Experimental comparison of probes for air discharge measurements Paasi J., Salmela H., Kalliohaka T., Fast L., * Smallwood J. + VTT Technical Research Centre of Finland, P.O. Box 1306, FI-33101 Tampere, Finland * SP - Electronics, P.O. Box 857, SE-50115 Borås, Sweden + Electrostatic Solutions Ltd., 13 Redhill Crescent, Bassett, Southampton, SO16 7BQ, UK We have studied the performance of three different kinds of passive resistor- transmission line probes intended for the measurement of electrostatic discharges (ESD): an unshielded ball probe, a shielded ball probe with the ESD current collected on a coaxial wire tip, and a needle-like probe. We evaluated them for possible application in assessment of ignition hazards or ESD damage risks to electronic components. The results show that there is no single probe type that is ideal for all kinds of situations. There are significant differences between the probe responses in their ability to initiate ESD, charge transferred, and peak ESD current. INTRODUCTION Rapid discharges of static electricity from insulating surfaces, metal objects and other materials are of concern in ignition of flammable materials and in the assessment of the risk of damage to electronic components and assemblies during manufacture and service maintenance. The nature of an electrostatic discharge (ESD) is determined by the electrostatic field distribution required to initiate a self-sustaining electron avalanche. The onset of discharge is governed by the electrical breakdown of the surrounding atmosphere. Accordingly, the waveform (peak current, duration, rise time, fall time, and charge transferred) of an ESD as well as the risks associated with the ESD depend on various factors, such as the amount and distribution of charge on the charged object, the conductivity of the charged surface, electrical and geometrical parameters of the discharging object, system geometry, etc. [1, 2]. A discharge between a charged surface and a discharging object is more likely to occur for short than long air gap lengths. Furthermore, at a fixed air gap, a discharge is more likely to occur to grounded needle than to a large grounded sphere. Different kinds of probes have been developed during the past years for the measurement of air discharges (spark and brush discharges, etc.) in order to evaluate the ESD risks, without the use of explosive atmospheres or destructive tests of electronics. In this work we have studied the performance of three different kinds of passive resistor-transmission line discharge probes: an unshielded ball probe developed at Physikalish-Technische Bundesansalt (PTB-probe) [3], a shielded ball probe having an earthed hemispherical shield with the ESD current collected on a coaxial wire tip, the probe developed at Electrostatics Solutions Ltd. (ESL-probe) [4], and a needle-like shielded probe developed at SP Swedish National Research and Testing Institute (SP-probe). ELECTROSTATIC DISCHARGE PROBES Ideally a probe for ESD measurements should be uniquely designed for the situation it tries to mimic. For general purposes, a probe for a faithful ESD waveform measurement should be relatively small in comparison with the size of the charged body, the impedance of the probe circuit should be solely resistive up to the highest rates of change of discharge current, and the value of the impedance should be small enough to avoid the generation of an excessive potential at the probe surface during the discharge