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
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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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