NL8302029A - A METHOD FOR STREAMLESS GOLD PLATING. - Google Patents

Description

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Title: A method for electroless gold plating.

The present invention relates to an improved method for the electroless or autocatalytic deposition of gold on substrates; and more particularly to the use of a new electroless plating bath for depositing gold on metallic and non-metallic substrates.

In recent years, quite extensive literature has developed regarding the electroless method of gold plating on surfaces. US patents of particular interest to both the electroless gold plating method and the problems associated with this procedure include 3,300.3 ^ 8 (Luce); 3,589,916 (McCormack); 3,389,916 (Gostir); 3,697,296 (Bellis); 3,700.1 * 69 (Okinaka); 3,798,056 (Ckinaka et al.); 3,917,885 (Baker); as well as the older patents and articles cited therein. Relevant articles include: Rich, D.W., Proc.

American Electroplating Society, 58 (1971); Y. Okinaka, Plating 57, 91-15 (1970); and Y. Okinaka and C. Wblowodink, Plating, 58, 1080 (1971) This amount of literature is applicable to the present invention to the extent that it describes alkali metal cyanides as a source of the gold or related metals in the electroless bath. as well as the use of alkali metal borohydrides and amine boranes as reducing agents are described. For example, the 1970 Okinaka article and U.S. Patent 3,700,1 * 69 described an electroless gold plating bath with the following ingredients: (1) soluble alkali metal gold complex; (2). excess free cyanide such as potassium cyanide; 25 (3). a basic substance such as potassium hydroxide; and (kj a borohydride or an aminehorane.

The 1971 article by Okinaka et al as well as Baker's U.S. Pat. No. 3,917,885 describe the problems associated with the use of these particular plating baths, especially when cyanide concentrations increased. Other problems were encountered when replenishment of the baths was performed and the baths became unstable when a plating rate of about 2.5 microns was approached. The need to avoid unwanted gold precipitation from the baths is also noted.

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In U.S. Pat. No. 3,98885, these problems were overcome by using an alkali metal imide complex formed from certain special imides as the source of the gold or related metals. In order to maintain the electroless gold plating at the desired pH 11 to 1U 5, the Baker patent discloses the addition to the bath of alkali metal buffer salts such as the citrates and so on.

The need to use special imides in the preparation of the gold imide complex is a clear commercial drawback.

Also, in a U.S. patent application No. 2k6.Vf2, it has been proposed to overcome the problems of the prior art by using a trivalent gold metal complex or compound as a source of the gold in the plating bath. Unfortunately, under large-scale commercial conditions, it has been found that replacement of the monovalent gold complexes or compounds with trivalent gold does not completely overcome the problems of improper bath stability, metal deposition and bath replenishment.

The object of the present invention is to provide an improved electroless or autocatalytic gold plating bath that avoids the problems and disadvantages of the baths proposed to date.

Another object of the present invention is to provide an improved electroless or autocatalytic gold plating bath that will easily deposit gold on gold as well as various metallic and non-metallic substrates with good adhesion.

Another object of the present invention is to provide an improved electroless or autocatalytic. gold plating bath, which will easily deposit ductyl, lemon yellow pure gold on substrates at highly desirable commercial speeds and thicknesses.

Yet another object of the present invention is to make a currentless. or provide an autocatalytic gold plating bath, which fuses. 3Q has stability and can be supplemented efficiently.

These and other objects will be readily apparent from the following description of the invention.

According to the present invention it has now been found that a further improvement in an electroless or autocatalytic. gold plating bath and gold plating procedure can be accomplished by using a mixture of (a) as the source of the gold in the plating bath. an 8302029 J 4 * - 3 - trivalent gold metal complex or compound such as alkali metal gold (HI) cyanide, alkali metal aurates, or alkali metal gold (III) hydroxides; and (¾) a monovalent gold metal complex or compound such as alkali metal gold (1) cyanide. More particularly, the present invention relates to autocatalytic baths and procedures, that is, gold can be plated on gold as well as other suitably-treated metallic or non-metallic substrates. Therefore, the therm used herein is "electroless" autocatalytic plating.

The electroless plating baths of the invention will also contain a suitable reducing agent such as an amino borane or an alkali metal horohydride, cyano borohydride, hydrazine or hypesulfite. The baths will have a pH of about 10 to 13 and may contain additional ingredients to achieve and / or maintain this pH, including a basic substance such as an alkali metal hydroxide and a buffer substance such as an alkali metal citrate. An alkali metal cyanide is a further optional ingredient to improve the stability of the bath.

In most operations, the electroless plating bath of the present invention will operate at a plating temperature of about 50 ° C up to a temperature at which the bath decomposes. The operating temperatures are typically about 50 to 95 ° C. and more preferably at about 60 to 85 ° C.

The substrates to be plated according to the present invention are preferably metals such as gold, copper, etc. No special pretreatments are required for this metal sustra-25 tea. In addition, metalized ceramic and non-metallic substrates can also be plated. Such substrates will, of course, be subjected to suitable pretreatment before plating, as are well known per se.

The invention also provides an addition to the electroless 3Q plating bath with trivalent gold in the form of an alkali metal acid or gold (II hydroxide solution) to maintain the desired gold concentration of the bath. This is an important feature of the present invention, which overcomes the prior art problem of improper metal deposition rates associated with the use of monovalent gold cyanide Additional base and reducing agent may also be added during bath replenishment without _____ 8302029% - 1 + - that one thereby experiences some adverse result.

As "described" above ", the essential feature of the present invention is the use as a gold source in the electroless plating bath of a mixture of trivalent and monovalent water-soluble gold complexes or compounds. Contrary to the prior art, teaching the use of complexes in which the gold was either in the monovalent state such as, for example, potassium gold (1) cyanide, or is in the trivalent state, for example potassium gold (III) cyanide In the present invention, the trivalent gold complex or the trivalent gold compound is an alkali metal gold (III) cyanide, an alkali metal aurate or an alkali metal gold (IIII hydroxide, the alkali metal gold (III) cyanides and alkali metal aurates being preferred; while the monovalent gold complex is an alkali metal gold (l ) is cyanide For most purposes, the alkali metal is 15-typical potassium or sodium, and the use of Particular preference is given to potassium as an alkali metal. Therefore, preferably potassium gold (III) cyanides, KAu (CH) .4, or potassium aurate, and potassium gold (1) cyanide are used to formulate the electroless gold plating baths of the present invention. It will be appreciated, however, that other alkali metal and / or ammonium trivalent gold and monovalent gold compounds or complexes can also be used and that the therm "alkali metal" used herein is intended to include ammonium compounds and complexes.

The reasons that the trivalent gold and monovalent gold mixture functions better than trivalent gold or monovalent gold alone in these 25 plating baths and in the autocatalytic plating process are not fully understood at this time. Possible explanations may be that use of the mixture (11 leads to better deposition rates due to an easier reduction of the mixed gold metals, (2i increases the stability of the bath, and (31 replenishes the bath using the trivalent alkali metal aurate or gold (III hydroxide facilitates, overcoming the problems of the prior art caused by high concentrations of cyanide ions in the plating bath. In addition, the bath is exceptionally stable to high concentrations of reducing agents , achieving faster stern casing speeds 8302029 «5 - 5 - than previously achieved, while maintaining the stability of the bath.

It will be clear that the alkali metal gold (monovalent and trivalent) cyanides used in the practice of the invention are water soluble. However, a variety of compounds which can give the gold component in trivalent and univalent condition can be used in the composition of the baths.

The amount of trivalent gold used in the mixture with the discrepant gold will be at least sufficient to stabilize the bath and to prevent cyanides from accumulating in the bath when replenished. Generally, the weight ratio of trivalent gold to monovalent gold in the bath will be at least about Q2: 1, while ratios of about 0.5 *. 1 to about k: 1 are typical and ratios of about 1: 1 to 3: 1 are preferred.

15 The maximum amounts of trivalent gold have not been found to be critical.

Therefore, while weight ratios of up to about ft: 1 are typically used, weight ratios of 10-15: 1 and higher can also be used without adversely affecting process operation.

The reducing agents used in connection with the present electroless plating baths include one or more of the borohydrides, cyano horohydrides or amino boranes that are soluble and stable in aqueous solution. Thus, for example, alkali metal borohydrides, preferably sodium and potassium borohydrides, are used, although various substituted borohydrides such as sodium or potassium trimethoxy horohydrides, (ia (X] 3 (0CH2) can also be used. Also preferred are the amine boranes such as mono and di-lower alkyl, for example up to C 6 -alkyl amine boranes, preferably isopropyl amine borane and dimetbylamine borane.

Other reducing agents such as hydrazine and hyposulfite can also be used,

The mixed valency electroless gold plating baths of the present invention should be maintained at a pE between about 10 and 13 to achieve the desired results. It is therefore preferred that an alkali metal hydroxide such as sodium or potassium hydroxide be used to maintain the pH at this level. pH control behind is considerably easier when next to the alkali metal. hydroxide 8302029-6-alkali metal buffer salts are used. Suitable alkali metal buffer salts include the alkali metal phosphates, citrates, tartrates, borates, metaborates and so on. In particular, the alkali metal buffer salts can be sodium or potassium phosphate, potassium pyrophosphate, sodium or potassium ciw. .o 5 trate, sodium potassium tartrate, sodium or potassium borate, sodium or potassium metahorate, and so on. The preferred alkali metal buffer salts are sodium or potassium citrate and sodium or potassium tartrate.

In order to further improve the electroless plating baths of the invention, it is desirable in some cases to further provide a chelating capacity by the addition of an organic chelating agent. such as ethylene damide tetraacetic acid, and the disodium, tri sodium and tetra sodium and potassium salts of ethylene damide tetraacetic acid, diethylene triamine pentaacetic acid, nitrilotriacetic acid.

The ethylenediamine tetraacetic acid, and its di-, and tetrasodium salts, are the preferred chelating agents, with the tri and tetrasodium salts being particularly preferred.

In addition to the above ingredients, the electroless plating baths of the invention may also contain alkali metal cyanides and more. especially contain the potassium or sodium cyanides. Such ingredients are added when greater stability for the autocatalytic process is required. When used, the amount of alkali metal cyanides can range from about 1 to 20 grams per liter, which is far above the minor critical amounts used by McCormack, which uses up to 50% milligrams per liter. In the electroless plating baths of the present invention, the gold compounds or complexes will be present in an amount at least sufficient to deposit gold on the substrate to be plated, up to their maximum plating solubility. The reducing agent is. present in an amount at least sufficient to reduce the gold, again to the maximum solubility in the bath.

The basic substance and the huffersxof are each present in an amount sufficient. to provide and maintain the desired bath pH.

More particularly, the components of the electroless plating baths of the invention will be present in amounts within the following ranges: 8 ~ 8302029-7 Ί.

9

Amounts g / 1

Components Normal Preferred (1) Gold (III) as alkali metal aureanide, aurate or auric hydroxide 0.5- ^ 1- ^ 5 (2) Gold (I) as an alkali metal aureocyanide 0.5-3 1-2 (3) Reducing agents like amino horane, alkali metal borohydride, cyanohorohydride, and so on. 1-15 2-10

10 (¼) Basic fabric 10-50 2Q-kQ

(5) Buffer substance as alkali metal salt 10-1-0 2Q-30 (6} Alkali metal cyanide (if present) 1-20 1-10 (7) Organic chelating agent 15 (if present I · 2-25 3-15 (8) Water to 1 litre

As mentioned above, the pH of the had is kept in a range of about 10 to 13s and in some cases between about 11 and 13. The typical operating temperature during plating is between about 50 and $ 5 ° C, Thigh preferably between 6Q and 85 ° C. For most purposes, plating rates will be up to 8 microns per hour, and preferably be at least about 2 microns per hour.

Although the invention has been described above primarily in dealing with electroless gold hats, it is to be understood that one or more alloying metals such as copper, zinc, indium, tin, etc. may be added to the electroless baths. When used, they are added to the bath as a suitable soluble salt in amounts sufficient to about 2G. weight% of the alloy metal (s) in the gold deposit.

According to the preferred features of the present invention, the substrates to be plated by the electroless gold hats are metals such as gold, copper, Soper alloy, electroless copper, nickel, electroless nickel, nickel alloys and the like. Thus, when a metallic substrate is used, the surfaces thereof include all metals which are catalytic for the reduction of the metal compounds dissolved in the baths described. In some cases, it is preferred that the substrate be further sensitized by treatments well known to those skilled in the art. In addition, it is possible to use nickel, cobalt, iron, steel, palladium, platinum, copper, brass, manganese, chromium, molybdenum, tungsten, titanium, tin, silver, etc. as metal substrates on which the gold is to be plated.

However, when using non-metallic substrates, these surfaces must be made catalytically active by producing a film of catalytic particles thereon. This can be done by the method of US Pat. No. 3,509-916 on surfaces such as glass, ceramics, various plastics and so on. When a plastic substrate according to the present invention is to be plated, it is etched first, preferably in a solution of chromic acid and sulfuric acid. After rinsing, the substrate is immersed in an acidic solution of tin (II chloride, such as tin (II) chloride and hydrochloric acid, rinsed with water, and then contacted with an acidic solution of a precious metal such as palladium chloride in hydrochloric acid, The now catalytically active non-metallic substrate can then be contacted with the electroless plating solutions of the invention.

The method of utilizing the present invention primarily involves immersing the metallic or non-metallic substrates in the electroless plating baths. These baths are described in the above. while plating is carried out at the above-mentioned temperatures. Commercially desirable thicknesses of gold metal deposits have been achieved without encountering bath instability problems and other known problems. Necessary adhesion properties were also readily realized in the practice of the present invention.

Another aspect of the present invention is an ability to supplement the bath without any problems. For example, it has been found that apart from the addition of additional basic substance such as potassium hydroxide and reducing agent, a replenishment of the gold content can be accomplished by adding an alkali metal gold (III hydroxide or alkali metal aurate) to the bath. This replenishment of the bath with water-soluble components is performed without adversely affecting the bath— 8302029 • i - 9 plating rate or the wash stabilitext.

The invention will now be explained more fully by means of the following examples of preferred embodiments.

Example! .1 5 An electroless plating bath was formulated from the ingredients listed below:

Ingredients Amount g / l

Gold as KA.u (CIT1 ^ 2

Gold as KA.u (CNl2 2 10 Potassium hydr0x7de 35

Citric Citrate 3G

Dimethylamino borane 6

The pH of the obtained had been about 11 to 32.

It had been used for plating gold on gold, copper and 15 copper alloys (31 δ cm per liter I he BQ C. The plating rate was 5 microns / hour. Deposits from this bath were duetile, lemon yellow, porium free pure gold with excellent adhesion to the substrates.

For a number of applications, the bath could be supplemented by the addition of gold as a gold (III J complex with alkali metal hydroxide or aurate. No accumulation of cyanide in the bath was observed and a constant metal deposition rate was maintained. Compared to known baths, excellent steadiness achieved, as evidenced by the consumption of approximately 1G0 # of the gold metal content in the bath, with 5 to 6 additions, without inaccurate metal deposits or bath resistance.

Example II

An electroless plating bath was assembled as follows;

Ingredients Quantity g / l 3G Gold as KAu (CTl ^ 3

Gold as KAu (C3Tj0 1

ammonium citrate 3Q

Potassium hydroxide 35

Dimethylamine borane. Deposits were obtained on copper and copper alloys at an 8302029-10 platinum rate approaching 2 to 8 microns per hour with the bath at a pH of 11-12 and a temperature of 80-85 ° C. The complement procedure of. Example I was used with equally good results.

Example III

5 Another electroless plating bath was assembled as follows:

Ingredients Quantity g / 1 gold as KAu02 3 gold as KAu (CN) _2 1 ^ potassium cyanide 10 potassium hydroxide 30 dimethylamine borane h

Deposits on gold were obtained at a rate of 3 microns per hour with the bath at 82 ° C and on a pE from 12 to 13. The additional procedure of Example I was followed using the gold (III) hydroxide , achieving a constant deposition rate and avoiding unwanted accumulation of cyanide ions.

The above data show that the improved electroless bath of the invention leads to superior results and avoids the problems or commercial disadvantages associated with the electroless gold metal baths hitherto proposed.

It will further be apparent that the above examples are illustrative only and that variations and modifications can be made without departing from the scope of the invention. For example, the plating bath can initially be formulated with univalent gold, i.e., an alkali metal gold (cyanide, and then supplemented with an alkali metal aurate or gold (III-hydroxide) to yield a bath containing both univalent and trivalent gold components.

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Claims (17)

An aqueous electroless gold plating bath comprising a water-soluble tri-gold component selected from the group of alkali metal gold (III) cyanides, alkali metal aurates and alkali metal gold (III hydroxides; a more soluble alkali metal gold (1) cyanide and a reducing agent selected from the group of alkylaminoboranes, alkali metal borohydrides, alkali metal cyanoborohydrides, hydrazine, and hyposulfite, the gold components being present in an amount at least sufficient to deposit gold on the substrate to be plated and the reducing agent is present in an amount at least sufficient to reduce the gold in the bath, which plating bath has a pH in the range of about 10 to 13. The electroless gold plating bath of claim 1, wherein also a basicizing agent and a basic buffering agent are present in amounts sufficient to maintain the pH of the bath within the indicated range. The electroless gold plating bath of claim 1, wherein the pH is kept in the range of about 11 to 12. if ·. Electroless gold plating bath according to claim 1, wherein the reducing agent is a dialkyl anhydride. The electroless gold plating bath according to claim if-, wherein the di-alkylamino borane is dimethylamino borane. The electroless gold plating bath of claim 1.1, wherein the reductant is an alkali metal borohydride. The electroless gold plating bath of claim 6, wherein the al-c-metal borohydride is potassium borohydride. The electroless gold plating bath of claim 11. wherein the reducing agent is an alkali metal cyanoborohydride. The electroless gold plating bath according to claim 8, wherein the alkali metal cyanoborohydride is potassium cyanohorihydride. 3G 10. An electroless gold plating bath according to claim 2, wherein the alkalizing agent is sodium hydroxide or potassium hydroxide. The electroless gold plating bath of claim 2, wherein the basic buffering agent is selected from the group of alkali metal phosphates, citrates, tarirates, borates, metaborates, and mixtures thereof. 12. The electroless gold plating bath according to claim 1, wherein as an additional ingredient 1 to 20 g / l alkali metal cyanide is added as an additional ingredient. The electroless gold plating of claim 1, wherein the trivalent gold component is an alkali metal gold (III) cyanide. An electroless gold plating as claimed in claim 13, wherein the alkali metal gold (III) cyanide is potassium gold (III) cyanide. The electroless gold plating according to claim 1, wherein the trivalent gold component is an alkali metal aurate. The electroless gold plating according to claim 15, wherein the alkali metal aurate is potassium aurate. 17- An electroless gold plating according to claim 3, wherein the trivalent gold component is an alkali metal gold (III) hydroxide. The electroless gold plating of claim 1, wherein the monovalent gold component is an alkali metal gold (11 cyanide. The electroless gold plating of claim 18, wherein the aLr, 15 potassium gold (1) cyanide potassium gold (1). cyanide. 20. Aqueous electroless gold plating with a pE within the range of about 10 to 13, comprising: Component amount of g / l (all trivalent gold as alkali metal aricyanide, aurate or ....: aurihydr oxide 0.5 ^ ^ (Hl univalent gold as the alkali metal aurocyanide 0.5 3 25 (dl an alkali metal huffing agent JO - ^ O (el an amino horane, an alkali metal horohydride, or an alkali metal cyanohorohydride. 1 - 15 21). Electroless gold plating according to claim 2Q, wherein the 3Q alkali metal is sodium or potassium. An electroless gold plating according to claim 2.1, wherein the alkali metal is potassium. 23, Electroless gold plating according to claim 2Q, wherein component (a1 are potassium gold (III cyanide; component (b) potassium gold (licyanide; 35 component (c1) potassium hydroxide; component (dl tripotassium citrate; and component (el dimethylamino-chloro. 8302029) - 13 - An electroless plating method for plating gold on a substrate, wherein the substrate is immersed in the gold plating bath according to any one of claims 1 to 23 and the substrate is kept in the bath for a sufficient period of time to the desired amount. gold on it. 8302029