By Jennifer Hazen, Editor
The art (or is it a science?) of grounding personnel
at ESD safe workstations is much more developed than even I realized
before my interview with Bette Palmisano of Worksurfaces Corporation.
Ms. Palmisano says that the use of top quality hardware allows
a very reliable grounding system with less chances for breakage
At an ESD safe workstation personnel must use
wriststraps to ensure they are grounded and therefore drain or
dissipate static charges which could wreck ESD sensitive
items. These wriststraps usually have a 1 Meg Ohm resistor in
their connector or wiring which protects the wearer from hazardous
electrical potentials. The connection of the workbench and ground
is usually through a "banana" plug or "alligator"
Worksurfaces Corporation as well as other companies
supply a variety of sophisticated connection hardware to provide
reliable grounding means for personnel and workbenches.
One major problem encountered by wrist strap users
is the stress placed on the connection and wiring during normal
use. This stress placed on the wire and plug of the wrist strap
as the user moves around eventually will cause a break and loss
of grounding. This renders the wrist strap inoperative. Most companies
require wrist strap users to check the wrist strap before beginning
work each day. Some require the user to check the connection each
time they return to their work area after breaks, lunch etc. Many
companies use continuous monitors on the wrist strap connection.
If a wrist strap wire or connector breaks between operational
checks, all parts handled since the last check are suspect and
probably should be tested if not discarded.
Worksurfaces Corporation has developed a unique
connector system to help eliminate the problem of stress on the
wiring and connector. They have a neat 3600 swivel
connector that moves with the user as they perform their job tasks.
We tested the connector over an extended period
of time and found it worked flawlessly. In fact we saw no change
in resistance during our entire test. The resistance varied between
0.1 Ohms and 0.2 Ohms. We looked at the connection with an oscilloscope
while the connection was loaded with a small amount of electrical
current about 10 mA. We saw virtually no "scratchiness"
in the signal - < 0.01 mV. This is a good test for the integrity
of the connection under use or as it swiveled. We found the swivel
connector to be highly functional and exceptionally reliable.
One other very important issue with grounding
of ESD workstations is the ability to connect the groundable point
to the work bench surface which is usually multilayer. In other
words, the ability to connect to the buried conductive layer is
difficult. Special connectors are available from Worksurfaces
Corporation which have "teeth" that bite into the conductive
layer and make very good contact.
Our tests of these connectors
proved as satisfying as for the swivel connector. We delaminated
a worksurface to expose the inner conductive layer so we could
connect directly to that layer. We installed one of the Worksurfaces
Corporation connectors and measured the resistance to the conductive
layer. We found the resistance to be approximately 1 Ohm at 10
Vaughan Roundtree who works at
Rockwell-Collins in Atlanta is a user of the Worksurfaces grounding
systems. She said that in the past they had to work hard
sanding etc. - at getting a good connection to the table top conductive
layer but with the Worksurfaces hardware, they always got good
connections with no problems. Vaughan said the systems are very
"3M friendly". This comment led to Dave Swenson of 3M
Corporation . Dave said the Worksurface hardware have been used
by 3M customers for several years with no complaints or problems.
He stated that this type of system provides the very reliable
connection required by constant monitoring systems.
The use of proper personnel grounding
is the most important part of a well designed ESD control program.
Today, connecting personnel and their wriststraps to ground is
as simple as acquiring the proper hardware. I guess the alligator
clip is gone for sure.