Avoiding the resistance panic button: troubleshooting resistance measurement discrepancies.

Most electrostatic discharge (ESD) control standards require some form of resistance measurement. We rely heavily on resistance for process monitoring and material selection. We base our specifications on resistance. And, when these measurements are outside of our specifications, our reaction is usually more than a casual, "Oops"

Sometimes we immediately conclude that the material is defective. Sometimes, however, the disparity is a measurement problem, not a material problem. Measurement problems can often be remedied quickly and inexpensively without pushing the panic button.

Here is a list of troubleshooting steps to assure the accuracy of disparate resistance measurements.

Resistance or Resistivity 

Resistance has rapidly replaced resistivity in materials evaluation as a measurement parameter for most ESD control products. Occasionally specifications are expressed in resistivity (ohms per square), and certain instruments measure resistivity rather than resistance. If the manufacturer's data sheet specifies resistance, be sure you are not measuring resistivity.

Instrumentation

Instruments should accurately measure at least one order of magnitude below and above the acceptance range of the materials in question. Multimeters will not give the correct resistance readings for static-dissipative materials. Be sure that your suppliers, laboratory or other facilities use instruments comparable to yours.

Are your instruments properly calibrated? The instruments used on the factory floor should be calibrated at least once a year as though they were highly sensitive laboratory instruments.

Electrode Configuration and Weight

Different electrode configurations and different weight electrodes will yield different resistance measurements. Lightweight parallel metal bars will result in a higher resistance measurement than circular five-pound electrodes, especially on non-homogeneous materials.

Measurement Methods and Procedures

Are you using the same test method as your material supplier? Do your various facilities use the same test method? Are you following the test method as described and measuring according to the generally accepted standards?

Contamination on the surface of the electrodes or the material being measured can affect resistance measurements. Be sure that both are clean before measuring.

Output (Test) Voltage

Many standards specify test instruments with open circuit voltages, as opposed to constant voltage instruments. The actual applied voltage of open circuit instruments will vary with the resistance of the material under test. To complicate matters, a low-test voltage typically yields a high resistance indication, and vice versa. Know the actual applied voltage of the instrument and the recommended voltage for the material under test.

Electrification Period

The electrification period is the optimal time required for an accurate measurement. If the test voltage is applied for a shorter period, the indicated measurement may be low. Applying test voltage longer than the electrification period may change the material's characteristics and result in a higher resistance indication.

Test Bed

Was the material tested on an insulated test bed? If the material is placed on a conductive test bed, the test voltage will "couple" with the conductor--yielding an abnormally low resistance indication.

Environmental Conditions

Under what environmental conditions are the measurements taken? In extremely high humidity, condensation may create a moisture layer across the material's surface, giving a lower-than-expected resistance measurement.

Resistance measurements can be problematic, but running a thorough check of procedures before jumping into product and process remedial actions can save time, money and aggravation.

Michael T. Brandt is a publicity consultant to the ESD Association and president of Marketing Resources Ltd., Rockford, IL; e-mail: [e-mail]. This column is a regular contribution of the ESD Association, Rome, NY; [phone]; e-mail: [e-mail].