Formation of voids in silicone RTV dispersion under beam-leaded silicon integrated circuits

The results of a study on void formation under simulated beam-leaded silicon integrated circuits (ICs) (chips) encapsulated with Dow-Corning 96-084 RTV silicone rubber dispersion are described. The simulated chips were made out of glass. Three types of voids were observed: voids originating at the center under a chip and progressively spreading outward (solvent vapor voids), voids formed by air entering under a chip through a tunnel in the RTV dispersion (tunnel voids), and voids originating at the sides under a chip and progressively spreading under it without tunnel formation (air diffusion voids). The process of formation of each void type was documented with the aid of photographs. Analyses have been made of the first two types of voids. The results of the analysis of the solvent vapor voids allow determination of the highest initial encapsulant viscosity for which void-free coverage can be obtained for a chip having a given side length. The results of the analysis of the tunnel voids give the smallest encapsulant height-to-chip-side-length ratio needed for tunnel-free coverage of chips with and without a tilt