Ohms per Square What?
by: Steve Fowler
The answer is: "It does not Matter." Square anything!
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, cross-section, etc.
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?
