[SI-LIST] Re: De-coupling capacitor
- From: Larry Smith <Larry.Smith@xxxxxxx>
- To: pradeepa@xxxxxxxxxxxxxxx, ruston_matt@xxxxxxx
- Date: Tue, 21 Jan 2003 09:48:18 -0800 (PST)
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
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> 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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