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Douglas C. Smith

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Technical Tidbit - July 2011
Measuring ESD Stress on a PCB
(When Conventional Measurements Don't Work)

Measured ESD stress

Figure 1.
Measured Current Through a Protection Device on a PCB
(Vertical Scale = 50 mA/div, Horizontal Scale = 20 ns/div)

Abstract: Measuring voltage stress on components on a PCB under ESD conditions can be difficult as the ESD noise can inject error in the measurement. Often this error is larger than the voltage of the stress to be measured. A method of measuring voltage stress is discussed that has very high common mode rejection and low ESD induced error.

Discussion: ESD induced error makes accurately measuring voltage stresses on a PCB difficult. In addition to high common mode noise, high amplitude time varying electric and magnetic fields can introduce error in the measurement loop formed by the voltage probe and the nodes to be measured unless this loop is kept very small. One way to accurately measure high frequency voltage stress is to replace the component that is being stressed, such as an IC, with a TVS or similar protector connected across the two nodes to be measured. In series with the protector, form a loop as small as possible to insert a current probe to measure the current through the protector.

As the ESD or other pulsed voltage stress is increased, an increase in current through the parasitic capacitance of the protector will be seen that follows the stress applied. When the breakdown voltage of the protector is reached, additional increase in current may be noted and the waveform will quickly show the current waveform as the stress is increased further for a protector that clamps to a specified value. For a protector that latches to nearly a short after turn-on, like an SCR, the current at the point of latching will be seen. Figure 1 shows the current through a protector that clamps to a specified voltage in a circuit with applied stress from a Fischer Custom Communications model TG-EFT high voltage pulser. The protector capacitance is helping form a resonant response with circuit inductance. The current waveform reaches a peak of about 80 mA.

In Figure 2, the stress is doubled and the peak current reaches about 150-160 mA, approximately increasing linearly with applied stress and keeping the same waveshape except the ringing frequency is slightly higher.

Measured ESD current at 2X applied stress

Figure 2. Measured Current Through  Protection Device on a PCB at 2X Applied Stress
(Vertical Scale = 50 mA/div, Horizontal Scale = 20 ns/div)

For Figure 3, the stress is increased to 3X the value in Figure 1. Note that the waveshape is beginning to change although the peak current is about 3X of the current in Figure 1, about 230 mA.

Measured current at 3X applied stress

Figure 3. Measured Current Through  Protection Device on a PCB at 3X Applied Stress
(Vertical Scale = 50 mA/div, Horizontal Scale = 20 ns/div)

In Figure 4, the stress is increased to 4X the value in Figure 1. Now the waveshape is definitely changed and is beginning to look like the current waveform I expect at this point in the circuit. The peak current is now almost 300 mA.

Measured current at 4X stress level

Figure 4. Measured Current Through  Protection Device on a PCB at 4X Applied Stress
(Vertical Scale = 50 mA/div, Horizontal Scale = 20 ns/div)

The stress that just begins to breakdown the protector is that of Figure 3 where the waveshape begins to change. Note that we are not trying to accurately measure the current, so some electric field response of the current probe is acceptable, we are looking for a change in the waveshape of the current as the stress increases. The current probe should be as small as possible so as to minimize the loop needed to insert it into the circuit.

The lower capacitance the protector has before breakdown, the larger the difference in amplitude and waveshape will be noted at the point of breakdown. Also, if the capacitance of the protector is much larger than the original device it replaced, the capacitance may reduce the voltage across the protector resulting in a larger applied stress being required to breakdown the protector. This will result in measurement error that will understate the stress on the original device to be measured. In general, the lower capacitance the protector has the better, but for sure it should not be much larger than the original device it replaces in the circuit.

Summary: Using a current probe and a protector connected in place of a device on a PCB to measure voltage stress on the device is a useful way to minimize error from sources like ESD in the voltage stress measurement. The capacitance of the protector used should be minimized and be on the order of the original device capactiance. The loop formed for the current probe should be as small as possible.

Additional articles on this website related to this topic are:
  1. Current Probes, More Useful Than You Think (~170K)
    • (1998 IEEE EMC Symposium paper)
  2. March 2011, Using High Frequency Measurement of ESD Current to Find Problems with an ESD Simulator
Equipment used in this Technical Tidbit:
  1. Fischer Custom Communications F-65 Current Probe
  2. Fischer Custom Communications TG-EFT High Voltage Pulse Generator
  3. Agilent DSO5054A oscilloscope

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Copyright 2011 Douglas C. Smith