B.5.FM:5.1 - Choose the heat path before commissioning a calculation

An engineer asks whether a stronger external fan can bring an overheating component below 65 °C. Inspection finds the component attached to a metal case through a pad; the fan blows over the outside of the case.

Trace the proposed path from component to case and then to the surrounding air. Keep the component, the case at its attachment and the air distinguishable: their temperatures can differ, and the fan acts on one part of that path.

Try a steady account with constant component heating, all generated heat passing through the pad, and an unchanged linear conductance between component and case. Carrying the same heat per second then requires the same temperature difference across that path. Treat outward case-to-air transfer as driven by temperature difference. A steady case receiving positive heat must remain warmer than the air.

Use supplied steady readings of 80 °C at the component, 25 °C at the case attachment and 20 °C in the air. The component-to-case difference is 55 °C. Even ideal external cooling of the case to the air temperature leaves the component at 75 °C under these assumptions. The fan-only proposal cannot reach 65 °C in this model.

The result directs work toward the component-to-case contact, another heat path or reduced heating. Direct airflow onto the component would change the assumed path; temperature-dependent heating or conductance would change the fixed-difference argument. Measurement uncertainty matters to the real-device conclusion.

A useful next request is: “Can this component stay below 65 °C after changing its contact to the case, and which observations distinguish the relevant heat paths?” The model supplies that request before a detailed equation set is commissioned.