Gold is used extensively in the electronics industry, particularly because of its exceptional electrical properties. Electroless gold plating is one method of application. It is advantageous because it requires no external current or elaborate equipment. Electroless plating methods include galvanic displacement, autocatalytic and substrate-catalyzed processes.
Galvanic displacement or immersion processes generally have no reducing agent in the bath.
In autocatalytic processes, deposition should be indefinite. Deposits should be thick and non-porous. Autocatalytic baths use several types of reducing agents. The most common are borohydride and amine borane systems.
Two types of borohydride baths are described in Table I (below). Concentrated solutions can be prepared and stored at ambient temperatures for up to three months.
|TABLE I—Borohydride Baths|
|Bath I||Bath II|
|KAu(CN)2||0.86 g/liter (0.003M)||5.8 g/liter (0.02M)|
|KCN||6.5 g/liter (0.1M)||6.5 g/liter (0.1M)|
|KOH||11.2 g/liter (0.2M)||11.2 g/liter (0.2M)|
|KBH4||10.8 g/liter (0.2M)||10.8 g/liter (0.2M)|
The plating rate of the two baths depends on agitation, temperature and gold concentration. It also depends on the concentration of potassium hydroxide, potassium cyanide and potassium borohydride.
Bath I plates at 1.5 microns per hr at 70C with mild agitation. The second bath has a plating rate of 0.5 microns per hour. However, vigorous agitation is needed to obtain satisfactory deposits. Bath II can be used with light agitation.
Electroless gold can be deposited from a borohydride system on palladium, rhodium, silver and gold as well as copper, nickel, cobalt and iron. These reactions are catalytic from the beginning.
The gold can also be deposited from a borohydride system on the active metals copper, nickel, cobalt and iron. With the active metals, deposition is initiated by galvanic displacement. Because of this, some of these ions are lost to the bath.
Loss of copper ions into the bath is not a problem; however loss of nickel, cobalt and iron into the bath causes bath decomposition. Nickel incompatibility is a particular problem, since nickel is used in numerous applications.
Electroless gold is compatible with silicon. However the solution actively attacks aluminum because of the high alkalinity. Some organics can also cause problems in the electroless gold bath. Polyethylene inhibits plating. Some surfactants and positive photoresists are also incompatible. Negative photoresists, polypropylene and Teflon are stable in an electroless gold bath.
The electroless gold bath can alsobe contaminated by trace organic contaminants in bath makeup water. Deionized water should be charcoal treated or distilled over permanganate.
The borohydride bath is stable for about 20 hours of operation, as long as the bath components are replenished. However, researchers discovered that EDTA and thano-lamine stabilize the electroless gold bath by forming strong complexes with metallic contaminants, making them less likely to react with the borohydride.
Increasing Plating Rate and Bath Stability
There are six methods for increasing plating rate: 1) increased agitation; 2) increased temperature; 3) decreased free cyanide concentration; 4) decreased potassium hydroxide concentration; 5) increased borohydride concentration; or 6) increased gold concentration to 0.003 Moles.
To try to increase the plating deposition rate to higher than three microns per hour using an original bath will not work, even with powerful agitation. This is because spontaneous bath decomposition starts under these conditions.
One way to enhance the plating rate beyond the three microns per hour limit is to add depolarizers. The plating rate can be increased up to 10 microns per hour using lead or thallium ions.
In another bath, containing trivalent gold cyanide complex as the source of gold and a small amount of lead chloride, a deposition rate of 2.5 microns per hour was achieved without agitation.
Researchers used thallous sulfate as a depolarizing agent to achieve a deposition rate of 10 microns per hour. However when the concentration of thallous sulfate exceeds 100 ppm the bath becomes unstable and the deposit is discolored.
An alternative is organic stabilizers that allow the bath to operate at higher temperatures. Using certain organic stabilizers, plating ranges from eight to 23 microns per hour can be obtained at 85-90C. However, at these temperatures borohydride is lost rapidly, causing problems with bath control and the quick accumulation of the hydrolysis product. This accumulation leads to instability of the electroless gold plating process if it is used with a replenishment of borohydride.
Plating on Nickel
Nickel, as stated, is not compatible with the borohydride or amine borane additive systems. However, there are methods for plating on nickel.
Preplate displacement of gold. Using a commercial gold plating bath, displacement gold can be deposited on nickel in a very thin layer, less than 0.3 micron. Because the deposit usually does not completely cover the nickel, a borohydride bath will become contaminated with dissolved nickel ions. This bath is not recommended.
Hypophosphate bath. This electroless gold hypophosphate bath has been around for many years and has undergone several modifications. It is not an autocatalytic bath. In the beginning, plating is carried out by galvanic displacement. Later in the plating process, exposed areas of nickel act as the catalyst surface where hypophosphite is oxidized anodically, causing the cathode deposition of gold. Gold deposition continues as long as nickel areas are exposed, however, the plating rate decreases. In studies, a maximum of 23 microns was obtained in 15 hours. Gold deposits from hypophosphate baths are porous.
Hydrazine bath. Other research was done on electroless gold plating baths with hydrazine as the reducing agent. The data show that gold plating continues much as in the hypophosphate system. It produces a deposit that is 25 microns thick in 20 hours. Further study of this process yielded three different reactions when the substrate is nickel: 1) galvanic displacement; 2) substrate catalyzed depositions; and 3) autocatalytic deposition.
Non-cyanide baths. A significant decline in plating rate will occur in a borohydride bath with the growth of free cyanide ions. A trivalent gold cyanide system can be replenished with cyanide-free gold compounds. The following are some cyanide-free electroless gold plating baths: Gold (III) chloride complex; and Gold (I) sulfite baths.
Electroless gold plating is becoming better understood and more widely used in the industry. Much research continues on the process in hopes of developing a continuous process for operation on a commercial scale.
- Y. Okinaka, in Electroless Plating: Fundamentals and Applications, edited by Glenn O. Mallory and Juan B. Hajdu, Chapter 15, American Electroplaters and Surface Finishers Society (1990).