This work aims to focus on a strategy to increase the operating time and decrease the thermal fluctuations simultaneously using Phase Change Material (PCM) based heat sinks in cyclically operating electronic devices. Whenever the electronic devices operate on periodic on-off duty cycles, they are subjected to thermal fluctuations. The thermal fluctuations cause thermal fatigue, which eventually can lead to the failure of electronic devices. A novel heat sink is necessary to dissipate the heat generated by those electronic devices and mitigate the thermal fluctuations. In this study, a design of PCM based square heat sink with vertical fins is proposed and is tested against heat loads with sinusoidal, triangular, and square profiles. Two sets of experiments, one without PCM and the other with PCM, are conducted to quantify the advantage of PCM in extending the operating time as well as in eliminating the thermal fluctuations simultaneously. A plate heater is attached to the bottom surface of the heat sink to mimic the electronic chip, and the base temperature is considered as the performance parameter to quantify the thermal performance of the heat sink. The heat sink is made of aluminium, and the PCM used is n-eicosane. To minimize the heat loss to the ambient, an insulating material, cork, is added at the bottom of the heater. To prevent the overflow of PCM from the heat sink cavity and to visualize the process, a transparent acrylic sheet is fixed on the top of the heat sink. All the experiments are performed at room temperature, which varies from 21°C to 22°C. In this investigation, the pin fin heat sink configuration with PCM is seen to perform from 68% to as high as 90% in enhancing operating time and reducing temperature fluctuations compared to the one without PCM configuration. From the in-house experimental studies, it is seen that the addition of PCM controls the sudden temperature rise during the heating phase and temperature drop during the cooling phase, simultaneously thereby regulating the thermal fluctuations significantly compared to a heat sink without PCM. With the addition of n-eicosane, the operating time of the heat sink is improved by at least 65%. With the addition of Expanded graphite (EG), the thermal fluctuations are reduced by as high as 15% in the initial cycles.