What is the difference between surface resistivity and surface resistance? Although there has been a lot of discussion centered around these parameters, they are probably some of the least understood in the ESD industry. ESD practitioners need to have a clear understanding of what the differences are to make informed material selections in their work environments.
Let’s start with the basics. Surface resistivity in ohms/square is used to evaluate insulative materials where high resistance characteristics are desirable. Surface resistance in ohms is a measurement to evaluate static-dissipative packaging materials where lower resistance characteristics are required. Now let’s explore the standards and tests that address these measurements.
ASTM D-257 Surface Resistivity
For many years, surface-resistivity measurements have been used to classify ESD-controlled packaging materials. The primary reference for this measurement has been ASTM D-257 Standard Test Methods for D-C Resistance or Conductance of Insulating Materials. ASTM D-257 measures the resistive or conductive properties of insulative materials rather than the dissipative characteristics of ESD-control materials. Regardless of its title, ASTM D-257 has been used throughout the military and commercial world for classifying the performance of static-control packaging materials.
Eliminating Misuse of ASTM D-257
In the late 1980s, the ASTM D-9 Committee notified the Electronics Industries Association (EIA) Packaging Electronic Products for Shipment (PEPS) Committee that using ASTM D-257 for evaluating packaging materials was technically incorrect. The D-9 Committee said using it for evaluating dissipative materials resulted in errors, and requested that EIA Standard 541 Packaging Material Standards for ESD-Sensitive Items eliminate ASTM D-257 as a test method for static-controlled materials.
The committee’s recommendation for change was quite correct. ASTM D-257 contains several procedures for the measurement and evaluation of high-resistance insulating materials, and provides a worst-case scenario for assessing an insulating material’s lowest surface resistivity, which indicates the material’s lowest insulating properties. The test method specifies high test voltages, moderate to high relative humidity (RH), and high test-fixture pressure to reduce contact resistance, all providing lower-resistance measurements of insulators.
When evaluating static-dissipative materials, you need to know the highest resistance that may prevent the movement of static charge to ground. In other words, the ASTM D-257 method for evaluating insulation properties provides an optimistic indication of the performance of an ESD-controlled material as well as inconsistent measurements of dissipative products.
To confirm the committee’s opinion, the EIA PEPS Committee’s ESD Task Force #1 performed a round-robin test among five laboratories, using five identical sample sets. Each lab tested the sample sets under controlled conditions using its interpretation of ASTM D-257. Analysis of the test data from the labs showed that using ASTM D-257 to evaluate static-controlled materials resulted in a 4th order of magnitude difference between laboratories. Task Force #1 went to work to correct the situation.
Assessing the Problem
The first goals were to define what was being measured, and why such a broad difference was found. The task force concluded that insulators were high-resistance homogeneous materials intended to prevent electron flow across their surface; static-dissipative materials were lower-resistance materials intended to allow electron flow across and below their surface.
Dissipative materials are not homogeneous because they may consist of any combination of conductors, organic chemicals or metals. These additives encourage electron flow. While insulators are made of purely nonconducting materials, the ASTM method assesses surface-resistivity characteristics of uniform insulators and is not suitable for evaluating complex, nonhomogeneous dissipators containing conductive elements. It is an error in application of an excellent tool, analogous to a carpenter cutting wood with a hammer.
The Effects of Humidity
ASTM D-257 recommends testing materials in 50% or greater RH. Humidity provides a conductive layer on the material’s surface, reducing an insulator’s protective properties by increasing the electron flow and the performance of static-dissipating materials. Conversely, low RH enhances an insulator’s resistive performance and reduces the function of a dissipative material.
ASTM D-257 discusses several fixture designs. The intent of fixture design is to provide a defined resistance of a square portion of the material’s surface; that is, resistivity in ohms/square. Any fixture may be used as long as its configuration and dimensions are defined. EIA Task Force #1 found merit in using the concentric rings for measuring static-dissipative materials, but different designs and material composition created inconsistencies when assessing dissipative materials.
High pressure (20 to 100 psi) is applied to the concentric ring or bladed test fixtures to reduce contact resistance between the electrodes and the insulator under test and to assess its most conductive properties. Electronic devices protected by ESD-controlled materials are relatively lightweight, often having additional contact resistance due to their configuration. As a result, high fixture pressure distorts the functional resistance of dissipative materials, often overstating the performance of the materials.
Test Voltage and Current
Test voltages of 500 V or higher are applied to the material for long periods before calculating surface resistivity. These higher test voltages are necessary to measure insulators accurately and to provide lower resistance measurements than low test voltages.
High test voltages applied to comparatively low-resistance ESD materials create significant current flow that easily modifies the characteristics of the material and provides a lower resistance indication. If 500 V is applied to a dissipative material as specified by ASTM D-257, the current flow is millions of times higher than that across an insulator.
High current flow causes material breakdown or carbon tracking between conductive elements and lamination layers. This results in irregular measurements, damaged material and a false indication of the useful performance of the material.
Applying the fixture test voltage for some time before recording a measurement is referred to as the electrification period. For insulator measurements, long electrification periods produce a lower resistance indication. When measuring dissipative materials, long electrification periods can affect surface characteristics and change intended performance.
The electrification period is not clearly defined by ASTM D-257 and practitioners use different periods to measure materials. The result is inconsistent measurement values.
The Modified Approach
After reviewing ASTM D-257 test data and discussing the intent of the measurement, Task Force #1 created a modified approach for measuring resistance of static-dissipative materials. A defined procedure was developed and tested by the initial five laboratories.
The final version of the procedure reduced total variance to ± 0.25 of 1 order of magnitude between labs, or a certainty of less than one-half order of magnitude. This was considered quite good and the new procedure was ultimately released to the industry in 1993 by the ESD Association as Surface Resistance Measurement of Static-Dissipative Planar Materials, EOS/ESD S11.11-1993.
Surface Resistance Per S11.11
S11.11 defines the surface resistance measurement procedure for a planar (flat) static-dissipative material. These are the critical elements:
Electrode Assembly and Related Equipment
The electrode assembly fixture design is based on a concentric ring (Figure 1). The S11.11 electrode assembly is defined by its size, electrode surface material and total weight. Any laboratory measuring surface-resistance characteristics of a dissipative material must use the same configuration.
Round-robin tests indicated that the surface resistance of a static-dissipative material varied considerably if the test specimen was measured on a conductive test bed. This variable was eliminated by specifying an insulator as the specimen support surface.
Instrumentation used for the surface-resistance measurement can consist of either a power supply and an ammeter or an integrated device that combines these instrument functions. The instrumentation must measure from 1.0 x 103 to 1.0 x 1013 W and have test-voltage capabilities of 10 and 100 V (± 5%). Because all systems are different, S11.11 uses two procedures to verify and characterize each system’s capabilities.
System Verification at the Low-Resistance Range
Critical low-resistance variables in surface measurement were eliminated by S11.11. The first challenge was to ensure that the electrode assembly made even contact with the material under test at low voltage (10 V). The second was to confirm that the entire system made an accurate low-resistance (1.0 x 106 W ) measurement at 10 V. If the electrode assembly was out of alignment or the system test voltage was abnormally low, the result was inaccurate high-resistance measurements.
A precise calibration ring (low-resistance range system verification fixture) is used to confirm electrode assembly alignment. If the electrode assembly is true and makes full contact with the verification fixture, the measured resistance of the system will be 5.0 x 105 W , ± 1% at 10 V. If accurately aligned, rotating the electrode assembly 90° will again provide 5.0 x 105 W , ± 1%.
System Verification at the High-Resistance Range
The capability of the system to measure high resistance (1.0 x 1012 W ) and to determine the appropriate electrification period was similarly confirmed. Each system could be different and have variations in power supply, current measurement, cables and electrode assembly connections. To assess these variables, an upper-resistance range system verification fixture is employed to confirm system performance at high resistance.
This second calibration fixture has two rings that match the material contact surface of the electrode assembly. A 1.0 x 1012 W resistor (± 5%) is installed between the inner and outer rings. This configuration allows the system’s electrification period to be determined by the following procedure:
The electrode assembly is placed on the upper-range verification fixture and connected to the instrumentation.
100 V is applied to the system and the electrode assembly to measure the verification fixture resistor. The actual resistance is defined as the upper stable resistance measurement of the system, which should be 1.0 x 1012, ± 5%. Once the upper stable resistance measurement is defined, it is used to determine the electrification period.
Five timed measurements (seconds) are made to within 10% of the stable resistance measurement.
The five measurement times are averaged and 5 s are added to it. The result is the electrification period for that particular system.
The calculated electrification period allows the system to make its maximum resistance measurement without undue exposure of the sample to high voltage. The electrification period is used for all resistance measurements greater than 1.0 x 106 W and ensures measurement consistency.
Sample Size and Preconditioning
S11.11 specifies that surface-resistance measurements be obtained on specimens that are a minimum of 3″ x 5″. Specimens should be slightly larger than the electrode assembly; and when comparing different materials, all test specimens should be the same size.
Before testing, the samples are conditioned for 48 h to 72 h at a defined temperature and low RH; that is, 23° C (± 3° C) and 12% RH (± 3% RH). Measuring the same size samples at a defined temperature and RH further maximizes repeatability and enhances measurement consistency.
Basic S11.11 Measurement Procedure
At least six specimens of the same material are measured to provide a minimum level of statistical reliability. After a test specimen is placed on the insulated test bed and the electrode assembly is positioned at its center point, 10 V is applied to the electrode assembly.
If the indicated resistance of the material is less than 1.0 x 106 W after 5 s, the resistance is recorded. If the indicated resistance is equal to or greater than 1.0 x 106 W , the test voltage is turned off and 100 V is used to re-energize the electrode assembly. The indicated resistance, after the system’s calculated electrification period is complete, is considered the specimen’s surface resistance in ohms per S11.11.
All six specimens are measured and the data is reported:
The minimum, maximum and mean surface resistance of all specimens.
The conditioning period, RH and temperature.
The test voltages and upper resistance range electrification period.
Comparing S11.11 Data to ASTM D-257 Data
Many organizations still refer to ASTM D-257 and specify that material resistive performance be reported in ohms/square. While this might seem to be a problem, the S11.11 electrode assembly geometry has a conversion factor of 10 that correlates to ASTM D-257 measurements. To convert S11.11 measurements to surface resistivity in ohms/square, simply multiply by 10, because S11.11 measurements are 1 order of magnitude lower than those obtained using ASTM D-257, as shown in Figure 2.
S11.11 warns that converting measurements to resistivity in ohms/square may not be valid for many types of materials. This includes laminated, plated, metallized and other materials incorporating conductive elements.
The new S11.11 procedure takes many resistance measurement variables into account and is a far superior method for assessing and classifying ESD materials between 1.0 x 104 and 1.0 x 1011 W . It enhances measurement repeatability and can greatly reduce confusion among suppliers, converters and users of ESD materials.
About the Author
Stephen A. Halperin is the founder and President of Stephen Halperin & Associates, an independent laboratory and consulting firm that specializes in electrostatic technology. He is known for his work in standards development with the EIA and ESD Association as well as his original concepts in facility evaluation of static-sensitive environments. Mr. Halperin received a B.S.B.A. degree from Roosevelt University, and has completed graduate work in organizational communication sciences as well as several military training courses and AMA programs. Stephen Halperin & Associates, 1072 Tower Lane, Bensenville, IL 60106, (708) 238-8883.
Explaining Ohms per Square
Those who are new to ESD-related measurements are often confused by the term “ohms per square.” The curious always ask, “Per square what? Inches? Feet? Yards?” The answer is: per any square, as long as the measurement is related to a square.
The simple beauty of ASTM D-257 is that surface resistivity is a calculation regarding resistance of an area—not a point-to-point measurement. In fact, measurements in ohms/square will always be the same regardless of the size of a material square.
To illustrate, assume we have a square of material 2″ x 2″, and use the 2″ bar electrodes to measure the resistivity of this sample. The ohmmeter indicates 5 W . Since we have accurately measured this 2″ square, we express the result as 5 W /sq.
Now, let’s attach another 2″ square of material to the first square and move the 2″ electrode bar to the end of the second square. In effect, this is like measuring two resistors in series, because we now have two 5-W materials forming a rectangle 2″ x 4″. It is no longer a square, and our instrument will indicate approximately 10 W across the 4″ length of the material.
RTseries = R1 + R2
RTseries = 5 W + 5 W
RTseries = 10 W
If two more 2″ x 2″ pieces of material are attached to the bottom of our 2″ x 4″ rectangle, we have two 10-W rectangles that form one 4″ x 4″ square. According to the fundamental resistance calculation for two resistors in parallel, if we had two 4″ electrodes placed along the edge of our new square, we should measure 5 W /sq across the larger sample, because:
RTparallel = R1 x R2
R1 + R2
where: RTparallel = total resistance of two resistors in parallel
R1= resistance of the upper rectangle
R2= resistance of the lower rectangle
10 W x 10 W 100 W 2
RTparallel = —————- = ——— = 5 W
10 W + 10 W 20 W
Any size square of the same insulative material will measure the same approximate resistance.
The term surface resistivity in ohms/square is the indication of this measurement calculation. The number of squares measured at one time is relative to the electrode assembly or fixture used for the measurement. Regardless of fixture design, the final calculation of surface resistivity in ohms/square relates to the resistive properties of a square.
Copyright 1996 Nelson Publishing Inc.