Heat dissipation in a sealed aluminum enclosure: fins, pads and when to vent

Start with the arithmetic

Heat leaves a sealed box by two paths: convection to the air around it, and radiation. For a first estimate, use a combined coefficient of h ≈ 6–10 W/(m²·K) for natural convection in still air, with the higher end for a vertical, finned, dark surface.

ΔT = P / (h × A)

A 100 × 68 × 50 mm housing has an external area of roughly 0.03 m². With h = 7 and P = 10 W:

ΔT = 10 / (7 × 0.03) ≈ 48 K

So a 25 °C room becomes 73 °C at the housing wall — and the junction inside is hotter still. The same box at 5 W runs about 24 K above ambient.

Two things follow. First, small boxes cannot dissipate much. Second, the internal path matters as much as the external area: if the heat source is not coupled to the wall, the wall stays cool while the component cooks.

The internal thermal path

Heat must travel: junction → package → PCB → (air gap or pad) → housing → ambient.

That gap between the component and the wall is usually the bottleneck. Air is a terrible conductor (0.026 W/m·K). A 1 mm air gap under a 10 × 10 mm package is roughly 380 K/W; a 1 mm gap pad at 3 W/m·K is about 3 K/W.

Options, cheapest first:

1. Move the hot part to the wall. Mount the MOSFET or regulator flat against the aluminum with a thermal pad and a screw. This is free and usually the biggest win.
2. Gap pad / thermal tape. 0.5–2 mm silicone pad, 1.5–6 W/m·K, die-cut to the footprint.
3. Thermal potting. Filled epoxy or silicone, 1–3 W/m·K, fills everything and adds IP protection — but it is permanent and adds mass.
4. Aluminum PCB or heat spreader clamped to the housing for high-power LED or motor drive designs.
5. Heat pipe or vapour chamber — only when nothing else fits.

External options

Fins. In an extrusion, fins cost nothing extra: they are part of the profile. Vertical fins spaced 4–6 mm with 10–15 mm depth roughly double the effective area. Fins work when the box is mounted vertically; horizontal fins trap hot air.

Surface finish. Black anodize raises emissivity from ~0.05 to ~0.85. Radiation is perhaps 20–30% of the total at these temperatures, so expect a 10–20% improvement — but it is free and also looks better.

Mounting. A housing bolted to a steel panel conducts heat into the panel. A housing on rubber feet in still air does not.

Vent. A membrane vent does not add much convective cooling, but it prevents pressure cycling and lets a small amount of vapour escape. It is a reliability measure, not a thermal one.

When to use a fan

Only when the arithmetic says you are 15 K over budget and there is no room for fins. A fan adds a failure mode, noise, dust ingress, and it fights the IP rating. A 40 mm fan at 8 m³/h is worth roughly 5–10 W of dissipation in a small enclosure — sometimes that is the whole problem solved.

A worked example

Target: 5 V, 3 A power module, 12 W dissipated, ambient 40 °C max, junction limit 105 °C.

1. Allowed rise: 65 K total.
2. Junction to case (module): 3 K/W × 12 W = 36 K.
3. Case to housing via 1 mm gap pad (3 W/m·K, 20 × 20 mm): 0.83 K/W × 12 W ≈ 10 K.
4. Housing to ambient: 65 − 36 − 10 = 19 K available.
5. Required h × A = 12 / 19 = 0.63 W/K. With h = 7, A = 0.09 m² — a 150 × 100 × 60 mm finned housing.

Change the gap pad to a direct screw mount and step 3 drops to 2 K, gaining 8 K of margin — enough to use a smaller box.

Practical checklist

• Mount power components to the wall, not in the middle of the board.
• Specify a finned profile if the load is above 5 W.
• Use a gap pad with a published W/m·K figure, not "thermal tape".
• Keep electrolytic capacitors away from the hot wall.
• Black anodize if the housing is visible.
• Add a vent if the box is sealed and cycles in temperature.
• Test at maximum ambient with the real load, not at 25 °C on a bench.