter 6 (calculated as metal), but the plating rate will be significantly effected'when the gold-content falls below 2.0 grams per liter. ideally, the gold content (as'mbtal) 'i's 'maintained within the range of'4-6 grams per liter.

The secofid component of the composition is a alkali metal cyan.ide sufficient to provide free cyanide in the bath. The amount of the cyanide compound may vary from as little 3 grams per liter to as much as 110 grams per liter as the bath is replenished from time to time. At makeup for the initial bath, the cyanide salt concentration is preferably in the range of about 4-10 grams per liter. The preferred cyanide salts are potassium cyanide, although sodium cyanide and lithium cyanide may also be employed.

Alkali metal hydroxide is required to provide the desired operating pH for the bath of 12.5-14, and is utilized to stabilize the cyanide and to participate in the reduction reaction with the boron compound. Thus, the ratio of hydroxide to cyanide should be within the range of 4.0-10.0. As in the case of the other salts, potassium hydr xide is preferred, although sodium hydroxide is a reasonable substitute therefor.

I The conventional boron compounds are used as the reducing agents in the composition. These may comprise alkali metal borohydrides and alkyl amine boranes within the range of 2-10 grams per liter and preferably 4 7 grams per liter. The preferred reducing agent is dimethyl amine borane, either alone or in combination with alkali metal borohydrides.

8 7 To provide the necessary stability for the composition, it is essential that there be included an organic stabilizer of the general formula wherein R 1 16 - COOH, -OH' -CH 2 OH, or -SO 3 H (or an alkali metal salt thereof), R 2 is - COOH, -OH, -Cl, -H, (or an alkali metal salt thereof and is disposed in the 2, 5, or 6 ring position, and the -No. group is in the 3 or 4 ring position, This stabilizer is incorporated in the amount of 0.1-0.3 gram per liter, and preferably in the range of 0.15-0.25 gram per liter. it has been observed that the stabilizer concentration in the solution may be depleted excessively between replenishment additions and this will cause the oxidation/reduction potential to exceed the limit of -700 millivolts. Accordingly, the potential is desirably monitored continuously and the stabilizer is added "in small increments of 0.05-0.1 gram per liter of the plating solution to maintain the oxidation/reduction potential within the operating range of -550 to -700 millivolts. As a result, the total amount of stabilizer added over the life of the solution may range as high as 10 grams per liter.

8 AS previously- indicated, the-"pH- of, the -'queoiis composition should - be within the rangfe'.of- IZ.5-14A:and pr7eferably 13.4-14.0.

TO obtain -a - desirable- 'plating rate,-. _the.. bath -should be maintained at a tempetatu.r-e'-of."8,5-950C,-and pre-ferably 88-930F.

Use- of the preferred compositions and temperatures will provide an effective plating rate of 3.75 - 6.75 microns per hour, and will produce a gold deposit of at least 99.9% purity, having a density of at least 19 grams per cc. (on the average) and a hardness of at least 85 Knoop (25 gram load maximum).

After the gold content- of the. solution diminishes to less than 2 grams per liter (as metal),,the plating-rate will begin to fall and it is necessary to replenish the composition. This is accomplished by adding alkali metal hydroxide, alkali gold cyanide and additional stabilizer. AS will be appreciated, the alkali metal hydroxide is -required to maintain the desired ratio of hydroxide to cyanide. Generally, the potassium hydroxide will be added to the gold repldni'dhet solution in an amount of 1-10 grams per liter, and the potassium gold cyanide at a range of 70-90 grams per liter (as gold metal). The amount of stabilizer added will be approximately 2-6 grams per liter. Generally, it has been found that the bath may be replenished up to ten turnovets before there is a significant loss of the desirable characteristics of the plating formulation. A turnover is defined as the plating out of the amount- of metal in a given volume of the solution.

9 In order to avoid contamination of the bath, the workpieces should be thoroughly cleaned before introduction thereinto in accordance with conventional gold plating practice.

When the workpiece is a synthetic resin or ceramic, it is necessary to initially produce an initial metallic deposit thereon and this will generally require etching with chromic acid, application of palladium/tin chloride, and immersion in an electroless copper or nickel bath.

Both such non-metallic workpieces, and metallic workpieces, must be subjected to an initial treatment to develop a immers ion gold strike. suitable compositions for developing the initial thin gold deposit include potassium gold cyanide, potassium dihydrogen phosphate and citric acid, and are maintained at a temperature of about 140-1600F. Following the deposition of the gold strike, the workpieces are rinsed, and then they may be introduced into the electroless gold plating compositions of the present invention to produce the desired deposit.

Illustrative of the efficacy of the present invention are the following specific examples, wherein all parts are parts by weight unless otherwise indicated.

EXAMPLE ONE

A prferred bath embodying the present invention was made by adding to a precleaned and leached tank, 43 grams potassium hydroxide, 4 grams of potassium cyanide,- 6 grams of potassium gold cyanide, 0.2 gram of mnitrobenzene sulfonic acid sodium salt, 6.5 grams methyl amine borane, and deionized water to produce 1 liter of solution. The ratio of potassium hydroxide to 'total cyanide as. potassium cyanide was 6.5, and the pH was 13.4.

The result-ant bath was heated to a temperature of about 91 0 C and the oxidation/reduction potential of the solution was monitored using an Orion Model SA 230 ORP meter and combination Redox Electrode Model 9678. W hen th e potential of the solution reached- -550 millivolts, -the solution was ready for use. EXAMPLE TWO The workpieces were flat.sheets of an alloy sold by Westinghouse Electric Company under the mark KOVAR and having a nominal composition of 29% ni!:kel, 17% cobalt, 0.3% manganesel and the balance iron. These sheets had a thickness of about 0.025 inch and were electrocleaned in a hot, caustic solution and then rinsed. The workpieces were then immersed in 50% by volume hydrochloric acid and rinsed, following after which they were introduced into a immersion plating bath comprised of potassium gold cyanide, potassium dihydrogen phosphate, and citric acid with a pH of approximately 2.5.. They were removed after they had developed ja uniform gold coloration upon the surface thereof.

These workpieces were then suspended in the bath of Example one, and%magnetic stirring was utilized to maintain agitation of the bath thereabout. The temperature of the bath was held at 910C.

1 1 1 11 After 20 minutes, the workpieces were removed from the bath, rinsed and dried. The deposit was found to have a thickness of approximately 72 microinches. The purity of the deposit was found to be 99.97. The deposit exhibited a satin matte finish and a lemon yellow color and, under microscopic examination, was uniform and amorphous.

EXAMPLE THREE

A Hull cell panel was thoroughly cleaned and immersed in the immersion gold plating solution of Example Two to -develop a uniform gold coloration thereover. It was rinsed and then suspended in the electroless gold plating composition of Example One for a period of 3.5 hours, following which it was removed, rinsed and dried.

A cross section of the plated panel was taken, and the microhardness was determined to be 93 Knoop at 25 grams load.

EXAMPLE FOUR

Ceramic workpieces comprising an alumina base with a sintered tungsten coating and a sputtered gold deposit thereon were obtained.

These workpieces were soaked in hot alkaline solution, rinsed and then immersed in boiling hot deionized water to bring then to temperature of the bath.

Thereafter, they were suspended in the bath of Example one for a period of 30 minutes, removed, rinsed, and dried. The deposit was found to be 102 microinches of electroless gold, and the light yellow colored gold deposit was found to be of uniform,'" matte finish with an amorphous structure.

12 EXAMPLE FIVE

The plating -solut:iOn of 'Example -One was subjected to an extended turnover test-invol-ving the plating of Hull cell panels. The composition ofthe-bath was monitored every hour to determine gold content.

upon depletion of the gold content to a level below 3 grams per liter, the bath was replenished using a formulation comprising an aqueous solution of 80 grams per liter potassium gold cyanide, 2 grams per liter potassium hydroxide, and 4 grams per liter of m-nitrobenzene sulfonic acid sodium salt. The amount of the replenisher solution added was that calculated to restore the gold content of the plating bath to 4 grams per liter.

- This procedure was repeated, and the plating rate was observed to remain essentially stable until 7 turnovers and then slowly began to decrease. The plating rate was found to vary within the range of 300 microinches initially to approximately 150 microinches per hour at 10 turnovers.

Thus, it can be seen from the foregoing detailed specification and examples that the electroless plating composition of the present invention provides -a stable and effective'bath for autocatalytic deposition of gold upon metallic and non-metallic workpieces. The deposits exhibit good amorphous structure, high purity and relative hardness, thus making them highly suitable for electronics applications.

1 1 13

Claims

CLAIMS:

1. An electroless gold plating composition comprising an aqueous solution of:

(a) alkali metal gold cyanide in an amount sufficient to provide from 1.0 to 16.6 grams per litre of gold (calculated as metal); (b) from 3 to 110 grams per litre of alkali metal cyanide; (c) from 2 to 10 grams per litre of a boron compound which is an alkyl amine borane andlor an alkali metal borohydride; (d) from 10 to 1100 grams per litre of alkali metal hydroxide; and ( \ e) from 0.1 to 0.3 grams per litre of a stabilizer of the formula R, fl -.-/ 0z 1 - 14 wherein R I is -COOH _-.OH, -CH 2 OH or -SO 3 H (or an alkali metal salt thereof.);! R 2 is -COOH, -OH, -Cl, -H, (or an alkali metal salt thereof) and is in the 2, 5, or 6 ring position, and -NO 2 is in the 3 or 4 ring position; said composition having a pH of from 12.5 to 14.0; the weight ratio OH-/CN- being from 4.0 to 10.0; and the oxidation/reduction potential of the solution being -550 to -700 millivolts.

2. A plating composition as claimed in claim 1 in which the boron compound is dimethyl amine borane present in an amount of from 4 to 7 grams per litre.

3. A plating composition as claimed in any one of the preceding claims in which the stabilizer is a nitrobenze-ne sulfonic acid or an alkali metal salt there6f.

4. A plating composition as claimed in any one of the preceding claims in which the stabilizer is present in an amount of 0.15 to 0.25 grams per litre.

5. A plating composition as claimed in any one of the preceding claims containing, at initial makeup, alkali metal cyanide in an amount of 4 to

6 grams per litre. and alkali metal hydroxide in an amount of 40 to 50 grams per litre.

1 1 6. A plating composition as claimed in any one of the preceding claims containing gold in an amount of 4 to 5 grams per litre, calculated as gold metal.

7. A plating composition as claimed in any one of the preceding claims having a pH of about 13.4 to 14.0.

8. A plating composition as claimed in claim 1 substantially as hereinbefore described with reference to the P.xamples.

9. A method for electroless plating of gold upon a workpiece, comprising the steps of:

(a) immersion plating upon the surface of a workpiece a thin deposit of immersion gold; and (b) immersing the plated workpiece in an electroless gold plating composition as claimed in any one of the preceding claims to plate gold thereon.

10. A method as claimed in claim 9 in which the solution is maintained at a temperature of from 85-950C.

11. A method as claimed in claim 9 or claim 10 including the additional step of replenishing the Composition, when Lhe gold content (as Metal) has decreased to 1.5 to 3 grams per liter, with a replenisher formulation comprising:

16 (a) alkali metal gold cyanide in an amount of 60 to 100 grams per litre (as metal); (b) alkali metal hydroxide in an amount of 1 to 10 grams per litre; and (c) stabilizer in an amount of 2 to 6 grams per liter.

12. A method as claimed in any one of claims 9 to 11 in which the electroless gold plating compositions is prepared by first preparing an aqueous solution of the _alkali metal hydroxide, alkali metal cyanide, alkali metal gold cyanide, stabilizer, and boron compound, then monitoring the heated solution until the oxidation/reduction potential has reached a value of -550 to -700.

13. A method as claimed in any one of claims 9 to 12 in which the oxidation/reduction potential is monitored during the plating step and increments of stabilizer in an amount of 0.05 to 0.1 gram per litre are added to maintain the potential within the range of -550 to -700 millivolts.

14. A method as claimed in claim 9 substantially as hereinbefore described with reference to the Examples.

Published 1990 at The PatentOffice.State House. 6671 High Holborn. LondonWClR4TP. Further copies mkv be obtainedfrom The Patent Office Sales Branch, St Ma-,-., Crky. Orpington. Kent BR5 3RD Printed by Multiplex techniques ltd. St Mary Cray. Kent. Con. 187 c