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

P

PeteS

Jan 1, 1970
0
Joerg said:
The problem is when they graduate from the HF short category into the DC
short category. KABOOM.

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
 
B

Boris Mohar

Jan 1, 1970
0
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.
 
M

Mateo EE

Jan 1, 1970
0
Thanks for all your input. Here are the possible fixes you'll gave:

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/ele...citors/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
 
J

John Popelish

Jan 1, 1970
0
Mateo said:
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.
 
J

Joerg

Jan 1, 1970
0
Boris said:
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.

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...
 
F

Fred Bartoli

Jan 1, 1970
0
Boris Mohar a écrit :
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.

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?
 
E

Eeyore

Jan 1, 1970
0
Mateo said:
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
 
B

Boris Mohar

Jan 1, 1970
0
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...

I had it in a covered ceramic clay pot. I also have three smoke detectors.
Your concern is appreciated.
 
B

Boris Mohar

Jan 1, 1970
0
Boris Mohar a écrit :

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?

Like I said "Statistic is crappy I realize"
 
T

TuT

Jan 1, 1970
0
[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.
 
E

Eeyore

Jan 1, 1970
0
TuT said:
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
 
T

Terry Given

Jan 1, 1970
0
Eeyore said:
TuT wrote:




Thanks.

Graham

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
 
J

joseph2k

Jan 1, 1970
0
Fred said:
Boris Mohar a écrit :

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?

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.
 
J

joseph2k

Jan 1, 1970
0
Bob said:
Mateo EE said:
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.
 
E

Eeyore

Jan 1, 1970
0
joseph2k said:
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
 
R

Ross Herbert

Jan 1, 1970
0
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


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.
 
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