An LED converts 30–50% of input power to light. The rest becomes heat, at the junction. If that heat doesn't leave the chip, the chip dies. The aluminium housing isn't decoration, it's the lifeline.
Four ideas to carry into the next specification conversation you have.
LED life data (LM-80) is reported at Tj. Every 10 °C above rated Tj cuts useful life roughly in half. Spec a maximum Tj, then size the heatsink.
Pure Al (1100) ≈ 220 W/m·K. Cast Al (A380) ≈ 100. Extruded 6063 (the usual heatsink) ≈ 200. Pick the alloy for the job.
A 1 kg fin-dense heatsink dissipates more than a 2 kg solid block. Fin spacing 5–8 mm for natural convection; closer for forced air.
Thermal Interface Material between LED MCPCB and heatsink. Without it, the air gap at the contact undoes everything. Spec a graphite/ceramic pad or thermal grease.
Thermal conductivity at 25 °C, typical use in lighting fixtures.
| Alloy | Conductivity · process | Typical use |
|---|---|---|
| 1100 (pure) | 220 W/m·K · sheet | Reflectors, decorative |
| 6063 (extruded) | 200 W/m·K · extrusion | Linear heatsinks · streetlight pole-arm |
| 6061 (structural) | 170 W/m·K · machined | Custom heatsinks · brackets |
| A380 (die-cast) | 96 W/m·K · pressure die-cast | Highbay shell · outdoor housings |
| A360 (die-cast premium) | 120 W/m·K · pressure die-cast | Premium downlight bodies |
| ADC12 (Japanese standard) | 92 W/m·K · die-cast | Asian-market fixtures |
| Copper (Cu) | 385 W/m·K · sheet/machined | PCB inserts · premium projects |
Five thermal-design ideas that separate good fixtures from great ones.
Heat travels Tj → solder → MCPCB → TIM → heatsink → ambient. Each interface adds thermal resistance (°C/W). A premium fixture totals < 5 °C/W; a budget one > 12 °C/W. At 10 W heat dissipation that's a 50 °C vs 120 °C rise, and the LED life ratio that follows.
A Metal-Core PCB has a thin dielectric on aluminium (1.5 W/m·K dielectric) instead of FR4 (0.3 W/m·K). Heat flows ~5× better. Premium fixtures use 2 mm aluminium-core MCPCB; budget ones use 1 mm FR4 with thermal vias, much worse thermal path.
Natural convection wants tall, vertically oriented fins with 5–8 mm spacing. The fin height should be ~3× the spacing for optimal boundary-layer thinning. A horizontal heatsink loses 40% capacity vs the same area vertical.
A bare aluminium heatsink has 0.05 emissivity, terrible at radiating heat. Anodised black raises it to 0.85, adding ~10% to total dissipation. Black powder-coat is fine; do not specify mirror-polished outdoor heatsinks. Inside an enclosed fixture, surface finish matters less.
Drill an M3 hole at the spec'd "tc" point. Run the fixture at rated current in a 25 °C ambient. After 30 minutes, the thermocouple reading should match the datasheet ±2 °C. If not, the fixture is under-spec'd thermally, even if it looks identical to a brand you trust.
Every NLC outdoor and industrial fixture is thermally validated at Tj target before mass production, not just calculated.
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