Ok Ok Bernhard,
Then lets go back to the beginning.
What are you trying to protect and from what?
Maybe you can use a Conformal coating, back fill with silica sand and top
off the result with a thin potting material. Last saw this in Ford Lansdale
in 1979.
Horses for courses. Give me a response with what your problem is and we'll
go forward from there - is that a lively discussion basis?
Regards,
Graham Naisbitt
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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: Wanner Bernhard <[log in to unmask]>
To: <[log in to unmask]>
Sent: Friday, July 02, 1999 7:32 AM
Subject: [TN] WG: [TN] Embedment, potting - reliability, thermal cycling
> Hi Werner and other interested technetters,
>
> Thanks for your responses. The products in question are small
> modules, potted with a two-component epoxide resin. Such resins are much
> harder than most silicones (rtv etc.). Worldwide there are tons and
megatons
> of small devices, often small DC/DC-converters, which are potted with
(more
> or less) a hard potting compound. Apparently there is no worldwide
epidemic
> of failures because of cracks.
>
> I am personally convinced that a rigrid potting mass kills (SMT-)
> system reliability. I estimate that there are many cracked solder joints
> which don't provoke an electrical failure, because often cracked solder
> joints make contact most of the time like a touch-button. Come on potting
> and lacquering (and MIL) guys honestly, am I the only one with this
opinion?
>
> To qualify a potted SMT-assembly I need to define an effective
test.
> Because the mechanical modulus of elasticity will increase dramatically at
> low temperatures (-25°C...-40°C), simply cycles at low temperature would
> provoke the failure mechanism occuring under real conditions. And this
would
> exactly the opposite to use the acceleration factors described in
IPC-D-279.
>
>
> Can anyone describe to me used/effective thermal cycling
parameters
> for potted SMT-assemblies?
>
> I hope this mail promotes a lively discussion of this subject.
> Bernhard
>
>
> ----------
> Von: Werner Engelmaier[SMTP:[log in to unmask]]
> Gesendet: Freitag, 25. Juni 1999 02:15
> An: [log in to unmask]
> Betreff: Re: [TN] Embedment, potting
>
> Hi Bernhard,
> To my knowledge, an I am the chair of the IPC Reliability
> Committee, there is
> no document that addresses the reliability of
> embedded/potted electronics,
> either THT or SMT. Certainly, the recommendations made in
> IPC-SM-785 for
> accelerated testing are useful in your situation; you just
> will not be able
> to use the acceleration factors.
> The possible problems posed by potting compounds have to
do
> with their CTE,
> their cure temperature [which sets the initial stress
> conditions (you want
> compressive on the solder joints and a cure temperature
> higher than the
> highest operating temperature)], and the operational
> temperature excursions
> (severity and number). These parameters will determine
what
> accelerated test
> conditions should be used and if there is an acceleration
> factor that can be
> applied.
>
> Werner Engelmaier
> Engelmaier Associates, L.C.
> Electronic Packaging, Interconnection and Reliability
> Consulting
> 7 Jasmine Run
> Ormond Beach, FL 32174 USA
> Phone: 904-437-8747, Fax: 904-437-8737
> E-mail: [log in to unmask], Website: www.engelmaier.com
>
>
>
> ----------
> Von: Wanner Bernhard
> Gesendet: Donnerstag, 24. Juni 1999 16:27
> An: [log in to unmask]
> Betreff: Embedment, potting
>
> Hi Technetters
>
> I have to consider and to take decision about the
> reliability of embeded (potted) assembled PCB's (THT and SMT). To evaluate
> the solder joint (and componenet) reliability I intend to perform thermal
> cycles (no load). IPC-SM-785 defines such cycling tests, but it seems
> Conformal Coating has been only with secondary importance and Embeding
> /Potting is not mentioned.
>
> Can anyone tell me if there is an IPC or other
> document handling the reliability of embedded/potted electronics
(especially
> after thermal cycling/lifetime)?
>
> At the good old MIL-world the Requirement 47 of
> MIL-STD-454K (a old version, I know) adresses some MIL-Std's (MIL-I-16923,
> MIL-M-24041, MIL-I-81550 etc.) for this purpose. Now I'm looking after
> equivalentes at the IPC-world (or J-STD)
>
> Bernhard
> [log in to unmask]
>
>
>
>
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