HP A4500A - VMEbus Single Board Computer 744 Motherboard Hardware Manual page 37

ESD Safety Standards
Static charges (voltage levels) occur when objects are separated or rubbed together. The voltage level of a
static charge is determined by the following factors:
• Types of materials
• Relative humidity
• Rate of change or separation
Effect of humidity on ESD charge levels
The following table lists charge levels based on personnel activities and humidity levels.
Activity
Person walking across a linoleum floor
Person walking across a carpeted floor
Person getting up from a plastic chair
a. Source: B.A. Unger, Electrostatic Discharge Failures of Semiconductor Devices (Bell
Laboratories, 1981)
b. For the same relative humidity level, a high rate of airflow produces higher static charges
than a low airflow rate.
c. Some data in this table has been extrapolated.
Static protection measures — computer room environments
Follow these precautions to minimize possible ESD-induced failures in the computer room:
• Maintain recommended humidity level and airflow rates in the computer room.
• Install conductive flooring (conductive adhesive must be used when laying tiles).
• Use conductive wax if waxed floors are necessary.
• Ensure that all equipment and flooring are properly grounded and are at the same ground potential.
• Use conductive tables and chairs.
• Use a grounded wrist strap (or other grounding method) and antistatic mats when handling circuit
boards.
• Store spare electronic modules in antistatic containers.
Static protection measures — office environments
Office areas generally do not offer the same degree of environmental control possible in a computer room.
However, some of the same precautions should be followed:
• Maintain recommended humidity level and airflow rates in the office.
Chapter
a
26%
6,150 V
18,450 V
24,600 V
b
Humidity and charge levels (voltages)
32%
5,750 V 4,625 V
17,250 V
23,000 V
ESD Safety Standards
c
40% 50%
3,700 V
13,875 V 11,100 V
18,500 V 14,800 V
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