Human Body, Machine, and Charged Device Models

Another way of modeling the ESD event is by an equivalent circuit model (Exhibit 2). To model a discharge from a human body, an RC network consisting of a capacitor discharging through a resistor is used. One may see differing component values used in the literature, reflecting different assumptions on the geometry and location (e.g., whether the discharge occurs when sitting or standing) of the discharge being modeled. For example, the IEC 61000–4–2 standard^[14] recommends a 150-pF capacitor and a 330-Ω resistor; although the ESD STM5.1 standard from the Electrostatic Discharge Association (an industry organization)^[15] recommends a 100-pF capacitor and a 1500-Ω resistor. To model a discharge from a small metal object, such as a conductive robotic arm, the so-called machine model has been used; however, there is some controversy over how well it represents ESD events encountered in factory equipment settings. Similar to the human body model, different component values have been proposed for this model. The IEC 61000–4–2 standard recommends a 200-pF capacitor and a 0-Ω resistor (i.e., no inductor or resistor); although the ESD STM5.2 standard^[16] recommends a 200-pF capacitor discharged through a 500-nH inductor.

Exhibit 2: ESD Models

A third model, the charged device model (CDM), is more widely used today.^[17] This model represents the discharge associated with a charged component, such as an integrated circuit, through one of its leads. It is viewed as more relevant than the machine model to simulate discharges that occur due to handling of devices in manufacturing processes prior to insertion into products. It is defined by a discharge waveform, not circuit elements, and so strictly speaking is not an equivalent circuit model, but is included here because the waveform greatly resembles the discharge of a capacitor through a very small series inductance and resistance