When
my career started (1973) I was a chem miller. We used 36 degree Baume
ferric
for
stainless and when the edges got ragged we used it for mild steels and
when
even
that didn't give smooth lines we diluted it to 32 Baume and etched
coppers.
The
first project I was given was to evaluate dry films as a replacement for
dip
coated
liquid resists. I took each metal from stock, tried about six variations
of
pre-clean processes and etched them all together. The choice of best
pre-clean
was based on how much resist lifting we saw before
the resist was stripped and
how
much burning was evident after stripping. The trick seemed to be not in
the Baume
of the
etchant but rather in the pre-clean. Higher Baume typically gives less
undercut
and
better sidewalls at a slower etch rate.
The
dulling you mention sounds like the early stage of typical resist adhesion
failure.
The
most successful pre-clean we used for stainless involved some form of
passivation.
I
suggest you dig out your metal finishing handbook and make sure your cycle
is
in
line with industry standards and then contact the cleaner rep for
recommendations.
I have a customer who is using standard
aqueous dry film on #301, #302 &
#316L stainless steel and then
etching with ferric chloride (this is actually
a chemical milling process,
not really printed circuits). He is having slight
undercut under the dry
film which dulls the surface (unacceptable to his
customer). He is using
45 Baume strength ferric chloride and has asked if
either lower or higher
strength would work better and not cause the undercut.
Any ideas out
there?
Regards,
Larry Fisher