Design and performance of new instrumentation to measure the shielding capabilities of materials against electrostatic discharges

Abstract

The shielding required to maintain a ‘static safe’ environment inside packaging used to transport semiconductor devices and microelectronic assemblies through uncontrolled environments needs to provide 1000:1 attenuation of electric field transients over the frequency range from 10 Hz to 1 GHz. The paper describes the principles, practical design features and the performance of a new method for assessing the characteristics of shielding materials and practical packages. The method described uses simultaneous positive and negative high electric field pulses of balanced amplitude and form to apply an electric stress across a defined width of material. With symmetrical pulses no nett charge is transferred to the layer under test. This means that the layer can be isolated from earth so the test is equally applicable to materials with surface or volume conductivity or with buried conducting layers. It also means that the signals observed on the reverse side of the layer have no common mode component. Attenuation performance is analysed as frequency components for each decade of frequency from 10 Hz to GHz. The problems of shot to shot variation of high voltage pulses are overcome by observing all frequency components simultaneously as the ratio of signals observed through the test material with those observed directly. The approach is effectively a single shot ratiometric spectrum analyser.