By Fred Tenzer
There are essentially two types of technologies used for monitoring operator wrist straps: Single-Wire Impedance or Dual-Wire Resistance Monitors. Both technologies monitor the function of an operator’s wrist strap, but it is important to understand how these technologies work and the shortcomings of each. Here is some basic historical information on the development of both technologies that will make my “preferences” a little more clear. If you choose to just know the answer to which technology I prefer, scroll to the bottom. All workstation monitors discussed will also monitor the worksurface ground circuit and while there are some differences, the worksurface part of the monitoring system is NOT discussed here.
When companies realized that operator grounding was the foundation of an ESD Control Program, wristbands and coil cords became the most common way to reliably ground an operator. The first ESD Association Standards meeting was held in 1982 and the first standard produced by the ESD Association was for wrist straps. The weakest part of the system was the coil cord and testing was developed for bending, also known as flex-life testing. The minimum flex life was established at 16,000 flex cycles and in the mid-1980’s it was tough to achieve.
Initially, wrist straps were tested while worn with some type of touch testing system where the operator would touch a button to activate a test of the wrist strap. Wrist strap monitoring technology was developed to detect initial flex fatigue while it was still in the “intermittent” stage, which is prior to a permanent “open” being created. Monitors provided operators with instant feedback on the status and functionality of their wrist strap. The normal periodic touch testing would almost never detect an “intermittent” failure. In addition, if a wrist strap system was touch-tested twice a day and an operator passed at 1:00 PM on Monday and failed at 8:00 AM on Tuesday, all the work that had been performed at that station after 1:00 PM on Monday would now be suspect and would be a cause for more detailed quality inspections by many companies. While discovering an “operator grounding problem” was a good thing, it was also costly due to increased quality control requirements.
Simple “AC capacitance” single-wire monitors were the first type of monitors to be developed. There were many shortcomings of the technology stemming mostly from false negatives (alarms sounding when they should not) and false positives(unit indicating the operator was grounded when they were not). This technology is still around today and is purchased by companies because of its low cost ($40-$50 per operator). The next stage in the single-wire evolution was “impedance” technology. Since the capacitance and therefore the impedance of the circuit will also vary with such things as the person’s size, clothing, shoe soles, conductance of the floor, chair, table mat, etc., these monitors often have to be adjusted or fine tuned to a specific installation and operator. Although impedance monitors have the same types “false positives” and “false negatives” as capacitance monitors, the technology was an improvement. Impedance technology is also still around as used due to its low cost ($50-$70 per operator).
The top of the line single-wire monitoring technology is referred to as “Wave Distortion” or vector impedance monitoring. Wave Distortion does not monitor at the impedance level, but at the waveform level generated by the circuit. Current will leak voltage at various points due to the combinations of resistance and capacitive reactance. There is a negligible amount of inductive reactance from the coil cord. By monitoring the “distortions” or phase shifts, Wave Distortion technology determines if the circuit is complete – the operator is in the circuit and the total equivalent DC resistance is within specifications. This technology is very reliable, with virtually no “false positives or “false negatives”. Additional benefits of Wave Distortion are the response time is very fast (<50 ms) and the wrist strap open circuit test voltage is very low at 1.2 volts peak-to-peak @ 1-2 Micro Amps. The cost for this technology is $125 – $140 per operator.
Dual-wire technology is a simple resistance measurement of the bridge formed by an operator wearing a wrist strap between the two leads in cord. The original dual-wire monitors used a 9 to 16 volt range in order to break the skin resistance. This level of voltage, if applied continuously, resulted in skin irritation. To eliminate the skin irritation, the voltage was pulsed at about every 2 seconds and voltage is applied for approximately 0.2 seconds. In order for the monitor to recognize an “Open”, the open would have to be measured over two pulse cycles and an “intermittent fail” would only be recognized if the duration was 2 seconds or longer. The ANSI/ESD S20.20, Appendix B describes the time needed for a ESD event to occur as follows “The discharge itself is a double exponential waveform with a rise time of 2-10 nanoseconds and a pulse duration of approximately 150 nanoseconds.” Since this is only 0.00000015 second, it is difficult to consider the pulsed technology being either “constant” or “continuous.” These pulsed style dual-wire monitors are still widely used and cost $100 – $150 per operator.
Recently the technology used in Dual-Wire Monitoring has improved to reduce the test voltage to as low as 50 mV to 1.25 volts and reduce the response time to <0.1 seconds. The new technology uses a dual polarity system that places a positive voltage on one lead and negative voltage on the other, resulting in near zero voltage at the operator. The constant test voltage on the operator may be no higher than 1.25 volts with no need for pulsing, resulting in a true constant monitor system. Additional benefits of the new dual-wire technology is that some of the monitors can detect Direct Current Voltage (VDC) greater than ±2.5 VDC that would be dangerous to an ESD susceptible item and some are equipped with ports for test data acquisition through software. The new technology of dual-wire monitors cost $175 – $300 per operator.
“Fred’s” Preference regarding Monitor Technology
Until recently, my preference for monitor technology has been the single-wire, Wave Distortion technology style. My preference was due to reliability, response time (really is constant, NOT pulsed), low voltage applied to the operator, and lower life-cycle cost. However with the improvements in dual-wire monitoring and with the new benefits of voltage detection and data acquisition, I am currently leaning towards the new style of Dual-Wire monitoring.
We know that new electronic devices are getting more susceptible to ESD, not less sensitive to ESD. There is a general demand to make electronic devices smaller, in turn device manufacturers are doing things such as removing “on-board” ESD protection circuitry to allow for more “device information real-estate” on the same size device and to increase device operating speed. Additionally there are more requirements for data tracking and corrective action reports from quality control departments of manufacturing facilities. The new technology of dual-wire continuous monitors has evolved to offer the same reliability benefits of the Wave Distortion technology and offer new features to meet to the increasing demands of ESD control for electronics manufacturers.
Fred is the National Sales Manager for the Desco Brand of Desco Industries, Inc. He is a founding member of the ESD Association (ESDA) and was a member of ESDA’s Standards Development for thirty years from 1982 to 2012. Fred was Vice-Chair of the ESDA’s Standards Committee. Fred can be reached at: Fred.Tenzer@Desco.com.