Nearly all IC encapsulating compounds require a postcure treatment to ensure integrated circuit (IC) package reliability. The issue of postcuring and this effect on IC package reliability performance are considered in this article. We examined the development of various encapsulating compounds＇ properties with various durations of postcure time. It was found that the mechanical strength, glass transition, and adhesion strength were increased with increasing duration of postcure time compared to as-molded samples. However, these properties could reach ultimate values after postcuring for 1-2 h. It was also seen that the moisture uptake was increased for samples that have been post-mold-cured due to increased crosslinking density causing a large free volume in the glassy polymer matrix. C-mode scanning acoustic microscopy (C-SAM) analyses were performed to investigate the effect of the duration of postcure time on the IC package reliability and they show a good relationship with the evolution of the compound＇s properties during the postcure process.