Expert: Mike Fulcher - 2/16/2007

Question
I've done some more experimentation and it does look like only acids aggressive towards nickel are really helping the solder bond and surfaces that are further abraded don't really show the improvement.  This is supporting the presence of the nickel oxide.  Would you have any recommendations on how to clean away the oxide with out deterioration to the substrate?  A 430 flux seems to do okay but doesn't give anywhere the amount of bond that I know is possible.  Are there safe cleaners that can cut the oxide?  One thing I have not yet tried but intend to is ammonia. It can be aggressive with some oxides but I'm not sure it will work with the nickel oxides.  Any input you have would be appreciated.
-------------------------------------------
The text above is a follow-up to ...

-----Question-----
I am trying to solder to an inconel shim pad and I've noticed that the solder strength is drastically improved by pre-etching the inconel with a couple second rub of ferric chloride.  I have measured the roughness of the inconel surface before and after etching and do not notice a difference in roughness.  So my question is, is the solder bond being improved due to increased surface area from the etching process as you would think or is it more from the cleaning of the oxides or is the opposite and I'm actually creating a small layer of oxides that is helping the solder bond.  I have tried to manually abrade the inconel surface with pumice but this doesn't seem to help the bond.  So now I'm back to the question...what is improving the solder bond in this case?
-----Answer-----
That is an awesome question.  My first response would be that you are removing the oxide layer on the inconel, which would help promote bonding.  Inconel has quite a bit of nickel in it, which forms a slippery (when it comes to soldering) oxide layer.  Ceramics in general (like nickel oxide) are almost impossible to solder to due to high surface energies (think surface tension).  You could confirm this using XRD or EDACS to look at surface composition of freshly treated and untreated samples if you'd like to be certain.  I hope this was useful for you.

Answer
We used to use nitric acid for removal of chromium oxides from stainless steel, so this should also work for the nickel.  I can't remeber the concentration off the top of my head, but I'd start low and work my way up.  Also, keep the nitric away from organic materials (solvents, etc.) as mixing of the two can be potentially explosive.