ESD Spark Behavior and Modeling for Geometries Having Spark Lengths Greater Than the Value Predicted by Paschen's Law

The insulation of plastic enclosures provides protection against direct electrostatic discharge (ESD) discharges to the system inside. However, seams between plastic parts are often unavoidable. To increase the voltage at which an ESD will penetrate the structure of the seam can be modified. Four plastic arrangements are constructed to investigate the spark length and current derivatives and to understand the ESD spark behavior for geometries having spark lengths longer than the values predicted by Paschen's law. A two to threefold increase of spark lengths was found for sparks guided by plastic surfaces compared to spark length expected from the Paschen value at the same voltage level. In spite of the longer path, a faster spark development is observed for sparks along the plastic surface. Plastic arrangements that provide detour and fold-back paths hardly reduced the total spark length. No significant effects of the plastic materials or the polarity were observed. The spark length increased as the (absolute humidity) Absolute humidity (AH) increased, and the current derivative decreased by about 20% as the spark length increased with (relative humidity) Relative humidity (RH) changing from 9% to 65% at 29 °C. The spark resistance is modeled by a modified Rompe and Weizel's law, which distinguishes the spark development in the air and along the plastic surface.

I. Introduction

One method to achieve ESD robust electronic system design is to prevent sparking into the system. This can be achieved by a sufficiently insulating barrier. While even 0.3-mm plastic is usually not penetrated by ESD up to 25 kV difficulties are introduced by seams between plastic parts and openings that expose the inner circuits. The design choice can influence the tightness of adjacent plastic parts, the plastic materials, the wall thickness, and the distance to the electronics. Further the designer can shape the seams of adjacent plastic parts such that detours lengthen the spark path, or introduce fold-back structures that force the spark to develop against the electrostatic field direction. This article analyzes the effect of such design choices and provides an improved simulation model for the spark resistance which also includes situations in which the spark develops parallel to plastic surfaces.