Engineering Reference
Thermal Resistance, Conductivity and Heat-Flow Reference
Quick-reference thermal resistance, conductivity, conductance, convection, heat capacity, time constants, interface layers, heat flow, units, and model boundaries.
- Reading Time
- 14 min
- Format
- Thermal lookup
- Updated
- September 27, 2026
Thermal Quantity Lookup
| Quantity | Symbol | Unit | Meaning |
|---|---|---|---|
| Heat flow / power | Q̇, P | W | Rate of thermal energy transfer |
| Temperature difference | ΔT | K or °C difference | Driving potential for heat flow |
| Thermal resistance | Rθ | K/W or °C/W | Temperature rise per watt between defined nodes |
| Thermal conductance | Gθ | W/K | Reciprocal of thermal resistance |
| Thermal conductivity | k | W/(m·K) | Material property used in conduction |
| Convection coefficient | h | W/(m²·K) | Boundary coefficient dependent on fluid, geometry, orientation, and flow |
| Specific heat capacity | cp | J/(kg·K) | Energy per unit mass per kelvin |
| Thermal capacitance | Cth | J/K | Stored thermal energy per kelvin; commonly mcp |
| Thermal time constant | τth | s | First-order product RθCth |
| Heat flux | q″ | W/m² | Heat flow per area |
| Interface thickness | t | m | Conduction path through a layer |
| Contact resistance | Rc | K/W | Interface contribution not captured by bulk conductivity alone |
Heat-Flow Relationship Index
| Model | Reference relationship | Boundary |
|---|---|---|
| Temperature rise | ΔT = P Rθ | Steady-state heat flow between the same two nodes |
| Series network | Rtotal = ΣRi | Same heat flow passes through each element |
| Parallel network | 1/Rtotal = Σ(1/Ri) | Paths connect the same temperature nodes |
| Plane-wall conduction | Rcond = t/(kA) | One-dimensional uniform layer |
| Convection | Rconv = 1/(hA) | Uniform effective h and surface temperature |
| Thermal capacitance | Cth = mcp | Lumped, approximately uniform body temperature |
| First-order transient | τ = RθCth | Single dominant resistance-capacitance model |
| Derating | Pallow = (Tlimit − Tambient)/Rθ | Only when the stated path and conditions apply |
Thermal Path Definitions
| Path | Nodes | Use | Caution |
|---|---|---|---|
| RθJC | Junction to case | Package internal path toward a specified case point | Case temperature must be measured at the defined location |
| RθJB | Junction to board | Package-to-board characterization | Board spreading and test method matter |
| RθJA | Junction to ambient | System-level estimate under stated board and airflow | Not a package-only constant |
| RθCS | Case to heat sink | Interface and mounting path | TIM thickness, pressure, flatness, and area matter |
| RθSA | Heat sink to ambient | Heat-sink convection/radiation performance | Orientation and airflow must match |
| Enclosure path | Internal air/surfaces to ambient | Cabinet temperature-rise estimate | Distributed sources and ventilation can invalidate a single-node model |
Worked Reference Examples
| Case | Inputs | Ideal result | Interpretation |
|---|---|---|---|
| Temperature rise | P = 5 W, Rθ = 8 K/W | ΔT = 40 K | Add to the correct reference-node temperature |
| Series path | 2 + 0.5 + 6 K/W | 8.5 K/W | All three must carry the same heat flow |
| Parallel paths | 10 K/W || 20 K/W | 6.667 K/W | Valid only between common nodes |
| TIM layer | t = 0.2 mm, k = 3 W/(m·K), A = 400 mm² | R ≈ 0.167 K/W | Contact resistance may be comparable or larger |
| Convection | h = 25 W/(m²·K), A = 0.04 m² | R = 1 K/W | h is an operating-condition estimate |
| Thermal transient | Rθ = 4 K/W, Cth = 50 J/K | τ = 200 s | About 63.2% of final rise after one ideal time constant |
Common Thermal-Model Mistakes
- Treating RθJA as a universal package constant.
- Adding thermal resistances that do not share one heat-flow path.
- Combining parallel paths that do not connect the same nodes.
- Confusing conductivity k with conductance or thermal resistance.
- Using bulk TIM conductivity without bond-line and contact resistance.
- Assuming a fixed convection coefficient across airflow and orientation.
- Ignoring radiation when it is material to the temperature range.
- Using steady-state temperature rise to predict transient warm-up.
- Assuming every system has one thermal time constant.
- Mixing absolute temperature with temperature difference units.
- Ignoring temperature-dependent power dissipation.
- Treating a simplified enclosure model as CFD or measurement.
Support reference
FAQ
What is thermal resistance?
Thermal resistance is the temperature difference between two defined nodes divided by heat flow, commonly in K/W or °C/W.
Are K/W and °C/W equivalent?
For temperature differences, one kelvin has the same magnitude as one degree Celsius, so K/W and °C/W are numerically equivalent.
What is thermal conductivity?
Thermal conductivity k is a material property in W/(m·K). Geometry converts it into a thermal resistance.
How are thermal resistances combined?
Series resistances add. Ideal parallel paths combine through reciprocal conductance, provided they connect the same thermal nodes.
What is a convection coefficient?
The coefficient h relates convective heat flow to area and surface-to-fluid temperature difference. It is condition-dependent, not a universal material constant.
What is thermal capacitance?
Thermal capacitance is stored heat per kelvin, often estimated as mass times specific heat capacity.
What is a thermal time constant?
For a first-order lumped model, τ = RθCth. Distributed systems can require multiple time constants.
Why does a thermal interface material need pressure?
Bulk conductivity alone does not define interface performance. Thickness, bond-line uniformity, contact pressure, surface finish, and voids affect resistance.
Does junction-to-ambient resistance apply to every PCB?
No. Published values depend on a stated test board, copper, airflow, orientation, and power distribution.
Can natural and forced convection use the same h value?
No. Airflow, orientation, geometry, fluid properties, and temperature difference can substantially change h.
Does steady-state temperature predict warm-up time?
No. Steady-state resistance sets the final temperature rise; thermal capacitance and network dynamics determine transient response.
Does this page cover RTD or thermistor curves?
No. RTD and NTC/TCR data are reserved for their dedicated References.
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