# 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

## 1 Answer

This is a good question: Tin is above copper in the 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. $\ce{2Sn(OH)3- -> Sn + Sn(OH)2-6}$, 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).

• So the metallic tin precipitated deposits on the surface . But in silvering of glass we see that whatever surface is exposed to the solution gets silvered but in this case only the copper surface gets tinned . Why is that? Jul 20 '17 at 10:11
• Another good question... but I am not sure of an answer. In the case of thiourea reduction, I assume copper is a catalyst, similar to the way a hot copper wire can catalyze methanol oxidation. Hopefully, someone more knowledgeable will comment on this and the disproportionation Jul 20 '17 at 20:44