Thermal: junction temperature, heatsinks and a board thermal map

Parts that dissipate power get warm, and their junctions must stay below the datasheet limit. The Thermal tab of Instruments → Circuit calculators… estimates temperatures in four ways. It uses the same engine as the ohmforge-thermal-calculator tool, so the tab and the tool give identical numbers. All results are steady-state estimates, not certified thermal design — measure critical parts.

1. Junction temperature and the heatsink you need

Open Instruments → Circuit calculators… and choose the Thermal tab. Calculation starts at “Junction temperature and heatsink needed” with a TO-220 regulator: Power (W) 10, Ambient temperature (°C) 40, θJC junction to case (K/W) 1.5, θCS case to heatsink (K/W) 0.5 and θSA heatsink to ambient (K/W) 4. The optional θJB junction to board 10 and θBA board to ambient 20 add a second heat path through the board.

The result: 5 K/W junction to ambient for both paths together, a junction temperature of 90 °C and a margin to the limit of 35 K against the 125 °C Maximum junction temperature (°C). Power at the limit says the part could dissipate 17 W. Clear θSA and enter a datasheet θJA junction to ambient without a heatsink to check a part without a heatsink.

2. A plate-fin heatsink in still air

Choose “Plate-fin heatsink estimate”. The example is 100 × 100 mm with 12 fins, 30 mm high and 1.5 mm thick, in aluminium (Thermal conductivity 200 W/(m·K)) with an anodised surface (Surface emissivity 0.8). The estimate is about 2.13 K/W; at 10 W the heatsink settles near 46.3 °C. The table shows the convection coefficient, the fin efficiency and how much heat leaves by convection and by radiation. Try 20 fins: they sit so close that still air flows poorly between them, and the resistance rises to about 3.55 K/W.

3. Temperature map of a plate

“Temperature map of a plate” spreads heat sources over a thin plate cooled from both faces. Heat sources takes one source per line — name, centre x and y, width, depth and power, in mm and W from the lower-left corner. With Clamped edge you can hold one edge at the Clamp temperature, for example where a plate is bolted to a metal case. The map colours the plate from cold to hot, and dashed boxes mark the sources.

4. Board thermal map

Choose Instruments → Board thermal map…. It opens the Thermal tab with “Board thermal map” selected and, under Board, the board you described in Board & enclosure… (see the enclosure tutorial). Under Part powers enter the power of each part that gets warm, for example 1.2 W for a relay coil. The copper fields describe the board: Copper layers, Copper thickness per layer (µm) and Copper coverage (%) — planes and pours spread heat far better than bare FR-4.

The sensor-node example board (70 × 50 mm, two layers, radio 0.5 W and microcontroller 0.3 W) reaches 41.7 °C, 16.7 K above ambient, near U1. The heat sources table lists the temperature below every part, and the energy balance confirms that all the heat leaves through the faces.

Press Save the part powers in this project to keep the powers with your board. Undo removes them again. Saved boards keep working in Board & enclosure; the enclosure ignores the powers.

5. Tips

• θJA in datasheets comes from a standard test board; your board may be better or worse.
• In still air the convection coefficient is about 5 to 15 W/(m²·K) including radiation; a fan gives 20 to 100.
• In a closed box, enter the expected inside air temperature as the ambient.
• Scripts and AI agents can run the same four calculations through the tool's JSON API and MCP.