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Model T101 Instruction Manual A Primer on Electro-Static Discharge
279
Any time a charged surface (including the human body) discharges to a
device. Even simple contact of a finger to the leads of a sensitive device
or assembly can allow enough discharge to cause damage. A similar
discharge can occur from a charged conductive object, such as a metallic
tool or fixture.
When static charges accumulated on a sensitive device discharges from
the device to another surface such as packaging materials, work
surfaces, machine surfaces or other device. In some cases, charged
device discharges can be the most destructive.
A typical example of this is the simple act of installing an electronic assembly into the
connector or wiring harness of the equipment in which it is to function. If the assembly
is carrying a static charge, as it is connected to ground a discharge will occur.
Whenever a sensitive device is moved into the field of an existing
electro-static field, a charge may be induced on the device in effect
discharging the field onto the device. If the device is then momentarily
grounded while within the electrostatic field or removed from the region
of the electrostatic field and grounded somewhere else, a second
discharge will occur as the charge is transferred from the device to
ground.
11.3. COMMON MYTHS ABOUT ESD DAMAGE
I didn’t feel a shock so there was no electro-static discharge: The
human nervous system isn’t able to feel a static discharge of less than
3500 volts. Most devices are damaged by discharge levels much lower
than that.
I didn’t touch it so there was no electro-static discharge: Electro-
static charges are fields whose lines of force can extend several inches or
sometimes even feet away from the surface bearing the charge.
It still works so there was no damage: Sometimes the damaged
caused by electro-static discharge can completely sever a circuit trace
causing the device to fail immediately. More likely, the trace will be only
partially occluded by the damage causing degraded performance of the
device or worse, weakening the trace. This weakened circuit may seem
to function fine for a short time, but even the very low voltage and
current levels of the device’s normal operating levels will eat away at the
defect over time causing the device to fail well before its designed
lifetime is reached.
These latent failures are often the most costly since the failure of the equipment in
which the damaged device is installed causes down time, lost data, lost productivity,
as well as possible failure and damage to other pieces of equipment or property.
Static Charges can’t build up on a conductive surface: There are
two errors in this statement:
Conductive devices can build static charges if they are not grounded. The charge
will be equalized across the entire device, but without access to earth ground, they
are still trapped and can still build to high enough levels to cause damage when they
are discharged.
A charge can be induced onto the conductive surface and/or discharge triggered in
the presence of a charged field such as a large static charge clinging to the surface of
a nylon jacket of someone walking up to a workbench.
07266B DCN6485