[eagle] Re: Jim's Phase Noise Number!!

Chuck Green greencl at mindspring.com
Mon Jun 11 21:51:41 PDT 2007


Hi Guys,

> There are issues with Tantalums.  From my perspective (and I could be 
> very WRONG!!!!) Lyle, and Chuck Green are AMSAT's corporate knowledge 
> on space-suitable components.  We should make no changes without 
> consulting them, and possibly Jan King as well.  Lou McFadin may have 
> some useful insight, also.
Don't look for consensus on this topic.  Like most subjects, you can 
find "experts" to support any position you want to take.

We've used a lot of Tantalums with good success.  My choice is to use 
Tantalum rather than Electrolytic whenever possible.  But I lay out PCBs 
and populate them, hence my bias.  I don't normally have to make the 
circuit work.

The position taken by Karl Meinzer is that you should never use 
Tantalums.  The argument I have heard to support this is that the 
failure mode of a Tantalum is a short.  Of course, depending on the 
circuit, if the capacitor fails, it might not make much difference what 
the failure mode is.  But for many circuits, if the capacitor fails, the 
circuit will probably keep working if the capacitor does not short.  I 
have read that this is no longer a problem for modern parts (newer than 
about 10 years) because it is no longer the failure mode.  I do have one 
recent experience with this when I put a part in backward which resulted 
in the predictable explosion.  The failure mode... shorted.

Note that the filter capacitors on the CAN-Do! are not Tantalum (except 
one).  This is because of Karl's rule.  He did allow the one when we 
severally de-rated the voltage rating.

Electrolytic capacitors have their own problems.  The biggest is their 
tendency to dry out, especially in a vacuum.  We address this by sealing 
the open end of the can.  Yes, this creates the potential for an even 
bigger bang, but we haven't seen this yet.  Epoxy has been used.  I also 
use the conformal coating for this purpose as appropriate.  You are not 
likely to use conformal coating on an RF circuit because of the bad 
things it's dialectic constant will likely do to your carefully tuned 
circuits.

My advice is to choose the technology that best suits your needs.  They 
both work and we have a lot of success with both of them when we take 
precautions as described above.

Chuck




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