Category Archives: ANSI/ESD S20.20
A Faraday cage or Faraday shield is an enclosure formed by conducting material or by a mesh of such material. Such an enclosure blocks external static and non-static electric fields. Faraday cages are named after the English scientist Michael Faraday, who invented them in 1836. An impressive demonstration of the Faraday cage effect is that of an aircraft being struck by lightning. This happens frequently, but does not harm the plane or passengers. The metal body of the aircraft protects the interior. For the same reason, a car may be a safe place to be in a thunderstorm.
ESD control products that provide a Faraday Cage or shielding include Statshield® Metal-In and Metal-Out Shielding Bags, Protektive Pak™ impregnated corrugated with shielding layer when using a lid, and Conductive Totes. Statshield® ESD Smocks creates a Faraday Cage effect around the torso and arms of the operator and shields charges from the operator’s clothing from damaging ESD sensitive devices. (Technically, suppressing the electrical field from clothing worn underneath).
There are standard tests measuring the energy penetration of electrostatic discharges to the interior. The Shielding test method per Packaging standard ANSI/ESD S541 is ANSI/ESD STM11.31 and the required limit is less than 50 nanoJoules of energy.
ESD shielding packaging is to be used particularly when transporting or storing ESD sensitive items outside an ESD Protected Area. Per Packaging standard ANSI/ESD S541 section 6.2 Outside an EPA “Transportation of sensitive products outside of an EPA shall require packaging that provides:
- Low charge generation.
- Dissipative or conductive materials for intimate contact.
- A structure that provides electrostatic discharge shielding.”
Definitions from the ESD Association Glossary ESD ADV1.0 include: Faraday cage “A conductive enclosure that attenuates a stationary electrostatic field.”
Electrostatic discharge (ESD) shield “A barrier or enclosure that limits the passage of current and attenuates an electromagnetic field resulting from an electrostatic discharge.”
Electrostatic shield “A barrier or enclosure that limits the penetration of an electrostatic field.”
Note: the ESD Association sells most documents, however, the Packaging standard ANSI/ESD S541 and the Glossary ESD ADV1.0 are free downloads from www.ESDA.org (click Standards and then Documents, scroll down to find documents that can be downloaded at no charge).
A company with an ANSI/ESD S20.20 ESD control program needs to document the program and define ESD protective packaging for ESD sensitive (ESDS) items. Packaging is to be defined for all material movement within the EPA and for outside the EPA. Best practice is to define the required packaging or material handling item on a product’s bill of materials. The ESD packaging is as important as a component part.
Customer contract packaging can take precedence, but otherwise “the organization shall define ESD protective packaging requirements, both inside and outside the EPA per ANSI/ESD S541.” [ANSI/ESD S20.20 section 8.4]
The ESD Association sells most of their documents. However, both ANSI/ESD S20.20 and ANSI/ESD S541 are available as complimentary downloads from www.ESDA.org.
The fundamentals of ESD control include grounding all conductors in the EPA. ESD packaging will have special material composition to lower the resistance so that when grounded, electrostatic charges will be removed to ground.
This is the Summary of ESD Protective Properties ANSI/ESD S541 Table 2:
|Low charging (antistatic)||Materials that have reduced amounts of charge accumulation as compared with standard packaging materials.|
|Dissipative or Conductive Resistance||Provides an electrical path for charge to dissipate from the package.|
|Discharge Shielding||Protects packaged items from the effects of static discharge that are external to the package.|
ANSI/ESD Table 3 lists Test Methods for Electrostatic Protective Packaging:
|Material Property||Test Method||Method Description||Limits|
|Low Charging (Antistatic)||ESD ADV11.2||Tribocharging of tubes, planar materials, bags, unit packs (vibration)||User defined|
|Conductive||ANSI/ESD STM11.11||Surface resistance of planar materials||< 104 ohms|
|ANSI/ESD STM11.12||Volume resistance of planar materials||< 104 ohms|
|Dissipative||ANSI/ESD STM11.11||Surface resistance of planar materials||> 104 to < 1011 ohms|
|ANSI/ESD STM11.12||Volume resistance of planar materials||> 104 to < 1011 ohms|
|ANSI/ESD STM11.13||Surface resistance 2-point electrode||> 104 to < 1011 ohms|
|Shielding||ANSI/ESD STM11.31||ESD Shielding of Bags||< 50 nanojoules|
Note that the unit of measurement for conductive and for dissipative is surface or volume resistance in ohms; not resistivity.
Statshield® bag film construction includes a metalized shielding layer. Shielding bags are designed to dissipate electrostatic charges over their surface, protecting ESDS contents from electrostatic fields and from ElectroStatic Discharge (ESD). Their surface resistance is dissipative, which is recommended when packaging in contact with the ESDS; per ANSI/ESD S541 section A.3 “Dissipative Material for Intimate Contact, to avoid rapid discharge to sensitive items, dissipative materials should be used as the layer of packaging that contacts the item.”
This is also true for enclosed Protektive Pak impregnated dissipative corrugated containers, either closed or with lid in place. The impregnated corrugated has a buried shielding layer that provides the shielding ESD control property so that ESD sensitive items can be stored or transported outside an ESD protected area.
Protektive Pak offers a number of ESD control packaging solutions, including Protektive Pak® Impregnated Dissipative Corrugated Products, Statshield Shielding Bags, Moisture Barrier Bags, Pink Poly Bags, and Labels for use within an ESD protected area (EPA) and for shipping outside an EPA. Protektive Pak products are designed to meet ANSI/ESD S20.20 and the ESD Association Packaging standard ANSI/ESD S541; they provide a great value balancing cost and durability.
ANSI/ESD S541 for the Protection of Electrostatic Discharge Susceptible Items Packaging Materials for ESD Sensitive Items