Low ESR Tantalum Failure due to Power On- Design Fix?

[Mateo EE]

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? 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]

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]

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? 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

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]

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

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]

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]

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.

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]

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

 

[Eeyore]

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

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]

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]

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? 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

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.

 

[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]

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]

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]

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

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

 

[The Real Andy]

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.

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 Larkin]

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??

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]

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? 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

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]

The tants I've tested were very good hf shorts. The leaded ones are
dominated by lead length inductance, not the slug itself.

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.

 

[Joerg]

The tants I've tested were very good hf shorts. The leaded ones are
dominated by lead length inductance, not the slug itself.

The problem is when they graduate from the HF short category into the DC
short category. KABOOM.