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Maximum Walking Voltage Range
Calculate the average and stand-ard deviation of maximum voltage peaks, multiply the standard deviation by three and add it to the average to obtain the probable top end of the maximum body-voltage range, then subtract three times the standard deviation from the average to obtain the probable bottom end of the maximum body-voltage range.

Typical Standing Voltage Range
Calculate the average and stand-ard deviation of minimum voltage values, multiply the standard deviation by three and add it to the average to obtain the probable top end of the standing body-voltage range, then subtract three times the standard deviation from the average to obtain the probable bottom end of the standing body-voltage range.

Referring to Figure 3, the maximum walking voltage is a negative value; that is, the footwear-floor combination generates a negative charge on the operator. At the moment the operator pauses after Steps 5 and 6, body voltage approaches and slightly exceeds the zero line. The average walking voltage is -47.29 V, and the standard deviation of the maximum peaks is 5.08 V.

Also from Figure 3, the probable maximum walking body-voltage generation range of plant personnel would approximate -32.0 V to -62.5 V. The minimum voltage would be about -5.5 V to 15.0 V while personnel worked standing at machines and workstations.

Knowing the probable voltage-generation ranges, we can adjust our process to maximize ESD protection from HBM events. For example:

• With a worst-case device HBM sensitivity of 100 V, we can safely handle the device while standing at the workstation or transport it through the plant environment where ESD footwear and floors are used.
• With an HBM device sensitivity of 25 V to 50 V, we can only handle the device safely while standing at the workstation but must place it in ESD protective packages or containers when transporting it through our environment.

Summary
Understanding personnel resistance to ground allows us to select and use wrist straps, footwear, and flooring in a manner appropriate to our most HBM-sensitive device and most compatible with our process. Simple resistance to ground measurements help verify that, once installed, these control products continue to perform at desired levels.

Certainly, the results of body-voltage generation analysis are based on ambient test conditions, operator care in performing the measurements, and the condition of footwear and flooring tested. However, this approach does characterize probable human-body voltage generation in a facility with greater than 90% confidence.

Given that we are aware of the voltage threshold of our most sensitive device, we can reliably predict the potential for HBM damage by human handling. Armed with this knowledge, we also can match our footwear to our floors to obtain the lowest possible voltage generation or know that we need to enhance our controls using effective static-control floor maintenance materials or transport procedures.

Once we have control of our HBM damage threat, we can focus on controlling CDM and MM failures to further reduce potential ESD losses.