ELECTRONICS COOLING CALCULATOR

Thermal Resistance Calculator

Calculate the maximum allowable heat-sink thermal resistance, estimate junction temperature, and check steady-state thermal headroom for a semiconductor cooling path.

Required RθSA Junction estimate °C / °F conversion
Start Calculation Thermal Validation
THERMAL NETWORK STEADY STATE · REV 2
PD 35 W HEAT SOURCE
TJ 87.3°C
RθSA 0.80°C/W
TA 40.0°C
THERMAL PATH TJ = TA + PD × (RθJC + RθCS + RθSA)

INTERACTIVE THERMAL CHECK

Size the Heat Sink and Check Junction Temperature

Enter the real heat load, ambient limit, package path, interface resistance, and selected heat-sink rating.

01

OPERATING LIMITS

Heat Load and Temperature

W
Heat generated at the sustained design workload.
°C
°C
°C
Subtracted from the maximum junction limit.
02

SERIES HEAT PATH

Package, Interface, and Sink

°C/W
Use the exact package and modeled case surface.
°C/W
Include TIM, mounting pressure, and contact quality.
°C/W
Use the rating at the intended airflow and orientation.
Do not enter RθJA as RθJC. These metrics use different thermal boundaries.
MAXIMUM ALLOWABLE HEAT-SINK RESISTANCE ≤ 1.02 °C/W Choose a lower RθSA value for more cooling margin.
Design Junction Target 95.0 °C
Estimated Junction 87.3 °C
Maximum Total Path 1.57 °C/W
Selected Total Path 1.35 °C/W
Thermal Headroom 7.8 °C
Budget Utilization 85.9%
Estimated Case73.3 °C
Estimated Sink Base68.0 °C
PRELIMINARY THERMAL CHECK PASSES

The selected path remains below the design target. Validate airflow, mounting, TIM application, and workload on finished hardware.

ACTIVE MODEL TJ = TA + PD × (RθJC + RθCS + RθSA)
Thermal Review

PRACTICAL WORKFLOW

Use the Result as a Thermal Design Screening Check

Move from measured heat load to heat-sink selection, then verify the complete enclosure under the worst operating condition.

01

Define the Heat Load

Use measured package power or a conservative loss estimate at the sustained design workload.

02

Screen the Cooling Path

Compare the calculated RθSA limit with heat-sink data at the intended airflow and orientation.

03

Validate the Assembly

Measure temperatures inside the final enclosure at worst-case ambient, fan condition, and workload.

CALCULATION METHOD

A Steady-State Series Thermal-Resistance Model

The available temperature rise is distributed across the package, interface, and heat sink.

01

Design Junction Target

TJ,target = TJ,max − Margin
02

Maximum Total Path

Rθpath,max = (TJ,target − TA) ÷ PD
03

Required Heat-Sink Performance

RθSA,max = Rθpath,max − RθJC − RθCS
04

Estimated Junction Temperature

TJ,est = TA + PD × (RθJC + RθCS + RθSA)
Model boundary: this calculator assumes one dominant, steady-state junction-to-case-to-sink-to-air path. It does not replace transient impedance, airflow, or full-system thermal testing.

INPUT QUALITY

Use Thermal Inputs That Match the Real Assembly

Accurate inputs matter more than extra decimal places.

PD

Heat Dissipation

Use package power or calculated device loss at sustained worst-case workload, not total system input power.

TA

Inlet Ambient

Use air entering the heat sink inside the enclosure, including recirculation and nearby heat sources.

RθJC

Package Path

Use the exact package value for the case surface connected to the cooling assembly.

RθCS

Interface Resistance

Account for TIM, bond-line thickness, surface flatness, contact area, and mounting pressure.

RθSA

Heat-Sink Rating

Use the rating at the intended airflow, ducting, orientation, and inlet temperature.

Highest confidenceMeasured configured hardware
Good screening inputMatched manufacturer curves
Early estimate onlyConservative assumptions

ENGINEERING SCOPE

What the Calculator Covers and What Still Requires Testing

Use the result as an initial thermal budget, not a qualification record.

Included in the Model

  • Steady-state device heat dissipation
  • Maximum inlet ambient temperature
  • Junction-temperature design margin
  • RθJC, RθCS, and RθSA series path
  • Estimated junction, case, and sink temperatures

Verify on Finished Hardware

  • Transient power bursts and thermal capacitance
  • Airflow, fan curves, bypass, and recirculation
  • PCB and enclosure heat spreading
  • TIM aging, mounting variation, and tolerances
  • Thermal throttling and production spread

ENGINEERING TRUST LAYER

Connect the Calculation to Documented Validation

Review internal test methods, reference configurations, and downloadable engineering guidance before design release.
Validation Methods Reference Platforms Engineering PDFs

THERMAL CONFIGURATION REVIEW

Need the Cooling Path Checked Against Real Hardware?

Send the device, sustained power, enclosure, ambient range, heat sink, fan, and mechanical constraints.

Request Engineering Review

ENGINEERING TOOL LIBRARY

Continue the Power, PCB, and System Design Check

Move from thermal sizing to power delivery, PCB design, mechanical clearance, and reliability planning.

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THERMAL DESIGN FAQ

Thermal Resistance Calculator Questions

Use these answers to avoid the most common heat-sink sizing errors.

Each answer focuses on the inputs, formulas, and limits used by this calculator.

Ask a Thermal Question
How do I calculate required heat sink thermal resistance?

Subtract RθJC and RθCS from the allowable junction-to-ambient resistance: (TJ,target − TA) ÷ PD. Select a heat sink with RθSA at or below that result.

What heat sink RθSA is needed for a 35 W processor?

It depends on ambient temperature, junction target, RθJC, and RθCS. With the default inputs, the maximum allowable RθSA is approximately 1.02 °C/W.

How do I estimate junction temperature from RθJC and RθSA?

Add RθJC, RθCS, and RθSA, multiply by device heat dissipation, then add inlet ambient temperature. The result is the estimated steady-state junction temperature.

Can thermal resistance be converted from °C/W to °F/W?

Yes. Multiply °C/W by 1.8 to obtain °F/W. The unit switch converts temperatures and thermal resistances while preserving the same physical design condition.

Why does higher ambient temperature reduce thermal headroom?

Higher inlet air temperature leaves less allowable temperature rise between ambient and the junction target, so the cooling path must provide lower total thermal resistance.

Can this calculator size a fan-cooled heat sink?

Yes, when the entered RθSA matches the intended airflow and system resistance. Verify fan curves, bypass, recirculation, degradation, and enclosure temperature on finished hardware.