Heatsinks may cause radiated emission and radio frequency interference problems when they are mounted on printed circuit boards. In this article, the radiation mechanism of heatsinks is systematically investigated using characteristic mode theory. The dipole moment is a commonly used equivalent source model for integrated circuits that drive radiated emission from heatsinks. On the basis of a simplified modal weighting coefficient formulation, the interactions between the dipole moment and the significant modes of the heatsink are efficiently evaluated, thus providing a clear physical insight into noise source placement. Finally, the grounding post design, a commonly used EMI mitigation method, is also discussed. The relative error of the mode-based field prediction is less than 3 dB compared with the full-wave simulation.