The evaluation of these important items requires good ESD test equipment. Indeed, this is a necessary adjunct to all the details of static control. Installation, daily monitoring, and periodic audits require state-of-the-art instruments to validate all the safeguards you have put into place.
To help you reach this goal, test standards developed by the ESD Association define three performance levels for test instruments: laboratory, acceptance/qualification, and audit. Each is differentiated by accuracy, range, size, and price.
A laboratory-grade instrument, probably a high-precision bench- or rack-mounted unit, is computer-controlled, quite complex, and possibly difficult to use. The acceptance/qualification instrument, portable or bench-mounted and somewhat simpler to use, likely incorporates a computer-compatible data port. Audit-grade equipment is very small, battery-operated, portable, and easy to use. The prices of the instruments vary with their complexity.
The three grades of performance can be illustrated by resistivity measuring equipment. “The range of a laboratory-level instrument typically is quite wide,” explained Stephen Halperin, president of Prostat. “It may measure 102 W to 1015 W, and the accuracy can be ±5% or better. The instrument could have self-contained calibration capability, and you can expect to pay $3,500 to $10,000 for it.
“The measurement range of the second category can be 103 W to 1011 W, and typical accuracy is 10% to 15%. The equipment can be mastered easily by an unskilled operator, and readings should correlate within 10% of their laboratory-level cousins. The price may be $600 to $1,500,” he said.
“Probably the simplest one is a set of decade-indicating LEDs. Resolution is generally one-half decade or less. These sell for just a few hundred dollars,” Mr. Halperin concluded.
This delineation of equipment is quite significant. Unfortunately, many of the instruments used now for measuring resistivity to ground or point to point on floors or worksurfaces don’t meet laboratory-level or acceptance/qualification-level criteria and should be replaced to maintain the integrity of ESD control programs.
To illustrate this point, the new ESD standards for resistivity testing require a constant-voltage system so the measurement potential does not drop when used in the rated resistance range. An audit-grade instrument may use open-circuit voltage but its readings may not agree with values obtained with constant-voltage systems.
“In case of a difference in readings, the laboratory-level instrument must serve as the standard,” noted David Swenson, the technical service specialist at 3M Electronic Handling & Protection Division. “Disagreement is not common, but it occurs sometimes when materials have resistance properties near the line that separates the conductive and dissipative ranges.”
Equipment Designs Driven by 20.20
It is no coincidence that the new ESD Association Document 20.20, ANSI/ESD S20.20-1999, The ESD Association Standard for the Development of an Electrostatic Discharge Program for Protection of Electrical and Electronic Parts, Assemblies, and Equipment, Excluding Electrically Initiated Explosive Devices, guides the design of today’s test equipment.
“It is the most important document for a static-sensitive industry,” said Mr. Halperin of Prostat. “It references all current standards as recommended guidelines for evaluation, installation, and periodic measurement of ESD-related materials and products. As a result, the search for measurable improvements in productivity, quality, and profits has intensified the demand for high-performance ESD control materials and instruments.”
Mr. Swenson of 3M agreed. “Documents such as 20.20 are definitely forcing instrument manufacturers to shape up. Providing the end user with the ability to run simple verification tests of instrument functionality is a basic requirement of the standard. It also specifies that the user establish a compliance verification plan and prove that instruments are working as specified.”
Designs Driven by Technology Advances
Since the role of ESD test equipment is to support the manufacture of electronic equipment, it follows that advancements in electronics technology on the production line are triggering some ESD test-equipment upgrades. For example, changes in ESD sensitivity for disk heads and high-performance semiconductors have prompted the redesign of ionization monitors for low-voltage measurements, according to Jim Hemingway, director of sales and marketing at Monroe Electronics. “We now use a contacting electrode instead of the field meter to adapt our products to these conditions,” he said.
This observation was echoed by Don Pritchard, an ESD applications engineer at Trek. “Because of the growing need for better accuracy in monitoring ionizer performance, we stopped using a field meter on the charged plate monitor. Instead, we use a high-voltage follower amplifier. We can reduce drift and error to less than ±1 V and provide suitable bandwidth for measuring the ripple from AC ionization. This also lets us use small plates so measurements can be made in tight places.”
Automated techniques for auditing conformance to ESD control standards continue to grow in popularity. For example, computers monitor ionizer operation. “At one time, it was common for a company to have just a few ionizers in the facility,” noted Mr. Hemingway of Monroe Electronics. “Monthly audits involved running a series of tests on each ionizer and writing the results on a pad. With just a few stations, this was not too difficult.
“Now a large manufacturer may have hundreds or even a few thousand ionizers,” he continued. “Monthly audits the old way are virtually out of the question. We have developed the capability to put a monitor under the ionizer, push a button, and have the instrument go through an automated test sequence for data collection and storage. The result can be analyzed by a central computer and evaluated by an experienced engineer.”
Tracking the performance of straps is automated as well. “We see a great future for a product that automatically monitors foot straps and wrist straps, shows the conditions, and simultaneously sends all signals to a central computer,” Fruhar Alavi, the president of Statico, commented. “This way, the bench supervisors don’t have the hassle of record-keeping. All test results are available to management for analysis, problem identification, and corrective action.”
What comes next? As always, ESD control specialists will continue to look for better ways to verify that their equipment and procedures are working properly. As one example of such vigilance, Brian Hohman, North American sales manager at Electro-Tech Systems, noted an increase in purchases of simulators during the past few months. This indicates that users are determined to validate ESD test equipment to get the most from it.
We also can expect instruments to be designated as laboratory-level and acceptance/qualification-level. This will reduce the amount of interpretation needed to select testers and allow manufacturers to set more stringent specifications for their higher-level instruments.
What about the 20.20 standard—is it set in concrete? Of course not. It will evolve to meet the needs of the static-control community. As Mr. Halperin of Prostat predicted, “It’s only a matter of time before constant voltage becomes an industry-wide requirement in support of 20.20 resistivity-related measurements.”
Published by EE-Evaluation Engineering
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