How does non-electrolytic tinning work?

Having made diy pcb 's at home I have used things like tinnit which is a clear liquid . When the pcb is dropped into them , a layer of tin forms on the exposed copper regions on the pcb . I was wondering if someone could explain what is happening or if someone could give an equation .


Here is a video of the process on youtube <https://www.youtube.com/watch?v=wOqkI-S-wdI>




This is a good question: Tin is above copper in the [galvanic series](https://en.wikipedia.org/wiki/Galvanic_series), so logically, copper should displace tin, not the other way around.


[Electroless (autolytic) tin plating can be accomplished by disproportionation of a tin compound.](https://www.jstage.jst.go.jp/article/sfj1950/38/6/38_6_240/_article) $\ce{2Sn(OH)3- -> Sn + Sn(OH)6^{2-}}$, where the less stable Sn(II) yields Sn(IV) + Sn(0), or metallic tin. The divalent tin is usually $\ce{SnCl2}$.


[Electroless tin can also be deposited by reduction of a tin compound](https://www.finishing.com/0200-0399/260.shtml), e.g. thiourea reducing $\ce{SnCl2}$ to metallic tin. [In a similar fashion, silver nitrate can be reduced to metallic silver to make a mirror on a glass surface.](https://www.youtube.com/watch?v=6vduHWV47Dg) Note that the silver is deposited without any effect on the glass (other than a nice, shiny coating).


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