Coating of Electronic Components by the RTV Dispersion—Part I: Physical Model of the Deposition Process Determined by Drop Tests

Abstract

This paper describes a study of the process of deposition of RTV dispersion on electronic components placed on substrates. The objective was to develop a technique for the consistent manufacture of encapsulant coating of a desired thickness and extent. In addition, it was desired to obtain an understanding of the phenomenon of run-over, or wicking, of the RTV dispersion onto external leads of circuits being encapsulated, and of means to control it. In this paper physical properties of the RTV dispersion which influence the deposition process were determined using a novel drop test method. These properties allow building of a physical model of the deposition process, and its analysis. The results of drop tests show that the RTV dispersion behaves like a plastic “false body” material which possesses yield stress after a long rest, and which retains residual yield stress after shearing. Part I of this paper is concerned with building of the physical model of the encapsulant deposition process. It also deals with the derivation of an equation relating the wall shear stress to the encapsulant volumetric flow rate.