by Doug Smith
Abstract: IEC 61000-4-2, the equipment level ESD testing standard, recently has been applied to individual solid state devices, a controversial use of the standard and despite there being no instructions in the standard on applying it to individual devices. In addition, some buyers are also insisting on an older 150 pF/150 Ohm network instead of the current 150 pF/330 Ohm network. Waveforms are presented comparing the current discharge of the two networks as supplied with a modern ESD simulator.
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Figure 1. Test Setup for Measuring ESD Current From an ESD Simulator
Abstract: IIEC 61000-4-2, the equipment level ESD testing standard, recently has been applied to individual solid state devices, a controversial use of the standard and despite there being no instructions in the standard on applying it to individual devices. In addition, some buyers are also insisting on an older 150 pF/150 Ohm network instead of the current 150 pF/330 Ohm network. Waveforms are presented comparing the current discharge of the two networks as supplied with a modern ESD simulator.
Discussion: Figure 1 shows the overall test setup for measuring the current waveform. The simulator ground lead was connected to the metal plane on the top of the table, the Horizontal Coupling Plane of the IEC 61000-4-2 test (as opposed to performing the test on the floor reference ground plane). The test was done near the edge of the table so the waveforms will not be exactly the same as the calibration waveforms, but a valid comparison of the two networks can be made. This same configuration was used to test a number of solid state devices. Current was measured with a Fischer F-65 current probe and Agilent 54845a oscilloscope.
Figure 2 shows a close-up of the measurement. The discharge tip of the simulator easily fits through the F-65 current probe. I am holding the current probe slightly off the ground plane so as to minimize common mode ESD current from traveling down the coax shield to the scope. There were several ferrite cores on the coax at the scope to suppress common mode ESD currents as well. The coaxial cable used was RG-142B/U. Its shielding effectiveness is extremely good and RG142B/U coaxial cable is well suited for making ESD measurements with the F-65 one GHz current probe.
Figure 3 shows the discharge current for a 1 kV contact discharge using the 150 pF/330 Ohm network and Figure 4 shows the discharge current with the 150 pF/150 Ohm network. There are both significant differences and similarities in the two waveforms.
Figure 3. Waveform for 150 pF/330 Ohm network
Note that the that the initial current peak and its di/dt are about the same for both waveforms at about 3 Amperes, but the slow part of the waveform, current level lasting tens of nanoseconds, peaks at about 3 Amperes for the 150 Ohm network compared to about 2 Amperes for the 330 Ohm network. As expected, the 150 Ohm network delivers significantly more energy to a device compared to the 330 Ohm network.
The "hash" starting about 7 ns before before the current waveform is due to EMI from the ESD simulator and current discharge radiating directly into the scope. The hash starts 7 ns earlier because the air path to the scope has 7 ns less delay than the current waveform traveling down the coaxial cable to the scope. This can be reduced to insignificance by averaging several waveforms. An example of this approach will be covered in next month's Technical Tidbit for December 2010.
Figure 2 shows a close-up of the measurement. The discharge tip of the simulator easily fits through the F-65 current probe. I am holding the current probe slightly off the ground plane so as to minimize common mode ESD current from traveling down the coax shield to the scope. There were several ferrite cores on the coax at the scope to suppress common mode ESD currents as well. The coaxial cable used was RG-142B/U. Its shielding effectiveness is extremely good and RG142B/U coaxial cable is well suited for making ESD measurements with the F-65 one GHz current probe.
Figure 2. Close-up of Current Measurement
Figure 3 shows the discharge current for a 1 kV contact discharge using the 150 pF/330 Ohm network and Figure 4 shows the discharge current with the 150 pF/150 Ohm network. There are both significant differences and similarities in the two waveforms.
Figure 3. Waveform for 150 pF/330 Ohm network
Figure 4.Waveform for 150 pF/150 Ohm network
Note that the that the initial current peak and its di/dt are about the same for both waveforms at about 3 Amperes, but the slow part of the waveform, current level lasting tens of nanoseconds, peaks at about 3 Amperes for the 150 Ohm network compared to about 2 Amperes for the 330 Ohm network. As expected, the 150 Ohm network delivers significantly more energy to a device compared to the 330 Ohm network.
The "hash" starting about 7 ns before before the current waveform is due to EMI from the ESD simulator and current discharge radiating directly into the scope. The hash starts 7 ns earlier because the air path to the scope has 7 ns less delay than the current waveform traveling down the coaxial cable to the scope. This can be reduced to insignificance by averaging several waveforms. An example of this approach will be covered in next month's Technical Tidbit for December 2010.
Summary: Comparison of the current discharge from both 150 pF/150 Ohm and 150 pF/330 Ohm networks in an ESD simulator shows that the 150 Ohm network delivers significantly more current and energy to the device under test but that the initial fast current peak is about the same in amplitude and di/dt.
