Copper plating - Copper per Mil-C-14550
Copper is the second most common metal plated, behind nickel. It provides a soft, red, ductile, solderable surface. Copper is an excellent electrical conductor. It is not often used as a final plate, however, because it tarnishes easily.
Because copper has excellent leveling properties and very high throwing efficiency, it makes an excellent undercoat for most other metals. In addition, because copper is ductile, it polishes easily to a high shine so that it supports a bright, shiny finishing metal above it.
Copper is able to fill sharp corners and surface imperfections allowing smooth and uniform coverage of the base metal. The throwing and leveling properties of copper insure that pinholes and subsequent blistering of finish metals will be avoided.
Copper makes an excellent undercoat on aluminum, which is a base metal that most other electrodeposited metals will not attach to.
Copper is also about the only metal that can be electroplated onto zinc die casts.
Generally copper is applied as an underplate in thickness between 100 and 200 micro inches.