From: Ken Cantrell ([email protected])
Date: Wed Dec 20 2000 - 12:09:07 PST
Mike,
I was on a team that installed this process in a manufacturing environment a
number of years ago. Quick overview first:
Gotta have the conductive floor (do the isles only to save money, the stuff
is very expensive)
Gotta have heel straps on your shoes, and wrist straps at the stations.
Static dissapative foam sheets at all stations where you are assembling,
disassembling, repairing, etc.
No one is allowed to walk off of the conductive isles into the manufacturing
area without putting on an ESD smock, including engineers. Don't care if
you designed the box all by yourself and are coming down to respond to a
work stoppage.
All parts in the warehouse designated as static sensitive must be marked
with highly visible label and be enclosed in a static dissapative bag. The
parts are placed on static dissapative carts for delivery to the line. It
goes on, but you get the idea.
The smocks must be static dissapative, and you have to wear them correctly.
Won't do you any good if you don't wear them right. Cuffs must be buttoned
with no clothing visible (regardless of material). All buttons except for
the top button must be buttoned, unless clothing protrudes beyond the last
button. If it does, button them all, or have the clothing assessed by your
ESD person for tribo or other effects.
And that's just the overview. However, it works if implemented correctly,
and has measurable impact to the bottom line.
In design areas, use best judgement, keeping in mind that you can inject or
receive a 2kV transient pulse and not feel it, which can cause latent fails.
So why static dissapative, and what does that mean? Static dissapative
material is between an insulator and semiconductive material. It captures
charge, but dissipates the charge slowly so that radiation does not occur.
I don't remember the ohms, it's high, but not at insulator levels.
Conductive material captures charge but discharges it too quickly causing
radiation. Insulators of course store charge to capacity and wait for an
unhappy victim to touch or walk too close to them. I always like the
Goldilocks analogy, and static dissapative is "just right".
The other big player is humidity. You have to have a controlled humidity
environment.
Furniture discharge, some say, is worse than human discharge. That's up for
debate. Point being, don't ignore it either. Metal cart with parts on it
touches, or comes near enough to couple to, the metal chair of a co-worker
or a file cabinet, for example.
For really unusual stories of transient "force at a distance", attend a Doug
Smith seminar. Real live things that happened to Doug that are both funny
and informative. I won't give it away, but my favorite is the jingling
change story. All his tall tales are empirically demonstrated/verified at
the seminar.
Ken
-----Original Message-----
From: [email protected]
[mailto:[email protected]]On Behalf Of Michael Nudelman
Sent: Wednesday, December 20, 2000 7:59 AM
To: [email protected]
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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