[SI-LIST] Re: Chip caps vs. Tantalum
- From: "Lewandowski, Bob" <blewandowski@xxxxxxxxxxxxxxxxxxxx>
- To: <Michael_Poimboeuf@xxxxxxxxxxxxxx>, <martin@xxxxxxxxxxxxxx>
- Date: Mon, 19 Aug 2002 20:10:09 -0500
There are also issues of a similar nature with very large cap value
ceramics in small package sizes. As capacitance goes up plate thickness
goes down, with a resultant decrease in reliability. The problems are
frequently caused by silver migration, which requires moisture, and/or
plate porosity. In principal ceramic capacitors should be "hermetic",
but it doesn't take much of a break in the surface to cause a problem.
A couple of lifetimes ago, we had some ceramic caps that were
affectionately known as "Blue Bombs" because a several actually
exploded. Surge can be a major issue as well as very low supply
impedance.
---Bob Lewandowski
Stratos Lightwave
> -----Original Message-----
> From: Michael_Poimboeuf@xxxxxxxxxxxxxx
> [mailto:Michael_Poimboeuf@xxxxxxxxxxxxxx]
> Sent: Monday, August 19, 2002 2:01 PM
> To: martin@xxxxxxxxxxxxxx
> Cc: si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] Re: Chip caps vs. Tantalum
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> There are lots of factors favoring ceramic over tantalum - as=20
> well as some the
> other way round.
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> The foremost factor in my experience is reliability and safety.
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> Tantalum caps are more prone to failure due to surge current=20
> from fast turn-on.
> If you search
> the web for surge current failure modes in tantalum=20
> capacitors I think you may
> be able to
> track down the papers regarding circuit designs to limit=20
> inrush current for
> tantalum applications.
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> For instance:
> https://www.onsemi.com/pub/Collateral/AN1542-D.PDF
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> I was involved in an unpublished study regarding computer=20
> system failure,
> explosions, fire and
> so forth due to inrush current failure... In my study we=20
> found that the tantalum
> capacitors failed
> because they were overheated by the contract manufacturer=20
> during assembly and
> rework and
> that inrush current caused them to fail (spectacularly) in=20
> the field afterwards.
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> If chip caps give you sufficient Xc over the frequency and=20
> temperature range
> you're designing
> for, and if you can tolerate microphonic noise and other=20
> issues with ceramics,
> then ceramics
> are better.
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> --
> Michael Poimboeuf
> mkp@xxxxxxxxxxxxxx
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> "Martin Euredjian" <martin@xxxxxxxxxxxxxx> on 08/19/2002 10:37:46 AM
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> Please respond to martin@xxxxxxxxxxxxxx
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> To: si-list@xxxxxxxxxxxxx =20
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> cc: (bcc: Michael K Poimboeuf/AM/Avid) =20
> =20
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> Subject: [SI-LIST] Chip caps vs. Tantalum =20
> =20
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> I've been trying to figure out what the differences might be between
> large-value (1 to 100uF) chip caps and similarly valued=20
> Tantalums. This, of
> course, as it pertains to PDS or switch-mode DC-DC converter design.
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> When/Why might one choose a chip cap over a Tantalum? What are the
> advantages/disadvantages in comparison?
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> I've seen pictures of modern DC-DC modules that seem to use several
> large-value chip caps. If that's the case, why did the=20
> designer go this
> route?
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> Thank you,
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> =
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=3D=3D=3D=3D=3D=3D
> Martin Euredjian
> eCinema Systems, Inc.
> voice: 661-305-9320
> fax: 661-775-4876
> martin@xxxxxxxxxxxxxx
> www.ecinemasys.com
> =
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