Electronics Cooling — Junction Temperature & Thermal Resistance
Junction temperature prediction, thermal resistance networks, PCB modeling, heat sink design optimization, and thermal interface material (TIM) selection.
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Quick Explainer
What is junction temperature and why is it the key metric?
Junction temperature (Tj) is the temperature at the semiconductor die — the hottest point in the device. Transistors fail or degrade rapidly above their maximum rated Tj (typically 125 C for silicon). The thermal resistance from junction to ambient (theta_ja) determines power dissipation limit: dT = P * theta_ja. Every 10 C reduction in Tj roughly doubles device reliability.
How do you build a practical thermal model of a PCB assembly?
Multi-scale modeling is standard. Components use compact thermal models (DELPHI or JEDEC 2-resistor) giving junction-to-case and junction-to-board resistances. The PCB is modeled with layer-by-layer effective conductivity. System-level airflow from CFD provides board-level heat transfer coefficients. Each scale informs the next.