ESD protection

[Magnus Danielson]

From: JWBarlow at aol.com
Subject: Re: ESD protection
Date: Sun, 10 Sep 2000 15:36:05 EDT

> Although Rob, John Blacet, and Harry have sufficiently scared me, I will not 
> let that deter me from making a fool of myself by asking: is it enough to be 
> barefoot while dealing with static sensitive parts? This is what I've always 
> done (I'm in California, so that might explain it), but I'd always imagined 
> that static damage would result in catastrophic failure of the affected 
> parts, not damage or degradation -- until reading this thread.

The thing with working in socks is that they want sufficient electric contact
with the anti-static floor. If you have a totally isolating floor, then you are
out of luck. It is the combination that counts!

At work we have installed anti-static floor in all labs, the hardware
department office area and then also my office ;)
We have given everyone antistatic shoes, not just the HW guys.
We have antistatic benches, antistatic chairs, antistatic waste baskets with
antistatic wastebaggs. We clean up the room with an antistatic vacuumcleaner.
The big lab requires cooling due to the amount of equipment in there, but the
humidity is usually acceptably high, but we should maybe pay some more
attention to that.

This is our way to play safe with all the more or less fragile equipment in
there. I can not recall that we have had any trouble that we can relate to
ESD. We have seen troubles due to rush-currents, but that was by incorrect
design and not ESD. A redesign solved that problem. Inserting a powerless card
can make you sad, since the protection diodes will pull current from all the
inputs in order to load the caps on the powerless card - OUCH!

Anyway, when I builded the original lab I went for ESD gear directly. Since I
started with it right from the start it has been natural to ensure that all
additions and extensions follow the same level, it is nice to have been setting
the standards fairly high for at least one company!

I strongly advice you to at least get an antistatic matt for your desk, add
also a wrist-wrap. Naturally, ensure that your matt and your solder-tip share
the same earth.

This is a reminder for myself, I really should get a table-top matt for my
home project soldering. It's not that expensive, but I really should kick my
lazy but to actually get it.

A few rule of thumbs:

It is the potential differance that provides the basis for inrush failure like
ESD. Touch (and preferably continously have contact with) the metal frame/earth
of the box you are fixing. Touch (and have contact with) the antistatic matt on
your table (you have one, right?). If you have been out walking around, rise
from the chair etc, go and grab that frame or matt before touching anything
sensitive.

	Keep in contact with ground!

It is the scrubbing of an conductor over an isolator which causes charge. It
may not be apparent when you have this situation, using the floor mat and
antistatic shoes (if you do not run around barefeet) is just one such thing.
Having nylon stockings (we do have females walking our labs) etc. is one of the
nonos. Syntetic materials in general is not a good thing.

	Do not use materials acting as good isolators anywhere near your
	labbench!

The humidity talk, having some humidity is good, since this will provide a
weak conduction path so that if charge have been collected, it will slowly
die out. Very low humidity is not good since this will remove virtually all of
the air-leakage effect, thus on dry days you have a much higher probability of
hurting your curcuits than on a damp day. Thus, for places where the humitidy
often goes below say 30 %, having an air humidifier may be a cheap way of
saving a lot of curcuit and thus a lot of troubles. Naturally, a very high
humidity environment is not good either.

	Ensure you have sufficient humidity!!!

Cheers,
Magnus