Thermal Chain Calculator

The idea of modeling heat flow as an electrical-circuit analog (temperature = voltage, heat current = electrical current, thermal resistance = electrical resistance) dates to Joseph Fourier\'s 1822 treatise on heat conduction. Applied to electronics, this analogy turns a three-dimensional thermal problem into a simple series-parallel resistor network, letting engineers use Kirchhoff\'s laws for heat just as they do for current.

Package-level thermal resistance specifications became standardized in the 1960s as discrete power transistors (TO-3, TO-220) entered mass production. Datasheets started listing R_θJC (junction-to-case) separately from R_θJA (junction-to-ambient) because designers needed to know which portion of the thermal path was fixed inside the package versus which they could improve by adding heatsinks, TIM, and airflow.

Modern multi-path thermal analysis extends the series model to include parallel paths: junction heat flows through the die-attach into the case AND through the wire bonds into the leads AND through the encapsulation into the air. For QFN and BGA packages with exposed thermal pads, this parallel-path accounting matters — the datasheet R_θJA assumes a specific PCB layout that real designs rarely match exactly.

Enter power dissipation, ambient temperature, and the three thermal resistances along the path: R_θJC (from datasheet), R_θCS (TIM and mounting — typically 0.1–1 °C/W for grease, higher for pads), and R_θSA (the heatsink\'s rated thermal resistance). The tool returns temperatures at each node: junction (T_j), case (T_c), sink (T_s), and the total R_θJA.

For the typical TO-220 linear regulator scenario: R_θJC ≈ 2 °C/W, R_θCS ≈ 0.5 °C/W (with grease), R_θSA depends on the heatsink (5 °C/W for small aluminum finned; 1 °C/W for large forced-air). Everything runs client-side; no values leave your browser.