Per ESD Handbook ESD TR20.20-2008 section 5.3.7.1 Workstation Shelving
"Shelving systems are frequently included as part of an ESD protective
workstation. The shelves in these systems can be used to store ESD sensitive
products (both packaged and unpackaged), documentation, manufacturing tools,
computer/printing equipment and test equipment such as oscilloscopes. If the
shelving system is designed to store unpackaged ESD sensitive products, then it
should be treated as an ESD worksurface. This means that the surface must be
properly grounded, possess a surface conducive to charge dissipation and be free
of unnecessary static generators. However, if the surface is not intended to be
used for storing unprotected ESD sensitive products, then the shelf can be
constructed from non-ESD protective materials."
Is the coating dissipative 1 x10E4 to < 1 x 10E11 ohms or above that and
insulative.
Shelving per Table 3 of ANSI/ESD S20.20-2007 required limit is < 1 x 10E9 ohms.
This should be the surface resistance of the material, not resistance in a
current limiting resistor in the ground cord.
A lower limit is recommended for worksurfaces and shelving if Charged Device
Model (CDM) failures are a concern. Per ESD Handbook ESD TR20.20-2008 section
5.3.1.7 Electrical Considerations "The most important functional consideration
for worksurfaces is the resistance from the top of the surface to the groundable
point. This establishes the resistance of the primary path to ground for items
placed on the surface. When worksurface materials are being selected,
consideration should be given to possible CDM damage to ESD sensitive products.
If CDM damage is a concern, then setting a lower resistance limit for the
worksurface should be considered. Typically, the lower limit for these types of
worksurfaces is 1 x 106 ohms."
Per Grounding standard ANSI/ESD S6.1 section 5.3.3 ESD Technical Element
Conductors "The grounding conductors (wires) from wrist straps, worksurfaces,
flooring or floor mats, tools, fixtures, storage units, carts, chairs, garments
and other ESD technical elements may or may not contain added resistance. Where
added resistance is not present, a direct connection from the ESD technical
element to the common point ground or common connection point is acceptable and
recommended.
Note: Manufacturers may add resistance to the grounding conductors for purposes
other than ESD (e.g. current limiting). Added resistance is acceptable for the
purposes of controlling ESD provided electrostatic accumulation does not exceed
specific EPA requirements. The typical added resistance in grounding conductors
is 1 megohm, although other values may be specified."
So, for ESD control, for a mat or shelf, a current limiting resistor is not
required in the ground cord.
Note: the ESD Handbook ESD TR20.20 has just been updated 2008 version, and is
available for purchase from www.ESDA.org; see
http://www.esda.org/esdhandbook.html
$200.00 LIST/$150.00 if ESD Association member
Provides guidance that can be used for developing, implementing and monitoring
an electrostatic discharge control program in accordance with ANSI/ESD S20.20.
It also includes Glossary of Terms (ESD ADV1.0-2004).
139 pages
Gene Felder
[log in to unmask]
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Dave Dixon
Sent: 2008-11-18 10:39 AM
To: [log in to unmask]
Subject: Re: [TN] ESD Shelving Question
Thanks Richard,
I was aware that a direct ground isn't acceptable, and assumed
that the lead had the resistance built in. Guess I need to go back over
and check for myself! I appreciate you taking the time to help explain
about the dissipative coating,
Dave
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Stadem, Richard D.
Sent: Tuesday, November 18, 2008 12:05 PM
To: [log in to unmask]
Subject: Re: ESD Shelving Question
True ESD shelving has a dissipative coating on it. Purely metal shelving
can be construed to be ESD shelving, provided that the only product
stored on it is in Faraday cages that will dissipate any charge to the
shelving. Having unprotected ESD assemblies sitting directly on a metal
shelf that is connected directly to ground does not constitute
protection. Metal shelves without benefit of dissipative coating should
be connected to ground through a 1 meg resistor. This prevents a direct
charge from going through the assembly, whether from ground to a
positively charged person or object, or vice-versa.
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Dave Dixon
Sent: Tuesday, November 18, 2008 11:10 AM
To: [log in to unmask]
Subject: [TN] ESD Shelving Question
Hi Gurus,
We just noticed that our supposed ESD shelving has a clear thin
insulative coating on it. The grounding clamps bite into this coating,
and if you poke a probe through the coating, you can see that the
shelves are connected to ground. This runs against all my beliefs about
shelving materials. Shouldn't they be conductive on the shelves'
surface area as well?
Thanks in advance for any input!
Dave Dixon
Engineering Technician
Aero-Mach Labs Inc.
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