Joe,

From a microelectronics standpoint [hermetic type of stuff] I have
always been under the impression that removal of silicone residue with
standard gases such as Argon, Oxygen, Nitrogen, or Argon+Hydrogen mixes
was not reasonably possible - talking RF plasmas in a partially
evacuated chamber, here.  Our best plan, was not to have it [silicone]
there in the first place - silicones have a really nasty tendency to
migrate all over the place, on occasion.

The greatest part of my fear of placing a silicone containing part
within the chamber is that some of what gets dislodged from the piece in
question may deposit on the chamber walls [instead of getting sucked out
the vacuum].  This can mean that the next part to go into the chamber
may get a smattering of this as it becomes knocked from the chamber
walls.  This would not be good for a part which was going to be wire
bonded, for example.

At our power levels, angstoms, or atomic layers, of contaminates are
removed.  I don't normally attempt to remove visibly thick layers of
contaminates with the plasma because doing so usually either takes a
long, long, time and/or the parts become hot and/or discolored [eg O2
will turn silver black.  Go long enough and SnPb solder will get dark
and dull looking, etc.].

A 'quicky' O2 plasma - say 5 minutes, at 150W, at about 150-200 mT will
do a nice job of activating the surface of many plastics for promotion
of adhesive adhesion though.  I have little doubt that it would also
improve the wetting characteristics of your bare laminate and/or solder
mask, but I would not necessarily lay claim that it would remove gobs of
organics [within a reasonable time frame, that is].  

I'm interested in hearing what Yoda Ingemar and D Hillman have to say on
the matter.

PS - Ingemar, I had -16F, outside air temp this morning [-14F
yesterday].  Getting more like your neck of the woods each day!  Have
had enough, thank-you. 

Steve



-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Kane, Joseph E (US
SSA)
Sent: Tuesday, February 06, 2007 11:23 AM
To: [log in to unmask]
Subject: [TN] Plasma Surface Treatment For Conformal Coat

While we're on the subject of CC, does anybody use plasma to
remove stubborn organic residues (e.g. silicones) from board
surfaces, or otherwise change the surface energy to promote
coating adhesion?

We're not talking about long or aggressive exposure here, just
enough to remove a thin layer of contaminant, or roughen the surface 
of a glossy soldermask.  So I'm not much concerned about removing
markings or anything, but are there any other potential pitfalls?

Joe Kane
BAE Systems
Johnson City, NY

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