Jon Lorber wrote:
Hello,

We use Ferric Chloride in a spray etching application to etch 304 Stainless
Steel. We are having problems and cannot determine what they are. We do not
have a scientific background and are just looking for a few simple answers.
When the bath is new it seems to work beautifully. Then at some point we
start to get peculiar undesirable results.

We use 45 Baume Ferric Chloride and etch at 45°. The Ferric is less than 3%
hyrdro-chloric acid when received.

The following website I put up clearly demonstrates the problems with
photos.

http://www.precisionsigns.com/badetchingindex.htm

1.) What is actually happening in these pictures? Is part of the steel
decomposing faster than other parts?

2.) What measuring equipment should we be using to monitor the Ferric
Chloride? We have been measuring it's Baume with a hydrometer but should we
also be measuring the pH or anything else? If so, what type of instruments
and what are the ranges that they need to be?

3.) I've heard of regeneration. Is regeneration of the Ferric Chloride safe
to do with HCL (Muriatic)? By safe I mean are there any harmful / flammable
fumes generated? Do I need to make sure that the HCL stays at 3%? And how
much tolerance is there with it?

I feel like I'm on the right track here but need some good advice or
confirmation.



What are you using for resist? Is there a possibility that some resist
is left in the large areas to be etched, and affecting the surface?

I only use FeCl to etch copper, as on printed circuit boards. But, I have
a good deal of experience doing that.

How old is your FeCl solution? Have you tried starting with new solution?
You should know that stainless steel is NOT pure iron, therefore, you can't
replenish the etchant with HCl (muriatic acid). You have a significant
Nickel concentration building up in any spent solution.

To test, take a piece of totally clean scrap and try etching it. If you
still get the knobby surface, then it is the etchant that must be the problem.
it is probably full of Nickel, and is preferentially etching the material
depending on small local variations in the nickel mixture, grain boundary
orientation, or something like that.

If it comes out smooth, then your problem is the resist masking process
is leaving a trace of something that blocks the etching action of the
FeCl. I've seen a LOT of that, even water spots left after rinsing
after the resist pattern is developed can do stuff like this. And, the
smooth near an edge but rough in the wide open spaces is something I've
seen before, too.

Jon