ESD Journal - Monitoring Ground

Continuous monitoring of workstation and employee grounding is necessary to maximize the safety of ESD-sensitive parts.

Monitoring of operator and work station grounds is needed throughout the manufacturing environment when handling ESD-sensitive devices, and indeed, it is often required by government contract or in-house practice. But depending on the sensitivity of particular parts, as well as local needs and practices, some types of monitoring may be more useful than others, and continuous monitoring of grounder integrity is often favored over periodic monitoring.

Periodic monitoring detects ineffective grounders, and regularly checking one's ground strap and cord may help to instill static awareness in employees. But the periodic approach doesn't sense failures immediately, missing those that occur between tests. By contrast, constant monitoring offers signals that immediately alert operators to intermittent or open grounds.

Long-tern cost savings can result from constant monitoring, reflecting the higher quality and lower rejection rates of devices produced when it is implemented within a facility. In fact, a company should begin to see quality effects as soon as constant monitoring is implemented, and the flaws of existing grounders and ground systems are detected for the first time.

Not unexpectedly, the government realizes the quality, cost, and also the reliability benefits of monitoring and frequently included monitoring provisions in its contracts. These contracts often reference DOD or other such specifications.

For example, DOD-HDBK-263 states that "periodic continuity and resistivity checks of personnel ground straps between skin contact point and ground connection, ESD-grounded workbench surfaces, conductive floor mats and other connections to ground should be performed periodically with suitable test equipment." (1)

If we agree that monitoring is useful and necessary, we must then compare the sorts of monitors available.

The most commonly used test device is the 9-V wrist-strap checker. This device tests the total resistance of a circuit, including the contact resistance at the skin/cuff interface. Both skin resistance and contact resistance will vary over a wide range due to chemical factors, moisture, and the amount of body hair between cuff and skin. (2)

After complaints about two types of 9-V testers form personnel at a General Dynamics manufacturing site, we researched the problem and scanned the market for testers and monitors that would satisfy their various contractual requirements.

The most basic 9-V testers can give misleading results. Today, it is widely accepted in industry that a tester powered by 10 V or less can give variable readings and is therefore not always indicative of the actual resistance to ground provided by a strap. (3)

Some periodic testers use 20-V power supplies. They are identical to 9-V testers in concept (i.e., checking total resistance), and they normally overcome the galvanic (human-battery) effects inherent in 9-V testers.

The 20-V testers also offer several tangential benefits. Since employees must test their grounders at least daily, and preferably more often, a failure would be detected after a single shift. Periodic testing can also remind operators to protect ESD-sensitive devices and thus might act as part of an awareness program.

On the other hand, periodic testing can eventually lead to boredom and inattention. Testing logs must be kept up-to-date, and supervisors must spend time verifying that operators are monitoring themselves and make sure that the logs are available for surprise ESD audits.

But the biggest drawback to periodic testing with most 20-V monitors is that the test takes place away from the work station; consequently, local pathways to ground must still be tested to determine if an individual is grounded.

Lastly, ground integrity is uncertain when the resistor, cord, connector, or the ground itself is intermittent. None of these items will be discovered immediately with periodic monitoring and only one of them, the open resistor, has a good chance of being detected as faulty.

The next type of tester is more generalized and much more economical. Affordable for every work station, the AC-line powered touch-and-test unit uses a small metal plate, which the individual touches from time to time to determine whether he or she is properly grounded. This tester also can check soldering irons, work surfaces, mats, and other ESD materials.

A disadvantage is that the operator must remember to touch the unit periodically and often must keep some kind of log, which can be time-consuming and bothersome. There must also be an electrical outlet available at the workstation and within easy reach for this kind of tester to be used.

Another useful device is the watchband type with a liquid-crystal display; it shows "OK" if the individual is properly grounded.This design merits consideration and is worth testing in your facility.

However, and drawback is that the monitor is worn on the wrist, and the operator must constantly look to see if he or she is properly grounded. The need for such continual policing could cultivate indifference. Adding an audible alarm to the device would make such monitors even more attractive since individuals would know of a ground failure immediately.

All of the previously mentioned devices have one important trait in common, namely that they are periodic in nature, and, as a result, have some disadvantages. They cannot achieve maximum reliability because an operator does not know immediately when ground integrity is lost. In a lapse of time before this discovery is made, ESD-sensitive parts might be damaged. Conversely, constant monitoring immediately signals when continuity or ground is lost, thus enhancing reliability.

There are at least four different types of constant monitors available and numerous companies are introducing new or improved products. The basic categories are these:

1. Transmitter-type units that see the body as a load or antenna.

2. Dual-cord units.

3. Combination capacitance-and-resistance monitors that check performance or cord, strap, and body as a whole.

4. Combination units with additional circuitry to monitor the resistance-to-ground of the worksurfaces.

All four types monitor ground integrity continuously, a matter of primary importance in high-reliability applications. However, some of the four have features that may make them less useful in some applications.

The transmitter-type device can be extremely sensitive and generate false alarms, which can confuse and irritate the operator.

Dual-cord monitors have two ground cords instead of one. Some dual-cord monitors use comparator circuitry to avoid the false alarm indications sometimes generated when ground cords rest on grounded worksurfaces. Others make each cord part of a loop and monitor total loop resistance.

Because both types of dual-cord monitors detect differences between the two cords, an alarm indication should precede any overall grounder failure. The monitor detects high resistance or an open circuit in one cord while the other is still intact, safely bleeding charge to ground.

Unfortunately, such monitors occasionally meet employee resistance since personnel must have two cords leading from their band, and one is often enough to make some operators feel "chained" to the workstations.

And finally, because the cord and its connectors form the least reliable part of most grounders, dual-cord monitors may give frequent alarms and require frequent cord replacement, although ground integrity itself won't normally be impaired.

As a result of this survey, the following items are considered of primary importance for proper operation and acceptance of constant monitors:

1. Monitors must be securely attached to the workstation for unobstructed viewing. Banana-jack boxes must be securely fastened to prevent the unit from falling or becoming lost.

2. Monitors must detect and warn of open circuits and high resistance-to-ground immediately.

3. Monitors must generate both visible and audible signals so that personnel can sense loss of grounding.

4. Monitors should accept a resistance range of 1M ohm to less than 10 M ohm resistance-to-ground

5. Monitors must have replacement parts that can easily be removed or interchanged.

6. There should be a warranty during which dependability and quality construction can be verified.

Monitors should also be able to pass the following tests:

1. Lifting one's feet off the floor should trigger the alarm, or not, as appropriate to the grounder type in use.

2. Simply resting the ground cord on a worksurface should not indicate effective grounding.

3. Change of operators should not, of itself, trigger an out-of-limit alarm.

4. A monitor should not leak significant current to the user.

5. Monitor performance should not drift over time.

6. The monitor's audible alarm should be loud enough to be noticed but should be adjustable so that employees with both sensitive and impaired hearing could use similar monitors.

7. Finally, monitors should work well with the type of grounding strap in use.

Some monitors work better with metallic bands than with fabric ones. In this case, they should be used with metallic bands that offer a wide surface area for minimal contact resistance- a good practice in any case.

If these conditions are met, and appropriate wrist bands are selected and worn properly, a constant monitoring system will generate both tangible and intangible savings. For example, in one year, an aerospace firm netted hundreds of thousands of dollars in savings following the installation of constant monitoring equipment.

Intangible savings also stemmed from prevention of the ESD strikes formerly incurred when operators failed to be grounded.

References
1. DOD-HDBK-263, p. 51.
2. W. Hunter, "construction Analysis and Testing Recommendations," pp. 11-12.
3. J.R. Huntsman, D. Yenni, Jr., "Test Methods for Static Control Products," EOS/ESD symposium Proceedings 1982, pp. 11-12.

Note: This paper draws upon work the author conducted while employed at General Dynamics, Pomona, CA, and is a revised version of a paper originally presented at the 1987 EOS/ESD Symposium.