Hi, Ramon
I have a strong opinion, based on testing of various smocks and smock
grounding methods I performed when I was an ESD site coordinator for an
EMS provider.
First, I tested the smocks by themselves using the methods in EOS/ESD
20.20-1999, in the "how-to document". This can be downloaded from the
EOS/ESD website http://www.esda.org/freedowloads.html.
I also tested several different vendor's smocks while they were worn by
various operators, using a field meter to measure the charge buildup
from different activities, including simply walking.
When testing smocks without elastic cuffs, a significant (>1000V)
triboelectric charge can easily be generated into the smock by a
sweater, the hood of a sweatshirt, or simply from hair contacting the
smock around the collar or shoulders, or just by walking. Because this
type of smock does not make contact with the skin of the wearer, this
triboelectric charge had no place to drain until it came in contact with
ground, which was often through the ESD-sensitive component or assembly.
In fact, it was found that unless the smock was grounded to the
operator's skin via a dissipative cuff or by grounding the smock ground
snap to the electrical ground connection used for the wriststrap, it was
actually better to not wear a smock at all in most cases. If the
operator is wearing ESD shoes or heel straps the charges built up on the
cuffed smock are dissipated through the wearer's body and into the ESD
flooring, or at least through the wrist strap when it is plugged in.
Even if ESD flooring is not used, bare concrete does have dissipative
properties.
So if you are evaluating smocks, I would specify one with elastic
dissipative cuffs. Some operators do not like them at first, but after
they wore them awhile they actually liked them better because their
sleeves never snagged on assemblies or other hardware, pulling them off
the bench or shelf or tote onto the floor. The other alternative is to
ground the smocks when seated or when standing and handling ESD devices
directly with a second ground cord. However, this is a royal annoyance.
Another method is to have a short ground strap that goes from the snap
on the end of the sleeve to a second snap on the wrist strap. However,
this little loop of wire often snags.
But whatever you do, make sure you pay three times the price of a
regular smock by getting the green ones that are "RoHS-Compliant". LOL
;o)
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Dehoyos, Ramon
Sent: Thursday, February 01, 2007 9:27 AM
To: [log in to unmask]
Subject: [TN] FW: January 2007 ESD Systems.com Newsletter
Technetters:
Does any body have an opinion on grounded Smocks?
Ramon
________________________________
From: [log in to unmask]
[mailto:[log in to unmask]] On Behalf Of Banana Jack
Sent: Tuesday, January 23, 2007 2:30 AM
To: Lusby, Jack R.
Subject: January 2007 ESD Systems.com Newsletter
Here is the latest ESD news from ESD Systems.com:
http://www.ESDSystems.com/Newsletters/v10issue1.html
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