Make PCB Tinning Solution ("Liquid Tin") by Redox Flipping





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Published on Oct 21, 2017

In this video we make a tinning solution for printed circuit boards.

Tinning solutions coat copper metal with a thin layer of tin. This is used to protect them from oxidation as well as make them somewhat easier to solder.

To make the solution we first get 1g of lead-free tin solder containing 95% or more of tin metal. Check the MSDS to be sure. Now to the solder we add 10mL of 30% hydrochloric acid. What we're making is tin chloride. The solution will bubble hydrogen as the tin dissolves. Leave it overnight and pour off the clear supernatant into 100mL of commercial metal polish that contains thiourea. I used "Tarn-X" brand for my experiments. You can also buy thiourea directly and use 5g per 100mL of water directly along with 1g of sulfuric acid. Anyway, however you obtain your thiourea solution, the added tin chloride will make it into a tinning solution.

Now just add in copper metal like that of a PCB and a thin layer of tin will coat it.

What's happening is that normally Tin chloride does not displace copper metal so you can't make tinning solution from just tin chloride. One perspective is that Tin has a lower redox potential than copper so it would actually consume energy to displace copper.

But thiourea forms very strong complexes with copper. This drives down the redox potential of copper to below that of tin. Now tin can displace copper.

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