I need to add a heatsink to a small product. It has a flat exposed flush heatsink now; I'm extending with fins.
But the attachment is an issue. Do the pads you put between a CPU & heatsink provide any stick? Or are the depending on external pressure to compress them? [I have no way to do that...]
For the record, this is a Mobius camera.
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What do you want to do, and why ?
Temporary or permanent ?
Pads don't usually have optimum heat transfer. Metal to metal contact is optimum, unless surfaces are really off.
Does the camera instructions say, add heat sink ?
Greg
On Fri, 11 Oct 2013 04:35:15 +0000 (UTC), David Lesher wrote as underneath :
Pads normally require mechanical pressure permanently. You can use epoxy resin as long as the layer is thin and you prepare the surfaces for mechanical key. Used this method on componants running too hot to last! Fast set epoxy is better because it never quite seems to set as rock solid as slow cure and thus shouldnt shatter under shock. C+
Some have "stick", some do not.
Most of the CPU-style pads for larger heatsinks and larger CPUs, aren't intended to provide structural strength. They couldn't do so adequately in these cases... some of the modern heatsinks weigh a good fraction of a pound... the heatsinks come with spring-loaded retainers which clip onto tabs on the CPU socket, or screws which mate directly onto the motherboard. The thermal transfer pads for these CPUs are optimized for high thermal conductivity - some of them are "phase change" compounds which (in effect) melt the first time the CPU heats up, and flow out to form a very thin interface layer.
Heatsinks for smaller embedded CPUs, GPUs, etc. are fairly light, and these are often fastened using a thermal adhesive (pad, or a paste or cream). Loctite used to sell tubes of a thermally-conductive adhesive to use used for this purpose... likely they still do.
That is contrary to everything I've ever known concerning how to couple a heat sink to a device. Almost without exception, the surfaces are, to use your term, "really off"; they never make contact at more than a very few small places. The purpose of the pad, or silicone grease, is to fill up the spaces where the metal does not touch. The pad or silicone grease is not a very good conductor compared to copper, but it is a whole lot better than still air, which is otherwise all that couples the heat from one piece of metal (source) to the other (sink).
In fact, because the pads are a lot easier to apply properly than grease, the pads usually perform a bit better.
If you really need to attach the sink to the device with an adhesive, try RTV silicone rubber, which has nearly as good thermal performance as silicone grease, and will not weaken with heat. Be sure to use as thin a layer as possible.
As a very general comment, small heat sinks attached to small devices will only provide marginal improvements. It's always surprising, when I have done thermal calculations, to find out just how large a heat a sink has to be, to do much good ...
Isaac
"David Lesher"
** Suggest you lightly sand the mating surfaces with fine emery paper and then smear with "super glue".Hold together tightly for several minutes.
Should be reasonably strong.
.... Phil
People who overclock their CPUs or GPUs will often "lap" both the top of the chip and the bottom of the heatsink, in order to reduce this surface roughness and improve the metal-to-metal or metal-to-ceramic contact. The preferred method seems to be something on the order of "a flat sheet of plate glass, with a sheet of very fine wet-and-dry abrasive paper taped to it", some water as a lubricant, and some manual work and care.
From what I've heard, the biggest problem with the use of heat-sink cream or grease, is that people tend to use much too much of it... too thick a layer. The method I've seen recommended, is to spread some on each surface, and then gently scrape or plane most of it off with a single-edge razor blade, so that you leave only a very thin film... enough to fill the "low spots" in the surface, but not much more than that.
Pads (especially the phase-change kind) can avoid the "too thick a layer" problem to some extent, I imagine, if they're made thin enough. Some of them are more "solid films" than "pads".
"isw"
** The opposite is true.
Even pressure is crucial to getting good thermal transfer, the usual bolt in the top part of a TO220 or TO3P metal tab is a horrible way.
... Phil
The camera has a reputation for running hot. In fairness, the original market was RC aircraft where forced air cooling is readily available. Other non-RC users have added heatsinks.
I studied the thermal conductivity issue a few years ago; I was coupling sensors to copper pipes & comparing approaches. What I found was any of the pastes & conductive epoxies I found had thermal resistances so high, vice metal-to-metal, they looked useful only against air gaps. In other words, they help by not being air, but lapping is a far better solution.
-- A host is a host from coast to coast.................wb8foz@nrk.com & no one will talk to a host that's close........[v].(301) 56-LINUX Unless the host (that isn't close).........................pob 1433 is busy, hung or dead....................................20915-1433
Buy a heatsink made for the CPU. It will have the proper spring. I use phase change pads with sticky on them, unless I can't match the surface. I also have the white stuff and mica insulators.
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