Pinpointing board bugs

Aug. 7, 2002
Improved inspection keeps fixable PCBs off the scrap pile.

Techno-Isel CNC gantries have 0.0005-in. resolution and repeatability, and 0.003-in./ft absolute accuracy, key metrics for tracking traces on densely packed PCBs. Gantries are built on steel stress-relieved bases with hardened-steel linear ways. Heavy castaluminum side plates supporting the Y axis further boost stiffness. Antibacklash ball screws and nuts give low friction, low wear, and long life. EPC 5500s come in three sizes and can handle boards to 64 X 47 in.

A repair station from Electronic Packaging Co., Dallas, helps technicians fix printedcircuit boards that conventional stations have deemed scrap. EPC's Model 5500 points out problem traces by illuminating them with a highresolution color LCD projector mounted on a CNC gantry from Techno-Isel, New Hyde Park, N.Y. The gantry steers the projector with sufficient accuracy and precision to accommodate today's densely packed PCBs. The equipment tracks traces on multilevel boards and locates recurring manufacturing problems.

Operators first perform a bed-ofnails test. Bed-of-nails testers check board electrical connections with a probe array that contacts board grid points. The array connects to automated test equipment programmed to check each network for opens and shorts and report their X-Y (nail) coordinates. The LCD projector then displays images of faulty traces, pads, components, along with step-by-step instructions for repair including test probe locations. Fault-prediction software queries board CAD data for all circuit elements associated with suspect coordinate pairs. Failureanalysis routines identify internal shorts and hence scrap boards. And fault-ranking software collects fault history to gauge repairability of subsequent PCBs.

Earlier methods had technicians poring over design drawings to locate faults. That was okay for single-layer boards where entire traces could be seen by turning the board from side to side. But traces in today's multilayer boards may go in one hole and travel inside the board before exiting the other side, making visual inspection impossible. "PCB inner layers are extensively tested before stack up and are mostly defect-free," says Evan Evans, engineering manager at EPC. "Up to 90% of shorts reside on the outer layers. At one board maker, 65% of boards thought to have internal shorts, didn't. IBM, for example, recovered the cost of its repair station in only six months by salvaging scrapped boards."

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