Heat Sink Thermal Resistance Calculator
Calculate required heatsink sink-to-ambient thermal resistance, junction temperature, sink temperature, case temperature, thermal path drops, and heatsink suitability using the simplified steady-state series path RθJC + RθCS + RθSA.
THM-002 is a generic heatsink sizing workflow. It accepts known device, interface and heatsink thermal resistances, but it does not predict package-specific RθJA, airflow from CFM, interface contact resistance, transient temperature, or multi-device thermal networks.
Engineering tool
Heat Sink Thermal Resistance Calculator
Calculate required heatsink RθSA, junction temperature, sink temperature, thermal path drops, suitability margin, and maximum steady-state thermal power.
Known ambient or reference air temperature around the heatsink.
Absolute junction temperature limit for the modeled device.
Heat flowing through the modeled heatsink path, not necessarily electrical input power.
Junction-to-case thermal resistance. Zero is allowed only as an idealized limit.
Case-to-sink interface resistance from pad, grease, washer, insulator, or mounting interface.
Result console
- Maximum Allowable RθSA
- 3.5°C/W
- Total Thermal Resistance Budget
- 5°C/W
- Package + Interface Rθ
- 1.5°C/W
- Requirement Status
- Feasible passive heatsink target
Select a heatsink with rated RθSA less than or equal to this maximum value under the relevant airflow, orientation, and mounting conditions.
Formula
Formula reference
Heat-sink thermal path formulas
Lower RθSA generally means better heatsink performance. The calculated RθSA,max is a maximum allowable value.
Rθ,total = RθJC + RθCS + RθSATj = Ta + P(RθJC + RθCS + RθSA)Tc = Ta + P(RθCS + RθSA)Ts = Ta + P RθSARθSA,max = (Tj,max - Ta) / P - RθJC - RθCSPmax = (Tj,max - Ta) / Rθ,totalMargin = Tj,max - TjVariable definitions
- RθJC
- junction-to-case thermal resistance
- RθCS
- case-to-sink thermal resistance
- RθSA
- sink-to-ambient thermal resistance
- P
- dissipated heat through the modeled path
- Ta
- ambient temperature
- Tj
- junction temperature
Heat Sink Thermal Resistance Formula Audit
| Thermal Path Definition | Junction -> Case -> Interface -> Heat Sink -> Ambient. |
|---|---|
| RθJC Definition | RθJC is junction-to-case thermal resistance. |
| RθCS Definition | RθCS is case-to-sink thermal resistance. |
| RθSA Definition | RθSA is sink-to-ambient thermal resistance. |
| Total Thermal Resistance Formula | Rθ,total = RθJC + RθCS + RθSA. |
| Junction Temperature Formula | Tj = Ta + P(RθJC + RθCS + RθSA). |
| Case Temperature Formula | Tc = Ta + P(RθCS + RθSA). |
| Sink Temperature Formula | Ts = Ta + P RθSA. |
| Required RθSA Formula | RθSA,max = (Tj,max - Ta) / P - RθJC - RθCS. |
| Maximum Power Formula | Pmax = (Tj,max - Ta) / (RθJC + RθCS + RθSA). |
| Temperature Margin Formula | Margin = Tj,max - Tj. |
| Rθ Margin Formula | Rθ margin = RθSA,max - RθSA,selected. |
| Thermal Path Drop Formulas | ΔTJC = P RθJC, ΔTCS = P RθCS, ΔTSA = P RθSA. |
| RθJA Boundary | RθJA is not added to the explicit JC + CS + SA path. |
| Power / Heat Definition | P is heat dissipated through the modeled path. |
| Steady-State Boundary | Transient thermal impedance and thermal capacitance are outside THM-002. |
| Parallel-Path Boundary | Real parallel heat paths are simplified into this one-dimensional model. |
| Multiple-Device Boundary | V1 assumes one equivalent heat source on the sink. |
| Airflow Boundary | RθSA must match actual airflow, orientation and mounting conditions. |
| Interface Boundary | RθCS is entered as known; THM-002 does not derive detailed TIM resistance. |
| Impossible-RθSA Policy | Negative RθSA requirements are reported as infeasible, not recommended. |
| Zero-RθSA Policy | Zero RθSA budget is flagged as no practical passive heatsink margin. |
| Absolute Temperature Convention | Ambient and junction limits are absolute temperatures. |
| Temperature Difference Convention | Thermal drops use temperature-difference units. |
Worked Examples
| Example | Calculation | Result |
|---|---|---|
| Ta = 25°C, Tjmax = 125°C, P = 20 W, RθJC = 1, RθCS = 0.5 | RθSA,max = 100/20 - 1 - 0.5 | 3.5°C/W |
| Selected RθSA = 3°C/W, same conditions | Tj = 25 + 20(1 + 0.5 + 3) | 115°C, margin 10°C |
| Selected RθSA = 3.5°C/W | Tj = 25 + 20(5) | 125°C, margin 0°C |
| Selected RθSA = 4°C/W | Tj = 25 + 20(5.5) | 135°C, margin -10°C |
| Ta = 25°C, P = 10 W, RθSA = 3.5 | Ts = 25 + 10(3.5) | 60°C |
| Same with RθCS = 0.5 | Tc = 60 + 10(0.5) | 65°C |
| Same with RθJC = 1 | Tj = 65 + 10(1) | 75°C |
| P = 10 W, RθJC = 1 | ΔTJC = 10 × 1 | 10°C |
| P = 10 W, RθCS = 0.5 | ΔTCS = 10 × 0.5 | 5°C |
| P = 10 W, RθSA = 3.5 | ΔTSA = 10 × 3.5 | 35°C |
| Path drops | 10 + 5 + 35 | 50°C total ΔT |
| Rθ values | 1 + 0.5 + 3.5 | 5°C/W total |
| Ta = 25°C, Tjmax = 125°C, Rθtotal = 5 | Pmax = 100 / 5 | 20 W |
| RθJC = 1, RθCS = 0.5, RθSA = 3.5 | Pmax = (125 - 25) / 5 | 20 W |
| 25°C absolute | TK = 25 + 273.15 | 298.15 K |
| 50°C temperature difference | ΔK = Δ°C | 50 K |
| 50°C temperature difference | Δ°F = 50 × 9/5 | 90°F |
| RθJC = 0 | Idealized component path | Allowed |
| RθCS = 0 | Idealized interface | Allowed |
| RθJC < 0 | Negative thermal resistance | Rejected |
| RθCS < 0 | Negative interface resistance | Rejected |
| RθSA < 0 | Negative heatsink resistance | Rejected |
| P = 0 in sizing mode | RθSA,max division by zero | Rejected |
| Tjmax <= Ta | No positive passive thermal budget | Rejected |
| Ta = 50°C, Tjmax = 100°C, P = 20 W, RθJC = 2, RθCS = 1 | Raw RθSA,max = 2.5 - 3 | Infeasible, not -0.5°C/W recommendation |
| RθSA,max = 0 | Only ideal 0°C/W sink could meet limit | No practical passive margin |
Engineering Notes
Heat Sink Thermal Resistance
Heatsink thermal resistance describes heat flow from the sink to ambient air.
Junction Temperature
Junction temperature is the highest modeled device node in this simplified path.
Case Temperature
Case temperature sits between the junction and the case-to-sink interface.
Heat Sink Temperature
Sink temperature is the temperature rise caused by RθSA above ambient.
Ambient Temperature
Ambient is treated as a known boundary condition, not an enclosure-rise calculation.
RθJC
Use the device datasheet value for the correct package and mounting boundary.
RθCS
Interface resistance depends on pad, grease, washer, mounting pressure and surface quality.
RθSA
Heatsink rating depends on airflow, orientation, fin geometry and test condition.
Thermal Resistance Budget
The total budget is split between package, interface and heatsink resistance.
Thermal Margin
Negative margin means the modeled junction temperature exceeds the limit.
Natural Convection
Natural-convection RθSA ratings assume no forced airflow and can be orientation-sensitive.
Forced Air
Forced-air ratings should be used only when the actual airflow is comparable.
RθJA Boundary
Do not add RθJA as another series term in the explicit JC + CS + SA path.
Steady-State Boundary
This calculator does not model startup, pulses or thermal capacitance.
Multiple Thermal Paths
Real products may also conduct heat through PCB, leads, chassis and airflow paths.
Common Mistakes
- Thinking larger RθSA means better cooling.
- Treating RθSA,max as a minimum required thermal resistance.
- Adding RθJA to RθJC + RθCS + RθSA.
- Forgetting RθCS in the thermal path.
- Forgetting ambient temperature.
- Using electrical input power instead of heat loss.
- Treating a heatsink rating as fixed under all airflow conditions.
- Recommending a negative RθSA instead of flagging an infeasible condition.
- Taking the absolute value of negative margin.
- Confusing sink temperature with junction temperature.
- Confusing case temperature with sink temperature.
- Using steady-state results as transient predictions.
- Calculating multiple devices independently on one shared sink.
- Treating °C/W as an absolute temperature.
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Open CalculatorFAQ
Support reference
FAQ
How do I calculate the required heatsink thermal resistance?
Use RθSA,max = (Tj,max - Ta) / P - RθJC - RθCS. Select a heatsink whose rated RθSA is at or below this value under the same airflow and mounting conditions.
What is RθSA?
RθSA is sink-to-ambient thermal resistance. It describes how much the heatsink rises above ambient for each watt of heat.
What is RθJC?
RθJC is junction-to-case thermal resistance. It describes the thermal path from the semiconductor junction to the package case.
What is RθCS?
RθCS is case-to-sink thermal resistance. It includes thermal pad, grease, washer, insulator and mounting interface effects.
How do I calculate junction temperature with a heat sink?
Use Tj = Ta + P(RθJC + RθCS + RθSA).
How do I calculate heat sink temperature?
Use Ts = Ta + P RθSA in the simplified series heatsink model.
How do I calculate case temperature?
Use Tc = Ts + P RθCS, or equivalently Tc = Tj - P RθJC.
What thermal resistance should my heat sink have?
The selected heatsink should have RθSA less than or equal to the calculated maximum allowable RθSA, with practical design margin.
Is a lower °C/W heat sink better?
Yes, lower sink-to-ambient thermal resistance usually means better cooling for the same airflow and mounting condition.
What does a 3°C/W heat sink mean?
A 3°C/W heatsink rises about 3°C above ambient per watt under its rated test condition.
Can I add RθJA to RθJC and RθSA?
No. RθJA is usually an equivalent junction-to-ambient path for a specific test setup. It should not be added as an extra term to RθJC + RθCS + RθSA.
What happens if the required RθSA is negative?
The package plus interface path already exceeds the thermal budget, so no positive passive heatsink RθSA can satisfy the entered limit.
What does zero required RθSA mean?
It means only an ideal zero sink-to-ambient thermal resistance would meet the model. That is not a practical passive heatsink target.
How does thermal interface material affect heat sink performance?
The interface contributes RθCS. Pad material, thickness, contact area, mounting pressure, flatness and voids can all change the actual value.
Does airflow change heatsink thermal resistance?
Yes. Heatsink RθSA often changes strongly with natural convection, forced airflow, orientation and air velocity.
Can I use electrical input power as dissipated heat?
Only if that electrical input power actually becomes heat in the modeled path. In efficient systems, the heat load is usually the loss.
Does this calculator include transient thermal impedance?
No. THM-002 is steady-state only and does not model Zθ(t), pulsed heating or thermal capacitance.
Can multiple devices share one heat sink?
Yes physically, but V1 assumes a single equivalent heat source. Shared-sink multi-device analysis needs additional thermal-network modeling.
What is the difference between this calculator and THM-001?
THM-001 uses one equivalent thermal resistance. THM-002 expands that into the explicit junction-to-case, case-to-sink and sink-to-ambient heatsink path.
What is thermal margin?
Thermal margin is Tj,max - Tj. Negative margin means the selected heatsink path exceeds the modeled junction limit.
Engineering Disclaimer
This calculator provides first-pass steady-state heatsink estimates. Production thermal design should verify component datasheets, heatsink ratings, airflow, orientation, mounting, interface materials, PCB heat spreading, transient behavior, electrical limits, SOA, reliability derating and measured hardware temperatures.