This work presents a numerical study on the thermal behavior of a PCM heat sink for the purpose of electronic cooling. Introducing copper fins inside a PCM heat sink was examined. A parametric study was performed to maximize the critical time elapsed before reaching the allowable temperature limit for different fin lengths, numbers, and thicknesses for the same copper amount. This amount was also evaluated as a perimeter envelope of the PCM. The enthalpy method was carried out. Natural convection in melted PCM was taken into account. The governing equations were solved by Comsol Multiphysics. This model was validated by comparing results with numerical data by Huang et al. (Int J Heat Mass Transf 47:2715–2733, 2004). The results indicated that the inclusion of fins can enhance the thermal performance of heat sink by increasing the exchange surface and ensuring better heat repartition inside the PCM. The fin geometry presented an important role in thermal control improvement. Although a significant difference was showed in temperature between the copper envelope and the copper fins, they present the same efficience for low heat flux.