RlL V Oj noEq

By definition, an autocatalytic system is substrate catalyzed. Consequently, there are no theoretical limitations to the thickness of gold that can be achieved with an electroless gold bath (Fig. 3).

Fig. 3 Thickness of gold as a function of time for an autocatalytic electroless gold plating bath. Theoretical curve at optimum conditions

Many electroless gold bath compositions have been reported in the literature. A typical system, developed by Y. Okinaka (Ref 1), uses potassium gold cyanide as the source of gold and sodium borohydride or dimethylamine borane as reducing agents (Table 1). These baths operate in strongly alkaline media; the strong alkalinity is necessary for the activity of the reducing agent. The suggested reaction mechanism (Ref 5) for the electroless deposition of gold in the presence of sodium or potassium borohydride is:

In fact, according to this mechanism, the hydrolysis of potassium or sodium borohydride leads to the formation of BH3OH- species, which ultimately provide the electrons for the reduction of the gold complex.

Table 1 Compositions and operating conditions for cyanide-base electroless gold plating baths

Reported by Y. Okinaka (Ref 1)

Constituent or condition

Bath A

Bath B

Potassium gold cyanide

0.003 M

0.02 M

Potassium cyanide

0.10 M

0.10 M

Potassium hydroxide

0.20 M

0.20 M

Potassium borohydride

0.20 M

0.20 M

Temperature °C (°F)

70 (158)

70 (158)

Rate of deposition ^m/h

1.5

0.5

A number of different gold salts are used in the formulation of electroless gold baths. In addition to the most commonly used--for example, potassium gold cyanide, KAu(CN)2, and potassium chloroaurate, KAuCl4--there is a growing interest in using sodium gold sulfite in the formulation of cyanide-free systems. Different reducing agents can also be used, and their activity influences both the rate of deposition and the stability of the bath. Other components of electroless gold baths include some chelating agents, stabilizers, buffers, accelerators, and wetting agents. Gold plating baths reported by H. Okudaira et al. (Ref 6) and C.D. Yacovangelo (Ref 7) are listed in Table 2 and Table 3, respectively.

Table 2 Composition and operating conditions for a sulfite-base electroless gold plating bath

Reported by H. Okudaira (Ref 6)

Constituent or condition

Amount

Sodium chloroaurate

0.20 M

Sodium thiosulfate

0.50 M

Sodium sulfite

0.05 M

N-Methylthiourea

0.0001 M

Pyrogallol

0.09 M

Ammonium chloride

0.20 M

Temperature, °C (°F)

70 (158)

pH

6.0

Rate of deposition, ^m/h

1.8

Table 3 Composition and operating conditions for a cyanide-base electroless gold plating bath

Reported by C.D. Yacovangelo (Ref 7)

Constituent or condition

Amount

Potassium gold cyanide

0.005 M

Potassium cyanide

0.035 M

Potassium hydroxide

0.80 M

Potassium carbonate

0.45 M

Dimethylamine borane

0.05 M

Hydrazine

0.25 M

Lead acetate, ppm

15

Temperature, °C (°F)

80 (176)

Rate of deposition, ^m/h

< 7.8

Process Variables and Parameters. Electroless gold baths are very sensitive to operating conditions; therefore, it is important to control all of the variables that can affect both the rate of deposition and the stability of these systems.

Gold Concentration. As the gold concentration decreases, the rate of deposition also decreases. In order to keep the rate of deposition constant, periodic addition of gold should be made in the form of a gold salt. Replenishment can be made based on analysis data and/or close monitoring of the quantity of metal deposited on the substrate.

Reducing Agent. The concentration of the reducing agent is almost as critical as the concentration of gold. The decrease in the reducing agent concentration is not only a function of the reduction of gold, but also is affected by possible degradation reactions, such as those that occur with sodium borohydride. The concentration level should be frequently analyzed and adjusted.

Stabilizers. The presence of stabilizers is necessary to the operation of most electroless gold baths. Free cyanide seems to have a beneficial effect on the stability of most cyanide-base electroless gold baths. However, excess amounts will overstabilize the bath and inhibit the deposition of gold. Other stabilizers can also be used to prevent the formation of gold particles and their accelerated growth, which can result in a significant drop in the ionic gold remaining in the solution.

Contaminants. Organic and inorganic contaminants can have very deleterious effects on electroless gold baths. For example, trace amounts of nickel ions are sufficient to cause a rapid decomposition of the borohydride bath reported by Okinaka. Because of the effects of nickel, the use of an immersion gold coating of about 0.25 pm over a nickel substrate is often recommended prior to electroless gold deposition. Some electroless gold baths (such as that given in Table 3) can deposit gold directly over nickel by combining an immersion process with a catalytic process. Traces of tin are also detrimental, whereas copper has no negative effect on bath stability.

Temperature. All electroless gold baths are temperature-dependent. In general, the higher the temperature, the faster the speed of deposition. An adverse effect of high temperature is bath decomposition. High temperatures also cause rapid consumption of reducing agents, creating the need for frequent replenishment.

Agitation. The rate of deposition of gold as well as the uniformity of the gold film will be influenced by the agitation. Both the solution and the workpiece can be moved to ensure good uniformity. Agitation is also required to prevent localized heating and possible decomposition of the solution.

The effect of pH is particularly important when plating on pH-sensitive substrates. For example, degradation of polyimide substrates will occur in very alkaline baths. Alumina-ceramic substrates are also incompatible with highly alkaline systems, and they should be treated in electroless gold baths operating at neutral pH (such as sulfite gold baths, which have a pH of 8.0).

Surface preparation is a key issue in any deposition process. Cleaning and activation of the gold substrate are necessary prior to plating in order to prevent contamination and to enhance adhesion. When plating gold directly over nickel or copper, the removal of the oxide layer is essential for achieving excellent adherence of the gold film. In this respect, a flash of nickel (electroless or electrolytic) may be beneficial.

Processing Equipment. Because of their sensitivity to high temperature and to contamination, electroless gold baths should be run in carefully controlled equipment. For operating very small units (3 gal or less), pyrex jars or jacketed beakers are practical. For larger units, polypropylene tanks or Teflon-coated stainless steel tanks are recommended.

The heat source plays a major role in controlling the stability and the performance of an electroless gold bath. Small water baths with thermostatic control are very practical for small-scale operations. For larger operations, heat-exchanger coil systems made of Teflon and operating with hot water or with steam are recommended. Teflon-coated immersion heaters can also be used, but care should be taken to prevent localized heating by vigorously circulating the solution in the vicinity of the heating coils.

In large-scale operations, pumping and filtration systems are also necessary. All pumps and filters should be made of chemically resistant materials, such as polypropylene or Teflon.

Environmental Considerations. Spent electroless gold baths should be treated both for environmental reasons and for gold recovery. All major suppliers of plating chemicals offer waste treatment guidelines that should be followed by their customers, particularly those operating the cyanide systems. In California, only cyanide solutions and IX resins are accepted by gold reclaimers. Aqua regia solutions are not accepted.

Health and Safety Considerations. All electroless gold baths contain chemicals that can be harmful to the user's health if not handled safely. The use of protective goggles, gloves, and lab coats is strongly recommended. The plating solutions should not be swallowed. They should be stored in cool and well-ventilated rooms and used under well-ventilated hoods. The Material Safety Data Sheets provided by the manufacturer should be read carefully before using any plating products.

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