Lux, Jim (337C) wrote:
-----Original Message-----
From: [email protected] [mailto:[email protected]] On Behalf
Of Bill Hawkins
I learned that the human body has a capacitance of 400 pico F.
Getting up from a chair could raise a couple of kilovolts. We
walked on conductive rubber floors wearing conductive rubber
shoes. Bench tops were conductive rubber. Nobody had thought of
the wrist strap yet.
If you're in and out of ESD areas, then shoes with conductive soles are easier to use than always wrist strapping. Ditto if you're working on something big where the cord for the wrist strap gets in the way. In some of our clean rooms, we have booties to go over your street shoes that have conductive coatings on them, and a conductive ribbon that you tuck into your sock to make contact. (And you go stand on a test pad to make sure, of course).
I like the conductive shoes approach, it's pretty screw up proof, because you
don't have to remember to plug your wrist strap in when you come to the bench,
but the floor needs to be conductive, too.
The first greeting a new collueage gets when comming first day to work
is "What shoesize are you?". We order ESD shoes for more or less
everyone. Our US sales-people is known to ask if it is real Birkenstock
shoes. :)
In those days, rubber was made conductive with carbon black. It
was almost as effective as a pencil at marking things. If the
anti-static material is not black, maybe it won't be a marking
hazard.
These days, the black bins are dissipative and not marking. The black foam is
history (we all have ICs with corroded leads in the garage where the black foam
turns to goo). Here at JPl, we don't use the pink bags/peanuts/stuff at all,
because apparently, the coating can flake or rub off. We use plastic that has
a very thin metalized layer, and I think that's pretty much industry standard
now.
This is why you guys don't have a pink-day at the office! :)
You got to have a silly pink-day every once in a while just for the fun
of it. I'll see if not the HW department can adapt it, as they have
tried non-casual-friday (usual dress code is... um... casual).
A megohm and 400 pF has a time constant of 400 microseconds, but
you do get the kilovolt spike. The wrist strap looks really good
as long as your motion is the only source of static electricity.
It keeps your body from ever reaching kilovolt potentials.
And the megohm is important to keep you from inadvertently dying when you
happen to accidentally contact the AC line.
There is a reason for EKG equipment being measured for isolation
properties. The medical staff wants to select the time and dosage of
larger currents through the heart, and preference is towards patients
that badly need it.
Cheers,
Magnus
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