Jim Marsico asks:
Just out of curiosity, what problems are you having with the coating
process
that warrants plasma cleaning?
*Doug, the two headed monster, replies:
We have several different kinds of solder mask (7) that are allowed on our
boards. We have a total of 12 suppliers. We have had some customer
rejections of late because our conformal coating does not stick to a
particular assembly as much as the customer would like.
So, as one of the process troubleshooters, I am working to find the root
cause: mask related itself, the way the fabricator processes the mask, the
way we process and clean the boards, the way we apply the coating, the way
we cure the coating, the way we control the process, etc. There is a very
large number of variables to consider in determining the root cause.
I am trying to remedy the problem, both as a long term solution (e.g.
material or process changes) and as a short term fix for work in progress,
which is considerable in volume. With most organic coatings, roughening
the surface of the solder mask will help improve the adhesion. I could use
something like a pumice scrub, but that is viable only at the bare board
level and it leaves lots of ionic residue. Plasma, with a proper selection
of gasses, power settings, and durations, can roughen all surfaces, thereby
improving adhesion. At least in theory. I am using Argon/Hydrogen for a
gentle etch. Running an agressive oxygen plasma clean after the board
exposure, with some other cleaning options, should solve any cross
contamination issues.
I call this work my Hydra project. Every time I cut off one head, two grow
to take it's place.
Doug
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