Well Iain, it depends. You knew it was coming.
There are a variety of approaches. Which one is used depends on whether
you are talking about a small area, such as over and around a chip cap, or
if you are trying to remove it from an entire assembly.
For very local removal, such as field repair of a component, you can
usually burn through the coating with soldering rework tools, such as
thermal tweezers or soldering irons, hot knives, etc. You will still have
to chemical strip the rework site to remove the residual polyurethane, but
removing coating from a flat unpopulated site is much easier than doing it
from components.
I would not recommend micro-abrasion. It works, but a good urethane
coating is very similar in hardness and composition to solder masks. When
you get an abrasive powder that works well enough to cut through the
urethane in a reasonable period of time, unless you are really really
really (and I mean really) careful, you etch off the solder mask with it.
Same consideration for chemical stripping. When you get a stripper
aggressive enough to dissolve the urethane, it can also attack the solder
mask, component markings, legend inks. The process has to be controlled
very tightly.
I am not familiar with the urethane you mention. We don't do urethanes
here at Rockwell, or only sparingly. Usually, when I want to know how to
remove a conformal coating chemically, I start with the manufacturer of the
coating. What do they recommend? Go from there.
Humiseal has a couple of stripping agents available, some made for
urethanes. Phil Kinner at Humiseal Europe in the UK should be able to get
you pointed in the right direction.
Dynalloy makes a material called UResolve 411, which I have found to be an
effective stripper.
If you are interested, the US Air Force has one of their rework and repair
manuals that are available on the web and it has a section on removal of
conformal coatings. Don't know if this is the most current version though.
http://www.robins.af.mil/logistics/LGEDA/documents/TechOrds/00-25-234rev2.pdf#search=%22%22Bond%20Breaker%20II%22%22
Hope this is more helpful than the goofball offline comments Hillman
provided you.
Doug Pauls
"Braddock, Iain"
<Iain.Braddock@MB
DA.CO.UK> To
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08/31/2006 01:31 [TN] Conformal coating rework?
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"Braddock, Iain"
<Iain.Braddock@MB
DA.CO.UK>
Hey Techies,
I'm interested in what processes people use to remove Polyurethane
conformal coating in particular Robnor resin 313C, local or full board,
with respect to IPC class 3 manufacturing.
Micro blast? chemical strip?
Regards,
Iain.
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