ESD HELP DESK

Provided by the ESD Association

Understanding Models Is Key

Q. Can you explain the difference between the human body model, the machine model, and the charged device model in regard to how a charge can be transferred to a static-sensitive item?

A. These models represent three different ways in which a static discharge can damage a sensitive device. They are used to describe the ESD event and to classify the ESD sensitivities of devices.

Because humans are a principal source of ESD, the human body model (HBM) is the most commonly used model to describe an ESD event. This model represents the discharge from the fingertip of a standing individual to a device. This type of discharge can occur from touching a sensitive component or printed circuit board. It is modeled by a 100-pF capacitor discharged through a switching component and a 1.5-kΩ series resistor into the device under test.

The charged device model (CDM) represents a discharge from a device, rather than to it. If a device becomes charged (e.g., by sliding down a feeder) and then is discharged by coming into contact with a conductive surface, a rapid discharge occurs from the device to the conductive object. Similarly, a charged device placed on a conductive work surface will discharge rapidly through the work surface, possibly damaging the device. For some devices, this type of event can be more destructive than the HBM event. Although the duration of the discharge is very short—often less than 1 nanosecond—the peak current can reach several tens of amperes.

The machine model (MM) is representative of a worst-case HBM event. Rather than the discharge occurring from the human body to the component, the machine model represents a discharge from an object to the component. Objects can be hand tools or production equipment. This model is characterized by a 200-pF capacitor discharged through a 500-nH inductor directly into a component with no series resistor. Because the magnitude of the discharge increases as the capacitance of the charged body decreases, this type of discharge can be quite damaging if delivered from a small capacitance object such as a pointed tweezers. Protecting products from the effects of static damage begins by knowing how they can be damaged and what their level of sensitivity is. With this key information, you can begin to design your control programs. For additional information, consult the following resources:

1. RG Renniger, "Mechanisms of Charged Device Electrostatic Discharges," in Proceedings of the EOS/ESD Symposium (Rome, NY: ESD Association, 1991).
2. LR Avery, "Charged Device Model Testing: Trying to Duplicate Reality," in Proceedings of the EOS/ESD Symposium (Rome, NY: ESDA, 1987).
3. LR Avery, "Beyond MIL HBM Testing—How to Evaluate the Real Capability of Protection Structures," in Proceedings of the EOS/ESD Symposium (Rome, NY: ESDA, 1991).
4. ESD STM5.1, "Electrostatic Discharge Sensitivity Testing—Human Body Model," ESDA, Rome, NY, 2001.
5. ANSI ESD STM5.2, "Electrostatic Discharge Sensitivity Testing—Machine Model," ESDA, Rome, NY, 1999.
6. ANSI ESD STM5.3.1, "Charged Device Model (CDM)—Component Level," ESDA, Rome, NY, 1999.
7. ESD TR 20.20, ESD Handbook, ESDA, Rome, NY.

To submit questions, go to http://www.ce-mag.com/esdhelp.html. Information on the ESD Association can be found at http://www.esda.org.