Martin,
I think that the crack you found may be from something unrelated to your
original problem, which is common - the effects of aging on ceramic
capacitors. The capacitance of these parts decreases as they age. It is a
logarithmic function - the reduction commonly being specified as X% per
decade hour beyond the last time the ceramic was exposed to its Curie
temperature (around 125C - 150C), which is why running these parts through a
hot bake, or over wave, will "reset" them. The rate at which they decrease
is strongly dependent upon the dielectric. Your parts are two years old,
and your dielectric is undoubtedly one of the semistable compositions.
Several other sources of information on this topic are":
EIA Standard, EIA-521: Application Guide for Multilayer Ceramic Capacitors -
Electrical
Passive Electronic Component Handbook, 2nd. ed., page 146, by Charles A.
Harper
Kemet Webpage: (This has my vote for the longest, most inscrutable URL
address that I've yet to find - any challengers?)
http://www.kemet.com/kemet/web/homepage/KFBK.nsf/vaFeedbackFAQ/58F4EDC0DEAB8
6BB8525683000558DF0?OpenDocument
Bruce Tostevin
Benchmark Electronics
Hudson, NH
> -----Original Message-----
> From: Martin Christie [SMTP:[log in to unmask]]
> Sent: Tuesday, February 01, 2000 12:42 AM
> To: [log in to unmask]
> Subject: [TN] Ceramic chip capacitors
>
> >
> >Technet guru's,
> >
> >We have a problem with a 1uF 10v -20/+80% (0805) chip capacitor. We have
> >seen some failures at in-circuit test for low capacitance (much less than
> >the -20% tolerance). Some of our test operators have found that by
> reflowing
> >(hand flowing) the component they can bring it back into tolerance. I've
> had
> >some of these components micro-sectioned and on one particular example,
> have
> >found a crack under the metallisation. After checking components off the
> >reel I've found them to be very low, in a lot of cases they were well
> below
> >the -20% tolerance (I've yet to get some micro-sectioned). I've not ruled
> >out the possibility that it could be our placement equipment or thermal
> >shock but we haven't found the same failure mode on any other similar
> value
> >components that we're placing. The reel has now been quarantined and
> another
> >reel (which is well within tolerance) fitted. I'm still awaiting the
> results
> >of the next production.
> >
> >To satisfy my curiosity that it wasn't variation in our meter, I measured
> >some (12 in number) similar value components straight off of another reel
> >from a different manufacturer. I found that they were within tolerance
> but
> >when measured over a period of 3 hours their capacitance increased by
> about
> >10% (I confirmed it wasn't our meter by measuring a known value component
> >over the same period and found less than 3% variation). After contacting
> the
> >manufacturer, I was told it was not unusual for reeled capacitors over 2
> >years old to exhibit this phenomena (well known in his industry) and that
> a
> >bake should be undertaken to bring the components up to their nominal
> values
> >(although he would not comment on the bake parameters). The components
> were
> >manufactured in 1998.
> >
> >I and my colleagues (including our Test Engineers) have never heard of
> this
> >before. Does anyone have any further information on this subject or know
> >what the recommended bake is?
> >
> >Regards,
> >Martin Christie
>
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