In response to Tamir Ben-Shoshan's question, I read your request on the 
TechNet and will be happy to help. Let me have your snailmail address and I 
will have some information sent.

I think Frank Hinojos reply should have the 4th category as silicone.

I would only add that we use the following as a guideline for users:

Acrylics: Good all purpose materials featuring high moisture and insulation 
resistance and easily repaired.

Urethanes: Better chemical/saline resistance but more difficult to repair. 
Generally longer curing schedules unless moisture sensitive/curing in which 
case they are unsuited for dip application.

Silicones: Excellent insulation resistance, low surface tension so better 
creep beneath components, now in solventless formulations. Generally poorer 
moisture resistance than Acrylics for the same given film thickness. Low mar 
resistance and difficult to repair.

Epoxies: Excellent chemical resistance. Generally poor moisture resistance and 
extremely difficult to repair. Not favoured these days unless the application 
is a really tough chemical environment.

Further, imagine looking at the coating film at molecular level. Then the 
molecules of Silicones and Epoxies would resemble a basketball whereas the 
molecules of Acrylics and Urethanes would resemble tennis balls.

Now, all coatings breathe! They allow air which may contain water vapour to 
migrate through them and the speed of migration will be faster through 
basketballs than through tennis balls. If you have a soluble ionic particle 
between the coating and the substrate, you will induce osmosis at anything up 
to 800 bar - and that's a lot! Enough to induce premature circuit failure so 
make sure you know the condition of the circuit surface before you coat.

As to your question about rework, please note that it was a recommendation in 
the US MIL-I-46058 specification, that Conformal Coatings should be reparable. 

This can be total coating removal by the use of proprietary strippers, 
selective repair by localised coating removal or particularly for the coating 
to be solderable. 

As an example, apply a hot soldering iron to the particular component and an 
Acrylic based material will be vaporised and facilitate the component removal 
and replacement. Cautionary note that the vaporised material should comprise 
only carbon monoxide and carbon dioxide but some urethanes may well contain 
isocyanates.

Once you have completed the replacement, make sure you clean the area of flux 
residues and then re-coat. 

Hope this helps, 
[log in to unmask]
Tel: +44 1276 691100
Fax: +44 1276 691227
CONCOAT LTD


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