Questions And Answers

Dissipative material handled by a properly grounded operator will have electrostatic charges conducted to ground. A clear dissipative bag or Metal In bag is preferable to a conductive bag or packaging. Per Packaging standard ANSI/ESD S541 section 7.2.2 Resistance of Dissipative Materials “A static dissipative material shall have a surface resistance of greater than or equal to 1.0 x 10^4 Ohms but less than 1.0 x 10^11 Ohms. Packaging materials that are in intimate contact with devices should be dissipative.”

Surface resistance in ohms is the required limit unit of measurement in ANSI/ESD S20.20 and ANSI/ESD S541. ElectoStatic charge is a surface phenomenon since like charges repel. So if an item has a positive charge, it will reside on the surface. “The normal stable structure of the atom shows that unlike charges attract and like charges repel. Therefore, a separated charge will be self-repellent and will reside only on the surface of a charged item. If the item were a perfect insulator or perfectly insulated, a separated charge would remain indefinitely. Since there are no perfect insulators and it is impossible to have perfect isolation, charges tend to leak away to join opposite charges in the immediate environment thus returning the item to an electrically balanced state.

Taking all of the above into account, static electricity is the set of phenomena associated with electrical charges on the surface of an insulator or insulated conductive body.” [ESD Handbook ESD TR20.20 section 2.3 Nature of Static Electricity Figure 2]

A conductive layer is required to provide ESD shielding property, however a product like Black Conductive Bag does not claim to possess the ESD control property of shielding. Statshield Metal In Bags that have a construction of inner and outer dissipative layers with a buried metal layer easily meets Packaging standard ANSI/ESD S541 Table 2 Shielding requirement of less 50 nanoJoules tested per ANSI/ESD STM11.31 energy penetration test. We preformed an energy penetration test on a black conductive bag and measured over 416 nanoJoules.

Will the small quantity of the unbalance electrical potential damage the sensitive components?
Answer: ESD sensitive items need to be tested to determine their ESD withstand voltage or sensitivity using three models, Human Body Model (HBM), Charged Device Model (CDM), and/or Machine Model (MM).

Is it too tight for a volume resistivity of no greater than 1.0 x 10 to the power of 8 ohm-cm?
Answer: The tolerance of no greater than 1 x 10E8 ohms volume resistance should be achievable depending upon the product. I am not aware of a transparent bag, such as a metal in or metal out ESD bag having that low of volume resistance. Note: the proper unit of measurement for resistivity is ohms. Per ESD Handbook TR20.20 section 2.6.1 Resistance and Resistivity “For surface treated materials, the conventional way to describe the electrical property has been ohms/square. As discussed in recent literature, this terminology is incorrect. The actual measurement is ohms only. Details of this discussion may be found in the book titled “Electrostatics” by Professor Niels Jonassen, Technical University of Denmark.”

Is there a spec. available in the ESD community to set the safe value of volume resistance or volume resistivity?

Answer: Surface resistance in ohms is the required limit unit of measurement in the ESD Association Packaging standard ANSI/ESD S541. You can specify volume resistance which is by definition less than 1 x 10E4 ohms. To test planar materials to verify volume resistance, the noted test method in ANSI/ESD S541 is ESD STM11.12. These documents can be purchased from www.ESDA.org. Note STM11.12 is the test procedure for a lab to test the material not advice on what the material should be.

As noted above, per ANSI/ESD S541 “Packaging materials that are in intimate contact with devices should be dissipative.”

Is it safe to use ESD dissipative containers for ESD sensitive components?

Answer: Yes, it is done all the time by leading companies. However, a conductive shielding layer is required to transport or store ESD sensitive items outside the ESD protected area. A dissipative surface is recommended for intimate contact with ESD sensitive items. For example, the inner layer of an ESD bag.

Per ANSI/ESD S541 section 6.2 Outside an EPA “Transportation of sensitive products outside of an EPA shall require packaging that provides:

1. Low charge generation.

2. Dissipative or conductive materials for intimate contact.

3. A structure that provides electrostatic discharge shielding.