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11.2. HOW ELECTRO-STATIC CHARGES CAUSE DAMAGE
or static. The most common example of triboelectric charging happens when someone
wearing leather or rubber soled shoes walks across a nylon carpet or linoleum tiled floor.
With each step, electrons change places and the resulting electro-static charge builds up,
quickly reaching significant levels. Pushing an epoxy printed circuit board across a
workbench, using a plastic handled screwdriver or even the constant jostling of
TM
Styrofoam
pellets during shipment can also build hefty static charges
Table 11-1: Static Generation Voltages for Typical Activities
MEANS OF GENERATION
Walking across nylon carpet
Walking across vinyl tile
Worker at bench
Poly bag picked up from bench
Moving around in a chair padded
with urethane foam
Damage to components occurs when these static charges come into contact with an
electronic device. Current flows as the charge moves along the conductive circuitry of
the device and the typically very high voltage levels of the charge overheat the delicate
traces of the integrated circuits, melting them or even vaporizing parts of them. When
examined by microscope the damage caused by electro-static discharge looks a lot like
tiny bomb craters littered across the landscape of the component's circuitry.
A quick comparison of the values in Table 11-1 with the those shown in the Table 11-2,
listing device susceptibility levels, shows why Semiconductor Reliability News estimates
that approximately 60% of device failures are the result of damage due to electro-static
discharge.
Table 11-2: Sensitivity of Electronic Devices to Damage by ESD
DEVICE
MOSFET
VMOS
NMOS
GaAsFET
EPROM
JFET
SAW
Op-AMP
CMOS
Schottky Diodes
Film Resistors
This Film Resistors
ECL
SCR
65-90% RH
1,500V
250V
100V
1,200V
1,500V
DAMAGE SUSCEPTIBILITY VOLTAGE
RANGE
DAMAGE BEGINS
CATASTROPHIC
OCCURRING AT
10
100
30
1800
60
100
60
2000
100
100
140
7000
150
500
190
2500
200
3000
300
2500
300
3000
300
7000
500
500
500
1000
10-25% RH
35,000V
12,000V
6,000V
20,000V
18,000V
DAMAGE AT
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