3-39
Table 3-2.—Typical Measured Statics Charges (in volts)
RELATIVE HUMIDITY
ITEM
LOW (10-20%)
HIGH (65-90%)
WALKING ACROSS CARPET
35,000
1,500
WALKING OVER VINYL FLOOR
12,000
250
WORKER AT BENCH
6,000
100
VINYL ENVELOPES FOR WORK INSTRUCT.
7,000
600
POLY BAG PICKED UP FROM BENCH
20,000
1,200
WORK CHAIR PADDED WITH URETHANE FOAM
18,000
1,500
Metal oxide semiconductor (MOS) devices are the most susceptible to damage from ESD. For
example, an MOS field-effect transistor (MOSFET) can be damaged by a static voltage potential of as
little as 35 volts. Commonly used discrete bipolar transistors and diodes (often used in ESD-protective
circuits), although less susceptible to ESD, can be damaged by voltage potentials of less than 3,000
electrostatic volts. Damage does not always result in sudden device failure but sometimes results in
device degradation and early failure. Table 3-2 clearly shows that electrostatic voltages well in excess of
3,000 volts can be easily generated, especially under low-humidity conditions. ESD damage of ESDS
parts or circuit assemblies is possible wherever 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.
Electrostatic discharge 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.
Technicians should be aware of the many sources of static charge. Table 3-3 lists many common
sources of electrostatic charge. Although they are of little consequence during most daily activity, they
become extremely important when you work with ESD material.
