rgtpa1

Topic: ENIG process-why does the gold solution turn bluish?

Good Day,

We ENIG plate PZT ceramic.  After a few runs, sometimes our gold solution takes on a bluish tinge.  It still seems to plate ok, but we remove it from production as a precaution.  What chemistry is at work here?  Is it ok to replenish the solution as we do the partially used gold solutions that do not turn bluish?

Thanks for your help.

DustinGebhardt

Re: ENIG process-why does the gold solution turn bluish?

I'm not familiar with the exact process, but if the gold bath contains cyanide, the bluish color you see may be the copper (or maybe nickel?) ions building up.  If there is enough free cyanide, the copper will remain in it's "colorless" state.  If the amount of free cyanide is too low, the copper will not have any cyanide to bind to and be readily visible as a blue compound.


You may be able to remove the blue color by adding back more free cyanide (usually sodium cyanide or potassium cyanide).  Check with your chemical vendor to be sure. 


In any case, I've done a quick check around the Internet and I find that the dissolved copper may be detrimental to the adhesion of the subsequent plating.  Copper will immersion plate onto nickel, but it has poor adhesion.  This may compete with the gold immersion process.  This should not happen in an alkaline solution, but I wouldn't take any chances. 

Last edited by DustinGebhardt (10/20/2009 - 12:33 PM)

-Dustin Gebhardt, CEF

Advanced Manufacturing/Finishing Engineer

Moen

Sanford, NC

rgtpa1

Re: ENIG process-why does the gold solution turn bluish?

Mr. Gebhardt,
Thank you for your response.  The gold is added as potassium gold cyanide, so the cyanide would be depleted as the plating process continues.  We should not have any copper in the solution, at least not deliberately.  If we replenish with PGC, perhaps that would remove the blue color, as you note.  We keep the solution at pH 5.5-5.8.  This has stumped our supplier, which is why I'm here.  Again, I appreciate your help.

DustinGebhardt

Re: ENIG process-why does the gold solution turn bluish?

Adding PGC probably will not fix your problem.  With a cyanide bath, you need to add extra cyanide to complex with the metal.  The cyanide in PGC is already accounted for.  I would recommend adding plain potassium cyanide.  You may want to play with the amount of KCN you add.  Adding KCN may also affect your plating efficiency, so again, test this first in your lab or another small sample. 

-Dustin Gebhardt, CEF

Advanced Manufacturing/Finishing Engineer

Moen

Sanford, NC