Julie,

LPI - UV resists can certainly contribute to poor conformal coating
adhesion.

Reason? Because these materials require de-bubbling and levelling agents
which are either silicone or glycol based surfactants - surface modifiers.

Naturally silicone based additives will probably leave unconnected molecules
to which nothing will stick - not even another silicone. Glycol's don't
generally cause this problem, but may contribute to certain board
reliability issues in subsequent process stages - e.g. fluxing.

The major issue is the way in which flux residues may inter-react with the
solder resist. Most modern fluxes have low solids and high liquid content.
So how do you stop them from dripping off the board when it upside-down? -
Add surface modifiers in the form of wetting agents.

More liquid = higher preheat = greater surface expansion = greater
absorption of fluid that has low surface energy = reliability problems?
Probably. At least, the conformal coating may not stick very well - but is
this a problem?

Well, if you expose your finished and coated assembly to full environmental
testing and it passes, but on subsequent analysis you find that the coating
adhesion isn't too clever - do you have a problem? In my opinion and
experience, not usually.

If however, you experience poor reliability test results, then these
unpredictable chemical inter-reactions will very probably be at fault. This
is why you folks need to seriously consider more effective reliability
testing using new SIR and/or IC measurement techniques.

Hope this helps. I do have loads more but at this point it gets real
commercial - so I will just ask that if you want more, please ask
off-TechNet and I will send.

Regards,
Graham Naisbitt

[log in to unmask]

WEB: http://www.concoat.co.uk

CONCOAT Ltd
Alasan House, Albany Park
CAMBERLEY GU15 2PL UK

Tel: +44 (0) 1276 691100  Fax: +44 (0) 1276 691227
----- Original Message -----
From: Julie Dixon <[log in to unmask]>
To: <[log in to unmask]>
Sent: 18 November 1999 18:04
Subject: [TN] Adhesion of Conformal Coating


> Dear Net,
>
> Has anyone ever heard of a problem with adhesion of conformal coating
being
> caused from UV soldermask? Will no-clean fluxes at assembly plants react
> differently to different types of soldermasks or LPI?
>
> Thanks,
> Julie Dixon
>
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