I stand in awed delight, gallant knight Sir Brian.
Pete Duncan
Brian Ellis
<b_ellis@PROTO To: [log in to unmask]
NIQUE.COM> cc: (bcc: DUNCAN Peter/Asst Prin Engr/ST Aero/ST Group)
Sent by: Subject: Re: [TN] Ionic contamination limit for PWA
TechNet with 0402 sizedcomponents
<[log in to unmask]
RG>
05/29/01 03:01
PM
Please respond
to "TechNet
E-Mail
Forum.";
Please respond
to Brian Ellis
Graham
Having worked in Continental Europe (but not in the EU) for donkey's
years, I sympathise with your point of view. I think that the real
terminological problem is that, in some continental languages, it is not
possible to find a suitable word for a coating. In French, for example,
the nearest word is "couche" (lit. bed, by extension, layer) but
"couchage" as coating is very slang. "Enduit" or "rev¨ētement" would be
more precise but still not the same as coating and are not often used in
this context. On the other hand, in the days when the only protective
coating was a phenolic varnish, the word "vernis" was commonly used, but
this can also mean a pigmented paint, so "laque" was used to denote that
it was transparent, even if it did not contain the sumac resin that
genuine lacquers contain. By extension, this has continued to include,
loosely, conformal coating. But the term "conformal coating", either as
the action of applying or the resulting layer, is really intraductible
in French. "Couchage conforme", for example, is the act of putting baby
to bed (or putting on its nappy/diaper), according to European Union
Directives (I'm sure there must be one on the subject!). :-) So, please,
don't be too hard on those who don't have the mot juste and who are too
intransigeant to accept foreign language terms into their sacred
language. Personally, I prefer "enduit de protection" but this doesn't
connote "conformal".
Brian
Graham Naisbitt wrote:
>
> Pete
>
> I hope you will excuse me commenting on your reply, but you make some
> statements that may be misleading:
>
> <I can't answer for the electronic side of your problem, though it does
> <sound like you have a problem with uncoated boards, humidity and ionic
> <contamination. You may have to kill one bird with two stones - if you
need
> <to put a barrier coating between the boards and the atmosphere
(conformal
> <coating), you will have to get the boards clean enough for a coating to
> <stick.
>
> Ahem. Conformal coatings are not intelligent. The question of adhesion
> always fills me with dread viz: "your coating does not stick to my
board!"
> It sticks to the component, the solder joint and other "bare" areas - so
how
> does it know the difference? Spooky ain't it? :>O
>
> It is fair to say that no-clean flux residues may:
>
> - react with the solder resist
> - provide an intermediate layer that inhibits adhesion
>
> this is often the result of silicone "contamination". Oooer - that could
> incite some severe reactions....
>
> <If you're using a no-clean soldering process, you will still have to
> <clean the boards, and by cleaning them sufficiently for conformal
coating,
> <you may find that your ionic contamination problem disappears, in which
> <case you may choose not to coat.
>
> Good and fair point. Consider that a car without paint may not "live" as
> long - and a conformal coating is a surface finish as well as a terrific
> filter against nasties.
>
> <Coating is good until you need to rework a board, and then it becomes a
> <pain as it has to be removed and that's usually messy and
time-consuming.
> <However, if coating proves to be your answer, an acrylic varnish such as
> <Humiseal 1B31 should do the trick. It can be applied by brush, dipping
or
> <spraying, and dries in ambient atmospheric conditions. Make sure the
board
> <is dry before coating.
>
> VARNISH? You heathen! The difference between that and a coating may well
be
> scientifically nil, but I categorise the difference, that a conformal
> coating is able to tolerate a far wider temperature variation (Tg) than a
> varnish. Then there are those in Europe who call them lacquers - and that
is
> an even more heinous crime - :-)
>
> Regards, Graham Naisbitt
>
> [log in to unmask]
> www.concoat.co.uk
>
> Concoat Limited www.concoat.co.uk
> <http://www.concoat.co.uk>
> Alasan House, Albany Park
> CAMBERLEY GU16 7PH UK
> Phone: +44 (0)1276 691100
> Fax: +44 (0)1276 691227
> Mobile: +44 (0)79 6858 2121
>
> <com@INVETECH To: [log in to unmask]
> .COM.AU> cc: (bcc: DUNCAN Peter/Asst
> Prin Engr/ST Aero/ST Group)
> Sent by: Subject: [TN] Ionic
> contamination limit for PWA with
> TechNet 0402 sized components
> <[log in to unmask]
> ORG>
>
> 05/28/01
> 12:04 PM
> Please
> respond to
> "TechNet
> E-Mail
> Forum.";
> Please
> respond to
> Chuck Mays
>
> I've checked the technet archive and have not found a similar problem, so
> here goes.
>
> I have a problem with a small mobile product that is failing in field
> trials. One area we have considered is a combination of RH and the level
> of ionic contamination affecting high impedance areas of the PWA to cause
> the problem. (The problem exhibits itself as a RAM corruption and the
S/W
> engineers are busily making sure it isn't their problem.)
>
> An external lab checked a sample of the PWA's using the Omega method and
> found the average level to be about 18 ug/sq.in. (I'm aware that 10
> ug/sg.in. is the recommended limit.) The PWA uses 0402 sized resistors
and
> is the finest pitched assembly that the manufacturer in China who builds
> this and other boards for us uses. A no clean process is used. The
other
> products built for us by this manufacturer are OK in terms of ionic
> contamination and field performance.
>
> Most of the failures in field trials have occurred not too far from the
bay
> in one of the suburbs of Sydney where the field trials are in progress.
> The temperature is not high (15 - 20 degrees C), but we are now in the
> rainy season. One day, when it rained in the suburb nearer the bay, and
> not in the other field trial areas, 6 of the field trial units failed in
> the one area.
>
> My questions are:
> 1. Does anyone have experience that indicates that the 10ug/sq.in. level
> for ionic contamination is OK for PWAs that use 0402 sized components?
Or
> does experience indicate that a lower level is more appropriate and if
so,
> what level? (It don't know the exact pitch of this board, but am waiting
> for the answer.)
> 2. Is it possible to determine an impedance above which one should be
> concerned with a level of ionic contamination for a given voltage?
> 3. If it turns out that for this design, RH and the level of ionic
> contamination are causing the problem, can the situation be improved by
> conformally coating the board or areas of the board? (If so, can someone
> please point me to the applicable standards for conformal coating?
> Thanks.)
>
> Best Regards,
>
> Chuck Mays
>
> Invetech Operation Pty Ltd
> [log in to unmask]
>
> Tel 613 9211 7700
> Fax 613 9211 7702
>
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>
>
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