Hi Graham
Currently I have to assess the reliability of small potted assemblies. Some
of this units failed after temp cycles The potting mass is IR401-A/B of a
far-east company. As immeadetly action the mix ratio of this potting mass
has been changed to lower the cure (at room tem.) from 90 Shore A to 45
Shore A. I'm not sure this is enough .
So I intend to qulify such units by
* Design consideration, this means by a simple mechanical model to
estimate the ocuring stress on SMT-solder joints as well to brittle
components as glass diodes and "filigrane" E/I-ferite cores.
* Qualification test besed on thermal cycling.
For potted SMT-assemblies there are some effects in addition to the
considered in IPC-SM-785. All considerations about CTE are also applicable
to any potting mass. To determine the stress on a solder joint (or glass
diode or E/I-ferrite core) beside the CTE also the mechanical modulus of
elasticity has to be considered. As lower the temperature, as more increases
the mechanical modulus of elasticity as more increases the mechanical
stress. This means for potted units a qualification test should cover
thermal cycling reaching into deep temperatures.
Because the trend of IPC-SM-785 to perform thermal cycling test in a temp
range +25°C...+100°C or 0°C...100°C (hi-rel appl.) and to avoid of deep
temperatures (-25°C or -40°C) I'm not sure how significante would be such a
thermal cycling reaching into deep temperatures.
Also it is not easy to determine "the" temperatures in application, because
application covers in house ambient as well as free exposure in a shell
(different clima regions) as well (sometimes) automotive. Because the mix
ratio (most in-house, sometimes free exp./automotive) I don't intend to
apply the hardest occuring parameters for such a qualification test.
I would be pleased if you could recommend cycling parameters, mechanical
considerations or models, and (softer) potting masses.
Bernhard Wanner
----------
Von: Graham Naisbitt[SMTP:[log in to unmask]]
Gesendet: Freitag, 2. Juli 1999 18:43
An: [log in to unmask]
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
[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: 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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