Characterizing Chairs for Use with Static Control Safe Workstations
According to NASA-STD 8739.6, “Implementation Requirements for NASA Workmanship Standards,” the relative humidity (RH) range within an ESD control area shall be between 30% and 70%. The first phase of our testing was performed at 50%+/-3% RH and the second phase was performed at 30% RH.
The ESD safe chair results were compared with a standard (non-ESD) conference room chair as illustrated in Photo 1. In our tests, a person would sit down and stand up from both types of chairs wearing a wrist strap on an ESD safe mat, followed by the same test using the same chairs placed on an insulative carpet. Photo 2 illustrates the different ESD readings from the non-ESD chair and the ESD chair. It is clear from Photo 2 that the Non-ESD chair is insulative and the ESD chair is static dissipative per ANSI/ESD STM 12.1 at 50% RH.
As a baseline for comparison, the non-ESD chair charged at 50% RH to -591 volts, while the ESD chair charged to a peak of -15 volts (see Figure 1). A limit of <200 volts is called out in ANSI/ESD S20.20-2014 (Table 3 of the standard) under ANSI/ESD STM4.2. Some organizations that handle Class 0A (<125 volts) ESD sensitive devices mandate not more than +/-100 volts at the ESD safe work station.
To represent actual conditions of an ESD safe work surface, computer interfaced “hands free” ESD sensing instruments were placed 12 inches from the edge of the work station as illustrated in Photo 3.
Testing took place in the following manner:
- Test ESD safe and non-ESD Chair per ANSI/ESD STM12.1 for resistance mapping at <1.0 x 109 ohms.
- Test ESD safe chair on a static control floor mat after standing up and sitting down while wearing a wrist strap (Chair is pushed back into the workstation).
- Test non-ESD chair on a static control floor mat after standing up and sitting down while wearing a wrist strap (Chair is pushed back into the workstation).
- Test ESD safe chair on an insulative carpet after standing up and sitting down while wearing a wrist strap (Chair is pushed back into the workstation).
- Test non-ESD chair on an insulative carpet after standing up and sitting down while wearing a wrist strap.
- ESD event antenna affixed to non-ESD chair to measure ESD during standing up and sitting down at 30% RH.
Through this series of tests, it became clear that voltages for both ESD safe and non-ESD chairs were less than +/-200 volts when the user was connected to a wrist strap. Table 1 illustrates the findings.
As can be seen in Figures 2 and 3, neither the ESD safe chair nor the non-ESD chair produced higher electrostatic fields at 50% RH. The electrostatic field meter did measure voltages greater than the proximity field antennas.
non-ESD chairs on ESD mat and insulative carpet.
Note: Blue = positive voltage, Green = negative voltage
As shown in Table 2, the ESD safe chair did not produce ESD events which were negligible at 0 volts. The non-ESD chair did produce ESD events when an operator was grounded but only at a peak of 9 volts. As shown in Figure 3, for each ESD event, there was a corresponding low electrostatic field.
In short, at 50% RH when an operator is wearing an ANSI/ESD S1.1-2013 wrist strap, the occurrence of high level electrostatic fields and ESD events measured at 12” were within acceptance levels for many organizations.
What happens with a non-ESD chair at 30% RH, 74°F using an ESD antenna connected to an oscilloscope?
The test set up shown in Photo 4 represents a person without a wrist strap. A coaxial cable (50 ohms) is interfaced with a 6” stiff wire as an extension of the center conductor through the BNC (Bayonet Neill–Concelman). The 6” length tunes the antenna to about 500 MHz, the bandwidth of the scope used. An insulative chair may generate radiation and this would be picked up by the wire antenna.
An operator stood up and sat down while allowing the chair to move a distance of 12” on a wool rug and plastic protective chair runner. Unlike the previous test, the operator is ungrounded, a factor which can certainly influence the readings.
Consequently, the reader can see from Figure 6 that the electrical signal from the stiff wire antenna goes off the scale of +/-400mV. This occurs from foam compression and expansion of the cushioning in the non-ESD chair, inducing a spark between two metal pieces of the chair that are not in contact but form a spark gap.
The foam compression and expansion causes the charge on the surface of the seat of the chair to move and the moving charge induces a spark within the chair. Thus, in mission critical aerospace systems, such as a satellite assembly, or in proximity to very sensitive devices like GMR disk drive heads, the induced voltages and currents from the fields radiated from the chair into nearby equipment could be catastrophic.
In a repeat test, the vertical scale was increased from 100 mV/div to 500 mV/div so the waveform would remain visible on the screen. The voltage recorded from the antenna was about 1.3 volts peak into 50 Ohms due to radiation from the chair as illustrated in Figure 7.
At -1.3 volts (20 NS per division), the radiation could damage exposed disk drive heads and possibly other sensitive components. Radiated EMI is known to create soft errors and lock ups in equipment.
Organizations should specify a chair that does not promote radiated EMI/RFI due to ESD. In addition, standards work in this area is needed.
In conclusion, both testing series indicate more work needs to be done in conducting tests at 30% RH or below to determine if similar findings can be secured at low RH conditions.
