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ESD protection

I'm having trouble understanding ESD protection. I hope somebody can
help.
Should an anti-static mat on a lab bench be connected directly to
ground or connected to ground through a 1Mohm resistor?
What is an anti-static mat made out of? Is it conductive? If so then
why don't I measure any resistance if I poke ohmmeter probes into two
spots on the mat?
Do I need to have an anti-static mat on the floor around my lab bench?
If I have a charge on me do I have to touch a circuit board to damage
it or can the charge jump through the air?
How do I safely move a circuit board from one lab bench to another lab
bench? (I see people carrying circuit boards around my lab by the
edges of the board with their bare hands and they think by not
touching any IC's with their hands that the board is safe.)
I'm often out in the field working on electronics and I have no ground
strap. Should I touch a ground point on a metal object first before
handling a circuit board, to remove any charge on myself?
Thanks for any help.
 
A

Archimedes' Lever

Jan 1, 1970
0
The mat direct to ground is fine as the mat has an inherently higher
resistance. It is your wrist strap cord that usually has one integrated
into it.

Many times, a carbon impregnated material. These days there are
several types, and most are multi-laminar in nature.
Is it conductive?

Yes... well... it has a high resistance, so it conducts "slowly".
If so then

Because the resistance is so high, you need a larger surface area of
contact than the probe tips provide. A couple of 3 inch foil patches
should allow you to see some motion (get a reading).

It is not required. If you are doing High Voltage work, it is not
recommended at all.

When you are in an elevated condition, you also carry a field. That is
why we wear smocks. They contain any field you would otherwise radiate.
I ALWAYS touch grounded structures before I touch a PCB or approach an
ESD work station with open assemblies on it.

In an ESD shielded container, such as a metallized or carbon black
dissipative bag WITH the top folded over. This can also be toted around
inside an ESD tote box with a lid on it, especially for longer hauls.
(I see people carrying circuit boards around my lab by the

If you were talking about power supplies, I'd say the parts are pretty
hardy.

Since you are likely referring to delicate digital circuit assemblies, I
would be sure that everyone involved takes every precaution possible.
Such failures do not immediately show themselves. Sometimes they take
time to propagate a failure mode.

With a PC as an example, I would:

Keep the chassis plugged in, as this keeps things grounded, and gives
you a place to be grounded as well.

Touch the chassis to equalize you body charge to the chassis. This
should also be ground, but equality is more important. NOW you are able
to touch circuit assemblies, etc.
Should I touch a ground point on a metal object first before

Yes. If you use the chassis for the device in question, you bring
yourself to its level, regardless of where ground is, though the two are
typically equal.

The mat does not have to be electrically conductive to dissipate a
electrostatic charge.

You're an idiot.

Conductor: WILL dissipate a electrostatic charge.

Insulator: Will NOT dissipate an electrostatic charge.

You were saying?
I conductive mat can cause problems with a circuit board under test on the
bench if the components are shorted out or the resistance between two nodes
are changed necessitation a piece of non-conductive mat on top of the
conductive one.

Only if one intends to energize circuit card assemblies at their ESD
safe workstation. It also depends greatly on the voltage in question.
Low voltage DC circuits have no problem with ESD mats.
The mat should be diretly connected to earth. Your wrist strap needa
1Megohm or greater so if you touch someting high voltage with respect to
earth ground the current is limited below the lethal level.

That is not the reason for it. It is there so that a charge does not
get dissipated to fast, which can cause damage to a circuit element as
well.
Antostatic floor mats are nice for when the boss walks in and starts poking
around the board without a wrist strap.

It doesn't mean a damned thing if he does not have on a heel strap.
Anyone around and ESD workstation should ALSO have an ESD smock on as
well.
Generally if you use a wrist strap
and bench mat you don't need a floor mat to unless you are working with some
very delicate stuff. (IMHO)

Three minimums for an ESD workstation.

Bench mat.

ESD Smock,

and to be grounded.

The best smocks have the cord integrated into them, and you ground that,
and there is no wrist strap.

Recommended:

Ionizer.
If you are charged up pretty good you will cause a noticable and audible
zap.

If you are causing zaps, you shouldn't be anywhere near an electronics
lab, and said lab should look into the humidity conditions.
Often times the "zap" is unnoticed but one port pin or logic IC start
acting funny on you.

I am done teaching... You're a goddamned retard!
In generalt its best to put on the wrist strap before
touching the board.

No shit.
Moving a board from one bench to another requires putting the board in an
ESD safe metalized bag or box.
Esd pretetion bags are silver or black in color. The pink one on do not
generate static when rubbed together but toe do not protect the board for a
static hit.

Pink are useless completely in low humidity.
there is almost always a ground point around. Any metal the goes into the
ground (plumbing is good) can be use to ground your wrist strap. They sell
ones with a alligator clip on the end for this purpose.

Walking around with a board held by the edge is a last option only
proposition. when you get to the other side you should discharge yourself
to the other bench with the opposite hand then place the board down.

Nice logic, but flawed... kind of like the old "duck and cover"
instruction for what to do if you see a nuclear blast.
 
So the 1Mohm resistor is only for protection against me getting
shocked? If there is no dangerous voltage around me or I'm just
working with 5V electronics then I can be connected directly to
ground?

When I'm carrying a board in an antistatic bag does it matter if the
bag is open, or does the bag need to be closed?

If I step on a grounded mat, any charge on me will flow through the
rubber soles of my shoes? Isn't rubber very resistive?
 
R

Richard Henry

Jan 1, 1970
0
So the 1Mohm resistor is only for protection against me getting
shocked?  If there is no dangerous voltage around me or I'm just
working with 5V electronics then I can be connected directly to
ground?

When I'm carrying a board in an antistatic bag does it matter if the
bag is open, or does the bag need to be closed?

If I step on a grounded mat, any charge on me will flow through the
rubber soles of my shoes?  Isn't rubber very resistive?

The mats, straps, smocks, etc and the instructions governing their
installation and use are designed to protect the electronics equipment
and personnel from damage, even if the people don't why they are doing
it. For people that do know why, they eliminate the need to think
about it.
 
E

Eeyore

Jan 1, 1970
0
mook said:
The mat does not have to be electrically conductive to dissipate a
electrostatic charge.

Yes it does. Dependes what you define as 'conductive'. ;~)

I conductive mat can cause problems with a circuit board under test on the
bench if the components are shorted out or the resistance between two nodes
are changed necessitation a piece of non-conductive mat on top of the
conductive one.

But the volume resisivity is usually so enormous, I don't recall any such
problem. Rememeber it's not zero ohms, it's in the megohms or tens or 100s of
same. Enought to discharge a few nanocoulombs but high enough not to interfere
with operation. Do check the data sheet though. They do come in various grades.

Graham
 
E

Eeyore

Jan 1, 1970
0
Do I need to have an anti-static mat on the floor around my lab bench?

Depends what's on the floor, what clothes you wear (try ALL pure cotton),
your shoes etc. You will acquire some charge simply by walking.You can get
ESD safe carpet, it has conductors woven into it, or spray the carpet with
ESD safe spray every so often. Some say dilute detergent does the job as
well but I stick with the Electrolube et all product in a ready to spray
dispenser.

Graham
 
E

Eeyore

Jan 1, 1970
0
So the 1Mohm resistor is only for protection against me getting
shocked? If there is no dangerous voltage around me or I'm just
working with 5V electronics then I can be connected directly to
ground?

As long as the equipment is Class II !

When I'm carrying a board in an antistatic bag does it matter if the
bag is open, or does the bag need to be closed?

The board needs to be totally enclosed in the bag but the zipper is
probably not that important.

If I step on a grounded mat, any charge on me will flow through the
rubber soles of my shoes? Isn't rubber very resistive?

That's why you have wall mounting grounding plates you touch with your
hands.

Graham
 
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