 |
||
|
ESD Controls
(Page 2 of 4)
Grounding of Personnel
People are
one of the primary generators of static electricity. The simple act of
walking around or repairing a board can generate several thousands of
volts on the human body. If not properly controlled or dissipated, the
accumulated static charge on a person can easily discharge onto a static
sensitive device. Such an event is known as human body model (HBM)
discharge.
LINKS:
Wrist
straps
are generally the primary means of controlling static charge build-up on
personnel. When properly worn and connected to ground, a wrist strap
keeps the person wearing it near ground potential. Because the person
and other grounded objects in the work area are at or near the same
potential, there can be no hazardous discharge between them. In
addition, static charges are safely dissipated from the person to ground
and do not accumulate.
Fig. 2.
Examples of wrist straps
Wrist straps
have two major components, the cuff that goes around the person's wrist
and the ground cord that connects the cuff to the common point ground.
Most wrist straps have a current limiting resistor molded into the
ground cord head on the end that connects to the cuff. The resistor most
commonly used is a one mega-ohm, 1/4 watt resistor with a working
voltage rating of 250 volts. This resistor would protect the person
wearing it from electric shock in case the point to which the wrist
strap is grounded accidentally gets 'live.'
Wrist straps
should be tested on a regular basis. Daily testing or continuous
monitoring is recommended.
Fig. 3.
Examples of wrist strap monitors
A second
method of controlling electrostatic charge on personnel is with the use
of
conductive floors
in conjunction with
conductive
shoes
or
foot straps.
Any charge build-up on a person wearing conductive shoes will be
dissipated to the conductive floors through the sweat layer between each
foot and shoe. The conductive floors must be properly grounded to
the common ground system.
In addition
to dissipating charge, some floor materials (and floor finishes) also
reduce triboelectric charging. The use of floor materials is especially
appropriate in those areas where increased personnel mobility is
necessary. When used as the primary personnel grounding system,
the resistance to ground including the person, footwear and floor must
be the same as specified for wrist straps (< 35 x 10E6 ohms) or the
voltage accumulation on a person must be less than 100 volts.
The use of
ESD-protective clothing is another way to control charge build-up on a
person.
Fig. 4.
Examples of heel and sole grounders, also known as foot straps
Fig. 5.
Conductive shoes and slippers
Copyright
©
2001-2005
www.SemiconFarEast.com.
All Rights Reserved. |
||