ESD Systems - Questions And Answers

When transporting PCBs in the manufacturing process, is there a significant difference between using conductive bins versus dissapative bins?

We are currently using conductive bins. Would it be cost effective or necessary to switch to dissapative?

- Anonymous, Madison, WI

The answer depends on how the bins are used in conjunction with your control program and the handling of your sensitive electronic devices. If all conductors are grounded, including personnel, carts, benches, floors, etc. and the control program is properly executed and well disciplined, then conductive bins may be okay for continued use.

The safest answer is that dissipative surfaces are preferred over conductive ones for ESD Control in the electronics industry. Conductive is defined 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).

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.

You can line the bins with a dissipative matting such as Type T2 to minimize the exposure of conductive surfaces or package the devices in Static Dissipative Bags when stored in conductive bins.

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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