[SI-LIST] Re: De-coupling capacitor
- From: "Gupta, Anurag x4500" <agupta@xxxxxxxxxxxxxxxxxxx>
- To: "'Larry.Smith@xxxxxxx'" <Larry.Smith@xxxxxxx>,pradeepa@xxxxxxxxxxxxxxx, ruston_matt@xxxxxxx, si-list@xxxxxxxxxxxxx
- Date: Tue, 21 Jan 2003 11:49:51 -0800
Hi All,
0.1uF 63V used to be a standard value in the European Catalogs; I have not
used one in the US in a long time
Probably Pradeep was using one of those in India.
As far a Shorting of a ceramic cap is concerned I have seen this happen on
physically large ceramic Caps
Sizes 2225, 1812( Used frequently in Telecom Circuits) etc
if manufacturer's - layout, assembly or rework instructions were not
followed.
..... but never on small sizes 0805, 0603
Pradeep - having worked in India and being aware of the constrains under
which the engineers work
- are you certain the cap was not from a "grey" market source ( ....like a
SJP Road outlet - in Bangalore)
Anurag Gupta
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx@ROCKWELLCOLLINS On Behalf
Of Larry Smith <Larry.Smith@xxxxxxx>
Sent: Tuesday, January 21, 2003 9:48 AM
To: pradeepa@xxxxxxxxxxxxxxx; ruston_matt@xxxxxxx
Cc: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: De-coupling capacitor
Pradeep - I'll agree with Matt, I have not seen ceramic caps
rated at
63 Volts. There are many tantalum and electrolytic
capacitors rated at
6.3V, but they usually have much more than 0.1 uF.
I was just telling my manager the other day that I have
never heard of
a ceramic cap shorting. The ceramic dielectric just does
not break
down. The capacitance value may deteriorate near rated
voltage, but
this is not a hard failure. Reduce the voltage and the
capacitance
returns. I don't believe we are anywhere near the ceramic
dielectric
breakdown voltage. (Is that correct? Can anybody confirm
the actual
dielectric breakdown voltage of ceramic?) I have seen
ceramic capacitors
fail for mechanical reasons, cracking, mishandling, etc.
But I have
never heard of a ceramic capacitor failing for any kind of a
wear out
mechanism. Does anybody have any evidence to the contrary?
On the other hand, 6.3 V tantalum and aluminum electrolytic
capacitors
are notorious for shorting, sometimes in spectacular ways.
This can be
caused by too much ripple current, over voltage, aging and
drying out,
reverse insertion, etc. Is it possible that the capacitor
in question
on this thread is one of the latter?
regards,
Larry Smith
Sun Microsystems
> Delivered-To: si-list@xxxxxxxxxxxxx
> From: "ruston, matt" <ruston_matt@xxxxxxx>
> To: "'pradeepa@xxxxxxxxxxxxxxx'"
<pradeepa@xxxxxxxxxxxxxxx>,
jrbarnes@xxxxxxxxx
> Cc: "<" <si-list@xxxxxxxxxxxxx>
> Subject: [SI-LIST] Re: De-coupling capacitor
> Date: Tue, 21 Jan 2003 10:18:47 -0500
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>
> Pradeep:
>
> Hi. A 63V capacitor does not sound right. It sounds like
a 6.3V cap.
> Ceramic caps usually have 6.3V, 10V, 16V, 25V, 50V, 100V,
etc. ratings. If
> it were a 6.3V cap and with the power supply at 5.5V
(max), you would be at
> 87% of rated voltage.
>
> As for the short, I'm no expert, but I can offer a
theory. The caps are
> using thinner and thinner dielectrics nowadays. Any little
defect in the
> dielectric can cause plate metal electromigration into the
defect under
> large voltage gradients (high voltage across thin
dielectrics). If the
> defect punches through the dielectric, this can cause a
plate-to-plate short
> (i.e. electrode-to-electrode).
>
> Regards,
>
> Matt
>
> -----Original Message-----
> From: Pradeep Amrithraj [mailto:pradeepa@xxxxxxxxxxxxxxx]
> Sent: Tuesday, January 21, 2003 4:35 AM
> To: jrbarnes@xxxxxxxxx
> Cc: <
> Subject: [SI-LIST] Re: De-coupling capacitor
>
>
>
> ** Proprietary **
>
> Hi,
> i desoldered the capacitor and tested it. it was a dead
short.The Vcc =
> (5V) is a regulated one. Iam using a DC to DC converter 24
to 5V. so i =
> think there is no chance of getting a higher voltage than
5.5V . My =
> capacitor rating is 0.1uF 63V.
>
> Regards
>
>
> Pradeep Amirtharaj
> Software Engineer
> L&T Emsys Mysore.
> Ph : 91 821 402561
> Fax : 91 821 403752
> Ext : 2728
> Email : pradeepa@xxxxxxxxxxxxxxx
>
>
>
> >>> John Barnes <jrbarnes@xxxxxxxxx> 21/01/2003 10:58:44
AM >>>
>
> Pradeep,
> Have you *verified* that the capacitor is shorted, by
removing it from
> the board, then measuring the capacitor by itself and the
board without
> the capacitor?
>
> Power-to-ground shorts are one of the hardest defects to
find, because
> so many components and so much copper area is involved.
Common problems
> are:
> * A solder bridge, sometimes like a spiderweb of solder
hiding under
> a capacitor or integrated circuit (IC).
> * A bent lead on a quad flatpack.
> * A long lead on a pin-through-hole component, bent and
touching=20
> another net.
> * A solder ball lodged under an IC.
> * A cracked capacitor, whose halves have shifted
slightly-- the=20
> dielectric layers in your capacitor may be < 0.001 inch
(25 microns)
> thick, so just a tiny shift can cause a short.
> * A copper sliver, from poor etching of the printed
circuit board.
> * A power trace run right through a ground via (I made
this error
> on the first pass of a 15" x 7.9" 6-layer board
populated with 950
> components).
> * A piece of wire laying on the card.
>
> If the problem is on the board, start with a thorough
visual inspection
> under a stereo microscope, at 7.5 magnification or so.
You may want to
> "twang" suspicious looking leads with a dowel that you
sharpened in a
> pencil sharpener, to see if they move. =20
>
> If nothing shows up visually, get a sensitive ohmmeter, a
heat gun, a
> can of coolant spray, and a co-worker. Put the ohmmeter
across one of
> the components that shows up shorted, where you can keep
good contact.=20
> Slowly scan the area of the board that has both nets with
the heatgun,
> on both sides of the board, looking for a change in the
resistance of
> the short. When you are close to the short the resistance
will rise OR
> fall, maybe by just a little bit, when it gets hot. Check
these areas
> under the microscope. If you still can't see anything
wrong, heat the
> suspicious areas of the board with the heatgun until you
see the
> resistance change, then use the coolant spray to cool off
small areas
> until you see the resistance shift the other way. It may
take you a
> couple of cycles heating an area with the heatgun, and
cooling it with
> coolant spray, to pinpoint the short.
>
> John Barnes KS4GL, PE, NCE, ESDC Eng, SM
IEEE
> dBi Corporation
> http://www.dbicorporation.com/=20
>
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