How to Calculate Heat Sink Thermal Resistance for an Extruded Aluminum Profile

Heat flows from the electronic junction or case through the interface into the Heat sink, then into ambient air. Every stage contributes thermal resistance. The sink-to-ambient requirement must leave room for upstream resistances, including the device package and thermal interface.

Here, R_θSA is allowable sink-to-ambient thermal resistance (°C/W); T_max is the temperature limit at the chosen reference point; T_ambient is surrounding air temperature; P is heat dissipation (W); and R_θother is the combined upstream resistance (°C/W). Keep the temperature reference point consistent with the resistances included.

Assume 100 W dissipation, a 100°C maximum reference temperature, 40°C ambient air, and 0.15°C/W upstream resistance. The requirement is (100 − 40)/100 − 0.15 = 0.45°C/W. The selected Heat Sink must achieve 0.45°C/W or lower at the specified airflow and mounting conditions. This is an initial estimate, not a guaranteed operating temperature.

Natural convection depends on buoyancy and installation orientation; forced convection depends on fan performance, ducting, bypass flow, and fin pressure drop. A thermal-resistance number measured at one airflow cannot be applied unchanged to another installation.

Fin height, spacing, thickness, base spreading, and the size of the heat-source footprint affect results. Taller fins are not always more effective because heat conduction along the fin and air movement impose limits. A manufacturer can review candidate extrusion profiles against die feasibility and available space.

Thermal simulation can narrow the options, but prototype testing should reflect actual power, airflow, ambient conditions, mounting pressure, and interface materials. Record the measurement location and test method so different profiles can be compared fairly.