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April 2006

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From:
Wayne Thayer <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Wayne Thayer <[log in to unmask]>
Date:
Fri, 21 Apr 2006 00:41:17 -0400
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I've had to do many jobs like this, and I agree that chemically is
definitely the wrong way to go.

A hot air jet can be pretty effective:  you hold the jet in one hand and
scrape with an xacto using the other.  Need very steady hands and you'll
probably get a few fingertip burns, but it is effective.  Just be
careful of the board material.  Believe me, I have done similar work at
tighter resolutions than your job.  You need a good stereomicroscope,
lots of hand supports, and controlled breathing.

If you have a bunch of these to do, the laser might be good, but don't
dismiss just plane old mechanical machining.  Mill bits are available
standard down to 5mils in diameter, and a real good machinist can get
the unwanted material reduced to about 0.001 inches thick, which is
usually pretty easy to scrape off.  The advantage is that no heat is
required.  The dis-advantage is the fixturing is kind of a pain to
build, but good machinists can do it.

Wayne Thayer

>>> [log in to unmask]  >>>
A couple of other things about Tetrahydrofuran.

Exposure to the chemical has been well covered.  However,  the TLV of
exposure in the US is still high for the properties outlined for
Europe.  200 ppm is
pretty high, but my guess is that will be lowered.  Anyway, the internet
says
a million pounds a year are used in the US.

THF is an excellent solvent, as Rudy has pointed  out.  I cannot imagine
it
will just attack parylene, and not attack the  epoxy of the board,
solder
mask/marking inks, and potting compounds on your  discrete components
from the
picture of your module. Some of the other solvents  Rudy offered you are
either
the stuff for dissolving epoxy so it can be coated  on to glass fabric,
or the
solvent used in the multilayer PTH line for removing  epoxy drill smear.

Finally, THF is pretty flammable.   I did not look  up ethyl ether, but
if
you use it, pretend you are using starter fluid for a  gasoline engine.

Denny Fritz
MacDermid, Inc.

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