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TO 1-1-700 - Robins Air Force Base

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<strong>TO</strong> 1-1-<strong>700</strong><br />

10.20 ELECTROSTATIC DISCHARGE (ESD) COMPO-<br />

NENTS.<br />

Figure 10-14. Magnesium Corrosion Products<br />

Figure 10-15. EMI Bonding Washers for Electronics<br />

Corrosion can degrade the electrical and mechanical properties<br />

of ESD protection systems. The exposure of ESD devices/<br />

ESD protection systems to various environmental conditions<br />

can significantly increase their susceptibility to damage from<br />

the discharge of static electricity. Under field service conditions,<br />

military avionics experience corrosion between metal<br />

surfaces such as electrical bonding/grounding connections.<br />

This problem is two-fold. First, corrosion between metal surfaces<br />

in avionics creates structural weaknesses that undermine<br />

its effectiveness to shield ESDS components from induced<br />

potentials. Second, the corrosive process creates nonconductive<br />

products that decrease the paths of conduction of static<br />

charge away from the device or assembly (see Figure 10-16).<br />

Although no valid method of evaluating the effect of corrosion<br />

on ESD sensitivity exists presently, certain trends can be identified.<br />

The performance of ESD protection for enclosed avionic<br />

equipment can degrade over time. This occurs often after<br />

only 3 or 4 months of exposure to the field environment and<br />

normal use. Galvanic corrosion is usually the principal form of<br />

attack. This type of corrosion can rapidly destroy a bonding<br />

connection if suitable precautions are not observed (refer to<br />

Chapter 7 of <strong>TO</strong> 1-1-689-3 and Chapter 8 of this manual).<br />

Damage to bonding/grounding hardware can cause the buildup<br />

of static potentials that could result in a spark discharge. Without<br />

regular maintenance, the buildup of metal oxides,<br />

absorbed atmospheric pollutants, dust, and field debris will<br />

also contribute to ESD shielding degradation of the avionic<br />

enclosure. This buildup introduces a nonconductive film<br />

between electrical contact materials which can often severely<br />

degrade ESD protection. See Chapter 9 of <strong>TO</strong> 1-1-689-3 and<br />

Chapters 3 and 8 of this manual for the corrosion effects on<br />

ESD and the corrosion prevention, treatment, and control for<br />

ESD.<br />

10-16

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