Warning: Hydrochloric acid is corrosive.
Thiourea based cleaners are toxic.
Wear gloves when handling them.
Greetings fellow nerds.
A number of electrical engineers have asked me to explore the chemistry of printed circuit board tinning solutions.
When immersed in these solutions copper metal will get a coating of tin metal.
I've also been asked if it was possible to make a version at home.
Turns out, unlike a lot of things i do, it is possible to make PCB tinning solution at home.
So let's get started.
First we need some tin.
I'm using this lead free tin solder.
Check the Material safety data sheet for the particular solder you're using and make sure it has more than 95% tin.
The remaining components can be silver, copper, antimony and so on.
They won't be a problem for our work as long as you have more than 95% tin.
Now make sure the solder is solid.
Don't get the solder that has a flux core like rosin or acid.
Plumbing solder like this one is a good choice.
We don't want the flux interfering with our work.
Now we need to convert it into tin (II) chloride.
Also known as stannous chloride.
We get 1g of tin solder and add to it 10mL of 30% hydrochloric acid.
It should start bubbling.
Let me zoom in for a better look.
What's happening is the tin is reacting with the hydrochloric acid to make hydrogen gas and tin (II) chloride.
Unfortunately this process is very slow so we'll have to leave it overnight.
Eventually it'll mostly dissolve and leave behind a residue of the 5% alloying metals in the solder.
Pour off the supernatant so we now have a clear solution of tin chloride in hydrochloric acid.
Now if you put in copper metal nothing will happen.
This makes sense since the reduction potential for copper is higher than that of tin.
We'll never be able to displace copper with tin.
But there is a trick we can do.
Get some silver polish.
I'm using this Tarn-X brand and check to make sure it has thiourea as one of the primary ingredients.
Other polishes may or may not have this so check to be sure.
It has to be thiourea.
Now get about 100mL of the polishing solution and add in our tin chloride solution.
Now it may precipitate some type of salt.
If this happens, heat up the mixture to drive it back into solution.
Now for the fun part, we put in some copper.
I'm going to put in this copper wire.
Now remember before that tin did not plate onto copper from tin chloride.
However we now have a nice smooth layer of tin.
Let's try that again but this time i'm going to use a printed circuit board.
Exactly what circuit this is i have no idea.
Anyway we're now depositing tin.
So how is this happening? As demonstrated before tin does not displace copper.
The reduction potential for tin is lower than that of copper.
But we must remember that standard reduction potentials only apply for idealized solutions containing only the ions we're examining and little else.
If we mix in other additives we can change the reduction potentials.
Copper forms very strong complexes with a substance called thiourea.
It's so strong in fact that this changes its reduction potential to below that of tin.
We can flip their redox potentials.
This redox flipping allows us to displace copper with tin and deposit it.
Anyway, you can leave the copper in the solution as long as you want.
Once it's coated with tin the reaction stops and proceeds no further.
I left it in for a few minutes.
And there you go, tin plated onto copper without electricity.
As you can see the difference is striking.
This process is used by electrical engineers to protect their circuit boards from air oxidation and make them easier to solder.
While not strictly necessary and not used by all engineers it helps in some cases.
Now a special note if you're doing this yourself.
You don't actually need to heat it until it's clear.
Now it works much faster when it's hot and you can see what's happening.
But the reactions still work while the solution is cold and precipitated.
So you can do this entire process by hand without a hot plate if you desire.
The only equipment you need really are beakers or jars.
As a side note i don't know the longevity of the solution in storage.
I only had a few days to test it.
But tin (II) chloride tends to oxidize in air to tin (IV) chloride so i wouldn't bet on prolonged storage.
Nonetheless the separate components seem to be usable even after prolonged storage.
Tin and hydrochloric acid do not degrade in storage and this bottle of tarn-x is six years old.
Aging doesn't seem to harm this process.
So rather than making a large amount and storing it.
Store the precursors and just make small amounts as you need it.
Anyway, that was homemade PCB tinning solution by redox flipping.
Thanks for watching.
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