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Electroless plating

In electroless plating, a metallic coating is deposited on electrically nonconductive plastics. Nickel and copper are the metals most frequently deposited in this way. The surface of the part to be plated is first etched with a strong oxidizing solution that partially erodes the plastic surface, creating microscopic holes. The enlarged surface area created makes the surface hydrophilic, enhancing the bonding of the plastic to the deposited metal. After etching, the part is immersed in a solution and a metallic coating is formed in a chemical reaction between the reducing agent present in the solution and metal ions.

The following steps can be distinguished into the process.

(1) Predips may be used prior to etching to overcome two problems in parts. The first reason is to improve the surface of poorly molded, highly stressed parts. By slightly swelling the surface, a more uniform surface attack in etching is possible, reducing non-uniform etch conditions and improving overall adhesion. Secondly, a predip is used to facilitate etching on normally hard to etch plastics, e.g. Xantar polycarbonate. This is also done by attacking and swelling the surface. A different solvent is needed for each polymer and high molded-in stresses should be avoided as they may lead to cracking during pre-dip.

(2) Etching. If the plastic contains butadiene rubber particles, which serve as an impact modifier, the etchant may be chromium acid, permanganate, chromium trioxide, or sulphuric acid. The butadiene is selectively removed, thus leaving small ball shaped holes or bonding sites for mechanical interlocking.

(3) After etching, the parts are rinsed in water and then put into a neutralizer, such as sodium bisulphite. Care must be taken that all etchants are completely removed, with no traces left in blind holes, as this may lead to poor metallizing if the etchant bleeds out in subsequent metallization steps.

(4) In the next step a catalyst (or activator) is applied by submersing the part in a palladium tin colloid bath. Palladium is deposited during the following reaction.

Palladium serves as a catalyst for the deposition of the nickel or copper.

(5) After rinsing following the catalysis step, metallic palladium is present on the surface of the part surrounded by hydrolized hydroxide. The excess stannous hydroxide must be removed from the part in an organic or mineral acid bath before the palladium can act as a catalyst.

(6) Finally an adherent metallic film, usually copper or nickel is deposited on the plastic surface a reduction reaction. This is accomplished by using a semi-stable solution containing a metal salt, a reducer, a metal complexer, a stabilizer and a buffer system. When a palladium-bearing surface is introduced into the solution, a chemical reduction of the metal occurs on the palladium sites, and, through autocatalysis, continues until the part is removed. The basic reactions for copper and nickel are:

Electroless plating is widely used to produce a conductive coating for subsequent electroplating.

Selective electroless plating

The surfaces of a part can be selectively electroless plated. This is done by replacing the etching step and the catalysis step by applying a lacquer that contains the catalyst onto the surfaces to be plated. The copper or nickel is only deposited where the lacquer is present.

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