ESD DISCHARGE TIMES:
Controlling your Discharges
Ryne C. Allen
ESD Systems
Desco Industries, Inc.
19 Brigham St., Unit#9
Marlboro, MA 017523170
Reproduced with Permission, EEEvaluation Engineering, January,
1998
I INTRODUCTION
The control of electrostatic discharge is an important aspect in the
manufacturing, assembling
and repairing of devices that employ electronics. Electrostatic discharges can
damage an electronic component at any stage of its production or application if
not controlled. The primary method of control is to ground (or bring to
the same potential) all conductors that come in contact or near proximity to
the electronic device(s). These conductors include humans, tools, ESD mats,
other electronic devices, boards, connectors, packaging, etc.
There are other components to a good ESD Control program including, removal of
unnecessary insulators, shielding, ionization, environmental controls,
training, education and top down compliance. This paper will talk about
controlling discharges to a grounded ESD mat on a workstation.
Of specific interest in controlling an electrostatic discharge is the time rate
of the discharge. A discharge will occur much quicker in/on a conductor with a
surface resistance of 10^{2} Ohms than in a conductor with a surface
resistance of 10^{9} Ohms. How fast or slow should the controlled
discharge be? Understanding the importance of discharge times will help you
choose the right ESD control materials in building, maintaining, or auditing
your own ESD Safe workbench(es).
The upper and lower boundaries of an ESD safe discharge rate are determined by
the application and materials used. To limit the discussion, the potential
energy sourced from the Human Body Model (HBM), [refer to ANSI EOS/ESD
S5.11993], is applied into an ElectroStatic Discharge Sensitive (ESDS) work
area or ESD mat.
II BODY & MOVEMENT You should be
familiar with the timing of the human body’s movements relative to handling or
working near ESDS devices to have a handle on the upper limit of the controlled
discharge. To reduce the likelihood of an operator discharging onto an ESDS
device, they should drain any charges before bringing an ESDS device in contact
with themselves or another conductor, whether floating or grounded.
Table I
Movement times (averaged) from typical operations.

Reaching 
Grabbing 
Lifting 
Relocation 
Landing 
Time (ms) 
455 
153 
231 
924 
247 
Std. Dev. (ms) 
48 
11 
61 
137 
73 
Table I depicts averaged times, in milliseconds for the handing of tools or
devices at a work bench with a corresponding standard deviation in millseconds.
The shortest time of 153 ms, or worst case, is the time that we will design our
ESDS workbench table top with. You want to be sure that your device is fully
discharged well before the 153 ms landing time. A good rule of thumb would be
to engineer a x2 safety factor. Therefore your device should be fully
discharged before reaching 76.5 ms (76.5 ms x 2 = 153 ms). The time constraint
of 76.5 ms for body movement defines the upper boundary of the controlled
discharge rate (not including the standard deviation of 11 ms).
III ENERGY CONSIDERATIONS
Table II
Typical Discharge times [t=R*C*ln(V/V_{0})] for an RC
circuit
where C=200 pF and V_{0}=249 Volts
R

10^{2}
W

2.2x10^{3}
W

10^{6}
W

10^{7}
W

8.3x10^{7}
W

10^{8}
W

10^{9}
W

10^{11}
W

Time 
92 ns 
2 m s 
920 m s 
9.2 ms 
76.5 ms 
92 ms 
920 ms 
92 s 
Table II shows calculated discharge rates for the human body model onto an ESD
grounded mat with surfacetoground (RTG) resistances from 10^{2} to 10^{11}
Ohms. The more conductive the ESD mat on the workbench is, the faster the
discharge, but there is another consideration too.
How fast is too fast? When does the discharge energy at any given time reach a
critical level that can damage a semiconductor? The answer depends on several
variables relative to the semiconductor’s construction such as line spacing,
composition, density, packaging, etcetra, all leading to an ESD component
classification [refer to table I in the ANSI EOS/ESD S5.11993 and the
manufactures’ device specifications].
For simplicity’s sake assume the worst case, class 0, which has a 0 to 249 Volt
tolerance. Applying the HBM, a conservative worst case capacitance would be 200
pF, twice that of the HBM and resistance of 10KW
. Therefore the maximum power (P) level based on Ohm’s Law is P=V^{2}/R
(J/s) and the worst case HBM is ((249)^{2}/10K)=6.2 Watts or Joules per
second (Js^{1}).
The maximum energy (E) stored in a worst case HBM capacitance (C) of 200 pF and
at a maximum voltage (V) of 249 Volts, (using E=1/2 CV^{2}
), yields 6.2 m J. The
next concern is to relate energy to time. The time constant (t
) is the measure of the length in time, in a natural response system, for the
discharge current to die down to a negligible value (assume 1% of the original
signal). For an RC circuit, the time constant (t
) is equivalent to the multiple of the equivalent resistance and capacitance.
In this case, the time constant (t
) of our RC circuit is (10KW
)(200pF) or t = 2
m s. Discharging this energy upon
touching a conductor at zero volts yields a current, (using I=P/V), of (6.2Js^{1})/(249V)
or 24.8 mA. To avoid damaging a class 0 ESDS device, the discharge current must
be below 24.8 mA. Engineering in a "2x" safety factor, the maximum discharge
current would be 12.4 mA. To maintain a discharge current below 12.4 mA, we
need to look at our grounding equipment on the ESDS workbench.
Table III
Discharge currents from a
6.2 m J lossless
energy source
(with C=200pF & V=249V) dependent on the discharge time.
Current

24,900 A 
24.9 A 
12.4 mA 
2.49 mA 
249 m A 
24.9 m A 
2.49 m A 
249 nA 
Time (s) 
1x10^{12}

1x10^{9}

2.01x10^{6}

1x10^{5}

1x10^{4}

1x10^{3}

1x10^{2}

1x10^{1}

The rate at which 6.2 m J
of energy discharges is very important. To fast a discharge will lead to an ESD
Event, which can electromagnetically be measured using a simple loop antenna
attached to a high impedance input of a highspeed storage scope. The faster
the discharge the greater the discharge current becomes as well as the emf (electromotive
force) on the loop antenna from the EMI (ElectroMagnetic Interference).
Table III depicts the discharge current for 6.2 m
J at varying discharge times. We are assuming lossless conditions during the
discharge for worst case. For our example, to keep the discharge current below
12.4 mA, the discharge rate [from Table III] must be no quicker than 2.01
m s. This energybasedtime
constraint forms the lower boundary of the controlled discharge rate.
IV MAT MATERIALS The upper and lower
boundary of our controlled discharge rate are now defined and can be used to
help in choosing the correct ESD mat for an ESDS workstation. ESD mat materials
come in many variations. In general, mats are either made from vinyl or rubber
material and can be homogeneous or multilayered. Rubber mats, in general, have
good chemical and heat resistance but vinyl tends to be more cost effective.
The electrical properties of an ESD mat are important to know in controlling
the electrostatic discharge.
An ESD mat will be either electrically conductive or dissipative. Both terms
mean that the mat will conduct a charge when grounded. The difference in the
terms is defined by the materials resistance, which effects the speed of the
discharge. By definition [ESD ADV1.01994] a conductive material has a surface
resistivity of less than 1x10^{5}
W /sq and a dissipative
material is greater than 1x10^{5} W
/sq but less than 1x10^{12} W
/sq. Anything with a surface resistivity greater than 1x10^{12}
W /sq is considered insulative and
will essentially not conduct charges.
Back to our example. If the maximum discharge current of our ESDS device is
12.4 mA, then the discharge time based on energy must be slower than 2.01
m s and based on body movement must
be faster than 76.5 ms. Using the discharge times from Table II and assuming
that the mat has a negligible capacitance relative to the HBM, then the mat
resistance must be greater than 2.2x10^{3} W
or 2.2x10^{4} W /sq
and less than 8.3x10^{7} W
or 8.3x10^{8} W /sq.
In other words, a very conductive mat for some applications may be to quick to
discharge and yield more dangerous ESD events whether properly grounded or
not.
Graph I
Graph I shows the natural response of a 249 Volt discharge
in an RC circuit using a capacitance of 200 pF (HBM) into resistances (mat) of
10^{4}, 10^{5}, and 10^{6} Ohms. The natural response
of the10^{4}
W curve is below 1% of its’
initial voltage in less than 10 m
s where the 10^{6} W
curve takes less than 1ms to discharge to less than 1% (V<2.49 V) of its
initial value (V_{0}=249 V).
V GROUND STRAPS Another defense, and
the most common method, to reduce the risk of creating an ESD event is wearing
a grounded wrist strap at the workstation. The wrist strap connects the skin (a
large conductor) to a common potential (usually power ground). Properly worn,
the wrist strap should fit snugly, making proper contact with the skin, to
reduce contact resistance. Refer to the manufactures specifications and
instructions of the wrist strap.
The wrist strap, since it is connected to ground, will quickly discharge any
charge the body either generates through tribocharging or becomes exposed to
through induction. Anytime the body directly touches a charged conductor, a
discharge will occur because the body is at a different potential (0 Volts).
Controlling this discharge is important if the conductor is an ESDS device and
in minimizing induced charges through EMI onto nearby ungrounded ESDS devices.
The electrical properties of the skin of an operator can have a wide range in
both resistance and capacitance depending on several variables. An operator’s
hand touching a charged device will initiate a discharge at the rate of the
time constant of the skin before including the RL properties of the wrist
strap. To reduce the potential of an unsafe discharge from a device to a very
conductive operator, adding resistance to the operator at the interface from
skin to device may be necessary. Some solutions are static dissipative gloves
or finger cots, which if worn properly, may add from 1 to 10 MW
to the RC circuit of the skin. This in turn slows down the discharge rate to
well over 2 m s.
VI CONCLUSION The upper and lower
boundaries of a safe discharge rate are determined by the application and
materials used. The movements of the operator define our upper boundary and the
max energy, as defined by the ESDS component classification, dictates our lower
boundary. We want to design an ESDS workbench to control the discharge rate
(via the circuit’s time constant) of our grounded or conductive materials
within these limits.
For the HBM, and a class 0 device, the materials chosen for a safe ESD
workbench should have electrical properties to support discharge rates between
2 m s and 76.5 ms. These
discharge rates, using worst case assumptions, equate to an ESD mat surface
with an RTG (Resistance To Groundable point) between 2.2x10^{3}
W and 8.3x10^{7}
W . This controlled discharge rate window will vary
depending on the class of semiconductor components used (class 0 to class 3B)
and the properties of production resources used (human vs. automated).
The numbers calculated were based on assumptions used to simplify the
explanation of the material. Real world applications are much more complex and
require a more detailed analysis, which was beyond the scope of this paper.
Copyright (c) 1998,
Desco Industries, Inc.
