2-14
3,000 volts can be easily generated, especially under low-humidity conditions. ESD damage of ESDS
parts or circuit assemblies is possible whenever two or more pins of any of these devices are electrically
exposed or have low impedance paths. Similarly, an ESDS device in a printed-circuit board or even in
another pcb that is electrically connected in a series can be damaged if it provides a path to ground. ESD
damage can occur during the manufacture of equipment or during the servicing of the equipment. Damage
can occur anytime devices or assemblies are handled, replaced, tested, or inserted into a connector.
Q-11.
A MOSFET can be damaged by an electrostatic discharge at approximately what minimum
potential?
ESD-sensitive devices can be grouped by their sensitivity to ESD. Semiconductors fall within the
following categories:
VERY SENSITIVE DEVICES. These include MOS and CMOS devices without input diode
protection circuitry on all input circuits; dielectrically isolated semiconductors with internal
capacitor contacts connected to external pins; and microcircuits using N + guard-ring
construction (with metalization crossing over the guard ring).
SENSITIVE DEVICES. These include all low-power Schottky-barrier and Schottky-TTL
devices; all ECL devices; high input-impedance linear microcircuits; all small-signal transistors
that operate at 500 MHz or higher; all discrete semiconductors that use silicon dioxide to insulate
metal paths over other active areas; MOS or CMOS devices with input diode protection on all
input terminals; junction field-effect transistors; and precision resistive networks.
MODERATELY SENSITIVE DEVICES. These include all microcircuits and small-signal
discrete semiconductors with less than 10 watts dissipation at 25º C, and thick-film resistors.
The following procedure is an example of some of the protective measures used to prevent ESD
damage:
1.
Before servicing equipment, you should be grounded to discharge any static electricity from your
body. This can be accomplished with the use of a test lead (a single-wire conductor with a series
resistance of 1 megohm) equipped with alligator clips on each end. After the equipment has been
completely de-energized, one clip end is connected to the grounded equipment frame; the other
clip end is touched with your bare hand. Figure 2-6 shows a more refined ground strap, which
frees both hands for work.
NOTE: When wearing a wrist strap, you should never use ac-powered test equipment because
of your increased chance of receiving an electrical shock.
