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a) Conductor

b) Human Body

Anonymous, Ampang, MalaysiaWe have the standard cloth roll-a-round adjustable type for all the production lines. We also have constant ESD monitors installed at all the ESD workstations. I was asked to check on the chairs that are listed as static controlled with a drag chain. We don't have any ESD wax or tile on the floors, just a grey painted surface. I know that with conductive casters the chairs won't generate static like the typical chairs do, but there are some that think you don't need to wear a Wrist strap if you are sitting in one of these "ESD Safe chairs. I'm having a hard time convincing them otherwise, can you help me out. –Anonymous, Radio Frequency Systems, Phoenix, Arizona

A quick answer is:

Very very fast for metal ( ~ 2x10

^{-18}seconds)

Sort of fast for human body (> 2x10

^{-6}seconds)

A more specific answer is: assume the worst case, class 0, which has a 0 to 249 Volt tolerance. Applying the HBM, a conservative worst case capacitance would be 200 pF, twice that of the HBM and resistance of 10K? . Therefore the maximum power (P) level based on Ohm’s Law is P=V2/R (J/s) and the worst case HBM is ((249)2/10K)=6.2 Watts or Joules per second (Js-1).

The maximum energy (E) stored in a worst case HBM capacitance (C) of 200 pF and at a maximum voltage (V) of 249 Volts, (using E=1/2 CV2), yields 6.2 ? J. The next concern is to relate energy to time. The time constant (? ) is the measure of the length in time, in a natural response system, for the discharge current to die down to a negligible value (assume 1% of the original signal). For an RC circuit, the time constant (? ) is equivalent to the multiple of the equivalent resistance and capacitance. In this case, the time constant (? ) of our RC circuit is (10K? )(200pF) or ? = 2 ? s. Discharging this energy upon touching a conductor at zero volts yields a current, (using I=P/V), of (6.2Js-1)/(249V) or 24.8 mA. To avoid damaging a class 0 ESDS device, the discharge current must be below 24.8 mA. Engineering in a "2x" safety factor, the maximum discharge current would be 12.4 mA. To maintain a discharge current below 12.4 mA, we need to look at our grounding equipment on the ESDS workbench.

The rate at which 6.2 ? J of energy discharges is very important. To fast a discharge will lead to an ESD Event, which can electromagnetically be measured using a simple loop antenna attached to a high impedance input of a high-speed storage scope. The faster the discharge the greater the discharge current becomes as well as the emf (electromotive force) on the loop antenna from the EMI (ElectroMagnetic Interference). Table III depicts the discharge current for 6.2 ? J at varying discharge times. We are assuming lossless conditions during the discharge for worst case. For our example, to keep the discharge current below 12.4 mA, the discharge rate [from Table III] must be no quicker than 2.01 ? s. This energy-based-time constraint forms the lower boundary of the controlled discharge rate.

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