Nonelectrolytic Displacement Alloys

Displacement alloys are applied without the use of a reducing agent and are deposited by a combination of oxidation and reduction reactions that occur as the base metal corrodes. The result of this chemical reaction is the formation of an alloy of two elements.

The displacement alloy coating is self-limiting in thickness, and the reaction will stop when the density of the corroding metal on the surface drops below the reduction potential of the primary metal. There are two coating systems used in electronic applications that incorporate this technique.

Displacement tin-lead alloys are available for limited use as solder deposits. Process control is critical in producing a consistent alloy and melting point in this system. The other system, which is also used on a limited basis, is an alloy of palladium and ruthenium used in the activation of copper for subsequent electroless nickel plating in printed circuit board applications. The addition of ruthenium reduces the number of palladium atoms on the surface and improves the adhesion of the nickel to the copper.

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