What is Immersion Tin?
Commonly referred to as White Tin, Immersion Tin is a chemical process which applies a very thin layer of tin to the copper. It is not used as often as other finishes because there are many other alternatives and not all printed circuit board manufacturers offer it.
Why is it used?
Immersion Tin has been primarily used as an alternative to a lead-based surface finish.
It has also used for its very flat and smooth finish which makes it ideal for fine pitched surface mount components that will be placed on the circuit board. Typically, the placement of these parts is completed with automated equipment.
Another reason to use immersion tin is because of sustainability. Other finishes such as ENIG or HASL use elements that may be difficult to source on a consistent basis. Additionally, it uses less water and chemicals in the application process.
It is also easy to do any kind of re-work in the event an error is made in the application of components on the board.
What is the process used to apply immersion tin?
Immersion tin is applied using an electroless chemical bath that will apply approximately 0.7 to 1.0 microns (or about 44 micro inches) of tin to the copper.
There are also a few disadvantages to using immersion tin. The tin finish is not very durable on its own and can be easily damaged if not handled properly. Also, if there is tin that is left exposed after the final assembly, it can corrode over time.
The process and method used for immersion tin falls under two US patents. Patent US2891871 and US2947639.