Transcript Thoughts and Theories on ESD

ESD Thoughts and theories
ESD things I think about (Rev. 1)
Do you have other ESD topics we should clarify better?
Graphics I did not create are ©1999 New Vision Technologies Inc.
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(c) 2000 Doug Mason
1
Introduction

I am a soldering trainer who focuses on the management of the
total process that impacts on the quality of the solder joint,
from design to delivery.
 One aspect I cover is the management of ESD. In researching
this topic, I am running into explanations that would not
satisfy my audience, who are: Process Workers, Stores people,
Purchasers, Managers, and so on.
 They do not want explanations that require an understanding
of Laws of Physics or any mathematics, formulae, and so on.
Explanations must be clear, simple and obvious, related to
their everyday experiences.
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(c) 2000 Doug Mason
2
Introduction

I want to know if my explanations make sense and
are correct.
 Even if I am wrong, hopefully I am a catalyst for the
generation of more lucid and logical simple
explanations that are understandable by the widest
community.
 When such people understand the reasons they need
to take such steps, this should surely help them
implement the required strategies.
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3
Introduction

I am interested in hearing from people who want to
make positive contributions.
 When you write, please tell me the Revision number
(see the first Title Slide)
 If you can make a useful contribution to my
knowledge, please do so. My address is
[email protected]
Many thanks,
Doug
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4
First, some basic principles . . .
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Atomic Theory
Basic Principles (1)
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An atom is made up of charges



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An atom is made up of
charges.
In its stable state, the size of
the positive charge at the
centre (nucleus) of an atom
is balanced by the size of all
the negative charges of the
electrons, making it neutral
overall.
The electrons whirl around
the centre like planets.
(c) 2000 Doug Mason
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Creating a charge imbalance
Electron “lost”
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

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When an atom loses an
electron, it has a charge
imbalance.
Since this atom has lost an
electron, which is a negative
charge, the atom is now a
positive charge.
When it gains an extra
electron, an atom becomes a
negative charge.
(c) 2000 Doug Mason
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A discharge
This powerful, rapid
movement of charges
can damage electronic
components.

Let’s see it
again!
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
When something which has
a charge imbalance is
brought close to or touches
something else, a stream of
charges might move, to try
to bring the atoms back to
their stable balanced
condition.
This movement of charges is
called a discharge.
(c) 2000 Doug Mason
9
Triboelectric generation
Basic Principles (2)
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When materials are in contact
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When materials are in contact
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When materials are in contact
When two materials are in intimate contact, they share
electrons which are at their surfaces.
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When materials are in contact
It is possible for electrons to be “stolen” from one
When two materials are in intimate contact, they share
material by nuclei in the other material, because they
electrons which are at their surfaces.
have a stronger force
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When materials are in contact
It is
possible
for electrons
“stolen”
fromnumber
one
Since
the
contacting
surfaces to
stillbehave
an equal
material
by and
nuclei
in the other
material,
they
of positive
negative
charges,
there isbecause
no overall
haveimbalance
a strongeron
force
charge
them.
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15
When the materials are separated
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When the materials are separated
Electrons “lost”
Electrons “gained”
When these materials are separated, electrons are
removed from one material and are transferred to the
other material.
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When the materials are separated
Electrons “lost”
Electrons “gained”
This
action
takes
place with
types of materials.
With
When
these
materials
areall
separated,
electrons are
insulators,
the charges
remain
atare
thetransferred
points of contact.
removed from
one material
and
to the
A charge spreads all
overmaterial.
an ungrounded conductor.
other
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When the materials are separated
Electrons “lost”
Electrons “gained”
This action
takes
with
all electrons
types of materials.
The loss
andplace
gain of
these
creates anWith
insulators,
theofcharges
at the points
ofon
contact.
imbalance
negativeremain
and positive
charges
the
A charge spreads
all over
an ungrounded
surface
of each
material. conductor.
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When the materials are separated
Electrons “lost”
Electrons “gained”
The loss and gain of these electrons creates an
When the surfaces are rough, this intimacy and
imbalance of negative and positive charges on the
separation is assisted by rubbing the materials together.
surface of each material.
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When the materials are separated
Electrons “lost”
Electrons “gained”
The size of the charge (imbalance) depends on the
When the surfaces are rough, this intimacy and
intimacy of the contact, how fast they were separated,
separation is assisted by rubbing the materials together.
the humidity and the kinds of materials.
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21
When the materials are separated
Electrons “lost”
Electrons “gained”
The size of the charge (imbalance) depends on the
The drier the air (lower relative humidity, RH) the
intimacy of the contact, how fast they were separated,
higher the generated charge.
the humidity and the kinds of materials.
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When the materials are separated
Electrons “lost”
Electrons “gained”
Separating similar materials can still produce a charge,
The drier the air (lower relative humidity, RH) the
such as when a smooth surface is separated from a
higher the generated charge.
rough surface (large rolls of plastic in the Store).
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23
The creation of a charge when
materials are separated is termed
“triboelectric charging”
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Charging by Induction
Basic Principles (3)
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Charging by induction

An explanation of the principle, as I understand it.
 This is required for the explanation of scenarios I
describe - (1) walking on carpet and (2) developing a
charge while driving.
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Charging by induction
The charge imbalance on a surface produces an electric field.
The presence of this very strong force causes similar charges
on the surfaces of nearby conductors to be repelled.
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Charging by induction
If the conductor is grounded while it is still influenced by the
electric field, these repelled charges go to earth, thus
maintaining a charge balance in the areas of the conductor
that are not affected by the electric field.
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Charging by induction
When the earth is removed and then the electric field is
removed, the conductor has become charged to the opposite
charge of the original source.
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Walking on carpet
Thoughts and theories (1)
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Walking on carpet

In one place, I read that as I walked on carpet, electrons
moved from the bottom of my shoe and up my leg, to create
the charge on my body. My question became: “If that path
exists, why is there then a need for a heel/foot strap to remove
those same electrons back to ground potential?”
 My current theory is described in the next few slides.
 I have not tested it, nor do I know how I would do it.
 This explanation shows why I showed the principle of
“charging by induction” on the previous slides.
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31
Walking across carpet

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Walking across a carpet
creates a charge
imbalance on the lower
surface of my shoes.
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32
A strong electric field
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
This charge imbalance
creates a strong electric
field that emanates in all
directions.

The material in the sole
of my shoe enables a
strong field to influence
charges within my foot.
(c) 2000 Doug Mason
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An induced charge
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
Since my body is relatively
conductive, the charges
that are repelled from the
lower regions of my foot set
up a charge throughout the
rest of my body.

But my body still has a
balanced charge overall (it
has not lost or gained any
charges).
(c) 2000 Doug Mason
34
Contacting a source of charges


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While I am still on the
carpet (still influenced by
the electric field on the
sole of my shoe), I briefly
touch a metallic object (a
chair, table, door knob,
metal stapler, etc.)
This enables charges to be
provided by the metallic
object.
(c) 2000 Doug Mason
35
Charging through the air
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
If the charge imbalance on
my body is large enough, I
do not need to touch the
object, as the imbalance
could allow a movement of
charges through the air.

This movement of charges is
thus producing an overall
charge imbalance in my
body, since I was previously
neutral overall.
(c) 2000 Doug Mason
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An overall charge imbalance

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Therefore, after that
brief encounter with
the metallic object,
my body now has an
overall charge
imbalance.
(c) 2000 Doug Mason
37
Removing that charge

The only way to remove the possibility of damaging
an ESD-sensitive device or assembly is to remove
the overall charge imbalance on my body.

This is done by connecting my body to the ready
source of charges that is at the same reference
potential as the item I wish to handle - using a wrist
strap or foot strap.
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38
Driving a car
Thoughts and theories (2)
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Charged while driving a car (part 1)
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
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I am having no small difficulty developing a model which describes
the actions that result in a charge imbalance being created on a
person while they drive a car.
Charge imbalances are created by the movement of the driver’s body
against their clothing, by the movement of that clothing against the
car seat and is increased by the action of leaving the seat.
I assume that the nett induced electric field and the (small)
triboelectric effect on the skin determine the charge imbalances
developed on the surfaces of the driver’s back and tail. The types of
materials worn by the driver and the material used on the car seat
have the greatest impact on determining the size of the electric field
that impacts the driver’s back and tail.
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Charged while driving a car (part 2)


This shows that the body does not assume a significant overall
charge imbalance. Maybe there is a potential gradient caused
by the presence of the charge near the driver’s back and tail,
with the driver’s fingers assuming an opposite potential, as the
charges are driven from the field induced by the seat, and the
fingers are outside that electric field. This is what I have
assumed.
This would mean, as I have described, that the observed
“discharge” between the person and the car body is actually a
neutralisation of those charges on the fingers, and this is thus
the time that the body receives an actual overall charge.
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41
Charged while driving a car (part 3)
Do the charges on the seat cover actually travel to the driver’s
skin through the material, at an atomic level? Or is this, as I
have described, a charge induced on the driver’s back?
 I need a description that can be understood by a reasonable
Process Worker - no maths, no “in-terms” - just a plain
English description that relies on basic concepts.
 This is all relevant to the creation and management of a charge
by an Electronics Assembler at their workplace.

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42
I am driving my car

As I drive my car, there is
constant movement between my
clothes and my car seat, and
between my body and my
clothes.

These movements create charge
imbalances on the surfaces of my
clothes and my skin.

These imbalances set up strong
electric fields that affect charge
distribution in my body.
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43
Charge distribution

Charge imbalances (and
strong electric fields)
appear at the:
– outer surface of my clothes
– parts of my body that are
influenced by the strong
electric field (opposite
polarity to the source)
– remote exposed parts of my
body (same polarity as the
source of the originating
charge imbalance)
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Further charging as I leave the car
 Further triboelectric
activity occurs as I turn
to get out of the car.
 Also, the Voltage of the
charge increases when I
stand up, since my body
now presents a smaller
area relative to the
ground.
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A discharge before exiting

If, before leaving the car, I touch a
ready source of charges (such as the
metal door handle), the charges
enter my body to balance the
charge imbalance on my fingers.
 This discharge creates an overall
charge imbalance on my body.
 This imbalance declines as the
charge on my clothes decline and I
continue to touch sources of ready
charges while this is occurring.
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Not discharging before leaving
 However, if I touch only non-
conductors as I leave the car,
the charge distribution
continues on my body for as
long as the charge imbalance
remains on the outer surface
of my clothing.
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47
Discharging to the car body
In this case, it is likely there
will be a movement of
charges between my body
and a source of ready
charges (such as the car
body).
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48
It’s the same at your work place
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If you are not provided with an ESD-safe chair at your
work place, the same actions occur as with the car driver.
Even though in most instances you do not feel the discharge
as you leave your car, it is likely that one exists.
The sensitivity level of many electronic components means
that they can be affected by a discharge produced by the
seat where you work, even though you do not feel it.
The only safe way is to always wear a properly connected
and functioning wrist strap.
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49
Is a smock a Faraday Cage?
Thoughts and theories (3)
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A smock as a Faraday Cage?

I consistently read that an ESD smock functions as a Faraday
Cage.
 I also read that a Faraday Cage is a hollow conductor that
distributes its charge imbalance on its outer surface, a fact
made use of in the shielding bag and tote box.
 But, I reason, if a smock holds its charges on its outer surface,
surely this is bringing it close to the components and
assemblies being worked on, which is not wanted.
 Or have I missed the point, is the inside of the garment the
outside of the Cage?
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A hollow conductor

A Faraday Cage is a
hollow conductor.
Michael Faraday
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Charges sit on the outside

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When a Faraday Cage is
placed in an electric
field, the charges sit on
the outside of the
conductive surface, and
the field does not
penetrate it.
(c) 2000 Doug Mason
53
Two practical applications

This fact is made use of in:
– the static shielding bag,
which has a continuous
conductive layer,
– the tote box, made of
conductive material.

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If there is an opening in
the conductive layer, some
of the electric field may
pass through it.
(c) 2000 Doug Mason
54
The “outside” of a Faraday Cage

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The “outside”
surface of a Faraday
Cage is the surface
that is closest to the
electric field.
(c) 2000 Doug Mason
55
A field placed inside a Faraday Cage


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When there is an electric
field inside a Faraday
Cage, the “outside” of the
conductor is thus the inner
surface of the Cage.
An electric field inside a
Faraday Cage does not
penetrate it, unless the
continuity of the
conductive layer is broken.
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56
Smock has a conductive layer
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
An ESD smock has a
continuous conductive layer.

This forms a Faraday Cage
around the wearer.

The electric field is inside the
garment. (Given the nature of
material, the term “outer”
could be seen as being quite
arbitrary, anyway.)
(c) 2000 Doug Mason
57
The need for a complete layer

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Electric fields resulting
from charge imbalances
produced by the wearer
do not pass through the
conductive layer - unless
the layer is not complete
(for example - if it is
unbuttoned or cuffs
protrude).
(c) 2000 Doug Mason
58
Sources of internal fields

Most movement occurs
with the arms, so the
sleeves carry the strongest
fields. These fields are
continually varying.
 The electric fields from
clothing not covered by
the Faraday Cage (such
as exposed sleeve cuffs)
enter the work area.
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59
Garments are dissipative

The high resistance of
the outer layer of
smocks is similar to that
of dissipative mats.
 This controls the
removal rate of any
charges, whether the
imbalance is outside the
garment or inside it.
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60
Garments are anti-static


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The garments are also
anti-static, meaning
that they resist the
creation of charge
imbalances.
The panels are sewn
with material that
ensures they are
electrically connected.
(c) 2000 Doug Mason
61
ESD Thoughts and theories
ESD things I think about (Rev. 1)
Do you have other ESD topics we should clarify better?
Graphics I did not create are ©1999 New Vision Technologies Inc.
[email protected]
(c) 2000 Doug Mason
62