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Static Discharge
and you!

How to kill computer and other electronic parts

INTRODUCTION

All objects carry a static charge. Most objects are loosely coupled
to their environment and other items around them, and so tend to
equalize their charge. The only time a danger exists is when the
difference in charges between two objects is large, and that
difference is abruptly canceled. In the computer service
technician's world, this usually takes the form of a spark, jumping
a gap across a bridge of ionized air molecules. This is often a
destructive process, because the charge takes the path of least
resistance, which can be through delicate electronic components.
KILLING CHIPS

This discharge may destroy semiconductor junctions and structures in
most common electronic parts, as they were often meant to carry a
few milliamps of current and less than 18 volts (often only 5
volts). By contrast, the voltages generated by walking across a
carpet in the wintertime can approach 50,000 (yes, fifty thousand)
volts. Table 1 lists typical generated voltages.

TABLE 1- GENERATED STATIC LEVELS

==Humidity======Generating=============================================
Low=====High====Activity===============================================
35,000v 15,000v Short walk across carpet
20,000v 1,200v Handling typical polyethylene parts bag
18,000v 1,500v Sitting on common work chair with polyurethane padding
12,000v 250v Walking on vinyl floor tile
7,000v 600v Handling plastic envelopes for work orders
=======================================================================
"Low" humidity is (here) considered to be 10 to 20%, "High": above 65%

MAKING & PASSING SPARKS

Static charges are generated and passed from object to object via
Direct discharge, Triboelectric charging, and Electrostatic fields.

Direct discharge is easy to visualize; anyone who has ever gotten a
spark to jump from fingertip to doorknob on a winter day knows what
it is.

The greek philosophers long ago classified certain types of objects
into a triboelectric series. With modern enhancements, this series
still serves today to predict the approximate static-generating
potential of various materials. Items that are far apart on the
list, when brought together, seem to have the most static-producing
capability. The greeks used wool and hard amber, and most elementary
school classrooms have a synthetic cloth swatch and a plastic ball.
Triboelectric charging occurs when objects far apart on the series
are brought into physical contact and then separated, or better yet,
rubbed together.

A charged object can transfer some charge to another object by
passing through the electrostatic field of the charged object. Wave
your hand very near the front of a color monitor or TV set, then
touch a grounded metal object. Or take my word for it, a large spark
is the likely result. You hand and thence your body picked up a
charge from the surface of the picture tube.

HOW MUCH IS TOO MUCH?

Various types of components have different tolerances for static
discharge. Large, high-current devices are typically unaffected by
it. Motors and other similar devices are not bothered by it. For the
computer person, the following table may be of some value. Most
IBMPC class computers do not make use of Gallium Arsenide, MOS, nor
much op-amp technology, but EPROMs and CMOS are common, and CMOS in
particular is becoming more so. The table shows the voltages at
which an unacceptably high number of parts will fail.

TABLE 2- VOLTAGES DAMAGING VARIOUS DEVICES

Volts===========Device Type=============

5-100 Gallium Arsenide
100-200 MOS and MOS FETs
100-150 EPROM
190-2500 Op-amp
200-3000 CMOS
300-3000 Film Resistors
300-7000 Bipolar Transistors
1000-2500 Shottky TTL

PROTECTIVE MATERIALS

Protective materials fall into several broad categories. What class
of protective materials you need depends a lot on what type of
equipment or parts you need to protect, and how the static is likely
to be generated or transferred. The appendix contains a CBBS dialog
which gives a list of various materials and some properties of them.

TABLE 3: SURFACE RESISTIVITY
Ohms per Square and class of protection

1 10 100 1000 10000 100000 10^6 10^7 10^8 10^9 10^10 10^11 10^12 10^13
|-----Conductive-------| | Static | | Anti |
|---Dissipative---| |-----Static------|

Above 10^13 is considered an insulator. Items with batteries, like
clock boards and CMOS ram banks, should not be wrapped in conductive
materials. Static Dissipative materials are best for most
applications.

TERMS: A short glossary.

semiconductors: Impure metallic oxides and other compounds used for
making diodes, transistors, and integrated circuits.

milliamp: one thousandth of 1 amp, unit of electrical current.

CMOS: complimentary metal-oxide semiconductor, a technology for
making semiconductors that run cooler and use much less power than
TTL parts.

MOS: Metal oxide semiconductor, a technology for making
semiconductors. MOS FET: a MOS field-effect transistor.

Gallium Arsenide: A newer semiconductor material with potential for
extremely high-speed, low-power operation.

EPROM: Erasable, Programmable, Read-only memory chip. A memory
device that can be erased via strong ultra-violet light, then
reprogrammed with special devices.

TTL: transistor-transistor logic. A family of logic chips, usually
numbered 74xxx. Faster than CMOS, although it runs hotter and takes
much more current.

Surface resistivity: measured from edge to edge of an arbitrary
square area on the surface of a material, in ohms per square. The
size of the square does not matter. Also called Rs.

Anti-static: Having an Rs of 10^9 to 10^13 ohms per square.

Static-dissipative: Having an Rs of 10^5 to 10^9 ohms/square.

Conductive: Material with an Rs of zero to 10^5 ohms/square.

APPENDIX:
Question and answer on static protection.

Mr. Bob Blair asked if I knew anything about protecting computer
parts and boards from ESD (electrostatic discharge) damage. Here is
the reply and the (slightly edited) message thread that followed.
Some additional data has been added.

Conference: HARDWARE #1938 08-17-88 17:23 (Read 96 times)
From: TOM PETERS (LEADER)
To: BOB BLAIR (Rcvd)
Subject: Reply to: BOARD PROTECTION Reply to #1933

That's a very good question, since many failed boards can be traced to
mishandling.

There are 3 or 4 types of plastics that one might be tempted to wrap
his/her boards and components in to protect them from dust and static.
Some are good, some are guaranteed to KILL your parts, especially CMOS
parts.

BLACK POLY: An opaque black polyethylene (or polypropyline? I forget)
substance with a kind of almost rubbery feel to it. This is probably
the best stuff. I think 3M invented it.

PINK POLY: Translucent pink tinted stuff. Also good, and you can see
the product. This is the stiff that's softer and not very shiny. 4-8
mil poly.

There is also a blue variant of this stuff, called BENSTAT(r) that is
designed to be a static-dissipative product. Is 6 mil poly. Should be
cheaper than Black poly, but in some catalogs it isn't. 50 bag lots
are $4 to $40 in one catalog I saw.

GRAY 3M: Smoke/grey translucent plastic. This stuff will have a shine
on its surface and not be as soft. I dunno how this material compares
to the black stuff, but it's probably almost as good. None of use work
in labs or other strange environments where stray high voltages,
static, rf, or other stuff is likely to be a problem, so I can't see
where it matters much. Protects against tribolelectric charging,
static discharge, and electrostatic fields. And those are the 3 main
ways things get damaged. I have seen this stuff in 50 bag lots from $8
to $100 depending on size.

CLEAR CELLOPHANE or polystyrene. BAD, BAD, BAD. This stuff generates
zillions of electron volts worth of static charge when you stare at it
hard. Wrap a board in this, even if it's bipolar (not MOS) and it'll
probably fail mysteriously.

WHITE POLYSTYRENE WRAP: Have you seen this white or tinted 1/8" to
1/4" thick foam wrap? I see it as an inner wrap on monitors and stuff.
VERY BAD. Generates much static.

PINK PEANUTS: Shipping peanuts made from pink poly. Much preferable to
white peanuts, but still, always use an anti-static bag around the
board or parts, carefully taped shut.

WHITE PEANUTS: Real bad. You could use them if the part was carefully
sealed in a black or grey 3M poly bag, but we don't. We save our pink
poly peanuts for such things. Can't hurt to be careful...

BLACK FOAM: For small parts with pins, like MEMORY CHIPS and other
chips. If you take a part out of a socket, stick in it some black
foam. You can get it at Radio Shack if desperate. Never let a chip sit
anywhere besides its socket or in black foam.

Anti-static wraps or bags really address 2 issues: 1. generating
static 2. dissipating static. If the stuff you wrap the board in makes
static, issue number 2 never even comes up. If the wrap doesn't make
static, but doesn't have the right surface conductivity (etc) to
dissipate safely static that already exists, it isn't suitable either.
It must also prevent things from picking up a static charge should it
pass through the field of another charged object, or at least
dissipate it safely.

HANDLING PRECAUTIONS: Contrary to popular belief, servicing a PC is
best done with the unit PLUGGED IN. This is assuming you know what you
are doing. Look real close at the plug on your PC. See that third
prong in the middle? The round one? That's called a GROUND. You didn't
cheat and use an adapter, so you could plug into a 2-wire outlet, now
did you? Because if you did, you threw away some of the best static
protection you could have. That's where the static charge goes, when
it gets transferred to the PC from your body after you walk across the
carpet. That's also where surges and spikes get shunted to when (not
"if"!) Wisconsin Electric sends all kinds of garbage down your power
line. So ground your equipment!

Even walking across a tile floor can build up a static charge on your
body, and even a small charge can do damage under the right
conditions. One should always touch something large and metal on the
computer, like the case, before hitting the switch or the keyboard.
Consider: The keyboard sends serial data and clocks back and forth
to/from the PC. These are usually TTL signals, and they vary from near
zero volts to 5 volts. You walk to the PC and try to touch a key. A
small spark jumps from your finger to a trace between two keys, and
fries the encoder chip. Perhaps a spark jumps from that trace to one
connected to the computer. In a fraction of a second, one smoked
keyboard and maybe a new motherboard, or at least some new chips.

If you service computers, invest in an anti-static mat, and ground in
per instructions. You can get tabletop and floor mats (very good when
used in combination), wrist straps, and many other products. We use
chip inserters for RAM chips, and one must be careful to get one that
is anti-static rated, and also one must GROUND IT. Especially for CMOS
or MOS parts.

c: HARDWARE #1942 08-18-88 07:05 (Read 86 times)
f: BOB BLAIR
t: TOM PETERS (LEADER) (Rcvd)
s: R: R: BOARD PROTECTION Reply to #1938

Now that's what I call a reply. I'll read thru this as time is

available and will probably understand "static" and protection.
Sounds as tho you have some experience with this Tom.
Thanks, Bob

>>>>>>>>>> Msg 1938 reply #1

c: HARDWARE #1949 08-18-88 21:12 (Read 84 times)
f: RUSS TERRY
t: TOM PETERS (LEADER) (Rcvd)
s: R: R: BOARD PROTECTION Reply to #1947

How would you like to play with mini/mainframe boards. Touchy stuff.
Toast one of those babes and it could cost a few duckies heh? I fried
(I think) one on a Honeywell mini a few years back when I started
servicing computers. 7K ! and that was a cheap one. That was the LAST
one too. 3-M grounding kits were ordered and on the way within usec's.
I couldn't believe they didn't have any already (I was new and they
TOLD us in school never, ever work on a machine without one), but when
the senior techs. were not using them I figured what the hey?

personally, when I take my PC under the knife, I don't use one, just
ground myself (with the cord plugged) before touching anything. Always
have people asking me why I had a hand in one pocket when probing or
the like. Learned that one by trial and error too. Got nailed by a
500vdc transformer tap on a microwave radio about 6-7 years ago.
P.S. An untreated concrete floor can also build up a substantial
charge. Weather (humidity levels, temp.) is also a contributing factor
in static buildup.

>>>>>>>>>> Msg 1938 reply #3

c: HARDWARE #1954 08-19-88 06:52 (Read 83 times)
f: BOB BLAIR
t: TOM PETERS (LEADER) (Rcvd)
s: R: R: BOARD PROTECTION Reply to #1947

Any sources for the products you mentioned in the first reply? I've
never noticed any offered in the ads I see. Perhaps 'cause I haven't
had this thought in my head, but Necessity is calling now. Thanks.
Bob

>>>>>>>>>> Msg 1938 reply #4

c: HARDWARE #1959 08-19-88 16:47 (Read 86 times)
f: TOM PETERS (LEADER)
t: BOB BLAIR (Rcvd)
s: R: R: BOARD PROTECTION Reply to #1954

1. Jensen Tools, 602-968-6231
2. JDR Microdevices
3. Digi-key
4. Radio Shack
5. Hamfest/Swapfest

For 1-4 see the back pages of BYTE or PC.

>>>>>>>>>> Msg 1938 reply #5

c: HARDWARE #1965 08-19-88 22:32 (Read 86 times)
f: RUSS TERRY
t: TOM PETERS (LEADER) (Rcvd)
s: R: R: BOARD PROTECTION Reply to #1959

Try also 3-M. They have more in their catalog than you will find in
Jensen; about the same prices. (Note: they may not sell small
quantities)

>>>>>>>>>> Msg 1938 reply #6

c: HARDWARE #1972 08-22-88 07:03 (Read 15 times)
f: BOB BLAIR
t: TOM PETERS (LEADER) (Rcvd)
s: R: R: BOARD PROTECTION Reply to #1959

Thanks, Tom. I get fliers from at least 3 of them. Appreciate all
the comments.
Bob

>>>>>>>>>> Msg 1938 reply #7

c: HARDWARE #1975 08-22-88 12:35 (Read 1 times)
f: TOM PETERS (LEADER)
t: BOB BLAIR
s: R: R: BOARD PROTECTION Reply to #1972

On static protection: I'm preparing a massive missive (chuckle) on the
topic, to be uploaded to the Mahoney collection early this week. Look
for STATIC.ARC or something similar. I have more hard facts and
numbers in the new piece, and plan to incorporate the one I sent to
you.

>>>>>>>>>> Msg 1938 reply #8




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