Detailed Requirements of an ESD Standard
A large number of ESD standards and test methods are
available.[18], [19] It is instructional to
consider the requirements of a particular standard in more detail. The IEC
61000–4–2 standard, from the International Electrotechnical Commission (IEC) is
typical. This document "defines the immunity requirements and test methods for
equipment which must withstand electrostatic discharges, from operators
directly, and to adjacent objects. Several severity levels are defined which
relate to different environmental and installation conditions."
The tests are to be made in a temperature (15–35°C) and humidity
(30–60 percent) controlled environment. The IEC 61000–4–2 standard specifies the
test set-up to include a metallic (copper or aluminum) ground reference plane
(GRP) on the floor of the test area. A wooden table 0.8 m high is placed on the
GRP. A horizontal coupling plane (HCP) is placed on the table, with the device
under test separated from the HCP by an insulating support that is 0.5 mm
thick.
The standard lists several types of discharge tests, and several
voltages for each type of discharge. All voltages listed should be checked. It
is quite possible for a device under test to pass at a high voltage, and fail at
a low voltage. One way, for example, could be if an IC had an improperly
designed ESD protection circuit that activated between the high and low voltage,
but the IC itself could fail at the lower voltage, where the protection circuit
did not protect it. It is also possible for a high voltage ESD discharge to jump
to a new path that bypasses an IC, where a low voltage ESD discharge may take a
different path through an IC, damaging it.
Field effect test. This test should be
performed before the discharge tests. The standard states, "The EUT [equipment
under test] shall be placed on the insulator on the HCP. The discharge probe,
charged to 15 kV, shall be passed over the entire exterior (top, bottom, front,
back, sides) at a distance of one-half inch."
Direct contact discharge. The tip of the
discharge electrode should touch the EUT before the discharge switch is
operated. The test voltages should begin at 4 kV, and continue in order through
6 kV and 8 kV, with at least ten discharges per polarity per voltage.
Direct air discharge. Air discharge is the
most common, and the least repeatable, of the ESD test methods. The test
voltages should begin at 4 kV, and continue in order through 8 kV, 10 kV, 12 kV,
and 15 kV with at least ten discharges per polarity per voltage.
Indirect discharge. In the indirect discharge test, 8-kV ESD
events are made to occur to the HCP and to a vertical coupling plane placed next
to the EUT. The purpose is to identify failures caused by the induced current
from the nearby discharge.
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