Ohms per Square What?
by: Steve Fowler
The answer is: "It does not Matter."
This term has been used for years by many who deal
in electrical measurements but it is still not clearly understood.
ASTM D-254 test method is used to measure the surface resistivity
of materials. The units of measurement are Ohms per Square. Even
though the ESD Associations Test Method S11.11 uses resistance measured
in Ohms, the resistivity of many materials is still required for
characterization and contract requirements. Most people do not ask
what it means for fear of showing their ignorance. Let 's try to
clear it up a bit.
Ohms per square is the unit of an electrical measurement
of surface resistivity across any given square area of a material.
It is the measurement of the opposition to the movement of electrons
across an area of a material's surface and normalized to a unit
square. This measurement is intended to be a basic material parameter
and not dependent on absolute area, length or thickness. Unlike
resistance, resistivity is not exactly a point-to-point measurement.
It is a measurement where the electrons can take multiple paths
across a uniform surface. However, the electrons are considered
to generally flow only on the surface.
The measurement of resistance is not the same as
resistivity. Resistance is the opposition to electron flow across
or through a material, and is measured from point-to-point . It
is not normalized. This measurement is very dependent on size, length,
The above figure shows the measurement of a material with the same
dimensions on all sides. It is a square area of the material. It
can be 1 inch or 1 mile on a side. When electrodes are attached
as shown, and the Ohm meter measures 1 Ohm, we can state that this
material has a surface resisitivty of 1 Ohm per square.
If two squares of the material (the same size as above) are laid
end to end and electrodes attached as shown below, the resistance
is the addition of these two squares or 2 Ohms. This measurement
is of two resisters in series.
Or if two of these squares are stacked as shown below, the resistance
is that of two resisters in parallel or 1/2 Ohm.
Now if we combine these two examples we have four
squares of the materials in an arrangement of two parallel resisters
in series. Or the resulting four resisters which now form
a square again read 1 Ohm on the Ohm meter.
If we take the examples to a higher level where
we have 16 squares of the material arranged as shown below, the
result is again 1 Ohm. It is again in this example a square.
If the first example is 1 square inch then this example is
4 square inches and the results are the same - 1 Ohm.
So you see that it really doen't matter. A
square inch or a square mile of a material with a surface resistivity
of 1 Ohm per square will read just 1 Ohm on an Ohm meter.
Strange isn't it?