in EOS/ESD Technology Dec 1988/Jan 1989
Instead, Part 1
decay measurements can mislead users
about the suitability of ESD-protective materials. Here's
some insight into this problem, as well as a method that gets
to Measure Resistance Instead, Part II
Lockheed Missiles & Space Co. Inc. Sunnyvale, CA
Editor's note: In the first part of this two-part article, Ben
Baumgartner discusses the nature of resistivity measurements and
shows why the results of such measurements can sometimes be misleading
and difficult to repeat. In part two, to be published in our next
issue, Baumgartner completes this discussion and outlines a simple
measurement technique that is less confusing.
Resistivity and static-decay
data are often used to describe the ESD-protective characteristics
of materials. However, because even engineers sometimes have difficulty
with the concepts and terminology involved, such data often gives
a confusing picture of the material being described. Indeed, resistivity
and decay data collected on the same or similar materials often
contain significant differences, and a number of papers have been
written addressing these discrepancies (Ref 1,2).
has become common to think of resistivity or static decay rate
as actually defining
the protection offered by a particular bag or tote, when this
is almost never the case. Reality is complex, and factors such
as device sensitivity, circuit configuration and the magnitude
of anticipated electrostatic events all effect the amount any
type of protection required.
Even though we know
that protection isn't a simple matter, we must often be satisfied
with measurements of the properties of barrier materials, bags,
totes, etc. that are thought to best describe the protection offered
to ESD-susceptable devices. Unfortunately, this is also complicated;
commonly used test methods frequently deliver the differing data
referred to above. What's needed is a test method that resolves
as many measurement and terminology problems as possible. We will
present such a method in Part II of this article.
materials through measurement can be difficult for at least two
reasons. The first, knowing what to measure, relies on understanding
what is meant by resistance o resistivity Unlike resistance, resistivity
is a property of an area or volume, and is not confined to wires
or leads, which can be accessed and measured directly and unambiguously.
Thus, if these terms are poorly understood and defined, measurements
can result in error, and the errors can find their way onto data
The second difficulty
is that many materials don't lend themselves to measurement by
conventional methods-- or to other available methods, for that
matter. For the data taken to accurately reflect a material's
properties, both the measurement method and the material tested
must conform to laboratory practice and adapt to the theory of
ideal resistivity measurement.
vary in shape, size, construction and composition. This is unfortunate
because for easy and accurate characterization, a material should
be homogeneous, have a well-defined surface and a solid volume.
Thus, resistance and resistivity measurements challenge our ability
to apply such concepts to ESD-protective materials. This is especially
true when data is taken on composites, laminates and foams; there
is often a good deal of confusion as to exactly what the measured
Resistance to the
flow of electric charge determines a material's ability to dissipate
a charge over time. The lower the resistance, the faster a charge
flows, and the more rapidly potential equalizes on or in the material.
Resistivity is the
most important property of a material for determining resistance
to the flow of these electric charges, both surface and volume.
Although resistivity determines the rate of charge dissipation
and the magnitude of static-charge field shielding, it theoretically
has no relation to a material's tribocharging properties (Ref
3, see sidebar).
Provided that certain
conditions exist, control and charge flow is assumed to protect
ESD-sensitive parts. The assumed conditions that charge (current)
neither flows through bipolar devices at a destructive rate nor
creates a voltage drop across the gate oxide of a metal oxide
semiconductor that succeeds its oxide-breakdown potential.
For certain surfaces
or volumes, such currents must be limited, while for others, they
must be relatively unlimited to provide optimum protection. Limiting
current by specifying resistance will limit dissipation rate if
the current travels through a device. Unlimited current on a surface
provides shielding to the device it encloses.
It'of the Fermi levels of
the two contacting materials, and has little to do with
By confusing the dissipation
rate with a lack of static charge generation, conductive
material is frequently (and infrequently) believed to be
antistatic (i.e., antribocharging). However, if charge dissipation
is slow for one of the contacting materials, equal and opposite
static charges can be observed on both surfaces.
When separating two materials
that are relatively conductive, no static charge is observed.
The tribocharging effect of a charge moving across material
boundaries may still be occurring where the materials are
in contact. This charge separation occurs across the contacting
surface, despite the low resistivity of the material.
s often assumed that resistivity
is related to tribocharging, but there is no direct relationship.
The tribocharging characteristics of a material is a function
Upon separation of the materials,
charge redistribution, or bleed back, occurs, and bleed
back is related to resistivity. Bleed back may be so fast
that little or no charge remains. Under these conditions,
charge generation may not be measured, and, therefore, tribocharging
can appear to be related to resistivity of the materials.
and conductive-range (see note) materials such as ESD gloves
(Ref 3) may tribocharge other materials and insulated areas
of PCBs. Thus, resistivity measurements do not provide all
the information needed to selects ESD-protective materials.
[Note: The Electronic Industries
Association (EIA) has defined static-dissipative material
(Ref 4) as having a surface resistivity equal to or greater
1 X 105W/square, but less than
1 X 1012W/square, or a volume resistance equal
to or greater than
1 X 104W, but less than
1 X 1011W - cm.]
is another way of describing ESD-protective materials. As measured
by Fed. Test Method STD 101C, Meth. 4046, static-decay time determines
the charge-dissipation rate of a material only when it is related
to surface resistivity.
As an aside, during
measurement the charge on the material (Ref 5) often was originally
created by tribocharging, but now it is applied using a 5-kV power
supply through the electrodes supporting the material being tested.
The original tribocharging method of electrifying the specimen
has led to the mistaken belief that the test also measured tribocharging
test is a dynamic-resistance measurement expressed in units of
time for a 3 x 5 in specimen. The test measures decay time, which
for an ideal material is related to surface resistance and resistivity
of the sample. When applied to MIL-B-81705 Type-2 polyethylene
materials, decay time has been shown to have a direct relationship
to surface resistivity (Ref 6).
When the decay test
is applied to nonhomogeneous materials, or to those having layers
with different resistivities, a field-suppression effect can cause
ambiguous measurements. This effect results in a mistaken indication
of low surface resistivity and a discrepancy in the measured decay
time vs. the actual surface-charge-dissipatioon time. For example,
a composite material's charge-dissipation time will be longer
than the indicated charge-decay time. This occurs because the
instruments don't measure charge directly, but instead measure
For some materials,
electric-field suppression time can be below the instrument's
measuring range, even though surface resistivity is the insulative
range. And the characteristic of materials in the dissipative
range can't be measured using this method because the instruments
can fail to respond to fast field changed. Despite these limitations,
some product data sheets report very fast decay times for ESD-protective
materials. These fast decay times cannot be useful in determining
charge dissipation for anything but a Method-4046-size specimen
unless the decay time is converted into a material property of
resistivity. Thus, the decay test has serious limitations (Ref
data cannot be generated by the decay-test method. To determine
the dissipation properties of any material, both volume- and surface-
resistance or resistivity measurements must be made. So the decay
test cannot be a substitute for a resistivity or a resistance
measurement (Ref 4).
Taking and Using
have both surface and volume paths that can carry current, but
neither surface nor volume current encounters a discrete resistance.
Instead, the sum of these resistive paths must be known in order
to calculate the total resistance that determines current. The
shape of the measurement electrode determines these paths and
the effective resistance through control of field-line configuration.
The resistance then measured is a function of current flow for
an applied voltage; it is converted to a resistivity value by
either the instrument or a calculation.
Although such resistivity
measurement method are theoretically sound, they work only when
measuring ideal specimens. Still, such resistivity ideal specimens.
Still, such resistivity data can be of value to package designers
when properly sized material specimens are prepared and measured.
In practice, the
field configuration in the material is seldom known or worth the
time to determine. Therefore, resistivity data have much less
value to the end user because arc-discharge points and the associated
paths are not known in advance.
Previous papers and articles (Ref 1,2) have
focused on resistivity measurements discrepancies while
misstating the problem. Solutions were offered without really
addressing the basic cause of the problem and, it addition,
one particular error must be highlighted and corrected.
The statement "...The ASTN, DIN, and IEC test method
equations are identical, but that the test methods do not
agree on the property they are measuring, or on its unitc,
creates a great deal of doubt in the correctness of the
equations used in the test methods" is incorrect.
These equations are identical and correct.
All instruments measure resistance between electrodes, and
this measurement must be numerically converted to a corrected
The use of surface resistance and surface
resistivity interchangeably causes a confusing terminology
This is why our textbook conventions and
the American Society for Testing and Materials provide a
warning by using the "per square" suffix, or "/sq.,"
(or, symbolically, W/ square) with the ohms unit of surface
resistivity. Other conventions rely on the reader understanding
when surface resistance in ohms is a unit of surface resistivity.
The units of "ohms" and "ohms/sq." are
equal and dimensionally correct1.
1 Any size square unit, or cell, is equal
to a dimensionless 1. If a 1 (/sq.) is divided into ohms,
the unit remains ohms.
Limits of Resistivity
produce data usable for engineering purposes only when the shape,
surface and thickness of the test specimen are controlled. Materials
so characterized also must be uniform and homogeneous. For example,
foam materials are not ideal and cannot have a true resistivity
because there are holes or cells throughout the material.
With ideal materials,
the presence of surface and volume currents does not cause large
errors in measurements, providing the material is an insulator.
A guard ring is used in some instruments to improve the measurement
The ASTM D-257, Standard
Test Method for DC Resistance or Conductive of Insulating Materials,
surface-resistivity measurement is intended for measuring high-resistivity
(volume) materials only. The surface resistivities of these insulative
materials tend to be lower than their volume resistivities.
This results in relatively
small errors in surface or volume measurements, even if the measurement
is unguarded. A guarded-electrode measurement system can properly
compensate for these conditions, and surface measurements will
produce usable results for ideal insulative materials.
This isn't always
the case for more conductive materials. Some instrumentation will
measure black carbon-loaded materials as insulators, while the
same material might measure about 20 kW with an ohmmeter and
pointed test probes. Thus, even guard-electrode instrumentation
can produce measurement errors in conductive-range materials beyond
the design range of the instrument. If the characteristics of
the material are not suspected in advance, such an error won't
be detected without additional measurements.
Except in theory,
there is no such thing as surface resistivity. Physics handbooks
list surface resistivity values for dielectrics (no values below
108W/square), but no surface resistivity values
are listed for conductive materials. Volume resistivity values
are given for both insulators and conductors.
Insulators have very
thin (i.e., several molecular layers thick) conductive surfaces;
but the surface conductivity of a conductor's surface is indistinguishable
from its volume conductivity. On an insulative substrate such
as sapphire, a thin conductive conductive film has a surface resistivity
related to its thickness. Generally, one should not assume that
surface and volume resistivities are related.
|Resistivity refers to a property
of a material, while resistance is what impedes the flow of
electrons in a circuit and can be used to calculate current
flow. Resistivity cannot be used with Ohm' s law; it must
first be converted to resistance after determining the configuration
of the circuit.
Even knowledge electrical engineers sometimes misuse impedance,
resistance and resistivity. This can cause confusion when
applied to ESD-protective materials. If resistivity is not
directly used in calculating current flow, why are ESD materials
specified in resistivity? Because it is a way to describe
a material without a discrete resistance between defined terminals.
Resistance is determined by both surface and volume resistivities,
and no absolute distinctions can be made about the various
paths through which current flows in a
In other words, the current flow is through
both surface and volume resistance, which can be determined
theoretically from the surface and volume resistivities
and the field configuration that forces charges along the
field line paths.
Unless current flow is limited to steady-state dc charge
movement, impedance should be used instead of resistance.
Impedance is a complex term made up of resistance (not resistivity)
and reactance. It governs the flow of charge current for
ac and non-steady-state conditions.
When charges move through dissipate materials, especially
materials in the high-dissipative range, reactance can be
ignored. However, we must keep in mind that electrostatic
discharge is not steady-state current flow; therefore, resistance
may not present a completely accurate analysis of the dissipation
a material provides.
To understand material
and instrumentation problems, we must review the electrode correction
and normalization factors used in resistivity measurement. Resistance
values are calculated from Equation 1, and surface resistivity
from Equation 2. The voltage, V, is divided by the current, I,
to find the measured surface resistance, Rs, in ohms.
Rho (RS) is the resistivity in ohms/square and K is a correction
1.) RS = V / I
2.)RS= K R5
A surface resistance
value assumes that no volume conductivity is involved. Any square
electrode configuration used on an ideal material will measure
a resistance in ohms that has the same numerical value as resistivity
1: Square electrodes (ASTM D-257 measurements)
Fig. 1 shows that
by adding equal resistance squares, the same value is obtained
as for the original square, providing the electrode configuration
remains square. To find surface resistivity (RS), RS is multiplied
by correction factor, K, which is the width-to-length ratio, and
which for a square electrode is always a convenient K=1, and thus,
RS = RS
For other shapes,
K must allow for electrode configuration. For circular electrodes,
account for the number
of squares being measured in parallel (Fig 2). Large spacing between
concentric-ring electrodes produce trapezoid-like sections that
result in a slight error for different-sized electrodes, but it
is not significant (Ref 1). K need not be a whole number. Equation
3 calculates K for concentric-ring electrodes for the diameters
given in 3a and 3b.
|Figure 2: Volume measurement
electrodes (ASTM D-257 measurements).
3) K = PDo
3a) Do = (D1+ D2) / 2
3b) g = (D1- D2) / 2
the average diameter, g is the gap between the rings, D1is
the outside diameter and D2 is the inside diameter.
If the rings don't have knife edges, the contact point nearest
the gap should be used as the measurement point. Fig 2 is to scale
for the correction factors given, and shows the cells that approximate
trapezoids. The cells are the proper length-to-width ratio when
a circle is tangent to all sides.
AND RESISTIVITY MEASUREMENT
[ raw data/
(converted data) ]
values are both raw and converted data.
Equation 4 can be
used to calculate volume resistivity rv is a normalized
value that is numerically equal to the resistance across the opposite
faces of a 1-cm cube.
4) RV= (L / A) rv
|Figure 3: Circular electrodes
)ASTM D-257 measurements).
is a normalized value since both thickness and area are not taken
into account in its calculation. Normalization results in a data-sheet
value that can be misleading (Fig 3).
In Equation 4, any
size (area) electrode used on any thickness of a material will
produce a resistivity value for the unit cube shown. Normalization
results if proper metrics are used in this equation. Resistivity
and actual measured resistance have the same numerical value only
if 1-cm2 electrodes are used to measure a 1-cm cube.
This allows a materials' resistivity to be compared with that
of others because it is normalized, and measurement data now refer
to an intrinsic property of the material, regardless of sample
value gives a false sense of actual resistance for a large or
small area. This value can be very misleading for an envisioned
current path when a volume current is not desirable. For a material
that is 4-mil (0.01-cm) thick, the actual resistance to a current
though a 1-cm2 electrode is 2 orders of magnitude below this.
For this 4-mil thick material, a 10 x 10-cm sheet would have 4
orders of magnitude less resistance.
Many problems, both
real and imaginary, are attributed to the electrodes used in measurement.
Should they have knife edges or offer broad contact? Should they
be circular or square, metal, or conductive rubber, large or small?
What pressure is needed to make a good contact?
The problems are
due to combinations of electrodes and materials, and thus blaming
the equipment does not address the problems of the measuring system.
Soft, hard or textured materials cannot be easily measured with
the same electrodes, and even greater problems are caused by composite,
laminate and foam materials since they are far from the homogeneous
There are many other
factors that could explain some of the variations in surface resistivity
values, but, in practice, it is difficult to separate the variables.
However, theoretical examples help us see what might cause measurement
discrepancies. Fig 4 shows how incorrect surface-resistivity measurements
might be caused by a buried conductive layer.
E1 is a plain knife-edge
electrode; E2 is a knife-edge electrode with a conductive-rubber
center. The volume current is Iv adds to the surface
current Is, and is not properly subtracted from the measurement
by the guard electrode since this subtraction depends on the volume
resistivity of the material contacting the guard electrode.
This condition can
exist where the top layer of a laminated specimen is relatively
conductive and the bottom layer is an insulator. The resistance
measurement is then practically independent of electrode spacing,
which results in a surface-resistivity-measurement error. Electrode
spacing determines the correction factor used to convert the resistance
measurement to resistivity.
|Figure 4: Surface measurement
current paths (circular electrodes).
The variation can
be greater for different electrode surface areas because the resistance
measurement is proportional to electrode area; it is as if the
measurement were one of volume. The dashed-line square in Fig
4 represents this larger effective electrode surface. For example,
the ratio of the areas of an 0.125-in. rubber electrode and a
knife-edged electrode is almost infinite.
to be repeatable, circumference must also be taken into consideration.
The area ration of a 25-in. circumference (approximately 8-in.-diameter
EMI-gasket electrode) to a 1.5-in.-circumference (approx. 0.5-in.-diameter
knife edge used in making measurements on small samples) makes
comparable measurements difficult. The data in Table 1 show variations
for several electrodes that are not as extreme as the above example.
|Figure 5: Volume measurement
current paths (circular electrodes).
also cause problems for volume measurements because the various
conductive layers often have different volume resistivities and
are of unknown thickness. Fig 5 shows how the volume current (Iv)
can be spread by a buried conductive layer if the top layer is
more conductive than the bottom material. Electrode E1, which
has a conductive-rubber center, is used to calculate the contact
area for the Iv current. The other electrodes are the plate,
E2, and the guard ring. This figure illustrates how, for composites
and laminates, layer thickness and electrode design both affect
The guard ring does
prevent surface current (Is) errors but is affected
by the sample's internal conductive layer, which increases to
Iv to the guard ring. The measurement may also be affected
by the guard contact area ( dashed-line square), which can increase
volume current to the guard electrode.
In open- or closed-cell
forms, volume measurements are confused because surface paths
shunt volume paths. For foam materials it's not possible to separate
the surface current from the volume current being measured. This
is also a problem for materials such as composites, which contain
random conductive wires, carbon-particle swirls (as in conductive
flooring) or layers of different materials.
1. D.C. Burdeaxu,
C.L. Mott, "An Analytical Approach to Surface Resistivity
Measurements," Evaluation Engineering, November 1986 (Table
3, p. 87; Summary, p. 97).
2. B.N. Stevens, "Determining the Surface Resistivity of
ESD-Protective Cellular Packaging Materials," Proceedings,
1986 EOS/ESD Symposium.
3. S.L. Fowler, "Triboelectricity and Surface Resistivity
Do Not Correlate," Proceedings, 1988 EOS/ESD Symposium.
4. Packaging Material Standards for ESD-Sensitive Items,
Electric Industries Assn., EIA STD IS-5-A (now EIA-541; 2.22,
5. V.E. Shashous, "Static Electricity in Polymers-- Theory
and Measurement," Journal of Polymer Science, 1958, 32:65-85.
6. G. Baumgartner, R. Havermann, "Testing of Electrostatic
Materials-- Fed. STD 101C, Method 4046.1," Proceedings, 1984
7. G. Baumgartner, "Static Decay Test versus Resistivity
Measurements," EMC EXPO, 1987 International Conference on