RE: [SI-LIST] : ESD Smocks

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From: Thomas Jackson (tjackson@fmi.fujitsu.com)
Date: Wed Dec 20 2000 - 10:17:52 PST


Back in my distant past, when I worked with some high voltage, high power
circuits, we always wore smocks with our minor arm (left for right-handed,
right for left-handed) inside the smock. This decreased the possibility of
forming a discharge path through one's chest. However, with 3.3 V logic, it
doesn't seem relevant.

Tom

Thomas L. Jackson, P.E.
Staff VLSI Design Engineer
Network Access Development
Systems Solutions Group
FUJITSU MICROELECTRONICS, INC.
3545 North First Street
San Jose, CA 95134-1804
telephone: (408) 922-9574
facsimile: (408) 922-9618
http://www.fujitsumicro.com
-----Original Message-----

From: Michael Nudelman [mailto:mnudelman@tellium.com]
Sent: Wednesday, December 20, 2000 6:59 AM
To: si-list@silab.eng.sun.com
Subject: [SI-LIST] : ESD Smocks

Hail to you, people that know electricity.

I have unrelated to SI question. And this question does not come from my
ancient animosity to ESD smocks. It comes but from the desire for pure
knowledge (and some distrust for businesses mongering ESD equipment).

Now, somebody please explain me the principle of the ESD smock's
operation. It is a gown, laced through by carbon fibers, forming a cage.
(St. Mike Faraday is smiling in heaven reading this, I'm sure).

As I remember from my Electricity course, any Faraday-cage-like device
(hollow metal object) is intended to protect inside from outside (a
static field inside the object is always zero if all the charges are
outside).
However any charge that is inside the object, upon touching the inner
surface will transfer ALL of its charge to the outside surface of the
object, thus
itself loosing 100% of the charge and the charge being distributed
evenly on the outside surface of the object. The object in our case is
the smock.

BTW, the Van-de-Graaf generator (the one capable of developing few
million volts potential) works exactly on the same principle.

For the smocks to work we need conductive floors and everybody wearing
straps. Otherwise, smocks are potentially harmfull. If you touch
something with it while not grounded, you may actually zap some poor
unsuspecting FET or a co-worker (unless he deserves it).

(And as I know, lots of companies do not have those floors. And ESD
businesses do not really insist on them - otherwise they would loose
some business, admitting, that the smock itself is not the remedy).

Unless there is some gimmick to the smock that I do not know of.

Anybody can explain this ... phenomenon?

Mike.

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