What are the dangers of using metal trays when holding electronic components
What is the difference between using a stainless steel tray and a black conductive material such as an anti-static tray that both have a resistivity below 1x10^4 [ohms]? What are the dangers of using metal trays when holding electronic components? Can I put a layer of dissipative foam on a metal tray for static sensitive devices?
Conductive is defined as having a surface resistance below 1x10^4 ohms. Because when the surface of an item is too conductive, a live device can easily discharge (an electrostatic discharge event) when it comes into contact with a conductive surface, whether it is grounded or not. This is the threat of the charging device model.
It is stated in the standard that a resistance lower than 2.2x10^3 ohms will trigger an ESD event due to low resistance. A stainless steel tray has a resistance well below 1 ohm and is very susceptible to a CDM ESD event, whereas a black conductive material may have a resistance to ground of greater than 2.2x10^3 ohms and is less dangerous, however, it is still possible to cause this CDM ESD event.
ESD-ADV53.1 Electrical Requirements: Workstation components shall be connected to common ground with electrical continuity as follows:
Working surface resistance: between 1 x 10^6 ohms and 1 x 10^9 ohms;
Shelf and drawer surface resistance for unprotected ESD sensitive equipment: 1 x 10^6 ohms and 1 x 10^9 ohms;
Personnel ground point resistance: less than 1 ohm;
Support Structure Resistance: Less than 1 ohm
You can lay a layer of foam of static dissipative material on the metal tray to minimize the risk of exposure to the tray's conductive surface, or pack the device in a static shielding bag when storing sensitive components on a conductive tray . By placing dissipative material between the tray and the device will slow down the energy transfer and greatly reduce the chance of an ESD event. Of course, you may also want to check the thickness of the static dissipative material before use, because the static dissipative layer is too thin, and the volume resistivity may be too low to provide protection.