Download Using Electrostatic Discharge Test Method for Full

Using Electrostatic Discharge Test Method for Full
Characterization of Dissipative and Conductive Materials
Ber-Chin Yap(1), Larry Fromm(2)
(1) Finisar Malaysia Sdn Bhd, Plot1, Kinta FIZ, Chemor 31200 Perak Malaysia. Email:[email protected]
(2) Finisar, 1308, Moffett Park Drive, Sunnyvale CA94089-1133. Email:[email protected]
For many years, DC resistance and charge decay testing have been used to characterize dissipative materials.
However, we have observed these tests to be sometimes inadequate in the face of non-ohmic behavior of some
dissipative materials. We propose new testing methods using an ESD pulsation technique. This technique could
determine an on-set threshold of strong discharge (> 10 mA in 100 nS) for a specific dissipative material.
Employing static bleed-down control methods above this threshold could potentially endanger ESDS products.
A Human Dissipative Glove Model simulation has established that a device could be damaged at a shifted
voltage level after the on-set threshold of a strong discharge through a dissipative glove.
I. Introduction
Dissipative materials have been widely used in ESD
protection areas. The principal objective is to
facilitate an effective way for prevention of static
charge accumulation at all times so that a relatively
“charge free” environment could be maintained at
ESD safe workstations. However, in the real world,
there are also possibilities of working fixtures or
ESDS devices that need to be in electrical isolation or
floating. In such scenarios, the fixtures and or the
ESDS parts may not be completely free of static
accumulation.
It is then desirable to prevent
subsequent ESD including Machine Model (MM),
Charge Device Model (CDM)[1], Charge Board
Model (CBM), and/or Charge Strip Model (CSM) [2].
To achieve this, dissipative material needs to be
placed in strategic locations to furnish a slow, safe
redistribution of potential static charge from fixtures
or from an ESDS device at the proper time.
With the evolution towards below 100 nano-meter
technologies, the discrete device ESD susceptibility is
expected to be more acute. For instance, the CDM
thresholds of the current GMR heads have been
already lower than 5 volts. Therefore, it is critical to
implement optimum ESD control measures for the
safe removal of even very low levels of static
accumulation on parts resulting from process tribocharging effects for example. Ionization has been
widely adopted as a key means for the safe reduction
of low charge accumulation levels. However, the
ionization control method has some limitations, such
as slow decay time and inaccessibility under certain
constraints during assembly process. In some cases,
one may find that the sole dependence on ionization
control environment may not be sufficient to mitigate
ESD risk during fast handling processes [3]. Better
process engineering design can create a slow, safe
discharge (ground) path using a dissipative material in
direct contact with a charged part or material in order
to reduce ESD risks. This engineering activity is
usually designed within the specific process and or
between two processes.
In current practice, the choice of dissipative materials
relies on the DC resistance measurement or test [4] at
a single voltage point.
This may also be
complimented by a charge decay time test using a
Charge Plate Monitor [5].
Both methods of
characterization are employed with the objective of
better working material achieving an optimized
“charge free” workstation environment.
Such
information is partial. It could leave some doubt
whether such data is adequate when one is employing
dissipative material tested in this manner to provide a
slow, safe path for potential static charge to dissipate
from the ESD sensitive part to ground. Furthermore,
another potential shortcoming in the above test
method has also been shown more recently [6]. The
DC standard test at a single fixed driving DC source
of a Mega-Ohmmeter at either 10 V or 100 V is likely
not sufficient to cover the critical characteristics of
materials in the low voltage regime below 10 V.
For most materials, the I-V relationship could be nonlinear over a wide range of currents and voltages. As
such there could be risks using a single, constant
value to denote the resistance over a wide I-V range.
This non-linearity is also known as a non-ohmic
electrical characteristic of materials. This non-ohmic
behavior is particularly more profound for dissipative
materials. For instance, a dissipative material may be
measured by a DC Mega-Ohmmeter to have a
resistance of around 1000 Mega-Ohms at 100 V, but
at 2000 V, its effective resistance might be more in
the range of 1000  or lower. The implication is that
the lower resistance at a high voltage could result in a
high current passing through a very thin layer of
dissipative conduction path between electrodes. The
Joule’s heat generated could cause certain materials
along the path to heat up excessively -perhaps close to
or beyond their melting point. Furthermore, in
association with such high current, the material would
turn into a better conductor compromising the
objective of a slow, safe discharge as originally
intended. The matter is further complicated by the fact
that an ESD even is an extremely fast pulsation in
most cases.
All these pose a major technical
challenge to any attempt to modify the existing DC
test method on dissipative materials over a wider
voltage range. Therefore, any modification based
upon the existing DC test meter may not be suitable or
practical. In our view, voltage and current pulsation
techniques, such as those that have been used for the
characterization of ESD protection devices on ICs or
sintered ceramic varistors at higher voltages and
currents, would definitely have advantages over
traditional DC testing methods.
In this paper, we shall present the results of our study
using the electrostatic discharge technique, which
could be viewed as a sub-set of pulsation techniques,
for the characterization of materials. For the present
characterization technique, we have adopted the
standard HBM simulator used in simulation of device
ESD failure threshold as a pulse source for our
evaluation testing of both conductive and dissipative
materials. This method appears to offer better insight
and information concerning the material’s true
electrical behavior during ESD events. We intend to
discuss both the test method and results from our
work.
II. Dissipative Materials
There are several types of dissipative materials [7].
One of the most common of these is the composite
material. Basically, it is made from a mixture of two
components. One is a base material from an insulative
plastic, and the other is a filler of conductive powder
or fibers. A conductive or dissipative floor tile is one
of the common composite materials that have been
widely used for making ESD safe flooring in
factories. In the fabrication process, tiny grains or
segment fiber filler will be added and pre-mixed
inside the melted viscous plastic base materials before
solidification.
A schematic micro-view of this
material structure suggests that the conductive grains
or fibers will form a matrix of “conductive islands”
that are surrounded by the base insulator. With the
increase of percentage (by volume) of fillers, the inter
layers of insulator between two adjacent conductive
grains or fibers could be very thin. These conductive
elements are irregular in both shapes and sizes.
Therefore, this inter insulator layer could vary in
thickness even for those very homogeneous mixtures.
There would be location(s) in the inter layer much
thinner than others. When a voltage is applied to a
pair of two close adjacent conductive elements
separated by the insulator layer, electrical conduction
can be initiated through those thinnest spots on this
inter insulator layer.
The physics of the electrical property in the formation
of two conductive elements separated by an insulator
layer could be a phenomenological equivalent to a
system consisting of two potential wells separated by
a potential barrier. Based upon the above physical
picture, it has been theorized that three possible
conduction mechanisms exist under the applied
voltage, or more appropriately, under the influence of
an external applied electrical field. In the low voltage
range, the conduction is believed due to electrons
moving across the potential barrier “wall” through
two possible kinds of charge transport mechanism.
Namely, the process could be a thermal activated
barrier hopping or a quantum mechanical barrier
tunneling or both. At high voltages, the applied
electric field could reach to a sufficient strength to
cause an onset of multiple impact ionizations. This
results an avalanche of dielectric breakdown across
the insulator [8]. These three conduction mechanisms
would cause the overall electrical property of the
material to be non-ohmic in behavior.
For other types of dissipative materials, such as some
compound thermoplastics, we are not as concerned
about the conduction mechanisms in the low voltage
or electric field regime. However, it is expected that
under high electric field or voltage, a similar,
avalanche impact ionization process will also take
over as the main conduction mechanism in these
materials. During the onset or initialization of high
intensity current conduction, the pulse technique
would have the advantage of limiting or lessening
Joule’s heating impact at a localized contact spot , i.e.
an extremely short pulse duration of less than 100 nS.
Such a short heat pulse would enable one to extend
the applied voltage to a much higher range, which
would otherwise be far beyond the safety capability of
the DC testing method applied to a material.
III. Measurement Techniques
We have adopted the standard ESD simulator with
Human Body Model or HBM discharge network
comprised of a 100 pF capacitor and a 1500  resistor
to perform ESD pulsation on the material under test.
A Thermo KeyTek Model 2000 MiniZap has been
used for the HBM simulation on conductive materials.
The current discharge pulse pattern have been
captured by a Tektronix CT-1 current probe coupled
into a Tektronix Digital Oscilloscope with a 2.5 GS/s
sampling rate and a bandwidth of 300 MHz. A
schematic test setup is shown in Figure 1.
Scope
HBM Simulator
CT-1
breakdown in conduction for a dissipative material at
higher voltages. A superposition of both the EMI
component and the actual discharge current pattern
could result in a distorted, captured signal. This low
EMI was generated from the trigger button and or
from its control circuitry of the simulator during pulse
activation.
The discharge ground wire passing
through the center of CT-1 transformer probe would
act as a dipole antenna to pick up such low level EMI.
To overcome the interference, we had taken a 20 pF
charge plate of a Monroe 268 Charge Plate Monitor
(CPM) which we connected in parallel with another
capacitor of 80 pF to this charge plate to ground.
Thus, the total capacitance of this set of parallel
capacitors is roughly 100 pF. We have made use of
the CPM power supply as our charge source for such
100 pF capacitor. The IEC ball shaped discharge tip
has been connected to ground. It is used as a
discharge tip during probe contact ESD discharge
testing of dissipative materials. During the test, the
sheet or layer of dissipative material is placed directly
on top of the charge plate and is allowed to charge.
The CT-1 is mounted at the discharge probe ground
wire. The 100 pF capacitor is always charged
positively. Therefore, the mobile charges, in this case
being the electrons, would be moving from the tip
through the layer of dissipative material towards the
capacitor. A schematic of the test configuration is
shown in Figure 2. The ball shape discharge tip is
chosen to prevent any mechanical puncture of the
material surface under test. The EMI level generated
at the instant of contact between the probe tip and
dissipative material during the static discharge is very
much lower [3].
Conductive Materials
Scope
Figure 1.
Setup
A Schematic Conductive Material Test
The above simulator is not specifically designed to
perform such discharge testing of dissipative
materials. The discharge resistance of the dissipative
path in series is much higher than the 1500  internal
resistor of a standard HBM simulator. It is obvious
that the discharge from the 100 pF capacitor through
such extremely high resistance path is no longer a
HBM simulation in its normal context. We have also
found that our simulator could generate a very low
level high frequency EMI that could be picked up by
the CT-1 current probe during a discharge through a
high resistance path.
This EMI level is now
comparable to the equally low magnitude peak
discharge current at the immediate onset of avalanche
CT-1
Ball Shape Tip
CPM
80 pF
Figure 2. A Schematic Dissipative Material Test
Setup
The current patterns at 1000 V discharging directly to
ground from both KeyTek 2000 Mini-Zap HBM
simulator and the modified method with a resistor of
1500  in series with the branch discharge (tip) are
shown in Figure 3a & 3b. The 1500  resistor of the
latter is to facilitate an identical discharge network of
a standard HBM simulator. Both discharge current
patterns are quite similar.
tip to the pre-charged 100 pF capacitor. This was also
to provide better control on a more consistent material
spot that would be under incremental ESD stress.
Finally we had also made an ESD simulation study on
the impact of a human wearing a dissipative glove
discharging to an ESD sensitive device. In such
study, we used the KeyTek 2000 HBM simulator with
the IEC ball shape discharge tip. The degradation
thresholds of the device under test were obtained for
cases with and without the nitrile glove wrapped
around this ball tip or probe. The results from such
testing are provided in the following section.
IV. Results
1. Conductive Material
A typical current discharge pattern from an HBM
ESD simulator performing a discharge at a distance of
roughly 3 cm from the end of the conductive surface
coated strip is shown in Figure 4. The ground wire is
clamped to the strip end (Figure 1).
Figure 3a. Typical Current Pattern from Discharge to
Ground of KeyTek 2000 at 1000 V
Figure 4. 2000 V HBM Discharge Current Pattern
across 3 cm Distant of Carbon Coated Conductive
Surface
Figure 3b. Typical Current Pattern from Discharge to
Ground by Our Designed Simulator at 1000 V
Note that for current below 10 mA and pulse width
below 10 nS, we are beyond the resolution and the
trigger threshold for our portable digital oscilloscope
to capture such a low magnitude, fast pulse.
During the characterization on the dissipative glove
materials, the sample arrangement had to be modified.
During this test, a glove material was first wrapped
around the ball shape discharge tip. The discharge
test was conducted by touching such glove wrapped
As shown in the captured current pattern, the peak
current amplitude is about 100 mA, and the current
decay time is approximately 4 S. The resistance is
about 1x 105  as measured by surface resistance
meter at 10 V DC driving voltage. Therefore the time
constant of the discharge from a 100 pF capacitor
through this equivalent resistor of 1x 105 ,  = RC ~
10 S which is comparable to the current decay time.
This is close to, but may be not fully identical to, the
discharge current pattern of a conductive resistor.
2. Dissipative Materials
We started the discharging of the 100 pF Capacitor
through the layer of dissipative material beginning
with 200 V and carrying on subsequent discharges
with an incremental voltage step of 100 V.
2a. Static Discharge Test on Materials in Thin
Flat Sheet Form
We have selected 3 dissipative materials for this
study. A piece of dissipative mat with its top
dissipative layer of ~ 1 mm thick and its surface
resistivity of ~ 2 x 108 /Sq. has been used to perform
the discharge to the ground. A typical discharge
pattern through the mat is given in Figure 5a. Note
that the current pulse is extremely short in the order
around 100 nS or below. There exist oscillations and
ringing in the current decay. When a 1500  resistor
is connected in series to the ground wire of the ball
discharge tip, such current oscillations in decay tail
have disappeared (Figure 5b). The 1500  may have
either suppressed such oscillations from being
reflected within the ground wire or raised the
resistance in the discharge path resulting in smoother
decay.
Figure 5b. Through Dissipative Mat: Discharge
Current Pattern at 5500 V with 1500 added in
series to Ground
However, bearing in mind the non-ohmic behavior in
this high voltage range, we could not use a single
value Req to denote the current decay behavior. The
actual value could be very much underestimated in
such a simplified model. A plot of the maximum
current peaks against the discharge voltages for this
material is given in Figure 6.
From the plot, we
could derive that the initialization or on-set of impact
ionization breakdown conduction mechanism to take
place around 1600 V. If we define the DC resistance
at a specific applied voltage as V/I, by taking the
discharge voltage V = 5500 V and I ~ 300 mA, the
apparent DC resistance at 5500 V works out to be
about ~ 18,000 .
Peak Current Versus Discharge Voltage of
Dissipative Table Mat
350
300
I (mA)
250
200
150
100
Figure 5a. Through 1mm thick Dissipative Mat:
Discharge Current Pattern at 5500 V
If one were to model the charge decay constant  =
Req C and if one were to take  ~ 100 nS, with C =
100 pF, one would work out the Req to be
approximately 100 nS / 100 pF = 1000 . This
simplified model would predict the equivalent
resistance along the thin discharge path of ~ 1 mm to
be around the order of 1000  or lesser.
50
0
0
1000
2000
3000
4000
V (Volts)
5000
6000
Figure 6. Peak Current Versus Discharge Voltage
A typical current discharge pattern for a dissipative
sheet sample made from PermaStat compound 1800
Ext is shown in Figure 7. Its thickness and its surface
resistivity are 0.5 mm and
~ 6 x 109 /Sq
respectively.
The corresponding maximum peak
250
200
I (mA)
current versus the discharge voltage is shown in
Figure 8. Another PermaStat compound, 1000A, has
also been tested. This also has a similar discharge
current pattern and its maximum peak current versus
the discharge voltage is provided in Figure 9. It has a
thickness of 0.54 mm and a surface resistivity around
~ 7 x 109 /Sq.
150
100
50
0
0
1000
2000
3000
4000
5000
6000
V (volts)
Figure 9. PermaStat 1000: Peak Current Versus
Discharge Voltage
2b. Glove Materials
For the first test, we selected one commercial type of
cleanroom nitrile dissipative glove for this study.
This glove had an un-stretched thickness around 0.15
~ 0.17 mm. The resistance of the glove in actual use
has been determined to be around ~108 . This is
done by a user placing and holding each 5 lb weight
electrode of the Surface Resistance Meter in each
palm. Only one hand would be wearing the glove,
and the other hand is bare during such measurement
with the meter.
For a glove, a typical pattern captured is provided in
Figure 10.
Figure 7. PermaStat 1800 Ext: Current Pattern
at 5600 V Discharge
160
140
I (mA)
120
100
80
60
40
20
0
0
1000
2000
3000
4000
5000
6000
V (volts)
Figure 8. PermaStat 1800 Ext: Peak Current Versus
Discharge Voltage
We did not see any visible mark of fusion or damage
at the discharge contact spot on the surface of the
material after being subjected to the highest voltage
discharge tests for all three types of samples. No
attempt was made to proceed with further
investigation on the potential change of material at
micro-structural level. Again, the nonlinear resistance
as a function of discharge voltage is noted.
Figure 10. Nitrile Glove: 3000 V Discharge Pattern
After the high voltage discharge, there existed a fused
spot on the discharge contact surface area of the glove
material, but there was no visible punctured hole
found at the fused spot under optical microscope
examination.
Figure 12. Some pink color dissipative finger cots
were also tested. In this case, the breakdown
threshold voltage could be initiated at voltages as low
as 500 V, and the material could rupture near 1000 V.
1200
1000
I (mA)
800
3. ESD Simulation through Dissipative Glove
onto ESDS Device – Human Dissipative
Glove Model (HDGM) Test
600
400
200
0
0
1000
2000
3000
4000
5000
V (volts)
Figure 11. Stretched Nitrile Glove: Peak Current
Versus Discharge Voltage
A typical maximum peak discharge current versus the
discharge voltage for the nitrile glove material is
shown in Figure 11. The glove had been stretched a
bit when it is wrapped around the ball shaped tip for
the discharge test. It has been observed that the
material further thinned out at the discharge contact
area after going through the 4000 V electrostatic
discharge test. Again, we did not find any sign of
rupture on such contact area under optical microscope
examination. This is further supported by the fact that
the discharge current pattern could not be detectable
when the discharge voltage was reduced below ~1000
V level again.
Figure 12. Red Dissipative Glove: 100 V Discharge
Pattern
In another case, a red color glove with a thickness
around ~ 0.1 mm and resistance in use of ~ 6 x 106 
had gone through the similar discharge test. This
glove material was ruptured at voltages as low as 185
V. A typical discharge current pattern is shown in
We have made a study to evaluate the incremental
tolerance margin one would enjoy with respect to
HBM ESD if cleanroom nitrile gloves were to be used
for persons handling ESD sensitive devices. We
choose a type of oxide VSCEL laser diode to conduct
this ESD simulation study. In this study, we also
used the IEC ball shape discharge probe with the
nitrile glove material wrapped around the tip. The
simulator was, again, the KeyTek 2000 MiniZap. We
first used the ball shape discharge tip to perform the
HBM ESD simulation of the oxide VSCEL laser
diode device. This laser device had been found to
consistently fail at roughly 350 V.
We then
performed a series of similar ESD simulations through
the nitrile glove layer onto good laser diode devices.
The simulation was done with incremental voltage
steps of 100 V starting from an initial voltage level at
200 V until device failure. This laser device failed
both electrical and optical specification at around
2800 V in this kind of test.
The above test results indicated that the device’s
effective failure threshold from a “gloved finger” had
been greatly “increased” with respect to that of
standard HBM ESD stress (standard HBM stress test
being a simulation of a charged person who performs
direct discharge from a bare finger to a device). In
general, this also implies that ESD sensitive devices
would encounter lower ESD risk from HBM
discharge for voltage levels below the on-set of the
high current peak by the wearing of specific glove
material. For nitrile gloves under study, the lowest
on-set is around 900 V. This is because below the
conduction breakdown threshold of 900 V, i.e. the
“on-set”, the effective material resistance is still
relatively high. This in turn restricts the peak
discharge current. Its magnitude is far below the
actual level that could cause serious degradation or
failure of the device during this form of “Human
Dissipative Glove Model” (HDGM) ESD. However,
after initialization, and in the regime of breakdown
conduction, the device could then be exposed to
higher peak pulse currents-greater in magnitude than
tens of mA. At a discharge voltage around 3000 V,
the peak current measured about 0.7A. This is
approximately the peak current level that could cause
the laser diode to degrade or to fail.
V.
2.
Summary
1. For conductive materials, the non-ohmic
characteristic for the type under study was not
shown to be very profound. This was
supported by the fact that the current time
decay was ranging from 100 nS to the order
of S.
However, for dissipative materials with a thin
conduction path around 1 mm or less, we
could only capture the discharge current pulse
above a certain threshold voltage range.
Moreover, above such voltage thresholds,
there existed a relatively strong, short decay
time (< 100 nS) current pulsation -injecting
through the thin layer of the materials. A
summary of such thresholds for a few
dissipative materials examined under the
present study is given in Table 1 below.
Table 1
Material Type
Dissipative Mat
PromaStat 1000A
PromaStat 1800
Nitrile Glove
(unstretched)
Red Glove
Pink Finger Cot
Thickness
Resistance
(at 100 V)
~ 1 mm
~ 0.54 mm
~ 0.5 mm
0.15~0.17 mm
~ 2 x 108 
~ 7 x 109 
~ 6 x 109 
~ 1 x 108 
Discharge
(>10 mA)
Threshold
~ 1700 V
~ 1600 V
1300 ~1500 V
1000 ~ 1200 V
0.1 mm
-
~ 6 x 106 
-
< 100 V
~ 500 V
3. One type of red dissipative/conductive glove
could be easily ruptured by a discharge in the
range of 185 V.
4. Because of the use of the dissipative nitrile
gloves, a VSCEL laser diode with a normal
HBM failure threshold of 350 V demonstrated
enhancement to about the 2800 V level. This
is in the context of direct human static
discharge through the glove layer to this
device. In the case of a human wearing
dissipative gloves, one may actually not be
facing the normal HBM ESD scenario.
Instead, it could be a Human Dissipative
Glove Model or HDGM in effect.
VI. Conclusion
The electrostatic discharge simulation method, as a
test employed for determining devices’ and products’
ESD susceptibility, could also be very useful for
dissipative material characterization to determine
effectiveness as a means of providing a slow, safe
discharge path.
This test method has been
demonstrated to be fairly precise in the determination
of the effective working range for dissipative
materials to act as a slow, safe draining channel for
charge accumulation. Furthermore, we have shown
that the method offers an improved method for the
selection of better dissipative glove and finger cots not only just in terms of better ESD performance in
HDGM context, but also in their thermal mechanical
performance. This study also suggests that the risk
for sensitive devices can be reduced within an
extended voltage range under the HDGM scenario -if
the gloves have been properly evaluated by the
electrostatic discharge test method.
We wish to propose further applications of the above
pulse test method beyond traditional simulation test
domains with devices and products. One could
actually extend the study into extensive evaluations of
any resistor intended for use in critical circuitry at the
sub-assembly and or at the printed circuit board level
on a product. For instance, a resistor may need to be
connected in series with an ESD sensitive device to
ground for better ESD immunity [9]. An ESD
simulation test could provide better assurance in
selecting a proper type of series resistor with
minimum, non-ohmic performance over a wide
voltage range. A product’s effective resistance range,
as designed for the enhanced ESD protection, could
also be better qualified through such simulation before
the products are released for manufacturing.
Acknowledgements
The authors wish to thank Finisar management for
their support of this work. The authors would also
like to express their appreciation to RTP Company for
providing material samples for this study.
References
[1] B. C. Yap and J. Turangan “Field Charging and
FIM ESD Tests on GMR Heads in Hard Disk
Assembly” EOS/ESD Symposium 2001, pp. 160
[2] A. Olney, etc. al. “A New ESD Model: The
Charged Strip Model” EOS/ESD Symposium
2002, 3A.2
[3] B.C. Yap, C.R. Patton, ‘Investigation of ESD
Transient EMI Causing Spurious Clock Track
Read Transitions During Servo-Write” EOS/ESD
Symposium 2000 pp. 233
[4] ESDA Standards: ESD STM 11.11, ANSI ESD
S4.1, etc. al.
[5] ESDA Standards: ANSI ESD STM 4.2
[6] B. Perry, etc. al. “Impact of Insulating
‘Conductive’ Materials on Disk Drive ESD
Robustness’ EOS/ESD Symposium 2002 4A.4
[7] R. B.Rosner, “Conductive Materials for ESD
Applications:
An
Overview”
EOS/ESD
Symposium 2000 2B.1.1
[8] L. V. Azaroff, J. j. Brophy, “Electronic Processes
in Materials” McGraw-Hill Book Company, pp.
368 – 370
[9] R. Zeng, etc.al. “ESD Damage of GMR Sensors at
head Stack Assembly” EOS/ESD Symposium
1999