Scott

A word of caution about underfilling a BGA with any coating - do please take
care of CTE mismatch - especially if you apply the coating too thick. Too
thick can lead to other issues you would prefer to avoid.

During a discussion at the SMART Conference in Brighton (UK) last week with
Rockwell Collins, they suggested that the use of a standard acrylic type
coating gives all the protection they require. Interestingly some of their
assemblies are situated in the same rack alongside other manufacturers who
used Paralene for the reason you gave - Rockwell continue to use the
acrylic.

The only time moisture would become a problem under such devices is if some
ionic residues are present: Moisture + Ionics = Electro-Chemical reactions
(Dendrites). Take any one of those out of the equation and you should not
have a problem.

As for a polyurethane v acrylic, take care with the selection as some
urethanes tend to be somewhat brittle under certain conditions. Thickening a
urethane might also create other problems.

I hope this helps - and if you would like more specific information, then
please contact me off-line cos it gets real commercial at this point!

--
Regards Graham Naisbitt

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Concoat Limited - Engineering Reliability in Electronics
A British Manufacturer

NEW WEB SITE: www.concoat.co.uk :NEW WEB SITE

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> From: "Newland, Scott" <[log in to unmask]>
> Reply-To: TechNet E-Mail Forum <[log in to unmask]>, "Newland, Scott"
> <[log in to unmask]>
> Date: Mon, 22 Nov 2004 17:38:45 -0500
> To: <[log in to unmask]>
> Subject: [TN] BGA Environmental Protection
>
> I'm looking for processes to augment our conformal coating in protecting
> non-underfilled BGA's against harsh wet environments (salt fog, condensation,
> etc).  Although it hasn't happened yet, I think it's a matter of time before
> moisture will penetrate my imperfect conformal coating and start a corrosion
> experiment under one of my fine pitch BGAs.  Parylene is a natural, but comes
> with its problems and currently would require going to a sub-contractor. Other
> suggestions have included thicken our traditional urethane conformal coating
> and
> dispense a fillet around the part, sealing it from the external environment.
> We
> have done some testing that shows using urethane conformal coating as an
> underfill does not lower reliability.
>
> Any suggestions / comments.
>
> Scott Newland
> Harris GSCD
> E-Mail [log in to unmask]
> For a brain teaser try:
> http://pao.gsfc.nasa.gov/science.html
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