There are two major functions of this electroplated tin lead mixture. First, the tin lead mixture acts as an etch resist for later processing. Another function is to protect the copper from oxidation. If the board is not solder mask over bare copper (SMOBC) then the tin lead may be fused in an oven to produce solder. Finally the solder coating allows for improved assembly as all exposed copper is already "wetted" with a thin layer of solder.