LED PCB Design and MCPCB Thermal

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In short: An LED board is a heat-removal problem first and a wiring problem second, so you put the LEDs on an aluminium-core MCPCB whose thin dielectric moves heat into the metal, drive them in series strings from one constant-current source, and spread the parts so no spot runs hotter than its neighbours.

Why LED boards are really thermal designs

A modern power LED converts roughly 30 to 45 percent of its input power into light. The rest, often 60 percent or more, leaves as heat from a die that is only a few square millimetres in area. A 3 W LED therefore dissipates close to 2 W in a tiny volume. Junction temperature then decides almost everything you care about: lumen maintenance (L70 life drops sharply above about 85 °C junction in many datasheets), colour shift, and forward voltage.

Key fact: Every 10 °C of extra junction temperature roughly halves the useful life of many LED packages, which is why a good thermal path is worth more than a brighter chip.

The board's job is to carry that heat from the LED's thermal pad to a heat sink with the smallest possible temperature drop. The general toolbox is covered in our guide to PCB thermal management; here we apply it to the specific case where the heat source is a dense array of LEDs.

Why aluminium-core MCPCBs beat FR-4

Conduction resistance follows R = t / (k × A), where t is thickness, k is thermal conductivity and A is the heat-flow area. Put a 3 W LED with a 10 mm × 10 mm (1×10^-4 m²) thermal footprint on standard 1.6 mm FR-4 (k about 0.3 W/m·K) and the board alone has R = 0.0016 / (0.3 × 0.0001) ≈ 53 K/W. At 3 W that is a 160 K rise before the heat even reaches the air. The die would be well beyond its limit.

A metal-core PCB replaces the thick glass-epoxy core with a three-layer sandwich: a copper circuit layer (35 to 70 µm), a thin thermally conductive dielectric (typically 75 to 150 µm, 1 to 3 W/m·K, premium grades up to 8), and an aluminium base (1.0 to 3.0 mm, alloys such as 5052 or 6061 at roughly 140 to 200 W/m·K). With a 100 µm, 2 W/m·K dielectric, the same footprint gives R = 0.0001 / (2 × 0.0001) = 0.5 K/W. That is about 100 times better than FR-4, and the dielectric, not the aluminium, dominates what remains.

Layout rules for thermal spreading

Once the stack is chosen, placement decides whether the aluminium is actually used well.

• Spread the sources. Heat spreads at roughly 45 degrees through the metal. LEDs placed closer than about twice the base thickness overlap their heat cones and raise each other's temperature. A 10 mm pitch on 1.5 mm aluminium is comfortable; 4 mm is not.
• Use the whole thermal pad. Match the copper land to the package's thermal pad dimensions and keep it unbroken. Avoid thermal-relief spokes under the pad; they are for soldering large copper to through-hole pins, not for heat paths.
• Keep copper pours, but isolate the electrical nets. Fill unused areas with copper on the circuit layer to spread heat laterally, respecting the creepage and clearance rules in our PCB design rules guide, especially on mains-connected luminaires.
• Do not use thermal vias on a single-layer MCPCB. The dielectric already is the via. Vias matter on FR-4 LED boards and are covered in the stackup discussion in PCB stackup design.
• Control the interface. A flat board, a 0.05 to 0.1 mm thermal interface layer, and screw spacing of 40 to 60 mm avoid air gaps that add 1 to 3 K/W of hidden resistance.

Key fact: Doubling the dielectric thickness doubles its thermal resistance, so specify the thinnest dielectric that still passes your hipot test (typically 1.5 to 3 kV AC for mains-class products).

Animation: how heat spreads into the aluminium

Current uniformity: strings, not parallel branches

LED brightness tracks current, and an LED's forward voltage falls by roughly 2 mV per kelvin. Wire two nominally equal LEDs directly in parallel and any small Vf difference (bins differ by 0.1 V or more) makes one take more current. It heats up, its Vf drops further, and it hogs more current: a positive feedback loop called thermal runaway.

The robust fix is a series string. The same current flows through every LED by definition, so brightness is matched regardless of Vf. When the supply voltage cannot cover a long string, use several equal-length strings, each with its own current regulation, or a multichannel driver such as an LED matrix controller that mirrors current into each channel.

Placing the driver and parts for uniform current

• Keep the driver off the hot zone. Buck converter inductors and the controller can reach 80 to 100 °C on their own. Place them at the board edge or on a separate PCB so they do not raise LED temperatures.
• Equalise string length and routing. Equal trace length and copper width mean equal series resistance. A 0.1 Ω mismatch at 1 A is only 100 mV, but on a low-headroom current mirror it can matter.
• Kelvin-sense the current resistor. Route the sense lines directly from the resistor pads to the driver, not through the high-current path.
• Cool the sense element and mirror transistors equally. Put mirror devices next to each other on the same copper area so their Vbe or Vgs tracks.
• Let simulation catch hot spots. Tools such as Cadence Celsius, Siemens Simcenter Flotherm and Ansys Icepak import the board layout and solve conduction and convection together.

Key fact: Hand calculation gets you to within about 20 percent; simulation then finds the real hot spot, which is usually the centre of the densest cluster.

Manufacturing and testing notes

Aluminium boards are scored or routed, not punched freely, and cannot be through-hole plated in a conventional way, so a single-sided layout with surface-mount parts is standard. Aluminium expands at about 23 ppm/°C against 6 to 8 ppm/°C for ceramic LED packages, so large boards need solder-joint fatigue checks. See PCB design for manufacturing for panelisation and assembly guidance. Verify the design by measuring the aluminium-base temperature with a thermocouple, then compute junction temperature as Tj = Tcase + P × Rjc. For design reviews and team projects, the engineers at Avecas apply these exact checks.

FAQ

Why not just use more thermal vias on FR-4?

Vias help, but a typical array only lowers FR-4 resistance to a few K/W in a small area. An MCPCB gives well under 1 K/W with no vias and spreads heat in a solid metal plate.

Does a thicker aluminium base help much?

Only for spreading. Going from 1.0 to 2.0 mm lowers lateral resistance, but it leaves the dielectric resistance unchanged, so gains flatten quickly.

Can I put LEDs in parallel if they come from the same bin?

Same-bin parts reduce the mismatch but do not remove the thermal feedback. Add per-string current regulation if you need parallel branches.

What dielectric conductivity should I specify?

Start at 2 W/m·K for general lighting. Choose 3 W/m·K or higher when LED power density exceeds roughly 1 W/cm² or the ambient is above 50 °C.