[sdiy] Tantalum manufacturing / assembly failures OT?

Ben Bradley ben.pi.bradley at gmail.com
Thu Nov 15 05:15:46 CET 2018


Indeed, I've "reformed" dozens of old electrolytics bought at hamfests
to 20 percent over rated voltage - if a capacitor charges up (slowly,
using low current, 1 to 10mA) to rated voltage without significant
leakage current, I've found it would always go to +10 percent or more
(for a few minutes) as well, without significant leakage. I expect
that sooner or later at some higher voltage it would fail, but I think
the highest I went was 20 percent over rated voltage. Maybe someday I
could see what happens if ...

It seems to me that, after seeing the second slide, such voltage
"ratings" on tantalums are flat-out misleading - I, as I presume most
designers would, have assumed that voltage means the same thing as it
does on aluminum electrolytics, but the second slide shows it does
not.
But then I've never designed tantalum cap into a product, so I've
never even thought of readomg a tantalum cap data sheet. This series
of slides mentions "this is in the data sheet" at least once.

I've heard/read a lot of lore about tantalums over the decades, mostly
about how they too easily fail short for some supposedly known or
unknown reason, and how they "can't tolerate even a volt over their
maximum rating." I can't help but wonder how much of this is because a
"6 volt" tantalum was put on a regulated 5V power rail when, by this
info, the cap should have had at least a 10V rating.

Aluminum electrolytics used to be (maybe some still are) marked with a
numeric voltage value and the letters "WVDC" for "Working Voltage
Direct Current." It's clear now that the true "working voltage" of a
tantalum, at least for Kemet, is HALF the voltage given.

On Wed, Nov 14, 2018 at 5:54 PM Richie Burnett
<rburnett at richieburnett.co.uk> wrote:
>
> Thank you.  That presentation is *very* interesting indeed.  The failed
> capacitors I witnessed weren't from Kemet but they are based on the same
> technology from a similarly well known brand manufacturer.
>
> The bit about what they consider an acceptable voltage de-rating factor is
> very interesting!  So basically a 25V "rated" device is only good for 12.5V
> service if you want an acceptable failure rate at initial power up and in
> the following 1000 hours of service.  And this is before you take into
> account any degradation due to exposure to reflow temperatures during
> assembly / rework, and any further degradation due to possible electrical
> design issues such as voltage transients, current ripple, etc. if they were
> to exist.
>
> I'm used to generously spec'ing the voltage ratings on ceramics because of
> the nasty voltage-coefficient effect in high-k ceramic caps, but had no idea
> that Tantalum caps needed that much safety margin on their voltage ratings.
> I guess I'm more used to aluminium electrolytics that run quite happily near
> their rated voltage, and are even characterised to tolerate significant
> over-voltage surges for many seconds without damage!
>
> -Richie,
>
>
> -----Original Message-----
> From: Ullrich Peter
> Sent: Wednesday, November 14, 2018 9:16 PM
> To: Richie Burnett ; SDIY List
> Subject: Re: [sdiy] Tantalum manufacturing / assembly failures OT?
>
> Hi Ritchie!
>
> We had some failures with Tantalum capacitors but not that much.
>
> The Austrian Kemet Sales Manager once gave us a presentation about derating
> you should take care of.
> I found this presentation online - you can have a look at it here at
> Digikey:
> https://www.digikey.com/en/ptm/k/kemet/derating-guidelines-for-surface-mount-tantalum-capacitors/tutorial
>
> Ciao
> Peter
>
> ________________________________________
> From: Synth-diy <synth-diy-bounces at synth-diy.org> on behalf of Richie
> Burnett <rburnett at richieburnett.co.uk>
> Sent: Wednesday, November 14, 2018 18:46
> To: SDIY List
> Subject: [sdiy] Tantalum manufacturing / assembly failures OT?
>
> Since we're discussing Tantalum capacitors, has anyone else who manufactures
> things in reasonable quantities, (either hobby, SDIY or in their day job,)
> had problems with SMD Tantalum capacitors failing short or partially shorted
> due to excessing heat during reflow?
>
> I've had a bunch of boards made by a well known UK PCB fab and population
> company, and several off them had 100uF/25V Panasonic SMD Tantalums fail
> short-circuit within about 10 seconds of powering up the board.
>
> Just to rule out a design issue, the two capacitors that were failing only
> saw 15VDC from a well regulated power supply, and charge slowly through a
> resistor so there is no significant inrush current or AC ripple current
> either.  And they weren't installed with incorrect polarity!  The two
> devices that failed on each board are right next to a D2PAK SMD power device
> that is clearly discoloured from excessive temperature during assembly, so I
> can only suspect that the use of a hot-air pencil during assembly is the
> culprit!
>
> I haven't used SMD tants that extensively in electronics, so would be
> interested to hear others experiences, either on list or via PM.
>
> -Richie,
>
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