There was an excellent method that was used in 3 PWB shops in
Scotland that eliminated the need for any pre-cleaning prior to
photoresist. At the end of the electroless Cu line, the boards in
the transport baskets were run through a CFC-113 displacement
drying system.  The cycle time was about 3 minutes, drying was
complete, and since the drying was done in a non-oxidizing
environment, the Cu surface did not oxidize. The boards were
taken directly to the yellow room for photoresist application.
Since the surface was clean and oxide-free, the pumice scrub was
step was totally eliminated. Without the need to plate extra
electroless Cu to scrub off, the boards spent less time in the
electroless Cu, thus reducing overall cycle time.
The process was in use at Prestwick Circuits, BEPI Circuits and
Exacta.
Today you could do the same thing with the new non-ozone
depleting fluorosolvents, such as HFEs or HFCs. Since this
displacement drying process was used to dry syringe needles-
which have an aspect ratio greater than any multilayer ever
built- drying vias (either through or blind; no problem) was a
piece of cake.

--Bill Kenyon
Global Centre for Process Change, Inc.
Tel/Fax:: 302-652-4272/-5701 

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