Based on years of experience with aqueous dry film solder masks, may I offer
some bits of information regarding ionic cleanliness with these kinds of
products.

1. You will never be able to achieve readings with dry film solder masks
(DFSM) as good as you can get with the LPI solder masks. They are thicker,
more porous and they are all one component materials (you can't make a two
component DFSM). Therefore, DFSM's tend to be less reactive and don't
crosslink as much and ionic components tend to "leach out" during the
testing.

2. The ionic cleanliness testers that used heated/agitated solutions (such as
the Omegameter 600 SMD) tend to give readings that are much worse with DFSM's
than the non heated testers.

3. You can achieve ionic cleanliness readings that will meet common specs
(<10.0 micrograms/square inch for the MIL-P-55110C and <6.45
micrograms/square inch for the Bellcore spec); however getting much better
than this is diffucult. Even reaching these levels with DFSM's in no "cake
walk" and may require:
     A. A UV cure step during the processing of the mask at the PWB fab site.
     B. Care that the boards are not subjected to harsh processes (such as
hot air
          leveling for multiple passes) after the mask has been fully cured.
     C. If aqueous fluxes are used at assembly and saponifiers are used in
the post clean
         it is a good idea to have a good DI water rinse after the tap water
rinse to get the 
         mask as clean as possible. A quick bake (say 10 min at 250 deg F)
after post 
         clean and prior to ionic cleanliness testing also helps.

Good luck and please contact me if you'd like additional information.

Larry Fisher
Dexter Electronic Materials
317-628-7401
[log in to unmask]

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