"w_tom" said in news:
[email protected]:
Grounding - even to electrical outlets - does not solve
static electric problems. For example, everything was
properly grounded. Static still existed.
Then everything was NOT grounded (or, more accurately, not everything was at
the same potential) and discharged to equal potentials before connected. A
discharge occurs only when there is a difference in potential. Me at 20,000
volts touching you also at 20,000 volts won't produce a spark. While the
equipment may be grounded, you approaching that grounded equipment with a
charge can still zap it until you have contacted the grounding device to
drain your potential *difference* (the equipment also has to be grounded; it
cannot float while you are grounded). You could short yourself and "enjoy"
the spark, or you discharge yourself more slowly (a second or two) by
touching an grounded anti-static device that drains through a 1 megohm
resistor so you don't get shocked. You want to get to the same potential
BEFORE touching the other object. You equalizing the charges by sparking to
it is what causes damage.
Note that I say YOU have been discharged because I was talking about YOU
touching the equipment. Static is a surface charge. You could be wearing a
synthetic shirt that insulates your wool sweater from your skin. Your wool
sweater has a static charge. You discharged your body but neglected to also
discharge your clothing that was insulated from you. That's why your lab
coats with conductive filaments will help because it contacts your clothes
along with contacting your wrist or hand and your neck, plus the filaments
enhance the corona effect to more quickly leak off the static charge.
Anti-static smocks do not bleed as fast as, say, a wrist strap so they
mostly provide a barrier to static that builds up on your clothes while
trying to bleed it off albeit slowly when you aren't so active. The smock
provides a barrier to prevent sudden ESD from your clothes while providing a
[slow] bleed off of the static.
Static electric is a charge typically across shoe soles.
Rubber is an insulator. That prevents the static from leaking off your body
and clothes onto the anti-static mat or treated carpet. A charged person
walking over and standing upon an anti-static floormat with rubber shoes
won't drain off the charge because they are insulated from the floormat.
That's why techs wear wrist or ankle straps.
Spark occurs only when a complete electric circuit is
created.
No, the spark is the result of the electron flow from the potential
difference due to an imbalance of charge. There is no "complete electric
circuit". The charge doesn't run from the topside of your shoe, through
you, and back to the bottom side of your shoe. You have so many coulombs of
charge which differs from the number of coulombs of charge of another
object. You touch the object. The difference in the charges then equalize
between you and the object. That's the extent of the electron flow. There
is no return path (to the bottom of your shoes). You're floating in space
and negatively charged to 30,000 volts in reference to another floating
body, say, your spacecraft will get you zapped when you provide a conductive
path between the two. If both were at 30,000 volts relative to Earth (or
zero volts relative to each other), no zap. Missiles and satellites have
been been damaged or destroyed because components weren't adequately
connected to prevent static buildup as they sped through the air or Van
Allen belt. So where was that complete electric circuit? The charges
equalize via electron flow between the two differently charged bodies. Or,
to look at it another way, the "complete electrical circuit" is just the two
imbalanced charged objects and the electron flow between them.
The point is that you want to be at the same potential as what you are
working on. Since the computer is grounded (or should be) then that's the
potential you want, too. If the computer were floating at a charge of
10,000 volts then you don't want to be touching it while you are grounded.
Your body as a tiny capacitor topside of your rubber shoes connecting to the
huge earth capacitor is where all the electron flow occurs, and there will
be neglible or no electron flow from the huge earth capacitor to the tiny
carpet capacitor at the underside of your rubber shoes.
You're using earth ground to suck up the static charge. You're not trying
to complete an electric circuit back to the bottom of your shoes. Your
rubber shoes render the anti-static mat ineffective. That's why you wear a
wrist strap (with 1 megohm resistor for safety). The carpet itself is an
insulator, too. You're not trying to discharge your body's charge to the
carpet under your feet. You're discharging it to a huge mass that can
receive your surplus electrons. The anti-static floormat is to drain any
charge *away* from you, just like the wrist strap drains the charge *away*
from you.
Some also
install ion generators specially designed to fill air with
charges that help discharge static.
Static is a buildup of electrons (i.e., negatively charged). Ions can be
negatively or positively charged atoms. Flooding the air with negatively
charged ions will not help to bleed off a negatively charged body because
that reduces the corona effect of letting the negative charge bleed off to a
differently charged environment. Make sure your ionizer produces positively
charged ions. Some ionizers only produce negatively charged ions. Some
produce both positive and negative ions, and some of those even have an ion
polarity balance adjustment.
Computers should be assembled so that static cannot damage
them. For example, that is why motherboard ground plane only
connects to chassis ground plane at one point.
At only one point? That's not what I read online, like at
http://www.dbicorporation.com/esd-art1.htm. I would think you would want
multiple discharge points to minimize the length for the discharge path and
thus its impedance so that any charge felt by components in the vicinity
would be under 20 to 30 volts. Why would you want to force an ESD to travel
an average of halfway across the motherboard? Why wouldn't you short it at
its point of entry, like every port (keyboard, mouse, parallel, serial, USB,
etc.) to thwart it getting further inside and at the ICs?
While reading through Google matches regarding mounting holes and grounding,
the arguments are because the folks don't look. Some argue that only
plastic standoffs are needed by citing the mobo will work while lying atop a
piece of cardboard (which has nothing to do with ESD and EMI prevention and
only utilitizes the ground path through the power supply). Some argue a
fiber washer is needed under a metal screw head for a metal standoff. They
don't bother to look to see that signal foils are too close to the mounting
hole but the screw head cannot be reduced in size to properly hold the mobo.
If the mounting hole has a metal pad and you can use a metal screw head that
doesn't exceed the size of the metal pad, then use a metal standoff. Some
mobos have a combo of all of these mounts: plastic (because there's no metal
pad), fiber washer because the metal head would short to nearby foils, and
metal standoffs. Logically the grounding via metal standoffs is a backup
ground path, but for ESD and EMI they are required. "Basically the
motherboard needs to be grounded to the chassis. Ground as many mounting
holes as possible, but be cautious. Some holes do not have a metal pad and
you will need to isolate the mounting hole. Make sure there is metal stand
to match each mounting hole on the motherboard. If the metal pad on the
board is smaller than the screw head, you may need to use an insulation pad
to avoid creating a short to any circuit near the metal pad."
(
http://www.supermicro.com/TECHSUPPORT/FAQs/Chassis_FAQ.htm#Q14).
