Thermal control of printed circuit board and integrated circuit package is challenging in microelectronics because the power density increases when smaller and more complicated packages are designed. Temperature rising due to power dissipation and hotspots worsens harmful clock skew and jeopardizes reliability of products. To overcome these risks, designers have to perform electromagnetic-thermal co-simulations at the early design stage. However, it is difficult to detect the hotspots in the package because very high-resolution simulations are needed. In this paper, an improved adaptive finite element method (FEM) is applied to detect hotspots, which only needs one-step adaptive refinement for a given error threshold, which is very fast and uses much smaller computation resources. Furthermore, instead of electromagnetic-thermal co-simulation, this method directly finds potential hotspots based on the volumetric heat generation. Test results show that the adaptive FEM only takes about 60 times the memory and CPU time to detect all hotspots when compared to the initial FEM solution.