A hot topic of debate involving ESD test equipment focuses on the usefulness and repeatability of decay-time measurements. There are several procedures for performing charge decay tests, and just as many results. This leads some to question the efficacy of the tests and to look to resistance measurements for useful material information.
The decay testing of static charges is applied per the Federal Test Method Standard (FTMS) 101C, Method 4046; the corona charge method and the direct-contact method (Figure 1). They all purport to deposit a charge on the surface, but often there are large differences in the decay times measured.
The EIA has improved Method 4046 by specifying a calibration procedure and requiring that the materials tested be homogenous. Still, the decay test usually does not measure what we think it measures, said G. Baumgartner of Lockheed Martin Missiles and Space.1 The models of decay still need to be understood, and the causes of unusual dissipation phenomena determined.
To add to the confusion, the results from some experiments have uncovered a difference in the relationship between decay curves and the time of charging.2 These tests showed that the longer the charge time, the flatter the decay curve and the more linear the relationship between decay time and the applied voltage.
Equipment Sophistication
Inherent in any discussion about measurements and correlation of results is the basic understanding of the levels of equipment performance. The sophistication of the test equipment you choose for making ESD resistance or decay measurements affects how accurate and detailed your results will be. You may only need a simple meter for a quick check of a work area, or you may require more advanced instruments for incoming inspection or laboratory research.
The laboratory-level instrument measures small tolerances, often requiring an experienced technician to operate it properly, said Stephen Halperin of Stephen Halperin & Associates. It is used in controlled laboratory conditions, and forms the basis for scientific investigation, material development, and laboratory testing for performance certification.
Inspection-level instruments are geared toward bench use and portability, continued Mr. Halperin. The measurements correlate to laboratory-level instruments, but the tolerances can differ from 5% to 10%. Typically, these instruments are easier to use and less expensive than lab equipment, but appropriate for experienced personnel needing defined instruments. Inspection-level equipment often is used for comparing material performance and for performing detailed facility evaluation and auditing.
Indicating instruments comprise another level of equipment that includes the handy, portable unit with an extensive measurement range rather than a specifically defined readout, said Mr. Halperin. For example, some units such as the Prostat PSI-870 indicate orders of magnitude with colored LEDs rather than specific measurements. These types of products are inexpensive and easy to use, and require little knowledge of the instrument to operate.
Charge Decay vs Resistance
The charge decay technique evaluates homogenous materials that are at the extreme limit of the dissipative range, said David Swenson, applications development manager at 3M. The ESD Association Standards Committee is not very supportive of charge decay measurements on materials because the association has determined that resistance methods provide the bulk of the necessary information.
Charge decay tests are performed according to the procedure you use. Method 4046, specified by military standards, and National Fire Protection Association 99 Health Care Facilities charges a sample conductively to ±5 kV. The triboelectric charge decay system used by NASA charges a sample by rubbing it on a rotating Teflon™ wheel. The corona process, originally designed for electrostatic copiers, charges a specimen from a high-voltage needle-point probe. Unfortunately, each procedure still needs fine-tuning before it produces acceptable and repeatable results.
Occasionally, materials have skin effects just beyond the dissipative electrical resistance range, said Mr. Swenson. In this situation, charge decay measurements may provide additional information on the material characteristics. For these rare products, the procedure described in Method 4046 is suitable as long as it is in a planar form.
Results from Method 4046 and the corona discharge procedure are dependent on the materials tested and the knowledge of the instrument operator, said Mr. Halperin. Neither method is entirely suitable for today’s ESD analysis needs. Method 4046 is influenced by material content and construction, while corona discharge is interpretive and erratic. Neither measures the dissipation of a charge across or through the material.
Method 4046 can measure static-decay times of homogenous materials under similar conditions, and can be used to resolve static-dissipative material characteristics falling into a range from 30 ms to more than 30 s, said Stanley Weitz, president of Electro-Tech. It also indicates the poorest performance of a sample as opposed to the resistivity/resistance test that shows the best performance of the product under test.
Unfortunately, 4046 only defines relative performance of a material, so the decay time of a finished material will be longer than the initial sample, said Mr. Weitz. The procedure is not usable for conductive (<107 W ) or insulative (>1014 W ) material, and it is not effective for most composite and laminated materials with a conductive layer.
Charge decay measures how quickly electrostatic charge can migrate over the surface or into the volume of a material, said John Chubb, Ph.D., proprietor of John Chubb Instrumentation. For example, users testing floors want to know how quickly a charge can be removed from the conducting object in contact with a person or object. For most other situations, the focus is on how quickly the charge disappears on the material.
The triboelectric method simulates actual use better than 4046, but affects the dissipative characteristics of the material and can remove topical antistats via the rubbing action, said Mr. Weitz. Additionally, the charge level is difficult to control and can vary from zero to several thousand volts.
The corona charge method analyzes materials that dissipate slowly and is used to check insulative objects. Unfortunately, the induced charging level is not consistent and also varies from zero to a few thousand volts. The corona can burn off antistatic additives with the high-voltage needle and change the material characteristics during testing.
Resistivity measurements described in the American Society for Testing and Materials (ASTM) D 991 Standard for Packaging of Electronic Products for Shipment are used by manufacturers for many materials, said Mr. Swenson. However, several techniques are indicated so exercise care to ensure you use the appropriate method for your material configuration.
The ASTM D 257 Standard Test Method for DC Resistance or Conductance of Insulating Materials is another test for surface resistivity, but it is not recommended for materials in the dissipative range, only insulative products, said Mr. Swenson. Numerous technical papers published in the last 15 years describe the difficulties using this method for evaluating the properties of materials with resistivities <1012 W /sq.
The only acceptable method for measuring surface resistance is S11.11, said Mr. Halperin. This procedure uses a defined fixture and measurement procedure and yields a result that is stated as surface resistance in ohms.
You can obtain an approximation of the surface-resistivity value, defined by the ASTM as ohms/square, by multiplying the resistance value determined by the ANSI/ESD Standard 11.11 Surface Resistance Measurement of Static Dissipative Planar Materials by one order of magnitude. For example, 102 W would become 103 W /sq.
Today, the trend is to conduct resistance measurements using the S11.11 test method for planar material, said Mr. Swenson. It is similar to D 257 since it uses concentric rings for contact electrodes, but the applied voltage, contact mass, and environmental conditions are precisely specified.
References
1. Baumgartner, G., “Electrostatic Decay Measurement Theory and Applications,” EOS/ESD Symposium, 1995, pp. 262-272.
2. Ehrmaier, B. and Schmeer, H., “Some Results in Measuring Static Decay,” EOS/ESD Symposium, 1996, pp. 259-264.
ESD Test Equipment
Simulator Offers Modules
For Specific Applications
The ESD30 Electrostatic Discharge Simulator provides ESD tests by changing the discharge module. The modules simulate the different capacitance and resistance models for automotive requirements. The 18-kV discharge network has interchangeable modules for air, contact, and furniture discharge applications. The automotive modules support 18-kV and 30-kV tests. Amplifier Research, (215) 723-8181.
Digital Resistance Meter
Conforms to Eight Test Methods
The ACL 800 Megohmmeter is a digital surface-resistance and resistivity meter for measuring resistance-to-ground, resistance point-to-point, and surface and volume resistivity. It conforms to UL, NFPA, EIA, ASTM, EOS/ESD, ANSI, military, and European test methods. The meter tests at 10 V and 100 V, and has a measurement range from 103 W to 1012 W . It also measures relative humidity from 10% to 100% and temperatures from 32°F to 212°F. ACL, (847) 981-9212.
Generator Provides
High-Voltage Pulses of <50 ps
The Model 632 Pulse Generator provides high-voltage pulses of <50-ps rise time for any load impedance. It helps calibrate ESD or CDM sensors. The unit generates a minimum pulse width of 750 ps. The output pulse amplitude of either polarity is adjustable from 500 V through 2,500 V. High-voltage attenuators are available for impedance matching and reduction of multiple reflections. Barth Electronics, (702) 293-1576.
Meter Shows Surface Voltage
To ±20 kV With 5% Accuracy
The A50015 Digital Static Fieldmeter indicates surface voltage and polarity up to ±20 kV at 1″ with 5% accuracy. It is chopper-stabilized, and features push-button auto-zero and hold functions. A flashing LED range-finder system provides accurate positioning information of the meter from the target. The case of the meter is conductive, providing a ground path via the operator or a grounded wrist strap. Desco, (909) 598-2753.
Shielded-Bag Tester
Provides 1-kV Pulse
The Model 431 Shielded-Bag Test System provides the 1-kV human body model discharge pulse. It also includes a Tektronix Model CT-1 Current Probe. The 431 performs the voltage test specified by the EIA-541 and MIL-B-81705C standards and the energy test for the ANSI ESD S11.31 standard. Electro-Tech Systems, (215) 887-2196.
ESD Tester Provides
Air Discharge to 16.5 kV
The PESD 1600 Electrostatic Discharge Tester offers changeable tips for contact with the EUT, and features a multifunction rotary knob to change test levels. It outputs an air discharge from 0.2 to 16.5 kV, a contact discharge from 0.2 to 9 kV at either polarity, and five repetition frequencies from 1 Hz to 20 Hz. Impulse test parameters also are performed. Haefely Trench, (703) 494-1900.
Static Detector Measures
20 kV at 100 mm
The JCI 100 Static Detector provides readings up to 20 kV or 200 kV with 10-V or 100-V resolution, respectively. The 3 ½-digit LCD meter has polarity and low-battery indicators and a zero-adjustment screw. The drift of the readings is <0.1%/s. The instrument also contains a ground-bonding socket with a cord. John Chubb Instrumentation, (011) 441 242 573347.
System Meets Waveform
Verification Requirements
The Model CM-ESD provides compliance-level capabilities to meet the IEC 1000-4-2 ESD testing requirements. It features test-routines software and front-panel control, and supports Windows® 3.1- and 95-based application software. Air discharge voltages range from 500 V to 8.8 kV with 1-V resolution and ±5% accuracy. Contact discharge extends from 500 V to 4.4 kV with 1-V resolution and ±5% accuracy. The unit is part of the CEMASTER™ testers, and may be configured to support IEC immunity standards. KeyTek, Division of Thermo Voltek, (800) 753-9835.
Meter Measures 20-kV
Static Electricity at 1″
The Model 282 Static Locator measures static on packaging materials and circuit assemblies. It has a range to 20 kV at 1″. A hold function and a low-battery indicator are provided. An increased measurement range is available at greater distances. Monroe Electronics, (800) 821-6001.
Voltage-Detection System
Monitors Workstation Charge
The Series 5300 Workstation Voltage Detection System provides an adjustable audible and visual alarm. It features an alternating polarity ground monitor, polarity and peak hold, an LED logarithmic display of voltages, a human body voltage measurement, and an ionization balance check. Input voltage ranges from 5 V to 500 V. Novx, (800) 728-NOVX.
Test Kit Checks
Ionizer Decay, Balance
The PIK-110 Ionization Test Kit checks ionizer decay and balance using the company’s PFM-711A Electrostatic Field Meter, the CPM-720 Charge Plate Monitor, and the PCS-730 ±1-kV Charging Source. Decay time is measured in tenths of a second by attaching the PDT-740 Static Decay Timer to the field meter. Airflow speed and temperature are checked with the PAN-750 Anemometer. The kit also includes a modified wrist strap, a common-point adapter plug, and the PHT-770 Hygro-Thermometer for temperature and humidity readings. Prostat, (630) 238-8883.
Charged-Plate Monitor
Uses High V, Z Circuitry
The Model 156A Charged Plate Monitor System provides quantitative measurements regarding the effectiveness of air ionization systems. It uses a high-impedance loading and a high-voltage follower circuit to monitor the ion collecting-plate voltage. The size and shape of the plate as well as the measurement capacitance can be custom-made to match specific ESD-control requirements. The monitor features a user-programmable decay mode and a float mode. Trek, (800) FOR-TREK.
Detection Kit Captures
ESD-Event Data
The 751K Detection Kit helps capture ESD-event data by identifying the location of static discharges. It is comprised of five static-event detectors, a magnetic resetting device, and the accessories needed to mount and use the detectors, including lead wires, mounting clips, and double-sided tape. The contents are packaged in a molded plastic case. The detector backplate indicates that an ESD event has occurred by triggering the LCD to change from clear to red when it senses a rapid change in potential. 3M Electrical Specialties, (800) 665-7862, ext. 74.
Copyright 1997 Nelson Publishing Inc.
July 1997