Susan,

Generally the Young's Modulus of the cured acrylic should be low enough that
any CTE mismatch does not produce much in that way of physical stress on the
board.

Mil qualified Type AR coatings will bend round a 3mm mandrel at room
temperature for example. Most conformal coating specifications will only
allow you to coat up to 75 microns (3 mil).

If you double the thickness of a solvent based coating, roughly speaking it
takes about 4 times as long to dry. Approximation, but a useful one just to
begin to imagine how long it will take to get the solvent out of your 'semi
potted' system.

I reckon the popping could due the pressure caused by heating solvent still
trapped under the devices?

Either that or the acrylic you are using is too tough (high YM) to be used
in this way.

Best Regards,

Alan Brewin
Chief Chemist
Concoat Ltd.
2C Albany Park, Frimley Road,
Camberley. Surrey. GU15 2PL. UK.
TEL: +44 (0)1276 691100
FAX: +44 (0)1276 691227

-----Original Message-----
Subject: [TN] conf coating used for potting


>What are the ramifications/advisability  of applying an acrylic conf
coating to a thickness of 20 mils over an assembly.  The various components
have this coating under as well as over the components so that it appears
"potted" instead of coated.
>
>During temp cycling the solder joints on surface mount are popping.
>
>Susan Mansilla
>Robisan Lab

>

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