Hi Julie, IF do you have the obligation to conformal coat, AND your
experience shows a lack of adhereance (as a main important parameter) THEN
this means there is no real qualification (and no awarness of all other
propertiers) of the chain: solder resist - fluxer - cleaning (resp. no
clean) - conformal coating.
Beside the adhereance, this chain influences also the SIR (Surface
insulation Resistance) CTI (Comparative Tracking Index) and especially the
ability to resist against electromigration (see also IPC-TR-476A). If there
there are no defined adhereance/Electromig./SIR/CTI-properties, you could
waive the conformal coating, jawohl!
May be you haven't defined the solder resist type (or only the
resist class. H, T resp. 1, 2, 3) and each pcb supplier is using not
specified solder resist. As consequence the electromigration properties will
be unknown (as the adhereance).
As consequence you should define the solder resist (keyword:
purchasing specification, requirement flowdown) and your internal process
(solder resist - fluxer - cleaning agents - conformal coating).
I've some things:
* solder resists secreting substances (unwashable) which decreases the
bond in a manner that the conformal coating has been flow away by pressure
air (no yoke).
* UV-solder resists not abolutely cured. The residual not-cured
chemistry has lowered the CTI from CTI600 below 250.
* Fluxer containing substances which lowers the bond (of CC) and
resists each practicable cleaning process.
* Conformal Coating with excellent - and others with miserable
bond/adhereance.
As consequence you should obtain Appendix D of your bible (J-STD-001B) and
consider all implications.
regards from Zurich
Bernhard
> Date: Fri, 19 Nov 1999 09:48:53 +0200
> From: Brian Ellis <[log in to unmask]>
> Subject: Re: Adhesion of Conformal Coating
>
> Julie
>
> The compatibility of coatings and masks is always a subject of doubt
> unless you use similar chemistry for both. In my book, I recommended not
> to use a solder mask if you intend to coat, provided you can get away
> with it. Certainly, "no-clean" fluxes require compatible masks and this
> is a subject for qualification testing: also on the degree of cure of
> the mask. As a general rule (but not absolute), the order of quality of
> masks are: (worst) dry film, LPI and UV screenable, 1-component
> screenable and 2 component screenable (best). BTW I do not recommend
> coating on top of "no-clean" residues, except under very specific
> circumstances where the residues form part of the polymerisation
> reaction chain -- and even then ...
>
> Brian
>
> Julie Dixon wrote:
> >
> > 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
> >
> > ##############################################################
> > TechNet Mail List provided as a free service by IPC using LISTSERV 1.8c
>
> Date: Fri, 19 Nov 1999 11:03:43 -0000
> From: Graham Naisbitt <[log in to unmask]>
> Subject: Re: Adhesion of Conformal Coating
>
> 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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