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Subject: developing photoresist problems

Date: 06/07/00 at 12:59 AM
Posted by: Adam Seychell
E-mail: aseychell@vet.com.au
Message Posted:

Hi all, I have a question about developing photoresists (negative dry
film types) that I hope someone who's had experience using photoresist
can help me. The problem I am having is an invisible residue (scum) left
behind on the copper surface after the developing process. This thin
film is resistant to the electro plating process that follows. From what
I understand scum is normally removed by copper etching but if thick
enough it will resists etching as well. Electroplating requires all
traces of scum to be removed since there is no self cleaning action like
there is with etching. After development the amount of scumming that
remains is enough to resists etching. I therefore cannot even do a pre
etching to remove the residue in order to expose 100% of the copper.
Development is done in 1% sodium carbonate at 25C in bubble agitated
tank. After about 3 minutes all visible signs of unexposed resists are
removed and shiny copper can be seen. At this point I place the board in
a tray of fresh water (room temp) and wipe over with soft cotton balls.
With this method the scum is still remaining, especially in narrow
crevices (< 6 mils). If I rub harder the scum is eventualy removed but
this also damages fine traces. The scum needs to be removed some other
way. Do I need to increase developing time, say to double or 6 minutes
? If left in developer for too long I notice the edges of the
photoresists start to swell and fine lines become very fragile, but this
returns to normal strength when rinsed down or placed in acid/etcher.

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