[sdiy] Tantalum manufacturing / assembly failures OT?

rburnett at richieburnett.co.uk rburnett at richieburnett.co.uk
Thu Nov 15 13:29:58 CET 2018


Totally agree with everything you said Ben.  Thanks for everyone's 
comments about this on and off list.  It's been a learning experience 
for me.

-Richie,


On 2018-11-15 04:15, Ben Bradley wrote:
> 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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