Understand the differences in ESD tests

Martin Rowe, Senior Technical Editor -- Test & Measurement World, 10/21/2008 12:58:00 PM

Whenever an electrically charged object discharges, it produces an ESD (electrostatic discharge) event. An ESD event can subject an electronic device to thousands of volts and several amperes. High voltage can cause breakdown in a device structure and high current can cause heat that damages the device.

The current discharged into a device depends on the ESD voltage and on the characteristics of the device’s current path. But, the amount of current that passes through a device can be drastically different for a given discharge voltage, depending on the test method. Unfortunately, the differences in current for a given voltage are not always well understood.

“ESD Device testing: The test determines the result,” a paper by Robert Ashton, senior protection and compliance specialist at ON Semiconductor, explains that the amount of current that can pass through a device depends on the test you apply to it. The paper covers the three test methods—HBM (Human-Body Model), MM (Machine Model), and CDM (Charged-Device Model)—by showing each model’s test circuits, which are designed to produce consistent test results.

“CDM produces a much shorter current pulse than either HBM or MM, but it produced a substantially higher peak current,” notes Ashton in the paper. “HBM and MM can deliver more energy to the device under test, but high peak currents are often the best predictor of damage.” Using waveform plots on different time scales, Ashton shows the differences in each model’s waveform. The paper also compares the current waveforms produced by tests that comply with two ESD system-level test standards, IEC 61000-4-2 and ISO 10605, which use different resistor and capacitor values in their test circuits.