In a lead-free world, the most common finish is tin because of its very good wettability properties. However, tin can be bright or matte. Matte tin is well adapted to high temperatures whereas bright tin tends to turn dark under high temperatures.
If brass (CuZn) or a zinc-containing pin alloy is used as the base material, the pin must first be plated with nickel to a minimum of 1.25 µm before the tin/gold is applied, as explained in the first chapter of this book.
Source: Wurth elektronik