Electroless Gold Plating,Electroless Gold Plating,Electroless Gold Plating,Electroless Gold Plating
THE DEPOSITION of a metal on a given substrate can be achieved by numerous conventional methods.
Electrodeposition is extensively used because it is technically convenient to operate and is cost efficient. Other
techniques, such as chemical vapor deposition and sputtering, are also used, but they require sophisticated equipment and
are more expensive to operate. In recent years, an unconventional technique, electroless deposition, has become
increasingly attractive, particularly in the electronics industry where the use of nonconductive substrates and the
miniaturization of the circuitry presents difficulties in using conventional techniques.
By definition, electroless deposition is the controlled autocatalytic reduction of a dissolved metal by a dissolved reducing
agent at an interface. This process requires a reducing agent to provide the electrons for the reduction of the metal ion to
the metal to be deposited on the metal substrate. Chemical deposition can also be accomplished by galvanic reaction
between a less noble metal and a more noble metal ion. The noble metal is deposited via this reaction. This form of the
process is known as immersion plating.
Several metals can be deposited using the electroless plating technique. Electroless copper and electroless nickel are
widely used (see the articles "Electroless Nickel Plating" and "Electroless Copper Plating" in this Volume). There is a
growing interest in the electroless plating of gold and, to some extent, other precious metals such as palladium and silver.
This article describes the electroless gold plating technique in terms of its advantages and limitations, applications,
processing, and the properties of the plated deposits.
Advantages of electroless deposition of gold include the following:
· Uniform, thin gold films can be deposited over electrically isolated tracks and bonding pads.
· Gold can be uniformly deposited on complex shaped and hollow articles, provided there is adequate agitation of
the solution and/or the articles to be coated.
· Gold can be deposited on nonconducting substrates such as glass, ceramics, polymeric materials, etc., provided
the substrate is activated.
· Gold films resulting from electroless deposition are 99.9% pure; therefore, they have excellent electrical,
soldering, and bonding properties.
Limitations of the process include:
· The electroless gold deposition rate is generally slow. The typical rate is 2 to 3 μm/h, compared to 10 or more
μm/h for electroplating baths.
· The chemistry of electroless gold baths is complex; the baths require tight control because they are very sensitive
to operating conditions as well as to organic and inorganic contaminants. Consequently, they have a shorter Applications. Electroless gold finds its application primarily in the electronics industry, where it is used for the
metallization of conductors and insulators in printed circuit boards (Ref 1). It is particularly appropriate to use electroless
gold on electrically isolated tracks and bonding pads that require uniform gold films, which are difficult to achieve by
electrolytic gold deposition. Electroless gold is also recommended to achieve good ohmic contacts on III-V
semiconductor materials such as n-GaAs, InP, etc. (Ref 2, 3). It can also be used in the fabrication of multilayer ceramic
packages (Ref 4) to improve their brazing, soldering, and wire bonding properties. Another major application for
electroless gold is for uniformly plating hollow articles that cannot be successfully plated by electrolytic deposition
because their shape results in bad electric field distribution.
Properties of Electroless Gold Films. Depending on bath chemistry, the color of electroless gold films varies from
pale yellow to dark brown; the surface texture can be matte, semibright, or reflective. The purity of electroless gold
deposits is 99.9% or better. This results in a soft gold film of less than 90 Knoop hardness, suitable for soldering and
bonding. The density of the deposited metal is about 19.3 g/cm3. The grain structure, as observed by scanning electron
microscopy (Fig. 1) exhibits a relatively tight distribution of fine particles. The porosity of such gold films depends on the
thickness, but above 1.5 μm the deposit is generally pore-free.

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