Acheson Colloids Company
1600 Washington Avenue
Port Huron, MI 48060
(800) 255-1908
www.achesonindustries.com
web.mail@nstarch.com
Also: Australia, Brazil, China, France, Italy, Japan, Korea, Mexico, Netherlands, Singapore, Spain, United Kingdom
Foreword
Measurement of electrical resistance during the processing of EMI coatings can be
confusing, especially when comparing numbers to the paint manufacturer’s method.
This report describes current industry standards and makes suggestions for proper
electrical resistance testing methods.
It is essential that coaters monitor film thickness, degree of cure, and conductivity on a
daily basis to control costs and ensure optimum coating performance.
Current Industry Standards
Point-to-Point
The most common and easiest resistance measurement is point-to-point testing
performed by simply touching ohmmeter probes to the painted surface. Locations of the
probes are agreed upon between the OEM and coater. Drawbacks of this method,
which affects resistance values, are uncontrolled normal force (pressure applied),
sharpness of the probes, distance and location of the probes and oxidation of the
probes. While point-to-point is a good and necessary practice, it should be
accompanied by film thickness monitoring.
Ohm/Square
The methods discussed below are erroneously referred to as ohm/square; a distinct
square(s) is not actually defined. A convenient “stethoscope” method is to attach an
ohm/square jig to the ohmmeter leads. Three ohm/square test jigs became popular in
the market over the last ten years: a point-to-point jig, a square block jig, and
*Versatronic™ probes held at 1 cm spacing.
Ω/Square
1cm
Versatronic
Probes
1cm
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Caution: These methods may not be accurate or sensitive enough for the highly
conductive coatings and conventional ohmmeter tolerances of today. Readings of 0.01
ohms are common while most conventional ohmmeters have a tolerance of ±0.01
ohms. Indeed, most readings with the square probe jig is < 0.01 ohms, accuracy
therefore unknown.
The one inch point-to-point jig was useful because it was spring loaded, included only
two points of contact and, for higher resistance coatings measured a value close to
double the ohm/square value (empirically determined). It can still be useful for
comparison measurements.
The ohm/square jig is based on two 1 cm square probes spaced at a distance of 1 cm.
Drawbacks are uncertain normal force (no springs), uncertain contact area (plastic
substrate is typically not flat and, therefore, does not conform to flat probes), and
oxidation of the probes.
Versatronic™ probes and meters have become popular for its high resolution (±0.001
ohm) and because IBM specified it for measuring conductivity on platings. The probes
alleviate the aforementioned drawbacks; normal force is controlled by spring pressure,
probes are rounded tip to provide somewhat consistent penetration, and gold plated to
reduce oxidation. Versatronic™ readings for conductive paints were originally reported
to give ohm/square readings at 1 cm spacing, leading to erroneous ohm/square/mil
values reported on paint manufacturers specifications. These errors have been
addressed as shown in the section on ohm/square/mil methodology.
The most common method in use today for measuring point to point resistivity is the use
of an ohmmeter with two leads, which can be placed virtually anywhere on the coated
surface. This method allows for readings to be taken from any two points on the coated
surface.
Versatronic is a registered trademark of Electronic Automation, Inc.
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Method for Determining Ohms/sq/mil of EMI Coatings
SCOPE:
To determine the dry film electrical resistivity of conductive coatings.
EQUIPMENT: 1. Ohmmeter with low resistance capability that can accept four leads.
2. Special compression contact jig (see Attachment #1, Drawings 1 & 2) for
measuring the electrical resistivity of a 1” x 3” area of coated surface.
METHOD:
1. A product to be tested for electrical resistance should be applied to a suitable
substrate (most commonly a glass plate or GE Lexan® panel) measuring 3” in width
and a minimum 4” in length. The coating needs to be properly cured before testing.
If the coating is applied onto a glass panel, this same panel can be used to
measure dry film thickness after resistivity has been recorded. If the coating is
applied onto plastic, a glass slide (typically a 1” x 3” microscope slide) needs to be
fixed onto the back of the panel, partially exposed, in the area where resistivity will
be measured (see Diagram #1)
NOTE: Identification of application method, cure, dilution ratio, if any, and
recommended film thickness can be found on the Product Data Sheet.
2. Product application for this test can be by spray or drawdown technique, as
specified on the Product Data Sheet. If drawdown technique is used, it must be
performed on glass substrate to accurately measure dry film thickness.
3. A one-inch wide section should be scribed into the cured coating. This can be
easily done using a 1” x 3” standard microscope slide as a template to measure the
1 inch width of the cut. Be certain that the scribe goes completely through the
coating.
4. The electrical resistance of the coating can then be tested using the special jig and
the ohmmeter. The ohmmeter must accept four leads to eliminate lead resistance
failures. Ensure the leads from the inner probes are wired to the source input on
the ohmmeter and the outer probes are wired to the sense input of the ohmmeter.
Before performing actual resistance measurements, check the electrical resistance
specifications of the product being tested and set the ohmmeter at an appropriate
scale.
5. Once the meter has been properly adjusted, insert the coated test panel into the
special jig ensuring the scribed area is lined up with the contacts. There is a
marked area on the jig for this purpose.
6. Obtain and record the electrical resistance reading from the ohmmeter. This value
represents ohms/square inch resistivity of the coating.
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7. Measure the thickness of the coated glass slide using a micrometer. The
measurement must be done in an area where the electrical resistance was made
(again, refer to diagram #1). Remove the coating using a razor blade (in the same
area the initial thickness measurement was made) and determine the thickness of
the glass slide with the micrometer. Subtracting the thickness of the glass slide
from the thickness of the glass slide and coating will give the thickness of the
coating.
8. The electrical resistance is expressed in units of ohms per square inch at a
thickness of 0.001 inch (one mil). The “ohms per square inch” is obtained as the
compression contacts on the electrical resistance jigs are one inch apart. This,
combined with the one-inch scribe, results in measuring one square inch of coating.
9. To calculate the electrical resistance, multiply the obtained electrical resistance by
the film thickness of the coating. This will give an electrical resistance value
normalized to a thickness of 0.001 inch, or “ohms/square/mil.”
GE Lexan is a registered trademark of General Electric.
DIAGRAM #1
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Attachment #1 – Drawing 1
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Attachment #1 – Drawing 2
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1600 Washington Avenue
Port Huron, MI 48060
(800) 255-1908
Fax: (810) 984-1446
www.achesonindustries.com
web.mail@nstarch.com
The information given and the recommendations made herein are based on our research and are believed to be accurate but no guaranty of their
accuracy is made. In every case, we urge and recommend that purchasers, before using any product in full scale production, make their own tests
to determine to their own satisfaction whether the product is of acceptable quality and is suitable for their particular purposes under their own
operating conditions. THE PRODUCTS DISCLOSED HEREIN ARE SOLD WITHOUT ANY WARRANTY AS TO MERCHANTABILITY
OR FITNESS FOR A PARTICULAR PURPOSE OR ANY OTHER WARRANTY, EXPRESS OR IMPLIED. No representative of ours has any
authority to waive or change the foregoing provisions but, subject to such provisions, our engineers are available to assist purchasers in adapting
our products to their needs and to the circumstances prevailing in their business. Nothing contained herein shall be construed to imply the nonexistence of any relevant patents or to constitute a permission, inducement or recommendation to practice any invention covered by any patent,
without authority from the owner of this patent. We also expect purchasers to use our products in accordance with the guiding principles of the
American Chemistry Council® program.
Printed in U.S.A.
08-752-RV120303
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