[SI-LIST] Re: De-coupling capacitor
- From: John Barnes <jrbarnes@xxxxxxxxx>
- To: pradeepa@xxxxxxxxxxxxxxx, si-list@xxxxxxxxxxxxx
- Date: Tue, 21 Jan 2003 00:28:44 -0500
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
another net.
* A solder ball lodged under an IC.
* A cracked capacitor, whose halves have shifted slightly-- the
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.
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.
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/
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