Fowler Associates Labs

First Published in EOS/ESD Technology Aug/ Sept 1991

A Proposed Test Methodology
for Floor Materials

Current flooring test procedures and the information they provide can be misleading. Here's one proposed solution to the floor-testing problem.

Michael T. Brandt- President, Marketing Resources Ltd., 305 Euclid Avenue, Sheboyan, WI 53083

Stephen A. Halperin- President, Stephen Halperin & Associates, P.O. Box 1225, Elmhurst, IL 60126

Floor Testing Series: This is the first in a series of two articles describing a possible test methodology for flooring materials. This method involves procedures for testing electrical resistance, body voltage generation, body voltage decay, voltage gradient, and voltage suppression.

Part 1 of the series will cover general test conditions, types of samples, and particular test methods. These test methods and procedures are for electrical resistance, body voltage generation and body voltage decay, voltage gradient, and voltage suppression tests. Part 2 will discuss the observations and results of these tests and procedures.

The test procedures of this proposed methodology are designed to specifically evaluate and compare flooring materials intended for very sensitive electronic manufacturing environments. They include footwear and human body factors encountered in the environment. The testing is conducted in a manner similar to the end use of the products in question.

Floor materials are often evaluated and selected based on inappropriate electrostatic properties and inadequate test procedures. Since floor material vendors have been working with ill-defined measures, they are poorly guided when it comes to developing and marketing products to meet user requirements. This puts the vendors at a loss to provide adequate and useful product information.

Current Evaluation Criteria

There are three traditional approaches to evaluating floor materials.
1.) Electrical resistance- indicates electrical continuity across a surface or from surface to ground.
2.) Body voltage generation- as measured according to AATCC/ANSI 134.
3.) Material decay testing- applicable to packaging materials.

These three approaches have their limitations and drawbacks. Electrical resistance testing does not necessarily indicate performance properties. The body voltage generation procedure was developed for less critical computer and consumer environments. Lastly, the material decay test does not indicate a floor material's ability to dissipate a charge from an object or person. The evaluation and discussion of ESD floor materials has often been limited to very specific materials ^8,9 or applications ¹⁰ and has often not included the human body in test procedures. ^1,2

This paper proposes that a through characterization of the electrostatic properties of ESD floor materials should include testing of electrical resistance, body voltage generation, body voltage decay, voltage gradient, and voltage suppression. Test methods for measuring these properties have been adapted from eh previously described approaches and modified to analyze floor materials used in highly sensitive environments. Where appropriate, the procedures simulate conditions of use.

Floor Material Samples

The tests evaluate various materials, such as epoxy floor coatings and resilient floor coverings, with and without floor finishes. A 4 ft. x 4 ft. sample of each floor covering was mounted on plywood or pressboard, according to manufacturer's instructions. If a conductive adhesive was recommended for installation, that adhesive was used to mount the samples. Floor finishes were applied on some samples. Ground connections (groundable points) were also provided on each sample.

A test bed of commercial vinyl-composition tile, installed on concrete below grade and finished with standard commercial floor finish, is used as a reference point and is considered the "uncontrolled" floor material.

Before testing, the floor material samples were cleaned twice with a solution of alcohol and water. Floor finishes were wiped with a dry, low-linting cloth. All samples were air dried and conditioned at least 48 hours at 20 to 24% relative humidity and 68 to 72 deg. F. The tests were then performed at 20 to 24% RAH and 68 to 72 deg. F

Footwear

The body voltage generation and decay tests are conducted using six different types of commonly-used footwear This footwear includes: 1) shoes with insulative soles (rubber or other synthetic soles); 2) shoes with insulative soles and conductive heel strap; 3) tennis-style shoes with static dissipative soles; 4) deck-style shoes with static dissipative soles; 5) dress shoes with leather soles; 6) Oxford-style shoes with conductive soles. Prior to testing, resistance to ground plate measurements are made of the footwear. Shoe samples are prepared by stroking the soles six times with 100 grit sandpaper, then cleaned twice with a solution of 70% alcohol and water.

Electrical Resistance Test

Although not a predictor of voltage generation, resistance indicates a path of electrical continuity allowing the flow of static charges across the floor surface or from surface to ground. The test procedure used here adapts the ASTM F150 and NFPA 99 procedures, but modifies the electrodes and the applied voltages. In addition to the traditional foil-covered, 5 lb., 2-1/2 in. diameter cylindrical metal electrodes, the procedure includes a second configuration that substitutes a conductive elastomer for the foil and non-conductive rubber contact surface. The procedure supplements the standard ohm-meter prescribed by ASTM F150 and NFPA 99 with a precision Dr. Theidig Milli-TO wide-range ohmmeter.

Procedure. An individual floor sample is placed on the test bed and lightly wiped with a low-linting cloth. For point to point measurements (R_tt), the electrodes are placed 36 in. apart on the surface of the test sample (Fig. 1). For electrode to groundable point measurements (R_tg) , one terminal of the ohmmeter is connected to the groundable point of the test sample. The other terminal is connected to an electrode placed on the surface o the sample (Fig. 2). The groundable point is defined as "the connection (such as a ground bolt, snap, wire, or conductive adhesive) used to attach a floor material to an appropriate ground"¹³.

The test voltage is applied and readings recorded five seconds after application of the voltage. Measurements are made at applied voltage of 10V, 100V, and 500V, with both the foil-covered electrodes and the conductive elastomer-covered electrodes. Five measurements are taken and the results averaged for each test condition. The samples are tested in an ungrounded condition; however, they are temporarily grounded between measurements.

Body Voltage Generation Test

The methodology for evaluating body voltage generation in a test subject wearing various types of footwear combines two procedures. The first procedure, an adaptation of the AATCC/ANSI 134 method, measures the maximum body voltage during a defined walking cycle. The second procedure measures the minimum voltage obtained at the end of the walking cycle, when the test subject has both feet placed on the floor. These two procedures illustrate the effects of capacitance, which is directly related to whether or not both feet are on the test surface.

Parallel Plate Capacitance. The formula for parallel plate capacitance is:

C = k (A/d)

where,
C= Capacitance
k= Constant (Permittivity)
A= Area of shoe surface
d= Distance between sole of foot and floor surface

At the end of the walking cycle, when both shoes are on the floor sample, shoe surface are (A) is maximum, and distance fro the floor (d) is minimum. Therefore, capacitance (C) is maximum with both feet on the floor and capacitance (C) is minimum when one or both shoes are removed from the floor surface during the walking cycle.

The second aspect of this relationship considers the primary electrostatic relationship of charge, capacitance, and voltage:

Q= CV

and Q= V/C

editor's note: This may have intented to be V=Q/C

where,

Q= Charge
C= Capacitance
V= Voltage

As capacitance (C) increased, voltage (V) decreases. Therefore, with both feet on the floor, capacitance (C) is at maximum and measured voltage (V) will be at a minimum. With one foot raised from the floor surface, capacitance (C) decreases and measured voltage (V) increases. Thus, the two procedures provide the minimum to maximum voltage range expected with the floor and footwear combination.

In addition to these basic voltage measurements, the procedure includes a statistical analysis to determine the probability of equaling or exceeding a defined body voltage.

The procedure requires the following test instrumentation: charge plate monitor with a voltage range of 0 to 20,000V DC connected to an XY plotter and a wrist strap (with a 1-megohm resistor); an electrostatic analyzer and personnel voltage tester (PVT); and a constant, indicating ground circuit monitor (Fig 3).

Procedure. After preconditioning, the sample's groundable point is connected to the constantly monitored earth ground. The test operator puts the wrist strap on and connects the discharge end to the charge plate monitor. The operator walks on the floor sample in front of the PVT. The step pattern requires forward and backward steps, and a cross-over step when changing directions. (Fig 4).

Two sets of measurements are obtained during the procedure. The first measures voltage generation during the walking cycle. This is defined as the maximum body voltage. The second, defined as the minimum body voltage, measures the voltage obtained at the end of the cycle with both feet on the floor sample.

During the walking cycle, the XY plotter records all operator body voltages as seen through the wrist strap by the charge plate monitor. At the end of the pattern, when both feet are flat on the floor sample, the operator touches the PVT device to record and enter the body voltage into the PVT memory. A test assistant also marks the XY plotter chart to indicate the moment of PVT measurement. The process is repeated for a total of 20 cycles, the footwear is changed, and the procedure is repeated until all footwear and floor material samples have been tested.

The 20 highest peak voltages from the chart recorder and entered into the PVT analyzer. Averages and standard deviations (n-1) are calculated for the walking cycle and the end of cycle data. A normal (Gaussian) probability distribution was assumed for determining the probability of equaling or exceeding a defined body voltage. To compare various floor and footwear combinations, the statistical analysis determines the body voltage above which there is less that 0.1% probability of being exceeded.

The probability function (f(N)) of the Gaussian distribution is given as:

and is pictured as the normal bell-shaped curve.

Body Voltage Decay Test

The body voltage decay procedure measures the time required for a charge to dissipate from the body through a given floor material and footwear combination. An ESD floor material should dissipate any accumulated charge from the body before exposure to sensitive parts can occur.

Body voltage decay measurements require the following apparatus: charge plate monitor with a voltage range of 0 to 20,000V DC connected to an XY plotter and a wrist strap (with a 1-megohm resistor); a constant, indicating ground circuit monitor; and an insulated, acrylic plate approximately 24 in. x 24 in. (Fig.5).

Procedure. The test subject wears the test footwear and wrist strap and stands motionless on the insulated acrylic plate next to the floor sample. The wrist strap is connected to the change plate monitor. The subject is charged to +/- 5,000V while standing on the acrylic plate. When the charging voltage is turned off, the subject steps from the plate to the floor sample. As the foot makes initial contact with the floor, the decay is tracked on the XY plotter until the voltage reaches zero or until no further decay occurs. This procedure is performed six times at +5,000V and six times at -5,000V for each floor material and footwear combination.

Voltage Gradient Test

Voltage gradient measurements indicate the practical dissipative characteristics of a floor material. Originally developed by G. Baumgartner to evaluate ESD worksurfaces, the procedure has been modified for use on floor materials. The method measures the voltage "seen" by an object resting on the surface when a nearby second object is charged to 2,000V. This measurement indicates whether dissipation is across the material's surface, through its bulk to ground, or both. If the path is across the surface, tote boxes, carts, or other objects on the floor can be exposed to the charge as it dissipates.

Procedure. An NFPA metal electrode is placed near the material's installed groundable point and attached to a metal plate isolated from ground. The plate voltage is measured with a non-contacting, precision electrostatic voltmeter connected to an XY plotter. A second electrode is placed on a diagonal line from one (grounded) corner to the opposite corner. Attached to this second electrode is a current-limited 2KV is applied to the electrode. The maximum voltage indicated on the electrostatic voltmeter is recorded (Fig. 6).

One measurement is made with the sensing electrode 5 in. from the material's groundable point and the powered electrode 5 in. from the sensing electrode. A second measurement is made with the sensing electrode 12 in. from the material's groundable point and the powered electrode 36 in. from the sensing electrode (Fig. 7). Six measurements at +2,000V and six at -2,000V are made for each electrode position. The test is repeated for each groundable point on the test sample.

Voltage Suppression Test

The test procedure for voltage suppression was developed by Dr. Joe Crowley in conjunction with preliminary work for the EOS/ESD Association Draft Standard 4 on worksurfaces. The test indicates a material's ability to drain a charge form a charged object, such as a person wearing specialized footwear. As the object contacts an ESD floor material, that charge should fully drain.

Procedure. A round, aluminum test plate (1/8 in. thick x 6 in. diameter with an attached insulated handle) is connected to the plate of a charge plate monitor. An XY plotter is also connected to the charge plate monitor to record the test voltages. With the floor sample grounded, the test plate is charged to +/- 5,000 V and lowered onto the sample's surface for approximately two seconds. The test plate is carefully tilted at an angle, then lifted free of the worksurface (Fig. 8). Any residual voltage remaining on the aluminum plate is recorded by the plotter. Any charge generated by lifting the plate free of the surface is also recorded. Six +5,000V and six -5,000 V test cycles are conducted at different positions on the sample.

Conclusion

Evaluation of ESD floor surfaces should be performed on parameters other than electrical resistance. The test methodology proposed here allows evaluation across a wide range of electrostatic properties: static generation, static decay, voltage suppression, voltage gradient, and electrical resistance. The procedures are designed to specifically evaluate and compare flooring materials which are intended for very sensitive electronic manufacturing environments. They include both footwear and the human body which are encountered in the environment. The testing is also conducted in a manner similar to the end use of the product in question. Combining the performance characteristics of the materials with appropriate statistical analysis can lean to a prediction of performance in the field and can lead to better selection o the most appropriate floor material and footwear combinations for the specified application.

References

1. ASTM F150, "Standard Test Method for Electrical Resistance of Conductive Resilient Flooring."
2. AATCC/ANSI 134, "Electrostatic Propensity of Carpets."
3. Chase, E.W., and Unger, B.A., "Triboelectric Charging of Personnel from Walking on Tile Floors," 1986 EOS/ESD Symposium Proceedings.
4. Crowley, Joseph M., and Halperin, Stephen A., "Resistance Testing of Static Dissipative Worksurfaces," 1988 EOS/ESD Symposium Proceedings.
5. Crowley, Joseph M., "Floor Covering Resistances Measured with Persons and Electrodes," and "Resistance Testing of Carpets and Other Floor Coverings," test reports prepared for the EOS/ESD Association Standards Subcommittee on Floor Materials, 1989.
6. EOS/ESD Association, "Standard for Protection of Electrostatic Discharge Susceptible Items: Worksurfaces."
7. Halperin, Stephen A., "How to Select Flooring," EOS/ESD Technology, February/March 1988.
8. Kolyer, John J., and Cullop, Dale M., "Methodology for Evaluation of Static-Limiting Floor Finishes," 1986 EROS/ESD Symposium Proceedings.
9. Kolyer, John M., Watson, Donald E., Anderson, William E., and Cullop, Dale M., "Controlling Voltage on Personnel," 1989 EOS/ESD Symposium Proceedings.
10. Lingousky, J. E., and Holt, V. E., "Analysis of Electrostatic Charge Propensity of Floor Finishes," 1983 EOS/ESD Symposium Proceedings.
11. NFPA 99, "Health Care Facilities."
12. Shah, B. M., Martinex, P. L., and Unger, B. A., "Test Methods to Characterize Tribolectric Properties of Materials," 1988 EOS/ESD Symposium Proceedings.
13. EOS/ESD Draft Standard 7.1 (Proposed), "Floor Materials-Resistive Characterization of Materials," June 7, 1990.