We know to be diligent to ensure proper handling of sensitive electronic assemblies. But a recent project provided a sobering reminder of exactly how diligent. In this case, an unlikely source of foreign-object debris (FOD) - ironically intended to provide protection - doomed an assembly before it ever left the factory floor.
A conformal-coated ECU field return was submitted for analysis. Upon visual examination, a fiber was observed that appeared to be bridging two SMT pads. While FOD is not particularly noteworthy, especially on a unit that has been in service, this particular fiber stood out for a couple of reasons: its presence on the board surface under the conformal coating material, and its metallic appearance.
The Culprit: A Conductive Fiber From a Protective Garment
The fiber was mechanically harvested from the assembly and assessed via SEM imaging and EDX analysis. Our analysis indicated that the object was a silver-coated fiber. The presence of the fiber under the conformal coat indicated that it was deposited on the board surface during assembly, prior to the conformal coat process. The most likely source of this fiber was an ESD smock or similar protective garment. These conductive fibers are woven into the material to provide the desired dissipative properties.
Remember to Be Diligent
This experience can remind us of the importance of monitoring and maintenance of all operational aspects. ESD control protocols are a critical aspect of assuring product quality, but we must also consider the maintenance of those items. As with tools, fixtures, and equipment guarding, ESD smocks and other protective garments should be regularly examined for damage and replaced as necessary. Worn or damaged garments can present the risk of shedding conductive fibers in your production environment and turning your friend into your enemy.