Transient Heatsink Simulator

Lumped thermal RC model that predicts how a heat source on a heatsink warms, cools, and responds to pulses or duty cycles. Reports time-to-limit, peak temperature, cyclic steady state, and cooldown time. For steady-state plate-fin sizing use the Heatsink Designer; for resistance- only network budgets use the Thermal Path Budget.

Inputs

Thermal Stack (Package On Heatsink)

Ambient temperature (°C)

Initial temperature (°C)

Starting temperature of all dynamic nodes.

Source max temperature (°C)

R_jc (K/W)

Source-to-case (junction-to-case + interface)

Sink properties source

R_sa (K/W)

Sink-to-ambient (steady-state)

Capacitance input mode

C_source (J/K)

Source/package thermal mass

C_sink (J/K)

Heatsink thermal mass

Source body

Material

Volume (cm³)

C = ρ V c_p = —

Heatsink body

Material

Volume (cm³)

C = ρ V c_p = —

R_sa and C_sink are computed from heatsink geometry using the plate-fin steady-state solver. You still enter C_source below.

Base length L (mm)

Along airflow

Base width W (mm)

Base thickness (mm)

Fin height (mm)

Fin thickness (mm)

Fin count

Heatsink material

Airflow

R_sa—

C_sink—

Sink volume—

Sink mass—

C_source (J/K)

Source/package thermal mass — not derivable from heatsink geometry

Power Profile

Profile type

Power (W)

Run time (s)

Pulse power (W)

Pulse duration (s)

Cooldown duration (s)

On power (W)

On time (s)

Off power (W)

Off time (s)

Total simulation time (s)

Long enough to reach cyclic steady state — try 8–10× the slowest time constant.

Cooldown Target (optional)

Cool-to temperature (°C)

Reports the time to drop below this value after the simulated peak. Only meaningful when the profile includes a cooldown phase (pulse or duty); reads "—" for steady-state step loads.

Sample Test Cases

Set inputs and press Calculate to simulate.

Nodes

Node	Capacitance (J/K)	Limit (°C)	Peak (°C)	Margin (K)

Segments

Segment	R (K/W)	τ = R·C_eff (s)

Time constants (per node)

Node	C (J/K)	Σ G (W/K)	τ = C / Σ G (s)

Cycle summary (duty cycle only)

Cycle	Peak (°C)	Min (°C)	Walk-up (°C)

Single-parameter sweep

Hold every other input constant and vary one parameter across a relative range. Useful for ranking design levers when peak temperature or time-to-limit is borderline. Run the main calculation first; the sweep uses those inputs as the baseline.

Parameter

Fractions of baseline

Comma-separated multipliers. Baseline (1.0) is always included.

Run a calculation first to set the baseline.

Resistance vs. thermal mass

Compares the peak-temperature drop from halving R_sa against the drop from doubling C_sink at the current baseline.

Governing equation

C_i \frac{dT_i}{dt} = Q_i(t) + \sum_j \frac{T_j - T_i}{R_{ij}}

Each dynamic node carries thermal capacitance \(C_i\) and exchanges heat with its neighbors through thermal resistances \(R_{ij}\). The right-hand side balances time-varying heat input \(Q_i(t)\) against conductive flow to the rest of the network.

Implicit-Euler discretization

\left(\frac{C_i}{\Delta t} + \sum_j G_{ij}\right) T_i^{n+1} - \sum_j G_{ij} T_j^{n+1} = \frac{C_i}{\Delta t} T_i^n + Q_i^{n+1} + \sum_{j \in \text{boundary}} G_{ij} T_j

Implicit Euler is unconditionally stable for the linear thermal problem, so the time step is chosen for resolution rather than stability. Boundary-node terms move to the right-hand side.

One-lump analytical reference

T(t) = T_\infty + Q R \left(1 - e^{-t/\tau}\right) + (T_0 - T_\infty)\, e^{-t/\tau}, \quad \tau = R C

For one dynamic node and a constant heat input, this is the closed- form response. The general numerical solver reduces to this case in the simple-lump limit.

Substituted result

Run a calculation to see the substituted form.

Resistance vs. capacitance — what each one controls

A heatsink design has two largely independent levers. Thermal resistance (R_jc, R_sa) sets the steady-state temperature: at steady state, \(T_{source} = T_\infty + Q\,(R_{jc} + R_{sa})\) regardless of any thermal mass in the system. Thermal capacitance (C_source, C_sink) sets how fast the system gets there. Their product, the time constant \(\tau = RC\), governs the rise and decay.

Practical consequences:

Long, continuous load — peak temperature converges to the asymptote, so resistance dominates. Mass only delays the inevitable.
Short, intense pulse — the system never reaches the asymptote. Peak rise is roughly \(Q\,t / C\) for a lumped sink, so capacitance dominates and resistance barely matters within the pulse.
Repeated duty cycle — both matter. Cycle-to-cycle walk-up is set by mean power × R; ripple amplitude is set by on-time × Q / C.

Why source temperature can rise much faster than the sink

If the source has a small thermal mass (C_source) and the path to the sink is resistive (R_jc > 0), its temperature can rise sharply while the heavier sink remains comparatively cool. For a constant positive load starting from a uniform ambient temperature, this passive linear RC model approaches its constant-power steady-state temperature from below; it does not overshoot that asymptote. A finite pulse can still violate the source limit long before the sink appears hot, so peak source temperature remains the safety criterion for transient loads.

Why mass delays without changing the asymptote

Doubling C_sink doubles the slow time constant — useful when the load duration is comparable to or shorter than \(\tau\). But for an infinite-duration load, the steady-state asymptote depends only on R and the heat input. Adding mass is therefore the right move for pulse and burst survival; it is the wrong move when continuous heat dissipation is the constraint.

Use the Sensitivity tab's "Compare design levers" button to see which lever applies to your specific profile.

Estimating capacitance responsibly

Heatsink C is usually defensible: \(C = \rho V c_p\), with the geometry-from-plate-fin mode giving you V directly. Source/package C is harder — package datasheets rarely publish capacitance, and "die" doesn't equal "thermal mass that participates." Common practice:

Use vendor-supplied transient impedance curves (Z_θJC(t)) when available; fit a single-time-constant pole to the early portion.
For an unknown package, estimate from observed warm-up time of a real unit at known power.
Treat C_source as approximate; a factor-of-2 uncertainty is common.

Limitations of lumped RC models

This tool models a single dominant temperature per body. Real heatsinks have spreading (hot under the source, cool at the edges), fin gradients (the fin tip is cooler than the base), and airflow non-uniformity (downstream channels are hotter than upstream). The lumped model is accurate when:

Bi < 0.1 inside each lump (the body is approximately isothermal)
Heat path topology is series-dominated, not multi-path
Material properties don't change much over the temperature range (typical for metals 25–150 °C)
Convection coefficients are roughly constant over the transient (may break under fan ramping)

When to escalate

3D temperature gradients matter — die-attach hotspots, fin-base spreading, embedded sensors at specific locations: use FEA.
Airflow is the unknown — bypass, recirculation, fan placement: use CFD or flow visualization.
Validation against hardware — pulse a real unit and instrument it. Lumped RC models are very accurate for averaged temperatures of components they model, and very weak for things they don't.

References

Incropera, DeWitt, Bergman, Lavine. Fundamentals of Heat and Mass Transfer. Wiley.
Çengel, Ghajar. Heat and Mass Transfer: Fundamentals and Applications. McGraw-Hill.
Bar-Cohen, Kraus. Advances in Thermal Modeling of Electronic Components and Systems. ASME Press.
JEDEC JESD51 family — thermal resistance and transient thermal impedance terminology for semiconductor packages.