TECHNET Archives

November 2008

TechNet@IPC.ORG

Options: Use Monospaced Font
Show Text Part by Default
Show All Mail Headers

Message: [<< First] [< Prev] [Next >] [Last >>]
Topic: [<< First] [< Prev] [Next >] [Last >>]
Author: [<< First] [< Prev] [Next >] [Last >>]

Print Reply
Subject:
From:
"Stadem, Richard D." <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Stadem, Richard D.
Date:
Wed, 19 Nov 2008 10:14:10 -0600
Content-Type:
text/plain
Parts/Attachments:
text/plain (204 lines)
Hi, Dave
Then you shouldn't have any issues. Good luck. 

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Dave Dixon
Sent: Wednesday, November 19, 2008 9:31 AM
To: [log in to unmask]
Subject: Re: [TN] ESD Shelving Question

Thanks for all of the great input.  The powers that be have decided to
directly ground the shelves, because "that is what company X does". I
guess I am not to be concerned with any issues that might arise with the
(urethane?) coating.  It sounds like it isn't enough of an insulator to
be a problem and is actually dissipative.  
In truth, I feel this is not an issue either way.  My thinking is, the
shelves could be made of rubber and lambskin, as everything on the
shelves is sealed within an ESD safe bag and then placed in an ESD safe
container! I appreciate all of the great advice, Dave


-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Gene Felder
Sent: Tuesday, November 18, 2008 6:18 PM
To: [log in to unmask]
Subject: Re: ESD Shelving Question

I think you make all good points.

The 1st problem Dave should address is "our supposed ESD shelving has a
clear thin insulative coating on it."  That is never a good thing.

Regarding a current limiting resistor in a shelf or worksurface ground
cord, it is not recommended for ESD control.  Per Table 1 Grounding /
Equipotential Bonding Requirements of ANSI/ESD S20.20-2007, the Required
Limit for Equipment Grounding Conductor is < 1 ohm impedance.  However,
if a Safety Officer believes it would be helpful, personnel safety is
more important than ESD control.

The presence of the 1 megohm resistor can aggravate troubleshooting.  If
a groundable point snap, say is loose, one might measure 2 or 3 ohms and
detect the problem.  If the ground cord includes a 1,000,000 ohm current
limiting resistor, the problem will not likely be detected.

Gene Felder
[log in to unmask]

-----Original Message-----
From: Stadem, Richard D. [mailto:[log in to unmask]]
Sent: 2008-11-18 11:59 AM
To: [log in to unmask]; [log in to unmask]
Subject: RE: [TN] ESD Shelving Question

Correct. However for shelving without any dissipative coating, as I
stated, it may not be "required", but it is certainly recommended if the
shelving does not have dissipative characteristics. Ask Roger Pierce.

[Note: 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."]
 

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Gene Felder
Sent: Tuesday, November 18, 2008 1:27 PM
To: [log in to unmask]
Subject: Re: [TN] ESD Shelving Question

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.

---------------------------------------------------
Technet Mail List provided as a service by IPC using LISTSERV 15.0 To
unsubscribe, send a message to [log in to unmask] with following text in
the BODY (NOT the subject field): SIGNOFF Technet To temporarily halt or
(re-start) delivery of Technet send e-mail to [log in to unmask]: SET
Technet NOMAIL or (MAIL) To receive ONE mailing per day of all the
posts: send e-mail to [log in to unmask]: SET Technet Digest Search the
archives of previous posts at: http://listserv.ipc.org/archives Please
visit IPC web site http://www.ipc.org/contentpage.asp?Pageid=4.3.16 for
additional information, or contact Keach Sasamori at [log in to unmask] or
847-615-7100 ext.2815
-----------------------------------------------------

---------------------------------------------------
Technet Mail List provided as a service by IPC using LISTSERV 15.0
To unsubscribe, send a message to [log in to unmask] with following text in
the BODY (NOT the subject field): SIGNOFF Technet
To temporarily halt or (re-start) delivery of Technet send e-mail to [log in to unmask]: SET Technet NOMAIL or (MAIL)
To receive ONE mailing per day of all the posts: send e-mail to [log in to unmask]: SET Technet Digest
Search the archives of previous posts at: http://listserv.ipc.org/archives
Please visit IPC web site http://www.ipc.org/contentpage.asp?Pageid=4.3.16 for additional information, or contact Keach Sasamori at [log in to unmask] or 847-615-7100 ext.2815
-----------------------------------------------------

ATOM RSS1 RSS2