Characterization of ESD Risk for Wearable Devices

The core difference between the posture assumed for IEC 61000-4-2 human metal discharge and a discharge to a wearable device is the impedance between the charged body and the grounded structure discharged to. Especially for a waist-worn device, a larger portion of the body is close to the grounded structure; thus the geometry forms much lower impedance which will lead to higher currents. Despite the variability for the air discharge, in most cases, the current will be higher than 3.75 A/kV as specified for contact mode electrostatics discharge (ESD) calibration. Even for the most slowly rising discharges having a spark length equal to the value given by Paschen's law, a 10 kV ESD from the waist will surpass 37.5 A for a waist-worn metal part discharged (e.g., to a door frame). Modeling the wearable device discharge provides predictions on the current derivative and the transient field a wearable device is subjected to. Observed failure levels of a wearable electronic device and comparing discharges according to the IEC 61000-4-2 test standard against discharges from the device while wearable showed that the IEC set-up may be insufficient to ensure the robustness of the wearable devices.