The current two-dimensional (2D) numerical study presents the melting phenomenon and heat transfer performance of the nanocomposite phase change material (NCPCM) based heat sink. Metallic nanoparticles (copper: Cu) of different volume fractions of 0.00, 0.01, 0.03, and 0.05 were dispersed in RT–28HC, used as a PCM. Transient simulations with conjugate heat transfer and melting/solidification schemes were formulated using finite–volume–method (FVM). The thermal performance and melting process of the NCPCM filled heat sink were evaluated through melting time, heat storage capacity, heat storage density, rate of heat transfer and rate of heat transfer density. The results showed that with the addition of Cu nanoparticles, the rate of heat transfer was increased and melting time was reduced. The reduction in melting time was obtained of − 1.36%, − 1.81%, and − 2.56% at 0.01, 0.03, and 0.05, respectively, compared with 0.00 NCPCM based heat sink. The higher heat storage capacity enhancement of 1.87% and lower reduction of − 7.23% in heat storage density was obtained with 0.01 volume fraction. The enhancement in rate of heat transfer was obtained of 2.86%, 2.19% and 1.63%; and reduction in rate of heat transfer density was obtained of − 6.33%, − 21.05% and − 31.82% with 0.01, 0.03, and 0.05 volume fraction of Cu nanoparticles, respectively. The results suggest that Cu nanoparticles of 0.01 volume fraction has the lower melting rate, higher heat storage capacity and heat transfer rate, lower heat storage density and heat transfer rate density which is preferable for passive cooling electronic components.