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 that the deposition of tin-hydroxide stables the scale.
  
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 dissolve considerably amount of scale?

Any suggestion to a procedure would be very welcome ! 


With best regards
CHEMITALIC A/S

Jan Thuesen
Process Engineer

##############################################################
TechNet Mail List provided as a free service by IPC using LISTSERV 1.8c
##############################################################
To subscribe/unsubscribe, send a message to [log in to unmask] with following text in the body:
To subscribe:   SUBSCRIBE TECHNET <your full name>
To unsubscribe:   SIGNOFF TECHNET
##############################################################
Please visit IPC web site (http://www.ipc.org/html/forum.htm) for additional information.
For the technical support contact Dmitriy Sklyar at [log in to unmask] or 847-509-9700 ext.311
##############################################################