Mail*Link(r) SMTP FWD>FW: ASSY: Smocks, are they necessary?
Leo and Norm -- why we use smocks in ESD controlled environments
As mentioned, smocks may not be necessary provided you have good ESD
protection and reduction procedures --- and use them. In general, if you
don't use smocks, then as a rule-of-thumb an persons sleeves should not extend
below the elbow on the arms and their clothing should (actually shall) not be
static generating synthetic clothing.
Test data has been reported that even with the person "strapped" in with a
wrist strap, depending on the persons clothing and normal body movements they
can still generate sufficient electrostatic charges on their clothes to zap a
part. Others have reported (and we have obtained correlation to their work)
static charges of up to 7 kV on the operators clothing due to moving the arms
away from being in contact with their chest of sides of the body. This means
that even though the person is soft-grounded significant electrostatic
voltages can be developed on the clothing they are wearing. This is why those
who really want to control ESDs use appropriate smocks.
The type of smocks we use have the elastized electrically-semiconductive wrist
cuffs. Both cuffs must be direct and full contact with the persons/operators
skin. Some people have 'dry-skin' and conductive creams are available. The
smocks have conductive fibers woven into the cloth forming a electrically
semiconductive grid that electrically connects all cloth panels of the smock,
including the cuffs. The smocks must be buttoned up by all personnel prior to
entering an ESD protected area. We have ESD static dissipative / antistatic
flooring in the ESD protected areas. A person must skin-touch by
long-arm-reach the ESD grounding terminal/system in the ESD protected area
prior to strapping up and sitting down. After they are strapped up they can
approach the workstation to sit down. Visitors to the ESD protected areas
must likewise ware an ESD smock and must remain behind the operator unless
soft-grounded and strapped in. Norm is very right -- you don't base an ESD
system on smocks, you use suitable smocks to supplement an existing good ESD
control system for that extra margin of control and reduced electrostatic
charge generation.
Ralph Hersey.
--------------------------------------
Date: 3/22/96 4:29 PM
From: Dill, Norm J
Leo,
The most value smocks have is marketing. With respect to image - smocks
are cool. With respect to ESD, their value is a good topic for discussion.
I would never want to base an ESD system on them.
For actual ESD protection you need real static conducting paths that
shunts the static voltage away from your components. As an enhancement to
your ESD foundation, you can add items or procedures that reduce the
generation of static.
Other than image, I see no reason why smocks could be a manditory
component of the shunt path.
----------
From: TechNet-request
To: TechNet
Subject: ASSY: Smocks, are they necessary?
Date: Friday, March 22, 1996 12:33PM
All,
We have a concern that I think is common to every electronic assembly
operation. Basically, the question is, "Is it necessary to have
veryone/anyone in the assembly area wearing smocks to protect static
sensitive devices?".
As a contract manufacturer, we have only one (potential) customer
requiring that we have all associaties wear smocks on the production
floor. Also, as a contract manufacturer, it is obviously a marketing
advantage to have the more professional look that smocks provide, but
we want to be sure we're dealing with good science as well as good
marketing.
The new interactive video from IPC on ESD covers the use of smocks as
something that may be used but is not required. Also, some of our
people have been in some very hi-test, Malcom Baldridge award winning
electronic assembly plants lately where they do not use smocks.
Thank you in advance for your comments.
Leo Reynolds
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Subject: FW: ASSY: Smocks, are they necessary?
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