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Question I was confused regarding the definition of conductive in ESD application. What is the difference using a stainless steel tray and the black conductive material such as tray since their resistivity are both below 1x10^4 [ohms]? What is the hazard of using metal tray when holding electronic components? Can I still use the metal tray and place a static dissipative inlay? - Anonymous, Taipa, NON, Macau
Answer Conductive is defined in the Static Control Business per ANSI/ESD S20.20, ADV 1.0, as having a surface resistance below 1x10^4 ohms. When a surface is too conductive, a charged device can easily discharge (undergo an ESD event) upon contact with a conductive surface whether it is grounded or not. This is the threat of the Charge Device Model, where a charged device comes into contact with a conductive material and discharges (an ESD event occurs). Standards imply that a resistance below 2.2x10^3 ohms has a low enough resistance to trigger an ESD event. Stainless steel has a resistance well below 1 ohm and could be very favorable for initiating CDM ESD events as opposed to a black conductive material that may have a resistance to ground greater than 22,000 ohms and not be as dangereous, but may still be a concern to the program.

ESD Protected Workstations ESD-ADV53.1 Electrical Requirements, Electrical Requirements. Workstation elements shall be connected to, and maintain electrical continuity to, the common point ground as follows: Worksurfaces - Resistance: Between 1 x 10^6 ohms and 1 x 10^9 ohms;

Surfaces of shelves and drawers intended to be used for unprotected ESD sensitive devices - Resistance: Between 1 x 10^6 ohms and 1 x 10^9 ohms;

Personnel Ground Connection Point - Resistance: less than 1 ohm; Support Structure - Resistance: less than 1 ohm

From ESD S4.1, standard on Worksurfaces - Resistance Measurements paragraph 8. Resistance Guidelines Due to wide variety of applications for worksurfaces, specific requirements that could be broadly applied are difficult to determine. However, the following set of guidelines can be used as a starting point for establishing local requirements for the resistance of worksurfaces.

Resistance-to-groundable point 1 x 10^6 to 1 x 10^9 ohms.

Yes, you can line the metal trays with a dissipative matting such as Type T2 to minimize exposure to the conductive surfaces of the tray or package the devices in Static Dissipative Bags when stored on conductive trays.

The threat is that the devices may undergo a Charge Device Model discharge when brought into contact or near proximity to the metal (very conductive) tray. By placing a dissipative material in between the tray and the device will slow down the transfer of energy and greatly reduce the chances of an ESD Event.

The thickness of the static dissipative inlay may need to be checked as the volume resistivity may be too low for thin inlays.

Most ESD Control programs are very good at protecting themselves from the Human Body Model (HBM), but not the Charge Device Model (CDM). The use of dissipative mats, surfaces and packaging material greatly reduces the problem of CDM.
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