Thermal resistance (Rθ) is the most critical specification for a heat sink. It measures the heat sink’s opposition to heat flow, analogous to electrical resistance. Specifically, it’s the temperature difference between the heat source (junction or case) and the ambient air per unit of power dissipated (units: °C/W). A lower Rθ means better cooling performance. When selecting a heat sink, you calculate the required Rθ based on: the component’s maximum power dissipation (P), its maximum allowable junction/case temperature (T_jmax or T_cmax), and the maximum ambient air temperature (T_amb). The formula is: Rθ_required ≤ (T_jmax – T_amb) / P. You then choose a heat sink with an Rθ value lower than this requirement, considering interface materials and mounting.