Telecom Equipment Cooling: Outdoor Cabinets and Radios
Short answer: Outdoor telecom equipment is cooled almost entirely by natural convection and radiation, because fans and vents compromise the IP seal that keeps rain, dust and insects out. That means the enclosure wall itself is the heat sink: die-cast or extruded aluminium bodies with vertical fins, sized for a typical 0.5–2.5 °C/W case-to-ambient resistance, black anodised or powder coated for emissivity, and sealed to IP55–IP66. A 40 W remote radio head in 45 °C ambient air needs roughly 2–4 kg of finned aluminium to stay under its 85 °C junction limit. BQUQ machines, casts and finishes these heat sinks in one Dongguan ISO9001 factory, with quotes in 12 working hours.
Why outdoor telecom cooling is a different problem
Indoor electronics cooling is mostly an airflow problem. You have a fan, a filtered intake, and a controlled room. Outdoor telecom has none of that. A pole-mounted remote radio head (RRH) in Dubai, a street cabinet in Singapore, and a microwave radio on a Norwegian mast all share the same constraints:
- No forced air. Fans draw in dust, salt fog, and insects. They also fail first — a fan bearing is the shortest-lived component in any outdoor enclosure.
- Sealed enclosures. Once you seal to IP55 or IP66, convection inside the box becomes negligible. Heat must conduct through the wall.
- Solar load. A dark cabinet in direct sun can see an effective ambient 10–15 °C above air temperature. Radiative and solar-absorptive properties of the coating matter as much as fin area.
- Wide ambient range. −40 °C to +55 °C is a common spec. Thermal design must survive both ends, including differential expansion between the housing and the PCB.
- Zero maintenance windows. A tower climb or a cabinet visit costs far more than the heat sink. Design life is 10–20 years.
The practical consequence: in outdoor telecom, the heat sink is the enclosure. Design decisions about fin geometry, wall thickness, alloy, and surface finish are mechanical, thermal, and environmental decisions at the same time.
How much heat do outdoor radios and cabinets actually dissipate?
Typical figures, useful for first-pass sizing:
| Equipment | Typical heat load | Cooling method | Notes |
|---|---|---|---|
| Remote radio head (RRH), 2–4 TX | 40–120 W | Natural convection, finned die-cast housing | Fins vertical, often 8–20 mm tall |
| Active antenna unit (AAU), massive MIMO | 150–400 W | Natural convection + internal heat spreader | Higher fin density, tighter ΔT budget |
| Microwave / mmWave radio | 20–60 W | Natural convection, sealed cast body | Small footprint, high fin efficiency needed |
| Outdoor street cabinet (BBU, power, battery) | 200–1500 W | Natural convection, heat exchanger, or AC unit | Often needs a door-mounted heat exchanger |
| Pole-mounted small cell | 15–50 W | Passive, integrated housing fins | Aesthetic constraints limit fin height |
| OLT / FTTx street box | 30–100 W | Passive or thermoelectric | Often buried or pedestal mounted |
These are indicative ranges. Real numbers come from the radio vendor's thermal spec, which usually gives a maximum case temperature and a maximum ambient. The design task is to close the gap between them.
The governing equation, and why it is simple outdoors
For a sealed outdoor enclosure, the thermal path is a chain of resistances:
1. Junction to case (R_jc) — set by the component and its interface, not by you.
2. Case to housing — set by the thermal interface material and mounting pressure.
3. Housing to ambient (R_sa) — this is the heat sink, and it is where you have leverage.
R_sa combines three mechanisms:
- Convection, natural, on the finned surface. Typically 3–8 W/m²·K for vertical fins in still air, rising with fin height and chimney effect.
- Radiation, which contributes 20–40 % of total dissipation for a black anodised surface and much less for bare or bright metal.
- Conduction through the housing wall and any internal spreader.
A useful rule of thumb for a well-designed vertical-fin aluminium heat sink in still air: R_sa ≈ 0.5–2.5 °C/W for a body in the 150 × 200 × 60 mm class. A 100 W load at 1.0 °C/W gives a 100 °C rise — which is why large AAUs use bigger bodies, higher fin counts, and sometimes internal heat pipes or vapour chambers to spread heat to the far end of the housing.
For a deeper look at how base thickness and spreader design affect this chain, see heat sink base thickness and spreading resistance.
Material and process choices for telecom heat sinks
| Process | Best for | Typical wall / fin | Relative tooling cost | Notes |
|---|---|---|---|---|
| Die casting (AlSi alloys) | RRH and AAU housings, complex sealed bodies | 2.5–4 mm wall, 1.5–2.5 mm fin tip | High | Best shape freedom, integrated bosses and gaskets |
| Aluminium extrusion | Cabinet wall panels, linear fin arrays | 2–3 mm fin, 10–40 mm tall | Low–medium | Constant cross-section only; excellent cost per kg |
| Skived fin | High-density copper or aluminium fin fields | 0.3–0.8 mm fin, 10–30 mm tall | Medium | Best fin density per unit volume |
| Bonded fin (epoxy) | Mixed materials, copper base + Al fins | 0.5–1.5 mm fin | Low | Watch out for thermal cycling and TIM pump-out |
| CNC machining | Prototypes, low volume, tight tolerances, sealing faces | Any | Low (no tool) | ±0.005 mm achievable on critical faces |
| Forging | High-volume small bodies | 3–6 mm | High | Good grain structure, limited fin aspect ratio |
For telecom specifically, die-cast aluminium dominates the radio housing, and extruded aluminium dominates cabinet panels and pole-mount brackets. Copper is used selectively — for spreaders, heat pipe envelopes, and where a small footprint must move a lot of heat.
Alloy choice matters more than most buyers expect. The trade-offs between 6063, 6061, 1050, and ADC12 are covered in aluminium alloys compared for heat sinks.
Sealing, coatings and the IP problem
Every vent you add to a sealed enclosure is a liability. That is why outdoor telecom cooling converges on a small set of architectures:
Fully sealed, passive
The housing is the heat sink. No openings at all. IP66 or IP67 achievable. Used for RRHs, small cells, and microwave radios. Thermal performance is entirely a function of surface area, fin geometry, and coating emissivity.
Sealed with a heat exchanger
Two isolated air paths: internal air circulates over a finned block, external air over a separate finned block, joined by a conductive wall. Used for larger cabinets where passive area alone is insufficient. No air crosses the boundary, so IP is preserved.
Sealed with a closed-loop chiller or AC
Used for battery cabinets and high-power BBU racks. Higher maintenance, but the only way to hold a tight internal temperature in extreme ambient.
Coating and emissivity
A black anodised or matte black powder-coated surface has an emissivity around 0.85–0.90, versus roughly 0.05–0.10 for bright bare aluminium. In still air, radiation can carry 25–40 % of the heat, so the finish is a thermal decision, not just a cosmetic one. The same coating must survive salt fog, UV, and 20 years of thermal cycling. Anodising and powder coat behave differently under UV and salt spray — the comparison is in anodising versus bare fin finishes.
Gaskets and interfaces
A compressed gasket is a thermal insulator. Where a PCB or module bolts to the housing wall, use a thin, high-conductivity gap filler rather than relying on the gasket. Bolt spacing and mounting pressure matter as much as the material — under-torqued interfaces are a common cause of field failures.
Design rules that hold up in the field
1. Fins vertical. Natural convection needs a chimney. Horizontal fins on a pole-mounted radio can lose 30–50 % of their capacity. If the mounting orientation is fixed, design the fin direction to match gravity, not the drawing's convenience.
2. Fin spacing 8–15 mm for natural convection. Tighter spacing increases area but chokes the boundary layer. Below about 6 mm, natural convection gains flatten out and dust bridging becomes a real risk.
3. Fin height 15–40 mm. Beyond about 40 mm, fin efficiency drops unless the fin is thick or the base is very isothermal.
4. Keep the base flat and thick enough to spread. A 3 mm base under a 40 mm fin is a bottleneck. Spreading resistance usually dominates in compact outdoor bodies.
5. Design for solar load. Add 10–15 °C to the ambient for a sun-exposed dark cabinet, or specify a light top surface and a shaded fin field.
6. Avoid thermal short circuits. A metal bracket that bridges the hot housing to a cold mounting plate can help — or can dump heat into a sensitive component. Model the whole assembly, not just the heat sink.
7. Protect the fin tips. Field handling bends fins. A 1.5–2 mm tip thickness or a protective rail at the fin ends pays for itself in yield and in service.
8. Plan the interface for TIM pump-out. Wide thermal cycling on a bolted interface can migrate grease away from the hot spot over years. Phase-change materials or pads are usually more stable outdoors.
Manufacturing tolerances that matter for telecom heat sinks
Telecom heat sinks are not just thermal parts — they are structural and sealing parts. Critical dimensions typically include:
| Feature | Typical tolerance | Why it matters |
|---|---|---|
| Gasket groove depth and width | ±0.05 mm | IP55/IP66 seal integrity |
| Mating flange flatness | 0.05–0.10 mm | Gasket compression uniformity |
| Fin tip thickness | ±0.10 mm | Handling damage and convection area |
| Mounting hole position | ±0.10 mm | Alignment with PCB and bracket |
| PCB interface flatness | 0.05 mm over 100 mm | TIM bond line thickness |
| Threaded boss depth | ±0.15 mm | Bolt engagement and torque |
BQUQ machines sealing faces and PCB interfaces to ±0.005 mm on CNC centres when the drawing calls for it, and holds cast or extruded features to the tolerances above. Because casting, machining, finishing, and inspection sit on four production lines in one Dongguan factory, tolerance stack-ups between processes are controlled in one place rather than argued across three suppliers.
Sourcing checklist for telecom thermal parts
Before you send an RFQ, confirm:
- Heat load and ambient — watts, maximum ambient, solar exposure, mounting orientation.
- Maximum case or junction temperature — this defines the allowable R_sa.
- IP rating and test method — IP55, IP66, IP67, and whether it is tested or declared.
- Coating spec — anodising thickness (typically 10–25 µm) or powder coat type, colour, and salt-spray hours.
- Alloy and process — die cast, extruded, skived, or machined; and whether tooling is amortised or owned.
- Interface details — TIM type, bolt pattern, torque spec.
- Volume and MOQ — prototype quantities versus production ramp.
- Documentation — material certs, dimensional reports, and finish certificates.
BQUQ quotes in 12 working hours and works with flexible MOQ, so a 10-piece prototype run and a 50,000-piece production order can go through the same process route. Explore the heat sink product range, extruded heat sinks, and CNC machined heat sinks for starting points.
Frequently Asked Questions
Q: Can I use a fan on an outdoor telecom cabinet?
A: Only with a filtered, IP-rated air path, and usually not for pole-mounted radios. Fans add a wear item, draw dust and moisture, and fail long before the electronics do. Most outdoor telecom designs use passive convection plus radiation, or a sealed air-to-air heat exchanger that keeps the two air streams separate.
Q: What fin spacing is best for natural convection?
A: For vertical fins in still air, 8–15 mm spacing is the practical sweet spot. Tighter spacing adds surface area but chokes the boundary layer, and below roughly 6 mm the thermal gain is small while dust bridging and cleaning problems grow. Fin height of 15–40 mm usually balances area against fin efficiency.
Q: Does the colour of the heat sink really matter?
A: Yes, for outdoor passive cooling. A matte black anodised or powder-coated surface has emissivity around 0.85–0.90 versus 0.05–0.10 for bright bare aluminium. In still air, radiation can carry 25–40 % of the heat, so the finish is a thermal specification, not just cosmetic. It must also survive UV and salt fog.
Q: How much aluminium does a 100 W remote radio head need?
A: As a rough guide, a sealed 100 W radio in 45 °C ambient with an 85 °C case limit needs roughly 2–4 kg of finned aluminium, depending on fin geometry, orientation, and coating. That corresponds to an R_sa of about 0.4–0.8 °C/W. Final sizing should come from CFD or a thermal test on the actual housing.
Q: What is the difference between die-cast and extruded telecom heat sinks?
A: Die casting gives shape freedom — integrated bosses, gasket grooves, and complex sealed bodies — which suits radio housings. Extrusion gives lower tooling cost and better conductivity per kilogram, which suits cabinet panels and linear fin arrays. Many outdoor products use both: a cast housing with extruded fin panels bolted or bonded on.
Related Resources
- About BQUQ and our Dongguan factory: /about/
- Heat sink product range and capabilities: /heat-sinks/
- Extruded heat sink profiles: /extruded-heat-sinks/
- CNC machined heat sinks for prototypes and tight tolerances: /cnc-machined-heat-sinks/
- Industry trends in telecom and electronics cooling: /industry-dynamics/
- Technical articles and engineering guides: /bquq-blog/
- Frequently asked questions: /faq/
- Case studies: /case/
- Contact the engineering team: /contact/
Authored by the BQUQ Engineering Team. BQUQ (Dongguan) runs CNC machining (±0.005 mm), metal stamping, custom springs, and heat sink production in one ISO9001 factory. Source-direct from Dongguan, China — quote in 12 hours: sc@bquq.com | WhatsApp +86 13713157787 | www.bquq.com
