Before this:Picking a board
Cases & cooling
Key takeaways An SBC’s speed is really a thermal budget: the SoC runs full speed only while it stays cool, and slows itself down when it isn’t. Passive cooling (a heatsink, or a metal case that is a heatsink) is silent and has no moving parts to fail — ideal for appliances when it’s sized generously. Active cooling (a fan) removes more heat but adds noise and a wear item. The case decides everything: a sealed plastic box traps heat, while a vented or all-metal one sheds it. For a 24/7 decoder running sustained CPU load, cool generously — you’re buying sustained speed, not peak speed.
The last purchase decisions are the box and what’s inside it. Cooling sounds like an enthusiast concern, but for a board running DSP around the clock it decides how much of the CPU you paid for you actually get to use.
Why does a computer slow down when hot?
The SoC monitors its own temperature. As it approaches a limit (around 80–85 °C on most boards), the firmware progressively reduces the clock speed to shed heat — protective, automatic, and silent. This is thermal throttling, and Unit 5’s Thermal throttling lesson covers measuring and diagnosing it. Today’s point is the design consequence: a board’s advertised speed is its cool speed. Whether you get that speed all day is decided by the cooling you bolt on. For a bursty desktop workload, throttling is a hiccup; for a real-time decoder that must keep up with the radio every second, a throttled CPU can mean falling behind and dropping samples.
You can watch the temperature any time:
$ vcgencmd measure_temp
temp=61.2'C
What are the cooling options?
| Approach | How it works | Pros | Cons |
|---|---|---|---|
| Bare board | Convection off the chip | Free | Throttles under sustained load |
| Stick-on heatsink | Metal fins spread heat into the air | Cheap, silent | Modest gain in still air |
| Metal case as heatsink | Whole case thermally bonded to the SoC | Silent, no moving parts, dustproof | Costs more; must be a designed thermal case |
| Fan (active) | Forced airflow over sink/board | Most cooling per dollar | Noise, dust, a part that wears out |
Two nuances. A heatsink in a sealed box helps far less than the same heatsink in moving air — heat has to leave the case, not just the chip. And fans on SBCs are small and cheap: they get loud, gather dust, and are often the first component to fail on an always-on build. Many appliance builders choose a generously sized passive metal case precisely to remove that failure mode; if you do use a fan, prefer one the board can speed-control so it runs only when needed.
What should the case do besides look good?
A case for an appliance has four jobs:
- Shed heat — vented, or all-metal and thermally bonded.
- Admit the cables — power, Ethernet, USB (your SDR), and the antenna feed routing near it, without strain on connectors.
- Protect the board — dust, knocks, curious pets, and accidental shorts from stray metal.
- Mount somewhere — shelf, wall, or rack; an appliance lives where the antenna cable and Ethernet want it to, often a closet or attic corner, so mounting points matter more than desk looks.
One radio-specific note: a metal case provides mild shielding of the board’s own RF noise (a real emission source — see USB SDR gotchas), but it also blocks the board’s built-in Wi-Fi. On wired Ethernet — the appliance default — that costs nothing; if you must use Wi-Fi, check the case’s antenna provisions.
How does ambient temperature change the math?
Cooling moves heat from the chip to the surrounding air, so everything shifts with that air’s temperature. The case that’s fine on a desk in a 20 °C room may throttle in a 35 °C attic in summer — precisely where scanner appliances often live, close to the antenna. Design for the hottest day in the worst spot: prefer over-sized passive cooling, leave convection space around the case, and confirm with measurement once deployed (Monitoring your board puts temperature on a dashboard so summer can’t surprise you).
Rule of thumb: under your appliance’s real sustained load, aim for a reported SoC temperature comfortably below 70 °C. That leaves margin for summer, dust, and an aging fan you haven’t noticed stopping.
Quick check: why does cooling matter more for a 24/7 decoder than for a desktop used in bursts?
Recap
- An SBC’s advertised speed is its cool speed: the SoC throttles itself as it approaches its thermal limit, and sustained decoding is exactly the load that gets it there.
- Passive cooling (heatsink or thermal metal case) is silent with no wear items — the appliance favourite when sized generously; fans cool more but add a failure mode.
- Heat must leave the case, not just the chip — sealed plastic boxes trap it.
- Design for the hottest ambient the board will meet, and verify with
vcgencmd measure_temponce deployed. - The case’s other jobs: cable access, protection, and mounting where the appliance actually lives.
Next up: When you need more than a Pi.