Stripping Hell: Small Holes and Thick Panels

by Rudy Sedlak

It is possible today, maybe even easy, to sign up to build boards that are almost unbuildable without a (good quality) spray machine to strip Tin or Solder. But there are some chemical methods to make this difficult process somewhat easier.

There are 4 different chemistries in use today for stripping Tin/Lead. All of these chemistries strip pure Tin better than they strip Tin/Lead. So the smart (or maybe real desperate) PCB fabricator, when faced with a job that is seemingly impossible to build because the holes can't be stripped, may want to consider using Tin as an etch resist instead of Tin/Lead. Desperate times call for desperate measures.

If you don't have a machine to spray the stripper, the best chemistry for stripping Tin in a tank is Nitric/Hydrofluoric (HF) Acid. This is also the most dangerous chemistry to the personnel. When Nitric/HF is used to strip pure Tin, the deposit dissolves completely, so no residues are formed to clog the hole, and thus stop stripping.

However if you have to strip Tin/Lead, Nitric/HF is not the best stripper, Nitric/Ferric Nitrate is the a much better choice. Nitric/HF will leave an insoluble residue of Lead Fluoride, and this residue can act as a barrier and stop stripping in very small, deep holes. The Nitric/Ferric Nitrate system does go through a stage where there is some residue during stripping, but ultimately the residue will dissolve. This dissolution makes this the chemistry of choice for Tin/Lead stripping.

The 4 different chemistries for stripping Tin or Tin/Lead are:
1. Ammonium Bifluoride/Peroxide (the original, & still used for Tips)
2. Two step, Nitric followed by ferric chloride
3. Nitric/Ferric Nitrate
4. Nitric/HF

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