Revisiting Electrostatic Discharge Behavior From Floating Metal Under Electron Irradiation

Floating metals are a known root cause of spacecraft anomalies and failures due to associated space radiation-induced electrostatic discharge (ESD). A recent publication called into question the standard size limit rules for ungrounded conductor applications on spacecraft, reporting that significantly larger ESD magnitudes compared to those used to derive the standard size limits may be possible if the discharge punches through the insulation separating the floating metal from a signal trace versus the previously observed blow-off (BO) mode ESD. That testing was done under the application of voltage directly to the floating metal and did not represent flight-like charging due to electron irradiation. We present testing of floating metal patches exposed to an electron beam. The dielectric surrounding the metal patches is also a source of ESD when testing with an electron beam; however, BO ESD either from the floating metal or the surrounding dielectric is distinguishable from punchthrough (PT) ESD due to differences in ESD pulse polarity, time constant, and magnitude. We confirm that large-magnitude PT ESD can occur due to electron irradiation of small floating metal patches. The worst case PT pulses are consistent with estimates of the maximum amount of energy of the equivalent capacitor at breakdown. That theoretical maximum is shown to be a good worst case bound if the correct material properties are known. We propose that floating metal design practices move away from generic conductor size limits to ESD threat-based criteria.