Dangerous parasitics of socketed CDM ESD testers

HBM, non-socketed and socketed CDM testing were performed on HF-ICs. By applying HBM and non-socketed CDM stress weak pins were located by selective stress. Socketed CDM, however, caused a damage at the weak pins, even when they were excluded from testing. Socketed and non-socketed CDM caused the same electrical failure signature. After insulating the weak pins from the tester circuitry, these pins survived the socketed CDM test without damage and the failure threshold increased. This behaviour is explained by the discharge of the parasitic tester capacitance through the weak pins, while other pins are tested.