ESD Purpose
Discussion
Electrostatic discharge (ESD) to electronic devices can occur
at any point from manufacturing, assembly, to installation into
a spacecraft. Numerous published papers from industry, military,
and aerospace organizations as well as alerts from the Government
Industry Data Exchange Program (GIDEP) have expounded on the failures
of electronic parts due to ESD. The cost of not implementing effective
ESD controls can be substantial in terms of lost dollars for failed
parts. For example, a failure of a part may cost in the area of
tens to hundreds of dollars; however, failures occurring at the
assembly levels can range from tens to hundreds of thousands of
dollars. Damage to an electrostatic discharge sensitive (ESDS)
device by an ESD event is determined by the device's ability to
dissipate the energy of the discharge or withstand the voltage
levels involved. This is known as the device's "ESD sensitivity".
ESD damage is usually caused by one of three events: direct electrostatic
discharge to the device, electrostatic discharge from the device
or field-induced discharges.
Discharge to the Device
An ESD event can occur when any charged conductor (including the
human body) discharges to an ESDS device. The most common cause
of electrostatic damage is the direct transfer of electrostatic
charge from the human body or a charged material to the ESDS device.
When one walks across a floor, an electrostatic charge accumulates
on the body. Simple contact of a finger to the leads of an ESDS
device or assembly allows the body to discharge, possibly causing
device damage. The model used to simulate this event is the Human
Body Model (HBM). A similar discharge can occur from a charged
conductive object, such as a metallic tool or fixture. The model
used to characterize this event is known as the Machine Model (MM),
which was designed as a worst case HBM. A charged HBM is a current
source that can be modeled by a 100 – 150 pF capacitor discharging
through a 1500 Ohm resistor into the device being tested. The discharge
has a waveform with a rise time of 2 to 10 ns and duration of about
150-160 ns, see Figure 1.
Discharge from the Device
The transfer of charge from an ESDS device, typically through
the pin contacts, is also a concern. Static charge may accumulate
on the ESDS device itself through handling or contact with packaging
materials, worksurfaces, or machine surfaces. This frequently occurs
when a device moves across a surface or vibrates in a package.
The model used to simulate the transfer of charge from an ESDS
device is referred to as the Charged Device Model (CDM). The CDM
is an energy source that can discharge through its lead into another
device of different potential. The rise time often is less than
0.5 ns, and the duration can be less than 2 ns, see Figure 1.
Figure 1. Idealized Current
Waveforms of HBM & CDM
Field Induced Discharges
Another event that can directly or indirectly damage devices is
termed field induction. Whenever any object becomes electrostatically
charged, there is an electrostatic field associated with that charge.
If an ESDS device is placed in that electrostatic field, a charge
may be induced on the device. If the device is then momentarily
grounded while within the electrostatic field, a transfer of charge
from the device occurs as a CDM event. If the device is removed
from the region of the electrostatic field and grounded again,
a second CDM event will occur as charge (of opposite polarity from
the first event) is transferred from the device.
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