Low ESR Tantalum Failure due to Power On- Design Fix?
Mateo EE
have found a lot of pages on Tantalum Capacitor failures, but very
little satisfactory fixes.
Here is set up: I have an 18.5 volt (7.2 amp) linear regulated power
supply. It is connected to a DC to DC power supply board with a number
of linear and switching power supplies. This power supply board is
connected to another electronic board. The overall electronics draw
about .5 amps at 18.5 volts. The caps that fail are on the input of the
DC to DC power supply board. I am using 47 uF 35 volt Tantalum Low ESR
capacitors. Actually, I have three of these caps in parallel and two
more in parallel individually shielded by a ferrite bead. All fail
over time.
How is the problem occurring? I have many other boards in a similar
set up that the caps are not failing, but in this setup the caps are
failing. I believe it is because this device has can be disconnected
between the regulated power supply and the DC-DC power supply.
Tantalum capacitors are sensitive to instantaneous voltage changes
(dv/dt). This means that they should never be connected to a
mechanical switch or in this case hot plugging the device. (See
http://www.maxim-ic.com/appnotes.cfm/appnote_number/1897/ under
"Component Selection.")
Possible solutions
1. Shoot and kill all service techs and clients that hot plug device.
Unfortunately, this is not an option my manager will let me pursue.
2. Exchange the Tantalum caps with Aluminum electrolytic. I see this
advice mentioned a lot, but the major problem is the life of
electrolytic. Also, the manufactures also don't recommend this, of
course it is not to their advantage... (See
http://www.kyocera.co.jp/prdct/electro/pdf/technical/dipptant.pdf) The
solution recommended is to fix the problem and not switch to a
different component with other problems. Also, my switching power
supplies on the DC-DC power supply board recommend tantalum caps... I
don't want to change to Aluminum electrolytic.
3. Add a series resistor between regulated power supply and rest of
input of DC-DC power supply board. The idea here is to limit inrush
current. But that resistor would need to be very high wattage (18.5
volts at .5 amps)... I don't think that is a feasible option in
this circuit.
4. Add a MosFET in series with input to DC-DC power supply board. The
idea here is to have a RC startup delay. (This is recommended
http://pdfserv.maxim-ic.com/en/an/AN658.pdf under section "keep a
handle on inrush current.") This seems to be the only viable option.
I was wondering if there is anyone that has tried this. If so could
you please discuss results? (This article also recommends using slow
turn on for power supply. This is already incorporated as mush as
possible on the board in question.)
5. Other options? Please let me know.
Thanks,
Matthew
Eeyore
Mateo EE wrote:
> I am having problems with the caps failing and shorting input power. I
> have found a lot of pages on Tantalum Capacitor failures, but very
> little satisfactory fixes.
>
> Here is set up: I have an 18.5 volt (7.2 amp) linear regulated power
> supply. It is connected to a DC to DC power supply board with a number
> of linear and switching power supplies. This power supply board is
> connected to another electronic board. The overall electronics draw
> about .5 amps at 18.5 volts. The caps that fail are on the input of the
> DC to DC power supply board. I am using 47 uF 35 volt Tantalum Low ESR
> capacitors. Actually, I have three of these caps in parallel and two
> more in parallel individually shielded by a ferrite bead. All fail
> over time.
>
> How is the problem occurring?
It's usually dV/dt creating a high I. I've seen the problem, albeit less so,
with ceramic MLCs and even mylar types used for decoupling. Heavily derating the
rated voltage seems to work empiricially
Graham
w_tom
> I am having problems with the caps failing and shorting input power. I
> have found a lot of pages on Tantalum Capacitor failures, but very
> little satisfactory fixes.
>
> Here is set up: I have an 18.5 volt (7.2 amp) linear regulated power
> supply. It is connected to a DC to DC power supply board with a number
> of linear and switching power supplies. This power supply board is
> connected to another electronic board. The overall electronics draw
> about .5 amps at 18.5 volts. The caps that fail are on the input of the
> DC to DC power supply board. I am using 47 uF 35 volt Tantalum Low ESR
> capacitors. Actually, I have three of these caps in parallel and two
> more in parallel individually shielded by a ferrite bead. All fail
> over time.
Put inductance in series on power line. I don't have numbers nor
experience with this problem. But series inductance (and not on ground
that is shared by both power and signals) would blunt the initial power
inrush. Ferrite bead would be no where near sufficient inductance.
Meanwhile, a typically design used a large aluminum electrolytic and
a smaller low ESR tantalum in parallel.
John Larkin
wrote:
Manganese dioxide tantalums are literally bombs. A bit of dv/dt pushes
current into them, a tiny spec of anode gets hot, and the MnO2 is the
oxidizer and the tantalum is the fuel. They don't just explode from
available electrical energy, they chemically ignite.
These caps are *not* reliable as supply bypasses. Replace them with
ceramics, polymer tantalums, or regular or polymer aluminum caps. I
think some of the new niobium caps may be ignition-free, but I have no
experience there.
Slow voltage rampup would help, but is a hassle. Derating the
tantalums by at least 2:1 on voltage may help some, but isn't a
guaranteed fix.
This is unpredictable between manufacturers and even batches.
John
Mateo EE
Graham,
I will look into using various other caps to see if they would work in
this situation.
I would agree that a larger rating on the cap would help solve issues
with over-voltage spikes. But one source (see
http://www.kyocera.co.jp/prdct/electro/pdf/technical/dipptant.pdf under
"Experience from Promotion Screening") seems to suggest that for
issues of dv/dt just the opposite: "...the results showed a slight
increase in failure rate with increased capacitor size and voltage
rating..." Thus increasing the rated voltage would actual cause more
failures due to surge currents!
Thanks,
Matthew
Ancient_Hacker
Mateo EE wrote:
> I am having problems with the caps failing and shorting input power.
So .............. don't use tantalums!
Anything that can't take dv/dt isnt really a capacitor in my book.
It's not like there are no alternatives-- good low ESR aluminum
electrolytics are cheap and not prone to shorting. You might need a
small ceramic in parallel to match the tantalum's HF performance. If
you need long life, there are high temp electrolytics with more hours
possible.
Could be worse, there were those old wet-slug tantalums where if the
seal failed, you got some really nasty acid spilling out all over.
Bob
"Mateo EE" <matthew_...@yahoo.com> wrote in message
news:1164665093....@j72g2000cwa.googlegroups.com...
>I am having problems with the caps failing and shorting input power. I
> have found a lot of pages on Tantalum Capacitor failures, but very
> little satisfactory fixes.
>
We've had similar problems. Here are the possible causes (the last one
should really piss you off):
1) Too high of ripple current due to the turn on voltage ramp rate
(I=CdV/dt), or the load's ripple current
2) Moisture absorbed by caps in storage cause physical deformation of cap
during reflow
3) Counterfeit parts (read on)
A recent batch of AVX 68uF/20V caps were subjected to only 100mA of startup
current (I=CdV/dt), small amounts of load-induced ripple current, and had a
max applied voltage of 12.3V (no overshoot or undershoot). We had about a
10% failure rate.
We had convinced ourselves that it was a moisture problem after reading
several documents -- including a study from NASA. There's one website that
show a SOT223 part blown of its pads due to outgassing from a moisture-rich
cap, and they did an experiment where they covered a board with some flour
(next to a cap) and then put the board through reflow. There was a jet trail
showing the path of the outgassing cap. Upon close inspection they were able
to see the bulge and crack where the gas came from.
Our failing parts were built in early 2006 but reflowed only a few weeks ago
so it seemed likely that could have had time to absorb enough moisture to
cause a problem. AVX told us (before seeing a sample from the "bad" reels)
that they now offer caps packed with a desiccant. They wouldn't directly
admit that there is a potential moisture problem but why else would they
offer them with desiccants now?
However, when we finally did send AVX some parts from the failing reels they
responded (in writing), "We did not manufacture these capacitors." Our jaws
dropped. The parts were purchased from a broker by our contract
manufacturer. That will never happen again.
Anyway, I'm convinced that if you don't exceed the rated ripple current, max
rated voltage, and have dry non-counterfeit caps that these tantalums should
be fine for supply bypassing -- both on the input and output of switchers --
with none of this bullcrap derating by 50% to 70% in voltage.
Bob
Eeyore
John Larkin wrote:
John, I've even seen MLCs look like they caught fire like this too ( 2 leads
left sticking out of the board ). Are you aware of a mechanism that would
explain that ? I always assumed they went low resistance and burnt through
resistance heating in this case.
Graham
Eeyore
Mateo EE wrote:
Hence my 'empirical' caution ! It's far better to address the issue at source. Is it
possible to ramp the supply volts a bit ?
Graham
Eeyore
Mateo EE wrote:
> I am having problems with the caps failing and shorting input power. I
> have found a lot of pages on Tantalum Capacitor failures, but very
> little satisfactory fixes.
As a follow-up, I recall that Epcos ( formerly Siemens ) have an app note on
using tantalums for decoupling.
I have browsed it in the past and amongst other things IIRC they advise derating
to 1/3rd of the rated voltage and fitting a series resistor or fuse !
Graham
TuT
news:1164665093....@j72g2000cwa.googlegroups.com...
I posted on here some time ago about a similar problem we were having with
around 2% of Arcotronics SFE solid tantalums failing short circuit on an
aircraft equipment. (Since there were 9 in parallel, this equated to around
18% equipment failures). The failure was not due to dV/dt issues, (failures
occurred even when we charged the caps through a 10k resistor), nor to high
voltage. We never did get an answer from Arcotonics as to the cause of the
failures - they actually stated that they considered our application benign.
Our solution was to replace the tants with multi-layer polymers and we ended
up using Evox Rifa's MDC part. We were only able to accommodate about a
quarter of the capacitance we had with the tants, but the greatly reduced
esr meant that the MDCs were actually better in our application.
--
T
If it's not broken, don't fix it.
Gerhard Hoffmann
>As a follow-up, I recall that Epcos ( formerly Siemens ) have an app note on
>using tantalums for decoupling.
>
>I have browsed it in the past and amongst other things IIRC they advise derating
>to 1/3rd of the rated voltage and fitting a series resistor or fuse !
They sold their tantal division last summer IIRC.
Gerhard
John Larkin
<rabbitsfriend...@hotmail.com> wrote:
>>
>> Manganese dioxide tantalums are literally bombs. A bit of dv/dt pushes
>> current into them, a tiny spec of anode gets hot, and the MnO2 is the
>> oxidizer and the tantalum is the fuel. They don't just explode from
>> available electrical energy, they chemically ignite.
>>
>> These caps are *not* reliable as supply bypasses. Replace them with
>> ceramics, polymer tantalums, or regular or polymer aluminum caps. I
>> think some of the new niobium caps may be ignition-free, but I have no
>> experience there.
>>
>> Slow voltage rampup would help, but is a hassle. Derating the
>> tantalums by at least 2:1 on voltage may help some, but isn't a
>> guaranteed fix.
>>
>> This is unpredictable between manufacturers and even batches.
>>
>> John
>
>John, I've even seen MLCs look like they caught fire like this too ( 2 leads
>left sticking out of the board ). Are you aware of a mechanism that would
>explain that ? I always assumed they went low resistance and burnt through
>resistance heating in this case.
>
>Graham
>
I don't see every repair we get, but I don't think we have any/many
ceramic cap failures. I'll ask the test folks. I have read that the
super-capacitance ceramics were unreliable when they were first
introduced, but I think they're better now. I think there were
problems with tiny defects in the microns-thick dielectric layers,
maybe a moisture thing, or maybe a result of water washing.
Anybody have experience with mlc failures?
I did recently test a sampling of surface-mount 10 and 25-volt creamic
caps, to see how much voltage they could stand. They all survived up
to the limits of my power supply, 120 volts DC.
John
Frank Miles
Mateo EE <matthew_...@yahoo.com> wrote:
>I am having problems with the caps failing and shorting input power. I
>have found a lot of pages on Tantalum Capacitor failures, but very
>little satisfactory fixes.
[snip]
After a fire started by a tantalum capacitor did some serious damage,
the insurance and legal people convinced Tektronix (over 25 years ago)
to stop using tantalum capacitors in most circumstances. Many others
did the same.
Like John L. said, these are incendiary devices. Maybe you can come up
with a cute turn-on circuit to limit inrush, but it would be better if
you have such control over the design to eliminate them before the boards
are assembled.
-frank
--
Jim Thompson
That's for sure. A client sent me his breadboard to evaluate, but
said there's a short somewhere on the VCC line.
So I power it up with current limit set to 100mA, while I ohm the
trace drops.
With my face right in there... kaboom! A Tantalum had been installed
backwards :-(
Thankfully I was wearing magnifiers which protected my eyes.
...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona Voice:(480)460-2350 | |
| E-mail Address at Website Fax:(480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |
I love to cook with wine. Sometimes I even put it in the food.
The Real Andy
Its been a while, but I had some SM ceramics fail in an 125kHz RFID
circuit. There were in a resonant circuit where the peak voltage was
up around 50V (my calculations only had 16V). IIRC they became OC
eventually, although it did take a while.
Funny enough, it was this verry circuit that put me off using tants
forever. For some strange reason tant's seem to be OC at 125kHz??
>John
John Larkin
The tants I've tested were very good hf shorts. The leaded ones are
dominated by lead length inductance, not the slug itself.
John
Tom Bruhns
If you insist on using tantalums, I think your best option will be to
use a combination of (4) and something like tantalums with built-in
fuses. One of the problems with tantalums is that they can short to a
low, but not zero, resistance and then get really hot if there's enough
available current. I understand some companies have a policy that you
will NOT design in a tantalum unless there is some value of resistance
between it and the supply. You're lucky that your supply is only 18 or
so volts; you can get OSCON caps which are very good (albeit a bit
pricey) that will handle that. I've worked on some boards that are
supplied with 24V, and would not try to put a 25V cap on the line as a
bypass.
You could consider multilayer ceramics these days, too. You might not
want to spend the money on enough ceramics to get the required bulk
capacitance, but some ceramics in parallel with aluminums or regular
esr tantalums might give you the performance you are looking for.
My experience is that tantalums are good for low voltage lines, but are
dangerous to try to use on high voltages, anything over about 10 volts.
Supposedly you'll get better reliability with tantalums that are lower
capacitance for a given case size; reliability of cutting-edge
capacitance-per-unit-volume is questionable. I've also seen some
emperical evidence that low ESR units are also more prone to failure
than standard ones.
I'm pretty sure CALCE has some data on tantalum failures, but I'm not
sure how much is available without a membership.
Hope this empirical stuff is helpful. Please post a followup sometime
when you're pretty sure you have the problem solved. That would be a
useful contribution to the group.
Cheers,
Tom
The Real Andy
<jjla...@highNOTlandTHIStechnologyPART.com> wrote:
I was using SMD tants. I ended up with a mod to replace them with a
leaeded Al Electro which worked a treat. I left not long after, but I
heard they made 10k + a never did change the design.
>John
>
Joerg
The problem is when they graduate from the HF short category into the DC
short category. KABOOM.
--
Regards, Joerg
PeteS
Tants have always been known to have issues. When I was in the Royal
Navy (worked on aircraft, a long time ago) we had posters in the
workshop extolling the problems of tantalum caps.
There's one rule I work to without fail: never, ever, use a tantalum at
the *input* to a power supply. I might, or might not, use one at the output.
Cheers
PeteS
Boris Mohar
<jjla...@highNOTlandTHIStechnologyPART.com> wrote:
>Anybody have experience with mlc failures?
>
>I did recently test a sampling of surface-mount 10 and 25-volt creamic
>caps, to see how much voltage they could stand. They all survived up
>to the limits of my power supply, 120 volts DC.
>
>John
Interesting. Recently I ordered 6800pF COG 0805 cap from Digikey.
495-1945-1-ND Epcos part number B37947K9682J62
http://www.epcos.com/inf/20/10/db/cc_03/01320139.pdf
I was not paying any attention to the voltage rating which I discovered was
16V. Since this was just over the working voltage I was somewhat concerned
so I took it to the 70V limit of my power supply and left it like that
overnight. No measurable leakage in the morning and the capacitance value
was the same as before. Statistic is crappy I realize. Maybe I should buy a
lottery ticket.
--
Boris Mohar
--
Posted via a free Usenet account from http://www.teranews.com
Mateo EE
1.Replace the low ESR Tantalum with:
a. Ceramic MLC (Graham, John, Tom)
b. Mylar (Graham)
c. Polymer tantalums (John)
d. Regular aluminum (John, The Real Andy)
e. Polymer aluminum caps (John,Tut)
f. Good low ESR aluminum (Ancient_Hacker)
g. small ceramic in parallel low ESR aluminum
(Ancient_Hacker, Tom)
h. High temp electrolytic for longer life (Ancient_Hacker)
i. OSCON cap (Tom)
2.Derating Tantalums 2:1 or 3:1 (John, Graham)
3.Counterfeit parts (Bob)
4.Moisture absorbed by caps in storage cause physical deformation of
cap
during reflow (Bob)
5.Surge Current
a. Slow voltage ramp-up (John)
b. turn on voltage ramp rate (I=CdV/dt),(Bob)
c. It's far better to address the issue at source. Is it
possible to ramp the supply volts a bit ? (Graham)
d. Inductor in series with power line (W_Tom)
Here is my response:
First off I don't have the caps in backwards. I am confident that my
suppliers have given us the manufactured caps... I have seen
mechanical issues and possible failure to moisture problems. But I
don't think this problem is related to these issues. I don't think
derating will help because the large the Tantalum the more prone it is
to problems...
I found the following precaution on Sanyo OS-CON caps:
http://www.sanyo.com/industrial/electronic_components/capacitors/os_con/downloads/oscon_precautions.pdf
I did look to much into these OS-CON caps, but basically the same issue
would happen with any low ESR cap...
Basically the inrush current with a low ESR cap is very large. In the
example they give 222 amps! Basically you can take the input voltage
and divide it by ESR value. In my case 18.5 volts / 2 ohms = 9.25
amps! I tested with a large series inductor (220 uH) and found that it
defiantly removes the large dv/dt jumps.
There are about another 30 low ESR Tantalums on the same electronic
setup that have no problems. They are not directly hot plugged and run
on lower voltages. I don't think that all low ESR Tantalums are bad,
but defiantly they should never be put on an unprotected input. On
existing electronics three of the Tantalums are used to regulate line
in and two more are isolated by 10 uH inductor (ferrite bead).
For a rework of existing boards: I plan to remove the three Tantalums
and replace with probably a long life (or high temp) aluminum in
parallel with a ceramic. On the other two caps I plan to increase the
inductance to about 100 uH to 200 uH.
In the future I plan to redesign the board with a large inductor
(100uH-200uH) in series with input power. I will probably also add the
fly back diode for safety.
At least that is the plan for now...
Thanks again for all the input,
Matthew
John Popelish
> In the future I plan to redesign the board with a large inductor
> (100uH-200uH) in series with input power. I will probably also add the
> fly back diode for safety.
Consider using a parallel resistor in place of a flyback
diode across the inductor to damp the overshoot. It will
change the ramp up to a step plus a ramp, but won't allow an
instantaneous capacitor dump through an input short.
Joerg
Overnight? Then one night there is a faint poof, little sparklers fly.
One of them lands in the gift basket that someone got for their birthday
at the office. Flames now lick the sides of it. This catches the stack
of schematics on the shelf of the work bench. A red light begins to
flash at fire station 5 in town, sirens wail...
Fred Bartoli
Did you check a significant batch?
Did you check significant batches from different production runs?
Did you checked it at the max rated temperature?
Did you checked it after significant aging tests?
--
Thanks,
Fred.
Eeyore
Mateo EE wrote:
> There are about another 30 low ESR Tantalums on the same electronic
> setup that have no problems. They are not directly hot plugged and run
> on lower voltages.
Your problem parts were hot-plugged ? Oops !
> I don't think that all low ESR Tantalums are bad,
> but defiantly they should never be put on an unprotected input. On
> existing electronics three of the Tantalums are used to regulate line
> in and two more are isolated by 10 uH inductor (ferrite bead).
>
> For a rework of existing boards: I plan to remove the three Tantalums
> and replace with probably a long life (or high temp) aluminum in
> parallel with a ceramic. On the other two caps I plan to increase the
> inductance to about 100 uH to 200 uH.
>
> In the future I plan to redesign the board with a large inductor
> (100uH-200uH) in series with input power. I will probably also add the
> fly back diode for safety.
>
> At least that is the plan for now...
>
> Thanks again for all the input,
I forgot to mention that although I've seen mylars 'burn up' I've never seen that happen to box polyester
types. Of course by it's connstruction you wouldn't *see* it but I've never been aware of any that failed.
Graham
Boris Mohar
I had it in a covered ceramic clay pot. I also have three smoke detectors.
Your concern is appreciated.
Boris Mohar
Like I said "Statistic is crappy I realize"
TuT
news:bokom2lfq7ovktdf4...@4ax.com...
[snip]
>
> I don't see every repair we get, but I don't think we have any/many
> ceramic cap failures. I'll ask the test folks. I have read that the
> super-capacitance ceramics were unreliable when they were first
> introduced, but I think they're better now. I think there were
> problems with tiny defects in the microns-thick dielectric layers,
> maybe a moisture thing, or maybe a result of water washing.
>
> Anybody have experience with mlc failures?
>
> I did recently test a sampling of surface-mount 10 and 25-volt creamic
> caps, to see how much voltage they could stand. They all survived up
> to the limits of my power supply, 120 volts DC.
>
> John
>
We use MLCs in parallel with the MDC multi-layer polymers on the project I
discussed on here some time ago. (The impedance of the MDCs starts to climb
above about 1MHz).
When the MLCs do fail, they burst into flames and get hot enough that the
PCB itself is a write off. AIUI, the failures are due to microscopic cracks
in the body of the cap, caused either by physical mishandling or by stresses
induced during soldering. Correctly handled and installed, their
reliability is not an issue.
Eeyore
TuT wrote:
> "John Larkin" <jjla...@highNOTlandTHIStechnologyPART.com> wrote in message
> >
> > I don't see every repair we get, but I don't think we have any/many
> > ceramic cap failures. I'll ask the test folks. I have read that the
> > super-capacitance ceramics were unreliable when they were first
> > introduced, but I think they're better now. I think there were
> > problems with tiny defects in the microns-thick dielectric layers,
> > maybe a moisture thing, or maybe a result of water washing.
> >
> > Anybody have experience with mlc failures?
> >
> > I did recently test a sampling of surface-mount 10 and 25-volt creamic
> > caps, to see how much voltage they could stand. They all survived up
> > to the limits of my power supply, 120 volts DC.
> >
> > John
>
> We use MLCs in parallel with the MDC multi-layer polymers on the project I
> discussed on here some time ago. (The impedance of the MDCs starts to climb
> above about 1MHz).
>
> When the MLCs do fail, they burst into flames and get hot enough that the
> PCB itself is a write off. AIUI, the failures are due to microscopic cracks
> in the body of the cap, caused either by physical mishandling or by stresses
> induced during soldering. Correctly handled and installed, their
> reliability is not an issue.
I've seen MLCs fail like that. Do you know what the mechanism is ?
Graham
TuT
news:456EFFC4...@hotmail.com...
Google throws up several articles on the subject. This is about the most
comprehensive I've found so far:
http://www.dfrsolutions.com/Articles/2003_Flex%20Crack_Hillman.Blattau.pdf
Eeyore
TuT wrote:
> "Eeyore" <rabbitsfriend...@hotmail.com> wrote in message
> >
> > I've seen MLCs fail like that. Do you know what the mechanism is ?
> >
> > Graham
> >
>
> Google throws up several articles on the subject. This is about the most
> comprehensive I've found so far:
>
> http://www.dfrsolutions.com/Articles/2003_Flex%20Crack_Hillman.Blattau.pdf
Thanks.
Graham
Terry Given
Marcon also wrote a nice app note re. mechanical cracking and the
associated failures.
both are talking about cracks that start from excess flexure, whereas
the other mechanism is due to thermal shock, caused (usually) by
hand-soldering. I've seen a 500V MLC explode with 200V across it as a
result of this.
as with purely mechanical cracking, it gets worse with bigger caps.
Cheers
Terry
joseph2k
Back in the day, when i was a test engineer, i tested many kinds of
capacitors including very many test lots of ceramics from a group of
manufacturers with all kinds of dielectric compounds. When we finally did
breakdown test on artificially aged ceramics the 50 V parts usually had
first breakdown at about 1 kV; 100 V parts typically had first breakdown
between 3400 V and 3600 V. The results were similar with newly made parts
as well. That was 20 and more years ago.
We tested other kinds of capacitors as well, including several plastic film,
various electrolytics, and some high energy density types. I tried to get
management to try testing some solid aluminum electrolytics that Phillips
pioneered, they would not do it because there was no second source at the
time.
--
JosephKK
Gegen dummheit kampfen die Gotter Selbst, vergebens.
--Schiller
joseph2k
>
> "Mateo EE" <matthew_...@yahoo.com> wrote in message
> news:1164665093....@j72g2000cwa.googlegroups.com...
>>I am having problems with the caps failing and shorting input power. I
>> have found a lot of pages on Tantalum Capacitor failures, but very
>> little satisfactory fixes.
>>
> [snip]
>
> We've had similar problems. Here are the possible causes (the last one
> should really piss you off):
>
> 1) Too high of ripple current due to the turn on voltage ramp rate
> (I=CdV/dt), or the load's ripple current
> 2) Moisture absorbed by caps in storage cause physical deformation of cap
> during reflow
> 3) Counterfeit parts (read on)
>
> A recent batch of AVX 68uF/20V caps were subjected to only 100mA of
> startup current (I=CdV/dt), small amounts of load-induced ripple current,
> and had a max applied voltage of 12.3V (no overshoot or undershoot). We
> had about a 10% failure rate.
>
> We had convinced ourselves that it was a moisture problem after reading
> several documents -- including a study from NASA. There's one website that
> show a SOT223 part blown of its pads due to outgassing from a
> moisture-rich cap, and they did an experiment where they covered a board
> with some flour (next to a cap) and then put the board through reflow.
> There was a jet trail showing the path of the outgassing cap. Upon close
> inspection they were able to see the bulge and crack where the gas came
> from.
>
> Our failing parts were built in early 2006 but reflowed only a few weeks
> ago so it seemed likely that could have had time to absorb enough moisture
> to cause a problem. AVX told us (before seeing a sample from the "bad"
> reels) that they now offer caps packed with a desiccant. They wouldn't
> directly admit that there is a potential moisture problem but why else
> would they offer them with desiccants now?
>
> However, when we finally did send AVX some parts from the failing reels
> they responded (in writing), "We did not manufacture these capacitors."
> Our jaws dropped. The parts were purchased from a broker by our contract
> manufacturer. That will never happen again.
>
Hot shit. Really good evidence that our worst nightmare is fully here.
Cheap Chinese knock-offs that look enough to fool anything short of
knowledgeable Destructive Physical Analysis (DPA). We need to get the
message out, to start raising some hell in DC.
> Anyway, I'm convinced that if you don't exceed the rated ripple current,
> max rated voltage, and have dry non-counterfeit caps that these tantalums
> should be fine for supply bypassing -- both on the input and output of
> switchers -- with none of this bullcrap derating by 50% to 70% in voltage.
>
> Bob
Correctly made solid tantalums should not outgas during wave soldering.
Eeyore
joseph2k wrote:
> Back in the day, when i was a test engineer, i tested many kinds of
> capacitors including very many test lots of ceramics from a group of
> manufacturers with all kinds of dielectric compounds. When we finally did
> breakdown test on artificially aged ceramics the 50 V parts usually had
> first breakdown at about 1 kV; 100 V parts typically had first breakdown
> between 3400 V and 3600 V. The results were similar with newly made parts
> as well. That was 20 and more years ago.
Are you saying that the dielectric strength is that much better ? It might
explain an alleged new development wrt 'super caps' !
> We tested other kinds of capacitors as well, including several plastic film,
> various electrolytics, and some high energy density types. I tried to get
> management to try testing some solid aluminum electrolytics that Phillips
> pioneered, they would not do it because there was no second source at the
> time.
You mean those Philips parts that looked like tantalum beads IIRC ?
Graham
Ross Herbert
wrote:
>I am having problems with the caps failing and shorting input power. I
>have found a lot of pages on Tantalum Capacitor failures, but very
>little satisfactory fixes.
>
Firstly, I would NEVER use solid or dipped tantalums in any power
supply circuitry. The only reliable tantalums to use in these
situations are hermetic sealed types. Of course these too are prone to
failure with excess voltage peaks but not nearly so badly as the solid
types.
My suggestion is that the tantalums (if they must be used for low ESR)
should be hermetic types and should always be permananently connected
at the output of the front end linear supply, not on the DC input of
the DC - DC stage following. The DC - DC stage following the linear
stage should have only low ESR aluminium electros on the input.