ESD Control/Prevention

ESD Controls (Page 4 of 4)

Air ionization is just one component of a complete static control program. It must not be a substitute for grounding or other methods. Ionizers are used when it is not possible to properly ground everything, or simply as a backup to other static control methods. In clean rooms, air ionization may be one of the few methods of static control available.

The ionization characteristics of air ionizers must be checked regularly, since an improperly working air ionizer can emit a unbalanced stream of positive and negative ions. If that happens, the air ionizer itself would accelerate a static charge build-up, and be the root cause of a serious ESD problem on the line.

Fig. 7. Examples of bench-top ionizers

Fig. 8. Examples of overhead ionizers

Protection of Products from ESD

Packaging and Handling

ESD-sensitive devices must never leave a plant unless they are properly protected from ESD. Direct protection of devices from ESD can be provided by properly selected packaging materials such as antistatic bags. Even intra- or inter-factory transport of devices must be done with the use of ESD-protective carriers.

ESD-protective packaging materials must: 1) be dissipative; 2) exhibit low triboelectric charging tendency; and 3) have the ability to shield their contents from electrostatic fields. The insides of these packaging materials have a low charging layer, while their outer layers have a surface resistivity that's within the dissipative range.

Dissipative materials have a surface resistance greater than 10⁴ but less than or equal to 10¹¹ ohms when tested according to EOS/ESD-S11.11 or a volume resistivity greater than 1.0 x 10⁵ ohm-cm but less than or equal to 1.0 x 10¹² ohm-cm when tested according to the methods of EIA 541.

Electrostatic shielding attenuates electrostatic fields on the surface of a package in order to prevent potential differences from being developed inside the package. Electrostatic shielding is provided by materials that have a surface resistance equal to or less than 1.0 x 10³ when tested according to EOS/ESD-S11.11 or a volume resistivity of equal to or less than 1.0 x 10³ ohm-cm when tested according to the methods of EIA 541. ANSI/ESD 11.31 is used to evaluate the shielding characteristics of bags.

Fig. 9. Examples of ESD-protective bags

Identification of ESD-sensitive Devices

The final element of a good static control program is the use of appropriate symbols to identify ESD-sensitive devices and assemblies, as well as products intended to control ESD. The two most widely accepted symbols for identifying ESDS parts or ESD control materials are defined in ESD Association Standard ANSI ESD S8.1-1993 - ESD Awareness Symbols.

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