Let's get a high five for Bright Tin Plating.
Tin plating is normally done to impart solderability to variety of base metal substrates. Tin is a silvery, blue-white metal that is ductile, solderable, and covers very well. The solderability of time can be affected by the substrate, since several metals tend to react with and migrate into the tin forming relatively non-solderable intermetallic layers. Of particular concern is tin plating over brass or zinc die-cast. The zinc will migrate into the tin and severely limit the shelf life of the finished parts. The migration can be mitigated by the common practice of applying an undercoat of copper or nickel or a combination of copper with a flash of nickel through which the zinc cannot migrate. Matte tin generally has better solderability, but bright tin is specified more because of its appearance. Tin does not tarnish easily, making it a good choice as a decorative finish.
Tin plating is addressed in Mil-T-10727. Although this specification calls for a a fairly thin plating for solderability (0.0005 in. or 1 um), common practice is to apply much more. This thinner plate was commonly used when "stannate tin" baths were in common use and practice was to reflow the tin periodically. The tin plate from these older-style tin baths tended to change spontaneously from "white" tin to "black or gray" after a few months, hence the need to reflow the tin. This process also caused not a few fires. Today, where long term shelf life is a consideration, 0.0003 in. (7.5 um) minimum tin is commonly used over an underplate of 0.0002 in. (5.0 um) minimum copper or nickel.
There are a great many other specification calling for tin combined with other metals being plated as an alloy. These include Mil-L-46064 and Mil-P-81728, both of which are for tin-lead, and Mil-P-23408, tin-cadmium. Tin may also be plated as a tin-nickel alloy, which is sometimes used as a substitute from decorative chromium. All of these alloy finished generally offer a better shelf life than pure tin plate.
Types of Tin Plating
Tin/Lead 60/40, 90/10, 93/7