In today’s sophisticated electronic manufacturing environment, measuring and simulating ESD events is critical to an effective ESD-control program. With these capabilities, you can monitor and update your ESD procedures and practices as well as qualify and test your products. But to get worthwhile results, you must use the right tools for the job.
Selecting the best tools requires a basic understanding of the various testing levels that exist. There are four levels of testing with four analogous categories of instrumentation, said Stephen Halperin of Stephen Halperin & Associates.
The most sophisticated class is the ESD lab level, which is used for design verification and requires very accurate instruments, he said. The next is the qualification level, which uses similar equipment as the lab to check and measure the accuracy of an ESD-control product.
The periodic/auditing level indicates if a product is within limits but is not for measuring its accuracy. The last category is the monitoring level. It uses equipment to determine if a product such as a wrist strap is functioning properly.
What tools work best in field applications and which are used for laboratory verification? The list of field equipment for measuring ESD includes surface resistivity meters, field meters, megohmmeters with probes, multimeters, wrist/foot strap testers and residual voltmeters.
Generally, the laboratory operation relies on equipment with more precise measuring capabilities. As a result, labs need all the field equipment plus charge plate monitors (CPMs), electrometers, Faraday cups, ESD simulators, static decay testers with a controlled humidity chamber, oscilloscopes, and power supplies with a 10-kW resistor in series with the high-voltage leads.
But before purchasing equipment, investigate your needs as well as the capabilities and differences of each instrument. Specifically, know the proper use and limitations of the instrument as well as the equipment criteria and standards it meets, said Carlos De La Isla of Desco.
Confusion abounds when testing is performed with inappropriate or inadequate test equipment, added Mr. De La Isla. Typically, the test misrepresents the ESD protective qualities or performance attributes of a product. For example, an operator may use a field meter that is not chopper-stabilized to check the performance of an ionizer and–conclude incorrectly–that it is out of balance because the meter is operating erratically.
An electrostatic field meter detects and measures the voltage and field strength associated with static charges on objects. It is performed indirectly by measuring the electric field or potential that a charge produces. Older field meters provide a continuous reading when activated. This is appropriate in most cases except when measuring an ionizing field.
“The nonchopper-stabilized instrument produces inconsistent responses because ionization imposes a charge on the meter’s sensing element,” said Steven Kelly of Monroe Electronics. Ions are deposited on the element, causing the unit to develop an offset. This forces the meter to display results that appear erratic, fluctuating with the ionizer charges.
Chopper-stabilized meters have internal circuitry to eliminate the effects of ionization. They continually reset to zero to allow accurate readings in ionizing fields.
Making accurate measurements with field meters also depends on maintaining the required distance from the object and having the object in a stand-alone condition, free from other objects. The distance usually is determined by the voltage range of the meter. For example, to measure a bag or box, suspend it at least 1 foot from a tabletop or vertical surface and ensure that the object is free from a charged field.
Chopper-stabilized field meters also are available with different accuracies for different tasks. For example, PROSTAT offers a dual-range field meter calibrated to a ±2-V accuracy. This level of accuracy is crucial to personnel measuring a charge or calibrating an ionizer, said Kimberly Becker, Operations Manager for PROSTAT. However, this accuracy may not be required to detect an electrostatic field in product receiving.
Lab Instruments
The more sensitive the environment, the greater the need for laboratory-quality calibration and documentation, added Ms. Becker. For example, with devices sensitive to 50 V, a very accurate field meter is important for measuring charge generation of less than 25 V on floors and work surfaces.
If you are searching for a megohmmeter or high-resistance ohmmeter, know the applicable industry standards, said Mr. Kelly. Certain standards dictate measuring voltages, probe configurations and electrification periods for these instruments.
The high-resistance ohmmeter applies a voltage of 10 V or 100 V and is used with probes to measure point-to-point resistance or resistance to ground of tabletops, mats and floors or floor finishes. Point-to-point resistance on tabletops can be measured by placing the probes 10 inches apart and reading the resistance. The exact procedures are outlined in ESD Standard #4 Work Surfaces.
Resistance-to-ground of tabletops also can be measured by using ESD Standard #4. Simply place one probe on the tabletop and connect the other lead to ground.
If you want to measure the resistance of floors and work surfaces, a wide-range ohmmeter set at 100 V using two 5-lb conductive rubber electrodes is the proper instrument per the ESD Association’s S7.1-1993 Flooring, said Ms. Becker. Some electronic facilities still measure floors at 500 V using foil electrodes per the National Fire Protection Association 99 Standard. This standard is not applicable for today’s sensitive manufacturing environments.
The point-to-point resistance of a floor or floor finish is measured by placing clean probes 30 inches apart on a portion of the floor that is dust and dirt free. Complete the operation by setting the voltage to 100 V and reading the point-to-point resistance.
To assess the risk of charges retained on materials, the instrument of choice is the static decay tester. It tests the decay time of a static charge from a given voltage to a predetermined percentage of that voltage under various humidity conditions. The procedure consists of placing a 3″ x 5″ sample in a holder and applying a charge followed by grounding the charge. The decay time is measured by a noncontacting voltmeter.
The International Electrotechnical Commission (IEC) is considering a method in which a high-voltage corona discharge would deposit a patch of charge on the surface of a material, said John Chubb, Proprietor of John Chubb Instrumentation. Studies reported in the International Conference, Electrostatics 1995 have shown that corona charge decay is a good model of tribocharge decay, but tribocharge performance is not revealed by resistivity measurements. A fast-response field meter, however, can measure the subsequent rate of decay of the surface voltage.
Surface resistivity is the ratio of DC voltage to the current that passes across the surface where the surface consists of square units of area. The intent of the resistivity measurement, calculated in ohms per square, is to project the behavior of the control product in terms of charge bleed-off, decay, shielding and, to some extent, triboelectric generation. It can be measured with several types of meters, including high-resistance meters and megohmmeters.
The difference between surface resistance and surface resistivity is often misunderstood, said Ms. Becker. These terms are used interchangeably. They should not be–they are two different types of measurements.
Surface resistance is a direct measurement reported in ohms. ESD Standard 11.11 Packaging requires the use of the surface resistance measurement. This standard provides the categories of <1.0 x 103 W as electrostatic shielding, <1.0 x 104 W as conductive, 1.0 x 104 W to 1.0 x 1011 W as insulative.
Another important piece of gear for the laboratory is the CPM. It monitors the balance and measures the efficiency of air ionizers and the rate of charge decay from a fixed voltage point to a desired endpoint (usually 10%) of the original charge. The CPM also can be used to measure the triboelectric charging of floor finishes, rugs, mats and ESD foot gear and to demonstrate induction charging.
To use the CPM for measuring charges requires a connection to the monitor plate. Before taking measurements, attach your wrist strap to the charge plate and set the plate for the float mode. Usually a wrist strap and a cord without a resistor are sufficient.
An essential part of static identification and control is the electrometer and the Faraday cup. Typical applications include testing the effectiveness of static-prevention materials, such as antistatic IC shipping tubes, antistatic bags and packaging materials.
ESD Simulators
A final category of ESD test equipment is the ESD simulator. It replicates the effects of ESD that occur during manufacturing and assembly. There are several models, including the human body model (HBM), the charged device model (CDM), the field-induced model, the machine model, the body-metallic model and the capacitive-coupled model.
The most familiar are the HBM and CDM. The HBM simulator produces an electrical pulse that resembles a charged person touching the lead of a grounded device. For this simulation, typical instruments use an RC network with a 100-pF capacitor and a 1.5-k W resistor.
The CDM simulator typically charges a device lying upside down on a 1/32″-thick insulating material by applying a voltage through a high-resistance probe and then discharging it to ground through a relay. The potential and the energy depend on the position and orientation of the charged device with respect to ground.
There is a significant amount of equipment on the market that performs specific functions, which can make informed decisions about buying tough. It is important to know exactly what those functions are and what they mean to you, said Ms. Becker. It also helps to know which test method the instrument meets.
Reference
1. Dangelmayer, G. T., ESD Program Management, Van Nostrand Reinhold, 1990.
ESD Test Equipment Products
Meter Measures Static
Charge to 19,990 V
The ACL 600 STATI-CHECK Meter measures static charge levels on personnel in a protected area. It shows charge levels between 0 and 19,990 V. The unit also determines the difference in static potential between two people. The meter indicates how well wrist straps, coil cords, ground leads, bonding plugs and heel straps are functioning. $318.50. ACL, Inc., (800) 782-8420.
ESD Meter Is Furnished With
Certificate of Calibration
The EOS 100 Electrostatic Field Meter is supplied with a certificate of calibration traceable to NIST standards. It indicates the presence, magnitude and polarity of static charges from ±5 V to ±3,000 V with four operator-selected sensitivity ranges. The unit measures typical workstation static charges using the ±100-V range. $395. Chapman Corp., (207) 773-4726.
Tester Checks Static
Shielding on Metallized Bags
The A50030 Shield-Check Bag Tester checks the ESD effectiveness of a metallized bag. A metal electrode pulses a 1,000-V discharge and any voltage that penetrates the bag is measured by a battery-operated sensor. LEDs indicate the measured voltage level . The unit provides eight detection levels from 10 V to 300 V. It simulates EIA-541 capacitive probe test procedures. $1,200. Desco Industries, Inc., (909) 598-2753.
Software Helps Store
Field Meter Readings
The JCILOG software package interfaces with the company’s JCI 111, JCI 121, JCI 140 and Model 404 Field Meters as well as the JCI 155 Charge Decay Test Unit. It emulates a paper chart recorder but includes real-time display and provides time-referenced storage of data. Data is recorded in a background mode, allowing inspection or editing of previous information. Data storage time steps are set from 0.25 s to 100 s. Stored information loads into spreadsheet programs for numerical analysis, graphical display and printing. John Chubb Instrumentation, (011) 441 1242 573347.
Monitor Locates and Measures
Static on Packaging Materials
The Model 281-1 Static Locator performs noncontact measurement of static on packaging materials and circuit assemblies. The meter offers a hold function to freeze the display and a low-battery indicator. The digital unit has a range of 20 kV at 1″. An increased range at a greater distance is available. $350. Monroe Electronics, (800) 821-6001.
Kit Provides Instruments
To Perform Field Measurements
The PROSTAT® PMK-150 Basic Resistance Kit contains the basic electrostatic instruments and accessories to perform general resistance and field measurements. The PFM-711A Field Meter is used to locate and measure electrostatic fields on charged surfaces. The PRS-800 Megohmmeter measures resistance from 1.0 x 104 W to 1.0 x 1011 W . Five-pound electrodes are furnished with low-resistance conductive rubber footings. The PHT-770 Hygrothermometer also is included for temperature and humidity readings. $1,895. PROSTAT® Corp., (708) 238-8883.
Copyright 1995 Nelson Publishing Inc.
July 1995