Re: [SI-LIST] : Plane short

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From: jrbarnes@lexmark.com
Date: Mon Apr 30 2001 - 07:24:57 PDT


Subas,
If other boards or raw boards do not have the power-to-ground short, you
probably have:
* A solder bridge.
* A solder splash.
* A solder ball.
* A component turned from its correct location.
* A component off location by one pin (quad flatpacks with power and ground
pins next to each other can easily
    create a power-ground short this way).

Your first step should be to look at the board under a binocular microscope,
starting with the tightest-pitch parts on that supply voltage, and checking the
entire topside and bottomside area. Be sure to check integrated circuit pins,
other component pins, and all vias in the area of the power plane. 7.5 to 10x
magnification is a good place to start, and you may need to go up to 20 to 30x
magnification to see some solder splashes (these look like metallic spider webs
under the microscope).

If you can't see anything suspicious visually, try a technique that was shown to
me by Ralph Tyras (a technician who retired from Lexmark a year or so ago)--
locate/borrow:
* A low-range ohmmeter (with sensitivity in the milliohm range), preferably one
with Kelvin connections and an audible
   output.
* A solder pencil or a heatgun with a small tip.
* A can of cooling spray.

You will probably need to get someone to help you:
1. Put one ohmmeter probe on a convenient voltage pad/via, and the other probe
on a convenient ground pad/via.
2. If the ohmmeter has an audible output, set it for a mid-range tone.
(Otherwise have someone keep a close eye on
     the meter, to tell you when the resistance changes.)
3. Slowly scan the board with the solder pencil or heatgun, looking for a place
where the resistance between the power
     plane and groundplane changes significantly up or down.
4. Use the cooling spray to try to force the resistance in the opposite
direction, then alternate between the solder pencil/
     heatgun and the cooling spray to narrow down the suspicious area.

This takes advantage of the fact that these accidental shorts usually have a
very-small cross-sectional area, thus heating/cooling them will have a big
effect on their resistance.

If you have access to an X-ray machine, you may want to X-ray any ball-grid
array (BGA) parts on your card.

If you are daring, you can hook a current-limited power supply between power and
ground and try to burn out the short. This will work only if the short has a
very small cross section, such as a copper sliver, and you are willing to risk
damaging components if you do manage to burn out the short.

If you have access to a sensitive infrared (IR) camera, you can hook a
current-limited power supply between the supply voltage and ground, and try to
find a spot on the card that shows a temperature rise. You may want to put the
card inside a cardboard box to keep from getting fooled by lab lights reflecting
off metal on the card, or turn off the lab lights in the area temporarily. I
accidently ran a 0.006" +3.3V trace through a ground via on a card that I
designed last year, and used this method to find the error after we had built 20
cards... We wound up painting a raw card with flat black paint, and by running
5 amperes through the short saw about a 1 degree C temperature rise.

                                                   John Barnes Advisory
Engineer
                                                   Lexmark International

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