ESD test equipment has a twofold purpose: to locate damaging static charges and to make the correct measurements. Defining the goals is easy. Selecting the right instrument for accurate results is more challenging. With so many instruments and testing procedures to choose from, a few pointers from industry leaders may help you find the right way to zero in on your static charge.
Measuring Electrostatic Fields
Topping the list of instruments for measuring static is the electrostatic fieldmeter. It checks for the presence and voltage level of electrostatic charge on products ranging from ICs on assembly lines to PCBs in shielded barrier bags. All fieldmeters perform quick and simple tests to indicate the voltage and field strength of static charges.
Fieldmeters give an approximation of the charge on a surface of a material or object, said David Swenson, International Technical Service Manager at 3M. The electric field emanating from a charged object produces a coulombic attraction force between the charged surface, other objects, or ground. A fieldmeter detects the force and converts the value to volts. While the values are not precise, they provide an estimate of the electrical potential of the charged object.
The measurements are made at a specified distance from the object and are usually displayed in kilovolts. Making accurate measurements depends on maintaining the distance required by the manufacturer. The magnitude of the indicated charge increases the closer the meter is to the DUT and decreases as it moves farther away.
For a correct measurement, the object must be in free space, which means the DUT must be kept away from other objects. For example, a bag or box must be suspended in air 1 ft from the tabletop or vertical surface, and the meter should be connected to equipment ground to prevent erroneous readings.
Not all fieldmeters are created equal, and you need to know which type is best before making any measurements. The electrometer or static-locator type is a capacitively coupled DC amplifier instrument, said Steven Kelly of Monroe Electronics. Like all amplifiers, it draws current and the measurements drift over time. The drift is accelerated when an ion balance is present. Electrometers are best used in non-ionized environments and where low cost is the main objective.
Most often, this instrument, also called a nonchopper-stabilized meter, determines where a static problem occurs in an industrial process, said Chandler Sinnet, Product Development Engineer at Chapman. This operation needs a static-detecting circuit with a quick response time and an easy-to-read, intuitive display. An instrument with a fast-moving analog needle allows you to move the meter while searching for charged areas or to hold the meter near a moving object and still make meaningful measurements.
A noncontacting, nonchopper-stabilized meter does not operate properly in ionized atmospheres because the air ions accumulate on the instrument’s electrodes and produce an incorrect measurement, said Lorna Finch of Trek. Unfortunately, there is no practical mechanism to prevent the air ions from accumulating on the sensitive electrodes of the nonchopper-stabilized unit.
The AC fieldmeter is a high-quality instrument which employs chopper stabilization, said Mr. Kelly. These instruments use a vibrating capacitor arrangement that moves the electrode perpendicular to the electric field to eliminate the effects of ionization.
For increased stability, look for units with a null-seeking feedback circuit that minimizes errors caused by modulator amplitude changes. Chopper-stabilized units are used in ionized environments as well as in long-term monitoring settings.
A third type of instrument uses a rotating-vane electrometer, said Mr. Swenson. A multibladed propeller rotates at a high rate over the measurement aperture, causing the DC electric field to be converted to an AC-type signal.
Rotating-chopper instruments have a higher sensitivity than the tuning-fork chopper-stabilized units because the sensing aperture is much larger, said John Chubb, Proprietor of Chubb Instrumentation. These fieldmeters provide a sensitivity of approximately 1 V at 100 mm and the long-term zero stability needed to easily assess critical static problems.
All three types of fieldmeters can be used in any environment for measuring surface potential, observed Mr. Swenson. They are particularly useful for estimating surface potential on moving systems such as plastic films during extrusion or coating operations.
The ESD Association is currently sponsoring research to develop fieldmeters with very fast response times, said Mr. Swenson. These are needed to evaluate charge generation on parts processed in automated handling equipment.
Measuring Buried Layers
Buried-layer shielding is currently measured using the capacitive probe test or the buried-layer test. The capacitive probe test requires a Human Body Model (HBM) discharge unit, a 150-MHz oscilloscope and a 2-MHz, 10-V function generator, said Mr. Kelly. The system measures the material’s capability to dissipate a charge to ground and typically costs from $14,000 to $30,000.
The buried-layer test method measures the conductive surface to a maximum depth of 0.005″ below the insulative surface, continued Mr. Kelly. The instrument emits an RF signal to the buried layer to assess the capacitance of the material. Based on the capacitance, sheet resistivity and fixture configuration, the resistivity measurement is displayed on the unit’s analog meter. It does not depend on the construction of the bag like the capacitive probe test and typically costs less than $1,000.
EIA -541 Standard for Packaging of Electronic Products for Shipment-1988 establishes the definition of static shielding based on the resistivity of the material. To meet the definition of the shielding, a material must have <1 ´ 104 W /sq surface resistivity for one layer in multilayer materials, or <1 ´ 103 W · cm of thickness for volume-conductive materials.
The buried shielding-layer meter typically comes with a test bed insulated on one side and a calibration plate on the other, said Kimberly Becker, Operations Manager at Prostat. To use this meter, you must calibrate it before inserting the test bed into a static-shielding bag. You then place the test unit on top of the bag, over the plate. Depress the test button and the instrument emits an RF signal through one side of the test plate and picks up the signal on the other side. The meter converts the signal into a resistivity measurement in W /sq. This check indicates when the shielding layer is damaged and helps you decide when to discard the bag.
The buried shielding-layer meter is used on single layers of film and cannot be used to determine if a package formed from the material is functional, agreed Mr. Swenson. In most cases, the weakest point in a bag formed from static-shielding material is the bottom fold.
To determine if the bottom fold has been compromised, the side seals must be cut off or the bag must be torn open and the film laid flat. The meter, with the transmitter on one panel and the receiver across the bottom fold, measures the apparent resistivity of the film across the fold.
EIA-541 Appendix E describes the earliest version of the test using a capacitive sensor with differential voltage probes connected to an oscilloscope. The procedure can be used on both metal-in and metal-out types of static-shielding materials.
The capacitive sensor detects the difference in potential between the top and bottom of a bag held in a fixture during an external discharge from a high-voltage RC network. Typically this is the HBM discharge circuit. The results of the test are given in volts. MIL-B-81705C Barrier Materials, Flexible, Electrostatic Free, Heat Sealable references this method and uses an upper limit of 30 V for Type III transparent static-shielding films.
According to Dr. Chubb, there are two basic problems to tackle in measuring the shielding capability of materials: how to cover the range of frequencies up to 1 GHz and how to make measurements that avoid problems from the common-mode signals.
The new version of EIA-541 overcomes the common-mode signal problem by using a current transformer, added Dr. Chubb. This is a good idea, but it does not provide the necessary frequency coverage.
The common-mode signal can be overcome by balancing the simultaneous positive and negative, 1-ns rise-time pulses. The signals are analyzed at each decade from 10 Hz to 1 GHz.
This approach is basically appropriate but, unfortunately, it is not easy to implement. Results from tests performed by John Chubb Instrumentation indicate that differences between materials are greatest at the highest frequencies.
The newest test is the ANSI/ESD S11.31-1994 Evaluating the Performance of Electrostatic Discharge Shielding Bags, said Stanley Weitz, President of Electro-Tech. This test measures the energy inside a static-shielding bag; and since most devices today are energy sensitive, it can correlate with device failure.
This test also uses a capacitive probe sensor; but instead of measuring the differential voltage, it checks the current through a 500-W resistor between the electrodes. The current waveform is measured using a 200-MHz, 1-GS/s oscilloscope. This waveform is used to calculate the energy.
The ANSI/ESD S11.31 bag-holding fixture, discharge circuit and capacitive sensor are similar to the items described in EIA-541, said Mr. Swenson. However, the ANSI/ESD S11.31 procedure uses a resistor connected between the top and bottom plates of the capacitive sensor and a current probe over the resistor wire when connected to the bottom plate. It measures the induced current across the resistor during a pulsed discharge to the exterior of a bag under test.
The current and resulting decay measurements are recorded on the oscilloscope. The current is converted to energy measured in nanojoules. A preliminary maximum energy for static-discharge shielding bags is set at 50 nJ. The pending IEC standard references this method and the maximum energy value allowed. EIA-541 is scheduled for revision and ANSI/ESD S11.31 will replace the existing Appendix E voltage measurement.
Another method is the frequency-sweep test specified in MIL-B-81705C, said Mr. Weitz. This procedure measures the attenuation characteristics of the shielded material over a wide frequency spectrum.
The buried-layer meter is appropriate for survey measurements in production applications. If you need to analyze bag construction or need in-depth measurement data, use the capacitive probe equipment and test.
Charge-Decay Measurements
Charge decay describes at least two situations and measurement techniques, said Mr. Swenson. One interpretation of charge decay is the process that a dissipative or conductive material goes through when it acquires an electrostatic charge and is subsequently grounded or bonded to other conductors. The charge becomes mobile on a dissipative or conductive material, resulting in current flow.
The charge on these materials approaches zero if they are attached to ground. You can determine the charge decay of these materials by applying a known charge and measuring the time for the material to become neutral.
The charge decay technique is also used to evaluate ionization systems, said Mr. Swenson. A known electrostatic charge is placed in an ionized air environment and the time needed for the charge to become neutral is measured.
Charge-decay measurements can encompass many types of procedures, agreed Mr. Weitz. The static-decay test specified in the Federal Test Method Standard 101C Method 4046 is the most widely used for measuring the static-decay characteristics of static-protective material.
This method calls for charging a 3″ ´ 5″ test specimen to 5 kV via the electrode assembly and measuring the decay time to the 1% (50-V) level. This test is best for evaluating homogenous and surface-coated insulative material.
To evaluate the static-dissipative characteristics of laminated materials, you must analyze the charge on the test specimen before the 5-kV charge is applied, then observe the accepted charge and measure the decay time at both polarities. Static decay in conjunction with a surface-resistance measurement provides a comprehensive analysis of dissipative material.
Other static-decay tests use triboelectric charging or corona charging, said Mr. Weitz. With these methods, the applied charge is not controlled and the amount of charge the sample accepts is not a function of its dissipative characteristics.
The corona-charging method uses a noncontacting electrostatic voltmeter to measure the decay rate of the material, said Mr. Kelly. Typically, the output of this charge device is monitored by a data recorder for long-term data analysis. It provides measurement decay rates from a few milliseconds to many seconds with excellent accuracy.
The corona-charging method, however, does not always imitate real-life tribocharging effects on material, continued Mr. Kelly. To offset these possible errors, the system usually limits the amount of charge applied and the duration of the charge cycle. This slows the migration of the charge through the material. It is also important to test the material at the same temperature and humidity.
Here are the basic requirements for a charge-decay test method, according to Dr. Chubb:
o The method of charging must ensure that the charge is placed on the surface, regardless of the dissipative or insulative qualities of the material.
o The observation of the charge application and dissipation must occur on the same side.
o The material ground connection must remain stable throughout the charging period and during the charge dissipation.
o The charge-decay observations must match the decay of the triboelectric charge for comparable conditions and several materials.
Charge-decay tests that fulfill these requirements easily meet real-world situations, said Dr. Chubb. Unfortunately, most of today’s methods fail on one or more of these points.
The decision to use charge-decay measurement techniques requires an understanding of the material properties and the intended application, said Mr. Swenson. Generally, charge-decay measurements on materials supplement resistance and resistivity measurements, especially if the resistance or resistivity is >1011 W or 1012 W /sq. If it is <1011 W , most charge-decay measurement devices will not provide accurate time measurements since the rate of decay will be too fast.
ESD Test Equipment
Unit Has Measurement Range
From 103 to 1012 W /sq
The ACL 385 Surface Resistivity Meter detects surface resistivity and resistance to ground with a test range from 103 W /sq to 1012 W /sq with an accuracy of ± 1/2 decade. The instrument measures conductive, static-dissipative and insulative surfaces using the parallel-bar method from the ASTM D-257 standard. Accuracy is ± 10% and repeatability is ± 5%. Calibration is recommended every 12 months. $331. ACL, (800) 782-8420.
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 generator is capable of 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. $7,850. Barth Electronics, (702) 293-1576.
Electrostatic Meter Has
Four Ranges With ± 5% Accuracy
The ESM 5000 Industrial Electrostatic Meter has four ranges with a full-scale accuracy of ± 5% from 1,000 V to 30,000 V. It is housed in a metal case for use in industrial applications. The analog readout indicates polarity and magnitude of the static charge. Measurements are taken 0.75″ and 3″ away from the object under test. $395. Chapman, (207) 773-4726.
Audit Kit Has Charging Source
And Resistance Probes
The Model 2015 Static-Control Audit Kit is a portable electrostatic measurement laboratory with a charging source and resistance probes for measuring small, uneven surfaces. The kit includes the Model 208A Dual-Voltage Power Supply with ± 500 V or ± 1,000 V, the Model 210 Static Meter, the Model 205A Charged-Plate Adapter and the Model 821 Surface Resistivity Concentric-Ring Probe. Accessory probes and plates are provided for testing chairs, footwear, gloves and finger cots. $4,595. Electro-Tech Systems, (215) 887-2196.
ESD Tester Provides
Air Discharge to 16.5 kV
The PESD 1600 Electrostatic Discharge Tester provides changeable tips for contact with the EUT and features a multifunction rotary knob to change test levels. It can output 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 are performed per IEC 1000-4-2. $7,695. Haefely-Trench, (703) 494-1900.
Charge-Decay Times Range
From 0.05 s to Several Hours
The JCI 155 Charge-Decay Tester uses proprietary brushless fieldmeter technology for noncontacting surface-voltage measurements. Charge-decay times can be measured from 0.05 s to several hours. The charging system generates 3-kV surface voltages of either polarity. Proprietary software allows you to set the corona voltage and duration. The software and a serial link to a PC provide automatic capture, analysis and graphical display of charge-decay curves. John Chubb Instrumentation, (011) 441-242-573347.
Fieldmeter Measures Static
Via Noncontact Method
The Model 282-1 Electrostatic Fieldmeter uses a noncontacting method to measure static on packaging materials, ICs, PCBs and workstations. The unit offers an auto-zero circuit and a distance-detection capability. The measuring range is 20 kV at 1″. Greater voltage readings are possible at greater distances. $495. Monroe Electronics, (800) 821-6001.
ESD Generator Uses
Piezoelectric Technology
The pQT ESD Generator is based on piezoelectric technology and fixed spark-gap arcing. The unit does not require user setup or adjustment. The generator, shielded with a metal case, produces single peak transients with a rise time of 2 ns and a pulse width of 10 ns. Generators are available in broadband and human-body models. Magnetic-field and electric-field probes are used to produce small radiated fields for digital products and PCB-level mapping for locating ESD-sensitive spots. $595. picoQ, (416) 626-9073.
Static-Decay Timer Measures
From 1 kV to 100 V
The PDT-740 Static-Decay Timer measures the decay time of ionizers from ± 1,000 V to <100 V in accordance with ESD Association Ionization Standard S-3.1. It also checks material decay time to <50 V. The unit provides decay-time measurements for ionizers, packaging materials, footwear and personnel grounding. The timer is used with the company’s Charge Plate Monitor Kit. $395. Prostat, (708) 238-8883.Detection System
Helps Locate ESD
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. $727. 3M Electrical Specialties, (800) 328-1368.
Charged-Plate Monitor Uses
Two-Conductor Cable
The Model 156 Charged-Plate Monitor System tests the effectiveness of ionization systems for work surfaces and laminar flow hoods. The system includes a 6″ ´ 6″ air ion collecting plate that has a 20-pF capacitance between the floating plate and ground. Plates with different capacitances between the collecting plate and the ground also can be used. The system operates in a decay or a float mode. $2495. Trek, (800) FOR-TREK.
Copyright 1996 Nelson Publishing Inc.
July 1996