The ESD threat

Materials and plant-including people-acquire and accumulate electrostatic charge during normal activity. The electric field produced at the surface of such a charged body or layer can exceed the normal electric strength of the ambient medium, about 3 MV/m for normal air, and so produce an electrical discharge (ESD). Injection of the suddenly released electrical energy into an electronic device can lead to catastrophic failure while the generation of such sparks in a flammable atmosphere can cause fire or explosion. Simplistically, therefore, the ESD threat can be modelled by the discharge of a capacitor charged to an appropriate potential. In many circumstances this is valid and is in fact used to assess vulnerability of electronic devices and sensitivity of flammable atmospheres. Difficulties arise, however, owing to variability in the sensitivity of the vulnerable system, which is not only dependent on the total energy of the discharge (as defined by the discharging capacitance and its voltage) but also the rise time, magnitude and duration of the discharge current. When the discharges arise directly from the surface of a charged insulator, all these factors are indeterminable and even the total energy in the discharge becomes unpredictable. Determination of the features of the ESD which define the actual threat in each particular circumstance is, therefore, required to establish unambiguous ESD test models. This is an important matter because critical decisions on safety and product quality protection are based on the data obtained in ESD model tests.