A detailed thermal simulation of the performance of FR-4 PCBs having various "via configuration" is made in this study. The results indicate the thermal resistance from the simulation is significantly affected by compact thermal via configurations. Thermal resistance can be improved by increasing via number and also with copper filled via. For further explanation of the significant drop of thermal resistance at PCBs with thermal via, the detailed thermal resistance distribution at the thermal module are further examined. The significant drop in thermal resistance mainly occur in FR-4 PCBs with the help of thermal via. However, there is observed a maximum number of via which contribute to optimized thermal resistance across the PCB.