Introduction
Welcome back! In our last post, we explained how it is possible to measure charge on the body without a physical connection to ground. It relies on the fundamental electrostatic principle that the local charge density and electric field are proportional on the surface of a conductor. So, by measuring the field, we obtain a direct measure of charge. And ultimately, it is charge that is responsible for any electrostatic discharge events that may damage your electronics.
Charge to Body Voltage
But, we decided that the quantity ‘nanoCoulombs per meter squared’ doesn’t offer a lot of intuition about one’s propensity for generating ESD events. With that said, even a few nanoCoulombs of charge transfer can absolutely fry a vulnerable circuit!
A more familiar measure is body voltage, since this is how thresholds for personnel are defined by the ESDA’s S20.20 document and other ESD Standard used across the world.
When we compared the charge density output of the StatIQ Band to conventional body voltage measurements, it turned out that they track with great linearity.
We won’t delve too deep into the math here, but the reason for this is: as the body’s voltage increases, the charge everywhere increases uniformly, as does the measured E-field. (This relationship holds true even with variations in body size because the body capacitance and surface area roughly cancel each other out).
Comparison to Existing Standards
The following plot shows the results of a ANSI/ESD STM 97.2 Body Voltage Walking Test, measured with the StatIQ Band and a conventional body voltage measurement probe. The charge measurement was linearly scaled to match the absolute body voltage, but otherwise unaltered.
When the measurement differs
You might think – “wait a minute, we learned in an earlier post that charge can be distributed non-linearly, especially when it is induced by nearby objects.” This is correct. In the case of the charged person standing near a wall, the StatIQ Band outputs a different result depending on which arm the user is wearing the Band.
This is why the output of the StatIQ Band is an Equivalent Body Voltage, which can be read as – “the Band is measuring the amount of charge that a person standing in open space has on their arm when at body voltage V.” This is a mouthful, but it’s a similar concept to using a static field meter to measure charge on an insulator. The output might be in Volts, even though voltage isn’t well defined on an insulator.
Even when the extrapolation to voltage doesn’t hold perfectly, the StatIQ Band always yields an accurate measure of the local charge, which is more indicative of impending ESD events than body voltage. And when the field measures zero, there is no local charge on the arm and it is impossible for an ESD event to occur!
Detecting ESD events
Speaking of ESD events, one of the coolest features of the StatIQ Band is its ability to detect low-voltage ESD events. The onboard field meter is sampling with such a high frequency (1200 Hz) that it is possible to identify rapid changes that can only happen by a sudden transfer of charge from a grounding event. The screenshot from our StatIQ Mobile App shows two of these 200-300V ESD events as red dots.
Ultimately, the goal of having an ESD compliance program in place is to prevent unnoticed low voltage discharges that cause invisible damage to electronics. At IONA, we believe that the first step in preventing these occurrences is the ability to reliably identify them, which the StatIQ Band is uniquely capable of doing. As you can see in the chart, the Band picks up low voltage ESD events that won’t be felt as a shock, because discharges become perceptible only around 2000V.
Conclusion
IONA Tech's wearable measurement equipment represents a significant advancement in ESD mitigation. By focusing on charge measurement and offering real-time monitoring, we provide a more comprehensive and proactive solution. This evolution in the industry is supported by solid scientific principles, even if it requires a shift in perspective. In our next post, we'll discuss the broader implications of this technology and how it can transform ESD protection practices.
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