by the ESD Association
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:
RG Renniger, "Mechanisms of Charged Device Electrostatic Discharges,"
in Proceedings of the EOS/ESD Symposium (Rome, NY: ESD
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.