In the 1400's European forts were using static control procedures and devices to prevent static discharge from igniting gun powder. Paper mills in the late 1800’s in the U.S. used various techniques to dissipate static electricity from the paper web during the drying process [i]. The graph below illustrates the level of static charge and its subsequent developed voltage of various materials as well as the effect of humidity on the ability to generate and retain charge.
Graph 1: ESD Voltage Level vs. Relative Humidity
Creating of electrostatic charge by contact and separation of materials is known as “triboelectric charging,” derived from the Greek word tribo meaning “to rub.” Once a material is charged and does not have a path to conduct the charge, its considered static. Static electricity is measured in coulombs. The charge “q” is determined by an object’s capacitance “C” and the voltage potential on the object “V”, therefore, q=CV.
Table 1. Static voltage level of materials
The amount of charge developed, electrical potential (voltage) and length of time the charge is retained is a complex matter of material properties, contact area, speed of separation between the materials, relative humidity as well as other factors. As shown in the table and chart, significantly high voltages can be generated and energy stored on an object’s surface. The amount of charge stored and the voltage combine to create a hazard for electronic components and circuitry. How devices fail due to an electrostatic discharge varies, but generally fall into two categories, catastrophic and latent although circuit transient malfunction is also possible.
When a device ceases for function, caused by junction breakdown, metal melting, or oxide failure for example, this is considered a catastrophic failure. Latent defects are more difficult to identify. A device may be partially damaged due to an ESD event, but continue to function, albeit at a reduced operating life. Latent defects are very difficult to find