Hydrogen Peroxide and Hydrochloric Acid Etching

Question

I am trying to make a PCB stencil by etching thin Aluminum using a toner transfer method.

I am using H2O2 3% and HCL 10% in a 1:1 ratio. I have also tried 3:1 and 1:2 ratios.

The alu has been cleaned with acetone.

I've tried room temp chemicals and putting them in a hot tap water bath.

In all cases the reaction is incredibly slow and weak. After 10 minutes the surface of the alu is covered in bubbles but no pitting.

Any ideas what I a missing? Others have reported this working for them (commonly with copper PCBs) so it seems strange. Perhaps this combination requires HCL at 30%?

Thanks.

Answer 1

Your HCL is a bit weak. You can speed up the reaction exponentially by warming the acid. Here is a complete tutorial on making solder stencils from old soda cans. He uses 31.5% HCL (aka Muriatic Acid) from The Home Depot.

Since Chinese suppliers can make beautiful professional laser cut stainless stencils for next to nothing (with welded frames) I've never been tempted to try it, but I admire his pluck.

Answer 2

Your big problem is that the first step which occurs is not dissolving aluminum - it's dissolving aluminum oxide (alumina) which has formed on the surface of the metal. Your acetone cleaning got oils and such off the surface, but it didn't touch the oxide layer. Copper, for instance, doesn't oxidize quickly, so cleaning is important. With aluminum the situation is different. And the oxide layer is self-renewing, as PlasmaHH has noted. Bare aluminum will grab oxygen out of water, producing hydrogen gas, so dilute acids don't do well.

You may be better off using a hot, saturated sodium hydroxide solution, but there are two caveats. First, that stuff is really nasty if you get it on you, and eye protection is an absolute must. Second, I have no idea how well your resist will hold up.