Of all of the component-level ESD tests available, the charged-device model (CDM) test is the closest to simulating real world events. CDM testing simulates ESD charging followed by a rapid discharge, similar to what is seen in the automated handling, manufacturing, and assembly of IC devices. Unfortunately, the CDM test sometimes gives confusing results. Some examples include the following:

·     Different results are seen when a part is stressed at different labs, even at the same voltage level.

·     A part tested at ±450 V fails, but later passes at +/-500 V.

·     A part tested at ±250 V passes, but another sample of parts from the same lot fails at  ±250 V.

We have investigated some of the more common causes of variability in the CDM test. Incorrect measurement of the current-versus-time waveform, which occurs during the discharge event, has been found to be the most important source of variation. Better measurement of the CDM waveform, using higher-bandwidth oscilloscopes to eliminate system-to-system variation, leads to more repeatable test results. This will become even more important in the near future when the ESDA and JEDEC release their joint CDM specification.