Re: finger-burningly hot = well designed for passive cooling
> > And the Quark chip runs finger-burningly hot.
> Presumably it is engineered to do so. As were Atoms before.
agreed! :-) My words exactly. Many ATOM-based fanless designs are a joke.
Compare that to the Vortex86. You can typically hold your finger on that, even without any kind of a heatsink if the board is large enough. On tiny boards, it takes a small passive heatsink that you can still keep your finger on after some runtime. That for the Vortex86DX clocked at 800 MHz at full throttle. With some power saving and underclocking, it doesn't take a heatsink.
> And any chip well-designed for passive cooling
> (because you need a fairly large delta-T before convection gets going).
Thanks for explaining the mindset of all the nameless Chinese R&D folks.
I'm on the other side of the barricade - I'm a troubleshooter with a small industrial/embedded hardware distributor, I'm effectively paid by system integrator people (our customers) to sooth their "burned fingers".
Imagine that you need an embedded PC board, at the heart of some book-sized control box. That box will be mounted in a cabinet. The folks at Westermo used to say that every "enclosure" adds about 15 degrees Celsius of temperature. And you have maybe 2-3 enclosures between your heatsink and the truly "ambient" temperature. In my experience, that 15 degrees applies to very conservatively designed electronics, with sub-1W-class ARM MCU's on the inside. For computers worth that name, where you aim for some non-trivial compute power, the 15* are a gross under-estimation. You have to calculate with Watts of power consumption = heat loss, thermal conductivity at surfaces and thermal capacity of coolant media (typically air) - even in the "embedded PC" business, far from the server collocation space.
Note that there are electrolytic capacitors on the motherboard, surrounding the CPU or SoC and VRM etc. They're not necessarily the solid-polymer variety. With every 10*C down, the longevity of these capacitors doubles. For low-ESR capacitors, It's typically specified at 2000 hours at 105*C. Twice that at 95*C etc.
Now... back to the mindset of our typical customer: it's fanless, right? so we can mount this in a tiny space in an unventilated box, run some cables in the remaining tight space on the inside... and sell that as a vehicle-mounted device... and remain unpunished, right? Let's go ahead with that...
(Where do you get convection around the CPU heatsink in that?)
The typical mindset of our suppliers' R&D is: let's sell this with the minimum possible heatsink, that will alow our bare board survive a 24hour burn-in test in open air, without a stress-load application running on the CPU (just idling).
Some particular fanless PC models are built in the same way. The most important point is to have an aluminium extrusion enclosure with some sexy fins on the outside. It doesn't matter if only a half of them is actually thermocoupled to the CPU and chipset, never mind the RAM and VRM and all the other heat sources on the inside (they'll take care of themselves). The enclosure needs to have fins and some cool finish, for eyewash effect - make it golden elox or harsh matte look. If it looks real mean, all the better - you can put the word "rugged" in your PR datasheets. Never mind if the surface of the fins is clearly insufficient to dissipate 15W of heat, on the back of an envelope (or just by the looks, to a seasoned hardware hacker). Perhaps also the computer maker's assembly team add a cherry on top, by optimizing the thermocoupling a bit: you can relax your aluminium milling tolerances a bit if you use 1 mm of the thermocouple chewing gum. Never mind that the resulting thermal coupling adds 20 Kelvins of temperature gradient. Even better, if the internal thermal bridge block designed by R&D is massive enough, you can probably just skip thermocouple paste or chewing gum alltogether, to accelerate the seating step on the assembly line... It takes maybe 20 minutes at full throttle before the CPU starts throttling its clock due to overheating, and the QC test only takes 2 minutes and is only carried out on every 20th piece of every batch sold.
Customer projects (close to the end user) that go titsup get later settled between purchasing and RMA departments and various project and support bosses and maybe lawyers - no pissed off troubleshooter techie has ever managed to wrap his shaking fingers around the faraway R&D monkey's throat :-)
If anyone is actually interested in practical advice, to get long-lived embedded PC's in production operation, I do have a few tips to share:
If you can keep your fingers on it, and it doesn't smell of melting plastic, it's probably okay. Do this test after 24 hours of operation, preferably in the intended target environment (enclosure, cabinet, ambient temp).
If you insist on playing with high-performance fanless stuff, do the heat math. You don't need finite-element 3D modeling, just back of the envelope math. What are the surfaces of your enclosures, times the heat transfer coefficients, times the wattage. What gradients can you come up with? Pay attention to all the heat-producing and hot components on your PCB's. All the parts inside your fanless enclosure principally run hotter than the "thermocoupled envelope". Putting sexy inward-facing heatsinks on hot components doesn't help much, inside a fanless enclosure. Consider that adding a tiny fan will turn this "roast in your own juice" feature of a fanless enclosure inside out.
If you intend to purchase third-party off-the-shelf fanless PC's for your project (complete with the finned enclosure), take a few precautionary masures: Take a look inside. Look for outright gimmicks and eyewash in thermal design, and for assembly-level goof-ups (missing thermocouple paste). Install some OS and run some burn-in apps or benchmarks to make the box guzzle maximum possible power. If there are temperature sensors on the inside, watch them while the CPU is busy - lm_sensors and speedfan are your friends. Some of the sensors (e.g. the CPU's coretemp) can be tricky to interpret in software - don't rely on them entirely, try opening the box and quickly touching its internal thermocoupling blocks and PCB's close around the CPU.
Single-board setups should be generally more reliable than a tight stack of "generic CPU module (SOM/COM) + carrier board" - considering the temperature dilatation stresses between the boards in the stack. In fanless setups, the optimum motherboard layout pattern is "CPU, chipset, VRM FET's and any other hot parts on the underside" = easy to thermocouple flat to the outside heatsink". Note that to motherboard designers, this concept is alien, it may not fit well with package-level pinout layouts for easy board routing.
Any tall internal thermal bridges or spacers are inferior to that design concept.
Yet unfortunately the overall production reliability is also down to many other factors, such as soldering process quality and individual board-level design cockups... so that, sadly, the odd "big chips on the flip side" board design concept alone doesn't guarantee anything...
If you're shopping for a fanless PC, be it a stand-alone display-less brick or a "panel PC", notice any product families where you have a choice of several CPU's, say from a low-power model to a "perfomance mobile" variety. Watch for mechanical designs where all those CPU's share a common heatsink = the finned back side of the PC chassis. If this is the case, you should feel inclined to use the lowest-power version. This should result in the best longevity.
If you have to use a "closed box with fins on the outside" that you cannot look inside, let alone modify its internals, consider providing an air draft on the outside, across its fins. Add a fan somewhere nearby in your cabinet (not necessarily strapped straight onto the fins).
Over the years, I've come to understand that wherever I read "fanless design", it really means "you absolutely have to add a fan of your own, as the passive heatsink we've provided is barely good enough to pass a 24hour test in an air-conditioned lab".
If your outer cabinet is big enough and closed, use a fan for internal circulation. Use quality bearings (possibly ball bearings or VAPO), possibly use a higher-performance fan and under-volt it to achieve longer lifetime and lower noise. Focus on ventilation efficiency - make sure that the air circulates such that it blows across the hot parts and takes the heat away from them.
Even an internal fan will cut the temperature peaks on internal parts that are not well dissipated/thermocoupled, thus decreasing the stress on elyt caps and temperature-based mechanical stresses (dilatation) on bolts and solder joints. It will bring your hot parts on the inside to much more comfortable temperature levels, despite the fact that on the outer surface of your cabinet, the settled temperature will remain unchanged!
If you merely want a basic PC to display some user interface, with no requirements on CPU horsepower, and for some reason you don't like the ARM-based panels available, take a look at the Vortex. Sadly, Windows XP are practically dead and XPe are slowly dying, and that's about the ceiling of what Vortex can run. Or you can try Linux. You get paid off by 3-5 Watts of power consumption and hardware that you can keep your finger on.
Examples of a really bad mindset: "I need a Xeon in a fanless box, because I like high GHz. I need the absolute maximum available horsepower." or "I need high GHz for high-frequency polling, as I need sub-millisecond response time from Windows and I can't desing proper hardware to do this for me." or "I need a speedy CPU because I'm doing real-time control in Java and don't use optimized external libraries for the compute-intensive stuff". I understand that there *are* legitimate needs for big horsepower in a rugged box, but they're not the rule on my job...