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Case Studies

For want of a flat washer, a telephone exchange died

A manufacturer of rack mount telephone switching equipment had a design flaw in their gear that caused a fire in a telecom facility. They had attached the backplane connectors with a #4-40 screw and a single tooth lock washer without a flat washer underneath. Compounding this mistake, the power plane was just below the external ground plane. The power plane did not have a keep-out zone. When the screw was tightened, a short between the two planes occurred.

Zonk Book
PCB is soft material
One tooth is the problem

3D Scanner to the rescue

MTSI was contracted to build a 3-D scanner that was used to scan the depth of the dent in hundreds of these small dents in the backplane. A laser rangefinder and a digital microscope were attached to a custom frame with stepper motors. This could be attached to the backplane to get a solid and stable reference point while the telecom equipment remained powered up and functional.

A LabView program scanned the area of the dent, drew a 3-D image of the dent, and measured the depth of the deepest dent after calculating the average surface as a reference point.