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November 2008

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Subject:
From:
Gene Felder <[log in to unmask]>
Reply To:
Date:
Wed, 19 Nov 2008 11:01:15 -0800
Content-Type:
text/plain
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The electrostatic charge is either positive or negative.  Like charges
repel, so electrostatic charge is a surface phenomenon.

Although only testing one size (8" x10"), a metalized shielding bag is
tested for effectiveness per ANSI/ESD S11.31.  It uses a computer, program,
oscilloscope, and a probe placed in the bag.  A 1,000 volt discharge is
applied and the amount of energy detected inside the bag is calculated.

The required limit, now per Packaging standard ANSI/ESD S541 (deleted from
ANSI/ESD S20.20-2007) for Shielding is < 50 nJ.

I suggest users take a couple 8" x 10" bags at random, and send to their
supplier asking for a test report.  You should expect to easily meet the
energy penetration test.

I think the ESD Handbook ESD TR20.20 has an explanation on how the shielding
bag works.  I'll see if I can find it.

Gene Felder
[log in to unmask]


-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Rex Waygood
Sent: 2008-11-19 12:47 AM
To: [log in to unmask]
Subject: Re: [TN] ESD Shelving Question

Just as a point of interest (and given that standards are not always
based on fact)
Why is an ESD metalised bag safe as I put my product into (is an open
bag a Faraday cage) the bag? This effectively simulates the earthed
metal shelf with an insulating layer. Can charge reside on an earthed
metal shelf with a thin metalised layer? Where is that old field mill? I
must check.

Regards
Rex 


Rex Waygood
Technical Manager
 
PartnerTech Poole Ltd 
Benson Road
Poole
Dorset BH17 0RY
United Kingdom
 
Tel: +44 (0)1202 674333
Fax: +44 (0)1202 678028
DDI: +44 (0)1202 338222
Mob: +44 (0)7887 997403
 
[log in to unmask]
www.PartnerTech.co.uk

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Gene Felder
Sent: 19 November 2008 00:18
To: [log in to unmask]
Subject: Re: [TN] 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.

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