# 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 .

Electroless (autolytic) tin plating can be accomplished by disproportionation of a tin compound. $\ce{2Sn(OH)3- -> Sn + Sn(OH)2-6}$, 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, 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. Note that the silver is deposited without any effect on the glass (other than a nice, shiny coating).