Surface Resistance
By William Klein, K & S Labs
Why electrode configuration
is important to the electrical testing of materials
Concentric electrodes may
not be used for determining resistances of materials which are not
homogeneous and for those with directionality in their surface conductivity
ASTM D257, AATCC 76 and
ESD S11.11 may be used to characterize materials if the technologist
understands the material being tested and its intended application
False assumptions are implicit
in most simple meters and test methods purporting to measure surface
resistivity directly. They employ electrode configurations valid
only for special cases. Surface resistivity measurements
assume a desire to know the electrical resistance from point to point
on the surface. This can be measured directly by the use of
a pair of electrodes, either circular or rectangular, on the surface
without regard to the path of flow. This is the basis of NFPA-type
resistance measurements for floors adopted by both ASTM and UL, and
for both working surfaces and walking surfaces by the ESD Association.
It is necessary to specify electrode size, spacing, and voltage.
The disadvantage of this method is that it does
not measure an inherent property of the material, but only of the
particular test specimen in the configuration employed. The
advantage is that it usually yields desired results without ascribing
false properties which may lead to serious design errors.
The use of only concentric electrodes does not
allow valid characterization of materials if the material has any
non-uniformity in its conductivity either in surface or volume directionality.
These electrodes may mask serious continuity problems.
ASTM D257: Most cited test method for surface
or volume resistivity, is more of a tutorial than a method, is specified
for insulating materials only, has broad applicability and gives wide
options on methodology both in electrode configuration and applied
voltage, based on intended use.
AATCC 76: Used by the textile industry as
a simple, direct method to measure the surface resistive properties
of fabrics. The results are meaningful for thin, flexible, relatively
homogeneous materials with conductive ability on or near the surface;
and for such materials having directional differences in surface conductance,
such as with textile materials. The method is a special case
of a surface resistivity measurement from D257 in which parallel electrodes
or concentric rings are used, with sizes and spacing appropriate to
the application, and no guard electrode employed.
ESD S11.11: Used for electrostatic control
applications of planar materials. The method is a specific case
of ASTM D257 with the electrode configuration, weight and applied
voltage fixed in an attempt to reduce lab-to-lab variations. The
electrodes are an inner disk surrounded by a concentric ring in a
fixed three-dimensional relationship. The applied voltage is
100 volts for materials in the dissipative range.
ASTM D257 users usually do not read it fully before they use it or
cite it. It is particularly important to note that "surface resistivity"
is often not definable or measurable, even in insulative materials
of the sort specifically dealt with in D257. Surface resistivity
does not exist as a basic material property in most of the materials
of interest in ESD protection. This is because surfaces do not
usually have electrical properties which are distinct from bulk properties
and the flow of current from point to point over a surface cannot
be described as a surface phenomenon only.
Figure 1 Parallel Electrodes
of the square, thus the term "ohms/square". Although
dimensionally the same, "ohms/square" differentiates surface
resistivity measurements from simple resistance measurements (R).
Figure 2 Concentric Ring Electrodes
The concept of surface resistivity is unnecessary and
often erroneous in materials used for ESD control. If the material
is thin, homogeneous and volume conductive as shown in Fig.1, then
the apparent surface resistivity is equal to the true
volume resistivity divided by the thickness.
Figure 3 Conductive Backing
A is the contact area of each electrode. If L
is much greater than t and the
circuit
can be approximately shown as in Figure 3b. It is actually volume
resistivity which is being measured and surface resistivity, in fact
a nonexistent material property, acquires its apparent value depending
on volume resistivity and specimen geometry. Note that R is
independent of electrode spacing.
In materials which are surface treated
to be conductive and the great bulk of which is much more resistive,
the concept of surface resistivity makes sense, although it is usually
unnecessary. This is indicated in Figure 4.
Figure 4 Conductive Surface Layer
If a material with directional differences
in conductivity, such as woven fabric or some fiber-loaded materials,
is being measured, then the property of resistivity may not be defined
or measured. The concentric electrode option is not valid if
there is any appreciable axial or directional non-uniformity. As
shown below, concentric electrodes mask serious continuity problems
while appropriate use of parallel electrodes allows material characterization.
Figure 5 Directional Conductivity
Figure 6 Random Fibers
