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The primary focus of this research was the measurement of ion diffusion in a common epoxy molding compound used to encapsulate microelectronic devices. Ion diffusion measurements were made by exposing the encapsulant materials to aqueous solutions of 2M sodium chloride, and making measurements by Dynamic Secondary Ion Mass Spectrometry. The diffusion dependency of temperature exposure of the encapsulants to aqueous salt solutions at 75degC, 85degC, 100degC, 125degC, 150degC and 175degC and post-mold curing for 2, 4, 6, and 8 h at 175degC was evaluated. The ion diffusion was found to depend on the glass transition temperature of the encapsulant. The rate of ion diffusion above the glass transition temperature of the encapsulant was faster than predicted by an Arrhenius plot of the diffusion coefficient as a function of temperature below the glass transition temperature. Post-mold curing of the encapsulant decreased the diffusion of chloride ions in the encapsulant. Ion diffusion was shown to be slower than moisture diffusion in the encapsulant by approximately nine orders of magnitude.