[time-nuts] Conducting Bench Top Material

[Lux, Jim (337C)]

> -----Original Message-----
> From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] 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.

> 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.



> 
> 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.