Deposition of aerosol (“hygroscopic dust”) on electronics – Mechanism and risk

In this paper the risk associated with deposition of aerosol (“hygroscopic dust”) on printed circuit boards is assessed for different equipment designs and deployment. The amount of deposited dust is related to (1) the total dust availability in a given type of equipment and (2) to the rate of deposition of dust due to different mechanisms. Of these, the Brownian diffusion, gravitational settling, thermophoresis and electrophoresis are of different relative importance depending on such design factors as air flow in the enclosure, temperature gradients between boards and walls and presence of electric fields. The expressions for electric fields in the vicinity of printed circuit boards relevant to electrophoretic deposition are derived. It is shown that not only the sealed Absolute Humidity Control Modules but also the breathing Relative Humidity Control Modules should not present a hygroscopic dust risk for either indoor or outdoor applications. Direct air cooled system with forced convection present a definite dust reliability risk both for indoor and outdoor applications while open natural convection cooled systems present a borderline case where each design has to be considered separately.