Is this a dumb idea of mine for greater cooling?

A heat sink is a thermal resistor. It improves heat removal because it provides better contact with the air, lowering the that thermal resistance. More airflow will greatly improve the thermal resistance of the heat sink to air interface and so the total thermal resistance.

Adding a heat sink on top of the existing heat sink will add more thermal resistance to the heat sink path, possibly significantly because of the smaller interface area of the heat sink to heat sink contact. The question is whether the increased resistance of the second heat sink will be offset by better airflow over the protruding heat sink. This is possible if you have the case oriented so the convection airflow is optimized or there is a natural breeze from some other source.

I doubt you will need to use alcohol to remove the existing heat sink, but I expect case disassembly is required in any event, so if that is a problem your idea might be the best. I'm not sure what is the best way to connect the two heat sinks. Possibly a thin piece of thermal pad. Thinner is better as all materials are thermal resistors and thinner is a lower value of resistance.

What's "too much"? How hot does it get? What environment is it running in that requires a heatsink at all?

A heatsink in a case may not work well, depending on the overall thermal design. The case will have thermal resistance itself, and beyond a certain point, better coupling the heat source to the air inside the case will not improve things.

You can use a case *as* a heatsink with the right design - aluminium case with a thermal path (copper, aluminium, or a heat pipe) carrying the heat to the case. This would normally be complete overkill for a Pi though.

Case temps of 100C are acceptable for some silicon chips

A sealed case is almost bound to result in semiconductor failure. I did a design once in a 19" rack case fully enclosed.

It took 4 hours for the temperature inside to get above 100C, but it did.

if you must have a sealed case you must provide a low thermal resistance path to the case itself and use that as a heatsink.

Exactly.

OK, so thats a good outflow hole for hot air, but how big are the hole(s) for cold air inflow and where are they located on the case? For optimum flow they should be as low as possible and IIRC, should be bigger than the hot air outlet. Note that, if there are several inflow holes, their effective area will be less than that of a single hole with the same area due to flow turbulence round the edges of the holes disturbing smooth flow through them.

IOW, if the inflow hole(s) are smaller than the outflow hole or badly placed, making more inflow holes in the case or enlarging the existing holes may solve the problem.

electronics in the 60's if possible, to increase lifespan.

Have you guys used the Pi 3? It isn't like prior versions that generally didn't need additional cooling. It's Broadcom BCM2837 SOC gets hot very easily and the chip speed throttles to cope.

Yeah, the vents seem adequate. Though I can't say it looks optimum to me. I've yet to see a case that really tries to optimise airflow over the entire board, top and underside.

all my systems during their downtime, and I hate to see wasted processing power due to excess heat.

I didn't mean that - just that inflow needs to be close to the top of the PCB as possible, assuming its horizontal, to get some (minimal) chimney effect past the heat sink. A case with inlets just under the top won't work well without a fan.

You can almost guarantee that a case designed for use with a fan isn't going to work well without one.

I have an RPi2B running BOINC 24/7 and the cpu temperature stays around

underclock your RPi 3 to match the speed of the 2B (900MHz).

Ray

The Pi2 is very different to a Pi3. The average no temperatures for mine

are:-

Pi 1B 256MB 50.7C No heatsink Pi 1B 512B 47.0C Heatsink Pi 2B 44.3C No heatsink Pi 3B 55.1C Heatsink

As you can see the Pi2 runs the coolest of all with no heatsink, and the

Pi3 runs hottest even with a heatsink.

---druck

How about fitting 6 inches of chimney directly above the existing heatsink and making sure that there is a good airflow over the board? That way you can use the chimney effect to accelerate the airflow to give better cooling.

It completely defeats the point of using something as small as the Raspberry Pi. If heat is that much a problem, use something else with proper passive or active cooling.

---druck

It also won't work. I tried measuring air flow in a 4 inch tube suspended over a 100 watt lightbulb. I could detect no airflow and the air at top of the chimney was the same as the room temperature. I'm sure the 2 watts of the rPi won't move much air either.

Pretend it's a ZX80 and put a carton of cold milk from the fridge on top of it.

Owain