"Electroless Plating"

When professionals speak of 'electroless plating, although they're referring to a plating process which doesn't use electricity, they usually mean it more narrowly: "autocatalytic" rather than "immersion/replacement" plating.

• Immersion/replacement plating is seen in science class when you put a galvanized nail into a solution of copper sulphate and it acquires a thin coating of copper on it. The reason is that copper is a noble metal and zinc an active metal. The copper exerts more pull on electrons than zinc does. So the copper in the solution which is touching the zinc covered nail "steals" electrons from it. As the electrons move from the zinc to the copper, the zinc is no longer electrically neutral but becomes positively charged and dissolves into the solution while the copper in solution is no longer positively charged an becomes neutral copper metal. Immersion/replacement plating has some disadvantages. First, adhesion is often poor because as the copper is depositing, the zinc is dissolving under it. Second, the thickness is very limited because as soon as no more zinc is exposed, no more copper will deposit.

• Autocatalytic plating has reducing agents built in which provide the electrons to convert the positively charged dissolved metal ions into metal. So the underlying metal doesn't dissolve and adhesion can be good; and much thicker plating is possible because it is not driven by replacement but by reducing agents. But we don't want the metal in the solution to just be spontaneously reduced and drop out, rather we want this reduction confined to the components; this is done by using reducing agents triggered or 'catalyzed' by the metal of the part. When the metal that is being deposited is also a catalytic surface itself, we call the plating process 'autocatalytic'.