Hi David,

A technique I had set up succesfully in a large corporation about 15
years ago had very good results, recovering over 70% of multilayer
boards with internal (power/ground) shorts.

First problem is in locating the short, as you essentially have large,
complex parallel circuits to isolate a single point. We had used a "low
ohms" meter (capable of measuring micro ohms) and left one probe
stationary, searching for the lowest resistance with the other probe
moving about the board. We could "zero in" on a single pin, usually a
signal thru hole shorted to an internal plane.

Once the suspect area was isolated, visual inspection was performed by
backlighting the pcb with a halogen microscope light. The FR4 is
transparent enough to project a reasonable image through the clearance
ring of the plane to see a darkened area where a short (usually an
underetch whisker) was present. We then would take a photomicrograph of
the image before the next step.

We would then begin to "excavate" the site using "micro-abrasive"
equipment. I recall (it was a LONG time ago) we experimented with
different materials  such as sodium bicarbonate (very sharp) and
crushed walnut shell (very dull) and found certain abrasives would
remove FR4 and not disturb the Cu, and others would be more aggressive
to the Cu, and less aggressive to the FR4. Once the site of the short
was exposed, we would again take a photomicrograph, this time
top-lighted with the same hallogen light. We'd then remove the short
with the micro-abrasive, clean it thoroughly with IPA, then backfill it
with 2 part epoxy.

We'd then take the photomicrogaphs, log them together with a time
sheet, and bill back the PCB supplier for the rework at burdened rate.

From a business standpoint, the repair was a "profit center" (billing
back to the supplier), so cost justifying the equipment and training to
execute was recovered in a short time, as well as recovering all the
labor and materials already in the assembly at that time, which would
have ended up scrap.

The one drawback at that time, which now I'm led to believe there is a
viable solution, was ESD. The micro-abrasive action created enormous
charges which are unnacceptable. I spoke briefly with one of the
micro-abrasive equipment suppliers several Nepcon's ago, and they claim
this is no longer an issue if ionized air is employed.

Sorry if I got a bit "long winded", folks, but sometimes I just can't
help myself!

If you would like to discuss this "offline" please feel free to contact
me at (714) 364-6614

Regards, and have a great weekend!

Ed Popielarski
QTA Machine

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