Mail*Link(r) SMTP FWD>DESIGN: Tantalum failure modes
Gary,
I believe you tantalum (Ta) electrolytic capacitors are being designed/used
for the wrong thing (misapplication).
The following are some of the general (albeit conservative) printed board
design rules/guidelines should be used as "good design
practices/recommendations" I use to help both circuit design and printed board
design personnel:
1) A good low-impedance ceramic (or similar dielectric) bypass capacitor
should be close coupled and used for each source of "current spiking", such as
an analog/digital integrated circuit. This is not the job for Ta's or other
electrolytic capacitors.
2) Tantalum electrolytic capacitors shall be "sprinkled" around the design
(generally based on current mapping) for each group of 10-20 IC's. The
purpose of the tantalum is not to supply the "surge" current for each active
component, instead it is to add capacitively stored energy to the power and
ground system to stabilize the voltage.
3) A low-impedance ceramic should be located as close to the input
terminals/contacts as possible and an additional Ta shall be located
"close-by".
4) Input power and ground terminals/contacts shall be located adjacent to
each other (close coupled) and not located on opposite edges of the printed
board.
Ta Electrolytic Capacitor Design Guidelines/Requirements.
* Voltage Derating - for all applications, Ta's shall be derated to ~66% of
their full rated voltage. For "serious" Class 3 applications, Ta shall be
used at no more than 50 % of their specified voltage rating, improves the
reliability by about 8X
* Dielectric Breakdown - Ta's in high impedance circuits will self-heal in
about 3 ns. This "self-heal" may/will cause a false signal to be coupled into
your functional circuitry with unplanned results. However, in low impedance
circuits dielectric breakdown will cause permanent damage (due to the I sqr'd
R heating). Therefore, a series resistance of ~3 Ohms/Volt is recommended in
all applications (the series resistance may include the impedances of the
power/ground distribution system.
* Capacitor Impedance - like most electrolytics, Ta's have a
resitive/capacitive impedance that lowers with increasing frequency to about
10 kHz, then there is a trough from about 10 kHz to ~1 MHz, then the impedance
becomes inductive with increasing frequency. At ambient (~25 degrees C) for
the same voltage-capacitance, Ta's have an impedance of about 0.07 Ohms over
the frequency range of 10-150 kHz increasing to about 0.15 at 1 MHz. In
contrast, aluminum electrolytics have a reasonably linear impedance of about
0.4 Ohms over the range of 10 kHz to 1 MNz. The "lower" ESR results in faster
current dumps which may result in greater dielectric heating (I sqr'd R loss).
* Ripple - ripple voltage/current and frequenct effect the reliability of the
capacitor (all types). Read the capacitor manufacturers specification and
application requirements/limitations.
* Voltage Derating and Operating Temperature - failure rate is directly
proportional to about the 3rd power of the ratio of applied voltage to rated
voltage. Operating voltage should be about 50% of rated voltage.
* Reverse Voltages - At ambient, Ta's will withstand a reversed voltage of
about 15% of rated voltage. Aluminum electrolytics don't like reversed
voltages.
If you need some more info. contact me off net.
Ralph Hersey
Lawrence Livermore National Laboratory
email: [log in to unmask]
--------------------------------------
Date: 4/16/96 3:41 PM
From: Gary Peterson
Does anyone have an opinion, comments or data to support a rumor I heard?
I'm told that tantalum caps can fail shorted if required to supply a current
surge. To prevent this failure aluminum electrolytics are preferred on
digital
boards.
I thought Aluminum electrolytics weren't very stable in capacitance over temp
and were not preferred to tantalum for that reason as well as size (more
Capacitance per volume in Ta than Al).
Gary P.
---
Gary D. Peterson
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Subject: DESIGN: Tantalum failure modes
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