Dear Jan:

I believe you are correct in ascertaining a chelated alkaline solution
would be effective in removing the scale you described.  I would
recommend a solution of 5% v/v EDTA and 5% v/v NaOH heated to 135 F and
recirculated through the spray manifold.  You may need to replace this
solution once or twice during cleaning.

Additionally, I think it would be wise to repeat this procedure
perodically as part of the routine maintenence on this equipment.

Regards,
Ted Stern

Jan Thuesen wrote:
>
> For resiststripping after plating we have a machine in stainless steel. Due to several years of using KOH as stripping solution it has built up a thick layer of scale, primarily of tin-hydroxide Sn(OH)2. The scale deposits inside the spraybars, pumps, sumps, nozzles etc. The scale always loosens/breaks up into small pieces (only to a certain degree) when we set up a fresh stripping-solution. It eventually stops when there has been processed panels enough to saturate the solution with tin, so tha
>
> Now we want to remove so much of the scale as possible before we shift to a proprietary resiststripper. The proprietary stripper will not dissolve tin from the panels, hence the scale will break into pieces and block the nozzles almost continuously and instantly. This could make production almost impossible for a period of time, testruns of the chemistry has indicated this.
>
> For Christmas the machine is taken out of production for a week and we hope at that time to have an effective procedure to remove the scale in the stripper.
> Experiments on the LAB has shown that only a solution of nitric acid and hydrochloric acid will dissolve the scale effectively, but this will also dissolve the machine.
> Other suggestions have been to use flouroboric-acid, but a acid-based solution will precipitate residues of photoresist, causing new problems.
>
> I would think that round-pumping of the machine every day for a week with a strong alkaline solution at high temperature added with a chelating agent like EDTA or ethanolamine, could remove some of the layer. Even though the solubility product for tin in alkaline solution is very low, the chelating agent should dissolve some. The solution then has to be made up fresh every morning, after a thoroughly high-pressure rinse with water of the inside of the machine. But will a week be enough to dissol
>
> Any suggestion to a procedure would be very welcome !
>
> With best regards
> CHEMITALIC A/S
>
> Jan Thuesen
> Process Engineer

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