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Choose the thermal question that matches the datasheet or prototype check.
Use a preset as a starting point, then keep the numeric fields tied to the actual part.
Enter the worst-case device heat load in watts.
W
Worst-case local air temperature around the heat sink.
deg C
Datasheet junction limit for the selected device.
deg C
The calculator sizes to Tj max minus this margin.
deg C
Package thermal resistance controlled by the part selection.
deg C/W
Pick a typical interface, then adjust thetaCS when the material data sheet gives a value.
Thermal resistance of the mounting interface.
deg C/W
Use an installed or candidate thetaSA for verification, charting, and maximum-power estimates.
Enter the heat-sink-to-air rating to check an actual design.
deg C/W
Use neutral free-air unless the datasheet rating and the product airflow are known to differ.
Guidance threshold for the estimated heat sink base temperature.
deg C
Adjust output precision without changing the calculation.
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Tags: Devtools, Diagnostics, Electrical
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Heat-sink thermal resistance is the temperature-rise budget between a semiconductor junction and the air around the product. A lower C/W value means the device warms less for each watt of heat it must throw away.

That budget matters when a regulator, transistor, motor driver, processor, or power LED is asked to dissipate several watts inside a real enclosure. The useful question is not only whether a heat sink is attached. The junction must stay below the working target after worst-case power, local ambient temperature, package resistance, interface material, mounting pressure, and airflow are counted together.

Thermal resistance path from junction through case, interface, heat sink, and ambient air.

The heat path is usually treated like a series chain: junction to case, case to sink, and sink to ambient air. If the package and interface already consume too much of the allowed temperature rise, no heat sink rating can fix the design by itself.

A thermal-resistance calculation is a first-pass steady-state estimate. Datasheet ratings are measured under stated conditions, and the finished assembly still depends on board copper, enclosure airflow, fin orientation, surface finish, mounting pressure, and the real power waveform.

How to Use This Tool:

Start with the thermal question you need answered, then keep every entered value tied to the actual device, interface material, and heat-sink rating you plan to use.

  1. Choose Solve for. Use Required heat-sink thetaSA when sizing a sink, Check installed heat sink when comparing a candidate, or Maximum power on installed sink when the sink is already fixed.
  2. Select a Package preset or choose Custom datasheet values. Replace Maximum junction temperature and Junction-to-case thetaJC with the part datasheet values before using the result for hardware decisions.
  3. Enter Power dissipated, Ambient temperature, and Design margin below Tj max. Use heat dissipated inside the device, the hottest local air near the sink, and the margin you want below the datasheet limit.
  4. Choose the Interface path or enter Case-to-sink thetaCS. A dry contact, thick pad, or poor mounting pressure can spend more of the thermal budget than a larger sink can recover.
  5. For check and maximum-power modes, choose a Heat-sink candidate or enter Installed or candidate thetaSA. Use Heat-sink rating context when the datasheet rating needs an orientation, enclosure, or forced-air adjustment.
  6. Use Advanced only when needed. Preferred sink base ceiling flags a handling or enclosure temperature concern, while Display precision changes reported decimals without changing the calculation.
  7. Fix validation messages before reading the result. Common causes are zero or negative power, a working junction target at or below ambient, negative theta values, a missing candidate thetaSA in check modes, or a sink base ceiling below ambient.
  8. Read Thermal Stack Ledger first, then use Fit Guidance and the Junction Margin Curve to decide whether the candidate is comfortable, tight, or too warm over the power range.

When the summary reports a tight or hot sink, change the physical design before relying on extra decimals: lower the dissipated power, improve the interface, reduce ambient temperature, add airflow, or select a lower thetaSA heat sink.

Interpreting Results:

Required thetaSA is the main sizing value. Choose a heat sink whose real rating is at or below that value after the mounting, airflow, and orientation conditions are accounted for. A lower C/W rating cools better; a higher C/W rating runs hotter.

Installed margin is the quick health check for an entered candidate. At 10 C or more, the result is labeled Installed sink has margin. From 0 C up to but not including 10 C, it is Installed sink is tight. Below 0 C, it is Installed sink runs hot.

Output What to check Common misread
Working junction target Confirm that Tj max minus design margin is still above the hottest local ambient temperature. Using the absolute Tj max as the working design target with no reliability margin.
Required thetaSA Select a heat sink at or below this C/W value after context adjustment. Thinking a larger thetaSA number means a stronger heat sink.
Estimated junction Compare it with the working junction target, not only the absolute datasheet maximum. Treating a calculated temperature below Tj max as safe without checking airflow and mounting.
Maximum power on candidate Use it as the steady-state power ceiling for the entered candidate stack. Applying it to pulsed loads or different airflow without recalculating.
Sink base ceiling Use the guidance row to flag handling, enclosure, or nearby-component heat concerns. Confusing a comfortable sink base with proof that the semiconductor junction is comfortable.

A positive margin is not a release test. Verify the final assembly with the real heat sink, interface material, board, enclosure, airflow, and temperature measurement method before approving production hardware.

Technical Details:

Thermal resistance uses the same series-resistance idea as an electrical resistor chain. Power dissipation is the heat flow, temperature rise is the drop across the chain, and C/W is the resistance between two points in the path.

For a heat-sinked semiconductor, the simple steady-state path is junction to case, case to sink, and sink to ambient. The package term depends on the part construction, the interface term depends on the mounting material and pressure, and the sink-to-ambient term depends strongly on fin geometry, air movement, orientation, enclosure clearance, and surface condition.

Formula Core:

The sizing calculation subtracts package and interface resistance from the total allowed junction-to-ambient resistance.

Ttarget = Tj max-M θJA allowed = Ttarget-TAP θSA required = θJA allowed-θJC-θCS Tj installed = TA+P⁢(θJC+θCS+θSA effective) Pmax = Ttarget-TAθJC+θCS+θSA effective

Here, P is device heat in watts, TA is local ambient temperature, M is design margin below Tj max, thetaJC is junction-to-case resistance, thetaCS is case-to-sink resistance, and thetaSA is sink-to-ambient resistance. Results are displayed to the selected precision from 1 to 4 decimals; changing precision does not change the underlying math.

With 7 W, 50 C ambient, 125 C Tj max, a 20 C margin, thetaJC of 4 C/W, and thetaCS of 0.2 C/W, the working target is 105 C and the allowed thetaJA is 55 / 7, or 7.857 C/W. Subtracting 4.2 C/W for package and interface leaves a required thetaSA of 3.657 C/W.

Condition Boundary Result meaning
Power dissipated Must be > 0 W Zero or negative heat flow cannot produce a thermal-resistance requirement.
Working junction target Tj max - margin must be > ambient No temperature-rise budget remains when the target is at or below local air temperature.
Before-sink budget Required thetaSA < 0 C/W The package and interface already exceed the allowed thermal rise.
Installed sink has margin Installed margin >= 10 C The entered candidate has useful steady-state headroom.
Installed sink is tight Installed margin >= 0 C and < 10 C The candidate fits the target but has little room for tolerance, airflow loss, or hotter ambient.
Installed sink runs hot Installed margin < 0 C The entered stack exceeds the working junction target at the entered power.

Heat-sink context changes the entered thetaSA before the installed calculation. The neutral datasheet context keeps the rating unchanged, horizontal fins or restricted airflow raise the effective C/W value, and forced airflow lowers it when the fan path is reliable. These multipliers are planning cues, not substitutes for a heat-sink datasheet curve or a chamber test.

Input Unit or choices Why it changes the result
Power dissipated W Multiplies total thermal resistance to produce temperature rise.
Ambient temperature C Sets the starting air temperature around the heat sink, not room temperature far away.
Design margin below Tj max 0 to 40 C in the visible control Lowers the working junction target before the budget is calculated.
Junction-to-case thetaJC C/W Consumes thermal budget before heat reaches the case surface.
Case-to-sink thetaCS C/W Represents paste, pad, mica, dry contact, flatness, and mounting pressure.
Installed or candidate thetaSA C/W Controls the sink-to-air part of the stack and drives installed margin and maximum power.

The junction-margin curve uses the chosen stack to show how estimated junction temperature rises with power. The target line and absolute Tj max line stay fixed for the entered ambient and margin, so the crossing point is a visual check on the reported maximum power.

Accuracy Notes:

This is a steady-state thermal planning estimate. It is useful for early sizing and sanity checks, but it is not a replacement for datasheet review, thermal simulation, or measurement on the finished assembly.

  • Use worst-case device heat, not output power delivered to a load.
  • Use the hottest local air temperature around the sink, including enclosure heat rise.
  • Confirm thetaJC, thetaCS, and thetaSA from the actual device, interface material, and heat-sink data.
  • Recheck pulsed loads with transient thermal impedance when the duty cycle is short enough that steady-state math overstates or understates risk.
  • Measure the prototype because natural convection, fin orientation, altitude, dust, nearby hot parts, and blocked airflow can move the real temperature.

Worked Examples:

TO-220 regulator in open air

A 7 W regulator at 50 C ambient with 125 C Tj max, 20 C design margin, 4 C/W thetaJC, 0.2 C/W thetaCS, and a 2.5 C/W heat sink gives Required thetaSA 3.66 C/W. The entered candidate estimates Estimated junction 96.90 C and Installed margin +8.10 C, so the verdict is Installed sink is tight.

Same sink inside a restricted enclosure

Keeping the same 7 W stack but changing Heat-sink rating context to an enclosed or obstructed airflow condition raises the effective thetaSA to 3.63 C/W. Estimated junction becomes 104.78 C and Installed margin falls to about +0.23 C, which is still not a comfortable design margin even though it has not crossed below zero.

Negative budget before choosing a sink

A hotter design with 10 W, 90 C ambient, 125 C Tj max, 20 C margin, 4 C/W thetaJC, and 0.2 C/W thetaCS leaves only 15 C of allowed rise. Required thetaSA becomes a negative value, and Before-sink budget explains that the package and interface already consume more rise than the target allows.

FAQ:

What does thetaSA mean?

ThetaSA is heat-sink-to-ambient thermal resistance in C/W. Lower thetaSA means a smaller temperature rise from the sink to the surrounding air for the same watts.

Why can required thetaSA be negative?

A negative required thetaSA means the working junction target is already exceeded by the package and interface rise. Lower power, lower ambient, reduce design margin only if appropriate, choose a better package, or improve the interface before selecting a sink.

Why does the same heat sink pass in one context and fail in another?

The entered heat-sink rating is adjusted by Heat-sink rating context. Horizontal fins, enclosure blockage, and forced airflow change the effective thetaSA used for Installed margin and Maximum power on candidate.

Can I use output power instead of dissipated power?

No. Enter heat lost inside the device. For a linear regulator, that is approximately voltage drop times load current; for a driver, LED module, or transistor, use the device loss from the datasheet or power calculation.

Do I need to enter proprietary part numbers?

No. The calculation uses thermal numbers such as watts, C, thetaJC, thetaCS, and thetaSA. Generic design values are enough when you do not want copied results to reveal a specific product or customer design.

Glossary:

Junction temperature
The semiconductor die temperature estimated from ambient temperature, power, and the thermal-resistance stack.
Ambient temperature
The local air temperature around the heat sink, which may be hotter than room air outside the enclosure.
thetaJC
Junction-to-case thermal resistance from the device datasheet.
thetaCS
Case-to-sink thermal resistance from the mounting interface, including paste, pad, insulator, contact quality, and pressure.
thetaSA
Heat-sink-to-ambient thermal resistance from the heat-sink rating and airflow condition.
Design margin
The temperature gap subtracted from Tj max before sizing the working thermal budget.

References: