ESD Threat Categories and Models

In all ESD events, charge flows rapidly either from or to a device. The effect this discharge has on a device is less dependent on the direction of charge flow than it is on the resistance to the charge flow in a given event and on the rise time of the event. ESD models have been developed to helpdistinguish between major types of stress in production and to provide a basis for the determination of relative ESD sensitivities of devices.

Threats in Electronic Production Lines

ESD threats in electronics manufacturing can be classified into three major categories:

 Charged personnel – When one walks across a floor a static charge accumulates on the body. Simple contact of a finger to a device lead of a sensitive device or assembly which is on a different potential, e.g., grounded, allows the rapid transfer of charge to the device.

 Charged (floating) conductor – If conductive elements of production equipment are not reliably connected to ground, these elements may be charged due to triboelectric charging or induction. Then these conductive elements may transfer charge to a device or assembly which is at a different potential.

 Charged device/boards – During handling, devices or boards can acquire a static charge through triboelectric charging or can acquire an elevated electrostatic potential in the field of nearby charged objects. In these conditions, contact with ground or another conducting object at a different electrostatic potential will produce a very fast ESD transient.

This categorization is useful in that each category implies a set of ESD controls to be applied in the workplace. ESD threats from personnel are minimized by grounding personnel through the use of wrist straps and/or footwear/flooring systems. Discharges from conductive objects are avoided by assuring that all conductive parts that might contact devices are adequately and reliably grounded. The occurrence of ESD involving charged devices or boards is minimized by a) preventing charge generation (low-charging materials, ionization) or b) by providing low-current “soft landings” using dissipative materials. Since these preventive measures are seldom perfectly deployed, the overall threat of ESD failure remains and the risk ultimately depends on how well the controls are maintained and the relative sensitivities of the devices being handled.