Tinning is the process of thinly coating sheets of copper or PCB with tin, and the resulting product is known as tin plated PCB. The term is also widely used for the different process of coating a metal with solder before soldering.
It is most often used to prevent rust, but is also commonly applied to the ends of standard wire used as electrical conductors to prevent oxidation (which increases electrical resistance), and to keep them from fraying or unraveling when used in various wire connectors like twist-ons, binding posts, or terminal blocks, where stray strands can cause a short circuit.
There are two processes for the tinning of the black plates: hot-dipping and electroplating.
Tinplate made via hot-dipped tin plating is made by cold rolling steel or iron, pickling to remove any scale, annealing to remove any strain hardening, and then coating it with a thin layer of tin. Originally this was done by producing individual or small packs of plates, which became known as the pack mill process. In the late 1920s strip mills began to replace pack mills, because they could produce the raw plates in larger quantities and more economically.
In electroplating, the item to be coated is placed into a container containing a solution of one or more tin salts. The item is connected to an electrical circuit, forming the cathode(negative) of the circuit while an electrode typically of the same metal to be plated forms the anode (positive). When an electric current is passed through the circuit, metal ions in the solution are attracted to the item. To produce a smooth, shiny surface, the electroplated sheet is then briefly heated above the melting point of tin. Most of the tin-plated steel made today is then further electroplated with a very thin layer of chromium to prevent dulling of the surface from oxidation of the tin.