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'[EE]: Aluminum caps vs. Tantalum'
2001\12\14@184342 by Jay Hanson

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When is it OK to use aluminum caps instead of the outrageously expensive
tantalum?

Jay

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2001\12\14@203527 by brandon

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  If you mean electrolytic when you say aluminum,
electrolytics are extremely lossy, have Higher
inductance and ESR (which makes them respond badly
to high frequency applications), and are very
inaccurate.

-
end

--- Jay Hanson <spam_OUTjayTakeThisOuTspamILHAWAII.NET> wrote:
>When is it OK to use aluminum caps instead of the outrageously expensive
>tantalum?
>
>Jay
>
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2001\12\15@101358 by Anand Dhuru

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If you are using the caps to generate precise (or predictable) timing, use
tanatalums; otherwise the aluminiums are fine, always assuming they are
rated for the right voltage.

Anand

{Original Message removed}

2001\12\15@165523 by Peter L. Peres

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> When is it OK to use aluminum caps instead of the outrageously expensive
> tantalum?
>
> Jay

When you do not care about high leakage, temperature dependence of the
cap. value, and rapid failure at high temperature (<1000 hours), as well
as the danger of leaking electrolyte (after 2-5years) and the effects of
them drying out and losing capacity (and increasing ESR) after 3-5 years.

In general in a consumer product you use Ta caps only for parts that need
to be highly stable and low leakage, low esr. All the others are Al caps.

In a higher end product you will probably be choosy about the make and
type of each part, not only caps.

Peter

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2001\12\15@170347 by Jinx

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From a list discussion a couple of years ago ISTR that there
were two points to consider when selecting either foil or Ta
caps

For foil caps, choose the voltage rating that is nearest (and
obviously above) your voltage rail. A 200V-rated cap on a
5V supply may not re-polarise if the circuit is not used for
some time

For Ta caps, allow a generous over-voltage margin (50%) as
they are wont to fail if pushed anywhere near their voltage rating.
For example use a 10V on a 5V supply, 25V on a 12V supply etc

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2001\12\16@032739 by Russell McMahon

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> For Ta caps, allow a generous over-voltage margin (50%) as
> they are wont to fail if pushed anywhere near their voltage rating.
> For example use a 10V on a 5V supply, 25V on a 12V supply etc

Let me tighten that specification (although I will be flamed for it :-) ).

Tantalum capacitors MUST have a rated voltage which is higher than the
highest voltage or transient voltage which will EVER appear across them.
When exposed to voltages not very much higher than their rated voltage they
will fail magnificently.

Failure is ultimately a very hard metallic short circuit but along the way
you can hope for bad smell, noise (shrieking, moaning etc), smoke, flame and
explosion. In the "best" case I have had all of these in the order listed.

For applications that DEMAND the Tantalum's capabilities consider using a
SOLID aluminium capacitor instead. They have most of the capabilities of
Tantalum and are of similar physical size for equivalent spec but do not
self destruct in the same manner and may be cheaper. Note this is SOLID
Aluminium - not wet film electrolytic. Philips, at least, make them.

If you want to produce a product which MIGHT smell, shriek, explode and
catch fire and/or short out various parts of its circuitry at indeterminate
future dates then Tantalum caps are definitely the way to go ! Because this
isn't what happens in all cases and because, if you design well and are very
careful and lucky you may not have any problems, you are now liable to have
people telling you what fine caps Tantalums are ;-)


regards


               Russell McMahon

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2001\12\16@044012 by steve

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> For applications that DEMAND the Tantalum's capabilities consider
> using a SOLID aluminium capacitor instead.

Just to be picky.
A Tantalum cap _is_ a Solid Aluminium electrolytic cap. Tantalum
is one type of the family 'Solid Al'. Another type is Manganese
Oxide (I think) which Philips market as Solid Al, preventing other
solid Al cap makers from using the term. Ain't trademarks et al
wonderful.

Since both are are solid Al, the performance is about the same and
they age about the same. The difference is that tantalum's
breakdown characteristics get worse with heat. So both ambient
heat and internal heat (especially due to ripple currents) effectively
reduce the operating voltage.
This characteristic is also a problem when they do fail. The Philips
parts will short with a bit of resistance and get really hot and stay
that way (pending any protection) and maybe blow the top off.
When a tant gets really hot, there is more breakdown which
generates more heat which causes more breakdown and so on,
leading to flames.

By the time you size them with some voltage overhead, temp
overhead and made allowances for ripple, you are probably just as
well off with a couple of wet electros and a decent sized ceramic.

Steve
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2001\12\16@155635 by Russell McMahon

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> > For applications that DEMAND the Tantalum's capabilities consider
> > using a SOLID aluminium capacitor instead.

> A Tantalum cap _is_ a Solid Aluminium electrolytic cap. Tantalum
> is one type of the family 'Solid Al'. Another type is Manganese
> Oxide (I think) which Philips market as Solid Al, preventing other
> solid Al cap makers from using the term. Ain't trademarks et al
> wonderful.

This is partly right but not quite as I understand it overall.
Here's a comment on how Tantalum caps are made

       http://www.ncc-matsuo.co.jp/product/tantalum/gijyutue.html

There are indeed process & chemistry differences between brands but Tantalum
caps use the metal Tantalum and Al caps use the metal Aluminum (or Aluminium
if you are down here or in UK :-) ) .Al & Tantalum are close on the periodic
table and have similar characteristics but for wehatever reason the Al metal
caps do not exhibit the extreme over-voltage sensitivity that tantalums do..

Here is what a friend of mine had to say in reponse to my last post -
He has had substantial experience with the two types.
_________________________________

Russell,

Agree whole-heartedly with your comments.

In the old days I used to design TC's into products  - every product turned
out to be a ticking timebomb.  A TC always failed eventually  - usually in a
catastrophic manner.  I have also had a lot of TC's fail in products
designed by other people (PC motherboards especially).

Since switching to solid Al caps, I have never had one fail despite the fact
that a good many of these products have a fairly hard life in terms of
temperature.  Several hundred thousand capacitors can't be wrong.

Regards,

   Ken Mardle

______________________________________



       regards


                  Russell McMahon

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2001\12\17@092458 by Roman Black

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Russell McMahon wrote:

> If you want to produce a product which MIGHT smell, shriek, explode and
> catch fire and/or short out various parts of its circuitry at indeterminate
> future dates then Tantalum caps are definitely the way to go ! Because this
> isn't what happens in all cases and because, if you design well and are very
> careful and lucky you may not have any problems, you are now liable to have
> people telling you what fine caps Tantalums are ;-)

I've seen resistors smoke and explode a lot
too, maybe we should all stop using resistors. ;o)
I don't know what bad experiences you had with
the tantalums but I haven't seen any trouble with
them. But then I never use a 16v cap on a 16v
power supply rail.
-Roman

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2001\12\17@100517 by Russell McMahon

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> Russell McMahon wrote:
>
> > If you want to produce a product which MIGHT smell, shriek, explode and
> > catch fire and/or short out various parts of its circuitry at
indeterminate
> > future dates then Tantalum caps are definitely the way to go ! Because
this
> > isn't what happens in all cases and because, if you design well and are
very
> > careful and lucky you may not have any problems, you are now liable to
have
> > people telling you what fine caps Tantalums are ;-)
>
> I've seen resistors smoke and explode a lot
> too, maybe we should all stop using resistors. ;o)
> I don't know what bad experiences you had with
> the tantalums but I haven't seen any trouble with
> them. But then I never use a 16v cap on a 16v
> power supply rail.

Resistors fail when you exceed there specifications substantially or if they
fail to meet specification.
Tantalum caps fail when you exceed their specs by a very small amount.
Also resistors usually miss out on the shrieking, smell much less evilly
impressive and the explosion from a tantalum puts the average resistor to
shame. Try it - you;; like it. Get a lab supply and a lowish voltage
tantalum cap. Apply increasing voltage above rated value and see what
happens. Great fun - as long as it isn't in a circuit that is meant to be
reliable and useful :-).

A PDF from Tantalum Capacitor maker NEC titled "Notes on the correct use of
Tantalum Capacitors" makes very interesting reading.
See -
   http://www.ic.nec.co.jp/compo/cap/english/products/c_tape/EC0332EJ1V0UM00.pdf

I find the first comment in their preface interesting in view of the rest of
the paper - something lost in the translation perhaps.

It provides a formula and nomogram for failure rates which would make many
designers blanch when they realised its significance.
They provide interesting guidelines
(following are as verbatim as possible but with some editing to reduce size)

eg

Operating voltage relative to rated voltage

- General applications - 70% max
- Power line or low impedance circuit 30% (50% max)
- Derate voltage by 1% per degree C above 85C
(many Al caps are only rated to 85 C and have very short lives at that
temperature so this is not a major problem).

Note that this means that, whereas an Al cap should be operated NEAR it's
rated voltage, a Tantalum should be operated at about 1/3 of its rated
voltage maximum or 50% if voltage transients are extremely well controlled.


Also

"Reliability is increased by inserting 3 ohms pere volt series resistance
into switching circuits, charge-discharge circuits etc".

"If the capacitor is in a low impedance circuit applied voltage should be
less than 1/2 to 1/3 of rated voltage.

"Do not connect two (or more) tantalum caps in series!!!"

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2001\12\17@101917 by Russell McMahon

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> I've seen resistors smoke and explode a lot
> too, maybe we should all stop using resistors. ;o)

Late breaking thought :-) -

Resistors require substantial external energy to destroy them and if run
inside rated specs requires substantial energy either as a very large
shortish spike or a sustained moderate overvoltage to die (provided voltage
ratings are not grossly exceeded). Failure mode is most often open circuit.

Whereas a Tantalum capacitor stores substantial energy within itself and
when subject to a low energy transient that causes it to break down, uses
its own stored energy to destroy itself. The transient can be VERY brief.
Once the oxide layer is gone the cap is too. They usually produce a superb
hard metallic short which makes for excellent crowbar protection. Few power
supplies have enough energy to break the short of a short-circuit Tantalum.
I have seen a 5v  several hundred amp supply manage to do so (DEC PDP11
computer) - there was a nice Tantalum metal bead rattling around inside the
case :-).

Al wet electrolytics when subject to substantial overvoltage (or reverse
voltage) will after a while blow out their end, eject a wad of innards and
spray caustic electrolyte everywhere. But they lack the satisfying true
explosion of a Tantalum and I have never seen an Al cap emit a jet of flame
as Tantalums sometimes do. .



           Russell McMahon

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2001\12\17@121707 by Douglas Butler

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I remember something about tantalum caps needing to be used at a goodly
fraction of their rated voltage.  I.E. on a 5V bus a 25V cap would fail
sooner than a 10V cap.

Does anyone else remember that?

Sherpa Doug

> {Original Message removed}

2001\12\17@123446 by Martin Peach

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I've never had any Ta cap fail except through reverse voltage (because they
were inserted the wrong way round). Then they pop and a bead of molten
tantalum pokes out of a pinhole in the case. Aluminum ones take a little
longer to manifest their discomfort and can sometimes still be used once
they are reoriented.
/\/\/\/*=Martin

----- Original Message -----
From: "Douglas Butler" <RemoveMEdbutlerTakeThisOuTspamIMETRIX.COM>
To: <spamBeGonePICLISTspamBeGonespamMITVMA.MIT.EDU>
Sent: Monday, December 17, 2001 12:12 PM
Subject: Re: [EE]: Aluminum caps vs. Tantalum


> I remember something about tantalum caps needing to be used at a goodly
> fraction of their rated voltage.  I.E. on a 5V bus a 25V cap would fail
> sooner than a 10V cap.
>
> Does anyone else remember that?
>
> Sherpa Doug
>
> > {Original Message removed}

2001\12\17@125045 by chris

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All I'll say on the subject at this point is that tantalum caps are great
when you are dealing with lots of noise in a digital circuit, when you just
can't seem to decouple well enough with cheap ceramic discs. I don't care
why, nor do I care how many more pennies they cost. They just make great
decoupling caps.

CL

{Quote hidden}

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2001\12\17@125936 by David VanHorn

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At 01:01 PM 12/17/01 -0500, Chris Loiacono wrote:
>All I'll say on the subject at this point is that tantalum caps are great
>when you are dealing with lots of noise in a digital circuit, when you just
>can't seem to decouple well enough with cheap ceramic discs. I don't care
>why, nor do I care how many more pennies they cost. They just make great
>decoupling caps.

If you look at what you're trying to decouple, you can do a better job,
rather than just shotgunning it.  Shotgunning can also induce problems.
(you have a 15 ohm resistor in series with the VCC supply, right)

Generally, smaller caps for higher frequencies.
It is also important that the PCB layout supports bypassing.
Layouts that have the cap from VCC on one chip to GND on another chip are
fatally broken.

I generally use a few zero ohm resistors to steer grounds, a few 1-10 ohm
resistors, a few 0.1 and/or 0.01uF caps, and that's about it.

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2001\12\17@170437 by David Lions

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----- Original Message -----
From: "David VanHorn" <dvanhornEraseMEspam.....CEDAR.NET>
To: <EraseMEPICLISTspamMITVMA.MIT.EDU>
Sent: Tuesday, December 18, 2001 4:58 AM
Subject: Re: [EE]: Aluminum caps vs. Tantalum


snip
> It is also important that the PCB layout supports bypassing.
> Layouts that have the cap from VCC on one chip to GND on another chip are
> fatally broken.
>
> I generally use a few zero ohm resistors to steer grounds, a few 1-10 ohm
> resistors, a few 0.1 and/or 0.01uF caps, and that's about it.
>
snip

On this subject, why, on a four layer board (or 6 or 8 or 16), do the power
planes usually go in the centre and the signal layers on the outside.  Isn't
it more logical to put plane on the outside to catch EMI/RFI?

Why have power planes at all.  If you used them as signal layers you would
have almost an extra 2 layers.

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2001\12\17@172136 by Scott Newell

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>On this subject, why, on a four layer board (or 6 or 8 or 16), do the power
>planes usually go in the centre and the signal layers on the outside.  Isn't

Any connections to the center layers have to go through vias.  Connections
to the top layer (SM) or outside layers (through-hole) don't need a via.
Signals usually outnumber power and ground.

Having signal traces on the inside makes it harder to debug a bad board.

Tradition.

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2001\12\17@172147 by Douglas Butler

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> On this subject, why, on a four layer board (or 6 or 8 or
> 16), do the power
> planes usually go in the centre and the signal layers on the
> outside.  Isn't
> it more logical to put plane on the outside to catch EMI/RFI?
>
Planes on the outside would be better from an EMI point of view, and
this is occasionally done in bleeding edge technology.  But normally the
planes are on inner layers so that they have fewer holes in them.  If
you put your ground plane on the top and you put a big BGA on your board
you would have to make a big hole in your ground plane for all the
signal pads.

Sherpa Doug

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2001\12\17@172420 by Eisermann, Phil [Ridg/CO]

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> -----Original Message-----
> From: David Lions [EraseMEyizgarnoffspamspamspamBeGoneOPTUSHOME.COM.AU]
> Sent: Monday, December 17, 2001 5:11 PM
> To: RemoveMEPICLISTKILLspamspamMITVMA.MIT.EDU
> Subject: Re: [EE]: Aluminum caps vs. Tantalum
>
>
> Why have power planes at all.  If you used them as signal
> layers you would
> have almost an extra 2 layers.
>

stated rather simplistically; in general, the more you break up your ground
plane, the less effective it becomes...

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2001\12\17@173438 by David VanHorn

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>
>On this subject, why, on a four layer board (or 6 or 8 or 16), do the power
>planes usually go in the centre and the signal layers on the outside.  Isn't
>it more logical to put plane on the outside to catch EMI/RFI?

You'd think so, but it makes modifying the PCB practically impossible, and
copper sheets are reflectors, not magic black holes.  Proper routing means
you don't radiate nearly as much to start with. I produce multi-processor
designs at >10MHz with switcher power supplies, that pass part 15 with only
a plastic enclosure and two layer boards. No shields, coatings etc.

>Why have power planes at all.  If you used them as signal layers you would
>have almost an extra 2 layers.

In my experience, layer three is worth about .5 layer for routing, and it
goes down from there.
I don't use blind or buried vias though.


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2001\12\17@173944 by chris

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I remember working with 'multi-wire' boards, which had layers of wiring
buried in the resin similarly. Heavier wires needed to be buried deeper.
Troubleshooting power 'layers' was a real bear, but it sure was a good thing
the signal wire layers were nearer the surface. ECN's seemed to focus on
them regularly.

CL

> Having signal traces on the inside makes it harder to debug a
> bad board.
>
> Tradition.
>

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2001\12\17@174312 by David VanHorn

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At 05:49 PM 12/17/01 -0500, Chris Loiacono wrote:
>I remember working with 'multi-wire' boards, which had layers of wiring
>buried in the resin similarly. Heavier wires needed to be buried deeper.
>Troubleshooting power 'layers' was a real bear, but it sure was a good thing
>the signal wire layers were nearer the surface. ECN's seemed to focus on
>them regularly.

That's a good "whatever happened to".. Like Space Food sticks, and fizzies!

I remember, the idea was that they just needed a netlist, no routing
information, and it was supposed to be faster and cheaper than prototype PCBs.

I suspect SMD is what happened to it.

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2001\12\17@181910 by steve

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> >I remember working with 'multi-wire' boards, which had layers of
> >wiring buried in the resin similarly.
> That's a good "whatever happened to".. Like Space Food sticks, and
> fizzies!
> I remember, the idea was that they just needed a netlist, no routing
> information, and it was supposed to be faster and cheaper than
> prototype PCBs.

They were still around a couple of years ago, being marketed
towards high speed, dense backplanes. They were cost effective
against the high layer count PCBs you might otherwise use.


======================================================
Steve Baldwin                Electronic Product Design
TLA Microsystems Ltd         Microcontroller Specialists
PO Box 15-680, New Lynn      http://www.tla.co.nz
Auckland, New Zealand        ph  +64 9 820-2221
email: @spam@steveb@spam@spamspam_OUTtla.co.nz      fax +64 9 820-1929
======================================================

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2001\12\17@185059 by Jay Hanson

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This conversation is priceless for us newbees!  For this of you who want
more, be sure to check the links these guys are supplying.  I followed
Dave's link and found an informative discussion on board layout at
http://www.dvanhorn.org/Micros/All/Bypass.php

Thanks again,
Jay

>{Original Message removed}

2001\12\18@020936 by Russell McMahon

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> I remember something about tantalum caps needing to be used at a goodly
> fraction of their rated voltage.  I.E. on a 5V bus a 25V cap would fail
> sooner than a 10V cap.
>
> Does anyone else remember that?

This is generally true for wet electrolytic aluminium caps which like to be
run near (but below) their rated voltage.
Tantalums (see NEC pdf I mentioned) are much better run at 1/3 to 1/2 their
rated voltage.


       Russell McMahon

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