We often used citric acid as an anti tarnish after electroless copper. The dry film was applied on the scrubbed copper surface and then went on to dry film lamination. The scrubbing was sufficient to produce a fresh copper surface. The citric acid only prevented tarnish in the pores of the electroless deposit.
Interestingly, one of the adhesion promotors used in dry film formulations is an aliphatic amine. I can't think of the particular chemical, but it wasn't MEA. The theory was that it would find its way to the surface, form a copper complex, and provide a chemical bond with the photoresist. The photoresist has acid groups all along the polymer chain. Aqueous processing dry films just have more that the old solvent process films. With the amine anchored to the copper, the acid group reacts with it and forms an ionic bond very much like amonium acetate. As long as the system is dry this is a very strong bond. Over development would cause the bond to fail. Getting the adhesion level just right was always a tradeoff. If you made it too strong you have great development latitude, but stripping was slow and had the potential to leave residues. Too weak and development becomes touchy, but it would strip quickly and cleanly.
Formulations have changed over the last 20 years and there may be more robust adhesion systems in modern dry films.
----- Original Message -----
From: Adam Seychell
To: Homebrew_PCBs@yahoogroups.com
Sent: Saturday, December 04, 2004 2:53 PM
Subject: Re: [Homebrew_PCBs] Cleaning copper with lemon juice
Thanks for the tips Earl, I do not know what the ingredients are in the
brightening additives I'm using, but they don't seem to help the tarnish
problem at all. I have both monoethanol amine (MEA) and citric acid
laying around the workshop. I'll try adding small amount of citric in
the rinse as you suggested. There is a triple cascaded rinse tank
following the acid copper plating. I also use these same rinse tanks for
rinsing off the ammonium persulfate microetch. After plating, the panels
are rinsed followed by application of dry film photoresist by wet
lamination. Would citric acid residue effect dry film ? MEA on the
other hand is alkaline and will probably be fatal to dry film. I did
read somewhere MEA can be used as dry film stripper.
I'll let you know what happens.
Adam
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