LED Thermal Design
Theory: a chain of thermal resistances
Overview
I thought LEDs were efficient and stayed cool. Does thermal design matter?
Very much. LEDs beat incandescent lamps, but over half the input often becomes heat, concentrated in a tiny junction and leaving mostly by conduction into the board rather than as infrared. Higher junction temperature means less light and shorter life. Representing the heat path as a chain of thermal resistances shows the bottleneck. Let's calculate a luminaire with ten 3 W LEDs and compare board and heat-sink choices.
Junction temperature
$R_{j\text{-}sp}$ is junction-to-solder-point resistance (datasheet value, here 4 K/W), $R_{board}$ through the board, $R_{hs}$ heat sink to air. The heat sink is shared by N LEDs, so it carries the total heat.
Board thermal resistance
On MCPCB the thin dielectric dominates and the aluminium below spreads heat well. On FR-4, via plating is the main path. Both are 1D estimates under the pad, excluding lateral spreading in copper.
From incandescent to LED
Incandescent lamps turn over 90% of their input into heat, but most leaves the bulb as infrared radiation in the direction of illumination. LEDs are far more efficient yet considered harder to cool, because their heat concentrates in a few-millimetre die and must be conducted out of the back rather than radiated. As white LEDs spread into lighting in the 2000s, luminaires integrating aluminium boards and heat sinks appeared.
Worked example: ten 3 W LEDs
Each LED: 3 W input, 35% wall-plug efficiency (1.95 W heat), 3 mm pad; ambient 40 °C.
Board and heat sink combinations
| Board | Board resistance | Forced air 2 K/W | Natural 4 K/W |
|---|---|---|---|
| MCPCB (0.1 mm dielectric) | 5.1 K/W | 96.6 °C | 135.6 °C |
| FR-4 + 9 thermal vias | 20.4 K/W | 126.6 °C | 165.6 °C |
Values are junction temperatures. FR-4 without vias, estimated under the pad alone, gives hundreds of K/W — impractical even allowing for lateral spreading.
Heat sink for Tj ≤ 105 °C
| Board | Required heat sink resistance |
|---|---|
| MCPCB | 2.43 K/W or less |
| FR-4 + 9 vias | 0.89 K/W or less |
Effect of efficiency (MCPCB, forced air)
| Wall-plug efficiency | Heat per LED | Junction temperature |
|---|---|---|
| 35% | 1.95 W | 96.6 °C |
| 50% | 1.50 W | 83.6 °C |
Changing the board alone shifts Tj by 30 K, and with FR-4 the heat-sink requirement is nearly three times tighter.
LED heat concentrates in a few-millimetre pad, so thin layers under it dominate. MCPCB's dielectric is only 0.1 mm thick and about seven times as conductive as FR-4, giving about 5 K/W; FR-4 must pass 1.6 mm through thin via copper, giving 20 K/W. At 1.95 W, that difference is about 30 K. The heat sink carries all ten LEDs' heat, so its budget is what remains after the board has taken its share; FR-4 uses up so much that little margin is left. Efficiency helps too: from 35% to 50% heat drops 23% and junction temperature 13 K.
Design tips
- Check datasheet resistances, allowable junction temperature and lifetime conditions.
- Estimate heat as input minus optical output (check efficiency).
- Review thin layers under the pad (dielectric, solder, TIM) first.
- With many LEDs, account for the shared heat sink and mutual heating.
- Inside fixtures air stagnates; use the in-fixture ambient temperature.
“It dimmed once recessed in the ceiling”
An LED downlight showed ample brightness and life in the lab, but recessed into an insulated ceiling its brightness dropped noticeably within months. The rear heat sink was surrounded by insulation, and ambient far exceeded 40 °C. In thermal-resistance terms, Ta itself rose and the heat sink's resistance worsened. Fixtures that may sit under insulation must be designed for that ambient.
Common mistakes
Mistakes and fixes
| Mistake | Effect | Fix |
|---|---|---|
| Treating input as all light | Heat underestimated | Compute heat from efficiency |
| Ignoring board resistance | Tj too low | Compute pad stack |
| Lab ambient | Overheats in use | Use in-fixture ambient |
| Sizing heat sink for one LED | Array overheats | Use total heat |
| Voids in TIM or solder | Local hot spots | Check assembly quality |
I'd like to learn related topics.
Try the LED junction temperature tool. Related pages include thermal vias, junction temperature, heat sink design and thermal interface materials.
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