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A Primer on Electro-Static Discharge Teledyne API - T100 UV Fluorescence SO2 Analyzer
318
+
+
Materials
Makes
Contact
PROTONS = 3
E
LECTRONS = 3
N
ET CHARGE = 0
P
ROTONS = 3
E
LECTRONS = 3
N
ET CHARGE = 0
Materials
Separate
+
PROTONS = 3
E
LECTRONS = 2
N
ET CHARGE = -1
+
PROTONS = 3
E
LECTRONS = 4
N
ET CHARGE = +1
Figure 14-1: Triboelectric Charging
If one of the surfaces is a poor conductor or even a good conductor that is not grounded,
the resulting positive or negative charge cannot bleed off and becomes trapped in place,
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
Styrofoam
TM
pellets during shipment can also build hefty static charges
Table 14-1: Static Generation Voltages for Typical Activities
MEANS OF GENERATION 65-90% RH 10-25% RH
Walking across nylon carpet 1,500V 35,000V
Walking across vinyl tile 250V 12,000V
Worker at bench 100V 6,000V
Poly bag picked up from bench 1,200V 20,000V
Moving around in a chair padded with urethane foam 1,500V 18,000V
14.2. HOW ELECTRO-STATIC CHARGES CAUSE DAMAGE
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 14-1 with the those shown in the Table 14-2,
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.
06807C DCN6650