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   TO:         I4235700 IBMMAIL   new address for ipc technet 25.6.96

   FROM:       DSTEWART EX2       D.Stewart        - Product Development Manager.

   DATE:       25 June 1996
   SUBJECT:    sulphuric acid and pumice!
   REFERENCE:  removal of nitric stains

    I'm back online again after a 6 month absence due to technical
    difficulties - I could read but not write. I am a voice to be heard
    again!

    We have an issue with nitric acid stains on boards after tin lead
    strip - the cause is known and relates to processing thin laminate
    cores through outer layer processing, but that's another story.
    Prior to soldermask, we cannot remove these stains by standard
    pumice, but we can remove them by passing through the 1% H2So4
    rinse on our developer prior to pumice. We are obviously trying to
    reduce handling and interference with a different process so we are
    trying to find an inline method of removing this stain.

    Our pumice machine supplier (IS) is not keen on us using sulphuric
    in the pumice machine, a nearby friendly shop with the same problem
    tried citric in the pumice but it turned the boards yellow! so does
    anyone out there have the same problem, or another solution?

    Dougal Stewart
    Exacta Circuits
    Scotland